JamesMNewton

The encoder is my very favorite part of the robot. I've written a page about how it works in detail: https://github.com/HaddingtonDynamics/Dexter/wiki/Encoders

In summary, it's a hybrid of digital and analog. To the standard quadrature encoder slit counter, we add sensing of the light levels as each slit covers or obscures the phototector. So we get e.g. 200 slits times 8096 light levels inside each slit equals over a million positions. It's so much data that we need an FPGA to process it fast enough. You can breath on the arm and it can sense it.

There is a constant battle between writing for sales/investment and writing for technical disclosure. Sometimes we get some chocolate in the peanut butter. It's certainly not phony, and I'm happy to answer any questions that anyone has about the real technical abilities.

The $2,999 price was for the Dexter 1 which was available at the time the article was written. The current generation is the Dexter HD which is more capable, much easier to build, and (as you noticed) more expensive.

The encoder / motor controller logic is a bit more complex than that. Because the encoders are analog / digital hybrid they generate far more data than standard quadrature encoders. The FPGA is doing huge table lookups, ATAN2, PID, Dithering the motor driver, and all of that at 5 MHz end to end, so we have a 200ns response.

Encoders at the joint only. Motors are steppers, so we have some idea where they are but that is open loop.

1. The encoders on the Dexter arm are at the joints because that gives us the ability to compensate for backlash in the drive system.

2. The motor dance issue happens because the feedback loop on most control systems isn't fast enough. With the FPGA, our control loop is 5 MHz / 200nS so we can correct before an oscillation has time to happen.

3. Our encoders give more than 1 million CPR (clicks per revolution) so we can sense an error before it has time to become an issue that might lead to oscillation.

(2 and 3) are the real magic of the arms design

4. Yes, Harmonic drives are hell. And they are not actually free of backlash, it's just "soft" e.g. they bend slightly. At this point, they are the best solutions, but we have prototypes of a cycloidal drive that costs less (in quantity) and gives us better performance. There are issues to work out, especially related to smoothness and wear, but we are making progress on it.