I am an resident Engineer working in an shiplift industry we are using hoist system in which loadcell connected the loadcell is communicating with Siemens s7-400 PLC through Analog Input (4 to 20 m amps, Siemens s7 331-7NF00-0AB0).

Problem:

we are using remote IOs in which Load cell communicating with Analog input. problem is there is sudden change in load but physically there is no load on hoist which Scada shows. We have measured mamps both the load cell and AI module all are with in the limit.

so please help on this issue to sort the problem.

 

If you change the load and the indicated load doesn't change, there's a good chance that the load cell was overstressed and the strain gage is unglued. If the scada shows change that didn't really occur then I would be looking at the analog input. Actually putting a current meter in the circuit is a really good way to divide and conquer. It's not clear from your message which is happening but with a meter in the circuit it's easy to tell which half is malfunctioning. change the load and watch the meter, no change or wrong change = loadcell. If the loadcell is working then wiring or AI.

Regards
cww

 

sir

actually i have checked the mA value in both load cell output and AI module. the value found to satisfactory. but when you look at the SCADA the value keeps on increasing Even though there is no change in load and mA. sir can give me the suggestion were i need to work on it.

 

What kind of load cell do you have that outputs a milliamp signal?

Most load cells have a mV/volt output.

Are you sure that your load cell has a conditioned current loop output?

 

Without a recent change in software, it sure sounds like your AI is bad. Not much to do but swap it.

Regards
cww

 

Did you check the earth connections?

Is the negative from the 24V supply connected to earth, Is this because you put a wire between the 24V negative terminal and earth, or is this by accident because the PLC connects the 24V negative to earth.

How is the negative supply of the analog output from the loadcell connected to earth?

Does the analog input have a reference terminal? Is this connected to earth? Check the manual what to do with this. When not connected properly this can cause drifting values from the analog input.

Just some ideas, hope it helps.

HeBa