Controlling Stepper Motors: Methods and Best Practices

Control Methods

1. Open-Loop Control: Stepper motors are often controlled using open-loop systems, where the controller sends command signals to the motor without feedback. This method relies on the motor’s inherent ability to move in precise steps in response to input pulses. Open-loop control is simple and cost-effective, as it eliminates the need for sensors and feedback circuitry. However, it does not compensate for disturbances or lost steps, which can lead to positioning errors.
2. Step and Direction Control: Modern stepper drivers simplify control by using two signals from a microcontroller: step and direction. The direction pin determines the rotation direction, while the step pin sends pulses to move the motor one step at a time. This method is widely used due to its simplicity and ease of implementation.
3. Microstepping: Microstepping divides each full step into smaller increments, increasing the motor’s resolution and smoothness. This is achieved by controlling the current in the motor windings to intermediate levels, allowing for finer positioning and reduced vibration. Microstepping drivers can divide a full step into as many as 256 microsteps, significantly improving accuracy.
4. Closed-Loop Control: In closed-loop stepper control, an encoder is attached to the motor to provide feedback on its position. The controller uses this feedback to adjust the motor’s current and ensure it accurately follows the commanded trajectory. Closed-loop control eliminates lost steps and the tendency to stall, providing greater energy efficiency and less heat generation.

Best Practices

1. Current Limiting: Stepper motors with low coil resistance cannot self-regulate current, so stepper controllers use sophisticated techniques to limit current to safe levels. Explicitly setting the maximum current allowed is essential to protect the motor and driver from damage. The driver should output at least 1.4 times the motor’s rated current.
2. Velocity Limiting: Every stepper motor has a resonant frequency at which it vibrates excessively, leading to overshoot and loss of torque. Setting a velocity limit can prevent the motor from operating at this frequency, minimizing vibration and improving performance.
3. Acceleration Control: Setting the acceleration too high can cause the motor to stall, especially with a heavy load. Using lower acceleration in high-torque applications helps prevent stalling and ensures smooth operation.
4. Minimize Current Ripple: Smoother operation occurs when sinusoidal current waveforms are applied to the windings. Any ripple in the current waveform causes an uneven torque and incremental angle of the motor, which manifests as vibration and poor accuracy.
5. Proper Driver Selection: Selecting the right stepper motor driver can greatly improve motion smoothness and accuracy. State-of-the-art drivers with better current matching and high levels of microstepping can significantly improve accuracy.
6. Careful Design of Driver Circuit: Every stepper motor is controlled via a driver circuit, which transmits current to the coils to activate them. Since the mechanism that controls the stepper motor’s movements begins with the motor controller, controls should be designed carefully and according to best practices.
7. Smart Tune Ripple Control: Smart tune ripple control, available in some drivers, allows for improved accuracy by operating a stepper motor on slow decay mode whenever possible, instead of fast or mixed decay.