When we build actuators in the Leg Lab, every actuator had some electrical feedback element. For example, leg length actuator had a resistive strip that was as long as the leg's travel. A fixed 'wiper' allowed us to therefore have a potentiometer, giving a leg-length-proportional voltage on the wiper when top and bottom were driven by a constant current source. This was used in a 500Hz (sometimes 1 kHz) servo loop for position control.
(somewhat related): We also had a velocity sensor on the legs -- a simple coil of wire and a magnet (but COTS). This gave us a Velocity signal directly -- rather than differentiating the position measurements. Velocity estimates made by differencing position measurements tend to be noisy, and filtering esp FIR filters introduces phase delay -- things you do not want. Lesson was: If at all possible, get a sensor that senses the quantity you want to sense, so you do not have to integrate or differentiate to get your desired quantity. It's the same reason we had a REAL gyroscope, and not a rate gyro (as in most MEMs IMUs -- in IMUs they call them 'gyroscopes' but they are actually 'rate gyroscopes' -- they only directly measure "theta-dot").
> Velocity estimates made by differencing position measurements tend to be noisy, and filtering esp FIR filters introduces phase delay
Yes. Which is why you see encoders for sale with a zillion points per rev. Not because the position is that precise, but so that you get enough events per second to calculate velocity. Actual tachometers, which are little DC generators, are sometimes used in servo systems, but not often. The last time I saw one, it was on a mainframe tape drive.
You do want velocity and force feedback if you're doing anything beyond pick and place in a totally controlled environment. Simple preprogrammed blind movement is most of industrial robotics, though. There are fancier systems, but most of them don't sell, because the engineering cost exceeds the value add. Sometimes you see a little force feedback to get something inserted properly, but more often you see some mechanical spring setup to get the same result.
I used to be into legged locomotion, but there's no profitable market. Many of the technical problems have been solved, though. The sensors and actuators are good enough. Electric motors finally are strong enough. (It's sad that Schaft went under. Google bought them and dumped them.)
