I rented out Tesla S recently with my friends to try out - doing SF to Monterey and back.

When I was driving, holding hands on a wheel all the time, the car kept giving me notices to put hands back on the wheel. After the third time it refused to reengage the autopilot.

I guess my grip wasn’t strong enough for it to notice.

On the way back my friend did a few short stretches letting go of the wheel completely (still fully alert and hands an inch away), and the car failed to notice.

It's based on wheel torque, not grip or anything. People have bypassed the check by wedging an orange in one of the gaps in the steering wheel (so its weight applies a constant torque to the wheel).

(This based on /r/teslamotors so it could be out of date or wrong. >.> )

The orange seems to be an urban legend, there are several Youtube videos showing it not working. Perhaps it used to. tesla have also recently update the software so that it requires more frequent applications of turning force. For a while after that the car kept disabling Autosteer because kit thought I wasn't attentive enough.

One of my friends has a Model S and attaches a wrist weight to the wheel when he's on long interstate trips.

I hate when I see replies from Tesla stating that so-and-so didn't have their hands on the wheel for X number of seconds/minutes. It's bollocks. As a model S owner, for a few weeks I didn't realise it was sensing turn pressure, rather than grip pressure. So I had numerous warnings about my hands not being detected, even though they were constantly on the steering wheel.

True, but usually just the weight of your arms applied on the hand is enough to issue micro-turns to the wheel.

I believe the detection relies on torque + angular position (the model S manual says it will sense resistance to wheel motion as well as motion itself), and the latter should be more more sensible because those mechanisms are usually made of encoder-motor-high ratio transmission-load, so a small turn on the load gets multiplied 10-100 times by the transmission before being sensed by the encoder.

I guess the lack of precision comes from Tesla wanting to avoid false positives

So the sensor is completely useless on a straight road.

Nah. Autosteer is constantly applying counter-torque unless you exceed the takeover limit, deactivating autosteer. Someone calculated the required force and it's something like 2.5 lbs. My commute has lots of straight freeway and resting my hand on the wheel works fine.

Suboptimal for sure.

There's a tiny in-cabin camera above the rear view mirror.