Choosing a Motor: DC, Stepper, Or Servo

How to choose a motor for your robot

Electric motors are used to “actuate” something in your robot: its wheels, legs, tracks, arms, fingers, sensor turrets, or weapon systems. There are literally dozens of types of electric motors (and many more if you count gasoline and other fueled engines), but for amateur robotics, the choice comes down to these three:

  • In a continuous DC motor, application of power causes the shaft to rotate continually. The shaft stops only when the power is removed, of if the motor is stalled because it can no longer drive the load attached to it.
  • In a stepping motor, applying power causes the shaft to rotate a few degrees, then stop. Continuous rotation of the shaft requires that the power be pulsed to the motor. As with continuous DC motors, there are sub-types of stepping motors. Permanent magnet steppers are the ones you’ll likely encounter, and they are also the easiest to use.
  • A special “subset” of continuous motors is the servo motor, which in typical cases combines a continuous DC motor with a “feedback loop” to ensure accurate positioning. There are many, many types of servo motors; a common form is the kind used in model and hobby radio-controlled cars and planes.

With three common motor types for amateur robots to pick from — DC, stepper, and servo — it can be hard to know which one is best. The answer is not simple, because each motor type has its own pros and cons.

Motor Type Pros Cons
Continuous DC
  • Wide selection available, both new and used.- Easy to control via computer with relays or electronic switches.
  • With gearbox, larger DC motors can power a 200 pound robot.
  • Requires gear reduction to provide torques needed for most robotic applications.
  • Poor standards in sizing and mounting arrangements.
  • Does not require gear reduction to power at low speeds.
  • Low cost when purchased on the surplus market.
  • Dynamic braking effect achieved by leaving coils of stepper motor energized (motor will not turn, but will lock in place).
  • Poor performance under varying loads. Not great for robot locomotion over uneven surfaces.
  • Consumes high current.
  • Needs special driving circuit to provide stepping rotation.
R/C servo*
  • Least expensive non-surplus source for gear motors.
  • Can be used for precise angular control, or for continuous rotation (the latter requires modification).
  • Available in several standard sizes, with standard mounting holes.
  • Requires modification for continuous rotation.
  • Requires special driving circuit.
  • Though more powerful servos are available, practical weight limit for powering a robot is about 10 pounds.

* The discussion is limited here to radio control (R/C) servo motors; there are other types of servo motors, but most are mondo expensive.

Bear in mind that all motors are available in different sizes.

  • Small motors are engineered for applications where compactness is valued over torque. While there are small high-torque motors, these tend to be expensive because they use rare earth magnets, high efficiency bearings, and other features that add to their cost.
  • Large motors may produce more torque, but also require higher currents. High current motors require larger capacity batteries, and bigger control circuits that won’t overheat and burn out under the load. Therefore, match the size of the motor with the rest of the robot. Don’t overload a small robot with a large motor when big size isn’t important.
  • When decided on the size of the motor, compare available torque after any gear reduction. Gear reduction always increases torque. The increase in torque is proportional to the amount of gear reduction: if the reduction is 3:1, the torque is increased by about three times (but not quite, because of frictional losses).