Chapter 13 Auto Tuning - Mitsubishi MELSEC-Q series Programming Manual

CHAPTER 13
Auto tuning is designed to make the initial setting of the PID constants.
The auto tuning of the CPU module can be used for processes that can be approximated with a primary delay plus dead time
represented by the following expression.
Example) Process with relatively slow response such as temperature adjustment
K -Ls
1+Ts e
K: Gain, T: Time constant, L: Dead time, s: Laplace operator
Auto tuning can be used for the loop that uses the S. PID or S. 2PID instruction.
Auto tuning is performed in the ZN process: stepped response process of Ziegler and Nichols.
[Outline of stepped response process]
With no control operation being performed, change the manipulated value (MV) step by step and look how the process
value (PV) varies.
1) When the manipulated value (MV) is changed step by step, the process value (PV) begins to vary slowly.
Soon, the PV will vary faster, then vary slowly again, and finally settle at a fixed value.
2) Draw a tangent line at the place where the process value (PV) varies fastest, and find the points of intersection
A, B where this tangent line crosses the horizontal axis corresponding to the first process value (
process value (
This provides the equivalent dead time (L) and equivalent time constant (T) as shown below.
From the equivalent time constant (T) and maximum process value width (Y), calculate the maximum
3)
slope (response speed) R
Apply the equivalent dead time (L) and maximum slope (R) to the Ziegler and Nichols' adjustment rule, and
calculate the proportional gain Kp (P), integral constant T
Manipulated value for
auto tuning AT1MV
Process value PV
( )
AUTO TUNING
).
1
Y/T.
=
X
1
Y
Maximum slope
R
(response speed) T
A
0
Equivalent dead Equivalent time
time L constant T
(I) and derivative constant T
I
B
Maximum measurement width
) and last
0
(D).
D
Y
(s)
Time
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