There are two main haptic feedback technologies. Eccentric Rotating Mass (ERM) and Resonant Linear Actuators (RLA). Fancy terms for pager motors and speaker coils. Well it is not exactly that simple. The RLA have a frequency that the prefer to work at while speaker coils are designed to work at a wide range of frequencies. 

We started with ERM actuators. They wabble around in a little circle as the motor tries to spin the off center weight. They are cheap but they are hard to control precisely. At low power the motors don't want to start so it is hard to get a large range of intensities out or them. They also take time to change power levels and tend to get warm at high power.

So we wanted to try LRA which oscillate up and down like they are bouncing. They start, stop and change amplitude quicker than the ERM devices do and they don't heat up. So we bought some.