ERM (Eccentric Rotating Mass) motors are a form of vibration motors that work by spinning an off-center mass. The radial forces that are generated from this movement cause the motor to oscillate back and forth, resulting in vibration. ERM motors are very affordable options for generating simple vibrations.

There are three main types of ERM motors: Shafted motors, encapsulated cylindrical motors, and shaftless motors.

Shafted motors are ERM motors for which the eccentric mass is visible. When using shafted motors, it is important to ensure that nothing comes in contact with the eccentric mass while it is spinning. If this does occur, it could result in a large jerk instead of a smooth vibration, in addition to damaging whatever came in contact with the eccentric mass..

Encapsulated cylindrical motors are similar to shafted motors, but they cover the rotating shaft in order to prevent interference of outside objects with the eccentric mass.

Shaftless motors, also known as “coin” or “pancake” motors, are thin coin-shaped motors in which the eccentric mass is enclosed. Because these motors can be much smaller than the other shafted motors, they are often used for wearable haptic displays.

ERM motors are fairly simple, reliable, and affordable, making them popular choices for haptic displays. The actual motor of an ERM motor is a DC motor, which makes it very easy to connect ERM motors to microcontrollers such as Arduinos.

The biggest disadvantage of ERM motors is that the frequency and amplitude are linked together. If the motor is spinning slowly, it will have a low frequency and amplitude, and if the motor is spinning quickly, it will have a high frequency and amplitude. For applications in which it is important for the frequency and amplitude to be independently set, ERM motors are not suitable choices.

There are many inexpensive ERM motors available on DigiKey. Three examples are shown below.

Shafted motor: https://www.digikey.ca/en/products/detail/vybronics-inc/VJP16-70E310/6009932

Encapsulated motor: https://www.digikey.ca/en/products/detail/vybronics-inc/VZ6DL2B0055211/6009925

Shaftless motor: https://www.digikey.ca/en/products/detail/seeed-technology-co-ltd/316040001/5487672

The above motors work as expected, but can sometimes have noticeable variations between identical motors. Higher quality ERM motors with more consistent motor characteristics are produced by companies such as Precision Microdrives. To see a catalogue of their available ERM motors, visit https://catalogue.precisionmicrodrives.com/product-catalogue/vibration-motor.

Paper by Choi https://www.precisionmicrodrives.com/ab-004