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Linear Resonant Actuators - LRAs

Within our Precision Haptic™ range of vibration motors, we offer several types of Linear Resonant Actuator (LRA) vibration motors (also known as linear vibrators). LRA motors are different to Eccentric Rotating Mass (ERM) vibration motors in the way that they work, they are used, and how long they last.

If you have an application that requires a durable vibration, or a high confidence MTTF rating, consider our Linear Resonant Actuator vibration motors as an alternative to long life brushless vibration motors. But, understand that that longer life and control does come at a slightly increased cost of complexity - continue reading to find out more.

General Construction for a Y-axis LRA Linear Vibrator

An exploded diagram of an LRA, with the internal magnetic mass with voice coil attached to a wave spring. The moving mass has a neodymium magnet
Precision Microdrives Y-axis LRA Linear Vibrator Motor

The illustration above shows the general arrangement of parts within a Y-axis LRA vibration motor. Those readers familiar with audio engineering will note that the voice coil drive is very similar to that loud speaker. However, instead of a cone that generates sound pressure waves, there is a mass that generates vibrations.

Below is another LRA which works in the same way, however the vibrations are directed in only the Z-axis. This offers users a greater choice in design as they can produce vibrations in either horizontal or vertical directions.

An exploded diagram of Z axis linear resonant actuator, where the springs are at the sides of the cases so the square magnetic mass moves horizontally
Precision Microdrives Z-axis LRA Linear Vibrator Motor

A magnetic field is generated by the voice coil which interacts with the magnet mass, which are suspended on a spring. As the magnetic field varies with the applied drive signal, the magnet and mass are moved up and down as they interact with the spring.

Resonant Frequency of Operation

A graph with the X axis showing frequency, the Y axis showing vibration amplitude - the line stays at 0 on the x axis until 145Hz where it sharply rises to a peak at 175Hz at returns to 0 at 205Hz
Linear Vibrator LRA Resonant Frequency Bode Plot

Again, those readers familiar with vibration, RF, or audio engineering, will quickly spot that attaching a mass to a spring causes a resonance effect. The combination of spring stiffness, mass and magnet / coil size will cause the linear vibrator to have a natural resonant frequency.

This natural resonant frequency is where the LRA is most efficient in its operation, as can be seen by the output amplitude vs frequency on the bode plot above for our C10-100 Linear Resonant Actuator.

Typically for Y-axis vibrators resonant frequencies are around 175 - 235 Hz, see our range here.

Driving Linear Resonant Actuators / Linear Vibrators

LRA linear vibrators needs to be driven with an AC signal, much like a loud speaker. It is preferable that this signal is a sinewave at the resonant frequency as shown below.

An oscilliscope graph showing a cyclical wave with the voltage amplitude of 2.06V and a frequency of 174.8 Hz
Linear Vibrator Driving Waveform

The driving waveform can of course be amplitude modulated to generate more advanced haptic feedback effects.

Take a read of our application bulletin about driving LRA vibration motors to learn more.

Extended Lifetime for Linear Vibrators

Unlike most vibration motors which have a electromechanical commutation, LRA vibration motors are effectively brushless as they use a voice-coil to drive the mass. This means that the only moving parts that are prone to failure are the springs. These springs are modelled with finite element analysis (FEA) and are operated within their non-fatigue zone.

With little mechanical wear, failure modes are restricted to aging of internal components which results in much longer MTTF failure modes than traditional brushed eccentric rotating mass (ERM) vibration motors.

We also sell an increasing range of brushless ERM motors which offer long-life vibration in two planes rather the single plane offered by the LRA motors.