Vehicle power steering system with electric motor

• US 20050096813A1
• Filed: 11/04/2004
• Published: 05/05/2005
• Est. Priority Date: 11/04/2003
• Status: Active Grant

• Current Assignee: Robert Bosch Automotive Steering GmbH (Robert Bosch GmbH)
• Original Assignee: ZF Lenksysteme GmbH (Robert Bosch GmbH)
• Inventors: Heilig, Arnulf, Hiegler, Stephan, Bischofberger, Michael
• Granted Patent: US 7,383,109 B2
• Time in Patent Office: Days
• US Class Current: 701/41
• CPC Class Codes: B62D 5/046 Controlling the motor
• CPC Class Codes: B62D 5/0472 for damping vibrations

Abstract

Vehicle power steering systems with an electric motor have an electronic control system which controls the electric motor dynamically so that it produces a motor torque which supports the steering force and is usually transmitted to the steering wheel mechanics over a gear connection such as a worm gear. However, gear connections cannot be constructed completely without backlash. Under certain driving conditions therefore, chatter develops in the form of mechanical, generally higher frequency oscillations, which cause unpleasant and irritating noises. In order to counteract such disturbances, there is provided a vehicle power steering system with an electric motor and a method for controlling the electric motor for which the control system evaluates the rotor rpm of the electric motor and increases the dynamic response time of the rotor, in the event that the evaluation of the rotor rpm indicates an unstable state of the control system. As a result the chattering is suppressed only when a critical driving situation arises in which the control system becomes unstable. These measures intervene selectively and directly in the motor dynamics so that the development of mechanical chattering is prevented effectively only when necessary if an unstable state occurs. However, in the normal state, the steering system exhibits all of its properties and is neither delayed nor limited in its dynamic behavior.

Claims

1. Vehicle power steering system comprising an electric motor and an electronic control system, which controls the electric motor dynamically, so that the latter, in addition to the steering torque exerted by the drive of the vehicle, generates a motor torque, which supports the steering force, said control system evaluating the rotor rpm of the electric motor and, if the evaluation of the rotor rpm indicates an unstable state of the control system, increases the dynamic response time of the rotor.

2. The vehicle power steering system of claim 1, wherein the electric motor is an asynchronous motor and that the control system, in the unstable state, increases the dynamic response time of the rotor by means of a field-oriented control of the asynchronous motor.

3. The vehicle power steering system of claim 1, wherein the control system monitors the speed of the vehicle and the rotor rpm in order to identify the unstable state only when the speed of the vehicle does not exceed a first specifiable value.

4. The vehicle power steering system of claim 1, wherein the control system evaluates the rotor rpm in that it monitors the change in the rpm of the rotor as a function of time, and the control system identifies the unstable state in the event that the rotor rpm fluctuates periodically over a specifiable period of time, especially with a frequency, which lies within a specifiable range.

5. The vehicle power steering system of claim 1, wherein the control system monitors the speed of the vehicle and, in the unstable state of the control system, increases the dynamic response time of the rotor only if the speed of the vehicle does not exceed a second, specifiable value.

6. The vehicle power steering system of claim 1, wherein the control system, in the stable state of the control system, decreases the dynamic response time of the rotor in that it acts upon the dynamic control of the electric motor with a first factor, which has a positive algebraic sign, and that the control system, in the unstable state of the control system, increases the dynamic response time of the rotor, in that it acts upon the dynamic control of the electric motor with a second factor, which is smaller than the first factor and/or has a negative algebraic sign.

7. The vehicle power steering system of claim 6, wherein the control system monitors the speed of the vehicle and determines the negative factor by means of a characteristic curve function, which depends at least on the speed of the vehicle and especially on the speed of the vehicle, the rotor rpm and a change in the algebraic sign of the steering torque.

8. The vehicle power steering system of claim 1, wherein the control system is a controller, which has at least one proportional element and at least one differential element, in that the controller controls the electric motor by means of a characteristic curve function, which is specified by its proportional element, and in that the controller, in the unstable state, acts upon the characteristic curve function with a time-delaying filter function.

9. Method for controlling an electric motor in a vehicle power steering system, for which the electric motor is controlled dynamically, so that the latter, in addition to the steering torque exerted by the driver of the vehicle, generates a motor torque, which supports the power steering, wherein the rotor rpm of the electric motor is evaluated and that the dynamic response time of the rotor is increased in the event that the evaluation of the rotor rpm indicates an unstable state of the control system.

10. The method of claim 9, wherein the method is carried out by means of a state machine, which comprises three states, the first state being assumed in order to activate or deactivate the recognition of the unstable state of the control system, the second or third state being assumed in order to evaluate the rotor rpm in order to carry out the evaluation of the unstable state.

11. The method of claim 10, wherein the speed of the vehicle is monitored in the first state and that, for identifying the unstable state, there is a changeover into the second or third state, in the event that the speed of the vehicle exceeds a specifiable first value, the positive or negative acceleration of the rotor being determined from the change in the rpm of the rotor as a function of time and compared with a limiting gradient value, the change into the second state being made in the event that the positive rotor acceleration exceeds the limiting gradient value and a change into the third state being made in the event that the negative acceleration of the rotor exceeds the limiting gradient value.

12. The method of claim 10, wherein the speed of the vehicle is monitored in the first state and that the identification of the unstable state is deactivated in the event that the speed of the vehicle exceeds a specifiable second value.

13. The method of claim 11, wherein the negative acceleration of the rotor is compared with the limiting gradient value in the second state in order to check whether this limiting gradient value is exceeded less frequently than a specifiable number and, if this is the case, there is a change into the first state in order to deactivate the identification of the unstable state or, if this is not the case, there is a change into the third state.

14. The method of claim 11, wherein the acceleration of the rotor is compared with the limiting gradient value in the third state in order to check whether this limiting gradient value occurs less frequently than a specifiable number and, if this is the case, there is a change into the first state in order to deactivate the identification of the unstable state or, if this is not the case, there is a change into the second state.