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Author Topic: G28 Overshoot  (Read 4627 times)

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G28 Overshoot
« on: December 28, 2009, 01:15:36 AM »
I am just finishing my DIY router and am having some trouble with the homing process. I have seperate home and limit switches (which work fine). The "ref all home" works ok. but if I enter "G0 X0" it goes too fast near the home switch and overshoots. I tried setting soft limits at mn 0 and max = travel (-ve for Z) and 0.5 for the slow zone but this did not help. Can someone help?

Thanks,
Simon

Offline Hood

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Re: G28 Overshoot
« Reply #1 on: December 28, 2009, 06:30:32 AM »
Sounds like your switches could be inaccurate, what kind are they?
 When you do a homing move the axis will move to the switch then back off and will set the zero position when the switch again closes, if you then move away and return to zero your switches should not be triggered. If however your switches are not accurate the closed position may vary considerably and cause the problems you are experiencing.

Hood
Re: G28 Overshoot
« Reply #2 on: December 28, 2009, 07:32:25 AM »
Hi Hood,

I read his problem differently, in that his machine homes OK

But if he then enters "G0 X0" presumably from the MDI line his machine overshoots.

Maybe he will come back and explain more fully

Phil
The Good Thing About Mach3, Is It's very Configurable

The Bad Thing About Mach3, Is It's Too Configurable

Offline Hood

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Re: G28 Overshoot
« Reply #3 on: December 28, 2009, 07:42:13 AM »
Yes the machine homes OK but if the switches are inaccurate then the X0 position will be different every time and when commanding a move back to X0 the switch may open sooner than it previously closed.

As for setting soft limits and the slow zone, that will all depend on what acceleration your axis have, if fairly low accel then you will need a larger slow zone so that Mach can decel from full rapid in time not to hit the switch.

Hood
Re: G28 Overshoot
« Reply #4 on: December 28, 2009, 08:04:58 AM »
Phil had it right...The home switches work fine when doing a "ref all home" which goes slowly. The G28 (entered from MDI) does not appear to slow down at all before hitting the home switch and then going past to the limit switch.

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Re: G28 Overshoot
« Reply #5 on: December 28, 2009, 08:38:58 AM »
So exactly what are you commanding from the MDI?
Hood
« Last Edit: December 28, 2009, 08:41:09 AM by Hood »
Re: G28 Overshoot
« Reply #6 on: December 28, 2009, 08:51:38 AM »
G28 X0

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Re: G28 Overshoot
« Reply #7 on: December 28, 2009, 08:53:05 AM »
And what does the DRO read in Machine coords when the limit is hit?
Hood
Re: G28 Overshoot
« Reply #8 on: December 28, 2009, 08:58:28 AM »
Simon from the MDI line you enter "G28 X0"

Doing G0X0 will take you at full speed the same as "G28 X0"

Only jogging near limits does the machine obey the slow zone

Your issue maybe that you have not set the motor tuning correctly maybe acceleration as the likely cause as either the two lines above you should end at Zero

Phil
The Good Thing About Mach3, Is It's very Configurable

The Bad Thing About Mach3, Is It's Too Configurable
Re: G28 Overshoot
« Reply #9 on: December 28, 2009, 10:37:50 AM »
Maybe I am using the wrong command to home the axis. What I was trying to do was get to the home switch a bit quicker than the "ref all", which takes a while. Is there a better way to home a single axis? Also is there a way to have the homing go faster until close to the switch and then slow down?