Machsupport Forum
Mach Discussion => General Mach Discussion => Topic started by: rkowash on April 22, 2008, 12:17:09 PM
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I am trying to use an NPN photo switch as an X axis home switch. I have it connected into a Campbell breakout board. The problem I am running into is that the input is flickering. I have a couple of thoughts that I am hoping somebody can either verify or kill.
1) First, the switch is rated 10-36V. The board is 11.6V, close to the bottom edge.
2) The other is the opto isolator may not be able to react appropriately with a 250Hz signal.
3) The Campbell board indicates it is setup for inductive or mechanical switches. I am sensing wood, so inductive is out, and I would prefer non-contact in this application.
I read on one post that a guy put a 1k ohm resistor between the signal and ground to limit the bounce, any thoughts?
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Try putting it in at another input if you can, I seem to remember someone having a problem with the Campbell board and an optical switch for the index pulse, think the inputs were too slow.
Hood
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Hi, Roger
Do you have a link or diag. for the photo switch.
You may need a pull-up or pull-down resistor "I read on one post that a guy put a 1k ohm resistor between the signal and ground to limit the bounce".
It would stabilize the "off" state until the sw has a solid "on" signal state.
Hood: I think the index pulse needed a mpg input that goes direct to the PP (no opto/direct) on the Campbell breakout board.
Hope this Helps, Chip
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rkowash, Post a link to the switch. We use some that measure distance. They put out an analog signal. Just wondering if that is the type you have.
Brett
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Here is a link to the switch. I am hoping that a resistor will do the trick!
Thanks
Roger
http://web1.automationdirect.com/adc/Shopping/Catalog/Sensors_-z-_Encoders/Photoelectric_Sensors/18mm_Round_-_Metal_(C18_Series)/Polarized_Retroreflective_(Axial)/C18P-AN-1E
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I think that is a Diffuse Reflective photo sw. with a sensitivity adjustment, out to 2 Meters ! I don't believe it would be a good choice for a Home switch. Might be OK for a limit if you can get it to calm down. The ones I used vary too much, they were nowhere near precise enough for homing.
However, these may be different........
A "Through Beam" or a "Retro-reflective" sensor would probably be a better choice. Still best is probably a snap action limit switch.
Sensing WOOD ? For homing ? Is your machine made of wood ?
RC
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I used a Photo Eye like this on a project last year as a home switch however, this is how we did it. The unit we used was the 'reflector style. We set up the photoeye and reflector at the home position, when the gantry broke the beam, the machine was homed. From your description thus far, it's hard to tell how your switch is mounted. Are you using a reflector. A pic or a drawing would probably help expedite a solution.
Regards,
Sid
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To answer some of the questions on the last two posts. I am cutting a SIP (structural insulated panel), it is a sandwich of 2 sheets of OSB and styrofoam in the middle. The parts are 8' x 24', and vary from 4.5" thick to 12.25" thick. The panels are vertical, and they move in the X, where the saw moves in the Y,Z, Beta, and tilt. This particular switch is fed off a powered coveyor into the saw. This trips the sensor. The X axis then clamps onto the part (now I have stepper precision) and backs the part up until this switch is clear. This will be my zero position.
I tried the retro-reflective sensors, but the skin of the part varies too much. I was having the same signaling problem on the Campbell board with a banner 312 photo sensor using through beam fiber optics. For using a mechanical limit switch, until the part is clamped, there is no guarantee that the part will be close enough to the switch to trigger it.
Ideally I can get this switch to work. If it is just too sensitive for the Campbell, I will probably hard wire this switch to the PLC, and code in that as soon as that input is latched, fire an output hooked to the Campbell. That way I maintain the speed, but can use the PLC to filter the signal.
Hope this helps explain my problem some.
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I tried the 1k resistor, and the sensor went to a constant on. I went with 100k resistor, and it slowed the pulse, but not enough. I tried about 10 different values in between, and while they all made a difference, none rocked the world. Any help would be great.
Roger
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Here's a thought... How about you add a relay to the mix. Have the photo switch trigger a relay (coil) and then use the switched relay circuit to trigger the BOB? I know it involves adding another piece of hardware but it's about all I can think of right now...
HTH,
Sid
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Thanks Sid, that is one of the first things I did. I put a 12v mini relay in there, but the switch does pass enough current to pull the contacts. I suppose I could go with a smaller, mini relay. I think at this point it would be easiest to burn an input and an output on the plc to make it work.
Roger
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Yeah, maybe. It's just kind of a shame to have to go that route if you don't already have the PLC integrated. Here is a link to a relay that has a 3V coil http://www.factorymation.com/s.nl/it.A/id.4962/.f?category=99 in case you wanna give that a go...
Regards,
Sid
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I agree, the PLC isn't the best. I will look into the relay, hopefully the draw is small enough the switch will let it pull in.