How to Control Stepper Motor Speed

Controlling stepper motor speed is essential in applications requiring precise motion control. With advancements in microprocessor technology, various control methods have emerged, but the fundamental principles remain consistent. This article explains how to control stepper motor speed effectively, focusing on two primary techniques: the software delay method and the timer interrupt method.

Understanding How to Control Stepper Motor Speed

The key to controlling stepper motor speed lies in regulating the duration of each energized phase. By adjusting this timing, the motor’s rotation speed can be precisely managed. Two widely used approaches are software delay and timer interrupt control, each with its advantages and limitations.

1. Software Delay Method for Stepper Motor Speed Control

The software delay method regulates speed by inserting deliberate pauses between each step phase. After powering a phase, the controller executes a delay subroutine before transitioning to the next phase. This delay duration directly influences the motor’s speed, allowing users to adjust speed by modifying delay parameters.

Advantages:

• Simple to implement with minimal hardware requirements

• Suitable for low-complexity applications and prototypes

Limitations:

• High CPU usage since the processor remains occupied during delays

• Inefficient for multitasking or real-time systems

For example, in microcontrollers like the SMK1702 series, the delay time can be calculated based on machine cycles and specific delay parameters. Adjusting these parameters allows fine-tuning of the motor's rotation speed.

2. Timer Interrupt Method for Precise Speed Control

To overcome the CPU load issues of software delays, the timer interrupt method uses hardware timers to generate precise timing events. The timer is loaded with an initial count value and, upon overflow, triggers an interrupt. The motor advances one step within the interrupt service routine, enabling consistent step timing without occupying CPU resources.

Benefits of Timer Interrupt Method:

• Efficient CPU utilization, freeing the processor for other tasks

• Higher accuracy and stability in speed control

• Ideal for complex and real-time control systems

For instance, using a timer on the 89C51 microcontroller with a 12 MHz oscillator, the timer can be configured to interrupt every 1 ms for a target speed of 1000 pulses per second. Fine adjustments account for interrupt response and processing delays to ensure exact timing.

Calculating Stepper Motor Speed

Accurate speed control also requires understanding the relationship between pulse frequency and motor speed. The formula to calculate speed (n) in revolutions per minute (rpm) is:

n = (60 × f × θ) / 360

Where:

• f = pulse frequency in Hertz (Hz)

• θ = step angle in degrees

This formula converts electrical pulses into mechanical rotation speed. When using microstepping (subdivision), the step angle is further divided, requiring adjustment in the formula to maintain accuracy.

General Speed Ranges of Stepper Motors

• Common speeds: Tens to hundreds of rpm for general industrial uses such as printing and automation

• High-speed applications: Thousands of rpm for rapid positioning and high-speed motion

• No-load speeds: Up to 2000 rpm for standard two-phase motors

• Rated load speeds: Usually up to 1000 rpm under load

Conclusion

Mastering how to control stepper motor speed is vital for optimizing performance in automation and precision tasks. The software delay method offers simplicity but with CPU overhead, while the timer interrupt method provides efficient and accurate speed control for advanced applications. Combining these methods with proper speed calculations ensures reliable and precise motor operation.