This cheat sheet compares the three most common robot actuators: DC motors, servos, and stepper motors. Students need it to choose the right motor for wheels, arms, grippers, and precise positioning tasks. It also connects motor behavior to the control signals used in robotics circuits and microcontroller code.
DC motors are best for continuous rotation and speed control, usually using PWM and an H-bridge. Servos use a control pulse to hold a commanded angle, often over a limited range such as 0 to 180 degrees. Stepper motors move in fixed angle increments, so position can be controlled by counting steps, but they need a stepper driver and enough current.
Key Facts
- A DC motor speed is commonly controlled by PWM, where duty cycle = on time / total period.
- A DC motor direction is reversed by switching the polarity across the motor terminals, usually with an H-bridge driver.
- A standard hobby servo position is controlled by pulse width, with about 1.0 ms for 0 degrees, 1.5 ms for 90 degrees, and 2.0 ms for 180 degrees.
- A stepper motor angle is found with angle moved = number of steps x step angle.
- For a stepper motor, steps per revolution = 360 degrees / step angle.
- Torque is the turning effect of a force, and torque = force x perpendicular distance from the rotation axis.
- DC motors usually need external feedback, such as an encoder, for accurate position control.
- Motors should not be powered directly from most microcontroller pins because motors require more current than pins can safely provide.
Vocabulary
- DC Motor
- A motor that spins continuously when direct current is applied, making it useful for wheels, fans, and fast rotating mechanisms.
- Servo Motor
- A motor system with internal feedback that moves to and holds a commanded position based on a control signal.
- Stepper Motor
- A motor that rotates in small fixed steps, allowing position control by counting step commands.
- PWM
- Pulse width modulation is a control method that rapidly switches power on and off to change the average power delivered to a motor.
- H-Bridge
- A motor driver circuit that lets a controller run a DC motor forward, reverse, brake, or stop.
- Encoder
- A sensor that measures motor rotation so a robot can estimate speed, distance, or position.
Common Mistakes to Avoid
- Connecting a motor directly to a microcontroller pin is wrong because motors can draw enough current to damage the pin or reset the board.
- Using PWM alone for precise DC motor position is wrong because PWM changes speed, not exact position, unless feedback such as an encoder is added.
- Treating a servo like a continuous DC motor is wrong because most hobby servos are designed to move to an angle and hold it, not spin freely.
- Ignoring stepper current limits is wrong because stepper motors and drivers can overheat if the driver current is set too high.
- Assuming a stepper always knows its true position is wrong because missed steps can occur under too much load unless feedback or homing is used.
Practice Questions
- 1 A PWM signal has an on time of 3 ms and a total period of 10 ms. What is the duty cycle?
- 2 A stepper motor has a step angle of 1.8 degrees. How many full steps are needed for one complete revolution?
- 3 A robot arm needs 0.40 N of force at a distance of 0.15 m from the joint. What torque is required at the joint?
- 4 A robot needs to drive two wheels forward and backward at variable speed. Which motor type and driver would you choose, and why?