A servo is really perfect for an application like that. You can set up the servo drive in torque mode where it will keep a specific tension on the feed spool as the coil takes up the wire. Another servo controls the mandrel in speed mode and is electronically geared to another servo that controls the laydown of the wire. You would use a pulse output of the drives encoder out trigger the step input on the layout motor which would be in position mode. Adjustable limit switches at the ends of the travel would flip-flop the direction input reversing the direction at the end of the pass. The laydown motor's built in electronic gearing function would control the pitch of the laydown. The index pulse out of the mandrel motor could be connected to a count down to stop at a pre-determined wind count. Other than the electronic gearing control the only other control you would need is a pot connected to the analog input on the mandrel motor and two more pots on the the reel motor to control torque and max speed. I bet you could reliably wind up to a couple thousand RPM even with very fine wire.
This could all be done with either 50 or 100watt motors off ebay. I would recommend the Mitsubishi MR-J2S-10A1 drives with 50 or 100 watt KFS series motors. They have 131,072 count encoders on the motors. They are going pretty cheap on ebay. The newest J3 motors have 262144 count encoders.
Back to the lathe:
The idea of adding constant surface speed is crossing my mind. Maybe using a quadrature to step/dir IC fed into a digital potentiometer IC to an op-amp might do what I want. Not sure.
Or maybe do all of it in an arduino. Use the analog out to control the drive. Its only 10 bits out but that is still about 2-1/2 rpm resolution. Stick a little control panel/readout on the DRO.
Finished the last part of the puzzle tonight. When the spindle stops it says servo locked, that is you can turn the chuck by hand and any attempt will be countered with resistance, it pushes back. This isn't too big of a deal but it sucks when you are trying to get to the chuck key socket.
On the drive there is a "Servo On" input. This is pulled low to activate the servo. I used two Omron H3FA time delay relays to turn off this input when the fwd/rev handle is in neutral. The reset of each relay is tied to the forward and reverse terminals through a small signal diode to prevent the relay itself from activating the drives forward and reverse optoisolators. The two NC contacts on the relays are in parallel and in series with the servo on input to ground.
When power is turned on the reset lines of the relays float and the timer's relay closes disabling the servo. When forward or reverse is selected the reset line is pulled closes as well as the direction input of the drive, the relay closes activating the servo and the motor runs in that direction. When the control is returned to off the reset line goes high and the timer kicks in and holds the contact closed for 1 second while the drive decelerates to a stop and then opens disabling the drive. You can plug reverse without any problems since the relay's outputs are in parallel.
I also found the drive's fans were not operating. Looks like the internal 24v power supply is dead. Looks to have always been that way as there is zero dust in the fan blades or on the heat sink. I routed 24v to the fans to keep the heat sink and breaking resistor cool.
Servo lock time out board by
macona, on Flickr