Specifications

SIMATIC Instructions
9-86
S7-200 Programmable Controller System Manual
C79000-G7076-C233-01
Since the digital computer must calculate the output value each time the error is
sampled beginning with the first sample, it is only necessary to store the previous
value of the error and the previous value of the integral term. As a result of the
repetitive nature of the digital computer solution, a simplification in the equation
that must be solved at any sample time can be made. The simplified equation is:
M
n
= K
C
< e
n
+
K
I
< e
n
) MX
+
K
D
< (e
n
–e
n–1
)
output = proportional term + integral term + differential term
where:
M
n
is the calculated value of the loop output at sample time n
K
C
is the loop gain
e
n
is the value of the loop error at sample time n
e
n - 1
is the previous value of the loop error (at sample time n - 1)
K
I
is the proportional constant of the integral term
MX is the previous value of the integral term (at sample time n - 1)
K
D
is the proportional constant of the differential term
The CPU uses a modified form of the above simplified equation when calculating
the loop output value. This modified equation is:
M
n
= MP
n
+ MI
n
+ MD
n
output = proportional term + integral term + differential term
where:
M
n
is the calculated value of the loop output at sample time n
MP
n
is the value of the proportional term of the loop output at sample time
n
MI
n
is the value of the integral term of the loop output at sample time n
MD
n
is the value of the differential term of the loop output at sample time n