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G31 Probe problem
« on: July 28, 2011, 05:10:12 PM »
I have a plasma table with a floating head. I have it set to go a G31 probe with a feed rate of 60 ipm.
The problem is that the motion is ignoring the accel / decel rates that is set it motor turning.

This causes the stop point to overshoot the sensor by almost 1 inch...

Any ideas?


Thanks
Dave

Offline BR549

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Re: G31 Probe problem
« Reply #1 on: July 29, 2011, 11:05:57 AM »
Are you sure it is not following the axis parameters?

Probing at 60 imp is quiet a feat to pull off as what you have found there can be a lot of overtravel due to the deacell rate.

What are your settings for accel in the axis config.

Try slowing down the probing to about 10 imp and see what the difference is.

IF it is still 1 inch then I think your probe is not setup correctly in pins/ports and the machine is not seeing the input from the probe. It is just stopping on the end of move parameter in the G31 call. G31 Z-1 means probe until you get a trip OR reach the end of move(Z-1)

Just a thought, (;-)TP
« Last Edit: July 29, 2011, 11:08:24 AM by BR549 »
Re: G31 Probe problem
« Reply #2 on: July 29, 2011, 12:34:41 PM »
I have accel set to 15.
It stops fast when I am using rapid.

With a slower probe feed it does not over shoot as much, but then I am waiting longer for each cut.

The probe trip is working fine. I have a M3 macro that reads the trip point DRO and uses that to zero the Z dro and then move up to the cut height.

The overshoot will sometimes hit the limit switch on thin plates... So I was looking for a solution.
Re: G31 Probe problem
« Reply #3 on: July 29, 2011, 12:50:30 PM »
Accel and decell are only really a factor on commanded moves because the Mach knows where it is stopping and plan the deceleration to that point.  When you probe, Mach has no idea when the probe will trigger.  When it triggers, Mach says stop, but it never stops exactly on the mark.  That's the same reason the THC commands overshoot.  The faster the THC feed rate the more overshoot you have.  The solution to both is to turn down the feedrate.  I probe at 30IPM with is a acceptable compromise between overshoot and speed.  The slower you make the feedrate, the closer you'll come to stopping on the mark.

Offline BR549

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Re: G31 Probe problem
« Reply #4 on: July 29, 2011, 05:09:42 PM »
HUM, Probing inside a Macro is always interesting(;-) you never quiet know what you will get.

Try moving it back to the Gcode side where things behave and happen at the speed of light.

I use the G31 here for the same thing you do BUT it is instant responce from the Gcode side.

You cannot compare teh G31 to the THC function they are totally different in the move aspect. The G31 will ALWAYS use the Deaccell curve to stop

The THC does NOT use any Accell/deaccell in its moves. It depends on the Arcvoltage PID to tell it to stop/go

Just a thought, (;-)TP
« Last Edit: July 29, 2011, 05:12:57 PM by BR549 »
Re: G31 Probe problem
« Reply #5 on: July 29, 2011, 09:59:08 PM »

 The G31 will ALWAYS use the Deaccell curve to stop


My point was that unless Mach knows where it is stopping, it can not decel to that point, thus when the probe is triggered, Mach stops, but because it has to decelerate, it never stops on the mark.  Slowing the feed rate gets it closer to the mark.  The OP wanted to stop on a dime at 60IPM, and that's not going to happen, regardless of where the code is.

The behavior is similar to the THC commands in that the slower you make the THC feed rate the less overshoot...or bounce, you get.
Re: G31 Probe problem
« Reply #6 on: July 29, 2011, 11:27:55 PM »
I understand that it will not stop on a dime. But over 1" of overshoot seems quite excessive.
When I job the Z at 150 ipm and let go of the jog button, it stops near instant.
I don't see why the G31 can not use this same deaccel rate.

Offline BR549

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Re: G31 Probe problem
« Reply #7 on: July 30, 2011, 12:39:05 AM »
IT DOES use the same rate. That is why I mentioned running the G31 from inside a macro it is not always the same as running it from the Gcode side.

I have a box full of broken probe tips and 1 broken probe from running the G31 from  macros. NEVER have I broken one from using G31 on the Gcode side.

Try your move from the MDI line and see IF it stops as it should. From the Gcode side it will always obey the acel/deacel parameters of the called axis's.

Do a simple axis move at your probing rate of 60 see how far it takes to stop.

Then make the same axis move with a g31 and trip the switch THEN see how far it takes to stop from the same speed.

(;-) TP



Re: G31 Probe problem
« Reply #8 on: July 30, 2011, 12:45:19 AM »
I will give that a try on monday and make a video if the problem still happens.
Would using G28.1 in a macro work better then G31?

Offline stirling

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Re: G31 Probe problem
« Reply #9 on: July 30, 2011, 05:54:11 AM »
probing does not NEED to "stop on the mark". When it trips, as long as it decels to a stop BEFORE it reaches the mechanical end of travel of the probe, then all is well and perfectly accurate because it is the trip point that is stored NOT the stop point. As Terry has said, G31 and I suspect G28.1 do not fair well (at least consistantly) in macros.

Ian