Machsupport Forum

Mach Discussion => General Mach Discussion => Topic started by: Graham Waterworth on January 08, 2006, 05:43:31 AM

Title: Zero Return (Home Setting)
Post by: Graham Waterworth on January 08, 2006, 05:43:31 AM

Hi All,

I have set up my lathe with home switches.  I am using servo motors with encoders and was wondering,

In Mach3 how is the home position determined?

Is it by sensing the home switch only or is it by sensing the switch and then looking for the zero mark on the encoder?

or is it done some other way?

Thanks

Graham.

Title: Re: Zero Return (Home Setting)
Post by: Brian Barker on January 08, 2006, 07:44:10 AM

Mach3 is going to be looking for the change in state of the home switch. It will hit the switch and then back off the switch till it chages state again.
You will be able to do homeing with the index pulse with the Grex in the future.

Hope that helps

Title: Re: Zero Return (Home Setting)
Post by: spc_aux on January 09, 2006, 12:36:00 PM

You don't need Grex to do homing with an index signal. If you have an index signal available you can route it through your mechanical home switch and then to the home input for that axis.

When homing the axis will travel in the home direction, when it reaches the mechanical home switch that will enable the index signal to provide the home signal and then when the index signal comes up it will provide the home signal to Mach.

You may have to keep the home speed a bit slow so Mach doesn't overshoot the short index home signal, but you'll get the precise homing of the index.

Pete C.

Title: Re: Zero Return (Home Setting)
Post by: Brian Barker on January 09, 2006, 03:09:52 PM

VERY SLOW!!!

Title: Re: Zero Return (Home Setting)
Post by: spc_aux on January 09, 2006, 03:41:21 PM

Ok, very slow. I think you can overcome that issue by tweaking the homing routine, but my latest CNC creation isn't quite complete enough to test that.

I think a routine like this would work:

move at 10% speed in the home direction
stop when you see the home/index signal go by
backup at .1% speed and stop when the home/index signal is active

Your home position would have to be a little off the physical limits to allow for a bit of overshoot, but that's good practice anyway. The commercial CNCs I've worked on have a seperate decel switch to indicate when to slow down and then use the limit/index combo for homing. You could do the same with Mach if you tweak it I think.

Pete C.

Title: Re: Zero Return (Home Setting)
Post by: Brian Barker on January 09, 2006, 04:54:44 PM

How about homing every axis to the switch and then home again coming away from the switch. You would only need to have one more input for the Home switch :)

I think it could be done...

Title: Re: Zero Return (Home Setting)
Post by: spc_aux on January 09, 2006, 05:12:33 PM

Because a standard limit switch is not a precision device and your home position can vary. By referencing an index signal you get your home position precise to a fraction of a degree of leadscrew rotation.

You can still do it with the usual four inputs, one for global limits (all axes) and one home input for each axis. You are simply integrating a coarse home signal from a switch with the index signal to get a more precise and repeatable home position.

The other advantage of this arrangement is that a piece of swarf that gets on the home switch will change it's trip point, but most often not by enough to change which leadscrew rev you will home on and thereby not affecting the home position at all.

Pete C.

Title: Re: Zero Return (Home Setting)
Post by: Graham Waterworth on January 10, 2006, 04:41:27 AM

Hi All,

now this is what I want on my machine, accurate homing, all the full size machines I have work like this.

How about a tweek Art,

Graham.

Title: Re: Zero Return (Home Setting)
Post by: HillBilly on January 19, 2006, 01:28:48 PM

I have not had a chance to test this but in theory it should work.

Darek Ashburn