NegentropicDev Posted July 17, 2020 Report Share Posted July 17, 2020 I'm seeing some unexpected output from a PID (Auto-tune) FB on a V1040. The PV is below the Setpoint, control direction is set to "0" (heating) as shown in the Config screenshot yet the output is going negative. This PID in question is using the TC 1 & 2 Avg as the process value (216.0°F) with a setpoint of 220°F yet the PID output is -10.4 with a low/high range of -50 to +50. The idea here is that an initial PID runs to derive an offset for the setpoint to use on the inlet air to help drive and stabilize the test fixture TCs that are in the oven. I was initially applying this offset directly to the setpoint and using that derived setpoint as the setpoint on the burner fuel PID but I quickly found out the PID FBs do not like constantly changing Setpoints. I'm now applying that as an offset to the inlet air PV for a similar effect but the output of the initial PID is causing this to drive in the wrong direction for some reason. When first starting the heat ramp the output starts off at +50 as expected but then runs down and eventually goes negative once the PV gets close to the SP. In the tuning screen you'll notice the P band column is specified in °F instead of just the %, it performs some quick math to calculate the % band which can be seen in the operands screenshot. In this case 10°F equates to 0.8% proportional band (-200 to 1100 input range) which is the value used by the PID block. The PID is reporting status value 4 "PID Running" so it's not reporting any problems. Anyone have any ideas why the output is going opposite the expected value? Cheers, Derrick Quote Link to comment Share on other sites More sharing options...
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