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α
2 Simple Application Controllers
Note:
Other PID systems may calculate proportional control based on the magnitude of the current
deflection only.
Table 6.27: Units for K
Units
· Analog input
· Function Block word output
0.1%
· Temperature input using the
Celsius scale.
· Temperature input using the
0.01%
Fahrenheit scale
Figure 6.22a. Characteristic response for different K
PV
K
P3
Integration Time, T
value not equal to the SV (this is called an offset error). With PI-control, a temporal integration
factor is added to deal with this problem. The idea behind PI-control is to ramp up the effect of
P-control, if an offset error persists over time. The T
automatically with the Auto-tuning function. The default value for T
be set to any value within the range of 1 to 32767 (in 100ms units). When a small value for T
set, the PV will reach the SV within a relatively short amount of time with greater searching
(e.g. more overshoot and more undershoot) for the SV.
Note:
Setting zero for this variable disables the integration effect.
Figure 6.22b. Characteristic response for different T
PV
Set value
(SV)
P
Type of Word data
K
P2
K
P1
. A common problem with P-control is that it tends to stabilize the PV at a
I
T
I3
T
T
I2
I1
Default Value
7.5%
0.75%
parameters
P
Set value (SV)
Process value (PV)
offset error
K
> K
> K
(Proportional gain)
P3
P2
P1
Time
parameter can be set manually or
I
parameters
I
0 < T
< T
< T
I3
I2
I1
Function Blocks 6
Range
0.1% to 3276.7%
0.01% to 327.67%
is 40 (4 seconds) but can
I
Process value in PI action
Process value in P-action
Time
6 - 59
is
I
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