New to servos, advice from the wise please

[erase42]

Hi guys, I have a basically homebuilt foamcutter that I use as my main source of income. It uses steppers and xylotex drivers. I havent really had any problems with it other than very slow speeds when positioning for the next cuts. Every now and then I get some missed steps for reasons Im not sure of and other times im very sure what caused it Sometimes a chunk of melted solid plastic will end up in the middle of a block of foam and the very delayed cutting of this chunk will cause enough problems to cause missed steps. Once it finally cuts, if its lost steps, the entire rest of the block is usually ruined since the missed steps cause the very closely nested parts that were cut before and after the missed steps to overlap in areas.
I digress...
Is a servo system subject to the same failure? Is a gecko or rutex driven servo considered a closed loop system? Will a closed loop system continue to fight to reach a location that its supposed to arrive at. What exactly is a servo fault? Does it just mean that there was an incident or does it trigger the system to stop? And finally what drive do you all reccomend?
ED
E3 Foam

[HuFlungDung]

Closed loop servos are more reliable because the controller is monitoring position feedback at all times. Indeed it must do so, because the nature of a servo control loop is that deviation of the current position from the commanded position creates an error signal which goes to the drive and forces the motor to attempt to keep moving until there is no more error in the position.

I'm not sure what to think about the problem with the delayed cutting. Most likely, even with servos, the control may shut the motor down if it has too much of a following error. If the servo motor stalls, the drive amp should trip out to protect the motor. The position is never lost, but if you are quick on the feedrate override, you can slow the machine to give it more time to cut. Worst case, you would return to a known earlier position in the program and cut the same path again.

I have used AMC drives, but that is just one of many companies.

[erase42]

What then is a servo fault? When people say the light blinks and the motion stops. Is that something other than a closed loop system?

THanks

[ger21]

Geckos and Rutex drives close the loop in the drive. When the servo lags behind more than a certain amount, it faults, or stops. I believe you can make a circuit to stop all the drives if one faults, but that may not help you. I'm assuming you can't just stop in the middle of the foam, and resume again later from the same spot. It sounds like your system is underpowered. An underpowered servo system most likely won't help you at all. If you had more power, would the chunk of plastic still cause problems? What software are you using to control the machine?

[erase42]

HI Gerry,
You are correct about not being able to start where i left off. If I do a controlled stop I can restart and continue with limited to no damage depending on the location of the wire. But a controlled stop is rare as there is no way to predict the problem untill after its become a problem. As far as being underpowered, that may be the case for the lost steps when the machine binds a little or some other source of friction (foam sliver in the rails) but more power wont cut the plastic contaminates any faster. Once the wire touches the solid plastic, the plastic sucks the heat out of the wire where the contact is made, slowing the cut. No amount of force (short of breaking the wire) will cause it to cut faster. Raising the heat can help but not much, I guess because an .014 wire cant hold very much heat? Under normal conditions the cut is done by the radiant heat around the wire, the wire never touching the foam. If the wire drags, it cools the wire and causes it to drag worse and so on. (sometimes the conatminate is a sliver of cardboard or something then im totaly screwed) The software im using now is deskcnc. It is a normal step direction output program although the step and direction signals are generated by its specialized control board. It can be connected to any type step dir drivers I am told. Desk cnc is now making their own servo driver units as well, saying that they are PID loop devices. Same as geckos?
Thanks for the help, my next machine will be servo powered no matter what, but im hoping for some other benifits over higher rapid transit speeds and quieter operation.

ED

[HuFlungDung]

Hmm, you might want to add some kind of spring loaded tensioner to your wire. Normally the wire gets longer anyways when it is turned on (I suppose), but it must reach an equilibrium length. At that time, then a microswitch could perhaps be rigged up on the tensioner, and if the wire starts to pull hard, it would activate the switch, send a feedhold to the control. If it burns its way through, then the feedhold would release and the cut might continue.

That is a lot of speculation, but it might be a way of getting feedback from the cutting force.

[ger21]

Mach3 can be set up with encoders and some programming to pause the machine if lost steps occur. You can but a ready to run set up for it from www.rogersmachine.net

[erase42]

HI HU,
I did have an idea like that at one point where the wire passed thru a tiny hole in a metal plate/electrical contact. So if the wire was deflected more than a fraction of a degree from perpendicular, it would touch the side of the holes closing a circuit which would either add a boost to the wire temp or send a signal to the feed speed control, slowing the wire down and triggering some sort of audio alarm so that I knew it had happened.
I was more concerned really to see to what extent i could loose steps with a servo and still have it return to an accurate location. If it would loose a couple inches worth of steps and still catch up that would be fine. If its going to fault after 100 steps or something it wont really help too much when I have problems.

Mach 3 is an awesome program but untill I can get one of the other software companies i use to write a 4 axis linear post for it, I am out of luck. I would like to use it tho.

[HuFlungDung]

Ed,
Depending on the control software, there is a following error parameter that can be set. This would determine how much position lag is allowable before an alarm stops the program. I have no idea how wide this can be set, but it should allow a good bit of flexibility for you.

I would still incorporate the sensor system as you have described, in hopes of keeping anything from getting too extremely far out of wack.

[erase42]

I guess i make it sound like its a tragic every day happening. but i burn about 20 fridge size blocks a month and i have problems about 1 out of 25 or so. Dont shed any tears for me

[nashyboy]

Have you considered converting it to a fastwire system instead of hotwire?