Machsupport Forum
Tangent Corner => Tangent Corner => Topic started by: Overloaded on December 14, 2009, 01:35:24 PM
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Hi all,
I could use some input.
I'm working on an apparatus to measure and sort small pre-cut parts by length.
Using an LVDT type probe and associated panel meter to determine good from bad.
To sort, I just need to shift a shuttle block either left or right which I thought I'd do with 2 Pull Solenoids......energise 1 for good parts and the other one for bad.
I also was thinking of an air cylinder with a 4 way solenoid pilot valve but that would just be more hardware to wear out.
Now the questions...
This will run several million parts through per year so what would be the best to use as far as durability/longevity as far as the electrical components go ?
Mainly, the coils in the solenoids. AC or DC ? Control with SSR ?
EDIT: Actually, the contacts in the relays that operate the coils...I'm afraid they wont last long with the arcs when they open.
Relay OUT or Transistor OUT PLC ?
I can make it work....but not sure of how to make it last.
Thanks for any input.
I assume all responsibility of course....and I've got the burned up screwdrivers to prove it. :D
Russ
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Hi Russ,
If it was me I would use an SSR or two SSR's.
The switched load must, of course, be AC but the low drive current (optical isolation), long life and extremely low noise (zero crossing point switching) make SSR's the preferred choice.
Tweakie.
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Tweakie,
Thanks for the input !
I will find suitable AC solenoids and go with the ssr's.
I tried arc suppressors which worked well but your sounds like a better plan.
Thanks again,
Russ
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You could, of course, always use the SSR output to switch a transformer with rectified DC output for your existing solenoids. The possibilities are endless ;D
Tweakie.
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Your're reading my mind Tweakie.......I thought only my wife could do that. lol
My next question was to be: What is the difference between a DC coil and an AC of the same rating and what would result from running AC through a DC coil ? Assuming a 50% duty cycle.
Thanks Tweak
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It will depend on the AC frequency and design of the armature (shaded pole etc.) but basically DC is quiet, AC is noisy.
Tweakie.
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Just another thought on your 50% duty cycle......
If you are thinking about switching 1/2 wave rectified AC through an SSR then it will not switch on reliably unless you provide a small load prior to the rectification (the SSR needs to see both the positive and negative going parts of the waveform to detect the zero crossing point).
Tweakie.
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Tweakie,
I just set up a quick test.
OPTO SSR- 110 to 24VAC transformer-rectifier-coil.
I used another little DC PS to control the OPTO through a tactile switch.
It seems to work flawlessly.
I vaguely recall a discussion about Zero Crossing SSR's.
Not sure but are there 2 types....one being NOT zero crossing....and that is maybe what I have ?
I have the SSR before the transformer. Are you meaning to put it between the rectifier and the coil ?
Can you estimate the resistor/load value that would be required ?
Thanks Tweakie
Oh...the duty cycle was in reference to the pull soleniod..only on 50% of the time.
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If you have the SSR before the transformer then the transformer is the load and no addition is necessary.
Good fortune with your project and I look forward to perhaps seeing a pic or two of the apparatus in operation.
Tweakie.
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Thanks for your help Tweakie,
I have a CRUDE model running now, will take a pic ... maybe a short vid when finalised.
My target was 1 - 1.2 seconds per part but this looks to be closer to .8 seconds.
Russ
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Russ, Looks like you have discovered the advantages of SSR. ;D
Tweakie.
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Hi Tweakie,
Here are 2 shots of the little trinket. I am waiting for a piece of 1" cast aluminum tool and jig plate to mount it to.
The parts will be delivered to the vertical guide chute by a vibratory feed bowl.
It's accurate to .01 mm. Can set the resolution to .001mm but not necessary for this app.
Was hoping for 1 sec per part, .8 would be better, this runs 85 parts per min. .7 sec ea. GREAT !
That will do the 110,000 per week easily. Will replace the DC solenoids w/AC to eliminate the transformers and rectifiers.
Thanks for your help,
Russ :)
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Very nice machine.
Tweakie.
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I much prefer DC solenoids than AC, AC only has the edge over DC when pulling in due to the inrush current making for fast pull in.
If an ac solenoid seizes, or fails to pull in it usually means game over for the SS operator or blows a fuse.
This does not happen with a DC coil.
Once pulled in, the DC solenoid is more efficient.
A back EMF diode is recommended across the coil, this not only prevents a spike at switch off, and saves relay contacts, but assists the use of unfiltered DC, if you so wish, as the free wheel action of the diode carries the current through at the point the voltage goes to zero in 120Hz ripple.
This has been the coil of choice in Europe for many decades, N.A. is now getting on board.
Nosmo.