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Author Topic: Opto switches behaving badly  (Read 8549 times)

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Re: Opto switches behaving badly
« Reply #10 on: January 20, 2010, 11:10:01 AM »
At 10 inches per minute the z can move 0.005" in 30 milliseconds.  The homing seems to be very consistent when I measure for absolute positioning.  My homing switches are no that close to the end of travel.  I have about .3" before I hit the end.

Offline simpson36

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Re: Opto switches behaving badly
« Reply #11 on: January 20, 2010, 05:13:27 PM »
If you have Mach3 set up to share homing and limits, you waste whatever the delta is between your trigger point and the actual physical limit.  Speaking for myself, I'm good to go on my X and Z, but I would not give up .3" of my Y.

Opto's are extremely accurate and allow you so set the trigger very close to the actual stop, that is one of the reasons I ditched the mechanical switches. Those with significant travel likely will home at a pretty good clip, so the need to be cautious about messing with the debounce is a real concern in many cases.  The relationship between debounce and homing is something useful to be aware of.
Re: Opto switches behaving badly
« Reply #12 on: January 20, 2010, 10:35:41 PM »
I don't have limit switches I have never reached end of travel for the work I do.  After thinking about it I don't see why a high debounce would effect the homing switches.  I did the above calculation incorrect.  If I use a value of 100 the manual says to calculate in milliseconds the debounce you divide the value by the speed you are running at in my case 25000 hz

100/25000 = .004 or 4 milliseconds and at 10 IPM that is .0006 movement.  I have seen reports that suggest 1000 or even 2000 for debounce.  Since the routine is to back off the switch after it triggers I think it would be fairly accurate even with a high debounce but I guess the only way to know is to actually test different values.   I did see a report that showed the higher the input voltage the more accurate the positioning using an optical switch.

Dan

Offline simpson36

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Re: Opto switches behaving badly
« Reply #13 on: January 21, 2010, 06:32:37 AM »
I don't have limit switches I have never reached end of travel for the work I do.  After thinking about it I don't see why a high debounce would effect the homing switches. 

If you read carefully, you may note that my comment was that if you share homing and limits in Mach. You may *think* that you don't have 'limit' switches, but that is simply a semantics argument that indicates you do not understand how Mach can handle 'home' switches.

Your assumption is theoretical and overly simplistic. You do not take deceleration into account in your calculations. In your case, you have the axis literally crawling up to the switch, but in any case, an axis is not going to stop instantly when the switch is triggered. The table weight (plus the vice, fixture and workpiece), the acceleration setting, the homing speed, 'cushion' between the switch and the hard stop, and finally the debounce setting all are part of the equation that predicts 'bang' or 'no bang'.

I would be interested to see a report that states an opto is 'more accurate' with higher voltage. Do you have a link to that?


Re: Opto switches behaving badly
« Reply #14 on: January 21, 2010, 07:49:59 AM »


I would be interested to see a report that states an opto is 'more accurate' with higher voltage. Do you have a link to that?




A 5 volt opto switch has a propagation delay of 4 nanoseconds but that same switch has a response to 110V of 1 gigliopiconanoumous second - but only one time.
Re: Opto switches behaving badly
« Reply #15 on: January 21, 2010, 01:26:42 PM »
 :) ;D :D ;D :)
Re: Opto switches behaving badly
« Reply #16 on: January 21, 2010, 01:49:51 PM »
Re: Opto switches behaving badly
« Reply #17 on: January 21, 2010, 02:58:01 PM »
That's interesting....wonder if that guy ever mounted any of those switches or did he go right into checking the accuracy of the pitch of the tap for the mounting screw threads.
Re: Opto switches behaving badly
« Reply #18 on: January 21, 2010, 03:53:43 PM »
I also found some interesting information here:

http://www.cnccookbook.com/CCCNCNoise.html
Re: Opto switches behaving badly
« Reply #19 on: January 21, 2010, 04:20:52 PM »
Good info. That makes me think how lucky I am to be past all of that - but I remember well....The machine is 'well' now and a joy of reliability.

 I had a noise issue and totally rewired the limit switches (mechanical) and a few other things that made the job of conversion tedious as hell. Then found that during the initial build, my daughter borrowed the computer that I had bought for the mill. She needed a printer so she installed one and I didn't know.....the problem was that she had left pages to be printed on the computer so the printer was being called to print - on the main parallel port that Mach3 was running the mill on. Was that noise - YES! That was resetting the Mach3 at random times. I wanted to kick the cat! (but we didn't have one)

This is totally (almost) off the subject but right in with a noise issue.

I must bring that up once in a while - it's so obvious - too obvious - and is totally overlooked.....clean up the computer first!