JPH01306353A - Device for measuring car body speed of automobile - Google Patents

Abstract

PURPOSE:To compute car body speeds accurately by switching a car body speed detecting means based on higher wheel speeds when the difference in wheel speed between two wheels is less than a value set in advance, and also based on wheel acceleration of higher wheel speeds when the difference is equal to or more than said value. CONSTITUTION:A processing circuit 4a computes respective wheel speeds based on signals from wheel speed sensors 2a and 2b of a right front and a left rear wheel. And when the difference in speed between them is less than a specified reference slip speed, the circuit judges that an automobile is running straight ahead wheel speed higher out of two wheel speeds is estimated to be car body speed. And when the difference in wheel speed is equal to or more than the reference slip speed, wheel acceleration of higher wheel speed is computed. And when the acceleration is less than a specified reference slip acceleration, the circuit judges that the automobile is cornering so that the average value of two wheel speeds is estimated to be car body speed. Furthermore, when the wheel acceleration is equal to or more than the reference slip acceleration, the circuit judges that driving wheels are slipping so that wheel speed lower out of two wheel speeds is estimated to be car body speed.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of the Invention The present invention relates to a vehicle body speed measuring device for estimating the running speed of a vehicle body from the wheel rotation speed of a vehicle, and is particularly suitably implemented in an anti-skid control device.

Conventional technology The rotational speed of the wheels (hereinafter referred to as "wheel speed") when the brake is depressed is carefully controlled to avoid a lock-like state, and to maintain maximum braking force between the wheels and the road surface. The anti-skid control device, which hydraulically controls the speed, detects the wheel speed and also determines the exact vehicle running speed (hereinafter referred to as "
"vehicle speed". ) is necessary to detect.

Conventionally, in consideration of the safety of brake operation, two piping systems have been provided to the hydraulic cylinders installed in each wheel, and they are generally located diagonally opposite each other when the car is viewed from above. Piping is used to connect two wheels to one system. Therefore, in a skid control device having such a brake system, two
It is desirable to determine the wheel speeds of each wheel, calculate and estimate the vehicle speed from these wheel speeds, and control the wheel speed of each wheel so that the frictional force between the wheels and the road surface is maximized.

As a means of estimating the vehicle speed from the two wheel speeds, a method is used that estimates the vehicle speed as the larger wheel speed out of the 52 wheel speeds. If the wheels slip significantly and the wheel speed or core increases, a large ((E) error will occur.

Is there a way to solve the above problems?
IIT c c l + old 5CtlE DERICIIT
E < Published February 1982 P.

Section 81.7.3.5>. This sentence II
According to H, if the wheel speed of the driving wheel is greater than the wheel speed of the non-driving wheel by a predetermined value, it is determined that the driving wheel is causing a slip phenomenon due to acceleration1, and the wheel speed of the non-driving wheel is A means for estimating the vehicle speed as the vehicle speed is disclosed. By estimating in this way, it is possible to prevent the vehicle speed from being estimated to an abnormally large value even if the driving wheels slip during acceleration and the wheel speed of the driving wheels becomes abnormally large. .

Problem to be solved by invention, problem 5: What is the condition of slip from the wheel speed difference between the driving wheel and the non-driving wheel?1
In the conventional method of estimating the wheel speed of the non-driving wheels as the vehicle speed, when slipping occurs, due to the slipping of the driving wheels during acceleration and the turning of the vehicle, There are cases where it is not possible to distinguish between cases where there is a difference in wheel speed between the outer circumference wheel and the inner circumference wheel. That is, for example, when a front-wheel drive vehicle makes a turn, the wheel speed of one bamboo wheel will be faster than the wheel speed of one wheat wheel, and if the wheel speed of the rear wheels is estimated as the vehicle speed, it will be slower than the actual vehicle speed. is estimated as the vehicle speed. Did you?),
Even if sudden braking is applied during a turn, the vehicle speed is set to a low value, so the wheels may begin to lock up (the start of the vehicle will be delayed, the wheel speed will drop significantly, and steering performance will be affected).

Furthermore, if the criteria for determining slip during wheel acceleration is set to a large wheel speed difference that does not occur during turning, the determination of whether slip has occurred will be delayed. If the brakes are operated before this 1-millimeter adjustment is performed, the vehicle speed is estimated to be high, so an anti-skid system is applied to prevent the wheels from locking. There is a risk that the pressure reduction operation of the wheel cylinder may be performed excessively.

The purpose of the present invention is to solve the above-mentioned problems, and therefore, it is necessary to remove the problem from a genuine car (twelve-speed An object of the present invention is to provide a vehicle body speed measuring device for estimating the speed of a vehicle.

assignment? ! - Means for Solving the Problem The present invention provides for driving one of the left and right front wheels and the left and right rear wheels, making the other a non-driving wheel, and each of the left front wheel, the right front wheel, the left rear wheel, and the left rear wheel. a wheel speed sensor that detects wheel speed; a first processing circuit that responds to outputs from a wheel speed sensor of the right front wheel and a wheel speed sensor of the left front wheel; a wheel speed sensor of the recording wheel and a wheel speed sensor of the right front wheel; and a second processing circuit that responds to the output from the speed sensor, and each of the processing circuits estimates the larger wheel speed as the vehicle speed when the speed difference between the driving wheel and the non-driving wheel is less than a predetermined value. means for determining the average value of the two wheel speeds when the speed difference between the recorded vehicle falling speeds is on the predetermined value side and the wheel acceleration of the larger wheel speed of the two wheel speeds is less than the predetermined value; a first estimating means for estimating the vehicle speed as the vehicle body speed; 218. A vehicle body speed measuring device for an automobile, comprising a 218I constant means for estimating the smaller wheel speed among the wheel speeds as the vehicle body speed.

Further, the present invention is characterized in that the second estimating means is executed in priority to the first estimating means.

Furthermore, the present invention is characterized in that the first estimating means and the second estimating means are not executed when the speed difference between the wheel speeds is less than a predetermined value.

In the present invention, when the speed difference between the driving wheels and the non-driving wheels is less than a predetermined value, the larger wheel speed is estimated as the vehicle speed. Then, when the speed difference between the driving wheel and the non-driving wheel is greater than or equal to a predetermined value, the wheel acceleration of the larger wheel speed of the two wheel speeds is calculated, and the wheel acceleration is less than the predetermined value. In this case, the average value of the two wheel speeds mentioned above is estimated as the vehicle speed. Furthermore, if the speed difference between the driving wheel and the non-driving wheel is greater than or equal to a predetermined value, and the wheel acceleration of the larger of the two wheel speeds is greater than or equal to the predetermined value, then The smaller wheel speed is estimated as the vehicle speed.

Further, in the present invention, the second estimating means is executed before the first estimating means.

Further, in the present invention, when the difference in wheel speed between the two wheels is less than a predetermined value, the first estimating means and the second tI flattening means are not executed.

Embodiment FIG. 1 is an electrical block diagram when an automobile body speed measuring device according to an embodiment of the present invention is used as an anti-skid control device. The anti-skid control circuit 1 is provided, for example, in a vehicle interior or a rear trunk. Wheel speed sensors 2a to 2d are provided for each wheel, and are sensors for detecting the rotational speed of the wheels. The wheel speed sensors 2a to 2d each include a detection plate made of a ferromagnetic material on which a large number of notches and protrusions are formed at equal intervals in the circumferential direction, which is fixed to a wheel shaft, and provided near the circumference of the detection plate. A wheel speed signal with a frequency proportional to the wheel speed is extracted using an electromagnetic pickup or the like. The wheel speed signals detected by the wheel speed sensors 2a to 2d are waveform-shaped into pulse signals by waveform shaping circuits 3a to 3d, and then provided to processing circuits 4a and 4b realized by a microcomputer or the like.

The switch 2e is, for example, a brake switch that sends out a signal indicating the manner in which the brake pedal is depressed,
The output of switch 2e is given to level conversion circuit 3e. The level conversion circuit 3e is a post-processing circuit 4a that converts the output of the switch 2e to a voltage level suitable for the anti-skid control circuit 1.

4b.

The processing circuit 4a calculates the respective wheel speeds based on wheel speed signals from the wheel speed sensor 2a attached to the right front wheel and the wheel speed sensor 2b attached to the left rear wheel. The difference between the two wheel speeds is the predetermined wheel speed, hereinafter referred to as "slip reference speed." ), for example 2~
If it is less than 3krn/8, it is determined that the vehicle is traveling straight, and the greater of the two wheel speeds is estimated as the vehicle body speed.

Further, when the difference in the wheel speeds is equal to or higher than the slip reference speed, the wheel acceleration, which is the rate of change of the larger wheel speed, is calculated, and the wheel acceleration is a predetermined value (hereinafter referred to as "slip reference acceleration"). ), it is determined that the vehicle is turning, and the average value of the two wheel speeds is estimated as the vehicle body speed.

Furthermore, if the wheel acceleration is equal to or higher than the slip reference acceleration, it is determined that the drive wheels are causing slip (hereinafter referred to as "acceleration slip") due to acceleration operation of the accelerator pedal, and one of the two wheel speeds is determined to be slipping. The smaller wheel speed is estimated as the vehicle speed. The vehicle speed determined in the above manner is transmitted to the processing circuit via the signal line s11.
be transferred.

The processing circuit 4b also performs the same processing as the processing circuit 4a, and the determined vehicle speed is transferred to the processing circuit 4a via the signal line s12.

After the control signal output from the processing circuits 4a and 4b is power amplified by the solenoid drive circuits 5a to 5d,
It is applied to the solenoid coils provided in the actuators 6a to 6d. The solenoid relay drive signals from the processing circuits 4a and 4b are applied to an AND circuit 7, and the output thereof is power amplified by a relay drive circuit 8 and then applied to a relay coil 9a of the solenoid relay. When the contact point 9b becomes conductive, the battery voltage of the battery 11 is applied to the other end of the solenoid coil incorporated in the actuators 6a to 6d via the connection point 10.

The motor 12 is a motor for driving a hydraulic pump, and when the motor relay 13 is turned on, electric power is supplied from the battery 11 and control hydraulic pressure is generated.

The motor drive signals from the processing circuits 4a, 4b are given to the OR circuit 14, and when the motor drive signal from either of the processing circuits 4a, 4b becomes high level, the motor relay drive circuit 15 is turned on, and the motor relay 13 is also turned on. Turn on.

When a lamp signal from the processing circuit 4a or 4b is applied to the lamp drive circuit 16, the alarm lamp 17 is turned on.

The warning lamp 17 lights up when some kind of abnormality occurs in the anti-skid control device to alert the driver.

The positive electrode of the battery 11 is connected to one power supply circuit via a power switch 18. The power supply circuit 1 converts the battery voltage to a desired voltage and then supplies the converted voltage to each circuit I\\ in the anti-skid control circuit 1.

2nd (2I) is a block diagram for explaining the hydraulic pressure path of the anti-skid system +31 device in which the vehicle body speed measuring device of an automobile, which is an embodiment of the present invention, is used.

The hydraulic pressure generated in the master cylinder 21 by pressing the brake pedal 20 is applied to the actuators 6a to 6d.
It is applied to the wheel cylinders 22a to 22d via. The actuators 6a to 6d control the braking oil pressure applied to the wheel cylinders 22a to 22d by the antiskid control circuit 1, and control the rotational speeds of the wheels 23a to 23d. The P valves 24b and 24d are valves for restricting the braking oil pressure applied to the rear wheel cylinders 22b and 22d so that it does not exceed a certain value.

FIG. 3 is a plan view of the automobile viewed from above, in order to explain the wheel speed difference between the outer circumferential wheels and the inner circumferential wheels when turning. When the outer wheels 23 of the automobile 25 (and the inner wheels 23d) turn in the direction of the trajectories 26c and 26d,
Since the outer wheel 23c travels a longer distance per nest time than the inner wheel 23d, the wheel speed of the wheel 23c is considered to be faster than the wheel speed of the wheel 23d. It is thought that the inner wheel 23d rotates faster than the wheel speed corresponding to the true vehicle speed of 25, and the wheel speed of the inner wheel 23d slips as if rubbing against the road surface (hereinafter referred to as "turning slip") and rotates slowly. It will be done.

Therefore, when the car 25 is turning, it is considered that the average wheel speed of the outer wheel 23c and the inner wheel 23d corresponds to the true vehicle speed. It will be done. Moreover, during a turn, it is thought that the difference between the wheel speed of the outer wheel 2)C and the wheel speed of the inner wheel 23d occurs slowly over time.

Therefore, if the wheel speed difference exceeds the slip reference speed and the wheel acceleration of the wheel with the higher wheel speed is less than or equal to the slip reference acceleration, it can be determined that the vehicle is turning.

The fourth (2I) is a time number l- for explaining the means for determining the vehicle speed when turning. When the automobile 25 starts turning from time t1, the line 11 representing the wheel speed of the outer wheel 23c and the inner A wheel speed difference occurs between the line 'l!2 representing the wheel speed of the peripheral wheel 23d.

The line a3 is a line representing the vehicle speed estimated from the wheel speeds of the wheels 23c and 23d. Until time t2 when the line β4 representing the slip reference speed is crossed, the wheel speed of the wheel 23c on the outer circumferential side where the wheel speed is higher is the same as that of the vehicle body. Estimated as speed. Then, at time t2, when the wheel speed of the outer wheel 23c exceeds the line 14, the wheel acceleration of the outer wheel 23c at time t2 is determined, and if the wheel acceleration is less than the slip reference acceleration, the turning The average value of the wheel speed of the outer wheel 23c and the wheel speed of the inner wheel 23d is estimated as the vehicle speed.

FIG. 5 is a time chart for explaining the means for determining the vehicle speed during acceleration. As a result of strongly pressing the accelerator pedal, the driving wheels begin to slip at time L3, and the line 15 representing the wheel speed of the driving wheels rises rapidly, resulting in a wheel speed difference between it and the line 16 representing the wheel speed of the non-driving wheels. occurs.

Line β7 represents the wheel speed of the driving wheels or the slip reference speed
Until time t4 is reached, the vehicle speed is estimated from the wheel speed on the driving wheel side. During the period from time t3 to time t4, the wheel speed of the driving wheels increases rapidly, so a certain limiting operation (a kind of filter) is performed when estimating the vehicle speed, and the vehicle speed does not necessarily match the wheel speed of the driving wheels. They do not match, and the increase in vehicle speed is suppressed.

Then, when the wheel speed of the drive wheel exceeds the slip reference speed at time t4, the wheel acceleration at time t4 is calculated, and if the wheel acceleration is greater than the slip reference acceleration, it is determined that the drive wheel is slipping, and the non-slip state is determined. The wheel speed of the drive wheels is estimated as the vehicle speed. Therefore, time t4
From now on, the line 18 representing the vehicle speed will be on the same line as the line 16 representing the wheel speed of the non-driven wheels.

FIG. 6 is a flowchart of processing performed to determine acceleration slip or turning slip. In step a1, it is determined whether or not the anti-skid control circuit 1 is performing anti-skid control. If anti-skid, li+ control is being performed, the following processing is not performed. When the anti-skid control is not executed, the process proceeds from step a1 to step a2, and in step a2, an acceleration slip determination flag A is set when it is determined that the drive wheels are causing acceleration slip.
It is determined whether P has already been set or not. If the acceleration slip determination flag AP, which is set when it is determined that an acceleration slip has occurred, has already been set, the following processing is not performed.

If the acceleration slip determination flag AF is not set, the process proceeds to step a3, and the difference between the two wheel speeds, for example, in a front-wheel drive vehicle, the wheel speed of the front wheel, which is the driving wheel, and the wheel speed of the rear wheel, which is the non-driving wheel, is determined. Difference Δ from wheel speed VIIIR
V is calculated. In step a4, step a3
It is determined whether or not the wheel speed difference ΔV calculated in ΔV exceeds the slip reference speed KV. If it does, the process proceeds to step a5, and the wheel acceleration AIIF of the front wheel exceeds the slip reference acceleration KG. It is determined whether or not there is. If the slip reference acceleration KG is exceeded, step a
6, the acceleration slip determination flag AF is set. If the wheel acceleration A□ of the front wheel is less than the slip reference acceleration KG in step a5, a turning slip determination flag CF is set, which indicates that the wheel speed difference Δ■ is caused by turning.

FIG. 7 is a flowchart of processing performed to cancel the determination of acceleration slip or turning slip.

The acceleration slip determination flag AP is set by the process shown in FIG.
Alternatively, when the turning slip determination flag CP is set, the determination flag is held until the acceleration slip determination flag AF or the turning slip determination flag CF is canceled by the process shown in No. 711ffi.

In step b1, the anti-skid control circuit 1
If it is determined that the anti-skid control is in progress, and if the anti-skid control is being performed, steps b2 and b3 are executed, and the acceleration slip determination flag AP and the turning slip determination flag CF are reset. If it is determined in step b1 that anti-skid control is not being performed, the process proceeds to step b4, where the wheel speed Vvy of the front wheel, which is the driving wheel, and the wheel speed Vw* of the rear wheel, which is the non-driving wheel.
The difference ΔV between the two is calculated. Then, in step b5, if the wheel speed difference ΔV calculated in step b4 is less than a predetermined wheel speed (for example, lkr//H), the process proceeds to step b2 and step b3, and the process proceeds to step b2 and step b3. Reset the quantitative flag.

FIG. 8 is a flowchart for calculating vehicle speed. In step C1, it is first determined whether the acceleration slip determination flag AF is set. If it is determined that the wheel speed VwR of the rear wheel, which is a non-driving wheel, is set, it is determined that the front wheel, which is a driving wheel, is experiencing acceleration slip, and the wheel speed VwR of the rear wheel, which is a non-driving wheel, is
Estimated to be 8.

If it is determined in step C1 that the acceleration slip determination flag AF is not set, the process proceeds to step C3, where it is determined whether the turning slip determination flag CF is set. If the turning slip judgment flag CF is set, the process proceeds to step C4, and if the car is running in a turning state, I'11 is detected, indicating that the front wheel speed is high. The average wheel speed of VW and l is estimated as vehicle speed ■3.

Next, if it is determined in step c3 that the turning slip determination flag CF is not set to 1~, the process proceeds to step c5, where the front wheel speed ■1 and the rear wheel speed V
The wheel speed which is larger between wR and WR is estimated as the vehicle speed V3.

Step c2. c4. Vehicle speed V estimated at c5
5, so-called software filtering is performed in step C6. This software filtering is performed, for example, at a certain time t. Based on the vehicle speed determined in , a limit is set within a certain range for the vehicle speed at the next time L0 when the vehicle speed is calculated, and if a vehicle body speed exceeding that limit is determined, the vehicle speed within the above range is set. This is filtering that estimates the upper limit value or lower limit value as the vehicle speed. By performing such filtering processing, it is possible to prevent sudden changes in vehicle speed due to noise, for example.

FIG. 9 is a functional block diagram for explaining the calculation of the vehicle speed performed in the processing circuit 4a. The driving wheel speed detected by the wheel speed sensor 2a and the non-driving wheel speed detected by the wheel speed sensor 21:) are given to the wheel speed difference calculation means 401, and the wheel speed difference between the two wheels is calculated. The wheel speed difference is provided to comparison means 402. In the comparing means 402, the wheel speed difference is compared with the slip reference speed stored in the first memory 403, and the result is sent to the acceleration slip turning determining means 404.

Further, the wheel speed of the driving wheels is given to wheel acceleration calculation means 405, the wheel acceleration of the driving wheels is calculated, and the result is given to comparison means 406. The comparing means 406 compares the wheel acceleration of the driving wheels with the slip reference acceleration stored in the second memory 407, and the comparison result is provided to the acceleration slip turning determining means 404.

Based on the output of the comparison means 402, the acceleration slip turning determination means 404 determines whether the wheel speed difference exceeds the slip reference speed stored in the first memory 403,
Based on the 0G output result, a judgment is made as to whether the cause of the wheel speed difference is due to slipping of the drive wheels or whether the vehicle is turning. That is, when the wheel acceleration of the driving wheel exceeds the slip reference speed stored in the second memory 407, it is determined that the driving wheel is in an acceleration slip state in which it is slipping, and the slip reference acceleration is determined. If the conditions are as follows, it is determined that the vehicle is turning. The determination result is given to the vehicle speed calculation means 408, and the vehicle speed is estimated. That is, when the wheel speed difference is less than the slip reference speed, the wheel speed which is greater between the driving wheel speed and the non-driving axle wheel speed is estimated as the vehicle body speed. In addition, when it is determined that the acceleration slip condition is present,
Vehicle speed calculation means 408 estimates the non-driving axle wheel speed as the vehicle speed. Furthermore, when it is determined that the vehicle is in a turning state, the vehicle body speed calculating means 408 estimates the average wheel speed of the driving wheel speed and the non-driving axle wheel speed as the vehicle body speed.

The vehicle speed estimated by the vehicle speed calculation means 408 is given to the vehicle speed comparison means 409, where it is compared with the vehicle speed estimated and transferred by the processing circuit 4b. The vehicle speed comparison means 409 sends the larger of the two vehicle speeds to the anti-skid control means 410 as the control vehicle speed.
Anti-skid control calculations are performed and the results are provided to the solenoid drive circuit.

The acceleration slip turning determination canceling means 411 calculates the driving wheel speed and the non-driving wheel wheel speed manually, and when the difference in these wheel speeds reaches the end determination criterion distance groove, sends the result to the acceleration slip turning determining means 404, and accelerates. Terminates slip and turning judgment.

Although the processing circuit 4a has been described above, similar processing is performed in the processing circuit 4b as well.

As described above, according to this embodiment, when a wheel speed difference occurs between the driving wheel speed and the non-driving wheel speed, the acceleration slip state and the turning state can be distinguished more accurately. Since anti-skid control can be performed based on the vehicle speed, malfunction of anti-skid control can be prevented.

Effects of the Invention As described above, according to the present invention, when a speed difference occurs between the two wheel speeds that are the basis for estimating the vehicle speed, if the speed difference is smaller than a predetermined value, the larger one is used. When the wheel speed is high, the vehicle speed estimating means is switched based on the wheel acceleration of the higher wheel speed, so it is possible to calculate a more accurate vehicle speed.

[Brief explanation of the drawing]

1 [21 is an electrical block diagram when the vehicle body speed measurement device of an automobile which is an embodiment of the present invention is used as an anti-skid control device, and FIG. 2 is an electrical block diagram of the vehicle body speed measurement device which is an embodiment of the present invention A block diagram for explaining the hydraulic path of the anti-skid control device in which the device is used. Figure 3 is a plan view of the car viewed from above to explain the wheel speed difference between the outer and inner wheels when turning. Figure 4 is a time chart for explaining the means for determining the vehicle speed when turning, Figure 5 is a time chart for explaining the means for determining the vehicle speed during acceleration, and Figure 6 is for accelerating slip or turning. FIG. 7 is a flowchart of the process performed to determine slip; FIG. Fig. 9 is a functional block diagram for explaining the calculation of the vehicle speed performed in the processing circuit 4a. 1...Anti-skid system (wholesale circuit, 2a to 2d...
Wheel speed sensor, 3a to 3d... Waveform shaping circuit, 4a. 4b... Processing circuit, 5a-5d... Solenoid drive circuit, 6a-6d... Actuator, 8... Solenoid relay drive circuit, 11... Battery, 12...
・Motor, 13...Motor relay, 15...Motor relay drive circuit, 18...Power switch, one...
Power supply circuit agent Patent attorney Number of strokes Keiichi Figure 2 Figure 6 Figure 7

Claims (3)

[Claims] (1) Wheel speed that detects each wheel speed of the left front wheel, the right front wheel, the left rear wheel, and the right rear wheel, with one of the left and right front wheels and left and right rear wheels being a driving wheel and the other being a non-driving wheel. a first processing circuit responsive to outputs from the sensor; a wheel speed sensor for the right front wheel and a wheel speed sensor for the left rear wheel; and a first processing circuit responsive to outputs from the wheel speed sensor for the left front wheel and the wheel speed sensor for the right rear wheel. a second processing circuit for estimating the greater wheel speed as the vehicle body speed when the speed difference between the driving wheel and the non-driving wheel is less than a predetermined value; first estimating means for estimating the average value of the two wheel speeds as the vehicle body speed when the difference is at least the predetermined value and the wheel acceleration of the larger wheel speed of the two wheel speeds is less than the predetermined value; When the speed difference between the wheel speeds is greater than or equal to the predetermined value and the wheel acceleration of the larger wheel speed of the two wheel speeds is greater than or equal to the predetermined value, the smaller of the two wheel speeds is set as the vehicle body speed. and second estimating means for estimating the vehicle body speed of an automobile. (2) The vehicle body speed measuring device of claim 1, wherein the second estimating means is executed in priority to the first estimating means. (3) The vehicle body speed measuring device of claim 1, wherein the first estimating means and the second estimating means are not executed when the speed difference between the wheel speeds is less than a predetermined value. .