JP5011813B2 - Electric power steering device - Google Patents

Description

  The present invention relates to an electric power steering apparatus, and more particularly to an electric power steering apparatus that prevents hunting of a motor output when a motor output limit is imposed due to an overcurrent or the like.

  An electric power steering device that applies a steering device of an automobile or a vehicle by a rotational force of a motor applies an auxiliary force to a steering shaft or a rack shaft by a transmission mechanism such as a gear or a belt via a reduction gear. It is like that. Such a conventional electric power steering apparatus performs feedback control of motor current in order to accurately generate assist torque (steering assist torque). The feedback control adjusts the motor applied voltage so that the difference between the current control value and the motor current detection value becomes small. The adjustment of the motor applied voltage is generally performed by a PWM (pulse width modulation) control duty. This is done by adjusting the tee ratio.

  Here, the general configuration of the electric power steering apparatus will be described with reference to FIG. 5. The column shaft 102 of the steering handle 101 is connected to the steering wheel through the reduction gear 103, universal joints 104 a and 104 b, and the pinion rack mechanism 105. Coupled to the tie rod 106. The column shaft 102 is provided with a torque sensor 110 that detects the steering torque of the steering handle 101, and a motor 120 that assists the steering force of the steering handle 101 is coupled to the column shaft 102 via the reduction gear 103. ing. Power is supplied from the battery 114 to the control unit 130 that controls the power steering device, and an ignition key signal is input from the ignition key 111. The control unit 130 is controlled by the steering torque Tref detected by the torque sensor 110 and the vehicle speed sensor 112. The assist command current command value Iref is calculated based on the detected vehicle speed Vel, and the current supplied to the motor 120 is controlled based on the calculated current command value Iref.

  The control unit 130 is mainly composed of a CPU, and FIG. 6 shows general functions executed by programs in the CPU. For example, the proportional-integral control unit 208 does not indicate a proportional-integral calculator as independent hardware, but indicates a proportional-integral function executed by the CPU.

  The function and operation of the control unit 130 will be described. The steering torque Tref detected and input by the torque sensor 110 and the vehicle speed Vel detected by the vehicle speed sensor 112 are input to the current command value calculation unit 204. The current command value calculation unit 204 calculates and determines a current command value Iref that is a control target value of the current supplied to the motor 120 based on the input steering torque Tref and the vehicle speed Vel. On the other hand, the motor current Im of the motor 120 is detected by the current detector 205 and fed back to the subtraction unit 207. The current command value Iref and the motor current Im are input to the subtraction unit 207, and the subtraction unit 207 calculates a deviation ΔI = (Iref−Im). The deviation ΔI is input to a proportional-integral control unit (PI control unit) 208, and a voltage command value is output from the PI control unit 208. When the drive circuit that drives the motor 120 is the inverter 211 that is PWM-controlled, the voltage command value indicates the duty ratio (Duty) of the PMW control unit 210.

  In general, a limiter 209 is disposed at the output of the PI control unit 208. The reason is that the duty ratio is 100% at the maximum, and it is meaningless for the motor output even if the PI control unit 208 calculates a value (100%) or more. Therefore, when the limit value (Dlimit) of the limiter 209 is set to 100% and the duty ratio (Duty) output from the PI control unit 208 is 100% or more, the limit value is limited, and the limited duty ratio (Dutlim) is output. The

In addition to the above-described reason, the limit value (Dlimit) of the limiter 209 is the reverse of the motor 12 as disclosed in Patent Document 1, for example, when the operation handle 101 hits the handle lock end strongly. Since the electromotive voltage is reduced and as a result, an overcurrent is generated in the motor current, the limit value (Dlimit) of the limiter 209 may be reduced from, for example, 100% to 50% to suppress the overcurrent. Specifically, when the motor current Im is detected by the current detector 205 and the motor current Im is larger than the set value in the overcurrent protection unit 212, the limit value of the limiter 209 is reduced from 100% to 50%, The motor current is prevented from becoming overcurrent.
Japanese Patent Application Laid-Open No. 11-240459

  By the way, when a digital operation process (discrete time system operation) is executed by combining an operation including an integral term such as PI control and a limiter having a variable limit value, the following problem occurs. For example, the handle comes into contact with the handle lock end and the motor current becomes overcurrent, and the limit value of the limiter is limited to 100% to 50% for reasons such as overcurrent. As a result of reducing the limiter value, the motor current is suppressed and the overcurrent state is released, so the limit value is returned from 50% to 100%.

  However, the value of integration by digital calculation maintains a value that generates overcurrent, and therefore the duty ratio that is the output of the PI control unit is a value that generates overcurrent, and the limit value of the limiter is 50%. When the current is returned to 100%, the motor current becomes an overcurrent state again. Therefore, the overcurrent protection is activated, the limit value of the limiter is reduced again from 100% to 50%, and the motor current is limited. Such a state is repeated, and a hunting phenomenon occurs in which the motor current changes at a high speed between 100% and 50%. As a result, vibration and noise are generated and the driver feels uncomfortable.

  The present invention has been made for the above-described circumstances, and an object of the present invention is an electric power steering apparatus using a control that digitally processes an arithmetic operation including an integral term and a variable limiter. Even if it exists, it is providing the electric power steering device which does not give the driver unpleasantness without suppressing the motor output which assist force gives, and suppressing a vibration and a noise.

The present invention includes a motor (120) that applies a steering assist force to a steering system, and a battery (114) that supplies electric power to an inverter (211) that drives the motor (120). A current command value (Iref) is calculated based on the vehicle speed, and a deviation between the current command value (Iref) and the motor current (Im) is PI-controlled by a PI control unit (10, 208), and the PI control unit (10 , 208) is limited or fixed by a limiter (209) and clamped at a fixed value, and PWM control is performed, and the motor (120) is driven by PWM control by the inverter (211). the object of the present invention, the PI control unit (10), a first intermediate variable by entering the deviation (W (k)) subtracting unit that outputs (11), the subtraction A first gain unit (15) and a unit delay unit (13) for inputting the first intermediate variable (W (k)) from (11), and a second intermediate variable (W from the unit delay unit (13)) (K-1)) is input, and the outputs of the second gain unit (14) and the third gain unit (16) and the outputs of the first gain unit (15) and the third gain unit (16) are added. An adder (12) that outputs a voltage command value (Vref), and a ratio unit (17) that divides the voltage command value (Vref) by a maximum value (Dmax) of a duty ratio. The first intermediate variable (W (k)) is calculated by subtracting the output of the gain section (14) into the subtracting section (11), and the second intermediate variable (W (k-1)) is calculated by the motor. Winding resistance value (Rm), battery voltage (Vbat) of the battery (114), The maximum value of the I ratio (Dmax), the changed by the current command value (Iref) and the motor current (Im), that the output of said ratio portion (17) is adapted to input to the limiter (209) Achieved by:

Further, the above object of the present invention can be achieved more effectively by setting the winding resistance value (Rm) and the battery voltage (Vbat) to fixed values .

Further, the object of the present invention is that the winding resistance value (Rm) is calculated based on a temperature sensor detection value or a temperature estimation value, and the battery voltage (Vbat) is detected by a voltage detector (20). This is achieved more effectively.

  Since the intermediate variable of the digital calculation including the integral term is changed in a feedforward manner based on the current command value, the motor current, and the limiter value, the output value (duty duty) of the current control means that inputs the current command value and the motor current. The ratio is already a value corresponding to the limiter value. Therefore, even if the limiter value changes, the output value (duty ratio) of the current control means changes based on the limiter value before the change, so that the duty ratio after passing the limiter suddenly changes according to the change of the limiter value. Therefore, the duty ratio after passing through the limiter does not cause hunting. A similar effect can be obtained by clamping at a predetermined fixed value instead of the limiter value. Therefore, it is possible to provide an electric power steering device in which the assist force is not hunted and the driver is not uncomfortable by suppressing vibration and noise.

  In addition, since intermediate variables of digital calculation including integral term are changed in consideration of motor winding resistance value and battery voltage value, calculation control is performed with higher accuracy, and assist force hunting, vibration and noise are further increased. It can be effectively prevented.

  An embodiment of the present invention will be described with reference to FIG. The same numbers as those described in FIG. 6 have the same functions. Deviation ΔI = (Iref−Im) between current command value Iref and motor current Im is calculated by subtractor 207. And deviation (DELTA) I is input into the PI control part 10 which concerns on this invention instead of the PI control part 208 of FIG. The interior of the PI control unit 10 will be described in detail later.

The duty ratio (Duty), which is the output of the PI control unit 10, is input to the limiter 209. From the limiter 209, the duty ratio ( Dutlim ) after being limited by the limit value (Dlimit) of the limiter 209 is output. When the inverter 211 and the motor 120 are modeled, the battery voltage value Vbat and the duty ratio ( Dutlim ) are input to the multiplication unit 21, and the output of the multiplication unit 21 is a transmission indicating the reciprocal (1 / Rm) of the motor winding resistance value Rm. The function 22 is input and the transfer function 22 outputs the motor current Im.

  There are cases where the battery voltage value Vbat and the motor winding resistance value Rm are handled as constants and as change values. In the case of handling as a change value, the battery voltage value Vbat is detected by the voltage detector 20, and the motor winding resistance value Rm is calculated using temperature estimation.

In such a control configuration and motor model, the relationship between the motor current Im, the battery voltage value Vbat, the motor winding resistance Rm, and the duty ratio (Duty) can be expressed as in Equation (1).
(Equation 1)
Im = (Vbat / Rm) · Duty
= (Vbat / Rm). (Vref / Dmax)
Here, Duty = (Vref / Dmax), Vref is a voltage command value,
Dmax is a value indicating a duty ratio of 100%.

Next, the inside of the PI control unit 10 will be described.

  First, the deviation ΔI that is the output of the subtraction unit 207 and the output of the gain unit 14 (gain = a1) are input to the subtraction unit 11, and the intermediate variable W (k) is output. The intermediate variable W (k) is input to the gain unit 15 (gain = b0) and the unit delay (Unit Delay) unit 13. The intermediate variable W (k−1), which is the value of the intermediate variable before one sampling, which is the output of the unit delay unit 13, is input to the gain unit 14 and the gain unit 16 (gain = b1). The output of the gain unit 15 and the output of the gain unit 16 are input to the adding unit 12, and a voltage command value Vref that is an added value of these is output. The voltage command value Vref is input to the ratio unit 17 and compared with a value Dmax representing 100% of the duty ratio. Specifically, the ratio unit 17 divides the input voltage command value Vref by Dmax, and the ratio (Vref / Dmax) = duty ratio (Duty) is output. That is, if the voltage command value Vref that is the output value of the adder 12 is Dmax, the duty ratio of the PWM control is 100%, and the voltage command value Vref that is the output of the adder 11 is (Dmax / 2). For example, the duty ratio (Duty) of PWM control represents 50%.

When the relationship among the gain units 14 to 16, the unit delay unit 13, the addition unit 12, and the subtraction unit 11 in the PI control unit 10 configured as described above is expressed by an expression, it can be expressed as the following formula 2.
(Equation 2)
Vref = b0 · (ΔI−a1 · W (k−1)) + b1 · W (k−1)
= B0 · ΔI + (b1−b0 · a1) · W (k−1)

That is, since the duty ratio (Duty) is expressed by Duty = (Vref / Dmax), if the voltage command value Vref is determined, the duty ratio (Duty) is also determined.

By the way, when the expression 2 is re-expressed around the intermediate variable W (k−1), the following expression 3 is obtained.
(Equation 3)
W (k-1)
= Vref / (b1−b0 · a1) −b0 · ΔI / (b1−b0 · a1)

Here, using the relationship of Vref = (Rm / Vbat) · (Im · Dmax) in Equation 1, it is substituted into “Vref” in Equation 3. Further, K1 = 1 / (b1−b0 · a1) and K2 = b0 / (b1−b0 · a1) are substituted, and K1 and K2 are also substituted into Equation 3. K1 and K2 are constants once the characteristics of the PI controller are determined. Therefore, Equation 3 can be rewritten as Equation 4 below.
(Equation 4)
W (k-1) = (Rm / Vbat) * (Im * Dmax) * K1-K2 * [Delta] I
= (Rm / Vbat) * (Dmax * K1) * Im-K2 * (Iref-Im)

Here, Dmax, which is a value indicating 100% of the duty ratio (Duty), is set to 1. When Dmax is defined as 1, the duty ratio (Duty) is 0% when the voltage command value Vref is 0, 50% when 0.5, and 100% when 1; and the limit value (limiter value) Dlimit is a value between 0 and 1.

When Dmax is defined as 1 and a limiter value (Dlimit) exists, Expression 4 is re-expressed as Expression 5.
(Equation 5)
W (k-1)
= (Rm / Vbat) * (Dlimit * K1) * Im-K2 * (Iref-Im)

The meaning of Equation 5 is that the voltage command calculated by Equation 2 is obtained by changing the intermediate variable W (k−1) in a feed-forward manner based on the current command value Iref, the motor current Im, and the limiter value Dlimit. The value Vref, that is, the duty ratio (Duty) can be changed in a feedforward manner to a limiter value, for example, a value that outputs 50%. Rewriting equation (4) to equation (5) can be considered by including Dmax in b0 and b1 in advance. If W (k-1) calculated in equation (5) is substituted into equation (2) and arranged, duty and dlimit are equal. The current becomes continuous.

  If the intermediate variable W (k−1) is not corrected according to Equation 5, the duty ratio (Duty) calculated based on the voltage command value Vref calculated by digital calculation including integration shows a value of 100%. Therefore, when the limiter value is released from 0.5 (50%) to 1 (100%), the duty ratio after passing through the limiter suddenly changes from 50% to 100%, so a large motor current is output. Is done. As a result, overcurrent protection is activated again and hunting occurs.

  However, when the intermediate variable W (k−1) is corrected in a feed-forward manner according to the above equation 5, the duty ratio (Duty) calculated based on the voltage command value Vref indicates a value of 50%. Even when the value is canceled from 0.5 (50%) to 1 (100%), the duty ratio after passing through the limiter will gradually change from 50%, avoiding overcurrent protection operation. it can. As a result, the hunting phenomenon due to overcurrent protection and the release of the limiter can be avoided. The present invention is based on the feed-forward correction of this intermediate variable.

  When correcting the intermediate variable W (k−1), there are cases where the motor winding resistance value Rm and the battery voltage value Vbat are treated as constants and as variables. Each embodiment in consideration of this difference will be described below.

  Here, an example in which (Rm / Vbat) is considered as a constant will be described. Although the winding resistance value Rm and the battery voltage Vbat of the motor are variables, since changes are small compared to the current command value Iref and the motor current Im, a large control error does not occur even if they are regarded as constants. First, an embodiment in which the winding resistance value Rm of the motor and the battery voltage Vbat are treated as constant will be described. As an example, the design value is used for the winding resistance value Rm of the motor, and the standard battery voltage value Vbat is 12V.

  The control of the motor 120 will be described with reference to the flowchart shown in FIG.

  First, the motor current Im detected by the current detector 205, the current command value Iref calculated by the current command value calculation unit 204, and the limiter value Dlimit set by the overcurrent protection unit 212 are read (step S1). In the first embodiment, the battery voltage value Vbat and the motor winding resistance value Rm are constants.

  Next, the limiter 209 determines whether or not the duty ratio (Duty) is limited (step S2). Being restricted means that the limiter value (Dlimit) is less than 1. For example, as an example in which the limiter 209 makes the limiter value (Dlimit) less than 1, a case where the above-described overcurrent protection unit 212 operates and the limiter value is changed from 1 to 0.5 is assumed.

  When the duty ratio is limited by the limiter (YES), it is intermediate during the period when the duty ratio is limited, or until the limit is released and for a predetermined period, that is, before the PI controller starts computation. After the correction of the variable W (k−1) is performed according to the above equation 5 (step S3), the process proceeds to the calculation of the voltage command value Vref (duty ratio), which is the next step. When the duty ratio is not limited by the limiter (NO), the process proceeds to the calculation of the voltage command value (duty ratio), which is the next step, without going through step S3.

  A voltage command value Vref that is a basis for calculating the duty ratio (Duty) is calculated according to the above equation 2 (step S4). After calculating the voltage command value Vref, it is determined whether the voltage command value Vref should be limited by the limiter 209 (step S5). If it is not necessary to limit the limiter value (NO), the process returns. When limiting (changing) the limiter value (YES), the limiter value (Dlimit) of the limiter 209 is set to a specified limiter value, for example, 50% (step S6).

  By performing the control process as shown in this flowchart, the following actions are brought about. Before that, the conventional problems will be explained again.

  The duty ratio (Dutlim) after limiting the limiter, which is the final duty ratio input to the inverter 211, is the duty ratio (Duty) calculated from the voltage command value Vref = (Vref / Dmax) below the limiter value. The duty ratio after limiting the limiter (Dutlim) = (Duty), but if the duty ratio (Duty) is equal to or greater than the limiter value (Dlimit), the duty ratio after the limiter limiting (Dutlim) = the limiter value (Dlimit). Therefore, when the limiter value is suddenly released from 50% to 100%, the duty ratio (Dutlim) after the limiter limit may be suddenly changed from 50% to 100%.

  However, in this embodiment, in step S3 and step S4, the voltage command value Vref that is the basis for calculating the intermediate variable W (k−1) and the duty ratio (Duty) is fed forward to a value corresponding to the limiter value 50%. Therefore, when the limiter value is released (100%) from the limit (50%), the duty ratio (Dutlim) after the limiter can smoothly change from the limiter value 50%. Therefore, it is possible to reduce the occurrence of the hunting phenomenon that has been caused by the limit release of the limiter value in the case of Dulim which is the duty ratio after the limiter. Therefore, the assisting force of the electric power steering device does not give a sense of incongruity to the steering wheel operation, and the generation of vibration and noise can be reduced.

  Next, an embodiment with higher control accuracy than the embodiment described above will be described with reference to FIGS. That is, in this embodiment, the winding resistance value Rm of the motor is controlled in consideration of a change in temperature due to the energization current and a change in the battery voltage value Vbat.

  In FIG. 4, first, the motor current value Im, the current command value Iref, and the limiter value (Dlimit) are read as in the first embodiment (step S1). Next, in the second embodiment, the battery voltage value Vbat is detected, and the winding resistance value Rm of the motor 120 is calculated (step S1A).

Here, calculating the winding resistance value Rm of the motor 120, so when energization current to the windings of the motor temperature of the winding resistance value is changed increases, corrects the resistance value in consideration of the temperature change It is necessary to calculate. Therefore, basically, it is basic to estimate the temperature of the winding that has changed due to the energization current. A technique for estimating the winding resistance value using such temperature estimation or a technique for detecting with a temperature sensor has already been disclosed, and Japanese Patent No. 3601967 is an example of a document disclosing temperature estimation. Specifically, in FIG. 3, the ambient temperature Ta of the motor is detected by a temperature sensor 30 installed in the vicinity of the motor 120, and the motor current Im is detected, and the temperature rise (ΔT) due to the motor current Im is detected. The temperature is estimated by the temperature estimation unit 31. In the winding resistance value estimation unit 32 with the winding temperature Tm as an input, Rm = Ra + α · (ΔT) (where Ra is the winding resistance value at the ambient temperature Ta and α is the temperature coefficient of the motor winding). The winding resistance value Rm of the motor 120 is estimated. On the other hand, the battery voltage value Vbat can detect the voltage value of the battery 114 using the voltage detector 20.

  Next, in FIG. 4, regarding each step from S2 to S6, except for the point relating to the calculation of the intermediate variable correction W (k−1) in step S3, the steps from step S2 to step S6 in the first embodiment are performed. Same as operation. That is, in the first embodiment, the battery voltage value Vbat and the motor winding resistance value Rm of Formula 4 are corrected as constants. However, in the second embodiment, the battery voltage value Vbat detected in step S1A and the estimated motor winding resistance are corrected. The calculation for correcting the intermediate variable W (k−1) in step S3 is also performed using the value Rm (step S3).

  As described above, in the second embodiment, the battery voltage value Vbat and the motor winding resistance value Rm are not treated as constants but are treated as variables, and the battery voltage value Vbat and the motor winding resistance value Rm are detected and estimated and calculated. Compared with the first embodiment, the intermediate variable W (k−1) can be corrected with higher accuracy. As a result, even when the limiter value is released, it can be expected that the duty ratio smoothly changes from the limiter value and the restriction due to overcurrent protection and the release hunting are not generated. Therefore, the assist force of the electric power steering device does not give a sense of incongruity to the steering wheel operation, and the generation of vibration and noise can be reduced more effectively.

  In the present embodiment, both the battery voltage value Vbat and the motor winding resistance value Rm are treated as constants or variables. However, even if an intermediate variable is corrected with one as a constant and the other as a variable, the intermediate accuracy is maintained. Needless to say, an effect can be obtained. For example, when the resistance is a constant, the battery voltage Vbat can be calculated in advance for the first term of Equation 5, and the calculation load can be reduced.

  In the above description, the limiter for limiting the output of the current control means with the limiter value is provided. However, an output means for clamping and outputting with a fixed value may be used.

It is a figure showing the digital calculation of the proportional integral control part which concerns on this invention. It is a flowchart for correct | amending an intermediate variable when a battery voltage value and a motor winding resistance value are made into a constant. It is a control block diagram to which a function of detecting a battery voltage value and estimating a motor winding resistance value is added. 6 is a flowchart for correcting an intermediate variable when a battery voltage value and a motor winding resistance value are variables. It is a block diagram of an electric power steering device. It is a control block diagram of the electric power steering device according to the present invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 Proportional integral control part 11 Subtraction part 12 Addition part 13 Unit delay part 14,15,16 Gain part 17 Ratio part 20 Voltage detector 30 Temperature sensor 31 Temperature estimation part 32 Winding resistance value estimation part

Claims (3)

A motor (120) that applies a steering assist force to the steering system and a battery (114) that supplies electric power to an inverter (211) that drives the motor (120) are provided, based on the steering torque and the vehicle speed. A current command value (Iref) is calculated, and a deviation between the current command value (Iref) and the motor current (Im) is PI-controlled by a PI control unit (10, 208), and the PI control unit (10, 208) In the electric power steering apparatus in which the output is clamped at a limit or a fixed value by a limiter (209) and PWM controlled, and the motor (120) is driven by PWM control by the inverter (211).
The PI control unit (10) inputs the deviation and outputs a first intermediate variable (W (k)), and the first intermediate variable (W from the subtraction unit (11)). (K)) is input to the first gain unit (15) and the unit delay unit (13), and the second gain is input from the unit delay unit (13) to the second intermediate variable (W (k-1)). An adder (14), a third gain unit (16), and an adder unit that outputs the voltage command value (Vref) by adding the outputs of the first gain unit (15) and the third gain unit (16). 12) and a ratio part (17) for dividing the voltage command value (Vref) by the maximum value (Dmax) of the duty ratio,
The output of the second gain unit (14) is subtracted and input to the subtraction unit (11) to calculate the first intermediate variable (W (k)), and the second intermediate variable (W (k-1)). Is changed by the winding resistance value (Rm) of the motor, the battery voltage (Vbat) of the battery (114), the maximum value (Dmax) of the duty ratio, the current command value (Iref) and the motor current (Im). The electric power steering apparatus is characterized in that the output of the ratio section (17) is input to the limiter (209) . The electric power steering apparatus according to claim 1, wherein the winding resistance value (Rm) and the battery voltage (Vbat) are fixed values . The winding resistance value (Rm) is calculated based on a temperature sensor detection value or a temperature estimation value, and the battery voltage (Vbat) is detected by a voltage detector (20). The electric power steering device described in 1.

Images