Electric Power Steering Motor Control Current Sensor Fault Mitigation
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Solution Overview
Problem
Current electric power steering systems using Permanent Magnet Synchronous Machines face instability in torque control due to current sensor faults, leading to reduced steering assist performance, as the transition from high-bandwidth current control to lower-bandwidth feedforward control results in less stable steering assist control loops.
Innovation Solution
A power steering system with a torque modifier module that generates a modified torque command in response to current sensor faults and a dynamic feedforward compensation module that modifies the motor current command, along with a stability compensator selector module to adjust the steering torque control loop, ensuring a stable frequency response during faults.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If transition from closed loop current feedback control to static feedforward inverse motor model based torque control is implemented when current sensor fault is detected, then loss of steering assist is mitigated, but the bandwidth of torque control decreases significantly
Solution Approach 1:
The patent applies dynamics by transitioning from a static feedforward inverse motor model to a dynamic feedforward model that incorporates the derivative of motor current. This dynamic approach maintains higher bandwidth during fault conditions by actively responding to current changes rather than relying solely on pre-calculated torque commands, thereby resolving the contradiction between reliability and control speed.
Solution Approach 2:
The patent changes the control parameters by introducing a derivative term of motor current into the feedforward torque command calculation. This parameter modification allows the system to maintain appropriate torque response during current sensor faults while preserving bandwidth, as the derivative component compensates for the loss of feedback information.
2Reliability
If stability compensation designed for lower bandwidth feedforward inverse motor model based torque control is used during current sensor fault condition, then torque control can be maintained, but the overall steering assist control loop becomes less stable
Solution Approach 1:
The patent modifies the feedforward torque command by adding a derivative term of motor current, which changes the dynamic characteristics of the control loop. This parameter change improves stability during fault conditions by providing proactive torque adjustment based on current rate of change, counteracting the destabilizing effect of the reduced bandwidth.
Solution Approach 2:
The derivative term in the feedforward control acts as a preliminary action by anticipating torque requirements based on the rate of change of motor current. This proactive adjustment stabilizes the control loop before instability can develop, preventing the oscillations that would otherwise occur during the transition to fault-mode operation.
Data Source
AI summary
A power steering system includes a torque modifier module that generates a modified torque command in response to a current sensor fault, a magnitude of the modified torque command changes over a time period. The power steering system also includes a feedforward selection module that applies a dynamic feedforward compensation to a motor current command, thereby generating a motor voltage that is applied to a motor of the power steering system, the dynamic feedforward compensation modifies a frequency response of the power steering system.