Stepper Motor Feed-Forward Voltage Control for BEMF Compensation
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Solution Overview
Problem
Stepper motors driven in voltage mode experience significant fluctuations in phase current at varying speeds, leading to potential loss of steps and motor stall due to uncontrolled back-electromotive force (BEMF) voltage, which is exacerbated by large electrical constants and high BEMF generation at low speeds.
Innovation Solution
Implementing a feed-forward voltage mode control characteristic that sets the drive voltage equal to the sum of estimated BEMF and the product of desired phase current and motor impedance, with a control curve approximated by straight line segments, and compensating for supply voltage fluctuations to maintain constant phase current amplitude across the speed range.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of operation
If voltage mode driving is used to control average phase voltage, then the control simplicity is improved, but the phase current fluctuations increase due to uncontrolled BEMF voltage
Solution Approach 1:
The patent applies feed-forward control by pre-calculating and applying the BEMF compensation voltage based on the known speed and electrical constant, before the BEMF actually affects the phase current. This preliminary action prevents current fluctuations rather than reacting to them, maintaining both control simplicity and current stability.
Solution Approach 2:
The patent dynamically adjusts the drive voltage parameter by adding a speed-dependent BEMF compensation term. The compensation voltage is calculated as a function of motor speed and electrical constant, changing parameters in real-time to counteract BEMF effects and maintain stable phase current throughout the speed range.
2Force
If the electrical constant is increased to improve torque production, then the torque capability is improved, but the BEMF voltage increases causing current control issues
Solution Approach 1:
The patent converts the harmful BEMF voltage into a useful compensation signal. By calculating BEMF as a function of speed and electrical constant, the system uses the same electrical constant that generates high BEMF to compute the exact compensation needed, turning the harmful effect into a beneficial control mechanism that maintains current stability.
3Measurement precision
If PWM switching frequency is increased to improve current regulation, then the current control precision is improved, but the computational complexity and switching losses increase
Solution Approach 1:
The patent performs BEMF compensation through feed-forward control, calculating the required compensation voltage in advance based on speed and electrical constant. This approach achieves precise current control without requiring high PWM switching frequencies or complex real-time iterative algorithms, thereby reducing computational complexity while maintaining control precision.
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
This approach effectively reduces phase current fluctuations and maintains torque consistency, preventing motor stall and ensuring accurate positioning and smooth operation across a wide range of speeds.
Implementation Method 1
the amplitude of the BEMF is equal to k E · ω EL , wherein ω EL represents the electrical frequency and k E is the electric constant of the motor
Implementation Method 2
When the windings of a stepper motor are energized, magnetic fields are generated and the rotor of the stepper motor turns to a certain position
Data Source
Figure 1~2
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AI summary
A method of driving a stepper motor in feed-forward voltage mode, comprises the step of setting, for a desired speed to be impressed to the stepper motor, the amplitude of a sinusoidal phase voltage of the stepper motor equal to the sum of the expected back-electromotive force (BEMF) amplitude estimated in function of the desired speed, and the product between a desired amplitude of a phase current (Iphase) and an estimated absolute value of the motor impedance. A relative system is also disclosed.