A stepper motor is a brushless DC electric motor that rotates in discrete, equal angular increments, called steps, each time its driver changes the current in the windings. Among actuators in robotics, it is valued because the shaft position follows the number of commanded pulses, so a controller can position an axis without a feedback sensor, a mode known as open-loop control.
Stepper motors drive 3D printer axes, camera sliders, laboratory automation, small Cartesian robots and pan-tilt units. They deliver high holding torque at standstill and at low speed, but their torque falls steeply as speed rises, which defines where they fit and where a servo motor is the better choice.
How a stepper motor produces steps
A stepper motor turns because its rotor aligns with the magnetic field created by energized stator windings. The driver energizes the phases in a fixed sequence, so the field rotates in small jumps and the rotor follows it one step at a time.
The common hybrid stepper combines a permanent-magnet rotor with fine teeth on both the rotor and the stator. With 50 rotor teeth and two phases, it produces 200 full steps per revolution, or 1.8 degrees per step. Each pulse on the driver’s step input advances the shaft by one step or a fraction of it, and the direction input selects the rotation sense.
| Construction | Typical step angle | Torque | Typical use |
|---|---|---|---|
| Permanent magnet (can-stack) | 7.5 to 18 degrees | Low | Valves, vents, low-cost indicators |
| Variable reluctance | 5 to 15 degrees | Low, no detent torque | Legacy and special designs |
| Hybrid | 0.9 or 1.8 degrees | Medium to high | Printers, CNC, robotic axes |
Drive modes, microstepping and key parameters
The behavior of a stepper motor depends heavily on its driver, which regulates winding current and sets the step resolution. Modern chopper drivers switch a supply voltage several times higher than the winding rating and limit the current electronically, which lets current build faster and keeps torque usable at higher speeds.
Microstepping shapes the phase currents as approximate sine and cosine waves, dividing each full step into 2 to 256 microsteps. It smooths motion and reduces resonance and noise, but it does not increase real accuracy in proportion: the incremental torque per microstep is small, so load torque can shift the actual position by a fraction of a full step.
| Parameter | Typical range | Effect |
|---|---|---|
| Frame size | NEMA 8 to NEMA 34 | Mounting face and available torque |
| Holding torque | 0.02 to 12 N·m | Maximum static torque at rated current |
| Rated phase current | 0.3 to 7 A | Determines driver selection and heating |
| Phase inductance | 1 to 50 mH | Lower inductance supports higher speed |
| Step accuracy | About ±3 to ±5 % of a step | Non-cumulative angular error |
| Detent torque | A few percent of holding torque | Small holding force with no current |
Winding connection also matters. Bipolar drive uses all of the copper and gives more torque, while unipolar drive simplifies the electronics at the cost of torque. Series windings favor low-speed torque, and parallel windings favor high-speed performance.
Stepper motor compared with a servo motor
The stepper motor and the servo motor are the two dominant electric actuators for precise positioning, and the choice between them is one of the most frequent decisions in robot axis design. The essential difference is feedback: a servo always measures and corrects its position, while a classic stepper assumes it has reached the commanded step.
| Aspect | Stepper motor | Servo motor |
|---|---|---|
| Control | Open loop by default | Closed loop with encoder |
| Torque at low speed | High, including holding torque | High, actively regulated |
| Torque at high speed | Drops steeply above a few hundred rpm | Nearly constant up to rated speed |
| Overload behavior | Loses steps without notice | Raises current, reports an error if exceeded |
| Power use at rest | Full current while holding | Only what the load needs |
| System cost and setup | Low cost, minimal tuning | Higher cost, loop tuning required |
Closed-loop steppers close part of the gap by adding an encoder, which detects and corrects lost steps and can reduce current when the load is light. A stepper may also drive a linear actuator through a lead screw, a common way to build compact positioning stages and end effector mechanisms.
Choosing and sizing a stepper motor
Sizing begins with the pull-out torque curve, which shows the torque available at each speed with a given driver and supply voltage. The required load torque, including acceleration torque from the load inertia, should stay at no more than about 50 to 70 percent of the curve at the highest operating speed.
| Situation | Recommended choice | Reason |
|---|---|---|
| Light axis, low speed, cost sensitive | Open-loop hybrid stepper | Simple and precise enough |
| Must never lose position silently | Closed-loop stepper or servo | Feedback detects missed steps |
| High speed or high dynamic loads | Servo motor | Torque stays available at speed |
| Holding a vertical load with power off | Stepper with a brake | Detent torque alone is insufficient |
| Quiet operation required | Stepper with high microstepping driver | Smoother current reduces noise |
Keep the load-to-rotor inertia ratio moderate, ideally below about 10:1, and use acceleration ramps rather than instant speed jumps. A higher driver supply voltage usually improves high-speed torque more than a larger motor does.
Limitations and common failure modes
The main weakness of a stepper motor is that a missed step is invisible to an open-loop controller. Once the load exceeds the available torque, the rotor slips behind the field, the position counter becomes wrong, and every following move inherits the error until the axis is homed again.
- Mid-band resonance: torque can collapse at certain speeds; microstepping and damping help.
- Heating: windings carry current even at rest, so motors run hot; current reduction at standstill is standard.
- Too aggressive acceleration: the rotor cannot follow and stalls.
- Low supply voltage: current cannot rise fast enough, and torque disappears at speed.
- Assuming microstep resolution equals accuracy: real position error remains a fraction of a full step.
Put simply
A stepper motor moves like the second hand of a clock: one small, exact jump at a time. Each electrical pulse from the controller moves the shaft forward by the same small angle, so counting pulses tells the machine where the shaft is.
That makes it a cheap and simple way to put something in an exact spot. The catch is that the motor cannot tell if it was pushed too hard and slipped. It is precise as long as it stays within its strength.
In practice
A laboratory builds a small Cartesian robot that moves a pipette head over a 96-well plate. Each axis uses a NEMA 17 hybrid stepper with 1.8-degree steps driving a belt, with a driver set to 16 microsteps and a 24 V supply, giving a theoretical resolution of about 0.0125 mm per microstep with a pulley that moves the belt 40 mm per revolution.
The engineers size the motors so that the required torque stays below half of the pull-out curve at the maximum travel speed and add gentle acceleration ramps. Each axis homes against a limit switch at the start of every run. The robot positions the pipette reliably over every well, and because it occasionally collides with a misplaced tube rack, the team later upgrades the vertical axis to a closed-loop stepper that reports a stall instead of losing position silently.