![]() Parallel Controller Algorithm Some academic textbooks discuss the parallel form of PID controller, but it is also used in some DCSs and PLCs. This algorithm is simple to understand, but not intuitive to tune. The reason is that it has no controller gain (affecting all three control modes), it has a proportional gain instead (affecting only the proportional mode). Adjusting the proportional gain should be supplemented by adjusting the integral and derivative settings at the same time. Try to not use this controller algorithm if possible (in some DCSs it is an option, so select the alternative). Significance of Different Algorithms The biggest difference between the controller algorithms is that the Parallel controller has a true Proportional Gain (Kp), while the other two algorithms have a Controller Gain (Kc). Controller Gain affects all three modes (Proportional, Integral and Derivative) of the Series and Ideal controllers, while Proportional Gain affects only the Proportional mode of a Parallel controller. This difference has a major impact on the tuning of the controllers. All the popular tuning rules (,,, and others) assume the controller does not have a parallel structure and therefore has a Controller Gain. To tune a Parallel controller using any of these rules, the Integral time has to be divided and derivative time multiplied by the calculated Controller Gain. The second difference between the controller algorithms is the interaction between the Integral and Derivative modes of the Series (Interactive) controller.
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