Schaublin 125-CNC retrofit.

My first idea was same as SMC.Collins with a stepper motor instead of the existing AC one on the variator, the second idea would be a linear potentiometer bolted to variator, and analog reading on a Mesa input, low resolution yes, but more that enough for this case.
Set some ranges in hal and be done with it, let the VFD do the fine adjustments.

I have taken many false starts at attempting this over the years. It is nice to finally do something about it.

I would love to know how Schaublin did this with no feedback other than the encoder, and less compute than a toaster. I am not sure if it did real CSS, or just lots of fixed, open loop timed steps. I will let you know how my last change software solution with such open loop, actually works (or not).

Assuming I fail... (good assumption ... and need to add a sensor...

Please share your argument for CVT lever feedback vs motor encoder.
I don't really see the advantage of feedback of the CVT positioning lever, given the non-linear relationship of position verses ratio, which would require an equation or LUT,  and have all the inaccuracies and slop in those linkages. That whole drive unit is beautifully engineered and made. Pulling that little 3ph CVT positioning  motor and replacing it with a servo (I have no step gens) adds a fairly complex  mechanical project, and the need for another servo driver (I dont have space for). 

A motor encoder would allow for a highly accurate, real time reduction ratio signal, which can be installed with just a mod to the motor cooling duct, a single cable (okay plus one for the electric fan), and connected to an existing spare 7i85 encoder channel. The back gear is only shifted stationary, so the ratio is only adjusted when running by the CVT. 
Cheers,
Mark
 

I vote encoder.

An encoder separate fan will allow:

• lower continuous motor speed without overheating
• higher continuous motor speed without the chance of a 45-year old fan leaving the chat
• you and LCNC not to guess what the motor speed is, adding confidence you aren't going to send the motor in to orbit

All giving you a wider safe frequency/speed range for your VFD/motor - and thus making each 'step' in the CVT more useful (wider range).

Actually, I vote for getting rid of the CVT.  Do what CNC mfgrs have been doing since the 80's & 90's and stick a big a$$ spindle-servo in there with an 8kRPM top end and appropriate-ratio single-speed belt drive.  Just horse the thing.  Simplify your control loop and save your sanity.

But I digress...image missing /media/kunena/emoticons/wink.png

I have a few points.

First: you refer to the fan multiple times. If your machine is like mine, and I have no reason to believe otherwise, the motor has no fan where you think the fan is, the thing there is the brake.

Second: regarding your solution 2, I can make a simple op-amp for you. Scaling to 24 V is easy, as you already have 24 V, and it would have 2.5 times the resolution of the system that gives satisfactory results for me. Yes, it has way less resolution than an encoder, but you do not need it. This would be the fastest and least invasive route.

Marti

My immediate reaction when I saw your problem was a cascade PID loop. These are two PID loops: fast and slow.
The fast loop is a normal encoder to VFD loop.
The second loop controls the CVT. That controller tries only to keep the output of the first loop at 6V (= 50Hz). I think you do not have to calculate the gear ratio, etc. Maybe it would work without any additional hardware.If the output of the fast controller (VFD) is below 5V, the CVT turns on until it hits the limit or the controller output is above 5V and below 7V. Above 7V, the CVT turns on in the other direction.

If this does not work and feedback from the CVT is needed, a Waterproof Wire Encoder (Aliexpress) can be considered. Then, the PV (actual position of the CVT) can be used to calculate / adjust the $K_p$, $K_i$, and $K_d$ of the fast VFD loop. Mark

my eyes glazed over looking at all the options but if its pneumatically actuated, why not a pressure switch? Cheap as chips!