The plant has several machines that combine and process various raw ingredients in measured amounts to produce particular kinds of plastics. They are numbered W1, W2, W3, and so on.
For the past several weeks, the W5 machine has been making plastic that is overly porous and of an uneven thickness. The plant’s chemical engineer says this is due to excessive temperature. “If it gets too hot, it gets too thin” he repeats every time he gives his analysis.
Yet, the PLC shows all of the process variables as having stayed in spec. The specs are the same ones used on W4 and W6, which make the same product. So what gives?
Your first step was to calibrate the temperature sensors. They were all in calibration. What now? The PLC relies not just on temperature sensors, but on the cabling and connections that form the temperature loop.
You probably don’t have a gross error here, just something that’s attenuating the temperature sensor signal(s) and causing the system to respond with more heat.
If you visually inspect the sensor wiring for sharp bends, or improper support (crimping), or corrosion at terminations, you may spot the problem quickly. If not, find the problem by measuring (e.g., loop simulator or point to point reference measurements on W4 and W6 compared to measurements taken on W5).
