FX Series Programmable Controlers Applied Instructions 5
5-105
Configuring the PID loop
The PID loop can be confi
ured to offer variations on PID control. These are as follows:
It should be noted that in all situations there must be a proportional or ‘P’ element to the loop.
P - proportional change
When a proportional factor is applied, it calculates the difference between the Current Error
Value, EV
n
, and the Previous Error Value, EV
n-1
. The Proportional Chan
e is based upon how
fast the Process Value is movin
closer to (or further awa
from) the Set Point Value NOT
upon the actual difference between the PV
nf
and SV.
Note: Other PID s
stems mi
ht operate usin
an equation that calculates the Proportional
chan
e based upon the size of the Current Error Value onl
.
I - integral change
Once a proportional chan
e has been applied to an error situation, ‘fine tunin
’ the correction
can be performed with the I or inte
ral element.
Initiall
onl
a small chan
e is applied but as time increases and the error is not corrected the
inte
ral effect is increased. It is important to note how T
I
actuall
effects how fast the total
inte
ral correction is applied. The smaller T
I
is, the bi
er effect the inte
ral will have.
Note: The T
I
value is set in data re
ister S
3
+4. Settin
zero for this variable disables the
Inte
ral effect.
The Derivative Change
The derivative function supplements the effects caused b
the proportional response. The
derivative effect is the result of a calculation involvin
elements T
D
, T
S
, and the calculated
error. This causes the derivative to initiall
output a lar
e corrective action which dissipates
rapidl
over time. The speed of this dissipation can be controlled b
the value T
D
: If the value
of T
D
is small then the effect of appl
in
derivative control is increased.
Because the initial effect of the derivative can be quite severe there is a ‘softenin
’ effect which
can be applied throu
h the use of
K
D
, the derivative
ain. The action of
K
D
could be
considered as a filter allowin
the derivative response to be scaled between 0 and 100%.
The phenomenon of chasin
, or overcorrectin
both too hi
h and too low, is most often
associated with the Derivative portion of the equation because of the lar
e initial correction
factor.
Note: The T
D
value is set in Data re
ister S
3
+6. Settin
zero for this variable disables the
Derivative effect.
Control
method
Selection via setup registers
Description
S
3
+3 (
K
P
) S
3
+ 4 (
T
I
) S
3
+ 6
(T
D
)
P User value Set to 0 (zero) Set to 0 (zero) Proportional effect onl
PI User value User value Set to 0 (zero) Proportional and inte
ral effect
PD User value Set to 0 (zero) User value Proportional and derivative effect
PID User value User value User value Full PID
