Eccentric rotating mass motors (ERM) and linear resonant actuators (LRA) are commonly used in smartphone and tablet applications to provide tactile feedback through haptic effects. While there are many characteristics to consider when designing for haptic feel, one of the most noticeable traits to users is the start and stop time of the actuator.
The start time of an actuator is the time it takes to go from 0% (or driver off) to 90% of the maximum acceleration. Likewise, the stop time is the time it takes for the actuator to go from when the driver waveform ends (or driver turns off) to 10% of the maximum.
| Figure 1: Start Time | Figure 2: Stop Time |
This start and stop time translates to a qualitative feel that a user will identify as “sharpness” or “crispness.”
The start time is analogous to a car’s “0-60” time. Let’s take two cars, one is a fast sports car and the other is an inexpensive compact. Both cars are stopped at a red light. When the light turns green, both cars slam the accelerator to the floor and begin moving. The sports car has a sharp burst of speed and quickly leaves the compact in the dust. Meanwhile, the compact is only halfway across the starting line and is far from reaching full speed.
Figure 3: Overdrive and braking is analogous to 0-60 speed times.
Likewise, some actuators will have very quick start times and others will have very slow start times. It depends on the design, manufacturing and type of actuator, all of which can be characterized empirically in the lab.
For haptics in touch screen smartphones, users may notice a keyboard click is sharper in one phone compared to another. This is due to the response time of the actuator.
To improve the actuator performance, the actuator driver can overdrive it to obtain a quicker start time and reverse drive for a quicker stop time. For ERMs, overdriving is achieved by applying a higher DC voltage at the beginning, and applying a negative voltage to brake.
Figure 4: ERM Overdrive and Braking Drive Waveform
For LRAs, overdriving is achieved by applying a higher AC voltage at the beginning, and applying a 180 degree out-of-phase signal to brake.
Figure 5: LRA Overdrive and Braking Drive Waveform
TI’s new DRV2605 haptic driver has a feature called “smart loop,” which uses closed loop feedback to apply the exact overdrive and brake signal to maximize the start and stop time of an actuator. Smart loop does this by actively monitoring the electromotive force (back-EMF) signal of the actuator to accurately control the drive voltage and acceleration.
The result is automatic overdriving and braking for ERMs and LRAs that simplifies software programming and reduces startup and braking time by 50 percent. And the DRV2605’s automatic actuator diagnostics and level tracking feature delivers consistent acceleration over a wide range of environmental conditions. If you want to get from 0-60 faster than the competition, take the DRV2605 out for a test drive. You’ll win, every time!
