I have a AB PLC that requires a input of 24v for my equipment to stay running. Its getting it return +24v from the sensor. This sensor we replaced with Mechanical that goes to ground when it sees no oil psi. I added a +24v ice cube relay and wired it N.C. When the sensor faults, it goes to ground, grounding the relay, which opens up the Contacts and kills the equipment. It works as intended but it is not fail safe. Meaning, if relay failed contacts would stay close and would not shut down in a faulted condition. I can't change the Digital Input to shutdown on low (0v) input. That would have been too easy. Anyhow, I need help coming up with something to make it fail safe.

 

You would need to have redundant sensors to be completely fail safe. Can you add a secondary sensor?
Do you have a spec sheet on the sensor?

 

You would need to have redundant sensors to be completely fail safe. Can you add a secondary sensor?
Do you have a spec sheet on the sensor?

I am more worried about the ice cube relay failing, staying latched when actually sensor (Kenco DNFT) called for a trip.
Its a mechanical (spring and ball) that grounds when no or low oil psi exist. In my yrs, these particular sensors have a very good life span.

http://kenco-eng.com/product/nfs-flow-safety-switch/#overview

 

Given that the sensor is closed on fault and open on good oil level, a broken wire from the sensor to the next black box is always going to look like a good oil level. So you need to get that next black box as close to the sensor as possible.

My suggestion is to shunt the sensor with a resistor - say 3.3K - right at the sensor. At the PLC end, use an analog input and a 12K resistor to 24 volts. Now the voltage will be around 5 V when the oil level is good, around 0 if it's low or if the wire is shorted to ground, and approaching 24 V if the wire breaks.

If you don't have an AI available, then you need some kind of window comparator. This needn't be solid state. There are ways to implement it with relays.

 

Have a pullup to +24V and wire the sensor between the input to 0V.
The pullup could be a lamp so it will light on oil pressure fail.
At least I think that is what you want. You say your equipments needs 24V to run. So is that 24V normally, and 0V on fail?

 

Have a pullup to +24V and wire the sensor between the input to 0V.
The pullup could be a lamp so it will light on oil pressure fail.
At least I think that is what you want. You say your equipments needs 24V to run. So is that 24V normally, and 0V on fail?

Simple pull-up won't work. He said he can't invert the PLC input. He probably can't use my idea either ...

 

I am more worried about the ice cube relay failing, staying latched when actually sensor (Kenco DNFT) called for a trip.
Its a mechanical (spring and ball) that grounds when no or low oil psi exist. In my yrs, these particular sensors have a very good life span.

http://kenco-eng.com/product/nfs-flow-safety-switch/#overview

 

You could use two ice cubes with the contacts in series and the coils on parallel.

 

I am still confused.
Is the input +24V on oil pressure fail, or +24V on normal running?
Does the PLC read the input active on +24V or what?
When the TS stated the equipment required 24V to keep running, that sounds to me that a fault condition would be 0V.
(It is early morning here and I am having trouble getting to sleep, so that may explain my dumbness)

 

Maybe I didn't explain the situation very well.

The previous sensor (if faulty) would not return the 24v back to the Digital I/O. The new sensor goes to ground (0V) on fault.

I am using relay (Y23234-A1004-X050) that is wired pin 30 to +24v and 87A back to the PLC digital input. I have +24 sitting on pin 86 and the new sensor is wired to 85. When the sensor sees 0 psi or low oil pressure, it will ground pin 85 on relay, causing the contact 30 and 87a to open, dropping the 24v back to the PLC, causing the equipment to die.

Now the failure point would be the relay. If it fails (which they rarely do) the contacts 87a & 30 will likely stay closed. which will cause $300K plus damage to the compressor. I am trying to figure out a way to make it fail safe.

My only option I am thinking would be to run a redundant relay.

 

I take it back. I mis-read the original post. I assumed an inversion was needed because of his relay. I concur with the pull-up suggestion. And I like the idea of using a light bulb as the pull-up load.

 

The way I am understanding it the sensor switch is the wrong logic for the AB input, is it not possible to make a simple program change to accommodate the correct logic?
Max.

 

The way I am understanding it the sensor switch is the wrong logic for the AB input, is it not possible to make a simple program change to accommodate the correct logic?
Max.

Not easily. I would have to rewire a Digital I/O card to down sinking which would change 1/2 my I/O's. I can't do that. I could add another digital I/O card. and change programming but then again $$$$.

 

I think I understand that the PLC needs to see +24 volt to enable operation. Do we have that right?

And the new sensor grounds when oil pressure is insufficient, right?

So you need to see 24 V on the PLC input UNLESS the sensor closes to ground, in which case you want the PLC input to be open but grounded should also work.

If so, then what's not to like about the pull-up? It even stops the system if the light bulb burns out.

 

I think I understand that the PLC needs to see +24 volt to enable operation. Do we have that right?

And the new sensor grounds when oil pressure is insufficient, right?

So you need to see 24 V on the PLC input UNLESS the sensor closes to ground, in which case you want the PLC input to be open but grounded should also work.

If so, then what's not to like about the pull-up? It even stops the system if the light bulb burns out.

I agree, Pull up method will work and a loss of 24V on the pull up will also shutdown the unit. Light isn't necessary as the equipment is un manned when running. PLC HMI shows the fault when it exist.

Thanks. ( i don't know why I didn't think about a pull up)

 

Not easily. I would have to rewire a Digital I/O card to down sinking which would change 1/2 my I/O's. I can't do that. I could add another digital I/O card. and change programming but then again $$$$.

I don't understand, why would sinking or sourcing matter if just changing the logic?
As I understand it, the sensor is just 2 wire contact closure?
Max.

 

The logic isn't reversed. The old sensor connected the DI to 24V when the oil level was good. The new sensor connects the DI to 0V when the oil level is bad. So the logic is the same. High = good, low = bad. His problem was that he needs to source current to the DI but his sensor is a sink. Solution is to add a source, in the form of a resistor, that the sensor can override.

 

The link shows a precision contact closure, with a 2 wire device, you should be able to use it sink or source, unless there is something in the sensor that requires anything different, a full explanation of the sensor would be required.
Max.

 

Reading the literature appears to confirm simple contact closure switch?
If so it should be able to use it in a sink or source arrangement, if the sensor logic is wrong then it should be simple to change the ladder logic for that input to accommodate it.
Max.

 

Could you use the sensor simply to blow a fuse or trip a cut-out in the +24V supply to your equipment?