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.

Eccentric Rotating Mass (ERM) Actuators

The eccentric rotating mass (ERM) motor is one of the most common types of haptic actuators. Since the late 1990’s, several video game controllers, cell phones, and sex toys have incorporated this actuator into their enclosures to produce vibrotactile effects. Though several of the initial applications of ERM motors were very simple in nature, recent developments have allowed hardware designers to create rich and diverse haptic effects using the technology.

An eccentric rotating mass motor behaves like a regular DC motor – it converts the flow of electrical current into a mechanical force that rotates the motor. Unlike most DC motors, the ERM motor also contains an off-center mass. Because the rotation of the off-center mass produces an asymmetric centripetal force, a non-zero centrifugal force is produced when the motor rotates. As long as the motor runs at a high number of rotations per minute, the consistent displacement of the force produces a perceivable lateral vibration.

Precision Micro Drives has filmed a video of an eccentric rotating mass motor with a high-speed camera. The video demonstrates the operating of the motor in different modes.

The primary benefit of using an ERM motor as an actuator is its simplicity: driving the motor requires providing power through a DC source capable of supplying a voltage within the motor’s specified range. It is important to note that the frequency and amplitude of vibration are both dependent on the voltage supplied to the motor: the frequency increases proportionally with the voltage, and the amplitude increases as a square of the voltage. The current draw of the motor will be proportional to the torque load produced by the rotation of the eccentric mass.

Often, a transistor or Darlington pair will be used to drive the motor from a microcontroller or processing unit that cannot supply enough current to drive the motor directly from its pins. For more control, an H-bridge from transistors or MOSFET’s can be used to quickly reverse the polarity of the motor and decrease its stop time by providing a “braking” voltage in the opposite direction of the driving voltage for a brief period of time. Pulse-width modulation (PWM) allows for rough control over the perceived intensity of the vibration when driving the motor using a transistor or H-bridge circuit. Now, more complex driver chips with sophisticated auto-braking and start-stop optimization provide a simplified interface to drive and control the intensity of vibration. The DRV2605 from Texas Instruments is an example of a recent chip capable of producing hundreds of built-in haptic effects from the Immersion Corporation while also providing more accurate PWM-based intensity control than an ordinary H-bridge driver chip.

Eccentric rotating mass motors allow quick integration of haptic effects into almost any consumer device, and the technology has become even more useful with the development of more advanced driver chips and ready-made haptic effects.