Linear resonant actuators (LRA) are currently considered the gold standard in haptic technologies. They enable better haptic experiences than older technologies like the eccentric rotating mass (ERM) motor. They are cost-effective and used in small packages. What's not to like?
Like every technology life cycle, once a technology is mature, it's also in danger of being disrupted by newer and better-performing technologies. In haptics, piezo technologies are lining up to be that disruptive agent. It's particularly true in applications where LRAs clearly show their limits: wearable devices.
Let's see why smaller devices, like smartwatches and fitness trackers, highlights LRAs' performance limits and why they can even make your device feel cheaper.
The natural frequency, or resonant frequency, of a haptic actuator, is how the mass creates the highest acceleration values. LRA actuators (like their name suggest) operate almost exclusively at their natural frequency. Otherwise, the vibrations aren't strong enough to create compelling haptics. As a general rule of thumb, the heavier the mass is, the lower the natural frequency will be, and vice-versa.
Since space is limited in wearable devices, the LRA needs to be smaller and uses a smaller mass. Therefore, the resonant frequency is higher than what we are used to feeling in a smartphone. Otherwise, we wouldn't be able to feel much haptic feedback.
Why is using a higher frequency bad for haptics? The accelerometer shows higher acceleration values, so everything must be good, right? Because humans aren't accelerometers!
While accelerometers can help you find the actuator's resonant frequency, it doesn't mean that this is the frequency your user will prefer. The human skin uses different mechanoreceptors to enable the sense of touch. The mechanoreceptors responsible for sensing vibrations are called the Pacinian corpuscles. These sensors can detect vibrations approximatively between 40 to 800 Hz, with a peak of sensitivity between 200 and 300 Hz. Haptic feedback at this frequency can quickly become intrusive and annoying. Unfortunately, smaller LRAs are close to this highly sensitive frequency bandwidth.
Sound, just like haptics, is based on vibration. The difference is that we don't use the same part of our body to feel those vibrations. When designing haptics, you have to consider the sound your solution will generate to create the best possible experience. A lot of noise will detract the user from the tactile feedback. Plus, it won't be enjoyable. Remember how much sound the ERMs did in the first smartphones? Everybody close to you knew you were receiving notifications, even if you set your phone to "did not disturb."
The human ear can hear frequencies between 20 to 20kHz. Haptic actuators vibrating at 100 Hz are audible but much less than smaller LRAs and ERMs over 200 Hz. It's best to target a lower frequency to have a quiet haptic solution.
On the other hand, having lousy haptics can make your device feel cheaper. Using Apple as an example again, look at the quality of haptics coming out of the Apple Watch versus the other smartwatches. Their decision to use a much bigger actuator (and more significant mass to lower the resonant frequency) than the competition pays nice dividends in user experience. Other smartwatches don't have that premium and luxury feel when they vibrate.
Now we know that Apple's Taptic Engine is a custom design that is quite big and that it isn't available publicly, so what can other smartwatches and wearables OEM can do to improve haptics in their devices?
Our engineering team tackled this challenge: How can you improve the haptic performance of an actuator small enough to fit in a wearable? The solution is the Boréas Piezo Haptic Engine (PHE)!
The Boréas PHE uses an off-the-shelf piezo actuator and uses the device's battery as a moving mass to reduces the required natural frequency without using a bigger actuator. It also generates higher acceleration (stronger haptics) while enabling much richer HD haptics thanks to faster rise and fall times. Plus, using the BOS1901 piezo driver, the power consumption is significantly reduced. Please visit this page to learn more about the Boréas Piezo Haptic Engine.
The key takeaway: piezo haptics makes your device feel premium. It represents a significant opportunity for wearables OEMs; you could be the one that seizes the opportunity and sets the industry standard.
Comparing haptics is hard to do with only specifications. We created multiple fitness tracker demonstrators to showcase our new Boréas Piezo Haptic Engine. Let us know if you'd like to try one by sending your request to info@boreas.ca