Hi,
OK that explains the origin of the different styles of loop closure.
I bought a big servo a while back for a spindle motor, 2.8kW and 14Nm. Its fitted with a resolver rather than an encoder so drives are a rare as rocking horse s*********t.
I decided to make one of my own, electronics is my thing and studied control engineering at University. I have had to learn and in many cases relearn stuff to do it.
I always maintain that the value of a hobby is best measured by the things you learn in pursuit of it. In which case my decision to make my own drive has been very valuable.
Amogst the things that you need to know about Field Oriented Control is control loops. There are some very good YouTube clips by Texas Instruments of Field Oriented Control, really well
done and a fascinating insight as to how AC servos work.....highly recommended.
The upshot is that for an AC servo to work you need three control loops. The innermost one is the acceleration (an expression of torque which is proportional to current) loop, then the next outer loop is the velocity control loop
and then the outer most one is the position loop. The advice is that if you are going to combine loops you have one loop per differential time. Thus the torque (or acceleration)loop is 'integrated'
(mathematically) to the velocity loop, which is in turn integrated (again in the mathematical sense) to the position loop.
Your drives AND the 7766 have three levels as well but you now have two acceleration loops, two velocity loops and two position loops. I suspect that the multiplicity of loops of the same
time 'dimension' will interact in a way that may be very hard to sort out.
I would certainly in the first instance reduce the complexity of your Z axis by having the one feedback control. Two different ways:
1) As I've already suggested use the 7766 as an open loop command generator and have the drive/stepper as the one feedback control element OR
2)Replace your drives with non-smart drives, ie just plain step/direction stepper drives like the AM882 from Leadshine and then have the 7766 close the loop.
If you already have plain ordinary drives 2) is perfectly doable but having to buy new drives just to avoid the complication of your current ones seems a real pain.
I am not aware that there is any advantage of one or the other. Having said that I would be tempted to guess that the manufacturer of your stepper and drive has made a combination
that works well together and would be easiest to achieve a good result. I do not mean any disparagement to the 7766 but it does lack the software driven scope and also its by
necessity more generalized in nature resulting in a more difficult tuning task.
I think this is the least confusing way forward. No matter how you work it out a good result is determined by the overall PID dynamics. Lets say you choose one of the above strategies
and find that the best performance of your machine is had with a P(roportional) gain of 1400, an I(ntegral) gain of 200 and a D(ifferential) gain of 175. If you now decide that you want
both the drive AND the 7766 to participate in the PID dynamics you at leat know the approximate distribution of gains required....so if you had the P gain of the 7766 at 14 then the
P gain of the driver would have to be about 100. (14 X 100 = 1400, our previously discovered sweet spot for Proportional gain) Alternatley you may decide the differential component
of the driver is numerically very noisy, a common problem with differentiators so you might set the drive D gain to 1 but have the 7766 D gain set to 175 and take advantage of the less noisy
differentiator Vital Systems built in.
Is your head spinning yet?.....mine sure as hell is!!!!
Craig