Motor Control Circuit for EPS Torque Ripple Reduction
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Solution Overview
Problem
Motor control circuits for electric power assisted steering systems with PWM-controlled multi-phase brushless motors face challenges in maintaining consistent torque output when one phase is in an open-circuit fault condition, leading to torque ripple and discomfort for the driver.
Innovation Solution
A motor control circuit that operates a three-phase star-connected motor as a single combined phase during an open-circuit fault, using a fault mode current controller to generate a single voltage demand signal and adjust polarity every 180 degrees, allowing the remaining phases to function smoothly and maintain consistent torque.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If the motor operates in normal three-phase mode, then the torque output is smooth and consistent, but the system cannot function when one phase is in an open-circuit fault condition
Solution Approach 1:
The control circuit dynamically switches between normal three-phase operation and fault mode operation based on the system state. When an open-circuit fault is detected in one phase, the controller automatically reconfigures the switching circuit to operate the motor as a single combined phase, allowing continuous operation under fault conditions without manual intervention
Solution Approach 2:
The switching circuit is designed to perform multiple functions: it can operate the motor in normal three-phase mode under healthy conditions, and automatically reconfigure to drive the motor as a single combined phase when one phase fails. This multi-functionality ensures the system can adapt to different operational states and maintain reliability
2Device complexity
If the motor operates with one phase open-circuit in normal mode, then the control circuit structure remains simple, but torque ripple occurs and motor performance deteriorates
Solution Approach 1:
Instead of treating the open-circuit fault as a failure condition to be avoided, the invention converts this harmful situation into a beneficial operational mode. By detecting the fault and automatically reconfiguring the switching circuit to drive the motor as a single combined phase, the system transforms the harmful open-circuit condition into a viable operating state that eliminates torque ripple and maintains smooth motor operation
Data Source
AI summary
A motor control circuit for an electric motor of an electric power assisted steering system comprises a switching circuit comprising a plurality of electrical switches, a current demand signal generator which converts the torque demand signal into a current demands signal; and a fault mode motor current controller that is responsive to an error signal that represents the difference between the current demand signal and the actual current flowing in the motor and is operable in the event of a fault where one phase is open-circuit to drive the remaining two phases as a single combined phase by generating a single voltage demand signal that is representative of the voltage to be applied across the combined phases, the voltage signal being in turn fed into a drive circuit for the switches that generates pulse width modulated switching signals for the switching circuit required to apply the voltage across the combined phases.