JPH08142615A - Method for detecting pneumatic pressure drop of tire - Google Patents

Abstract

PURPOSE: To surely detect the drop of the pneumatic pressure of a tire based on the physical phenomenon that the wheel speed ratio is fluctuated according to the vehicle speed change when the pneumatic pressure of the tire is dropped. CONSTITUTION: While the vehicle stable driving condition where the pneumatic pressure drop judgement based on the wheel speed ratio is not impeded is achieved, and the estimated condition flag is raised by an estimated condition judging part 162, a by-speed range wheel speed ratio calculating part 163 calculates the by-speed range wheel speed ratio referring to the vehicle speed based on the wheel speed ratio calculated by a wheel speed ratio calculating part 161. A pneumatic pressure drop judging part 165 raises a pneumatic pressure drop judging flag when the difference between the maximum value and the minimum value of the by-speed range wheel speed ratio calculated by a wheel speed ratio fluctuation calculating part 164 exceeds the judgement value.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for detecting a decrease in tire air pressure, and more particularly, to accurately detect a decrease in tire air pressure in a four-wheeled vehicle from the wheel speed to improve safety in driving and steering stability. The present invention relates to a method for detecting a decrease in tire air pressure.

[0002]

2. Description of the Related Art When the tire pressure is lowered, the tire may be damaged or the steering stability of the vehicle may be impaired.
Therefore, as disclosed in JP-A-3-135806 and the like, it is known to detect tire air pressure based on an output signal from a pressure sensor incorporated in a wheel.
It is also known to indirectly detect the tire pressure from the detection result such as the variation of the wheel speed or the variation of the distance between the vehicle body and the road surface. Further, as disclosed in JP-A-63-305011, the angular velocity of each wheel is detected, and the sum of the angular velocity of a pair of wheels on a diagonal line and the sum of the angular velocity of a pair of wheels on another diagonal line is detected. It is known that the deviation between the two and the average angular velocity are obtained, and the tire pressure drop is determined based on the comparison result between the above-mentioned deviation and the average angular velocity and the comparison result between the angular velocity of each wheel and the average angular velocity.

[0003]

However, in order to detect the tire pressure based on the output of the pressure sensor, a pressure sensor is required and the wheel side processing for incorporating the sensor into the wheel is required, resulting in a high cost. Further, when the tire air pressure is detected based on the change in the wheel speed, the change in the wheel speed due to the decrease in the tire air pressure is extremely small, and the influence is exerted when the vehicle is turning or accelerated / decelerated. It is difficult to accurately detect the tire pressure.

Further, as suggested in Japanese Patent Laid-Open No. 63-305011, it is conceivable to calibrate the wheel speed signal to improve the accuracy of air pressure drop determination.
When the determination system is configured to perform some manual operation on the driver side during this calibration, if the manual operation on the driver side is incorrect, accurate air pressure drop determination will be hindered.

The present inventor has conducted various experiments on tire properties in connection with detection of a decrease in tire air pressure, and as a result of repeated studies, it has been found that when the air pressure of any wheel decreases, the wheel speed ratio changes in accordance with the change in vehicle speed. Has been recognized that fluctuates. Therefore, the present invention reliably detects a decrease in tire air pressure without requiring calibration by determining whether or not there is a change in the wheel speed ratio due to a change in vehicle speed, preventing tire damage, and improving vehicle steering stability. It is an object of the present invention to provide a method for detecting a decrease in tire air pressure that can achieve the above.

[0006]

A tire pressure drop detecting method according to a first aspect of the present invention is a wheel speed detecting process for detecting two or more wheel speeds of front, rear, left and right wheels of a vehicle, and front and rear, left and right wheels. The vehicle acceleration / deceleration state detection process for detecting one or more acceleration / deceleration states, the steering state detection process for detecting the steering state of the vehicle, and the vehicle body speed of the vehicle from two or more wheel speeds of the front, rear, left, and right wheels. Based on the vehicle speed detection stroke for detecting the vehicle speed and the vehicle speed detected in the vehicle speed detection stroke, a wheel speed ratio is calculated for each vehicle speed range from the wheel speeds of two or more wheels detected in the wheel speed detection stroke. Based on the vehicle speed, the wheel acceleration / deceleration state, the steering state, and the braking device operating state, the vehicle motion state is calculated based on the wheel speed ratio calculation process, the braking device operating condition detection process for detecting the operating condition of the braking device of the vehicle. Judgment vehicle movement state judgment process , The difference between the maximum value and the minimum value of the plurality of wheel speed ratios calculated for each speed range in the wheel speed ratio calculation process that is repeatedly executed while the vehicle stable driving state is determined, and the maximum When the difference between the value and the minimum value becomes larger than the first predetermined value, at least one of the front, rear, left and right wheels is determined to be in a low air pressure state.

According to a second aspect of the present invention, in the method of the first aspect, the wheel is repeatedly executed while the tire air pressure drop determination stroke is determined during the vehicle motion state determination stroke to determine that the vehicle is in a stable driving state. When the wheel speed ratio calculated in the speed ratio calculation process exceeds the second predetermined value continuously for a certain period of time,
It is characterized in that it includes a second determination stroke for determining that at least one of the front, rear, left and right wheels is in a state of low air pressure.

According to a third aspect of the present invention, in the method according to the first or second aspect, the wheel speed ratio calculation step is executed when the tire pressure drop determination step determines the vehicle stable operation state for the first time in the vehicle motion state determination step. The difference between the wheel speed ratio calculated in step S4 and at least one of the plurality of wheel speed ratios calculated in the wheel speed ratio calculation step that is repeatedly executed while the vehicle stable operation state is subsequently determined is the third. When it becomes larger than the predetermined value, the third determination process including the third determination process that determines that at least one of the front, rear, left, and right wheels is in the air pressure lowering state is performed, and the third determination process is performed every time the vehicle travels with engine start. It is characterized by

According to a fourth aspect of the present invention, in the method of the first aspect, the wheel acceleration / deceleration state detection process includes a process of detecting acceleration of each of the front, rear, left and right wheels, and the steering state detection process includes a process of detecting a steering angle. Including that, in the vehicle operation state determination process, the vehicle speed detected in the vehicle speed detection process is larger than the fourth predetermined value, the operation of the braking device is not detected in the braking device operation state detection process, and The detected steering angle is the fifth
Acceleration of each of the front, rear, left, and right wheels detected in the wheel acceleration / deceleration state detection stroke is smaller than the sixth predetermined value, and the acceleration of each of the front, rear, left, and right wheels detected in the wheel acceleration / deceleration state detection stroke. Is smaller than the seventh predetermined value, the process includes a step of determining that the vehicle is operating in a stable state.

According to a fifth aspect of the present invention, in the method of the fourth aspect, the vehicle motion state determining step changes the fifth and sixth predetermined values in accordance with the vehicle speed detected in the vehicle speed detecting step. It is characterized by including. According to a sixth aspect of the present invention, in the method of the first aspect, the wheel speed detection process includes a process of detecting the wheel speed of each of the front, rear, left and right wheels, and the wheel speed ratio calculation process is on one diagonal line of the vehicle. The value obtained by subtracting the sum of the wheel speeds of the pair of wheels on the other diagonal from the sum of the wheel speeds of the other wheels is divided by the average value of the wheel speeds of the front, rear, left, and right wheels to obtain the diagonal sum wheel speed ratio. Is included as a wheel speed ratio.

[0011]

According to the method of claim 1, during traveling of the vehicle, two or more wheel speeds of the front, rear, left and right wheels, an acceleration / deceleration state of one or more of the front, rear, left and right wheels, and a steering state of the vehicle, The operating state of the vehicle braking device is detected. Further, the vehicle speed is detected from the wheel speed, and the wheel speed ratio is calculated for each speed range of the vehicle based on the wheel speed while referring to the vehicle speed. Further, the operating state of the vehicle is determined based on the vehicle speed, the wheel acceleration / deceleration state, the steering state, and the braking device operating state.

Then, the difference between the maximum value and the minimum value of the wheel speed ratio calculated for each speed range is calculated while the vehicle is in a stable driving state. Then, if the difference is larger than the first predetermined value, it is determined that the tire air pressure has dropped. When the vehicle is in a stable driving state, factors other than the tire pressure have little influence on the wheel speed ratio, and therefore a wheel speed ratio that well reflects the tire pressure is required. Therefore, while the vehicle is in a stable driving state, the error factor for the air pressure drop determination based on the wheel speed ratio is removed.

If the tire pressures of the four wheels are not reduced, the tire rolling radius of the four wheels and thus the wheel speeds similarly change as the vehicle speed changes, so that the wheel speed ratio does not change even if the vehicle speed changes. On the other hand, when the tire pressure decreases, the tire rigidity decreases, and when the vehicle speed increases, the centrifugal force applied to the tire increases, so when one of the tire pressures decreases and the tire rigidity decreases. In addition, as the rolling radius of the wheel having the decreased air pressure and thus the wheel speed increases, the wheel speed changes more greatly than the wheel speeds of the other wheels. Therefore, the wheel speed ratio fluctuates more in the higher vehicle speed range.

According to the method of claim 1, since the wheel speed ratio variation due to the vehicle speed change is reliably detected, the tire air pressure decreases more than a certain degree while the vehicle traveling with the vehicle speed change is performed. Then, this decrease in air pressure is reliably detected. According to the method of claim 2, the wheel speed ratio calculated while the vehicle stable driving state is achieved is continuously maintained for a predetermined period of time.
If the predetermined value is exceeded, it is determined in the second determination step that at least one of the front, rear, left, and right wheels is in the air pressure reduced state. As described above, the requirement for the air pressure drop determination is that the wheel speed ratio fluctuation representing the air pressure drop continues for a certain period of time, so that the influence of noise added to the determination system is eliminated. Therefore, according to the method of the second aspect, even if the wheel speed ratio is not calibrated, when the tire air pressure decreases by a certain degree or more, this decrease in air pressure is reliably detected. In other words, it is possible to determine the decrease in air pressure based on the absolute value of the wheel speed ratio.

According to the third aspect of the present invention, the third determination step is executed every time the vehicle is run with the engine started.
In the third determination process, the difference between the wheel speed ratio calculated when the vehicle stable driving state is first determined and each of the wheel speed ratios calculated after that while the vehicle stable driving state is determined. Is required. If any of the differences is larger than the third predetermined value, it is determined that the air pressure has dropped. As a result, the change in the wheel speed ratio over time is detected using the wheel speed ratio when the vehicle stable driving state is first achieved as a criterion. Therefore, according to the method of claim 3, when the tire air pressure decreases by a certain degree or more for some reason during the passage of time after the vehicle starts traveling, this decrease in air pressure is reliably detected.

According to the method of claim 4, the acceleration of each of the front, rear, left and right wheels is detected, and the steering angle is detected. The vehicle speed is higher than the fourth predetermined value, the braking device is not activated, and the difference between the steering angle, the acceleration of each wheel, and the acceleration of each wheel is one of the fifth to seventh predetermined values. If it is smaller than the above, the stable driving state of the vehicle is determined. When the vehicle speed is low, when braking, or when the steering angle, the wheel acceleration, or the wheel acceleration difference is large, the wheel speed ratio fluctuates, and the tire pressure cannot be properly expressed by the wheel speed ratio. According to the method of claim 4, the determination of the decrease in the air pressure is prohibited in such a case, so that the erroneous determination is prevented.

In the method of the fifth aspect, the fifth and sixth predetermined values are changed according to the vehicle body speed. Therefore, it is properly determined whether or not the vehicle is in a stable driving state, and thus whether or not there is a decrease in air pressure.
According to the method of claim 6, the diagonal sum wheel speed ratio is calculated. When the diagonal sum wheel speed ratio is taken, the difference in the wheel running state (for example, acceleration / deceleration state, turning state) between the front and rear wheels and between the left and right wheels is canceled to some extent, and the influence of the vehicle operating state on the wheel speed ratio is affected. Will be alleviated. Therefore, the air pressure drop determination is appropriately performed.

[0018]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to FIG. 1, a vehicle is equipped with a controller 5 for an antilock brake system (ABS), and a right front wheel FR, a left front wheel FL, and a right rear wheel RR of the vehicle.
The front left wheel speed sensor 1, the front left wheel speed sensor 2, the rear right wheel speed sensor 3 and the rear left wheel speed sensor 4 are connected to the rear left wheel RL.
Are installed respectively. The ABS controller 5 obtains the wheel speed signals FVFR, FVFL, FVRR, FVRL shown in FIG. 2 from the wheel speed pulse signal outputs sent from the wheel speed sensors 1 to 4 as the wheels FR to RL rotate, and at the same time, the wheel speed signals. Based on the wheel speed signal and the estimated vehicle body speed signal, the ABS control for applying an appropriate braking force to each wheel is executed from the vehicle body speed signal VREFB. This ABS
During the execution of the control, the ABS control signal ABSOR is transmitted from the ABS controller 5.

Reference numeral 6 indicates an electronic control unit (ECU) including a computer, a memory, an input / output circuit, etc. In the ECU 6, an ABS controller 5 and a steering sensor 8 attached to a steering device 7 of the vehicle, An alarm lamp 9 provided on an instrument panel (not shown) or the like of the vehicle and a brake lamp switch 10 interlocking with a brake pedal (not shown) of the vehicle are connected. The ECU 6 cooperates with the elements 1 to 5 and 8 to 10 to configure a device for carrying out the tire pressure drop detecting method according to the embodiment of the present invention.

Although not shown in detail, the steering sensor 8 is provided with, for example, two sets of photo interrupters and a neutral point detection unit, and in response to a steering wheel operation by a driver,
Two steering pulse signals ST1, which have a phase difference and are used to determine the steering wheel rotation direction and the steering wheel rotation amount.
In addition to sending ST2, it sends a neutral position signal STN each time the steering wheel is in the neutral position.

The ECU 6 controls the wheel speed signals FVFR, FVFL, FVRR, FVRL from the ABS controller 5,
Estimated vehicle speed signal VREFB and ABS control signal ABS
OR, the steering pulse signals ST1, ST2, STN from the steering sensor 7 and the brake lamp signal BLS from the brake lamp switch 10 are input (see FIG. 2), and the tire pressure drop is determined based on these signals. Sometimes the alarm lamp 9 is turned on.

In order to detect the above-mentioned decrease in air pressure, the ECU
Functionally, 6 has various functional units shown by blocks in FIG. Reference numeral 61 is a wheel speed signal FVFR, FVFL, FVRR, which is input from the ABS controller 5.
The filter part which performs the filter process for removing the high frequency component contained in FVRL (the fluctuation | variation amount of the wheel speed signal by the variation of the tooth gap precision of the wheel speed rotors (not shown) of the wheel speed sensors 1-4) is shown. Filtered wheel speed signals VFR, VFL, VRR and VR from the filter unit 61
L is a wheel speed ratio calculation unit 62 and a wheel acceleration calculation unit 63.
Sent to

The wheel speed ratio calculation unit 62, based on the filtered wheel speed signals VFR, VFL, VRR and VRL, for example, calculates the diagonal sum wheel speed ratio VXPERF from the following equation.
Calculate (%). VXPERF = {(VFR + VRL)-(VFL + VRR)} ÷ {(VFR + VFL + VRR + VR
L) / 4} That is, from the sum of the wheel speed VFR of the right front wheel and the wheel speed VRL of the left rear wheel, the wheel speed VFL of the left front wheel and the wheel speed VRR of the right rear wheel.
(More generally, when the vehicle is viewed in a plane, the sum of the wheel speeds of a pair of wheels on one diagonal of the vehicle to the wheels of a pair of wheels on the other diagonal of the vehicle) Diagonal sum Wheel speed ratio VX
Find the PERF.

The diagonal sum wheel speed ratio VXPERF shows a constant value when the steering wheel angle is substantially neutral and the wheel acceleration is substantially zero, and the wheel speed of one of the four wheels decreases because the air pressure of one of the four wheels decreases. The value fluctuates as it increases. Wheel speed ratio V
XPERF takes a positive value when the air pressure of the right front wheel or the left rear wheel decreases, and takes a negative value when the air pressure of the left front wheel or the right rear wheel decreases. This diagonal sum wheel speed ratio VXPERF
Is sent to the reference wheel speed ratio setting unit 66, the wheel speed ratio by speed range calculating unit 67, and the air pressure drop determining unit 68.

The wheel acceleration calculation unit 63 includes a filter unit 61.
The wheel speed signals VFR and VF of each wheel input successively from
The wheel acceleration (acceleration / deceleration state) of each wheel is calculated based on L, VRR, and VRL. In the figure, the symbols DVFR, DVF
L, DVRR, and DVRL represent right front wheel acceleration, left front wheel acceleration, right rear wheel acceleration, and left rear wheel acceleration, respectively. The steering angle calculation unit 64 outputs the steering pulse signals ST1, ST2, S sent from the steering sensor 8.
Based on TN, the steering angle (steering state) STR from the steering handle neutral position is obtained.

The air pressure drop estimation condition determination unit 65 determines based on various determination parameters whether the vehicle is traveling in a stable state in which the wheel speed ratio does not fluctuate due to factors other than the tire air pressure drop. When it is determined that the vehicle is in a stable running state, that is, the condition for estimating the decrease in air pressure is satisfied, the flag F_
X is set to the value "1", which allows the determination of the decrease in air pressure.

In this embodiment, the steering angle STR, the wheel acceleration DVFR to
DVRL, ABS control signal ABSOR, estimated vehicle speed VR
The EFB and brake lamp signals BLS are used. Then, (i) vehicle speed is high, (ii) brake operation is off, (iii) ABS control is off, (iv) wheel acceleration of four wheels is almost zero, (v) wheel acceleration difference of four wheels is almost zero, and ( vi) Steering angle is almost zero 6
When the two conditions are satisfied at the same time, the vehicle stable traveling state (the air pressure drop estimation condition is satisfied) is determined.

When the vehicle speed is low, the calculation accuracy of the wheel speed ratio decreases,
If the wheel speed is disturbed when the brake operation is turned on or the ABS control is performed, and the wheel acceleration of the four wheels is large, the slip ratios of the four wheels are not the same, and if the wheel acceleration difference of the four wheels is large, the vehicle is running on a bad road. If the steering angle is large, the turning radii of the four wheels are not the same. In either case, the wheel speed ratio is affected, and it becomes difficult to properly perform the air pressure drop determination based on the wheel speed ratio. On the other hand, when the above six conditions are satisfied at the same time, the influence of such an erroneous determination factor is eliminated, and the determination accuracy in the air pressure drop determination based on the wheel speed ratio is secured.

More specifically, the air pressure drop estimation condition determination unit 65
The ECU 6 executes the estimation condition determination subroutine shown in FIG. This subroutine and each of various subroutines described later are executed at a predetermined cycle for each subroutine. In the subroutine of FIG. 4, the wheel acceleration DV of the four wheels input from the wheel acceleration calculator 63
The maximum and minimum values of FR, DVFL, DVRR, and DVRL are obtained and stored in the A register and the B register (both not shown) of the ECU 6 (step S1,
S2). Then, the wheel acceleration initial value and the predetermined acceleration absolute value X preset and stored in the memory of the ECU 6 are stored.
X_DV is read from the memory and added, respectively, and then the A register value TMP_A (maximum value of wheel acceleration of four wheels) is compared with this added value (step S3).

When it is determined in step S3 that the A register value is less than or equal to the added value, a value obtained by subtracting the above acceleration absolute value XX_DV from the above wheel acceleration initial value and the B register value TMP_B (of the four wheel acceleration) (Minimum value) is compared (step S4). It is determined in step S4 that the B register value is greater than or equal to the subtracted value, and accordingly, steps S3, S
If the result of the determination in 4 indicates that the wheel acceleration of the four wheels is within the predetermined range and is substantially zero, the estimated vehicle speed data VREF is determined from the map (illustrated in FIG. 5) stored in advance in the memory. The acceleration band determination value YX_DVB is obtained, and the value obtained by subtracting the B register value from the A register value is compared with the acceleration band determination value (step S5).

Register value difference (TMP_A-TMP_
B) is equal to or less than the acceleration band determination value YX_DVB, and therefore, when it is determined in step S5 that the wheel acceleration difference of the four wheels is within the predetermined range and is substantially zero, the value TX_DV of the acceleration timer of the ECU 6 is decremented. (Step S6). This acceleration timer value TX_DV is calculated in step S
If the determination result of any one of S3, S4, and S5 is opposite to the above case, the predetermined acceleration timer value XX_DVT is set in step S7.

Subsequent to step S6 or S7, the steering wheel angle determination value YX_STR corresponding to the estimated vehicle speed data VREF is obtained from the map illustrated in FIG. 6, and the absolute value of the value obtained by subtracting the neutral point angle from the steering angle STR | STR
-The neutral point angle | is compared with the steering wheel angle determination value YX_STR (step S8). If absolute value | STR−neutral point angle | is less than or equal to the determination value YX_STR, and therefore it is determined in step S8 that the steering angle is substantially zero, the value TX_STR of the steering wheel angle timer of the ECU 6 is decremented (step S9. ). This timer value T
X_STR is when the absolute value exceeds the judgment value,
In step S10, the predetermined steering wheel angle timer value XX_ST
Set to RT.

Following step S9 or S10, the estimated vehicle speed VREFB is compared with a predetermined estimated vehicle speed determination value VREFB (step S11). Estimated vehicle speed VREF
If B is equal to or greater than the determination value VREFB and it is determined in step S11 that the vehicle speed is high, the value of the acceleration timer TX_
DV is checked (step S12). It is determined in step S12 that the acceleration timer value is "0", and therefore the state in which the wheel acceleration of the four wheels and the wheel acceleration difference are substantially zero continues for a predetermined time corresponding to the above-described predetermined acceleration timer value XX_DVT. Then, the value of the steering wheel angle timer TX_STR is determined (step S1).
3). When it is determined in step S13 that the steering wheel angle timer value is "0" and the steering angle is substantially zero for a predetermined time corresponding to the above-described predetermined steering wheel angle timer value XX_STRT, the brake lamp signal BLS is determined. Is checked (step S1)
4). If it is determined in step S14 that the brake lamp signal value is "0" and the brake operation is off (the brake pedal is not depressed), AB
The value of the S control signal ABSOR is further determined (step S15). When it is determined in step S15 that the ABS control signal value is "0" and the ABS control is off (the ABS control by the ABS controller 5 is not executed), the value of the flag F_X indicates that the vehicle is in a stable traveling state (pneumatic pressure). It is set to "1" indicating that the deterioration estimation condition is satisfied) (step S16). That is, when the high vehicle speed, the wheel acceleration of the four wheels and the wheel acceleration difference are substantially zero, the steering angle is substantially zero, and the brake operation OFF and the ABS control OFF are sequentially determined in steps S11 to S15, the air pressure drop estimation condition is satisfied. Is determined.

On the other hand, if the determination result in any one of steps S11 to S15 is opposite to the above case, the value of the flag F_X is reset to "0" (step S1).
7). That is, one of low vehicle speed, large wheel acceleration of four wheels and large wheel acceleration difference, large steering angle, brake operation ON, or ABS control ON is step S1.
When it is determined in 1 to S15, it is determined that the air pressure drop estimation condition is not satisfied. The value of the air pressure drop estimation condition satisfaction determination flag F_X is sent to the reference wheel speed ratio setting unit 66, the speed range-specific wheel speed ratio calculation unit 67, and the air pressure drop determination unit 68.

The reference wheel speed ratio setting section 66 is a wheel speed ratio VXPERF from the wheel speed ratio calculating section 62, a flag value F_X from the air pressure drop estimating condition judging section 65, and an ignition key (not shown). A signal IG from a switch (not shown) is input, the ignition key is turned on to start the engine, and the value of the flag F_X is set to "0" after the ignition switch signal IG becomes, for example, high level "1". From the wheel speed ratio VXPERF at the time of the first change from "1" to the reference wheel speed ratio V
Set as X_IGON. That is, the reference wheel speed ratio VX_IGON represents the wheel speed ratio when the vehicle stable traveling state is first achieved after the vehicle traveling with the engine start is started. A suitable method for setting the reference wheel speed ratio will be described later.

The wheel speed ratio-by-speed-range calculating unit 67 calculates the wheel speed ratio VXPERF from the wheel speed ratio calculating unit 62, the flag value F_X from the air pressure drop estimating condition determining unit 65, and the estimated vehicle body speed from the ABS controller 5. Enter VREFB,
When the vehicle is in a stable traveling state (F_X = 1), it is determined based on the estimated vehicle body speed VREFB to which of a plurality of pre-divided speed ranges the current vehicle speed belongs. The speed range is, for example, a first speed range of 30 to 60 km / h, 60 to
70km / h second speed range, 70-80km / h third speed range
Speed range, fourth speed range of 80 to 90 km / h, 90 to 10
It is divided into a fifth speed range of 0 km / h, a sixth speed range of 100 to 110 km / h, a seventh speed range of 110 to 120 km / h, and an eighth speed range of 120 km / h or more. Then, the wheel speed ratio VXPERF at the speed range determination time point is obtained as the wheel speed ratio in the determination speed range.

As described above, by obtaining the wheel speed ratio for each speed range, it is possible to detect a change in the wheel speed ratio due to the vehicle speed and thus a decrease in air pressure. The reason for this is that if the air pressure of the four wheels is standard, the wheel speed ratio becomes almost constant over the entire speed range, but if the air pressure of one of the wheels decreases, the wheel speed ratio fluctuates as the vehicle speed changes. By comparing the wheel speed ratios for each region, it is possible to detect a change in the wheel speed ratio due to a change in the vehicle speed, and to determine a decrease in air pressure.

In connection with this point, FIG. 18 illustrates a measurement example in which the tire rolling radius is expressed as a function of the vehicle speed and the tire air pressure. In FIG. 18, marks ◯, ×, □, and ● are added to distinguish the tire air pressure, and the tire air pressure takes a small value in this order. As is clear from FIG. 18, the lower the air pressure, the more the tire rolling radius and thus the wheel speed change with the change in vehicle speed. The reason for this is that the smaller the tire rigidity due to the decrease in tire air pressure, the greater the degree of increase in the rolling radius when the centrifugal force applied to the tire increases as the vehicle speed increases.

The above-mentioned speed range classification is set considering that the vehicle speed is not easily affected in the low speed range and the memory capacity, and the range of 30 to 60 km / h is set in the low speed range (first range). 1 speed range) and 10 other speed ranges
It is divided by the km / h step. More preferably, the entire vehicle speed range up to the maximum speed is divided into suitable steps. The speed range-specific wheel speed ratio calculation unit 67 of the present embodiment executes the speed range / calculation condition determination subroutine shown in FIGS. 7 and 8 and the speed range-specific wheel speed ratio calculation subroutine shown in FIGS. 9 and 10.

In this subroutine, the estimated vehicle speed V
The value of REFB is checked and estimated vehicle speed data VR
It is temporarily stored in the memory as EF (step S2
1). If the value of the estimated vehicle speed VREFB is not zero and therefore the vehicle is traveling, the value of the flag F_X is checked (step S22). Further, if the value of the flag F_X is "1", the estimated vehicle speed data VREF is checked (step S23). The estimated vehicle speed data VREF is 60k.
If m / h or less, the value "6" is set as the filter status S_X used in the process described later (step S
24). In addition, the estimated vehicle speed data VREF is 60 km.
/ H or more and less than 70 km / h, 70 km / h or more and less than 80 km / h, 80 km / h or more and 90 km / h
Less than 90 km / h and less than 100 km / h, 10
0 km / h or more and less than 110 km / h, 110 km /
h or more and less than 120 km / h, or 120 km /
If h or more, one corresponding value "7" to "13" is set as the filter status S_X (step S
25 to S31), this subroutine ends.

On the other hand, when it is determined in step S22 that the value of the flag F_X is "0", the filter status S
The value "0" is set in _X (step S32), and the present subroutine ends. It is step S that the estimated vehicle speed data VREF is "0" and the vehicle is stopped.
When the determination is made in 21, the value "60" is set as the vehicle speed index V used in the following processing (step S33). Next, the value “0” or the initial value is the integration buffer V_vSU.
M, integration counter C_vSUM, filtering status data S_v, and speed range wheel speed ratio VX_vF are set (steps S34 to S37). As described above, the integrating buffer, the integrating counter, the filtering status data and the speed range wheel speed ratio for the first speed range are initialized.

The integration counter C_vSUM is used to integrate the speed range wheel speed ratio, and this integration value is stored in the integration buffer V_vSUM. The filtering status data S_v is used for determining whether or not the filter constant and the filter value (speed range wheel speed ratio VX_vF) are stable, and is composed of, for example, 7 bits. The lower 4 bits of the stator data constitute a sign determination counter, and the 6th bit is a sign flag. As will be described later, the sign of the value obtained by subtracting the average wheel speed ratio from the current wheel speed ratio is positive. Is reset to the value "0" and is set to the value "1" when the sign is negative. The 7th bit is a filter value stable flag, which is set to a value "1" if the filter value is stable, and a value "0" if the filter value is not stable.
Is reset to.

Next, the value "10" is added to the vehicle speed index V to update the vehicle speed index V (step S38), and it is determined whether the updated value of the vehicle speed index V is "140". (Step S39). If the determination result in step S39 is negative, the control flow proceeds to step S34. As a result, the accumulation buffer, the accumulation counter, the filtering status data, and the speed range wheel speed ratio for the second speed range to the eighth speed range are sequentially initialized. When the initialization for the eighth speed range is completed, step S3 immediately after that is completed.
Since the value of the vehicle speed index V is updated to "140" in 8 and the determination result in the next step S39 becomes affirmative, this subroutine ends.

Next, in the wheel speed ratio calculation subroutine for each speed range shown in FIGS. 9 and 10, the contents of the accumulation buffer V_v
The wheel speed ratio VXPERF is added to SUM to update the buffer contents (step S41), and the value C of the integration counter is calculated.
_VSUM is decremented (step S42).
Next, it is determined whether or not a borrow (negative carry) has occurred, that is, whether or not the integration has ended (step S).
43). If the integration has not ended, this subroutine ends.

After that, when it is determined in step S43 that the integration has been completed, an initial value (for example, 128) is set in the integration counter C_vSUM (step S44), and the integration buffer value corresponds to the initial value (for example, "128"). The average value of the speed range wheel speed ratios obtained by dividing by () is set in the integrating buffer (step S45). In this way, when the wheel speed ratio is sampled a predetermined number of times (for example, 128 times),
The following filtering process is started.

That is, the filtering status data S_v is checked (step S46), and if it is determined that the value is "0" and the filtering is the first time, the value V_vSUM in the accumulation buffer is set to the speed range wheel speed VX.
_VF is set (step S47), and the value is "01".
Is set as the filtering status data S_v (step S48). Next, the control flow is shown in FIG.
To step S60 in step S60 and the accumulation buffer value V_vSUM
Is cleared to "0" and the present subroutine ends.

In step S46 of the next subroutine execution cycle, since it is determined that the filtering status data S_v is not "0", the speed range wheel speed ratio VX is determined.
_VF and the accumulated buffer value V_vSUM representing the average value (for example, 128 times average value) are compared (step S4).
9), whereby the code determination is performed. If the wheel speed ratio is larger than the cumulative buffer value, a value "0" indicating a positive sign
Is set in the register A (step S50),
If the wheel speed ratio is smaller than the integration buffer value, the value "1" representing a negative sign is set in the register A. Following step S50 or S51, or step S49
If it is determined that the wheel speed ratio and the accumulated buffer value are equal, the control flow advances to step S52 in FIG.

In step S52, the value of the 7th bit of the filtering status data S_v is examined.
If this value is "0" and it is determined that the filtering is not yet stabilized, the value of the 6th bit of the same status data S_v representing the previous code determination result and the current code determination result A The value of the register is compared (step S53). If the sixth bit value and the A register value are not equal and therefore it is determined that the sign has been inverted, the lower 4 bits of the status data S_v as a sign inversion counter are counted up (step S54). Following this step S54, or if it is determined in step S53 that the sixth bit value and the A register value are equal and there is no sign inversion, the control flow is step S55.
Proceed to and the lower 4 bits are compared with the value "4".

If it is determined in step S55 that the lower 4 bits of the status data S_v are smaller than the value "4" and therefore the sign inversion has not been performed three times, the control flow advances to step S58. This step S58
Then, in the speed range wheel speed ratio VX_vF, the integration buffer value V_
The value obtained by dividing the sum of vSUM by the value “2” is
The speed range wheel speed ratio VX_vF is set. That is, the 1/2 filter processing for improving the followability to the target value is performed. Next, the A register value TMP_A is set to the 6th bit of the status data S_v for the next code determination (step S59), and the integration buffer value V_vSUM is cleared for the next wheel speed ratio integration (( (Step S60), this subroutine ends.

On the other hand, if it is determined in step S55 that the lower 4 bits of the status data S_v are the value "4" or more and the sign inversion is performed three times, the status data S_v of the status data S_v is determined for the next determination of the number of code inversions. After the lower 4 bits are reset to the value "0" (step S56), the value "1" indicating that the filter value (wheel speed ratio by speed range) has been stabilized becomes the 7th bit of the status data S_v. It is set (step S58). Next, the above step S
58 to S60 are sequentially executed, and the present subroutine ends.

When the filter value is stabilized and the value "1" is set in the 7th bit of the status data S_v as described above, the control flow proceeds from step S52 to step S52.
Proceed to step 61, and the sixth bit and A of the status data S_v
The register value TMP_A is compared. 6th bit and A
If the register values do not match, the lower 4 bits of the status data S_v are reset to the value "0" (step S62), the above steps S59 and S60 are sequentially executed, and the present subroutine ends.

Then, in step S61, the sixth bit of the status data S_v and the A register value TMP_
If it is determined that A matches and the same code continues twice,
The value "1" is set in the lower 4 bits of the status data S_v (step S63), and the ± 1 filtering process for reducing the influence of noise is started. That is, the wheel speed ratio VX_vF and the accumulated buffer value V_ representing the average value thereof.
vSUM is compared (step S64), and if the wheel speed ratio is larger than the integration buffer value, the value obtained by subtracting the value "1" from the wheel speed ratio is set as the wheel speed ratio (step S65), while the wheel speed is set. If the ratio is smaller than the accumulation buffer, the value obtained by adding the value "1" to the wheel speed ratio is set as the wheel speed ratio (step S66). Then, in step S65
Or following S66, the above-mentioned steps S59 and S6
0 is sequentially executed, and this subroutine ends.

The graph of FIG. 11 illustrates changes of the wheel speed ratio, the sign reversal counter, and the filter value stabilized flag over time during the above-described filtering process. In FIG. 11, the V mark indicates sign inversion. Referring to FIGS. 2 and 3, the air pressure decrease determination unit 68 determines the maximum value of the speed range wheel speed ratio based on the speed range wheel speed ratio VX_vF and the filtering status data S_v from the speed range-specific wheel speed ratio calculation unit 67. VX_MAX and minimum value VX_MIN
It has a wheel speed ratio fluctuation value calculation unit 681 for calculating Further, the determination unit 68 includes an estimated vehicle speed VXPERF from the ABS controller 5 and an air pressure drop estimation condition determination unit 6
No. 5 from the first air pressure decrease determination unit 682 that determines whether or not there is a decrease in air pressure based on the flag F_X, and the maximum and minimum wheel speed ratio VX_MA from the wheel speed ratio variation value calculation unit 681.
A second air pressure drop determination unit 683 that determines whether or not air pressure has dropped based on X, VX_MIN and the reference wheel speed ratio VX_IGON from the reference wheel speed ratio setting unit 66, and a calculation unit 68.
A third air drop determination unit 684 that determines whether there is a drop in air pressure based on the maximum and minimum wheel speed ratios from 1;
An air pressure drop comprehensive determination unit 685 that makes a final air pressure drop determination based on the air pressure drop flag is included.

EC as the wheel speed ratio variation value calculation unit 681
In order to obtain the maximum value and the minimum value of the speed range wheel speed ratio calculated by the speed range-specific wheel speed ratio calculation unit 67, U6 executes the wheel speed ratio variation value calculation subroutine shown in FIG. 12 in the present embodiment. In this subroutine, the values of the maximum value register and the minimum value register (both not shown) of the ECU 6 are reset to "0" (steps S71, S72), and the vehicle speed index V indicates the value "60 (km / km / h) "is set (step S73), and then the vehicle speed index V
The 7th bit of the filtering status data S_v corresponding to is checked (step S74). If the value of the 7th bit is "0" and the speed range wheel speed ratio VX_vF corresponding to the vehicle speed index V has not been calculated, the maximum and minimum wheel speed values in the speed range corresponding to this vehicle speed index V It is determined that the calculation cannot be performed, and the control flow is step S described later.
Proceed to 82.

On the other hand, the value of the 7th bit of the status data S_v is "1", and the speed range wheel speed ratio VX_vF corresponding to the vehicle speed index V has already been calculated.
If it is determined at 74, the value of the minimum value register TMP_MI
N is checked (step S75). Immediately after the register is reset, the register value TMP_MIN is "0". In this case, the speed range wheel speed ratio VX_vF (= VX_60F) corresponding to the vehicle speed index V (= 60 km / h) is sequentially stored in the maximum value register and the minimum value register (steps S76 and S77). Speed ratio VX_
vF is compared with the value TMP_MAX of the maximum value register. Here, since both values are equal, the control flow advances to step S80, and the speed range wheel speed ratio VX_vF and the value TMP_MIN of the minimum value register are compared. Since both values are the same here, the control flow is step S82.
Then, the value "10" is added to the vehicle speed index V, and it is determined in the next step S83 whether the vehicle speed index V is the value "140". Here, the determination result in step S83 is negative, and the control flow returns to step S74.

Next, the vehicle speed index V (= 7) representing the second speed range.
7th of status data S_v corresponding to 0 km / h)
If the bit value is "1" and the speed range wheel speed ratio VX_70F corresponding to the second speed range has already been calculated, the control flow proceeds to step S75. In the previous cycle, when the speed range wheel speed ratio VX_60 corresponding to the first speed range is stored in the maximum value register and the minimum value register, it is determined that the value TMP_MIN in the minimum value register is not "0", and control is performed. The flow proceeds to step S78 and the speed range wheel speed ratio VX_70F of the second speed range and the value TMP_MAX of the maximum value register (speed range wheel speed ratio VX_ of the first speed range).
60F) is compared. Value VX_70F is value TMP_
If it is larger than MAX, the value in the maximum value register is the value VX.
It is updated to _70F (step S79). On the other hand, the value V
If X_70F is less than or equal to the value TMP_MAX, then the value VX_
70F and the value TMP_MIN of the minimum value register are compared (step S80). If the value VX_70F is smaller than the value TMP_MIN, the value of the minimum value register is updated to the value VX_70F (step S81). Following step S79 or S81, or step S
If it is determined at 80 that the value VX_70F is greater than or equal to the value TMP_MIN, the control flow proceeds to step S82 described above.

The speed range wheel speed ratio VX_70F has already been calculated, and the speed range wheel speed ratio VX has been calculated in the previous cycle.
If _60F was not stored in the maximum and minimum registers, the value VX_70F is stored in both registers. After that, the vehicle speed index V is set to the values “80” and “9.
0 ”, ...,“ 130 ”are updated while the corresponding one of the above steps S74 to S81 is executed, whereby the value of the maximum value register or the minimum value register becomes the comparison result in steps S78 and S80. Will be updated or maintained accordingly. Vehicle speed index V (= 130km / h), that is, 8th
When the corresponding one of steps S74 to S81 is executed for the speed range, the vehicle speed index V is updated to the value "140" in the next step S82, so that the determination result in step S83 is affirmative. In this case, the control flow is step S
In step 84, the maximum value register value TMP_MAX is set as the maximum value VX_MAX of the speed range wheel speed ratio, and in the next step S85, the minimum value register value TMP_MIN is set.
Is set as the minimum value VX_MIN of the speed range wheel speed ratio, whereby the present subroutine ends.

The first air pressure drop determination unit 682 determines the air pressure drop when the wheel speed ratio VXPERF deviates from the predetermined range continuously for a predetermined time. The predetermined range is set to, for example, ± 1% of the standard air pressure. This is because if the four wheels have standard air pressure, the wheel speed ratio is usually ± ±, even if there are individual tire errors due to differences in wear rates.
This is because if it is about 0.3%, it does not exceed ± 1%.

To determine whether the air pressure has dropped, the first air pressure drop determination unit 682 of this embodiment executes a first air pressure drop determination subroutine shown in FIG. In this subroutine, the flag F_X from the air pressure drop estimation condition determination unit 65
Is checked (step S91), and if the value of the flag F_X is "0" and the air pressure drop estimation condition is not satisfied, the air pressure drop determination timer is set to XXPER for a predetermined time in step S92, and this subroutine ends. To do.
On the other hand, when the value of the flag F_X is "1" and it is determined that the estimation condition is satisfied, it is determined whether or not the wheel speed ratio VXPERF is not less than the first predetermined value and not more than the second predetermined value (step). S93). The magnitudes of the two predetermined values may be the same, and in this case, it is determined in step S93 whether the absolute value of the wheel speed ratio is less than or equal to the predetermined value. Step S93
If the determination result in step 1 is positive, the control flow proceeds to step S92 described above. That is, the air pressure drop determination is not performed.

On the other hand, if the decision result in the step S93 is negative, the value TXPER of the decision timer is decremented (step S94), and then the timer value TXPER is examined (step S95). If the timer value TXPER is not "0", the deviation of the wheel speed ratio from the predetermined range has not continued for a predetermined time, so the determination as to whether or not the air pressure has dropped is suspended and the present subroutine ends. On the other hand, the timer value TXPER becomes "0", and therefore the wheel speed ratio V
When it is determined that XPERF has continued to deviate from the predetermined range for the predetermined time XXPER, it is determined that the air pressure has decreased, and the first air pressure decrease flag FAIR_P is set to a value "1" representing the air pressure decrease (step S9).
6).

The first air pressure drop determination subroutine of FIG. 13 determines that the air pressure has dropped when the wheel speed ratio deviates from the predetermined range while the vehicle is running.
Unlike the case of the second air pressure drop determination subroutine described later, the presence or absence of air pressure drop is determined based on the wheel speed ratio itself without setting the reference level of the wheel speed ratio (calibration of the wheel speed ratio). . That is, this determination of decrease in air pressure is made based on the absolute value of the wheel speed ratio, so to speak. According to the subroutine of FIG. 13, for example, the tire of one of the wheels is in a completely punctured state (more generally, the air pressure is reduced by about 70% or more), or a tire of a different size such as a temper tire is attached to the left and right wheels. It is possible to detect the state. When the present subroutine is completed as described above, the flag value FAIR_P (= 1) is sent to the air pressure drop comprehensive determination unit 685.

The determination of the air pressure drop by the second air pressure drop determination unit 683 is performed by determining the maximum and minimum wheel speed ratios VX_MAX and VX_MIN from the wheel speed ratio fluctuation value calculation unit 681 and the reference wheel speed ratio VX_IGON from the reference wheel speed ratio setting unit 66. And based on. The setting of the reference wheel speed ratio has been described above, but a suitable example thereof will be described below. Reference wheel speed ratio V
In setting X_IGON, the reference wheel speed ratio setting unit 66
ECU 6 preferably executes the reference wheel speed ratio setting subroutine shown in FIG.

In this subroutine, the value of the IG on-edge flag FIGON is checked (step S10).
1) If the flag value FIGON is "0" and the on-edge of the ignition switch signal IG is not detected, this subroutine ends. After that, the ignition switch is turned on to start the engine, and when the ignition switch signal IG rises and its on-edge is detected, the flag value FIGON is set to "1". As a result, the determination result in step S101 is Become affirmative. In this case, the seventh bit of the filtering status data S_60 corresponding to the vehicle speed of 60 km / h or less, that is, the first speed range is checked (step S1).
02). The value of the 7th bit is "0", so the first
If the calculation of the speed range wheel speed ratio VX_60F corresponding to the speed range has not been completed, this subroutine ends.

On the other hand, when the value of the seventh bit is "1" and it is determined that the wheel speed ratio VX_60F has been calculated, this wheel speed ratio VX_60F is set as the reference wheel speed ratio VX_IGON (step S103). Next step S1
In 04, the flag value FIGON is reset to "0",
This subroutine ends. The wheel speed ratio VX_60
Since F is calculated when the flag F_X = 1, that is, the air pressure drop estimation condition is satisfied, the above-described reference wheel speed ratio VX_IGON is set when the condition F_X = 1 is satisfied.

In the present embodiment, the second air pressure drop determining unit 68
The ECU 6 as No. 3 executes the second air pressure drop determination subroutine shown in FIG. In this subroutine, the reference wheel speed ratio VX_ from the reference wheel speed ratio setting unit 66
The value of IGON is checked (step S111), and if this value is "0" and the setting of the reference wheel speed ratio has not been completed,
This subroutine ends. On the other hand, if the value of the reference wheel speed ratio is not "0" and the reference wheel speed ratio has already been set, the minimum value VX_MIN of the speed range wheel speed ratio from the wheel speed ratio variation value calculation unit 681 is checked (step S112). If the minimum value is "0" and the calculation of the speed range wheel speed ratio has not been completed, this subroutine ends, while if the minimum value is not "0" and the speed range wheel speed ratio has been calculated, Reference wheel speed ratio VX_IGON and maximum speed wheel speed ratio VX_MAX
And the minimum value VX_MIN of the speed range wheel speed ratio are compared, the maximum value and the minimum value of these three values are respectively obtained, and stored in the maximum value register and the minimum value register of the ECU 6, respectively (step S113). , S114).

Next, the value TMP_MAX of the maximum value register
Value (TMP_MAX-TMP_MIN) obtained by subtracting the value TMP_MIN of the minimum value register from the judgment value XAIR
_T (for example, 0.2%) is compared (step S11).
5) If the difference between the maximum value register value and the minimum value register value is smaller than the determination value, the wheel speed ratio has not changed from the reference wheel speed ratio by the determination value or more, so it is determined that there is no decrease in air pressure, and this subroutine is executed. finish. On the other hand, the difference between the maximum value register value and the minimum value register value (TMP_MAX-TMP
_MIN) is greater than or equal to the determination value XAIR_T, it is determined that the air pressure has decreased because the wheel speed ratio has fluctuated by more than the determination value, and the second air pressure reduction flag FAIR_T is set to a value "1" indicating a reduction in air pressure (step S116).

As described above, in the second air pressure drop determination subroutine of FIG. 15, the wheel speed ratio is changed from the reference wheel speed ratio as time elapses during vehicle traveling that starts with engine start and ends with engine stop. A predetermined value (for example, 0.
2%) or more, and whether or not the air pressure has dropped is determined based on the determination result. Therefore, according to this subroutine, it is possible to detect the decrease in the air pressure while the vehicle is traveling, for example, when the decrease in the air pressure of about 20% or more corresponding to the fluctuation of the wheel speed ratio of 0.2% occurs. Then, when such a decrease in air pressure is detected, this subroutine is ended. The flag value FAIR_T (= 1) is sent to the air pressure drop comprehensive determination unit 685.

EC as the third air pressure drop determination unit 684
U6 executes the third air pressure drop determination subroutine shown in FIG. In this subroutine, the minimum value VX_MIN of the speed range wheel speed ratio sent from the wheel speed ratio fluctuation value calculation unit 681 is checked (step S121), and the minimum value is "0" and the minimum value has not been calculated. When it is determined that the process is completed, this subroutine ends. On the other hand, the minimum value V
When it is determined that the value of X_MIN is not "0" and the minimum value has been calculated, the control flow proceeds to step S122, and the maximum value VX_MAX of the speed range wheel speed ratio is changed to the minimum value V.
The value obtained by subtracting X_MIN and the determination value XAIR_V are compared. The determination value XAIR_V is, for example, the value VX.
It is set according to the speed range to which each of _MAX and VX_MIN belongs, and is set to a value near 0.15%, for example.

The difference between the maximum value and the minimum value (VX_M
If AX-VX_MIN) is smaller than the determination value XAIR_V, it is determined that the air pressure has not decreased because the wheel speed ratio has not changed by the determination value or more due to the vehicle speed change, and this subroutine is ended. On the other hand, if this difference is equal to or greater than the determination value, it is determined that the air pressure has decreased because the wheel speed ratio has changed with the vehicle speed change, and the third air pressure decrease flag FAIR_V is set to the value "1" indicating the air pressure decrease. (Step S
123).

In the third air pressure drop determination subroutine of FIG. 16, the degree to which the centrifugal force applied to the tire overcomes the tire rigidity and the rolling radius of the tire increases, the higher the vehicle speed range,
Further, it is based on the physical phenomenon that the larger the degree of decrease in tire air pressure, the larger it becomes.In this subroutine, as described above, it is determined whether or not there has been a change in wheel speed ratio above a predetermined value due to a change in vehicle speed. A decrease in air pressure is determined based on the determination result. The degree of decrease in tire air pressure that can be detected by this subroutine varies depending on the vehicle speed, such as 20% or more in the high vehicle speed range and 60% or more in the low vehicle speed range. This subroutine ends when a drop in air pressure is detected. Flag value FAI
R_V (= 1) is sent to the air pressure drop comprehensive determination unit 685.

EC as the air pressure drop comprehensive determination unit 685
U6 executes the air pressure drop comprehensive determination subroutine shown in FIG. In this subroutine, the first air pressure drop flag F sent from the first air pressure drop determination unit 682.
AIR_P is checked (step S131), and if the value of the first air pressure drop flag is "0", the second air pressure drop flag FAIR_T is checked (step S132).
If the value of the second air pressure drop flag is also "0", the third air pressure drop flag FAIR_V is checked (step S13).
3). Then, if the value of the third air pressure decrease flag is also “0”, it is determined that there is no air pressure decrease, and this subroutine ends.

On the other hand, the first air pressure drop flag FAIR_P
Is determined to be "1" in step S131, or the value of the second air pressure reduction flag FAIR_T is determined to be "1" in step S132, or the value of the third air pressure reduction flag FAIR_V. Is determined to be "1" in step S133, it is determined that the air pressure has decreased, and the control flow is step S134.
Then, the air pressure drop flag FAIR is set to the value "1" representing the drop in air pressure, and the present subroutine ends. The air pressure drop flag value FAIR (= 1) is sent to the alarm lamp drive unit 69 (FIG. 2).

ECU as alarm lamp drive unit 69
Reference numeral 6 monitors the value of the air pressure drop flag FAIR in an alarm lamp output subroutine (not shown). If the flag value is "0", the alarm lamp drive flag FAIR is displayed.
If the value of the air pressure drop flag FAIR is "1" while the value of OUT is reset to "0", the flag FAIROU
Set the value of T to "1". The alarm lamp drive flag FAIROUT is sent to the alarm lamp 9 (FIG. 1), and when the flag is set to the value “1” and the drive power is supplied, the alarm lamp 9 lights up and the driver etc. is informed that the air pressure has dropped. Inform.

The operation of the tire pressure drop detecting device having the above construction will be described below. When the ignition key of the vehicle is turned on, the ignition signal IG rises, the flag FIGON (FIG. 14) is set to "1", and the engine is started to start traveling of the vehicle. While the vehicle is traveling, the ABS controller 5 controls the front, rear, left and right wheels FR to R.
Wheel speed signals FVFR to FVR based on the wheel speed pulse signals supplied from the wheel speed sensors 1 to 4 with the rotation of L.
Obtain L and the estimated vehicle speed signal VREFB. The wheel speed signal is filtered by the filter unit 61, and the filtered wheel speed signals VFR to VRL are output to the wheel speed ratio calculation unit 62 and the wheel acceleration calculation unit 63. Also,
The steering angle STR is calculated in the steering angle calculation unit 64 based on the steering pulse signals ST1, ST2, STN sent from the steering sensor 8 in accordance with the steering wheel operation while the vehicle is traveling.

In the wheel speed ratio calculation unit 62, the diagonal sum wheel speed ratio VXPERF is calculated based on the filtered wheel speed signal.
Is calculated and output to the reference wheel speed ratio setting unit 66, the speed range-specific wheel speed ratio calculation unit 67, and the air pressure decrease determination unit 68. The diagonal sum wheel speed ratio is suitable for the determination of the decrease in air pressure because it can cancel out the difference in the wheel traveling state between the wheels. Then, the wheel acceleration calculation unit 63 uses the wheel speed signals VFR to VR.
Wheel accelerations DVFR to DVRL are calculated based on L,
The wheel accelerations DVFR to DVRL are supplied to the air pressure drop estimation condition determination unit 65.

In the judging section 65, the wheel accelerations DVFR to DFR.
VRL, steering angle STR from the steering angle calculator, brake lamp signal BLS from the brake lamp switch 10, ABS control signal ABSOR from the ABS controller 5, and estimated vehicle speed VREFB.
Based on (i) high vehicle speed, (ii) brake operation off,
Whether or not six conditions (Iiii) ABS control off, (iv) wheel acceleration almost zero, (v) wheel acceleration difference almost zero, and (vi) steering angle almost zero are simultaneously established, that is, the air pressure. It is determined whether or not the deterioration estimation condition is satisfied. Then, when the condition is satisfied, that is, when the vehicle is in a stable traveling state in which the air pressure decrease determination based on the wheel speed ratio can be appropriately performed, the air pressure decrease estimation condition satisfaction determination flag F_X is set to the value "1".
This flag value is output to the reference wheel speed ratio setting unit 66, the wheel speed ratio calculation unit 67 for each speed range, and the air pressure drop determination unit 68.

The first air pressure drop determination unit 682 of the air pressure drop determination unit 68 monitors the wheel speed ratio VXPERF from the wheel speed ratio calculation unit 62 while the air pressure drop estimation condition is satisfied (see FIG. 1).
3) If the wheel speed ratio deviates from the predetermined range for a predetermined time XXPER, it is determined that the air pressure has decreased. As described above, the determination unit 682 makes the air pressure drop determination based on the absolute value of the wheel speed ratio. This makes it possible to detect a considerable decrease in air pressure, such as a complete puncture state, immediately after the vehicle starts traveling.

In the wheel speed ratio calculation section 67 for each speed range, the estimated vehicle speed VR from the ABS controller 5 is satisfied while the estimation condition is satisfied.
Which of the first to eighth speed ranges the current vehicle speed belongs to is determined based on the EFB, and the wheel speed ratio VXPERF successively input from the wheel speed ratio calculation unit 62 at the speed range determination time.
Is subjected to a filtering process (FIGS. 9 and 10), and the filter value obtained as a result is the wheel speed ratio VX_v in the determination speed range.
Required as F. Also, when the filter value is stable,
The seventh bit of the filtering status data S_v is set to the value "1".

In the reference wheel speed ratio setting section 66, the ignition key is turned on and the IG on-edge flag FIG.
When the ON value becomes "1", the first speed range (vehicle speed ≤ 60k
It is determined whether or not the wheel speed ratio VX_60F of (m / h) has been calculated, and the calculated wheel speed ratio VX_60F is set as the reference wheel speed ratio VX_IGON (see FIG. 14). The reference wheel speed ratio is output to the second air pressure drop determination unit 683 of the air pressure drop determination unit 68.

The wheel speed ratio variation value calculation unit 681 of the air pressure decrease determination unit 68 inputs the speed range wheel speed ratio VX_vF and the status data S_v from the speed range-specific wheel speed ratio calculation unit 67, and then the first to eighth values are input. Maximum and minimum values VX_MAX, V of the speed range wheel speed ratios already calculated for the speed range
Determine X_MIN. The maximum value and the minimum value are output to the second and third air pressure drop determination units 683 and 684.

In the second air pressure drop determination unit 683, the reference wheel speed ratio VX is established while the air pressure drop estimation condition is satisfied (F_X = 1).
_IGON, the maximum wheel speed ratio VX_MAX, and the minimum wheel speed ratio VX_MIN, the difference between the maximum value and the minimum value is compared with the determination value XAIR_T (FIG. 15). When the speed ratio changes from the reference wheel speed ratio by a certain amount or more, it is determined that the air pressure has decreased.
This makes it possible to detect a relatively slight decrease in air pressure.

In the third air pressure drop determination unit 684, the difference between the maximum value VX_MAX and the minimum value VX_MIN of the speed range wheel speed VX_vF is the determination value XA while the air pressure drop estimation condition is satisfied.
It is compared with IR_V (FIG. 16), and if this difference is greater than or equal to the determination value and the wheel speed ratio fluctuates by a certain amount or more due to the vehicle speed change, it is determined that the air pressure has decreased. As a result, when the vehicle travels over a wide range of vehicle speeds, the decrease in air pressure is reliably detected.

In the air pressure drop comprehensive determination unit 685,
When any of the third air pressure drop determination units 682 to 684 determines that the air pressure has dropped, the air pressure drop is determined. As described above, in the present embodiment, the absolute value of the wheel speed ratio, the presence / absence of a change in the wheel speed ratio from the reference wheel speed ratio with the lapse of time after setting the reference wheel speed ratio, and the wheel speed associated with the vehicle speed change Three types of determinations are made based on whether or not there is a change in the ratio, and when any one of the determinations determines that the air pressure has dropped, the alarm lamp 9 is turned on to notify that the air pressure has dropped.

The method for detecting a decrease in tire air pressure according to the present invention is not limited to the above embodiment, but can be variously modified. In the above embodiment, when the tire pressure drop occurs in any one of the four wheels, it is possible to detect the decrease in the tire pressure more accurately.
Not only the change in the wheel speed ratio due to the change in the vehicle speed is detected as a decrease in the air pressure by detecting the decrease in the air pressure, the absolute value of the wheel speed ratio (the wheel speed ratio without calibration) changes relatively greatly. In this case, the first air pressure drop determination unit 682 determines that the wheel speed ratio fluctuation is the air pressure drop.
Further, based on the reference wheel speed ratio VX_IGON detected when the vehicle stable operation state is first achieved after the vehicle traveling with the engine start, the wheel speed is changed when the wheel speed ratio changes with time. The second variation in air pressure determination unit 683 detects the ratio variation as a reduction in air pressure. However, in the method of the present invention,
It is not essential to perform the air pressure drop determination based on the absolute value of the wheel speed ratio or the air pressure drop determination based on the wheel speed ratio variation over time.

That is, in the method of the present invention, it is only necessary to determine the decrease in air pressure based on the presence / absence of a change in the wheel speed ratio associated with a change in vehicle speed, which can be implemented using the apparatus illustrated in FIG. The apparatus of FIG. 19 includes a wheel speed ratio calculation unit 161 corresponding to the wheel speed ratio calculation unit 62 of FIG. 2 and an estimation condition determination unit 16 corresponding to the air pressure drop estimation condition determination unit 65 of FIG.
2, a wheel speed ratio-specific wheel speed ratio calculating section 163 corresponding to the wheel speed ratio-specific wheel speed ratio calculating section 67 in FIG. 2 and a wheel speed ratio fluctuation value calculating section corresponding to the wheel speed ratio fluctuation value calculating section 681 in FIG. 164
And an air pressure drop determination unit 165 corresponding to the third air pressure drop determination unit 684 in FIG.

As in the case of the above embodiment, the estimation condition judging section 1 is based on the vehicle speed (estimated vehicle speed), wheel acceleration, steering angle, ABS control signal and brake lamp signal.
In 62, it is determined whether or not the vehicle stable driving state is achieved and the air pressure drop estimation condition is satisfied, and the estimation condition flag indicating the determination result is sent to the speed range-specific wheel speed ratio calculation unit 163. In the calculation unit 163, the ignition signal IG
Wheel speed ratio calculation unit 161 while the estimation condition is satisfied after input
The wheel speed ratios successively sent from the vehicle speed ratio VX_ for each speed range according to the vehicle speed when the wheel speed ratios are sent.
60F to VX_130F are respectively set. The wheel speed ratio variation value calculation unit 164 calculates the maximum value and the minimum value of the calculated wheel speed ratios for each speed range. Then, the air pressure drop determination unit 165 calculates the calculation unit 1 while the estimation condition is satisfied after the ignition signal IG is input.
The difference between the maximum value and the minimum value of the wheel speed ratio for each speed range calculated in 64 and sent from the same calculation unit is compared with the determination value. Then, if this difference is equal to or more than the determination value, it is determined that there has been a certain or more wheel speed ratio variation due to the vehicle speed change,
As a result, it is determined that at least one of the front, rear, left and right wheels has a reduced air pressure.

In the above embodiment, the diagonal sum wheel speed ratio is calculated as the wheel speed ratio used for the air pressure drop determination, but this is not essential. For example, the front wheel wheel speed ratio instead of the diagonal sum wheel speed ratio may be calculated according to the following equation, and whether or not the air pressure of one of the left and right front wheels has decreased may be determined based on this calculated value. Front wheel speed ratio = (VFR-VFL) / {(VFR + VF
L) / 2} Alternatively, the rear wheel speed ratio may be calculated according to the following equation, and the decrease in the air pressure of either the left or right rear wheel may be determined based on the calculated value.

Rear wheel speed ratio = (VRR-VRL) /
{(VRR + VRL) / 2} In the above embodiment, the high vehicle speed, the wheel acceleration, the acceleration difference and the steering angle are small, and the ABS control and the brake operation off are the air pressure drop estimation conditions.
Other requirements may be included in the estimation condition in order to improve the determination accuracy. For example, it is possible to include a requirement that the vehicle lateral acceleration is a certain value or less and / or the vehicle longitudinal acceleration is a certain value or less.

Further, in the above embodiment, the tire pressure drop detecting device is mounted on the vehicle equipped with the ABS so as to share the elements used for the ABS control, but this is also not essential, and therefore the method of the present invention is the ABS. It is also applicable to non-equipped vehicles. Since the device configuration used in this case is clear from the above description, the description thereof will be omitted.

[0090]

As described above, according to the tire pressure drop detecting method of claim 1 of the present invention, the wheel speed detecting process for detecting the wheel speed of two or more of the front, rear, left and right wheels of the vehicle, and the front, rear, left and right wheels. A vehicle acceleration / deceleration state detection process for detecting an acceleration / deceleration state of one or more of the above, a steering state detection process for detecting a steering state of the vehicle, and a vehicle body of the vehicle from two or more wheel speeds of the front, rear, left, and right wheels. Based on the vehicle speed detection stroke for detecting the vehicle speed and the vehicle speed detected in the vehicle speed detection stroke, the wheel speed ratio is calculated for each vehicle speed range from the wheel speeds of two or more wheels detected in the wheel speed detection stroke. Based on the vehicle speed, the wheel acceleration / deceleration state, the steering state, and the braking device operating state, the vehicle motion state based on the wheel speed ratio calculation process, the braking device operating state detection process for detecting the operating condition of the vehicle braking device, A vehicle motion state determination process for determining The maximum value and the maximum value are calculated while calculating the difference between the maximum value and the minimum value among the plurality of wheel speed ratios calculated for each speed range in the wheel speed ratio calculation process that is repeatedly executed while the two stable operation states are being determined. And a minimum value that is greater than a first predetermined value, a tire air pressure drop determination process that determines that at least one of the front, rear, left and right wheels is in a low air pressure state. Therefore, based on the physical phenomenon that the wheel speed of the wheel that has reduced the air pressure changes more greatly than the wheel speed of the other wheels as the vehicle speed increases, it is possible to reliably reduce the air pressure such that the wheel speed ratio changes with the vehicle speed change. It can be detected, and the setting (calibration) of the judgment standard related to the wheel speed ratio becomes unnecessary. In addition, since the error factor for the air pressure drop determination based on the wheel speed ratio can be removed, the determination accuracy is high.

According to the second aspect of the present invention, the tire air pressure drop determination process is performed by a wheel speed ratio calculation process that is repeatedly executed while it is determined in the vehicle motion state determination process that the vehicle is in a stable driving state. When the wheel speed ratio continuously exceeds the second predetermined value for a certain period of time, a second determination step of determining that at least one of the front, rear, left, and right wheels is in a reduced air pressure state is included. In this way, since the air pressure drop determination requirement is that the wheel speed ratio fluctuation representing the air pressure drop continues for a certain period of time,
The influence of noise or the like added to the judgment system is removed. Therefore, even if the wheel speed ratio is not specifically calibrated, it is possible to reliably detect a decrease in tire air pressure based on the absolute value of the wheel speed ratio. Therefore, the air pressure drop determination logic can be simplified. Further, even if the air pressure drops immediately after the vehicle travels, the air pressure drop determination can be performed.

According to the third aspect of the present invention, the tire pressure drop determining step is performed when the vehicle stable operation state is first determined in the vehicle motion state determining step, and the wheel speed ratio calculated in the wheel speed ratio calculating step and then the wheel speed ratio are calculated. When the difference from at least one of the plurality of wheel speed ratios calculated in the wheel speed ratio calculation process that is repeatedly executed while the vehicle stable driving state is determined becomes larger than the third predetermined value, the front-rear direction A third determination step is included that includes determining that at least one of the wheels is in a low air pressure state, and the third determination step is performed each time the vehicle travels with engine start. Therefore, a change in the wheel speed ratio over time is detected with the wheel speed ratio when the vehicle stable driving state is first achieved as a criterion. Therefore, it is possible to reliably detect a decrease in air pressure that causes the wheel speed ratio to fluctuate more than a certain amount.

According to a fourth aspect of the present invention, the wheel acceleration / deceleration state detecting step includes a step of detecting accelerations of the front, rear, left and right wheels, and the steering state detecting step includes a step of detecting a steering angle.
In the vehicle driving state determination process, the vehicle speed detected in the vehicle speed detection process is larger than the fourth predetermined value, the operation of the braking device is not detected in the braking device operation state detection process, and is detected in the steering state detection process. Steering angle is smaller than the fifth predetermined value,
The accelerations of the front, rear, left, and right wheels detected in the wheel acceleration / deceleration state detection process are smaller than the sixth predetermined value, and the difference between the accelerations of the front, rear, left, and right wheels detected in the wheel acceleration / deceleration state detection process is the seventh predetermined value. If the value is smaller than the value, the process includes the step of determining that the vehicle is operating in a stable state. Therefore, the air pressure drop determination in the vehicle operating state in which it becomes difficult to determine the air pressure drop based on the wheel speed ratio is automatically prohibited, and therefore an erroneous determination can be prevented.

According to the fifth aspect of the present invention, the vehicle motion state determining step includes a step of changing the fifth and sixth predetermined values in accordance with the vehicle body speed detected in the vehicle body speed detecting step. Can be set more appropriately, and the detection accuracy in the air pressure drop detection performed while the vehicle is in a stable driving state can be further improved. The method according to claim 6, wherein the wheel speed detecting step includes a step of detecting the wheel speed of each of the front, rear, left and right wheels, and the wheel speed ratio calculating step is the sum of the wheel speeds of a pair of wheels on one diagonal line of the vehicle. The diagonal sum wheel speed ratio is calculated as the wheel speed ratio by dividing the value obtained by subtracting the sum of the wheel speeds of the pair of wheels on the other diagonal line from the average wheel speed of the front, rear, left, and right wheels. Since the stroke is included, the difference in the wheel traveling state between the front and rear wheels and between the left and right wheels is canceled out, and the accuracy in detecting a decrease in air pressure is improved. Further, when a decrease in air pressure occurs in any of the front, rear, left and right wheels, such decrease in air pressure can be reliably detected.

[Brief description of drawings]

FIG. 1 is a schematic diagram showing an apparatus for implementing a tire pressure drop detecting method according to an embodiment of the present invention together with an ABS controller.

FIG. 2 is a functional block diagram showing in detail the function of an electronic control unit (ECU) shown in FIG.

FIG. 3 is a functional block diagram showing in detail the air pressure drop determination unit shown in FIG.

FIG. 4 is a flowchart showing an estimation condition determination subroutine executed by an ECU as an air pressure drop estimation condition determination unit shown in FIG.

FIG. 5 is a graph illustrating an estimated vehicle speed / acceleration band determination value map used during execution of the subroutine shown in FIG.

FIG. 6 is a graph illustrating an estimated vehicle speed / steering wheel angle determination value map used during execution of the subroutine shown in FIG.

FIG. 7 is a flowchart showing a part of a speed range / calculation condition determination subroutine executed by the ECU as the speed range-specific wheel speed ratio calculation unit shown in FIG. 2.

FIG. 8 is a flowchart showing the remaining part of the subroutine shown in FIG.

FIG. 9 is a flowchart showing a part of a speed range wheel speed ratio calculation subroutine executed by an ECU as a speed range wheel speed ratio calculation unit.

10 is a flowchart showing the remaining part of the subroutine shown in FIG.

11 illustrates an example of changes over time in a wheel speed ratio, a sign reversal counter, and a filter value stable flag during filtering processing executed in the speed range-specific wheel speed ratio calculation subroutine shown in FIGS. 9 and 10. It is a graph to do.

FIG. 12 is a flowchart showing a wheel speed ratio fluctuation value calculation subroutine executed by an ECU as a wheel speed ratio fluctuation value calculation unit shown in FIG.

FIG. 13 is a flowchart showing a first air pressure drop determination subroutine executed by the ECU as the first air pressure drop determination unit shown in FIG.

FIG. 14 shows E as a reference wheel speed ratio setting unit shown in FIG.
It is a flowchart which shows the reference wheel speed ratio setting subroutine performed by CU.

FIG. 15 is a flowchart showing a second air pressure drop determination subroutine executed by the ECU as the second air pressure drop determination unit shown in FIG. 3.

16 is a flowchart showing a third air pressure drop determination subroutine executed by the ECU as the third air pressure drop determination unit shown in FIG.

FIG. 17 is a flowchart showing an air pressure drop comprehensive determination subroutine executed by the ECU as the air pressure drop comprehensive determination unit shown in FIG. 3.

FIG. 18 is a graph illustrating tire rolling determination, vehicle speed, and tire pressure characteristics.

FIG. 19 is a functional block diagram showing a main part of a tire pressure drop detecting device according to a modification.

[Explanation of symbols]

 1, 2, 3, 4 Wheel speed sensor 5 ABS controller 6 Electronic control unit (ECU) 8 Steering sensor 9 Alarm lamp 10 Brake lamp switch 61 Filter unit 62 Wheel speed ratio calculation unit 63 Wheel acceleration calculation unit 64 Steering angle calculation unit 65 Air pressure drop estimation condition determination unit 66 Reference wheel speed ratio setting unit 67 Wheel speed ratio calculation unit for each speed range 68 Air pressure reduction determination unit 69 Alarm lamp drive unit 161 Wheel speed ratio calculation unit 162 Estimated condition determination unit 163 Air pressure reduction determination unit 681 Wheel Speed ratio fluctuation value calculation unit 682 First air pressure decrease determination unit 683 Second air pressure decrease determination unit 684 Third air pressure decrease determination unit 685 Air pressure decrease comprehensive determination unit

Claims (6)

[Claims] 1. A wheel speed detection process for detecting two or more wheel speeds of front, rear, left and right wheels of a vehicle, and a wheel acceleration / deceleration state detection for detecting an acceleration / deceleration state of one or more of the front, rear, left and right wheels. A stroke, a steering state detection stroke for detecting a steering state of the vehicle, a vehicle body speed detection step for detecting a vehicle body speed of the vehicle from two or more wheel speeds of the front, rear, left and right wheels, and a vehicle body speed detection step. A wheel speed ratio calculation step for calculating a wheel speed ratio for each speed range of the vehicle from the wheel speeds of two or more wheels detected in the wheel speed detection step based on the vehicle speed detected in A vehicle motion state in which the motion state of the vehicle is determined based on a braking device operation state detection stroke that detects the operation state of the device, the vehicle body speed, the wheel acceleration / deceleration state, the steering state, and the braking device operation state. Judgment process and above While calculating the difference between the maximum value and the minimum value of the plurality of wheel speed ratios calculated for each speed range in the wheel speed ratio calculation process that is repeatedly executed while both stable operation states are being determined, When the difference between the maximum value and the minimum value becomes larger than a first predetermined value, a tire air pressure drop determining step for determining that at least one of the front, rear, left and right wheels is in a low air pressure state is provided. Air pressure drop detection method. 2. The tire air pressure drop determination step is calculated by the wheel speed ratio calculation step that is repeatedly executed while it is determined in the vehicle motion state determination step that the vehicle is in a stable driving state. When the wheel speed ratio exceeds a second predetermined value continuously for a certain period of time, a second determination step of determining that at least one of the front, rear, left and right wheels is in a state of reduced air pressure is included. The method for detecting a decrease in tire air pressure according to. 3. The tire pressure drop determination step is the wheel speed ratio calculated in the wheel speed ratio calculation step executed when the vehicle stable operation state is first determined in the vehicle motion state determination step, and then the When the difference from at least one of the plurality of wheel speed ratios calculated in the wheel speed ratio calculation process repeatedly executed while the vehicle stable driving state is determined becomes larger than a third predetermined value, It is characterized in that it includes a third determination step of determining that at least one of the front, rear, left and right wheels is in a state of low air pressure, and the third determination step is executed every time the vehicle travels with engine start. The method for detecting a decrease in tire air pressure according to claim 1 or 2. 4. The wheel acceleration / deceleration state detection process includes a process of detecting acceleration of each of the front, rear, left and right wheels, the steering state detection process includes a process of detecting a steering angle, and the vehicle driving state determination process includes: The vehicle speed detected in the vehicle speed detection stroke is larger than a fourth predetermined value, the operation of the braking device is not detected in the braking device operation state detection process, and the steering detected in the steering state detection process is detected. The angle is smaller than a fifth predetermined value, the acceleration of each of the front, rear, left and right wheels detected in the wheel acceleration / deceleration state detection process is smaller than a sixth predetermined value, and the acceleration is detected in the wheel acceleration / deceleration state detection process. The step of determining that the vehicle is being driven in the stable state is included when the difference between the accelerations of the front, rear, left, and right wheels is smaller than a seventh predetermined value. Tire pressure Below detection method. 5. The vehicle motion state determination step is the fifth step.
The tire pressure drop detecting method according to claim 4, further comprising: a step of changing the sixth predetermined value according to the vehicle speed detected in the vehicle speed detecting step. 6. The wheel speed detection process includes a process for detecting the wheel speed of each of the front, rear, left and right wheels, and the wheel speed ratio calculation process includes a wheel speed of a pair of wheels on one diagonal line of the vehicle. The value obtained by subtracting the sum of the wheel speeds of the pair of wheels on the other diagonal from the sum of the two is divided by the average value of the wheel speeds of the front, rear, left and right wheels to obtain the diagonal sum wheel speed ratio. The tire pressure drop detecting method according to claim 1, further comprising a step of calculating as a ratio.