Robots move by converting electrical energy into controlled mechanical motion, and the motor choice shapes how accurate, strong, fast, and affordable a design can be. Servo motors, stepper motors, and DC motors all spin, but they are optimized for different jobs. A robot arm joint, a precision linear rail, and a drive wheel may each need a different motor because control requirements are different.
Choosing the right motor helps a robot move reliably without wasting power or adding unnecessary complexity.
A servo motor usually uses feedback to hold a commanded angle, making it useful for joints, grippers, and steering mechanisms. A stepper motor moves in small fixed angle increments, so it can position a mechanism precisely when loads are predictable. A DC motor spins continuously and is simple, fast, and inexpensive, but it needs extra sensing and control for accurate position.
In robotics, engineers compare motors by feedback, torque, precision, speed, cost, and the type of motion the mechanism needs.
Key Facts
- Servo motors are best for controlled angles, such as 0 degrees to 180 degrees in many hobby servos.
- Stepper motor step angle formula: step angle = 360 degrees / steps per revolution.
- Motor speed relation: rpm = revolutions per minute, and angular speed omega = 2 pi rpm / 60.
- Wheel speed formula: v = 2 pi r rpm / 60, where r is wheel radius in meters.
- DC motor torque is approximately proportional to current: tau = k I.
- Power in rotational motion: P = tau omega, where tau is torque and omega is angular speed.
Vocabulary
- Servo Motor
- A motor system that uses a control signal and usually feedback to move to and hold a commanded position.
- Stepper Motor
- A motor that rotates in fixed angular steps, allowing precise incremental motion without continuous position feedback in many applications.
- DC Motor
- A motor that spins when direct current is applied, commonly used for wheels, fans, pumps, and fast continuous motion.
- Feedback
- Information from a sensor, such as position or speed, that lets a controller correct the motor's motion.
- Torque
- A rotational force effect that measures how strongly a motor can twist a shaft or move a load.
Common Mistakes to Avoid
- Using a DC motor for exact position without feedback is wrong because a basic DC motor does not know its own angle or location.
- Assuming a stepper motor never loses position is wrong because missed steps can occur when the load torque is too high or acceleration is too sudden.
- Choosing a servo only by its angle range is wrong because torque, speed, voltage, and duty cycle also determine whether it can move the robot part.
- Comparing motors only by maximum rpm is wrong because robotics tasks often depend more on torque, control accuracy, gearing, and load inertia.
Practice Questions
- 1 A stepper motor has 200 steps per revolution. What is its step angle in degrees?
- 2 A robot uses a DC motor to spin a wheel of radius 0.04 m at 120 rpm. What is the wheel's linear speed in m/s?
- 3 A small robot arm joint must move to a commanded angle and hold that angle while carrying a changing load. Which motor type is usually the best choice, and why?