Dexter droops, loses position, zaxis error.

[JamesNewton]

While working with Dexter, you might notice problems with it not being at the correct height when reaching down to the surface, or "drooping" or not lifting when commanded. Most of this is caused by problems with Joint 2 (the main "shoulder" joint) or Joint 3 (the "elbow"), but we've seen several things that cause this problem so we are documenting all the possible causes here:

1. The stepper motor shafts on Joints 2 and 3 can easily slip inside the 3D printed adapter from the motor shaft to the inside of the "harmonic drive". Since the motor shaft is what holds the wave generator in, when the shaft comes loose, the generator can fall out.* To fix: Carefully remove the drive or it's wave generator* and adapter and then mung (MUtilate, Nurl, Gorge) the motor shaft with small end cutters or other available tool. Apply a small amount of a high end adhesive to the inside of the adapter (the DP420 Epoxy is recommended, but the metalized JB weld, #8276 has been used) and carefully slide it back in. Take care not to push it too far in as that will cause binding. Let cure before moving.

Note: Later model (2018) Dexters have a motor with "D" shaft which has a flat on one side and is longer to avoid this issue

2. The adapter can also come loose from the wave generator. This is fairly rare, but can happen especially if there is an alignment problem. This, again, causes the joint to lose traction and/or the wave generator to fall out.* In this case, the solution is a bit of low viscosity super glue carefully applied between the adapter and inner hub of the wave generator without disassembly or after re-assembly. Be very careful to avoid getting any glue in the wave generator. Let sit.

*Be very careful re-inserting the wave generator. It is best to try slightly different positions until you find one that slips in easily. However, it can be accidentally inserted such that the teeth are not evenly divided between the two nodes of the wave generator. In that case, the joint will jam up and you won't be able to "back drive" it. To fix it, you should be able to manually spin the inner motor shaft until the teeth pop over into the correct position. At that point, the joint may start working correctly again. Or the wave generator may be damaged.

3. The harmonic drives will bind or jam if they are not perfectly aligned. Loosening, moving and tightening the mounts can help. Also manually rotating the inner shaft / adapter / wave generator can work out the "kinks"

4. Stepper motor driver weakness. In extreme heat, especially when the motor board has been modified to increase the stepper drive, the driver chip can overheat and go into thermal shutdown to prevent burn out. This shows up as a "pulsing" where the joint grips, then sags at a rate of about once per second. Blowing on the chip will temporarily stop the issue. Note that the slipping issues mentioned above can present as a weak stepper motor, where a little "help" will resolve the issue; but this is almost certainly because the joint is slipping and not due to a motor or driver problem. When the driver shuts down, it cycles on and off and is not just steadily weaker. Also, a stepper motor which is losing steps makes a distinctive vibrating noise; it is not quiet, and the slipping is usually very quiet. To fix:
   
   - Add small heatsinks to the driver chips with thermal adhesive. Take care not to short the surface mount components around the driver chips.
   
   - Add a small fan directly over the motor drivers.
   
   - Reduce the drive current back to the original spec's if the motor board was modified to increase it.

Note: Due to the fact that the Joint 2 (and 3) motors are buried inside the 3D printed housings, it is a really bad idea to get them very hot, especially if the housings were printed in PLA or other plastic which can deform when heated. Although the structure of the robot is all the stronger materials, some motor mounts, shaft adapters, etc... are plastic and a loss of rigidity can cause slipping, alignment, and binding issues.

5. Intermittent Z axis position creap. Some users have notice and reported that even a fully functional arm can occasionally drift on the Z axis. E.g. a job that was working at the correct height above a surface, will suddenly move to a slightly different height. Adjusting the job script to compensate results in many more cycles at the correct height, but then suddenly it will move to the wrong height again. We have not been able to reproduce this under test conditions and so are not sure what might cause it. If you can reliably reproduce this problem, please contact us. It is important to eliminate all of the issues mentioned previously before arriving at this diagnosis.