Stepper Motor Average Current Control for Torque Ripple Reduction
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Solution Overview
Problem
In stepper motors, inaccuracies in coil current magnitudes lead to inaccuracies in rotational position due to the reliance on peak current control, which introduces torque imbalances and delays in controlling the H-bridge transistors, resulting in ripple that varies with supply voltage and motor inductance.
Innovation Solution
A stepper motor driver system that includes an H-bridge with low-side transistors, a reference current circuit, an averager circuit, and a comparator to determine and regulate the average coil current, adjusting the input offset of the comparator to maintain the average coil current at a desired level, thereby reducing torque imbalances and control delays.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Force
If peak current control is used in H-bridge transistors, then the motor can achieve sufficient torque output, but torque imbalances and control delays occur resulting in rotational position inaccuracies
Solution Approach 1:
The patent implements a feedback mechanism where the average coil current is continuously measured and fed back to the control system. The averager circuit calculates the average current over a complete switching cycle, and this feedback signal is used to adjust the PWM duty cycle, thereby eliminating torque imbalances and improving rotational position accuracy
Solution Approach 2:
The patent replaces traditional peak current control mechanisms with an average current control system. By substituting the control parameter from peak current to average current and using an averager circuit instead of peak detection circuitry, the system eliminates control delays and achieves more accurate torque control
2Device complexity
If traditional control methods are used, then the system structure remains simple, but control delays occur due to H-bridge transistor switching
Solution Approach 1:
The patent applies preliminary action by pre-calculating and averaging the current over the complete switching cycle before using it for control adjustments. The averager circuit continuously computes the average current in advance, allowing the control system to respond without delays caused by real-time peak detection and transistor switching variations
3Force
If peak current is controlled, then torque can be maintained, but ripple varies with supply voltage and motor inductance causing position inaccuracies
Solution Approach 1:
The patent changes the control parameter from peak current to average current. By controlling the average coil current instead of peak current, the system eliminates ripple variations caused by changes in supply voltage and motor inductance, thereby improving rotational position accuracy while maintaining adequate torque
Data Source
AI summary
A stepper motor driver includes an H-bridge including first and second outputs. The H-bridge includes a low-side transistor coupled between the first output and a ground. A reference current circuit is configured to produce a reference current. The reference current circuit has a reference output. An averager circuit includes an input and output. The input of the averager circuit is coupled to the first output of the H-bridge. A comparator includes first and second comparator inputs. The first input of the comparator is coupled to the output of the average circuit and the second input of the comparator is coupled to the reference output.