Unwinding Rotary Axis for Homing

Hi All, 

Just working on finishing up a 5/6 axis router build and I'm trying to wrap my head around homing the C axis on the head.  I'm aiming to have 720deg of rotation, maybe more.  What I'd like to do is automatically unwind the head during homing, so I don't wind up the cables over time.  So the controller would store the last position (it's really difficult to back drive so I'm not worried about it moving much while the machine is off),  then move in the the correct direction to get back to 0deg (not just the shortest way) and then start the homing process using a switch and encoder index pulse ,this part is no problem.  I had a few ideas on how to accomplish this, but I was wondering if there's some easy solution I'm overlooking.   

Your project is very interesting. Where did you get the spindle mount?

The spindle mount is just something I came up with and made.  Right now it fits a cheap air cooled 80mm 2.2kw spindle, but I can fit a 100mm diameter spindle if I need to.  I machined a hole in the side of the spindle to feed the wiring out of and through the center of the side bearing.  I hope that will save me from tangling up the wiring.   

Could you share your 3D CAD model? If not, it's not a problem.

It might be better to use an absolute encoder on the C axis in your case. I also have a 5-axis head/head design with ±185° of rotation, and the unwinding is handled by the postprocessor in CAM.

Looking at your design, did you integrate harmonic drives on these two rotary axes?

Here's the file for the head assembly. It's a fusion archive, so hopefully it included all the components properly. If you want a different format just let me know.

drive.google.com/file/d/1jZTfEVjwmL9wT5n...b-K/view?usp=sharing

I've considered picking up an absolute encoder for it. I already had the servos, so I was just trying to use them up. Each axis has a 100:1 harmonic reducer that's belt driven. The belts made the package a bit smaller and allowed me to route the cables through the center of the gear reducers. I'm not concerned about unwinding during a program. I just don't want to shut down the machine at say 400deg and then add another 320deg by always homing in the same direction.

I've got a half baked solution in mind. A while back I made a tool changer and spindle homing procedure from the home all button by calling a halui mdi command and then using g code and I think I might be able to do something similar for this.

Few ways of doing this.

The way I handled it on a 4th axis plasma build was with a magnet on the rotary and a hall effect sensor sensor to read it. Then sending the rotary back to 0 at the end of the gcode via the post processor.

It's not fool proof (power failures) but it works well enough as I just drive past the home switch then home again the one in 1000 times I have a power failure during running

Could use a 3d printed gear reducer to get a 1:4 rotation and put the sensor on that....so 720 turns into 180.