Figure 3 – Industrial batch reactor P&I diagram
Most of old-fashion PID tuning methods require PID data from which clear
step change of PID OP in manual mode is visible or data showing
oscillatory PID controller behavior, as shown in Figure 4. All these
tests are time consuming, especially in the case of a slow TC, and they
can seriously harm the process and produce off-spec product. Based on
the conducted step-tests in manual mode the user will often need to
calculate (visually ) several parameters from the trends,
depending on the used old-fashion PID tuning method.
Trial-and-Error method will require to know time-to-steady-state (TTSS)
based on which the process time constant (parameter which explains
the speed of the response ) can be calculated. This parameter can be
correlated with integral tuning parameter, but P&D parameters will be
set just as a guess value based on the user past experience, as shown by
Table 1. For using closed-loop Ziegler Nichols PID tuning method user
needs to mark ultimate proportional gain and ultimate oscillation
period. For open-loop Ziegler Nichols PID tuning method the user needs
to calculate process reaction curve and its parameters. Relay method
needs high and low peak values of PID OP which made PID controller to
oscillate in manual mode. Lambda method requires to know closed-loop
dead time and closed-loop time constant. IMC method needs to have
open-loop dead-time, gain and time constant, based on visual trend
analysis. Some other methods require the calculation of decay ratio
(typically set as 1:4 ).