Closed Loop Driver vs. Closed Loop Computer

[ojibberish]

I understand that you can use servos and a closed loop system with the Gecko g320, and if the encoder is x steps off from the signal, it gets a fault. I also understand that you can connect encoders directly to the parrallel breakout board to create a closed loop using computer software. Are there advantages and disadvantages to these techniques? Can they be combined?

In a seperate issue, are Gecko G320 drives compatable with linear glass scale type encoders instead of rotary encoders?

Thanks for your expertise!

-Christopher

[snaggletto]

Hello,
With the Geckos or Rutex servo drives, the encoder feedback loop is closed at the drive. The software (Mach2 etc. or any other Windows/DOS based cnc software) has no clue. The software says move some number of steps/encoder pulses, and simply has to trust that the motor moves that far. If the motor gets bogged down or even stalls out, you will have lost your machine home and part zero postions. Geckos will fault out with +/- 128 encoder counts, I *think* that you can rig the software to read the fault and go into e-stop, but that's it. Any errors within the +/- 128 counts is simply lost motion (accuracy). The Rutex drives have a much higher error trip point +/- 20,000 counts. If you calculate your resolution (inches per encoder count) you can see how much you would be off. For example if 1 encoder count is .001" then you would be off .128" (a bit over 1/8").

Linux EMC does have the capability to close the feedback loop in software with the right hardware. You can also run open loop (closed at the servo drive) like above. The most cost effective way I've seen is at http://www.pico-systems.com/motion.html

These boards only work with EMC, but have some awesome features. You would be interested in the Universal Stepper Controller and the Gecko Servo Interface. You wire the encoders to the USC and then to the Geckos. EMC can read the encoders from the USC giving true closed loop. It also acts as a real DRO with no power to the motors (if you leave handwheels on your machine). It also has a hardware pulse generator giving 300,000 pulses/sec (vs. 45,000 p/s or less for Mach2 or similar software pulse generators with a fast computer which can greatly limit your max speed if your encoder resolution is high).

If you want to run in Windows, you are looking at $1000+ IO boards, VERY expensive software and expensive servos/drives (no step/dir Geckos/Rutex drives).

I don't know about liner scales. I've heard the DRO types you see have to slow of an update rate to be of any use for real time CNC.

[Al_The_Man]

Originally posted by snaggletto I don't know about liner scales. I've heard the DRO types you see have to slow of an update rate to be of any use for real time CNC.

The DRO may be slow, but practically all linear scales are constructed the same as the rotary encoder and operate exactly the same way, one thing you have to watch for if you use one in place of a rotary encoder is that some are sine-wave out and need conditioning (squaring up), Heidenhain use both types alot.
Al

[ESjaavik]

Right Al. Heidenhain glass scales are just like a rotary encoder straightened out. There are others that don't use glass scales, but still output quadrature pulses in realtime.

Snagglet may be referring to the capacitance type of DRO scales that typically have a readout on the slider with a possibility of connecting a remote readout. They have a serial transfer of position at a much too slow update rate for a feedback loop. It's the same as the digital calipers that came as a blessing to us which grew too short arms to see the traditional calipers without hanging glass on our noses.

There is one point though. Using a rotary encoder, it is (usually) right on the motor = very stiff connection. While the linear scale is on the axis. Then you (machine buider/retrofitter) need to make sure the connection motor-axis is stiff. Otherwise the drive will hunt continously and be impossible to tune. So beefy preloaded ballscrews and solid construction is a requirement. With encoders the motor can be tuned to follow closely to where it should be, and the flexibility will just result in the axis not being exactly where it should be.