A Sensitive Kind of Motor
Linear resonant actuators, typically used to make your phone buzz, can be repurposed to also work as touch-sensitive input devices.
Reducing the part count of a device saves on cost, size, and power consumption — all of which are particularly important considerations in the mobile world. Nearly all consumer targeted gadgets have some form of input and output built in, so this is a natural area to target for simplification.
A group at Google Research took a fresh look at linear resonant actuators — the little motors that make your phone buzz — with the goal of repurposing them as input sensors, in addition to the traditional output applications they are typically used for.
To accomplish this, they took advantage of a property of electric motors called back-EMF. In summary, back-EMF voltage increases as the speed of the motor increases, and a stationary motor produces zero back-EMF voltage. When a spinning motor is touched, the velocity of the inside mass changes as energy is dissipated into the contacting surface.
An off-the-shelf linear resonant actuator will produce the back-EMF voltage needed to sense touches, however, they do not contain the circuitry needed to measure that back-EMF. The researchers built a relatively simple circuit that can be built around an existing linear resonant actuator to retrieve this data. The circuit consists of an audio amplifier and a pair of switches that feed data into a Teensy 4.0 microcontroller development board (featuring an Arm Cortex-M7 microcontroller) for processing.
New measurements are stored in a buffer and are compared with previous back-EMF voltage measurements. Applying a custom algorithm to this data, the team was able to detect touching, tapping, and pressure levels applied.
Running at a fraction of a milliamp, this new IO method is very mobile-friendly, and will have minimal impacts on battery life.
In addition to sensing active input (e.g. touch, tap), the device is also capable of sensing information about its environment. For example, a smartphone equipped with the device can tell if it is being held, resting on a solid surface, or is on top of a piece of foam.
Linear resonant actuators do tend to be a bit on the larger side, however, the group is exploring tiny piezoelectric actuators as an alternative for a future device. They also suggest that extending the sensing beyond back-EMF voltage, to also look at frequency sweep and phase differences, could improve sensing further.