Radar sensors getting stuck

What would cause a radar level sensor to get stuck at a certain level, IE stop updating?

I have a customer which uses both Endress-Hausser, Omega and Vega (Vegapuls 62?) radar level sensors, and it appears, at random times they get stuck at certain readings. When measuring the signal ill read a voltage corresponding to the level they are stuck at, and will hover around them a +/- few milivolts. I am unsure what the models are at the moment. I was able to solve the problem by disconnecting the signal wire and re-inserting it into the plc and it would give the correct readings again. Before it was just the Omegas doing it, now it appears they all do it from time to time. This problem has been happening for about a year.

The PLC is a Omron CJ1M, another which sensors are connected to is an older Omron CQM1H-CPU51,. The problem happens to any radar sensors connected to either of the PLCs.

Does anyone know what causes this? And how I can fix it?

Thanks!

For example, it would read something like 69.9%, and I would have voltage reading of 3.8V. And it would stay there, even though the tank would be completely emptied. The sensor would get unstuck only after I disconnect the - or + signal wire and re-insert it.

One solution is to use buttons to break contact momentarily, but I am looking for a cause of this problem so I can fix it.

Random occurrences? Can or have you checked the local device display if it has one compared to the PLC?
Current loop signals? Can you verify the actual milliamps during false reading?
Are these HART signals? If so do you have a way to connect via HART to check readings and current output?
Some transmitters have test meter terminals to verify current output. Another way is to put a temporary resistor (250 ohm precision) in series and calculate the current from measuring the voltage across the temp resistor. This would give you a chance to verify without breaking the loop and resetting the transmitter.
Is it only level signals? Are there any other analog signals? Different tanks?
Can you correlate it to something like after a pump runs, mixer running, or whatever? Possible ground loop? Any hardware, wiring, power changes just prior to false readings?

That is strange. I assume the signals are 4-20ma and not voltage. Is the cable from the sensors screened? If so is it only grounded at one end or not grounded? Should be grounded at the PLC card. Is the DC supply grounded? I never ground my DC supplies and leave them floating and they are also isolated supplies and have not seen this problem.

Are you within the sensors allowed input impedance of the analog card? Just a random thought, odd issue..

BobB said:

If so is it only grounded at one end or not grounded? Should be grounded at the PLC card. Is the DC supply grounded? I never ground my DC supplies and leave them floating

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WOW, I think you may be onto something!!! I have a location that uses 3 - 4-20ma sensors and every now and then the readings will stop responding. I have replaced PLC, Analog module, all 3 sensors are grounded at the PLC, and the power supply is GROUNDED.

I have exactly the same setup at another location and the power supply is NOT grounded, and I have NOT had the first problem???

I wonder!!!!

BObB:

Can you explain to me your theory on why you leave your power supplies UNGROUNDED?

Thanks

damica1 said:

BObB:

Can you explain to me your theory on why you leave your power supplies UNGROUNDED?

Thanks

Click to expand...

When you ground your power supply you lose isolation from ground noise and can cause issues if the parameters are right but is not an issue 99% of the time but that 1% that it is can be a real pain to find and if the supply is used for other things like controls and digital I/O leaving it ungrounded can be a PITA for troubleshooting.

This is why sensitive equipment / circuits should have their own supply and can be left ungrounded if necessary.

A programmer that I run into quite a bit on projects was having a similar issue with an EH radar unit over an open manure pit. He told me they start getting a little wacky when they get subjected to the cold weather sometimes. Just an idea...

I've seen him take portable magnetic block heaters and stick them to devices to keep them warm before.

http://www.amazon.com/Kats-1153-Han...3344295&sr=8-1&keywords=magnetic+block+heater

I've seen this happen on a Vegapuls due to condensation in the radar cone. It would lock on a fixed value until it was power cycled. Essentially, the radar is picking up obstructions in the cone and gets "confused". I would look at setting up false signal suppression to suppress that nuisance signal.
As Cow mentioned, a small heater on the cone would probably give you the same result.

If I left the power supply ungrounded would I still attach the "Drain" wire from the sensor to "Earth Ground"?

Before you decide to leave the power supply ungrounded, search the site about the pros and CONS of doing so. I may be showing a little bias there.

Another problem I had, has added to the CONS. You might think that you are leaving the system floating by removing the ground connection on the power supply, but it ain't necessarily so. I had reason to check the 0V to GND on a system and so removed the ground connection whilst testing resistance. Most of the system came in at 125K ohms, which is almost floating, I had two flow meters that gave 5K ohms each and a modem that was 0ohms. If I hadn't grounded the supply I would have been relying on a connection of unknown current capacity to ground the system. So you don't create an ungrounded system, you create an unknown system.

On to something like your problem. I had two guided radar systems that caused problems, the sensor wasn't isolated and even though they were in separate tanks they interacted with each other. I ended up adding loop isolators to separate the signals.

so yes unground the PSU, and have the grounds on one place for each cable, that is mostly at the sensor.
the inputs should have isolation so NO grounding is needed, nor better.

Alright, so there has been some development on this. We still don't know for sure what is causing the problem.

What we know so far. There are 6 tanks indoors, and 20 outdoor.
Half the tanks outdoor are Vegapuls 62s, the other half are Endress + Hauser Micropilots.
The tanks indoor are Vegapuls 53s.

All of the radar sensors seem to get stuck indiscriminate of brand. We have had a similar issue with a micro pilot at another location, but it has only happened once.

The radar sensors inside the facility, vegapuls 53s, VERY FREQUENTLY get stuck at levels.

I have phoned VEGA, and basically the only possible cause they gave me for this occurring is due to something with the False Signal Suppression accumulating over time or something along those lines. I didn't fully understand the description of the cause and im currently reading into it.

Not knowing anything about radar sensor this is only a guess but it sounds like false echo’s are stored somewhere (I couldn’t begin to tell you why) and maybe when that buffer gets full the unit stops reporting. Disconnecting the signal wire might put the unit into some sort of mode that (among other things) dumps the buffer so that when you reconnect the sensor it starts working again. Again, just a guess.