Accuracy Question

[CarbideBob]

jubee
Quote: "From what I have read I think 2048 cpr ,mounted on the motors should get the required accuracy. "
Normally I'd jump up and down with agreement on this one. However there have been some posts that imply that the 320 drives don't like too high of an encoder count.
This may be due to the fact that the "I" term in the PID loop is not adjustable or it may just be servo mismatch/tuning errors.
By changing the encoder resolution you change the amount of correction from the integrator.

BTW , I never use encoder counts like 1024, 2048, etc. I almost always use 1000 or 2000 line encoders as this works out to an even number of counts for the system resolution.
an example:
1000 line x 4 = 4000 counts per motor rev.
10mm lead with 2:1 pulley = 5mm motion per motor revolution.
5mm/4000 counts = .00125 mm per count or 800 counts per mm.

A 1024 line encoder gives you 0.0012220703 mm per count or 819.2 counts per mm. I also rarely use 3:1 belt ratios for the same reasons.
Of course sometimes you'll want to use whatever you can get for the lowest price. You just have to be sure that the software handles the rounding errors correctly. Many high end systems use different screw/pulley/encoder combinations for metric and english models so that the counts per distance comes out to an even number.

I'm thinking a 1000 line encoder on the motor with a 2:1 reduction would be a good starting point for testing. I would never attach the encoder to the ballscrew as this raises stability problems in the servo loop forcing you to use "softer" tuning values.

Quote: "And the best I could do in a single jog would be .05mm (0.002`) ,the axis wouldn't move for anything less."
This is very interesting. The real question is does the servo amp know that this error exists? By checking the voltage at the "Position Error" test point you should be able to tell if the amp has been able to close up the error.
I would run your test while monitoring this signal. I suspect that you will see a corresponding error between this test point and what you indicator says. ( 5 volts = zero error, 5.04 volts = 1 count error, 5.08 volts = 2 counts error, .. etc). This would tell us that the integrator isn't putting out enough power to overcome the friction or drag torque of the system.
The integrator is responsible for closing up or zeroing the following error of the servo. The way it works is basically as follows:
Say you command a move of .005 mm. The integrator circuit sees an error of 4 counts (using the above encoder and pulley). This 4 counts is converted to a small output current which is supposed to move the motor. Now since there is some fixed friction that we have to overcome this is probably not enough power to get things moving.
After some fixed amount of time the integrator looks at this error and sees that it has not changed. So the circuit then applies more power to the motor. This loop continues until the motor finally gets enough power to move.
Most integrators will have some fixed or adjustable limit on the amount of power that will be fed to the motor.
It's looking like you're not getting enough output from this part of the circuit to get the motor to move. Overly tight gibs or high friction loads will cause this problem. It's important that the servo be able to start the motion with very little effort. You should be able to move the table at low speed by rotating the pulleys by hand with very little force. Even overly tight belts can contribute to these errors.

Quote:"Pulled the z axis apart and couldn't even move the section that holds the motor/gearbox "
Woops, did I miss something? This is the first mention I've seen of a gearbox involved in this system.

Quote:"Iam thinking of going with the g100 grex from gecko drives"
I sure do like this idea. There is a definite limit to what you can pump through a printer port even if you get underneath the operating system by using real time extensions. This circuitry was never designed for high speed pulse use. However at this point we aren't worried about top speed yet so anything that will output pulses to the drive will work.

You're really not testing accuracy. What you're testing is positional repeatability. But you've got to get this repeatability working first before you worry about accuracy.

In summary I'd try a higher count encoder mounted on the motor, retune the drive, and then repeat your test while monitoring the error output voltage. Try to eliminate as much drag as possible so the motors can move the axis with a very small amount of force. The P, I, and D factors interact with each other to form a stable servo loop. Higher P (gain) adjustments on the amp will also help reduce this error but will cause oscillation if it gets too high. Higher D "damping" helps control this oscillation but too high of damping values causes rough running at high speeds and high freq oscillations (buzzing motors).
Bob