Stepper Motor Back-EMF Measurement During PWM Off-Time
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Solution Overview
Problem
Stepper motor control systems face challenges in accurately measuring back electromotive force (BEMF) voltage due to limitations in time availability during operation modes, especially at higher speeds, which hinders precise positional feedback and rotor movement sensing without additional complex hardware.
Innovation Solution
A stepper motor control system employing a fixed delta current regulation method, allowing BEMF voltage assessment throughout the commutation cycle without direct measurement and independent of motor voltage variations, using equations to calculate BEMF voltage based on off-time periods and known coil parameters.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If traditional BEMF measurement methods are used in stepper motors, then measurement can be made when current is zero, but no measurement time exists in full step mode and available time is very short in other modes
Solution Approach 1:
The patent introduces an intermediary approach by measuring the voltage across the coil during the off-time period when current is flowing, rather than waiting for current to reach zero. This intermediary measurement during the transition period enables BEMF detection without requiring complete current cessation, effectively resolving the time availability contradiction.
Solution Approach 2:
The patent performs preliminary action by capturing the voltage measurement during the off-time period before the current fully decays to zero. This preliminary measurement approach allows the system to obtain BEMF information earlier in the commutation cycle, extending the effective measurement window and enabling full step mode operation.
2Reliability
If rotational encoder or linear encoder is added to confirm positional movement, then positional feedback is obtained, but system complexity and hardware requirements increase
Solution Approach 1:
The patent applies the self-service principle by enabling the stepper motor system to measure its own position and speed using its existing coil structure and BEMF voltage. The motor's own electrical characteristics are utilized to generate positional feedback information, eliminating the need for external encoders or additional sensing hardware while maintaining closed-loop control capability.
Solution Approach 2:
The patent substitutes the mechanical sensing system (rotational encoder or linear encoder) with an electrical measurement system based on BEMF voltage detection. By replacing mechanical position sensors with electrical field-based measurement, the system achieves positional feedback with reduced mechanical complexity and fewer moving parts.
3Loss of information
If BEMF voltage is measured directly in full step mode, then positional feedback is available, but measurement cannot be made because current is never zero
Solution Approach 1:
The patent applies dynamics by adapting the measurement approach to the dynamic characteristics of the coil current waveform. Instead of requiring a static zero-current condition, the system dynamically captures voltage information during the transient off-time period when current is changing, enabling measurement in full step mode by exploiting the dynamic behavior of the electrical system.
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
Enables accurate and robust BEMF voltage determination, reducing the need for external position sensors and associated hardware, and allowing precise rotor movement tracking across various stepping modes and speeds.
Implementation Method 1
When a motor is driven with a current, a counter electromotive force pushes against the current. In electric motors, the motor has coils that cut across a magnetic field as the coils rotate. The motor therefore acts as a generator even at the same time as it rotates as a motor. The BEMF voltage that results is proportional to the motor speed and position
Data Source
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AI summary
In a described example, an apparatus (800) includes: a FET driver circuit (812) configured to supply current to a coil in a stepper motor, the FET driver circuit (812) configured to regulate the current to the coil using a fixed delta current; a current chopper PWM circuit (810) coupled to the FET driver circuit (812) configured to supply pulses corresponding to a step control signal and a direction control signal; a back electromotive force (BEMF) monitor (822) coupled to the current chopper PWM circuit (810) configured to measure an off time pulse and to output a BEMF monitor signal; and a controller (801) coupled to the current chopper PWM circuit (810) to supply the step and direction control signals and coupled to receive the BEMF monitor signal.