Motor Control Device Limit Gain for Torque Preservation
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Solution Overview
Problem
Excessive limitation of q-axis current leads to a substantial reduction in motor output torque, deteriorating the performance of electrically driven actuator products, such as electrically driven power steering systems, which affects their rotation following capability.
Innovation Solution
A motor control device that computes a limit gain to restrict both q-axis and d-axis current command values, ensuring the power supply current is limited to a predetermined upper value while minimizing the reduction in motor output torque by using a limit gain that adjusts based on the motor's angular velocity and current values, thereby maintaining optimal torque production.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If the q-axis current is excessively limited to prevent overcurrent, then the power supply current is restricted to a predetermined upper limit value, but the motor output torque is substantially reduced and motor performance is deteriorated
Solution Approach 1:
The patent applies dynamics by making the current limitation adaptive rather than fixed. The control device dynamically adjusts the limitation of q-axis and d-axis currents based on the motor's operating state, specifically using the relationship between angular velocity and current values. This allows the system to prevent overcurrent faults while maintaining optimal torque production across different operating conditions, resolving the contradiction between reliability and power output.
Solution Approach 2:
The patent changes the parameters of current limitation by computing a limit gain that varies with motor angular velocity and current values. Instead of applying a fixed limitation to q-axis current, the system adjusts the limitation parameters dynamically based on operating conditions. This enables the motor to maintain higher output torque when needed while still preventing overcurrent faults, thus resolving the contradiction between reliability and power.
2Reliability
If the q-axis current command value is restricted to limit power supply current, then the power supply current becomes equal to or lower than an upper limit value, but the rotation following capability of the electrically driven power steering is reduced
Solution Approach 1:
The system uses dynamic current limitation that adapts to the motor's angular velocity and operating state. By computing the limit gain based on real-time operating conditions, the system can maintain adequate current for good rotation following capability while preventing overcurrent that would damage wiring and switching elements. This dynamic approach resolves the contradiction between protection and ease of operation.
3Reliability
If only the q-axis current is limited without changing the d-axis current, then the q-axis current is restricted, but the overall current optimization is insufficient
Solution Approach 1:
The patent segments the current control into separate q-axis and d-axis components, each with its own limitation strategy. By computing individual limit gains for q-axis and d-axis currents based on the motor's operating state, the system optimizes each current component independently. This segmented approach allows for more efficient current utilization while maintaining overcurrent protection, resolving the contradiction between reliability and productivity.
Data Source
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Figure 3A~3C
AI summary
To lighten reduction in motor output torque occurring when power supply current supplied from a power supply to a motor is limited to a predetermined upper limit value or lower. A motor control device (30) including: a power supply voltage detection unit (64) configured to detect a power supply voltage; a rotational angular velocity detection unit (50) configured to detect a rotational angular velocity of the motor; a command value calculation unit (40) configured to calculate a q-axis current command value and a d-axis current command value; a gain computation unit (51) configured to compute, based on an upper limit value of power supply current supplied from a power supply to the motor, the power supply voltage, the rotational angular velocity, the q-axis current command value, and the d-axis current command value, a limit gain reducing the power supply current to the upper limit value or lower by limiting the q-axis current command value and the d-axis current command value; and a driving circuit (48) configured to drive the motor, based on the q-axis current command value and the d-axis current command value limited by the limit gain.