Effect of PID Parameters

Tuning of a PID controller should ideally lead to values of the P, I, and D terms of the controller that result in the most favorable actual control performance. The P-term is described by the proportional gain Kp or the proportional bandwidth Xp. The effect of changing the proportional band is shown in Fig. 4.41. 

A narrow proportional band can result in strong oscillations, while a wide proportional band will result in a large offset, in the absence of reset function. The I-term provides the reset function and is described by the integration time constant t, or reset time constant. The effect of different values of the reset time constant is shown in Fig. 4.42. 

When the reset time constant is too long, the process will come back to the setpoint very slowly. On the other hand, when the reset time constant is too short, oscillations will occur. The reset time constant is generally considered optimum when the temperature returns to setpoint as rapidly as possible without overshoot. 

The derivative term is characterized by the derivative time constant td or rate time constant. The effect of different rate time constants is shown in Fig. 4.43. 

When the rate time constant is too iong, the temperature changes too rapidly, resulting in overshoot and oscillations. 

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