Haptic Motor Driver Hook-Up Guide
ERM and LRA Motors
The DRV2605L is capable to driving two different types of motors. So what are they? How do they work? How are they different?
Precision Microdrives published application notes on using both Eccentric Rotating Mass, ERM and Linear Resonant Actuator, LRA type motors. The default firmware for the DRV2605L is set for use with ERM type motors. There are six effects libraries for the ERM type and only one for LRA.
The difference between the two motors is how the movement of a mass is displaced. LRA vibration motors require an AC signal, driving a sine waveform that is modulated to get multiple effects. ERM vibration motors use a DC motor with a counter weight attached. The DC voltage controls the speed of the motor.
The ERM has an off-centre load, when it rotates the centripetal force causes the motor to move. The rotation is created by applying a current to the armature windings attached to the motor shaft. As these are inside a magnetic field created by the permanent magnets on the inside of the motor’s body, a force is created causing the shaft to rotate. To ensure the rotation continues in the same direction, the current in the windings is reversed. This is achieved by using static metal brushes at the motor terminals, which connect to a commutator that rotates with the shaft and windings. The different segments of the commutator connect to the brushes during rotation and the current is reversed, maintaining the direction of rotation.
In a similar method, LRAs use magnetic fields and electrical currents to create a force. One major difference is the voice coil (equivalent of the armature windings) remains stationary and the magnetic mass moves instead. The mass is also attached to a spring, which helps it return to the centre. Driving the magnetic mass up and down causes the displacement of the LRA and thereby the vibration force.1
More Suggested Reading
- ERM Vibration Motors
- LRA Vibration Motors
- Solutions for Haptics
- How Devices Provide Haptic Feedback
1: "AB-020 : UNDERSTANDING LINEAR RESONANT ACTUATOR CHARACTERISTICS." Application Note. Https://www.precisionmicrodrives.com. N.p., n.d. Web. 29 Nov. 2016.