Closed loop positioning in Mach 3

[saabman]

I am curious to know if anyone has connected X and Y dros to Mach 3 and used them to provide closed loop positioning. The current glass scales from Accurite output square waves and it would seem to be simply a matter of creating an interface box with a power supply in it to hook the scales to Mach through parallel port inputs.

[Al_The_Man]

For servo's you would need Mach to close the PID loop.
For steppers it would not really apply.
The loop is closed if using Mach and the Galil card.
Al.

[saabman]

Let me rephrase, proxy closed loop. That is Mach knows where it want to go (calculated value), directs servo/stepper to get there, and DRO ultimately says if it go there. Mach adjusts as needed.

[ger21]

Mach3 can't "adjust as needed". You can write a custom macro to stop if it's not where it's supposed to be. But that's about it.

There's actually a commercial product to do this, with a Mach3 plugin.

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[saabman]

The problem I see with the closed loop acting at the servo/encoder level is it is not measuring table/sadle movement but ball screw movement. Any backlash will come in to play. My thinking is a physical DRO acts as an independent measure of table/saddle movement.

I have a scale mounted on X (Y is in the works), and I can attest that there is quite a bit of backlash difference caused by gib tightness (duh). My point is with the DRO I can see it and I can adjust for optimum performance quickly and easily (sure you can do this with measuring blocks, etc) Mach could see if too (if properly coded) and automatically adjust for it. I have read where DRO external inputs can be copied to Machs main DRO. Mach would need to look at the DRO differently than it does, namely that it represents the true position not a computed display value.

[ger21]

Mach3 doesn't have the capability to correct for position errors.

[saghirecho]

Hi,
It is possible to connect with mach3 with servo as closed loop, but it is necessary that your hardward can support with this. If you want to do so u can build your own servo controller or purchase if available with EMC2 you can easly attached closed loop. I suggest you that dont use MACH(X) but use EMC2.

saghirecho@yahoo.com

[MJ_cnc]

What you're asking about is often referred to as position maintenance for steppers. Make a move and then adjust at the end of the move. My only comment is that if you overshot your move, position maintenance doesn't do you much because you have already cut too much, then you move back.

I have thought about doing this w/ Galil as was mentioned previously. You would need to do it with Servo's. However, I have not done it. W/ the Galil, you can input 2 encoders per axis, one would be for the motor (ON THE MOTOR) and the 2nd would be the glass scale. I would do it w/ Renishaw. anyway, the cool part about doing this is that you CAN compensate for backlash. The Galil takes care of it, you need to have at least 2:1 ratio for the motor to glass scale encoder. The motor needs to have finer adjustment to compensate. I had done some quick calcs and a 1000 line or 2000 line encoder w/ quadrature is enough for anything we'd need.

Good luck and good question.

Mark

[saghirecho]

Mach3 doesn't have the capability to correct for position errors.

Ok but it has suspend button which can easly control close loop.

[saghirecho]

I am curious to know if anyone has connected X and Y dros to Mach 3 and used them to provide closed loop positioning. The current glass scales from Accurite output square waves and it would seem to be simply a matter of creating an interface box with a power supply in it to hook the scales to Mach through parallel port inputs.

U can easly use mach3 for close use. But if your hadware can support with it.

saghirecho@yahoo.com