Here’s an app note from Precision Microdrives on different techniques of motor vibration that can be used as haptic feedback. Link can be found here
Advanced signal waveforms are predominantly used in haptic feedback, but in fact the effects of simple vibration alerting can be greatly improved with relative ease. With vibration alerting the output waveform need not be restricted to just a continuous vibration, we will cover three simple techniques that can be used to extend the capability of a vibration alerting system, without implementing fully a haptic feedback solution.