MOTOR DRIVEN POWER STEERING CONTROLLER

Abstract: The steering controller consists of a car speed sensor (10) for sensing the car speed, a torque sensor (3) for sensing the rotational force of a steering wheel (1) and an identifier for discriminating the lock of the motor and of a mechanical system from steering inthe proximity of a steering wheel lock position. A control unit (9) inputs the signals of the car speed sensor (10), the torque sensor (3) and the identifier. A D.C. motor (13) provides auxiliary actuation force and an electromagnetic clutch (16) disposed at a mechanical connection portion between the output of the motor (13) and a rack shaft (6). Under the control of the control unit (9), the lock of the motor and of the mechanical system is discriminated from the steering near the steering wheel lock position and, on the basis of this discrimination result, breakdown of the motor (13) and a power device due to the temperature rise resulting from continuous power feed to the motor (13) is prevented. The motor current is set to zero and the electromagnetic clutch (16) is cut off so that the steering wheel operation can be made at the time of mechanical lock. When steering is made near the steering wheel lock position, the motor current is reduced to zero in the course of a predetermined time to reduce any offensive feel of steering.

Description: This invention relates to a motor driven type power steering control device for assisting an operation of a steering device of an automobile with rotational force of a motor, such as defined in the preamble of claim 1 and known for example from JP-A-60-209365. A conventional power steering device of this type assists an operation of a steering shaft or a rack shaft with driving force of a motor through a reduction mechanism and a transmission mechanism such as gears or belts. Since, in such a conventional power steering device, it is impossible to distinguish between a malfunction of the motor or other mechanical systems, such as locking thereof, and a locking of a steering wheel, there may be a case where a current supply to the motor is continued for a time long enough to heat power elements for controlling the motor to high temperature, causing them to be damaged and/or a steering becomes impossible by such mechanical locking, causing a severe accident to occur. This invention was made in view of such problems as above and an object of this invention is to obtain a motor driven power steering control device capable of preventing a motor and power elements from being damaged by overheating and of making a steering wheel operation possible when a mechanical locking occurs to thereby assure a safety driving of an automobile. This object is achieved with a steering control device having the features defined in the characterising part of claim 1. Thus, when the steering feels heavy due to a motor locking or a locking of a mechanical system, the locking of the motor or the mechanical system is distinguished from the case of steering around wheel locking positions, motor current is reduced to zero and an electromagnetic clutch is disengaged, so that damages of the motor and power elements are prevented and a steering operation becomes possible even if the mechanical locking occurs. Further, in a preferred embodiment, motor current is made zero and, simultaneously, the electromagnetic clutch is disengaged. Then, when the steering is performed around the wheel locking positions, motor current is made zero within a predetermined time, so that the power steering can be performed with minimized abnormal feeling of steering. The invention will now be described in more detail by way of example with reference to the accompanying Figures in which: Fig. 1 shows a construction of a motor driven power steering control device according to an embodiment of this invention, Fig. 2 is a block circuit diagram of a control unit, Fig. 3 is a steering torque vs. motor current curve showing a control characteristics, Fig. 4 shows a vehicle speed vs. motor current curve and a vehicle speed vs. voltage applied to an electromagnetic clutch curve showing control characteristics, Fig. 5 is a flow chart of a control program, Fig. 6 is a construction of a motor driven power steering control device according to a second embodiment of this invention, Fig. 7 is a block circuit diagram of a control unit in the second embodiment, Fig. 8 is a flow chart of a control program for the secondembodiment, Fig. 9 is a motor current vs. time curve showing a current reducing control characteristics when a supply of current exceeding a predetermined value to the motor is continued for a predetermined time period or longer, Fig. 10 is a motor current vs. time curve showing a current reducing characteristics in another embodiment and Fig. 11 is a flow chart of a control program for another embodiment. Fig. 1 shows a construction of a motor driven power steering control device according to one embodiment of this invention, together with a block circuit diagram, in which 1 is a steering wheel to be operated by rotational steering force given by a driver, 3 is a torque sensor for providing a signal corresponding to rotational force applied to the steering wheel 1, 4a is a first universal joint, 4b a second universal joint, 2a is a first steering shaft connecting the steering wheel 1 and the torque sensor 3, 2b is a second steering shaft connecting the torque sensor 3 and the first universal joint 4a, 2c is a third steering shaft connecting the first universal joint 4a and the second universal joint 4b, 5 is a first pinion shaft connected to the second universal joint, 6 is a rack shaft having a first rack teeth portion 6a meshed with the pinion shaft 5 and a second teeth portion 6b, 7a is a ball joint connecting a tie rod 8a and an end of the first rack teeth portion 6a and 7b is another ball joint connecting another tie rod 8b and an end of the second rack teeth portion 6b. 9 is a control unit, 10 is a vehicle speed sensor for detecting a vehicle speed, 11 is a vehicle mounted battery, 12 is a key switch and 13 is a d.c. motor having a shunt field or magnet field and adpated to be driven by the battery 11 through the control unit 9. 14 is a worm shaft connected to an output shaft of the motor 13 and forming a reduction device, 15 is a worm wheel meshed with the worm shaft 14 for driving the latter, 16 is an electromagnetic clutch for mechanically engaging or disengaging a connection between the wheel shaft 15 and a second pinion shaft 17 meshed with the second rack teeth portion 6b according to an instruction from the control unit 9 and 18 is a steering angle sensor for detecting a steering angle of the steering wheel 1. Fig. 2 shows a block circuit diagram of the control unit 9, in which 9a is a steering torque measuring means responsive to an input from the torque sensor 3 to measure the steering angle, 9b is a steering angle measuring means responsive to an output of the steering angle sensor 18 to measure the steering angle of the steering wheel, 9c is a vehicle speed measuring means responsive to an input from the vehicle speed sensor 10 to measure a vehicle speed, 9d is an abnormal motor current judge means for deciding a motor current as abnormal when a motor current indication corresponding to the steering torque exceeds a predetermined current value where the steering angle is within a predetermined angle range near the steering wheel locking position, 9e is a motor current determination means for normally setting the motor current to a value corresponding to the steering torque and setting it to zero when an output of the abnormal motor current judge means 9d indicates an abnormal, 9f is a motor current control means responsive to an output of the decision means 9e for controlling the motor current and 9g is an electromagnetic clutch control means for on-off controlling the electromagnetic clutch 16 according to a condition determined by at least the vehicle speed. An operation of this will be described with reference to Figs. 3 to 5. Fig. 3 shows a control characteristics of motor current with respect to steering torque, Fig. 4 shows control characteristics of motor current and voltage applied to the electromagnetic clutch with respect to vehicle speed and Fig. 5 is a flow chart showing a control program of the control unit 9. Firstly, when the key switch 12 is turned on to start an engine, the electromagnetic clutch 16 is engaged to mechanically connect the worm wheel shaft 15 to the second pinion shaft 17. When the steering wheel 1 is turned under this condition, the control unit 9 controls a current of the motor 13 as shown in Fig. 3. In Fig. 3, when the steering torque is increased rightwardly, the motor 13 is energized at a point a and a current I OF ( in the order of 2 to 10 A ) flows for reduction of an effect of inertia of the motor and other mechanical system. With a further increase of the steering torque, the motor current is increased linearly with respect to the steering torque from a point b to a point c at which the motor current becomes 100 %. When the seering torque is decreased to the point c, the motor current starts to decrease and it becomes I OF at the point b. With a further reduction of the torque to the point a, the motor is deenergized. This control is the same in a leftward variation of the torque. A relation of transmission torque to motor current is linear. Therefore, when the torque increases in Fig. 3, the motor is energized at the point a and the motor current I OF flows. Since, with a further increase of the torque, the control is to increase the motor current gradually from the point b, an output torque to the worm shaft 14 increases gradually and thus an auxiliary torque corresponding to force applied by the driver to the steering wheel is transmitted through the worm wheel shaft 15, the electromagnetic clutch 16 and the second pinion shaft 17 to the second rack teeth portion 6b. Therefore, the steering wheel 1 becomes light. The above is the operation of this device when the vehicle is stationary. When the vehicle becomes a running state, the control of motor current is to reduce it to I OF at speed ( vehicle speed = V₂ ) at a point d and to zero, together with a reduction of the voltage applied to the electromagnetic clutch 16 to zero, at speed ( vehicle speed = V₁ ) at a point e, as shown in Fig. 3. Since, therefore, the worm wheel 15 is separated from the second pinion shaft 17, the driver steers the steering wheel without the auxiliary energization. On the other hand, in a case where the steering wheel 1 is operated around the locking position such as in the case of steering while the vehicle is stationary, it is impossible to distinguish the situation from a case where the steering operation is heavy due to that it is performed around the locking position when an accident such as locking of the motor 13 or other mechanical system, so that the motor 13 may be supplied with current continuously. Therefore, there may be a possibility of damage of the power elements constituting a portion of the motor 13 and the control unit 9 which controls the motor by heat generation of these elements. Further, there may be a case where it becomes impossible to steer the steering wheel by mechanical locking, leading to a dangerous accident. In this invention, a situation where an absolute value of the steering wheel angle is within a predetermined angle ϑ M which is close to the steering wheel locking position and an instruction value of the motor current corresponding to the steering torque is decided as an abnormal condition, upon which the motor current is made zero and simultaneously the electromagnetic clutch 16 is disengaged to prevent the motor 13 and the power elements from being damaged by overheat and to allow the steering in the machanical locking condition, as shown by the flow chart in Fig. 5. In the embodiment mentioned above, the steering angle sensor is used as means for distinguishing the locking of the motor and other mechanical system from the steering around the steering wheel locking position. A second embodiment which uses another means as this distinguishing means will be described. Fig. 6 shows a construction of the second embodiment of the motor driven power steering control device according to this invention, in which same reference numerals used in Fig. 1 depict same or corresponding components, respectively. A portion of the embodiment in Fig. 2 which is different from the device in Fig. 1 is a lock position sensor 19 for detecting opposite lock positions of front wheels during a steering which is provided instead of the steering angle sensor 18 in Fig. 1. A circuit construction of the control unit 9 is different from that in Fig. 1 necessarily due to the difference shown in Fig. 6. The circuit construction of the control unit 9 is shown in block in Fig. 7. As shown in Fig. 7, 9a is a steering torque measuring means responsive to an input from a torque sensor 3 for measuring a steering torque, 9c is a vehicle speed measuring means responsive to an input from a vehicle speed sensor 10 for measuring a vehicle speed, 9h is a lock position detecting means for detecting lock positions of respective front wheels during a steering, 9d is an abnormal motor current judging means for judging an abnormality of situation when an output of the lock position detecting means 9h is outside the lock position and an indicated value of motor current corresponding to a steering torque exceeds a predetermined value, 9e is a motor current setting means for normally setting the motor current to a value corresponding to the steering torque and seeting it to zero when an output of the abnormal motor current detecting means 9d indicates an abnormality, 9f is a motor current control means responsive to an output of this setting means 9e for controlling motor current and 9g is a control means of an electromagnetic clutch 16 for engaging or disengaging the electromagnetic clutch 16 according to a condition determined by at least the vehicle speed. In the second embodiment, as shown in a flow chart in Fig. 8, a situation where an output of the lock position sensor 19 for detecting a left and right lock positions during a steering indicates out of lock position and an instructed value of the motor current which corresponds to the steering torque exceeds a predetermined current value I MO is decided as abnormal upon which the motor current is made zero and the electromagnetic clutch 16 is disengaged to prevent the motor and power elements from being damaged by overheating and to allow a steering during a mechanical locking. Further, another embodiment will be described, in which the motor current setting means 9e of the control unit 9 in the second embodiment shown in Fig. 7 is constituted such that it normally sets the motor current to a value corresponding to the steering torque, to zero when the output of the abnormal motor current judging means 9d indicates an abnormality and to a value which reduces to zero or a predetermined value steppingly within a second predetermined time period after a first predetermined time from a time instance at which the lock position sensor 19 provides an output indicating a detection of lock position. In this embodiment, as shown in Fig. 11, a situation where an output of the lock position sensor 19 for detecting a right and left lock positions of the front wheels when the steering wheel is operated indicates a detection of no lock position and an instructed value of the motor current corresponding to the steering torque exceeds a predetermined current value I MO ( e.g., 15 to 25 A ) is decided as abnormal, upon which the motor current is made zero and the electromagnetic clutch 16 is disengaged to prevent the motor 13 and the power elements from being damaged by overheating and to allow a steering during the mechanical lock. Then, as shown in Fig. 9 which shows a reduction control characteristics of motor current with time when the supply of current exceeding a predetermined value to the motor 13 is continued for a time exceeding a predetermined time, when the output of the lock position sensor 19 indicates a detection of a lock position and the motor current exceeding the predetermined current value I MO is continued for a predetermined time T MS ( e.g., 10 sec to 1 min ), a control shown by the flow chart in Fig. 7 is performed to prevent the power elements which constitute a portion of the motor 13 and a portion of the control unit 9 and control the current supply to the motor from being damaged by increase of their temperatures. That is, the motor current is controlled on the basis of a value which is a product of an attenuation constant K T which reduces gradually with time and a motor current I M which corresponds to the steering torque. As a result, the motor current is reduced gradually and becomes zero within a predetermined time period ( e.g., 30 sec to 5 min ) and the voltage applied to the electromagnetic clutch 16 is also controlled to zero to disengage the worm wheel shaft 15 from the second pinion shaft 17, resulting in a steering without assist. Fig. 10 is a reduction control characteristics of the motor current with time according to another embodiment. It is possible to obtain the same effect by reducing the motor current during the motor current reducing control to not zero but a predetermined current value to be held as shown in this figure. POSSIBILITY OF INDUSTRIAL UTILIZATION As described hereinbefore, according to this invention, the motor lock and the mechanical lock are distinguished from the steering around the steering wheel lock positions to reduce the motor current to zero and simultaneously disengage the electromagnetic clutch. Therefore, it is possible to prevent the motor and the power elements from being damaged and to steer the steering wheel even during the mechanical lock, resulting in a highly safe power steering. Since the motor current is controlled such that it is gradually reduced for a steering around the steering wheel lock positions, a feeling of physical disorder of steering is minimized, resulting in a highly safe power steering.

Claims: A motor driven power steering control device comprising a vehicle speed sensor (10) for sensing a vehicle speed, a torque sensor (3) mounted on a steering shaft (2a,2b) for detecting a rotational force of a steering wheel (1), a control unit (9) adapted to be supplied with signals from said vehicle speed sensor, and said torque sensor, a d.c. motor (13) to be driven by a vehicle mounted battery (11) through said control unit and an electromagnetic clutch (16) connected directly to an output shaft (14) of said d.c. motor and on-off controlled by said control unit, characterized by: means for detecting when steering is performed in the region of a wheel locking position and for supplying a first corresponding signal to said control unit; means for detecting a motor lock or a lock of a mechanical system and supplying a second corresponding signal to said control unit; and said control unit being operable such that motor lock or mechanical lock detected when steering is not in said region of a wheel locking position is judged abnormal. The motor driven power steering control device as claimed in claim 1, characterized in the said steering lock positions detecting means is a steering angle sensor (18) for detecting an angle of said steering wheel. The motor driven power steering control device as claimed in claim 2, characterized in that said control unit comprises a steering torque measuring means (9a) responsive to an input from said torque sensor to measure the steering angle, a steering angle measuring means (9b) responsive to an output of said steering angle sensor to measure the steering angle of said steering wheel, a vehicle speed measuring means (9c) responsive to an input from said vehicle speed sensor to measure a vehicle speed, an abnormal motor current judge means (9d) for deciding a motor current as abnormal when a motor current indication corresponding to the steering torque exceeds a predetermined current value where the steering angle is within a predetermined angle range near the steering wheel locking position, a motor current determination means (9e) for normally setting the motor current to a value corresponding to the steering torque and setting it to zero when an output of said abnormal motor current judge means indicates an abnormal, a motor current control means (9f) responsive to an output of said determination means for controlling the motor current and an electromagnetic clutch control means (9g) for on-off controlling said electromagnetic clutch according to a condition determined by at least the vehicle speed. The motor driven power steering control device as claimed in claim 1, characterized in that said steering lock positions detecting means is a lock position sensor (19) for detecting a left and a right lock positions of front wheels during an operation of said steering wheel. The motor driven power steering control device as claimed in claim 4, characterized in that said control unit comprises a steering torque measuring means (9a) responsive to an input from said torque sensor to measure the steering angle, a vehicle speed measuring means (9c) for measuring a vehicle speed, a lock position detecting means (9h) responsive to an input from said lock position sensor for detecting a left and a right lock positions of front wheels during an operation of said steering wheel, an abnormal motor current judge means (9d) for deciding a motor current as abnormal when a motor current indication corresponding to the steering torque exceeds a predetermined current value where an output of said lock position detecting means indicates no detection of the lock position, a motor current determination means (9e) for normally setting the motor current to a value corresponding to the steering torque and setting it to zero when an output of said abnormal motor current judge means indicates an abnormal, a motor current control means (9f) responsive to an output of said decision means for controlling the motor current and an electromagnetic clutch control means (9g) for on-off controlling said electromagnetic clutch according to a condition determined by at least the vehicle speed. The motor driven power steering control device as claimed in claim 4, characterized by that said control unit comprises a steering torque measuring means (9a) responsive to an input from said torque sensor to measure the steering angle, a vehicle speed measuring means (9c) for measuring a vehicle speed, a lock position detecting means (19) responsive to an input from said lock position sensor for detecting a left and a right lock positions of front wheels during an operation of said steerng wheel, an abnormal motor current judge means (9d) for deciding a rotor current as abnormal when a motor current indication corresponding to the steering torque exceeds a predetermined current value where the steering angle is within a predetermined angle range near the steering wheel locking position, a motor current determination means (9e) for normally setting the motor current to a value corresponding to the steering torque, setting it to zero when an output of said abnormal motor current judge means indicates an abnormal and setting it to a value which is reduced to zero or a predetermined value steppingly within a second predetermined time period after a first predetermined time period from a time at which an output of said lock position sensor indicates a detection of lock position, a motor current control means (9f) responsive to an output of said determination means for controlling the motor current and an electromagnetic clutch control means (9g) for on-off controlling said electromagnetic clutch according to a condition determined by at least the vehicle speed.

Inventors: KOUGE,SHINICHI;, MORISHITA,MITSUHARU;
Applicants: MITSUBISHI DENKI KABUSHIKI KAISHA
Application year: 1987
Main classification: B62D5
IPC codes: B62D5