Stepping Motor Current Control via Dynamic PWM Thresholds
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Solution Overview
Problem
Existing motor control devices for stepping motors face challenges in predicting current waveforms due to varying drive voltage, rotational speed, load torque, and temperature, leading to torque loss, oscillation, and noise, with solutions like using two comparators or shortening PWM periods increasing costs.
Innovation Solution
A motor control device with an H-bridge circuit and a controller that selects operation modes based on current comparisons before and after a predetermined time within each PWM period, using a single comparator to manage charge, fast decay, and slow decay modes, reducing costs and improving current followability.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If two comparators are provided to compare the measured current value with two reference values, then the current followability is improved, but the device cost increases
Solution Approach 1:
The patent applies dynamics by making the comparator reference values time-varying rather than static. The first reference value and second reference value change dynamically based on the PWM period phase, allowing a single comparator to achieve the functionality of two comparators by adapting its threshold throughout the switching cycle. This resolves the contradiction by maintaining measurement precision through dynamic adaptation while avoiding the cost of additional hardware comparators.
Solution Approach 2:
The patent changes the parameter of reference values from fixed to variable. By making the reference values dependent on the PWM period timing (first reference value before current control re-execution time, second reference value after), the system achieves dual-comparator functionality with a single comparator. This parameter transformation allows the same hardware to operate at different threshold levels at different times, resolving the cost issue while maintaining current followability.
2Measurement precision
If the PWM period is shortened to suppress current ripple, then the current followability is improved, but the controller cost increases due to high-speed operation requirements
Solution Approach 1:
The patent applies dynamics by implementing time-dependent reference values that adapt to different phases within the PWM period. Instead of uniformly shortening the PWM period, the system dynamically adjusts the comparison threshold based on timing, allowing effective current control at standard PWM frequencies. This resolves the contradiction by achieving current ripple suppression through intelligent threshold modulation rather than brute-force frequency increase, avoiding high-speed controller requirements.
Solution Approach 2:
The patent applies preliminary action by pre-setting the first reference value to be applied before the current control re-execution time. This anticipatory approach allows the controller to prepare the appropriate reference value in advance, enabling effective current control without requiring high-speed real-time adjustments. The preliminary setting of reference values reduces the computational burden during the PWM period, avoiding the need for high-speed controllers.
3Productivity
If the measured current value deviates from the predicted value, then the current ripple increases to compensate for the drop, but torque loss, oscillation, and noise occur
Solution Approach 1:
The patent applies feedback by continuously comparing the measured current value against time-dependent reference values and adjusting the H-bridge circuit operation accordingly. The system uses the comparison results to determine when to switch between charge mode and decay modes, creating a closed-loop control that responds to current deviations. This feedback mechanism prevents excessive current ripple by making controlled, incremental adjustments rather than large compensatory changes, thereby reducing torque loss, oscillation, and noise.
Solution Approach 2:
The patent applies periodic action by structuring the control around the PWM period cycle, with reference values that reset and repeat each period. The first reference value is used before the current control re-execution time, and the second reference value is used after, creating a periodic control pattern that synchronizes with the motor's electrical cycle. This periodic structure allows the system to anticipate and prevent current deviations at regular intervals, maintaining smooth current flow and reducing harmful effects like torque loss and noise.
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
The solution improves current followability with respect to target values while maintaining a low-cost configuration, reducing torque loss, oscillation, and noise by efficiently switching operation modes within a PWM period.
Implementation Method 1
control based on a 'charge mode', a 'fast decay mode', and a 'slow decay mode' is repeated for each PWM period
Implementation Method 2
the inductance of the stator winding varies depending on the positional relationship between a rotor and a stator, the decay speed of the current in the decay mode also varies according to this positional relationship
Data Source
AI summary
This motor control device has an inexpensive configuration and enhances motor current target value tracking. This motor control device has an H bridge circuit that has a switching element and is connected to a motor coil provided in a motor, and a control means that drives the switching element at each prescribed PWM period and specifies an operation mode for the H bridge circuit from among a charge mode for increasing the motor current (Icoil) flowing through the motor coil, a fast decay mode for decreasing the motor current, and a slow decay mode. In each PWM period, the control means selects one of the operation modes on the basis of the result of comparing the motor current and a current reference value (Iref) before the time that has passed from the start of the PWM period reaches a prescribed current control re-execution time (Tr) and selects one of the operation modes on the basis of the result of comparing the motor current and the current reference value after the time that has passed reaches the current control re-execution time.