Chapter 10 Built-in PID Functions
10 - 3
10.3.3 P control
As shown in the following expression (10.3.7), MV in P control is composed of the proportional term operation
MV_p only. The proportional term operates in the form of the multiplication of the error by the proportional
coefficient. The user must set the proportional coefficient properly according to the system. The greater the
proportional coefficient is set, the more sensitive the system becomes to the error.
)
7
.
3
.
10
(
)
6
.
3
.
10
(
p
p
p
MV
MV
E
K
MV
=
=
The development of P control of any virtual system has the following characteristics.
The virtual system below is designed for better understanding by the user, but may be different from an actual
temperature system.
In the simulation above, SV is 50.0. the K_p value is properly adjusted to obtain the PV development above.
Four seconds after the operation starts at the initial temperature of 20
℃
, the system settles into the stable
state and thereafter remains constant at 46.2
℃
. The offset is 3.8
℃
(around 7.6%). The reason there exists a
permanent offset in P control is that, as PV approaches SV, the E gets smaller and also MV gets smaller and
comes into equilibrium at the equilibrium point with K_pat the equilibrium point (46.2 abov
℃
e). The offset
inherent in the P controller can be compensated by using PI control.
0.0
10.0
20.0
30.0
40.0
50.0
60.0
0
2
4
6
8
10
12
14
16
Second
T
e
mp
eratu
re
