National Instruments 371685C-01 用户手册

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:

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)

P

0.5K

—

—

PI

0.4K

0.8T

—

PID

0.6K

0.5T

0.12T

SP

e

+
–

P(I) Controller

Process

PV

Relay

+
–

τ

( ) = 

τs

–

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