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Engineering
Grade 9-12
Stepper, Servo & DC Motor Comparison Cheat Sheet
A printable reference covering stepper motors, servo motors, DC motors, torque, speed, precision, and feedback for grades 9-12.
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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?