Automation Direct DL05 User Manual page 89

Chapter 8: PID Loop Operation
Select Forward/Reverse
1
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 control loop means that whenever the
control output increases, the process variable will also increase. The control output of most
2
PID loops are forward acting, such as a heating control loop. An increase in heat applied will
increase the PV (temperature).
3
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
4
increase in the cooling input causes a decrease in the PV (temperature).
The Transfer Mode
5
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 control output is switched from Manual to Auto. If this is not desired, choose
6
BumplessII.
The characteristics of Bumpless I and II transfer types are listed in the chart below. Note that
7
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
8
velocity form of the PID algorithm.
TransferType
9
Bumpless
10
Transfer I
Bumpless
11
Transfer II
The transfer type can also be selected in a RLL program by setting bit 3 of PID Mode 1,
12
V+00 setting as shown.
13
14
SP/PV & Output Format
This block allows you to select either Common format or Independent format. Common
A
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.
B
The reason for this is that they become independently selectable in the SP/PV and the Output
dialogs.
C
Common Data Format
Select either Unipolar data format (which is positive data only) in 12 bit (0 to 4095), 15 bit (0
D
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 compliment. The bipolar selection is
not available when 16-bit data format is selected.
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DL05 Micro PLC User Manual, 6th Edition, Rev. A
Transfer
PID Algorithm
Select Bit 3
Position
0
Velocity
Position
1
Velocity
PID Mode 1 Setting V+00
Bit 15 14 13 12 11 10 9 8 7 6 5
Manual-to-Auto
Transfer Action
Forces Bias = Control Output
Forces SP = PV
Forces SP = PV
Forces Bias = Control Output
none
Bumpless Transfer I/II Select
4 3 2 1 0
Auto-to-Cascade
Transfer Action
Forces Major Loop Output =
Minor Loop PV
Forces Major Loop Output =
Minor Loop PV
none
none