2 ms·
The reason encoders go on the motor and not at the far end of the gears is because inexpensive gearing has backlash, which will cause the motor to dance back an
by joshu 8y ago
The reason encoders go on the motor and not at the far end of the gears is because inexpensive gearing has backlash, which will cause the motor to dance back and forth to keep the encoder at a specific point.
So how are they solving the gearing problem? Harmonic drives are expensive and patented.
- cbames89 8y agoI have a Dexter. They have harmonic drives from China which were deemed out of spec for normal usecases but are fine for the 3kg realm. Their latest version is running a custom cycloidal gear system.
- JamesMNewton 8y ago1. 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.