JP2009120035A - Electric power steering system - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To properly set a charging and discharging threshold value while restraining cost increase of an auxiliary electric power source on an electric power steering system using the auxiliary electric power source. <P>SOLUTION: This electric power steering system shares electric power only by a main electric power source and makes the auxiliary electric power source in a state free to charge when electric power source output required for steering assistance is less than a prescribed charging and discharging threshold value and shares the electric power with the main electric power source by also using the auxiliary electric power source, when the electric power source output exceeds the charging and discharging threshold value. The charging and discharging threshold value which is larger than a value when a second-order derivative value of energy by the charging and discharging threshold value becomes a peak value, is set in a control circuit in accordance with a characteristic curve of energy required for the auxiliary electric power source when the charging and discharging threshold value is changed. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to an electric power steering apparatus that generates a steering assist force by a motor, and more particularly to a configuration of an electric circuit thereof.

  The electric power steering device is a device that generates a steering assist force by a motor in accordance with a driver's steering torque. In recent years, the demand for electric power steering devices for large vehicles has increased rapidly, and the steering assist force required for such large vehicles has also increased. Therefore, more power must be supplied to the motor. However, there are cases where such a large amount of power cannot be adequately provided by using only a main power source constituted by a battery. Therefore, an auxiliary power supply is provided separately from the main power supply. Usually, only the main power supply can be used. When larger power is required, the main power supply and the auxiliary power supply are connected in series to supply power from both power supplies. (For example, refer to Patent Document 1). Note that the auxiliary power supply is charged by a battery when only the main power supply is used.

FIG. 3 is a diagram illustrating an example of a change with time of electric power necessary for the electric power steering apparatus. In the figure, until time T1, the electric power P to be supplied to the motor is equal to or less than a predetermined electric power _Pth . Therefore, necessary power is borne only by the main power source. At times T1 and T2, power exceeding a predetermined power P _th is required, so the main power bears the power P _th out of the total power, and the auxiliary power bears the power exceeding that (shaded portion). To do. Then, at time T2 to T3, since the power required is less than a predetermined power P _th, bear power only in the main power supply, and, in surplus of power (inverse shaded area), discharged auxiliary power To charge. Similarly, at time T3 to T4, the power P _th and the main power burden, minute power beyond that is borne by the auxiliary power supply. From time T4 to T5, power is borne only by the main power source, and the discharged auxiliary power source is charged with surplus power.

Japanese Patent Laying-Open No. 2005-287222 (FIG. 1)

  In the electric power steering apparatus as described above, it is determined whether or not the auxiliary power source is used with predetermined power as a boundary point, in other words, whether the auxiliary power source is discharged or can be charged. . Therefore, the power (or current) at this boundary point is referred to as a charge / discharge threshold. Depending on where this charge / discharge threshold is set, the maximum battery load and the energy required for the auxiliary power supply differ. The energy required for the auxiliary power source is closely related to the cost of the auxiliary power source.

However, no specific proposal has yet been made as to what criteria should be used to set the charge / discharge threshold, which is an important numerical value. Under such circumstances, it is difficult to set the charge / discharge threshold to an appropriate value.
In view of such a conventional problem, an object of the present invention is to appropriately set a charge / discharge threshold while suppressing an increase in cost of an auxiliary power supply in an electric power steering apparatus using the auxiliary power supply.

  The present invention is an electric power steering device that generates a steering assist force by a motor, and includes a main power source that supplies power to the motor, an auxiliary power source that can supply power to the motor, and the main power source. A first output state for charging the auxiliary power supply based on the power supply to the motor only by the main power supply, and a second output state for supplying power to the motor from the main power supply and the auxiliary power supply. And when the power output required for steering assist is equal to or lower than a predetermined charge / discharge threshold, the charge / discharge circuit is set to the first output state, and when the charge / discharge threshold is exceeded, the charge / discharge circuit is A control circuit for setting the charge / discharge circuit to the second output state, and the control circuit is based on a characteristic curve of energy required for the auxiliary power supply when the charge / discharge threshold is changed. , Is characterized in that the charging and discharging threshold greater than the discharge threshold value when the second order differential value of the energy due to charging and discharging threshold reaches a peak value and the predetermined charge and discharge threshold.

In the electric power steering apparatus configured as described above, the characteristic curve has a form in which energy decreases while changing from a steep slope to a gentle slope as the charge / discharge threshold increases, and in the middle of such a change. Thus, it was found that there is a unique point at which the rate of change of the slope is maximum. Therefore, by setting a charging / discharging threshold value larger than the charging / discharging threshold value when the second-order differential value of energy becomes a peak value in the control circuit,
(1) A region where energy required for the auxiliary power source is relatively large can be avoided. As a result, the cost of the auxiliary power source can be kept low. Also,
(2) It is possible to avoid a region where the energy change is steep, and use a charge / discharge threshold value in a region where the energy changes slowly. Thereby, the dispersion | variation of the energy actually required with respect to the set charging / discharging threshold value becomes comparatively small, As a result, a charging / discharging threshold value can be set appropriately.

  According to the electric power steering device of the present invention, the cost of the auxiliary power supply can be kept low, and the charge / discharge threshold value can be set appropriately.

  FIG. 1 is a circuit diagram showing a schematic configuration mainly including an electric circuit of an electric power steering apparatus 1 according to an embodiment of the present invention. In the figure, a steering device 2 includes a driver's steering torque applied to a steering wheel (handle) 3, and a steering assist force transmitted from a motor 4 to a steering shaft 2a via a speed reducer (not shown). Driven by. The motor 4 is a three-phase brushless motor and is driven by a drive circuit 5. The control circuit 6 includes a microcomputer, determines a necessary steering assist force based on a vehicle speed signal input from the vehicle speed sensor 7 and a steering torque signal input from the torque sensor 8, and the steering assist force. Is applied to the steering shaft 2a.

  The drive circuit 5 is given a voltage of the main power supply 9 or, if necessary, a DC voltage obtained by adding the voltage of the auxiliary power supply 10 to the drive circuit 5. The main power source 9 is composed of a battery 9B and an alternator (having a rectification and regulator function) 9A, and the discharge circuit is switched in the middle of electric circuits L1 and L3 for guiding the voltage of the main power source 9 to the drive circuit 5. A switch 11 is provided. On the other hand, the auxiliary power source 10 is constituted by an electric double layer capacitor, for example, and is connected in series with the main power source 9. The output voltage obtained by connecting the auxiliary power supply 10 to the main power supply 9 in series is guided from the electric circuit L2 to the drive circuit 5 via the switch 11 and the electric circuit L3.

  The switch 11 can switch the connection under the control of the control circuit 6. In the first output state shown by the solid line, the electric circuits L1 and L3 are connected to each other, and in the second output state shown by the dotted line, the electric circuits L2 and L3 are connected to each other. The switch 11 is actually configured to realize a circuit switching function as shown in the figure using a semiconductor switching element such as a MOS-FET.

  A charging circuit 12 is connected in parallel to the auxiliary power source 10. The charging circuit 12 has a function of boosting the voltage of the main power supply 9 and charging the auxiliary power supply 10. Whether or not the charging circuit 12 performs charging is controlled by the control circuit 6. In addition, the electric circuit L1-L3 and the switch 11 comprise the discharge circuit which can comprise either the said 1st, 2nd output state, and also added the charging circuit 12 to this, and the charge / discharge circuit 13 is comprised. Yes.

  Based on the steering torque signal sent from the torque sensor 8 and the vehicle speed signal sent from the vehicle speed sensor 7, the control circuit 6 estimates the required current for obtaining the required steering assist force, Is compared with a predetermined charge / discharge threshold. When the required current is less than or equal to the charge / discharge threshold, the control circuit 6 sets the switch 11 to the first output state indicated by the solid line in FIG. Accordingly, the voltage of the main power supply 7 is supplied to the drive circuit 5 via the electric circuits L1 and L3, and the drive circuit 5 drives the motor 4 based on the control signal from the control circuit 6. In this case, the power of the auxiliary power supply 10 is not supplied to the drive circuit 5. Further, the control circuit 6 monitors the voltage of the auxiliary power supply 10 by a voltage sensor (not shown) built in the charging circuit 12, and if the voltage does not reach a certain voltage, the control circuit 6 drives the charging circuit 12. To charge the auxiliary power supply 10.

  On the other hand, when the required current exceeds the charge / discharge threshold, the control circuit 6 sets the switch 11 to the second output state indicated by a dotted line. As a result, the high voltage is supplied to the drive circuit 5 with the main power supply 9 and the auxiliary power supply 10 connected in series with each other. As a result, the required large power can be supplied to the drive circuit 5 while limiting the output current of the main power supply 4 to the charge / discharge threshold.

  Next, the criteria for setting the charge / discharge threshold will be described in detail. FIG. 2 is a graph showing the relationship between the charge / discharge threshold and the energy required for the auxiliary power supply 10. The solid line portion of the graph in FIG. 2 is a characteristic curve showing how the energy E [J] required for the auxiliary power supply 10 changes with respect to the change in the charge / discharge threshold value I [A] expressed in current. is there. The inventors obtained this result by obtaining the energy required for the auxiliary power supply 10 at each threshold while changing the charge / discharge threshold from 5 A to 45 A in small increments, for example.

As a method for obtaining energy, for example, in a state where a certain charge / discharge threshold is set, a certain operation (for example, garage entry, parallel parking, stationary stop) with a large amount of required power is performed, and at that time, the auxiliary power supply 10 loses power. Energy:
(1/2) C · (V ₀ ² −V ² )
(C: capacitance, V ₀ : terminal voltage before discharge, V: terminal voltage after discharge)
Was measured and the result of taking the measurement variation into account was defined as “energy required for the auxiliary power supply 10”.

  According to FIG. 2, the characteristic curve (solid line) has a shape in which the energy E decreases while changing from a steep slope to a gentle slope as the charge / discharge threshold I increases, and in the middle of such a change, The knowledge that there is a specific point where the rate of change of the slope is the maximum is obtained.

  In FIG. 2, in the region of 10 A or less, the required energy is large (approximately 5000 [J] or more), and therefore the cost of the auxiliary power supply 10 is increased. In addition, since this region is steep, the variation in energy actually required with respect to the set charge / discharge threshold becomes relatively large. Therefore, it is desirable to avoid an area of 10 A or less as the charge / discharge threshold. On the other hand, in the region of 20 A or more, the required energy is small (about 1000 [J] or less), and thus the cost of the auxiliary power supply 10 is reduced. Further, since this region has a gentle gradient, the variation in energy actually required with respect to the set charge / discharge threshold is relatively small. Therefore, an area of 20 A or more as the charge / discharge threshold is suitable for use.

However, in order to lighten the burden on the battery 9B, it is also desirable to use the charge / discharge threshold region as low as possible. Then, where the lower limit value of the charge / discharge threshold can be set in the region from 10A to 20A.
In view of this, the point where the slope of the characteristic curve changes most rapidly from the steep slope to the gentle slope (or vice versa) is taken, and a current value larger than the current value corresponding to that point is set as the charge / discharge threshold. did.

Specifically, the second-order differential value of the energy E with respect to the charge / discharge threshold I, that is, the rate of change d ² E / dI ² of the energy gradient is obtained by second-order differentiation of the energy E with the charge / discharge threshold I. The broken line is a graph showing the characteristic of the rate of change d ² E / dI ² with respect to the charge / discharge threshold I. The two-dot chain line is a graph showing the characteristic of the energy gradient dE / dI with respect to the charge / discharge threshold I as a reference. Then, a charging / discharging threshold value larger than the charging / discharging threshold value (15 A in this example) at which the peak value of the energy gradient change rate d ² E / dI ² appears is set in the control circuit 6. As a result, a region with a steep change in energy E is avoided and a region with a gentle gradient is used, so that the actually required energy variation with respect to the set charge / discharge threshold becomes relatively small. Note that the upper limit of the charge / discharge threshold can be set based on the assumed maximum power and varies depending on the vehicle and the vehicle type (45A in this example).

As described above, by setting a charging / discharging threshold value larger than the charging / discharging threshold value when the second-order differential value of energy becomes a peak value in the control circuit 6,
(1) A region where the energy required for the auxiliary power supply 10 is relatively large can be avoided. As a result, the cost of the auxiliary power supply 10 can be kept low. Also,
(2) It is possible to avoid a region where the energy change is steep, and use a charge / discharge threshold value in a region where the energy changes slowly. Thereby, the dispersion | variation of the energy actually required with respect to the set charging / discharging threshold value becomes comparatively small, As a result, a charging / discharging threshold value can be set appropriately.

When importance is attached to reducing the burden on the battery 9B, the value is larger than the charge / discharge threshold (15A in this example) at which the peak value of the energy gradient change rate d ² E / dI ² appears, and A range as close as possible to the charge / discharge threshold (for example, 15 to 30 A) may be selected.

  In addition, the numerical value shown in the graph of FIG. 2 is an example, and a numerical value changes with vehicles and vehicle types.

1 is a circuit diagram showing a schematic configuration mainly of an electric circuit of an electric power steering apparatus according to an embodiment of the present invention. It is a graph which shows the relationship between a charging / discharging threshold value and the energy required for an auxiliary power supply. It is a figure which shows an example of a time-dependent change of the electric power required for an electric power steering apparatus.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Electric power steering apparatus 4 Motor 6 Control circuit 9 Main power supply 10 Auxiliary power supply 13 Charge / discharge circuit

Claims (1)

An electric power steering device that generates a steering assist force by a motor,
A main power supply for supplying power to the motor;
An auxiliary power source capable of supplying power to the motor;
A first output state for charging the auxiliary power source based on the main power source and supplying power to the motor only by the main power source; and a second output state for supplying power to the motor from the main power source and auxiliary power source A charge / discharge circuit that selectively configures the output state of
When the power output required for assisting steering is equal to or lower than a predetermined charge / discharge threshold, the charge / discharge circuit is set to the first output state, and when exceeding the charge / discharge threshold, the charge / discharge circuit is set to the second output state. And a control circuit that
The control circuit has a charge / discharge threshold when the second-order differential value of energy by the charge / discharge threshold becomes a peak value based on a characteristic curve of energy required for the auxiliary power supply when the charge / discharge threshold is changed. An electric power steering apparatus characterized in that a larger charge / discharge threshold is set as the predetermined charge / discharge threshold.

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