Kurt Posted March 14 Share Posted March 14 Hi AzeoTech and forum users, We have a number of heating elements supplied with controllable DC power supplies whose current output is set by a DAQFactory PID loop. This system has worked well for us, including autotuning. However, we recently increased the size of the system being heated, resulting in a larger current needed to achieve similar temperatures. For these larger systems, autotuning no longer works well. When we try to autotune using the larger systems, the autotuning appears to work, but post-tuning control (including at the same setpoint used for tuning) is poor. We see large oscillations (sometimes undamped). We tried increasing the relay height to ca. 80% of steady-state output (i.e., if the steady-state output at the setpoint is 100, the relay tuning sweeps from an output of 20 to an output of 180), but the resulting autotuned parameters still result in oscillation. As a result, we've had to manually tune by trial and error. We suspect the autotuning problem is because of increased process lag with our larger system. As one example, once power is cut, the larger system takes 30-45 s for its temperature to decrease by half. Is there another autotuning method, or a way to adjust the methods used by the relay tuning method, that we could try to improve our results? We were wondering if there might be a way to fit our data to a "first order plus deadtime" model (whether in real space or Laplace space). Or if there might be other known tuning correlations we could use. Thanks for your help! Quote Link to comment Share on other sites More sharing options...
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