National-instruments PID Control Toolkit 371685C-01 Instrukcja Użytkownika Pobierz pdf (Strona 15)

Chapter 2 PID Algorithms

LabWindows/CVI PID Control Toolkit User Manual 2-6 ni.com

Figure 2-2 illustrates the autotuning procedure excited by the setpoint relay experiment,

which connects a relay and an extra feedback signal with the SP. Notice that the PID Library

autotuning functions directly implement this process. The existing controller remains in

the loop.

Figure 2-2. Process under PID Control with Setpoint Relay

For most systems, the nonlinear relay characteristic generates a limiting cycle from which the

autotuning algorithm identifies the relevant information needed for PID tuning. If the existing

controller is proportional only, the autotuning algorithm identifies the ultimate gain K

and

ultimate period T

. If the existing model is PI or PID, the autotuning algorithm identifies the

dead time and time constant T

, which are two parameters in the integral-plus-deadtime

model, as follows:

Tuning Formulas

This package uses Ziegler and Nichols’ heuristic methods for determining the parameters of

a PID controller. When you autotune, select one of the following types of loop performance:

fast (1/4 damping ratio), normal (some overshoot), or slow (little overshoot). Refer to the

following tuning formula tables for each type of loop performance.

Table 2-1. Tuning Formula under P-Only Control (Fast)

Controller K

P 0.5K

— —

PI 0.4K

0.8T

—

PID 0.6K

0.5T

0.12T

–

P(I) Controller Process

Relay

–

s() =

τs–

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National-instruments PID Control Toolkit 371685C-01 Instrukcja Użytkownika Strona 15