Simple 4th axis

To switch a stepper between position and speed control, the simplest way is probably to run the stepgen in velocity mode and run two different PID controllers. One has input and feedback in position, and the other velocity, but both output a velocity command. (to achieve this I would anticipate the position PID being strong on P and with a bit of I, and the velocity one being mainly FF0 and I term)

RoberCNC wrote:

Thank you very much Andy, you will forgive my ignorance but I do not understand exactly what you are saying, would you be so kind to explain more in detail?

Possibly because I over-complicated it. 
You do not need a velocity PID for velocity control of a velocity stepgen, you can run that part open-loop. 

You will need a PID for position mode, though. 

So, configure the stepgen as velocity-mode. 
Connect a mux2 to choose between velocity command and the output of the position PID. 
The position PID takes position (C axis?) as input and stepgen position as feedback. 
Use some logic to simultaneously switch the mux and enable the PID when you want position control. 

You might need something a bit clever if you want to switch from velocity mode to position mode after many revolutions, by default the position PID will want to unwind all the current stepgen turns.