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Author Topic: Homing position is not absolute  (Read 14195 times)

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Re: Homing position is not absolute
« Reply #20 on: May 02, 2011, 02:29:52 PM »
Really though you don't have to find the best speed, you just have to use one and not change. Granted if you have slow accel and are closer than your axis can accel to full speed then that distance may change but the double homing should prevent that being an issue.

Hi Hood!

You are wrong!
If you are to close to the switch (closer than the distance needed to accel to homing speed), the axis stops exact at the same distance from where you started, because the accel and break distance are the same. You can do the homing 10 times. If you are to close at the first one, you will be also on the last run. I have tried it!

Do the homing from a distance. Watch the stepper position. Make a very littel move towards the switch without activate it. Do the homing again. You will see, it will be a different position. Do the homing again and you will get the same position as the last one.

If I optimise my stepper driver I want to increase speed.

Burkhard
« Last Edit: May 02, 2011, 02:34:31 PM by JimKnopf »

Offline Hood

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Re: Homing position is not absolute
« Reply #21 on: May 02, 2011, 03:47:17 PM »
Well afraid I dont follow, you will probably be best contacting Andrew at machsupport and giving him an exact test method so he can replicate there and I am sure Brian will then be able to find the issue. Homing  however is likely to be a driver function so that would then fall to Art to supply a solution.
Hood

Offline BR549

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Re: Homing position is not absolute
« Reply #22 on: May 02, 2011, 07:15:09 PM »
Now it was years ago that it was tested  AND it was on a scope BUT I remember that it set the position from the trigger point of the switch not where it stopped.

It really would not make sence to do it any other way.

I will have to dig out the old trouble notes again.

Not to worry Ian, I respect all opinions and occasionally I am almost kinda maybe sorta incorrect(;-). AND every once in a long while I am flat out WRONG.

(;-) TP

Offline BR549

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Re: Homing position is not absolute
« Reply #23 on: May 02, 2011, 08:52:05 PM »
OK JUst a question are we talking PP or SS??  (;-)  I do know that homing is control through the driver on the PP.  SS??? should be on the board.

(;-) TP
Re: Homing position is not absolute
« Reply #24 on: May 03, 2011, 03:09:26 AM »
OK JUst a question are we talking PP or SS??  (;-)  I do know that homing is control through the driver on the PP.  SS??? should be on the board.

(;-) TP
Hi TP!

I have a very simple machine, self constructed. I use stepper motors (dir/step controlled) using the parallelport. The homing sequence is full controlled by Mach3. The optos are connected to the parallelport.

It really would not make sense to do it any other way.
That is what I mean, but it does not work that way.
It is so easy to test, I can only repeat it. As long as during the homing, a acceleration curve take affect, the machine will travel on after releasing/leaving the switch. You can see, that the machine doesn't travel towards the switch again. Look at the machine coordinates. If they show zeros that will be ugly, and that is what I see on my machine. And it is not to clear the jitter area.  It is not the speed of the parallelport it is the software.

@hood
that was the first thing I wanted to do, but I did not find an email adress.

Burkhard
« Last Edit: May 03, 2011, 03:11:27 AM by JimKnopf »

Offline stirling

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Re: Homing position is not absolute
« Reply #25 on: May 03, 2011, 05:30:40 AM »
Hi Burkhard

I think we're agreed that this issue is amplified the lower the accel capabilities of the machine. Putting my practical hat on, I think the accel issue alone is probably going to prove your real problem. I don't know how far down the road you are with your build, but CV for example must be (or will be) a PITA. Although I put home and limits on machines I build for others, I don't have either on my own wood router and to be honest I've never missed them, but I'd be lost without CV and to get that to perform you need hot acceleration.

Just an (alternative) thought.

Ian
Re: Homing position is not absolute
« Reply #26 on: May 03, 2011, 06:39:01 AM »
Hi Ian!

Now you got me :)
I'm not familiar with all terms, especially in English. What means CV. I'm now ready with my second machine. The first one was build of wood :D. And it was working. Not very stable but OK for PCB milling. I was using cheap materials like "trapeze" screws. Now I'm using quality components, like supported rail, ball screws.
The only things missing now are the limit switches and the toolsetter.
Basically I can live with the effect I described, because I know how to prevend it. But I think it will be simple to change the code in the program to avoid these things.
Burkhard

Offline stirling

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Re: Homing position is not absolute
« Reply #27 on: May 03, 2011, 10:08:30 AM »
Hi Burkhard

I'm darn sure your English is waaaaaaay better than my Deutsch  ;D

CV - is so called "Constant Velocity" contouring. See 10.1.16 Path Control Modes in http://www.machsupport.com/docs/Mach3Mill_1.84.pdf

Cheers

Ian

Offline BR549

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Re: Homing position is not absolute
« Reply #28 on: May 03, 2011, 10:41:42 AM »
OK I will have to setup a machine here to run MACH. The last couple of years I have been on the dark side (EMC2).

(;-) TP
Re: Homing position is not absolute
« Reply #29 on: May 03, 2011, 01:35:41 PM »
Hi Ian!

Thanks for the explanation, now I know about CV :-). Your Deutsch was perfect :)
I will increase my acceleration, when I finish the new drive circuit wit FETs. But I will drill and engrave the PCB with my machine ;-)

I just thought that the way of the homing with dir/step controlled steppers could be improved. I will write to Andrew when I get the email address.
Burkhard