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View Full Version : Cheap 0 Clifton and servo drive selection (advice needed)



Konstantin
04-12-2004, 11:13 PM
From what I have read through the scattered posts about using Gecko or Rutex servo drives with a Clifton 9.99 precision motor is as follows:

To use Rutex if the encoder you re using isnt the best.
Go Gecko but use USdigital encoders.

The main problem resides in the error handling of Gecko 320 wich is 127 something, and because of that one has to use a good encoder like E2-250-250-G USdigital to avoid problems.

Did I made my homework properly?


I got my cliftons and now looking for a drive. Rutex way will cost $502USD plus S&H ( 3x R990H servo drives and 1x R990mb morherboard) and Gecko way is 3x114=$342USD plus S&H.

Budget is limited.


Meanwhile I have to research into PSU options as well.

Konstantin.

Mariss Freimanis
04-13-2004, 12:00 AM
Originally posted by Konstantin


The main problem resides in the error handling of Gecko 320 wich is 127 something, and because of that one has to use a good encoder like E2-250-250-G USdigital to avoid problems.


Konstantin.

Our drive as well as Rutex (good drive from what I hear) are PID servos. This means there is no static following error to speak of.

Dynamic following error occurs when you ask something from the motor it cannot physically do such as instantly reversing from 500 RPM clockwise to 500 RPM counter-clockwise.

What our drive does (and I assume Rutex as well) is to decelerate to a stop, then acellerate away in the new direction and catch up with where it should have been had it been able to do it instantly.

During this time, the servo develops, then takes up the following error so that it equals zero again. Following error is the difference between where the motor is and where it was supposed to be.

In our opinion, if +/-128 counts isn't enough then +/- 1 million counts won't be enough either. Remember, the number is how many increments of motion the motor is out of position.

Using a 500-line encoder, this would be a maximum of +/- 23 degrees out of position with our drive. With a +/- 32,000 count lock range the same encoder would allow a +/- 16 revolution error. What application is there where you could be out of position by that much?

If you don't want a following error (either 128 counts, 32,768 counts or a million counts), don't ask more from a motor than what it has to deliver. Don't overload it, don't over acellerate it.

To put this into perspective, during the testing of the G2002, we acellerated a pretty ordinary NEMA-34 sero motor to 3,000 RPM in 1/2 a motor revolution, then back to zero in the remaining 1/2 turn. It took 0.04 seconds start to stop, your eye couldn't see it. The G320 developed a maximum 70-count following error at the 1/2 rev mark. Most people use saner rates of acelleration.

Mariss

Konstantin
04-13-2004, 12:28 AM
Thank you Mariss for promth answer.
So in your professional opinion, which encoder would you suggest to couple on my cliftons and to use with G320 servo drives.

I do not need high speeds in IPM, I am planning a minimill cnc conversion. Have no mill yet. I enfatise on precision and repeatability.



Konstantin.

Mariss Freimanis
04-13-2004, 01:07 AM
That is a different question entirely. The consideration here is to match the motor to the load, meaning finding the correct reduction ratio from the motor to the driven element (leadscrew, pinion, toothed-belt, etc.).

This is very important with servo motors. Bad mechanicals will give crappy performance with step motors but it will not hurt them. Bad mechanical design will also give crappy performance with servos with the added feature of burning the motors (and drive) out as well. REALLY IMPORTANT; you have to be much more careful with servos for that reason.

If you already have a motor adequate to the task:

1) Start by finding out what your motor's no-load speed is at its rated voltage. Use a strobe tach if you don't have the data.

2) Design for a speed equal to 80% of your motor's no-load RPM when your mechanism is moving at its maximum speed.

3) Calculate the RPM of the driven element (leadscrew, etc, etc) when the mechanism is moving at its max speed.

4) Divide that RPM into 80% motor no-load speed. That will be the optimal reduction ratio from motor to load. Use a toothed-belt for anti-backlash reduction.

5) Determine what is your software's maximum step pulse rate. It'll be published somewhere. Convert your motor no-load RPM into revs per second (divide RPM by 60) and divide that by 4. Divide that into your software's max step frequency. The result will be the maximum encoder line count. Pick the next lowest standard mfg encoder line count. In other words, if your result is 566.66 lines, pick a 500-line encoder.

Mariss

Konstantin
04-14-2004, 11:21 AM
Thank you again Mariss.
Now I see what people mean when they say that you give great support.


Konstantin.