Audio to Haptics

Now, it’s common to use an audio signal to drive a haptic actuator and produce tactile effects that correspond to sounds. The Apple Watch and Taptic engine use this technique to render haptic feedback.

In general, the best approach for most consumer electronics devices involves the implementation of a haptic driver capable of consuming audio signals as input. The DRV2605 from Texas Instruments can provide this form of feedback with an eccentric rotating mass motor or linear resonant actuator.

Another option, which is less suited for most portable electronic devices, is to use a surface transducer. A surface transducer will frequently be used in speakers to produce vibrations in an enclosure that result in sound. Unfortunately, surface transducers consume a lot of power, but they are able to produce vibrations that propagate over a hard surface very quickly and consistently.

The Lilypad Vibe Board

The Lilypad Vibe Board is an excellent way to quickly integrate haptic effects into a wearable project. It uses an eccentric rotating mass (ERM) motor and can be driven directly from a general purpose IO pin from an Arduino board. It relies on 5-volt logic and places a 33-ohm resistor in series with the DC motor to reduce the current draw below the 40ma maximum that can be drawn from an output pin of an Arduino board. It also contains a protective diode to prevent damage to a connected IC.

Although the Lilypad Vibe Board can be integrated very quickly into a project and uses a well-designed circular PCB with sewable and solderable pads, the board cannot be used to produce advanced haptic effects easily. Because its current draw is limited to the output of an Arduino output pin, its vibration does not reach the rated maximum of the motor. Likewise, it contains no pre-programmed effects, and the reduced power consumption also reduces the effectiveness of pulse-width modulation for creating custom effects. We recommend using it for projects and prototypes that require simple alerts – its ease of integration makes it very useful when advanced effects aren’t necessary.

DRV2605 Breakout Board from Adafruit

Adafruit’s DRV2605 Breakout Board is an excellent way to get started experimenting with haptic effects. It not only works really well in a breadboard prototype but has been a quick way to hack together haptic prototypes with an Arduino without creating a custom PCB. This allows you to quickly integrate haptic effects using an ERM or LRA into your electronics projects!

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.