Touch / Continuity Sensing

[sbaum]

Looking for some ideas on sensing contact between 2 metal parts.

The application is a part gauge fixture. The gauge and part are both conductive metals. There is an aluminum test tool that is isolated from the rest of the gauge. We need to sense if that tool comes in contact with the part and turn on a light.

We have been using Omron D5C-1DA0 touch sensors for this. Works reasonably well, although can be a bit sensitive to set up. These sensors are now being discontinued so we're looking for another solution.

The toolmaker's suggestion has been to use a 5vdc source to a relay coil. Could connect the 0vdc to the fixture chassis and run the 0vdc of the relay coil to the isolated tool. Of course this would work but in my view this is a shock hazard for the operator when loading the part. Unless there's a way to limit the current to <1mA. (Still don't know what my electrical inspector would say)

Seems there must be a solution on the market for this but I've search several industrial sensor suppliers and so far no success.

[joseph_e2]

I've seen these in use for something similar:
https://www.se.com/us/en/download/document/6008346/
https://tesensors.com/global/en/prod...erence/ST08164

I'm not sure if they're still available or not, but I remember buying some as recently as 2016 or so. With Schneider having bought Telemecanique who appears to have bought R.B. Denison, support may be spotty. And there isn't a whole lot of info on their website, but I've attached a slightly better datasheet that I had saved.

One application was detecting when a metal part reached a gate on a conveyor. The gate was plastic with 2 metal pads that the part would bridge. Another application was as a jam-up detector. The stack of parts would buckle, contacting a plate above them which would cause the switch to close.

[5618]

They're marketed for level sensing of conductive liquids, but if you have a dry process maybe you could use something from Warrick Controls to detect conductivity when parts touch.

https://www.gemssensors.com/product/...ivity-controls

Edit: I would consider 16MA... for low value 4.7K threshold. I don't think you would need something high like 1M ohms. Another one to consider is 16VM... for field settable threshold if you want to try different settings.

[sbaum]

Thanks for the suggestions so far.

I'd prefer 24VDC controls but the Telemecanique unit is close to what I'm looking for. Now will need to see if available...

I had also thought of the Warrick Controls units. I may need to investigate those further as well.

[joseph_e2]

You might also reach out to Keyence. Just be prepared to hear from them pretty frequently afterwards...

[I_Automation]

I have used level sensors designed for 3 probes in liquid. You can jump the Low level terminal to COM probe terminal and wire almost anything between COM & High that will give you a relay output.


The sensors put out a safe signal that doesn't energize the fluid it's in and is safe to the touch.

[rdrast]

Around here, I just tie the 'Sensor' part to +24VDC through a 2.2K pull-up resistor, and also tie the 'Sensor' part to a PLC Digital input. The product is grounded through the machine frame.



It is normally high, but goes to low if the product touches the sensor at all.

[sbaum]

Originally Posted by rdrast

Around here, I just tie the 'Sensor' part to +24VDC through a 2.2K pull-up resistor, and also tie the 'Sensor' part to a PLC Digital input. The product is grounded through the machine frame.

It is normally high, but goes to low if the product touches the sensor at all.

This solution is attractive but I question the safety in my application as the operator is in direct contact with the part. There's 10mA current available between the 'Sensor' and the frame. (1mA can be felt, >6mA can cause loss of muscle control per OSHA)

[I_Automation]

Originally Posted by sbaum

This solution is attractive but I question the safety in my application as the operator is in direct contact with the part. There's 10mA current available between the 'Sensor' and the frame. (1mA can be felt, >6mA can cause loss of muscle control per OSHA)

The operator will only be in contact with the grounded 0V, and the operator creating contact between ground and the object tied to the 0V side of a relay contact will not feel anything from the 24VDC power supply.



I have even seen older systems using the ground the same for 120V coils without anyone getting zapped

[OkiePC]

I've seen it done with 24 volts and a resistor and with liquid level relays like this:

https://www.automationdirect.com/adc...ers/clc1-d-120

I prefer the liquid level relay because a resistor on a terminal block is sometimes problematic if it gets messed with or lost during repairs and the liquid level relays use low voltage and a very low current.

The one in my link is "5 volts DC pulsed".

The liquid level relays also have a sensitivity pot, so if things get grungy and give a false positive, or fail to detect contact, you can adjust them easily.

[KingPresident]

Originally Posted by joseph_e2

You might also reach out to Keyence. Just be prepared to hear from them pretty frequently afterwards...

LOL.
I had to catch my breath after that one!

[joseph_e2]

Originally Posted by KingPresident

LOL.
I had to catch my breath after that one!

We actually use a fair amount of their products, so I've managed to "train" them that I'll reach out when I need help. It took a bit, though...

[I_Automation]

Originally Posted by joseph_e2

We actually use a fair amount of their products, so I've managed to "train" them that I'll reach out when I need help. It took a bit, though...

I have a lot of respect for my Keyence guy. Calls ahead and asks to schedule a visit if going around with someone from national or international, doesn't flood me with spam, etc. And does help out.



Now Cognex? I won't login to their website or download any datasheet or catalog. Plus especially do not respond to the 3 times weekly emails to win a free vision system.


They now have you in their database and will try to get to you every way they can - email, call, show-up at your office, show-up looking for you anywhere they find out you are going.


I will not spec Cognex components in any quote and if a customer requests Cognex I try to find a compatible product from another source or tell them they have to buy the Cognex stuff themselves.