The Linear Resonant Actuator (LRA)

Though most vibrotactile feedback provided in devices is produced using an eccentric rotating mass (ERM) motor, a new type of actuator has emerged as a more effective alternative to ERM actuators. The linear resonant actuator, or LRA, consists of a voice coil pressed against a moving mass on a spring. The voice coil produces a vibration according to the frequency and amplitude of the electrical signal provided to it through two electrical terminals. When the frequency of vibration approaches the resonant frequency of the spring contained within the actuator, the device produces a crisp and powerful vibration.

Mechanically, the only part of the LRA that experiences wear over time is the spring – since the spring is operated within its non-fatigue zone, an LRA will have up to five times the lifespan of a similar-size ERM. An LRA is effectively brushless. Unlike an ERM, the LRA can only be driven by an AC signal that can often be more complicated than the simpler DC circuit to drive an ERM, but this AC signal also allows for direct control over the frequency and amplitude of vibration produced by the actuator.

Benefits of LRA-type Actuators

  • Lower power consumption (up to 50% less current draw)
  • Greater force (up to 2x)
  • Frequency and amplitude may be independently controlled
  • Start time as low as 5ms
  • Stop time with braking as low as 10ms
  • Vibrations produced vertically (Y-axis)

Benefits of ERM-type Actuators

  • No need for a complicated driver chip (can be driven by a transistor, H-bridge, or DC source)
  • Less expensive (5-10x less expensive depending on purchasing source)
  • Ubiquitous and found in most modern vibrotactile interfaces
  • Useful for low-fidelity effects (start-stop time on the order of 30-50ms)
  • Vibrations produced laterally (X-axis)

Precision Micro Drives has a more detailed analysis of linear resonant actuator performance. Likewise, Texas Instruments provides information about their SmartLoop Architecture for driving haptic actuators.

Author: Shantanu Bala

Shantanu Bala graduated from Arizona State University in 2014 with a double B.S. in Computer Science and Psychology.

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