M
Hi, We build a cascaded loop involving three input variables, and used simulators to input signal to the three channels. With the simulated inputs constant we, the output of the second PID is oscillating, between three points, and not full scale. The loop that we are having a problem with is a three term control loop, looks like this : 1. The input from a flow transmitter, call it FT1 goes into the first PID block, call it PID1. 2. The output from PID1 goes into exp block as a_in_0 3. Another input from another transmitter, call it FT2 goes into exp block as a_in_1. 4. The exp block executes the following a_out_0 = a_in_1 - a_in_0 + 50 5. The output a_out_0 on the exp block goes into the second PID2 CAS_IN. 6. A third flow transmitter FT3 also feeds PID2 as IN. 7. The BKCAL_OUT on PID2 is wired to BKCAL_IN on PID1. 8. The output OUT on PID2 is outputed to a control valve, 4 -20 mA. PID2 is in cascaded mode, and PID1 is in auto mode. We are using simulators for FT1, FT2, and FT3. All are set at about %50. We have a current meter to measure the output. The output oscillates, and is not stable, although all three FT are constant. When we change the value of any of the FT, the output behaves correctly, but the oscillation is still there. To test this we also changed the expression in the exp block, so as not to include the minus sign between a_in_1 - a_in_0, i.e. we changed the minus sign to a plus sign, and the output was stable. We have done multiple tests and it seems that the only case where the oscillation goes away is when we write an expression without a minus sign. Does anybody know of reason or reasons for the oscillation on the output. Thank you.............Monah