Rockwell Automation 1771-PD PID MODULE (+DU) - Manual (Page 120)

Programming
Chapter 3
3Ć77
The PID module can be user-configured to provide many types of closed
loop control. The PID module can perform single element control,
cascade, feedforward, cascade and feedforward, decouple (multi-loop,
multi-variable) and PC processor interactive control. Other features such
as scaling, digital filtering, and lead/lag can be selected to supplement the
basic control function.
Expanded features of the module are described below.
Scaling
Scaling is the linear conversion of raw data to engineering units and vice
versa, such as gallons/minute, degrees centigrade, and pounds/square
inch. Unscaled data in the PID module has a range of 0 through 4095.
The resolution of this data is 12 bit binary corresponding to 1 part in
4095. The resolution of scaled values is the same as the raw data, 1 part
in 4095 regardless of the chosen scaling.
Scaling is an expanded loop feature which can be performed on the
process variable, set point and/or error terms. Scaled process variable and
error values can be read from the PID module and/or scaled set point
values can be written to the PID module in scaled units by the PC
processor. Each loop can perform scaling independent of the other.
The scaling feature is implemented by selecting expanded features by
setting W01 B14; by setting bit B17 for the process variable, B16 for set
point and/or B15 for error in word W30(W50); and by entering the
minimum and maximum scaling values SMIN and SMAX. SMIN in
word W32(W51) is the value corresponding to the bottom of the scale.
SMAX in word W32(W52) is the top of the scale. When the minimum
and maximum values are chosen and scaling is selected, the scaled values
will be found in the following words.
set point W06(W13)
loop error W62(W69)
process variable W65(W72)
Scaling Example 1
This example describes the scaling of flow transmitter data in loop 1.
READ PV values are reported to the PC processor in word W65. Selected
values with and without scaling are tabulated in table 3.M.
Expanded Features