Stepper Motors

A stepper motor moves in discrete electrical steps. It is popular because it can position open-loop in many low-speed systems. The tradeoff is that a stepper can miss steps without telling you unless the system includes feedback or verification.

Learning Objectives

By the end of this lesson, you should be able to:

• explain step angle, steps per revolution, and microstepping;
• relate step rate to speed;
• understand current-limited chopper drivers;
• identify resonance and missed-step risks;
• plan homing and position verification.

Step Angle and Speed

A common stepper has 200 full steps per revolution, or 1.8 degrees per step.

$$
\theta_\text{step}=\frac{360^\circ}{N_\text{steps}}
$$

Speed from step rate:

$$
RPM = \frac{f_\text{step}}{N_\text{steps}} \times 60
$$

For 1000 steps/s and 200 steps/rev:

$$
RPM = \frac{1000}{200}\times60=300\ RPM
$$

Driver Basics

Modern stepper drivers regulate winding current with a chopper circuit. The supply voltage may be much higher than the winding's simple I x R voltage because higher voltage helps current rise quickly at speed. Current limit must be set correctly.

Microstepping

Microstepping drives winding currents in intermediate ratios. It reduces vibration and improves smoothness, but it does not magically multiply mechanical accuracy under load. Torque per microstep is small, and friction may prevent tiny commanded moves.

Practical Checks

1. Set driver current limit from motor rating.
2. Start with low speed and acceleration.
3. Check motor and driver temperature.
4. Verify the mechanism can move without binding.
5. Add a home switch to establish reference position.
6. Use feedback for safety-critical or high-reliability positioning.

Common Mistakes

• Connecting a stepper directly to a microcontroller.
• Setting current limit too high or too low.
• Assuming microstepping guarantees accuracy.
• Using no acceleration ramp.
• Running open-loop where missed position is dangerous.

Summary

Stepper motors are excellent for controlled incremental motion when load torque, speed, current limit, acceleration, resonance, and homing are handled correctly. They are not self-verifying unless you add sensors or process checks.

Further Reading

• Trinamic stepper-driver application notes.
• Pololu stepper motor and driver guides.
• Oriental Motor stepper motor basics and torque-speed curves.

Mind Map