Electric Power Steering Current Control for Time Constant Accuracy
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Solution Overview
Problem
In electric power steering devices, discrete time control of vector control systems often results in deviations from the designed time constant of the current control system, leading to noise and discomfort due to inadequate voltage command updating periods.
Innovation Solution
A control device for electric power steering systems, where the q-axis proportional gain is set as Kpq=Kq/τq×L, with Kq being a constant greater than 0 and less than 1, to reduce deviations in the actual time constant from the designed value, even under discrete time control.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Manufacturing precision
If the voltage command updating period is not sufficiently smaller than the time constant of the current control system, then the control system is easier to implement, but the actual time constant deviates from the designed value causing noise and discomfort
Solution Approach 1:
The patent modifies the proportional gain parameter from the conventional ωc×L to a corrected value that compensates for discrete time control effects. This parameter change allows the actual time constant to match the designed value even when the voltage command updating period is not sufficiently smaller than the time constant, thereby reducing noise and discomfort without requiring excessively fast updating rates.
2Ease of manufacture
If the proportional gain is set according to conventional design (ωc×L), then the control design is simpler, but the actual time constant deviates from the designed value leading to abnormal sound
Solution Approach 1:
The patent introduces a corrected proportional gain parameter that accounts for discrete time control effects. By changing this parameter from the conventional ωc×L to a value that incorporates the voltage command updating period, the system eliminates abnormal sound caused by time constant deviation while maintaining practical design simplicity.
3Manufacturing precision
If the voltage command updating period is made sufficiently smaller than the time constant, then the time constant accuracy is improved, but the control frequency and processing requirements increase
Solution Approach 1:
Instead of requiring the voltage command updating period to be extremely small, the patent changes the proportional gain parameter to compensate for finite updating periods. This allows accurate time constant control at practical updating rates, avoiding the need for excessively high control frequencies and reduced processing time.
4Speed
If the proportional gain is increased to reduce time constant deviation, then the response speed improves, but the system becomes more sensitive to noise and instability increases
Solution Approach 1:
The patent uses a specifically derived proportional gain parameter that balances response speed and stability. The corrected parameter accounts for discrete time effects while maintaining appropriate damping, avoiding the noise sensitivity and instability that would result from simply increasing the gain.
Data Source
AI summary
A control device for an electric power steering device includes an inverter converting direct current voltage into alternating current voltage, and apply to a synchronous motor; and a controller for converting three-phase current flowing in the synchronous motor into d-axis and q-axis currents, performing proportional integral control for a deviation between q-axis current command and the q-axis current so that the q-axis current follows the q-axis current command, to calculate q-axis voltage command, performing proportional integral control to calculate d-axis voltage command, converting the d-axis and q-axis voltage commands into three-phase voltage command, and controlling the inverter based on the three-phase voltage command. A q-axis proportional gain used in the proportional integral control for the q-axis current is obtained using a time constant of a q-axis current control system, a q-axis or d-axis inductance, and a constant greater than zero and less than one.