Three families, three different jobs
Most hobby robots move with one of three kinds of motor. A plain DC motor spins when you apply voltage. A hobby servo turns to a commanded angle and holds it. A stepper moves in small, fixed increments. They are not rivals so much as tools for different jobs, and the right choice depends on whether you need continuous rotation, a position, or precise repeatable steps. Because they behave so differently, the full set of motor types shapes the electronics and code you need as well as the mechanics.
Brushed DC motors
A brushed DC motor is the simplest of the three. Apply a voltage and the shaft spins; reverse the voltage and it spins the other way. As a rough rule, speed rises with voltage and torque rises with current.
Strengths
- Cheap and widely available in many sizes.
- Simple to control: speed is usually adjusted with pulse width modulation, and direction by reversing polarity.
- Good for wheels, fans and anything that should spin continuously.
Limitations
- It has no idea where its shaft is. To know position or speed accurately you must add an encoder or another sensor.
- Bare motors often run too fast and with too little torque for a robot, so they are usually paired with a gearbox.
- They need a driver, since a controller pin cannot supply the current. The reasons are covered in the article on motor drivers.
Brushed motors also generate electrical noise, which can disturb nearby sensors if the wiring is careless.
Hobby servos
A hobby servo packages a small DC motor, a gear train, a position sensor (often a potentiometer) and a control circuit in one case. You send it a repeating pulse whose width tells it the target angle. Typical hobby servos expect pulses lasting roughly one to two milliseconds, repeated at around fifty times a second, but check the documentation for the specific model since ranges vary.
Strengths
- Position control is built in. You send an angle and the servo does the rest.
- Compact, with a gearbox already included.
- Easy to connect: usually power, ground and one signal wire.
Limitations
- Most standard servos move over a limited range, often around half a turn, unless they are a continuous-rotation version.
- Precision and strength vary a lot between cheap and better units, and cheap ones can show jitter or slack in the gears.
- Holding a position against a load draws current continuously, and a stalled servo can get warm and drain a battery quickly.
- The internal control is a black box. You cannot easily tune it or read back the real position.
Servos suit steering linkages, small grippers and the joints of lightweight arms, where a limited range and modest force are enough.
Stepper motors
A stepper divides each rotation into many equal steps. A common design has 200 steps per turn, which is 1.8 degrees each, though other step counts exist. A stepper driver energises the motor coils in sequence, and each pulse you send moves the shaft one step. Many drivers also offer microstepping, which splits a step into smaller parts for smoother motion, although the extra resolution does not always translate into equal extra accuracy.
Strengths
- Repeatable positioning without a sensor, as long as the motor keeps up.
- Good holding torque when stationary, and good torque at low speeds.
- Well suited to machines that move along axes, such as 3D printers and small plotters.
Limitations
- If the load or acceleration is too high, the motor can skip steps. In a simple open-loop setup the controller does not notice, so the position error persists.
- Torque tends to fall as speed rises, so steppers are usually not the best choice for fast-spinning wheels.
- They draw current even when holding still, and they need a dedicated driver and a suitable supply.
- They can be noisy and warm.
A comparison in general terms
| Aspect | Brushed DC | Hobby servo | Stepper |
|---|---|---|---|
| Position without extra sensors | No | Yes, within its range | Yes, if no steps are lost |
| Continuous spinning | Yes | Only continuous-rotation versions | Yes, but speed is limited |
| Typical use | Drive wheels | Joints, steering | Axes, precise positioning |
| Control effort | Low, needs a driver | Lowest, one signal wire | Moderate, needs a driver and step signals |
| Typical cost | Low | Low to moderate | Moderate, plus driver |
These are general tendencies. A strong, well-made servo can outperform a weak stepper in some respects, and specific parts differ widely.
How to choose
Start from the motion, not the motor.
- Driving a wheeled robot? Geared DC motors are the usual choice, often with encoders for speed feedback.
- Moving a joint to a set angle with light loads? A servo is the quickest route.
- Repeatable positioning along an axis? A stepper fits, provided you leave margin in torque so it does not skip.
- Heavy loads or high precision? Consider that you may need a gearbox, a closed-loop arrangement, or a larger motor class than these three.
Gear reduction changes the picture for all of them. The way gearing trades speed for force is explained in the article on gears, belts and torque, and it often matters as much as the motor itself.
Practical points that apply to all three
- Read the datasheet. Rated voltage, stall current and torque figures determine your supply and driver. Careful reading of datasheets prevents most mismatches.
- Plan power first. Motors draw sharp current peaks, particularly at start-up and when stalled. A hobby power planning approach, including battery capacity and a supply that can deliver peak current, avoids resets and brown-outs. Estimating a battery runtime estimate from realistic average load, not the label rating, gives a more honest answer.
- Wire for the current. Thin wire carries limited current safely, so wire gauge and fusing should match the worst case.
- Be careful with batteries. If you use lithium polymer cells, follow the manufacturer’s guidance on charging and protection.
- Keep fingers clear. Even small motors can pinch or drag. Test with the robot lifted off the ground, and give any moving machine a reliable way to cut power.
Motors are only one stage of a larger loop of sensing, deciding and moving, and the best choice is usually the simplest one that meets the job with some margin to spare.