User manual

PID Loop Operation
(DL450 Only)
Maintenance
8--27
PID Loop Operation (DL450 only)
DL405 User Manual, 4th Edition, Rev. A
Select Forward/Reverse
It is important to know which direction the control output will respond to the error
(SP--PV), either forward or reverse. A forward (direct) acting c ontrol loop means that
whenever the control output increases, the process variable will also increase. The
control output of most PID loops are forward acting, such as a heating control loop.
An increase in heat applied will increase the PV (temperature).
A reverse acting control loop is one where an increase in the control output results in
a decrease in the PV. A common example of this would be a refrigeration system,
where an increase in the cooling input causes a decrease in the PV (temperature).
The Transfer Mode
Choose either Bumpless I or Bumpless II to provide a smooth transition of the control
output from Manual Mode to Auto Mode. Choosing Bumpless I will set the SP equal
to the PV when the c ontrol output is switched from Manual to Auto. If this is not
desired, choose Bumpless II.
The characteristics of Bumpless I and II transfer types are listed in the chart below.
Note that their operation also depends on which PID algorithm you are using, the
position or velocity form of the PID equation. Note that you must use Bumpless
Transfer type I when using the velocity form of the PID algorithm.
Transfer
Type
Transfer
Select Bit
PID Algorithm
Manual-to-Auto
Transfer Action
Auto-to-Cascade
Transfer Action
Bumpless
0
Position
Forces Bias = Control Output
Forces SP = PV
Forces Major Loop Output =
Minor Loop PV
B
u
m
p
l
e
s
s
Transfer I
0
Velocity Forces SP = PV
Forces Major Loop Output =
Minor Loop PV
Bumpless
1
Position Forces Bias = Control Output none
B
u
m
p
l
e
s
s
Transfer II
1
Velocity none none
The transfer type can also be selected in a RLL program by setting bit 3 of PID Mode
1, V+00 setting as shown.
PID Mode 1 Setting V+00
013456789101112131415 2Bit
Bumpless Transfer I / II select
SP/PV & Output Format
This block allows you to select either Common format or Independent format.
Common format is the default and is most commonly used. With this format both
SP/PV and Output will have the same data structure. Both will have the same
number of bits and either bipolar or unipolar. If Independent format is selected, the
data structure selections will be grayed out. The reason for this is that they become
independently selectable in the SP/PV and the Output dialogs.
Common Data Format
Select either Unipolar data format (which is positive data only) in 12 bit (0 to 4095),
15 bit (0 to 32767), or 16 bit (0 to 65535) format, or Bipolar data format, which ranges
from negative to positive (--4095 to 4095 or --32767 to 32767) and requires a sign bit.
Bipolar selection displays input/output as magnitude plus sign, not two’s
complement. The bipolar selection is not available when 16--bit data format is
selected.