It difficult to find motors to power small projects. For robotics and other actuators, one would usually want a servo motor. They have two important things: large torque and position sensing. A small motor by itself is not particularly useful, since both things are missing. There are a lot of small DC motors, but without the reduction gears, such small motors would spin really fast and provide almost no torque. This is not particularly useful for moving small robots. The gearbox reduces the speed of the motor and converts it to torque. While larger motors have built-in or clip-on encoders, small DC motors don’t have anything. An encoder is a term encompassing various position sensing sensors, usually magnetic fields (Hall effect) or infrared.
There are more exotic actuators such as shape memory alloy, electrorheological fluids, and piezo motors. However, in practice usually nothing beats a standard electric motor. Electric motors are innexpensive, easy to get, and easy to control.
Those motors are small with 6mm diameter. They are made by many manufacturers in China, and sold by multiple online retailers They come with different gear ratios: 136:1, 26:1, and 700:1.
We used the 136:1 motors for Rovables project. They have plastic gears inside, and can easily break, for example, if they are blocked when rotating. Because of low price ($10), small size, and good torque those motors are the best deal for early prototypes. On the downside, those motors will need custom encoders to sense the rotations. In Rovables, we attached a disk with a reflective pattern and tracked it with an infrared sensor. It is a lot of work to get it to work reliably.
The DC motors are easy to control. If motor moves in one direction, it can be controlled with a simple power switch. If motor moves forward and backwards, am H-bridge circuit is usually used. Such circuits can be bought as integrated chips. I like using the DRV8835 chip. It is small, easy to interface with PWM, and can drive two motors.
I have not tested those motors personally. They are the smallest and still affordable gear motors on the market. I heard that they have a wobbly plastic shaft, making the motor difficult to use.
Those linear servos are made for small RC airplanes and helicopters. I think they are made in China, and also sold by many online retailers. Since its a servo, they have integrated DC motor, gears, position sensing and position control electronics into one package. They come in different versions, designated by their weight 1.5 grams, 1.9 grams, 2.3 grams, and 2.9 grams. Those servos are fairly inexpensive ($15). They have plastic gears, so they will break fairly easily.
Those are probably the smallest geared motors on the market and impressively engineered. They are made in Germany and are hard to get. Each motor costs about $200. Since they are brushless motors, they require additional electronics to drive them, unlike DC motors which are controlled with a simple H-bridge. I have been using this chip for controls, for example such as LV8806QA. Those motors don’t need encoders since the rotations are automatically counted using back EMF. The motor and the gears are very high quality and are made from metal.
Small stepper motors
Stepper motors come in very small sizes. Small stepper motors are used for applications such as autofocus lenses. They are sold in various shapes on amazon (1, 2, 3). They are good for controlled, low torque, and precise movements.
Shape memory alloy
Shape Memory Alloy (SMA) is a metal wire that contracts when it heats up.
Shape memory alloy servo
A Japanese company made a servo motor with shape memory alloy. The specs could be found here
Unlike normal servo motors, it is very thin (3mm) and light (1g) , but it’s not that small in the other dimensions.
Those are small servo motors, used for RC toys. It is probably as small as servo can get with the standard servo designs. We used those motors in the ChainFORM project. As they are made of plastic. the gears in those servo motors tended to occasionally break.
|Planetary gear motor
|6mm diameter, 14 mm length
|DC or PWM
|22 x 17 x 8 ,mm (2.3 g)
|Yes, back EMF
|Closed-loop Brushless Motor controller
|Hard to get
|Open-loop brushless motor controller
|Shape memory alloy (SMA)
|DC or PWM
|38 x 9 x 3 mm
|Hard to get