PWM Current Sensing Offset Correction for Electric Power Steering
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Solution Overview
Problem
Existing current detection systems in PWM control for electric motors suffer from offset current errors, leading to inaccurate current detection and potential torque ripples in electric power steering systems, causing an unnatural steering feel.
Innovation Solution
A current detection device that includes a current detection unit, a correction value calculation unit, and a correction unit to accurately detect and correct offset current by calculating an offset correction value during PWM control, especially for multiphase inverters, with specific conditions for updating the correction value based on duty ratio limits.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If offset correction value is updated during PWM control, then current detection accuracy is improved, but detection accuracy deteriorates when duty ratio is outside valid range
Solution Approach 1:
The patent applies preliminary action by checking whether the duty ratio is within the valid range (greater than lower limit and less than upper limit) before updating the offset correction value. This preliminary check prevents unreliable detection data from corrupting the correction value, ensuring that only valid offset current measurements are used for correction.
Solution Approach 2:
The patent implements feedback by continuously monitoring the duty ratio and using it to control the update condition of the offset correction value. When the duty ratio falls outside the valid range, the system feedback prevents updating, and when it returns to the valid range, updating resumes, ensuring reliable current detection accuracy.
2Manufacturing precision
If offset correction is applied during PWM control, then current control precision is improved, but torque ripple may occur if offset error is not properly corrected
Solution Approach 1:
The patent converts the potentially harmful effect of offset current into a beneficial correction process. By detecting the offset current during PWM control and applying correction values under appropriate conditions, the system eliminates torque ripple caused by offset errors, transforming what could be a source of vibration into a means of improving control precision.
Solution Approach 2:
The patent applies parameter changes by dynamically adjusting the offset correction value based on the detected offset current and the duty ratio conditions. This parameter adjustment ensures that the correction is applied only when reliable detection is possible, improving current control precision while preventing torque ripple.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
Accurate detection and correction of offset current in PWM control, improving current control precision and reducing torque ripples in electric power steering systems.
Implementation Method 1
a current detection unit configured to detect current flowing through a switching element of one arm of either an upper-side arm or a lower-side arm of a multiphase inverter on which PWM control is executed, based on a voltage drop of a resistor element connected in series to the switching element
Data Source
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AI summary
There is provided a current detection device including a current detection unit (35a to 35f) configured to detect current flowing through a switching element of a lower-side arm (or an upper-side arm); a correction value calculation unit (50b) configured to calculate an offset correction value, based on a detection value of the current detected by the current detection unit in a period during which the switching element off during PWM control, and to hold and update the calculated offset correction value; and a correction unit (50c) configured to correct, by the offset correction value held by the correction value calculation unit, the detection value of the current detected by the current detection unit in a period during which the switching element is on. The correction value calculation unit (50b) is configured to not update the offset correction value in a case where a duty ratio of the PWM control is equal to or less than a duty ratio lower limit value (or equal to or greater than a duty ratio upper limit value).