MOTORIZED POWER STEERING APPARATUS

Abstract: A motorized power steering apparatus for a vehicle has a torque sensor (3) which generates an output voltage proportional to the steering torque exerted by the driver of the vehicle and a supply voltage. A motor controller 9 controls the output torque of a drive motor 7 which generates an auxiliary steering force corresponding to the output voltage of the torque sensor. The supply voltage for the torque sensor is decreased as the vehicle speed increases, so that the output torque of the motor for a given steering torque decreases as the vehicle speed increases.

Description: BACKGROUND OF THE INVENTION This invention relates to a motorized power steering apparatus for automobiles or other vehicles. In the past, power steering apparatuses for automobiles were usually driven by hydraulic power generated by a pump. However, as hydraulic systems are bulky and heavy, in recent years, there has been a trend towards the use of electric motors to provide the drive force for power steering. A power steering apparatus which employs an electric motor to generate an auxiliary torque to assist the steering of the vehicle is referred to as a motorized power steering apparatus. In a motorized power steering apparatus, a torque sensor measures the steering torque applied by the driver to a steering wheel. An electric motor which is connected to a suitable portion of the steering gear is then controlled in accordance with the measured torque to impart an auxiliary steering force to the steering gear. The auxiliary steering force lessens the force which need be applied to the steering wheel by the driver. Generally, as the speed of a vehicle increases, the resistance between the road surface and the tires decreases, so the force required for steering the vehicle also decreases. If the auxiliary steering force which is generated by the electric motor were the same at high and low speeds, the resistance to steering felt by the driver would become too light at high speeds. If the driver exerted a sudden torque on the steering wheel at a high vehicle speed, the wheels of the vehicle could turn too sharply, possibly upsetting the stability of the vehicle. Therefore, in a motorized power steering apparatus, the electric motor is controlled so that the torque which it outputs in response to a given steering torque is lower at high vehicle speeds than at low vehicle speeds. In many conventional power steering apparatuses, the motor is controlled by a microcomputer equipped with an internal memory in which is stored a table of data relating the motor output to the steering torque for all vehicle speeds. When the microcomputer receives signals indicating the vehicle speed and the steering torque applied by the driver, based on the data in the memory, it calculates the appropriate amount of torque to be generated by the motor and controls the motor current accordingly. In order to perform fine, continuous control of the electric motor over the entire speed range of the vehicle, a large amount of data is necessary. The internal memory of the microcomputer must have a large capacity to accommodate all the data, and it therefore tends to be expensive. Taking everything into consideration, conventional motorized power steering apparatuses which employ a microcomputer are complicated, unreliable, and costly. SUMMARY OF THE INVENTION It is an object of the present invention to provide a motorized power steering apparatus which can perform fine control of the auxiliary steering force which it generates without employing a microcomputer. It is another object of the present invention to provide a motorized power steering apparatus which is reliable, simple in structure, and inexpensive. A motorized power steering apparatus in accordance with the present invention has a torque sensor for generating an output voltage proportional to a supply voltage and the steering torque exerted by a driver of the vehicle. A voltage supply supplies the torque sensor with a supply voltage which is inversely proportional to the vehicle speed. An electric motor for applying an auxiliary steering force to a steering gear is controlled by a motor controller so as to generate a torque proportional to the output voltage of the torque sensor. As the vehicle speed increases, the supply voltage which is supplied to the torque sensor by the voltage supply decreases. The output voltage of the torque sensor for a given steering torque therefore decreases with increasing vehicle speed, and the output of the motor accordingly decreases. As a result, the motor is prevented from generating too great a torque at high vehicle speeds. Any device capable of generating a voltage corresponding to the steering torque can be employed as the torque sensor. In a preferred embodiment, the torque sensor comprises a torque-displacement converter which produces a twisting displacement which is proportional to the steering torque imparted to a steering wheel by the driver, and a potentiometer having a wiper arm which is moved by the displacement of the torque-displacement converter. The output voltage of the torque sensor is the voltage at the wiper arm of the potentiometer. Preferably, the torque sensor 3 has a right turn potentiometer which generates an output voltage proportional to the steering torque only when the steering torque is such as to steer the vehicle to the right and a separate left turn potentiometer which generates an output voltage proportional to the steering torque only when the steering torque is such as to steer the vehicle to the left. The motor controller controls the voltage which is applied to the motor so that the output torque of the motor is proportional to the output voltage of only one of the potentiometers at a given time. The motor controller preferably includes a logic circuit which prevents the motor from operating when both of the potentiometers simultaneously generate an output voltage of above a prescribed level. There are no restrictions of the structure of the voltage supply. In a preferred embodiment, it comprises a vehicle speed sensor which generates an electrical output having a frequency proportional to the speed of the vehicle, and a frequency-voltage converter which generates an output voltage which is inversely proportional to the frequency of the output signal of the speed sensor. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a block diagram of an embodiment of a motorized power steering apparatus according to the present invention. Figure 2 is a circuit diagram of the control unit of the embodiment of Figure 1. Figure 3 is a graph showing the output of the torque sensor as a function of the steering torque applied to the steering wheel by the driver of the vehicle. Figure 4 is a graph of the output of the frequency-voltage converter as a function of the speed of the vehicle. DESCRIPTION OF THE PREFERRED EMBODIMENTS A preferred embodiment of a motorized power steering apparatus according to the present invention will now be described while referring to the accompanying drawings. As shown in Figure 1, which is a block diagram of this embodiment, a steering wheel 1 of an automotive vehicle is mounted on the upper end of a first steering shaft 2a. The lower end of the first steering shaft 2a is connected to the upper end of a second steering shaft 2b by a torque sensor 3. The torque sensor 3 includes a torque-displacement converter 3a which produces a displacement which is proportional to the steering torque applied to the steering wheel 1 by the driver of the vehicle. Any conventional torque-displacement converter can be employed, such as the torque-displacement converter disclosed in U.S. Patent No. 4,666,010 in which a torsion bar which connects two steering shafts undergoes angular displacement due to twisting which is proportional to the steering torque applied to a steering wheel. The torque sensor 3 also includes a right turn potentiometer 3d and a left turn potentiometer 3e. Each potentiometer has a wiper arm which is moved along a resistive element by the displacement of the torque-displacement converter 3a. The voltage at the wiper arm of each potentiometer constitutes an output voltage of the torque sensor 3 and is provided to a control unit 9 as an indication of the steering torque to the right or to the left, respectively. The lower end of the second steering shaft 2b is connected to the upper end of a third steering shaft 2c by a first universal joint 4a, and the lower end of the third steering shaft 2c is connected to the upper end of a drive shaft 5 by a second universal joint 4b. A pinion gear 5a is formed on the lower end of the drive shaft 5. The pinion gear 5a meshes with a rack 6 of the steering gear of the vehicle. An auxiliary torque can be applied to the drive shaft 5 by an electric motor 7 such as a shunt-wound or permanent magnet DC motor. The motor 7 is connected to the drive shaft 5 by a reduction gear 8 which reduces the rotational speed of the motor 7. The operation of the motor 7 is controlled by the control unit 9, which provides the motor 7 with a pulse width modulated drive signal. The control unit 9 is powered by the vehicle battery 10, which typically is a 12-volt battery. Portions of the control unit 9 are connected directly to the battery 10, while other portions are connected to the battery 10 via a key switch 11. A vehicle speed sensor 12 generates an output signal having a frequency which is proportional to the vehicle speed. This output signal is supplied to the control unit 9. Figure 2 is a circuit diagram of the control unit 9 of Figure 1. The control unit 9 constitutes a self-excited pulse width modulation circuit with feedback from the motor 7. A first comparator 911 has a positive input terminal which is connected to the wiper arm of the right turn potentiometer 3d and a negative input terminal which is connected to the junction of two series resistors 924 and 925. A second comparator 912 has a positive input terminal which is connected to the wiper arm of the right turn potentiometer 3d and a negative input terminal which is connected to ground through a resistor 942. A third comparator 913 has a positive input terminal which is connected to the wiper arm of the left turn potentiometer 3e and a negative input terminal which is connected to the junction of two series resistors 926 and 927. A fourth comparator 914 has a positive input terminal which is connected to the wiper arm of the left turn potentiometer 3e and a negative input terminal which is grounded through a resistor 945. Resistors 924 and 925 are connected in series between the output terminal of a frequency-voltage converter 950 and ground. Resistors 926 and 927 are likewise connected in series between the output terminal of the frequency-voltage converter 950 and ground. A terminal at one end of the resistive element of each potentiometer 3d and 3e is connected to the output terminal of the frequency-voltage converter 950, while the terminal at the other end of the resistive element is grounded. The output terminals of the first comparator 911 and the third comparator 913 are connected to the output terminal of a 5-volt voltage regulator 902 by pull-up resistors 928 and 929, respectively. The output terminals of the second comparator 912 and the fourth comparator 914 are connected to the output terminal of a 26-volt power supply 901 by pull-up resistors 930 and 931, respectively. The output terminal of the first comparator 911 is also connected to the input terminal of a first inverter 916 and to one of the input terminals of a first AND gate 918. The output terminal of the third comparator 913 is connected to the input terminal of a second inverter 917 and to one of the input terminals of a second AND gate 919. The output terminal of the first inverter 916 is connected to the other input terminal of the second AND gate 919, and the output terminal of the second inverter 917 is connected to the other input terminal of the first AND gate 918. The electric motor 7 is driven by first through fourth power MOSFET s 920 - 923. The gate of the first MOSFET 920 is connected to the output terminal of the second comparator 912, its drain is connected to the battery 10, and its source is connected to a first terminal 7a of the electric motor 7. The gate of the second MOSFET 921 is connected to the output terminal of the first AND gate 918, its drain is connected to a second terminal 7b of the motor 7, and its source is grounded. The gate of the third MOSFET 922 is connected to the output terminal of the fourth comparator 914, its drain is connected to the battery 10, and its source is connected to the second terminal 7b of the electric motor 7. The gate of the fourth MOSFET 923 is connected to the output terminal of the second AND gate 919, its drain is connected to the first terminal 7a of the electric motor 7, and its source is grounded. The first terminal 7a of the motor 7 receives a positive input voltage when the motor 7 is being driven so as to steer the wheels of the vehicle for a right turn, and the second terminal 7b received a positive input voltage during a left turn. A resistor 941 is connected between the first terminal 7a of the motor 7 and resistor 942, and a capacitor 943 is connected from the junction of resistors 941 and 942 to ground. Similarly, a resistor 944 is connected between the second terminal 7b of the motor 7 and resistor 945, and a capacitor 946 is connected from the junction of resistors 944 and 945 to ground. Capacitors 943 and 946 determine the frequency of self-excited oscillation of the control unit. Via resistors 941 and 944, the second and fourth comparators 912 and 914 receive feedback signals indicating the motor voltage. The power supply 901 and the voltage regulator 902 are connected to the battery 10 by the key switch 11. The above-mentioned frequency-voltage converter 950 receives the output signal of the vehicle speed sensor 12 and generates an output voltage which is inversely proportional to the frequency of the speed signal. The output voltage of the frequency-voltage converter 950 is provided to the potentiometers 3d and 3e and resistors 924 and 926 as a positive supply voltage. Figure 3 illustrates the output characteristics of the torque sensor 3 as a function of the steering torque applied to the steering wheel 1 by the driver. When no steering torque is applied to the steering wheel 1, the output of both potentiometers 3d and 3e is zero volts. When a rightward torque is applied to the steering wheel 1, the output of the right turn potentiometer 3d increases linearly with increasing torque until the steering torque reaches a value of approximately 0.8 kgf-m, at which the output voltage saturates at a voltage V SR . When the right turn potentiometer 3d has a non-zero output, the output of the left turn potentiometer 3e remains at zero volts. Conversely, when a leftward steering torque is applied to the steering wheel 1, the output of the right turn potentiometer 3d is zero volts, while the output of the left turn potentiometer 3e increases linearly with increasing steering torque until the steering torque reaches approximately 0.8 kgf-m, upon which the output saturates at a voltage V SL . As will be explained below, the levels of V SR and V SL depend upon the vehicle speed. Figure 4 is a graph illustrating the output characteristics of the frequency-voltage converter 950 as a function of the vehicle speed. At a low vehicle speed of up to approximately 5 km/hr (point f in Figure 4), the frequency-voltage converter 950 generates a constant voltage of approximately 5 volts. At a vehicle speed of greater than approximately 20 km/hr (point g), the frequency-voltage converter 950 generates a constant output voltage of approximately 2.5 volts. At a vehicle speed between approximately 5 and 20 km/hr, the output voltage of the frequency-voltage converter 950 linearly decreases as the vehicle speed increases. The output voltage of the frequency-voltage converter 950 is supplied to the potentiometers 3d and 3e as a positive supply voltage. Since the output voltage of the frequency-voltage converter 950 varies in accordance with vehicle speed, the output characteristics of the torque sensor 3 are also dependent on the vehicle speed. As shown in Figure 3, when the vehicle speed is below that corresponding to point f of Figure 4 (approximately 5 km/hr), the output characteristics of the potentiometers 3d and 3e are as shown by curves a and a′, respectively, which have a steep slope and a high saturation voltage V SR(L) and V SL(L) of approximately 5 volts. When the vehicle speed is above that corresponding to point g of Figure 4 (approximately 20 km/hour), the output characteristics of the potentiometers 3d and 3e are shown by curves b and b′, respectively, which have a more gradual slope and a lower saturation voltage V SR(H) and V SL(H) of approximately 2.5 volts. When the vehicle speed is between 5 and 20 km/hour, the output characteristics of the potentiometers 3d and 3e are described by curves lying somewhere between curves a and b or a′ and b′. For example, when the vehicle speed is that corresponding to point h of Figure 4 (approximately 12.5 km/hour) midway between points f and g, the output characteristics of the potentiometers 3d and 3e are as shown by curves c and c′, respectively, lying midway between curves a and b (or a′ and b′). When the unillustrated engine of the vehicle is started and the key switch 11 is closed, 12 volts are supplied by the battery 10 to the power supply 901 and the voltage regulator 902 and the control unit 9 begins to operate. It will be assumed that the vehicle is initially stationary, so the vehicle speed sensor 12 generates a signal corresponding to a speed of 0 km/hour. As shown in Figure 4, the frequency-voltage converter 950 therefore generates a maximum output voltage of approximately 5 volts. This output voltage is supplied to the potentiometers 3d and 3e and to resistors 924 and 926 as a positive supply voltage. If the driver then applies a steering torque to the steering wheel 1, the torque sensor 3 generates an output voltage corresponding to the magnitude and direction of the steering torque as shown in Figure 3. When the driver applies a steering torque to the steering wheel 1 to turn the wheels of the vehicle to the right, the voltage of the wiper arm of the right turn potentiometer 3d is input to the positive input terminals of the first and second comparators 911 and 912. If the output voltage of the right turn potentiometer 3d exceeds the voltage V N at the junction of resistors 924 and 925, the output of the first comparator 911 goes high. As no voltage has yet been applied to the motor 7, the voltage at the junction of resistors 941 and 942 is low, and the output of the second comparator 912 goes high. The output voltage of the left turn potentiometer 3e is 0 volts, so the outputs of the third and fourth comparators 913 and 914 are low. Therefore, the output of the first inverter 916 is low and the output of the second inverter 917 is high, so the output of the first AND gate 918 is high, the output of the second AND gate 919 is low, and the first and second MOSFET s 920 and 921 are turned on. Therefore, a voltage from the battery 10 is applied to the motor 7 through the first MOSFET 920, and the motor 7 begins to conduct. The positive input voltage for the motor 7, i.e., the voltage at terminal 7a, is also applied across resistors 941 and 942. Therefore, the voltage at the junction of resistors 941 and 942 rises exponentially with a time constant determined by capacitor 943. When the voltage at the junction of resistors 941 and 942 exceeds the voltage at the positive input terminal of the second comparator 912, which is the output voltage of the right turn potentiometer 3d, the output of the first comparator 911 remains high but the output of the second comparator 912 goes low, so the first MOSFET 920 is turned off, and the supply of current to the motor 7 is interrupted. When this interruption takes place, the voltage at the junction of resistors 941 and 942 falls exponentially with a time constant determined by capacitor 943. When the voltage at the junction of resistors 941 and 942 falls below the voltage at the positive input terminal of the second comparator 912, the output of the second comparator 912 again goes high, and the first MOSFET 920 is again turned on to drive the motor 7. In this manner, the second comparator 912 is repeatedly turned on and off, and a series of pulses are supplied to the motor 7 as a supply voltage. A current which is determined by the supply voltage and the counter emf corresponding to the rotational speed of the motor 7 flows through the motor 7. The motor 7 generates an auxiliary torque to turn the wheels of the vehicle to the right, and the steering torque which need be exerted by the drive of the vehicle is reduced. The width of the pulses which are supplied to the motor 7 is dependent on the output voltage of the potentiometer 3d. The greater the output voltage of the potentiometer 3d, the longer is the pulse width. When the driver exerts a steering torque to turn the vehicle to the left, the third and fourth comparators 913 and 914 are controlled in a manner similar to that described above with respect to the first and second comparators 911 and 912, and a voltage is supplied to the motor 7 to steer the wheels of the vehicle to the left. As shown in Figure 3, the output voltage of the potentiometers 3d and 3e corresponding to a given steering torque decreases as the vehicle speed increases. Since the torque generated by the motor 7 decreases as the output voltage of the potentiometers 3d and 3e decreases, it follows that the auxiliary torque generated by the motor 7 in response to a prescribed steering torque decreases as the vehicle speed decreases. This decrease in auxiliary torque compensates for the decrease in the resistance to steering as the vehicle speed increases. Therefore, the feel of the steering wheel remains comfortable without becoming too light at high vehicle speeds, and the safety of the vehicle is maintained. The AND gates 918 and 919 prevent the motor 7 from operating unless the output of either the first comparator 911 or the third comparator 913 is high. This state occurs when the output voltage of the right turn potentiometer 3d exceeds the voltage V N at the junction of resistors 924 and 925, or when the output voltage of the left turn potentiometer 3e exceeds the voltage V N at the junction of resistors 926 and 927. If the driver exerts only a very low torque of less than T1 on the steering wheel 1, the outputs of the potentiometers 3d and 3e will not exceed V N , so the motor 7 will not be turned on. Accordingly, there is a dead band of steering torque on either side of a neutral torque in which power steering is not performed. The magnitude of the dead band, i.e., the magnitude of T1, depends on the value of V N and on the output charactertistics of the potentiometers 3d and 3e. In the present embodiment, V N is a function of the output voltage of the frequency-voltage converter 950, and so V N varies with the vehicle speed. As shown in Figure 3, at a low vehicle speed below that corresponding to point f of Figure 4, V N has a maximum value of V N(L) , and at a vehicle speed above that corresponding to point g of Figure 4, it has a minimum value of V N(H) . When the vehicle speed is somewhere between that corresponding to points f and g, V N has a level somewhere between V N(L) and V N(H) . Thus, V N decreases as the vehicle speed increases. However, as V N decreases, the slopes of the curves defining the output characteristics of the potentiometers also decrease, so T1 and the magnitude of the dead band remain constant at all vehicle speeds. Normally, only one of the potentiometers generates an output voltage at a time. However, if the potentiometers should malfunction (due, for example, to noise generated by the torque sensor itself, bad contacts, broken wires, or short circuits) and both generate an output voltage of greater than V N at the same time, the outputs of both AND gates 918 and 919 will go low and prevent MOSFET S 921 and 923 from conducting. Therefore, the motor 7 will not be able to operate, and there will be no possibility of the motor 7 exerting an auxiliary torque in a direction opposite to the direction in which the driver wishes to steer the vehicle. Although the steering will feel heavy when the motor 7 does not operate, as the steering wheel 1 is mechanically linked to the rack 6, the driver will still be able to safely steer the vehicle. A motorized power steering apparatus in accordance with the present invention can perform fine control of the auxiliary torque which it generates over the entire speed range of a vehicle without employing a microcomputer. It is therefore inexpensive, reliable, and easy to manufacture.

Claims: 1. A motorized power steering apparatus for a vehicle comprising: torque sensing means (3) for generating an ouput signal dependent on the steering torque exerted by a driver of the vehicle; a motor (7) for generating an auxiliary steering force; and motor control means (9) for controlling the output torque of the motor in accordance with the output signal of the torque sensing means; characterised in that the torque sensing means (3) generates an output voltage signal proportional to a supply voltage and to the steering torque, and voltage supply means (12,950) provides the torque sensing means with a supply voltage which decreases as the vehicle speed increases. 2. A power steering apparatus as claimed in Claim 1, wherein the voltage supply means comprises: a vehicle speed sensor (12) which generates a vehicle speed signal having a frequency proportional to the vehicle speed; and a frequency-voltage converter (950) which generates an output voltage which decreases as the frequency of the vehicle speed signal increases and which is supplied to the torque sensing means (3) as the supply voltage. 3. A power steering apparatus as claimed in Claim 1 or 2, wherein the torque sensing means comprises a right turn potentiometer (3d) for generating an output voltage corresponding to a steering torque exerted by a driver of the vehicle only when the steering torque is in a direction to turn the vehicle to the right and a left turn potentiometer (3e) for generating an output voltage corresponding to a steering torque exerted by the driver only when the steering torque is in a direction to turn the vehicle to the left. 4. A power steering apparatus as claimed in Claim 3, wherein: the torque sensing means includes a torque-displacement converter (3a) for generating a displacement proportional to a steering torque exerted thereon; and each of the said potentiometers (3d, 3e) has an input terminal which is connected to the voltage supply means (950), a resistive element, and a wiper arm which is linked to the torque-displacement converter so as to be moved along the resistive element by the displacement of the torque-displacement converter, the output voltage of each potentiometer being the voltage at its wiper arm. 5. A power steering apparatus as claimed in Claim 3 or 4, further comprising means for preventing the motor from operating when both of the potentiometers generate an output voltage of above a prescribed level at the same time. 6. A power steering apparatus as claimed in any preceding Claim, further comprising limiting means for preventing the motor from being driven when the output voltage of the torque sensing means is below a prescribed voltage which decreases as the vehicle speed increases. 7. A power steering apparatus as claimed in any preceding Claim, wherein the motor control means comprises a self-excited pulse width modulation circuit with feedback from the motor for supplying the motor with drive pulses having a pulse width which is modulated by the output voltage of the torque sensing means. 8. A power steering apparatus as claimed in Claim 7, wherein the pulse width modulation circuit comprises: a bridge circuit consisting of a plurality of switching elements, the motor being connected across the bridge circuit; a capacitor and a resistor connected in parallel between the motor and ground; and a comparator having a first input terminal to which the output voltage of the torque sensing means is applied and a second input terminal connected to the junction of the capacitor and the resistor, the output voltage of the comparator being supplied to a gate one of the switching elements of the bridge circuit.

Inventors: MORISHITA, MITSUHARU C/O HIMEJI SEISAKUSHO
Applicants: MITSUBISHI DENKI KABUSHIKI KAISHA
Application year: 1989
Main classification: B62D5
IPC codes: B62D6, B62D5