The watch taps your wrist, and touch becomes a channel
The part of the Apple Watch announcement I keep thinking about is a small mass on a spring, and what it says about how little computers use touch.
Most of the Apple Watch coverage last week was about the crown, the gold edition, and whether anyone needs a smaller iPhone on their wrist. I’ve been thinking more about the Taptic Engine, the small linear actuator that taps you.
Apple says turn-by-turn directions will tap differently for left and right turns, so you can walk around without looking at the screen. With “Digital Touch” you can send someone a tap or your heartbeat. The screen also senses how hard you press, separately from where. In the keynote it came across as a gimmick, but phones have only ever used vibration as an alarm, and this is the first mass-market device I know of that uses touch to carry information.
Phones buzz with an eccentric rotating mass, a small motor spinning an off-center weight. It’s cheap and strong and also sluggish, since the motor needs time to spin up and down, so every buzz has a soft start and a tail and about the only thing you control is duration. A linear actuator is a mass on a spring driven back and forth by an electromagnet, textbook stuff. Drive it at resonance and it moves a lot, brake it and it stops fast, so you get short, sharp pulses and can make several distinguishable kinds. You need that before touch can be anything like a vocabulary.
Skin has four main kinds of mechanoreceptors. Merkel cells handle sustained pressure and edges, Meissner corpuscles light touch and low-frequency flutter (a few tens of Hz, useful for noticing slip), Pacinian corpuscles high-frequency vibration, most sensitive around 250 Hz, which is how you feel texture, and Ruffini endings skin stretch. They’re spread very unevenly. The palm side of the hand has something like 17,000. A fingertip can separate two points a couple of millimeters apart, while on the forearm they need to be several centimeters apart. So the wrist is bad at detail, and Apple sensibly sticks to simple patterns. The upside of the wrist is that a tap there is private and needs no looking, and nobody next to you knows you got it. I can’t think of a screen or sound notification that works that way.
The force-sensing screen matters too. Right now a touch is an on/off event at a coordinate. Add force and every touch carries a continuous value as well. I’d bet this reaches phones within a couple of years, and that haptics becomes something designers actually work on once they have hardware that can do more than buzz.
The part I care about most is robots, though. At the DARPA Robotics Challenge trials last December, some of the most advanced robots around took minutes to turn a valve or open a door. They work mostly by sight: look at the handle, plan, move, look again. People open doors largely by feel, noticing the latch catch and the weight of the door as it starts to swing. Most robot hands are grippers with maybe a force sensor at the wrist, which is like tying your shoes in oven mitts while watching your hands on a monitor.
Vision is having a great couple of years, and networks are getting close to people at recognizing objects and faces. But whether something is heavy, slippery or stuck is mostly something you feel. I suspect a lot of physical intuition comes from years of grabbing things as a kid, and I doubt robots get it from cameras alone. My guess is they’ll need tactile skin on their hands and a lot of data about how things feel.