Sremac Posted August 17, 2011 Report Share Posted August 17, 2011 For some time I get complaints from my customers that my PID controlled heater loops overshoot a lot at the beginning of the operation, and then functions OK. Talking about V570. PID is connected to a physical output driving SSR thru a PWM FB. At the begining of the heating cycle PID Control Value is 100% and PWM output bit is permanently ON, of course. But later on, as the temperature rises, CV naturally starts to fall (99%, 98%, ..., 57%, ...) but PWM output bit is still permanently holding ON state, without any blinking, so my SSR is fully open, providing 100% average power to the hater, overshooting the setpoint a lot. Then, when CV becomes 0, output bit is finnaly reset, and from that moment everything is working fine - the temperature is falling, CV is rising, PWM is producing modulated pulses and the temperature is finaly under control. Also, I searched for something else that could keep output bit clamped to ON - absolutely nothing, because it occurs only two times in my program - in PWM block, and in a plain different net, driving the physical output. Also, PWM scan FB is placed in the Main Routine, so it executes all the time. This is why I added a few ladder nets that are forcing CV to zero for 0.1 second every 10 seconds during initial heating of the system, and that solves the problem, i.e. PWM output bit is pulsing immediately after CV decreases below 100%, so the regulation is fine all the time, without noticable overshoot. The question is why such a strange behaviour? (Of course, I use VisiLogic 9.3.0 with the latest OS 3.4.0) Quote Link to comment Share on other sites More sharing options...
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