Electricity: motive power systems – Induction motor systems – Primary circuit control
Reexamination Certificate
1999-12-16
2001-08-14
Nappi, Robert E. (Department: 2837)
Electricity: motive power systems
Induction motor systems
Primary circuit control
C180S443000
Reexamination Certificate
active
06274999
ABSTRACT:
The present invention relates to a method and system for induction motor load compensation for power steering applications.
BACKGROUND
The assignee of the present invention designs and develops electric vehicles. In pure electric vehicles, there is not internal combustion engine from which to draw motive power to a hydraulic pump of a hydraulic power steering system. One method to compensate this is to provide a motor, such as an induction motor, as the motive input. The conventional hydraulic system, though, uses mechanical or belt drive from the mechanical input and pressure sensing of the power steering system to provide feedback and control system pressure. This type of system does not provide optimum efficiency and tends to use power to keep the steering system pressurized when not utilizing the pressure to ensure that the system has desired response when a steering input is provided.
It would therefore be desirable to have an improved power steering system for use in electric vehicles. It would also be desirable to have an improved induction motor load compensation algorithm or method for use in electric vehicle power steering systems.
SUMMARY OF THE INVENTION
The present invention provides a method and system for induction motor load compensation for power steering applications particularly suitable for use in an electric vehicle. In an example advantage, the present invention may be used in a system in which the motor for providing power steering system hydraulic pressure operates without the need of a hydraulic pressure sensor or a pressure relief valve as found in certain conventional systems, or motor position sensor feedback.
In an advantage provided by an example implementation, the control method or system provides for induction motor load limiting to avoid excessive power consumption while providing sufficient system pressure and flow over varying loads.
In an advantage provided by an example implementation, the controller and software algorithm or processing method adjusts the output power of the power steering motor by adjusting the output voltage of the power supply to maintain necessary flow and pressure for steering maneuvers. The output power of the power supply changes at different output frequencies. One or more (three, for example) software lookup tables are generated empirically by characterizing the output power of the power supply at different system pressures over a range of motor speeds. The software lookup tables provide a power band that implements a power adjusting state machine. Power calculations are performed at the output voltage signal frequency.
In an advantage provided by an example implementation, the use of the software algorithm or processing method provides relatively fast power steering system response while exerting lower power consumption by matching the output to the load. The relatively fast response maintains sufficient hydraulic system pressure and flow over a range of power steering loads without excessive power consumption. Thus the example system eliminates the need to run in a mode where high hydraulic pressure is maintained but not used.
In an example operation, the software algorithm or processing method dynamically adjusts the output voltage applied to the power steering motor to provide sufficient torque over a range of motor loads. An open loop or open load command (i.e., corresponding to a desired “on center” steering input) is received by the system, typically having been generated as a function of vehicle speed. This command is converted to a voltage command (Vrms) to the switching unit (such as an IGBT power stage) for the induction motor. The output phase current is monitored and used as a correspondence to hydraulic pressure to dynamically adjust the voltage command to maintain the desired pressure.
Advantageously, according to a preferred example, this invention provides an induction motor control system for use in a power steering system, comprising: a power steering induction motor coupled to a power supply and providing motive input to pressurize a hydraulic steering subsystem; a phase current sensor for sensing the phase current output by the power supply; and a power steering controller coupled to the power supply for processing a commanded frequency input, the sensed phase current, and data contained in a plurality of software lookup tables using a transfer function to adjust the motor voltage produced by the power supply to control the speed of the power steering motor.
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Char Wade E.
Downer Scott D.
Fujii Stanley K.
Duda Rina I.
General Motors Corporation
Nappi Robert E.
Simon Anthony Luke
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