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andypugh wrote:
Ah Andy, you have espied my cunning plan. And also found that pratice is different than theory.

The U.S.Digital encoder components seem to be quite a deal pricewise, 4 sets (1 spare) would be less than $150, and it does seem to be the most robust option in the feedback chain.

On the topic of the nested PIDs. I still intend to pursue this. That is why I asked about LinuxCNC having a native velocity component. 'cmorely' opines in a post about that the velocity component is derived. Does that mean that dV/dT and integral(dV/dT) are being calculated by LinuxCNC? I need to find time to look at that.

I still intend to use the HH scales for 'ground truth', even though their resolution is 1/4 that of the rotary encoders I suppose they are more accurate for a number of reasons.

cmorley wrote:Yes, I'll cross that bridge when I get there. However the documentation I have is pretty clear that the interpolation is adjustable. There are jumper blocks in the device. Sadly, thats all I've got.



With regared to the scales, the reference marks are documented as "Selectable by magnet every 50mm"? Does anyone here know how to perform this adjustment?



Its not over 'till its over.

John
.

from memory:
look inside the housing you find a flat magnet that can be moved along the length of the scale.
it strengthens the index signal somehow.

if you figure out how to adjust the 602 please let me know - I had the manual for it and it didn't say how.

velocity is derived either from position and thread period or some of the hardware cards (Mesa and PICO) can supply a high quality velocity number.
They do this by using position and an internal time clock.

Personal experience:

I have had to drive both a rotating and a linear servo motor with linear encoders.

In case of the linear motor I picked up the encoder signals coming off the 7i33 card. I used a monostable arrangement on both A channel and B channel to generate a constant pulse width train A and train B.

This was then passed through suitable low pass filter on both channel and summed to generate a analogue tacho signal where the signal of +/- 10 V represented slightly higher than my max desired linear speed.


On my rotary motor I had to include a "tacho" signal to implement a solution based on Advanced motion control servo amplifier.

Fortunately his amplifier accepts encoder pulse train directly and converts it to analogue tacho signal.
Major problem is my linear scales are @0 micron resolution ( with quadrature processing down to 5 micron.
The on-board converter would provide too low a signal at my maximum velocity to be useful.
Fortunately I had purchased dual shaft servo motors so I git a few rotary shaft encoders ( AMT 102 from CUI Inc) ) which had a much higher effective resolution per unit travel than the linear encoders .
The output from rotary shaft encoders was fed directly into the servo amp pulse input providing an order of magnitude increase to tacho sensitivity circuit.

The loop settled down immediately and within a few minutes of tweaking the velocity loop was tuned and shortly thereafter the positional loop settled down and PID and feed forward elements got tuned.


Just a word of warning. When driving linear encoder positional system eliminate any and all flexibility in the mechanical structure between the encoder reader and motor shaft.
If you do not the loop will constantly buzz. I was using helical aluminium couplings between motor shaft and lead screw. The loop used to buzz and carry on like a two bob watch.

I removed decided to pour urethane compound into the shaft space of the couplings and extrude it through the grooves of the coupling. Naturally excess material was removed and the remainder cured.
This resulted in a flexible albeit stiffer coupling. I am pleased to say the stiffer coupling did the trick.

I hope this helps some.

Zig wrote:
Good point!

I suppose dealing with the slack and elasticity in the belting will be interesting.

Something more to think about. Thanks.

edit: And thanks also for the reference to the AMT encoders.