The shift supervisor complains that batches are running on the dry side, despite the fact that no recipe changes have been made. The only liquid in the batch comes from a vat that is supposed to reach 75% full then dump into the mixer. To get a handle on this problem, what are some steps you should perform?
You need to obtain “as –found” data. In this case, one of the things you need to find is the level of the vat when it's dumping. To do that, simulate the level transmitter at the analog input module for this loop, and watch the indication on the PLC. You want to see what the PLC's level indication is at the moment it provides the open signal to the outlet valve on that vat.
If this functions properly, stroke the valve on the vat from that same point. If the valve responds properly, you've verified everything downstream of the PLC input. If the valve doesn't respond properly, stroke the valve at the valve controller and calibrate as needed. Continue with the output side until it works correctly.
If the loop works with a simulation at the input module but not with the level transmitter connected, simulate the transmitter from the transmitter output terminals. A common way to do this is to simulate 25%, 50%, and 75%, and radio the operator for the PLC readout. If the PLC doesn't correctly respond, it’s a wiring issue. If it correctly responds, calibrate the level transmitter.