This cheat sheet compares stepper motors, servo motors, and DC motors for engineering motion control. Students need it to choose the right motor for robot arms, wheels, conveyors, camera mounts, and automated mechanisms. It highlights the main differences in torque, speed, precision, feedback, cost, and control complexity.
The goal is to make motor selection faster and more logical during design work.
Key Facts
- A DC motor rotates continuously when voltage is applied, and reversing the voltage polarity reverses the direction of rotation.
- Motor speed for a basic DC motor is approximately proportional to applied voltage, so higher voltage usually gives higher rpm within safe limits.
- Motor torque is calculated by torque = force x radius, where force is in newtons and radius is the perpendicular distance in meters.
- Mechanical power is calculated by power = torque x angular speed, where power is in watts, torque is in newton-meters, and angular speed is in radians per second.
- A stepper motor moves in fixed angle steps, so steps per revolution = 360 degrees / step angle.
- A 1.8 degree stepper motor has 360 / 1.8 = 200 full steps per revolution.
- A servo motor uses feedback to compare commanded position with actual position, then corrects error using a control circuit.
- Choose a stepper for repeatable open-loop positioning, a servo for precise closed-loop motion, and a DC motor for simple continuous rotation.
Vocabulary
- Stepper motor
- A motor that moves in small fixed angular steps when driven by electrical pulses.
- Servo motor
- A motor system that uses feedback to control position, speed, or torque accurately.
- DC motor
- A motor that converts direct current electrical energy into continuous rotary motion.
- Torque
- A turning force that depends on the applied force and the distance from the axis of rotation.
- Feedback
- Information from a sensor that tells a controller the actual position, speed, or motion of a system.
- Pulse width modulation
- A control method that rapidly switches power on and off to adjust the average voltage sent to a motor.
Common Mistakes to Avoid
- Choosing a DC motor when exact position is required, because a basic DC motor does not know where it is without an encoder or other feedback sensor.
- Assuming a stepper motor cannot lose position, because missed steps can occur if the load torque is too high or acceleration is too sudden.
- Comparing motors only by rpm, because torque, power, voltage, current, gearing, duty cycle, and control needs also determine performance.
- Ignoring stall current, because motors can draw much more current when starting or blocked than they draw while spinning freely.
- Using torque = force / radius, because torque equals force times radius when the force is perpendicular to the lever arm.
Practice Questions
- 1 A robot wheel needs 0.40 N of tangential force at a radius of 0.06 m. What torque must the motor provide at the wheel?
- 2 A stepper motor has a step angle of 1.8 degrees. How many full steps are needed for 3 complete revolutions?
- 3 A motor produces 0.25 N m of torque at an angular speed of 120 rad/s. What mechanical power does it produce?
- 4 A camera mount must hold a position accurately even when disturbed by wind. Which motor type is usually the best choice, and why?
Understanding Stepper, Servo & DC Motor Comparison
Motors turn electrical energy into motion, but the useful result depends on the load. A motor may spin freely on a desk yet stall when it must lift a mass or start a vehicle. Torque describes its turning effect.
A force applied farther from the axle produces more torque, which is why a long wrench feels stronger than a short one. Gears can trade speed for torque. A gear reduction makes an output shaft turn more slowly, but it can move a heavier load.
Students should check the torque needed at the actual shaft, not just the motor’s advertised rating. Starting torque matters because friction and inertia are greatest when a mechanism is at rest.
A basic DC motor needs a driver when a microcontroller controls it. A microcontroller pin cannot safely supply the current required by most motors. An H bridge driver sends current through the motor in either direction.
It can use rapid switching, called pulse width modulation, to control average power. This gives useful speed control, though the real speed still changes when the load changes. Wheels, fans, pumps, and small conveyor belts often use this arrangement.
A DC motor can create electrical noise as its brushes make contact with the commutator. Good designs use a separate motor power supply when needed, common ground connections, and noise reduction parts near the motor.
Stepper motors contain coils that are energized in a sequence. The changing magnetic fields pull the rotor from one resting position to the next. A driver controls this coil sequence and limits current to protect the windings.
Microstepping divides a full step into smaller commanded movements by changing current smoothly between coils. It can reduce vibration and make motion appear smoother. It does not guarantee equally accurate tiny movements, especially under a changing load.
If the load is too heavy, the motor can miss steps without reporting the error. This is a major concern in 3D printers, plotters, and sliding mechanisms. Acceleration must be gradual because a stepper has less available torque as its speed rises.
A servo system measures motion with a sensor such as an encoder or potentiometer. Its controller compares the measurement with the target and changes motor power to reduce the difference. This feedback allows the system to respond when a load pushes the shaft away from its intended position.
Many classroom servos are position servos with a limited travel range, while industrial servos may rotate continuously and report position, speed, or both. Feedback improves accuracy, but it adds wiring, tuning, and cost. Poor tuning can cause oscillation, where the mechanism repeatedly overshoots its target.
During testing, students should watch for heat, stalling, gear backlash, loose couplers, and power supply voltage drops. These practical limits often decide whether a mechanism works reliably.