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Sudden Lock Down of Load Cell Values
I faced an issue in my plant where the load cells suddenly lock down to zero while the process starts.

I'm having an issue in my batching plant where one of the load cells of the materials locked down to zero while the batch starts. As a result the material overflowed and the batch had to abort. This happened with other load cell for different hoppers. This problem is not associated with just one load cell. It is happening to all load cells on random basis. Sometimes no issue at all and system works fine. Even after the issue if done with a complete restart of the system, then after the re-boot the system works fine.

Could anyone suggest me what i can do?

First thing comes to mind is noise on your cable. As I understand it the cable should be grounded on one side only. Interestingly enough we just developed a problem with ours as well, and I will watch for responses for others to see what they think and offer for advice.

Chuck

Hello Chuck,

Thank you for the response.

As you said about earthing i checked my panel. I found that the cables are earthed on one side. but i found a voltage of 35v between the neutral and the earth terminal while the system is online. this voltage is appearing only in the batching control panel, and it is not seen on the main incomer power panel. On the batching control panel there is a control transformer where two phases are its input to the primary, and 230v is taken as output. This output is used inside the control panel. Also this 230v is also feeding another isolation transformer for a secondary batch panel where the loadcells are connected. the primary of this panel is 238v, and secondary is 228v as measured. when i checked the neutral to earth voltage in isolation transformer, i see 63v and phase to earth is 150v.

Good morning

That is an interesting find, could be bonding issue or not. The first thing I would do is find out if it is an induced voltage or (ghost voltage). A high impedance digital voltmeter will indicate small voltages that really are nonexistent because of the sensitive qualities of the meter. Have you checked it with what I call an iron meter like a Simpson? Any of the meters with analog meter movement will give you a better reading of actual voltage. You may be on to something there.

Chuck

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Your transformers don't seem to have the secondary grounded like they normally do in Nth America. Since you mention 238 Volts, I assume you are in a different part of the world. What do your regulations call for?

Normally our instruments are powered from 24 Volts DC which has the negative grounded, and also bonded to the input I/O modules Negative. I suspect your input signals are floating, and perhaps being pushed too high or too low for your control system I/O module to read, basically above 24V or below zero.

Is there anything else in the plant that seems to act strangely at the same time?

Yes, yes. I totally agree Roy. I run into floating supplies on our
equipment troubleshooting it makes me crazy. Thanks for posting.

Chuck

By Nobel Mandaza on 4 February, 2018 - 8:08 am

I do't get it when you say loadcell lockdown to zero. But if it's as you say, then is seems like an fail_safe trigger. Which is very rare in batching systems unless it's a system that i am not familiar with, or it could be auto_tare. However, check your zero config and tare settings. Also adjust your accumulated zero to typical value of 300kg per 4000kg target. You do the math according to your target weight and see if the problem persist.