Rockwell Automation 1771-PD PID MODULE (+DU) - Manual (Page 6)

Introduction
Chapter 1
1Ć3
download an output bias from the PC processor
Expanded control features
perform scaling on the process variable, set point and/or error
set minimum and maximum scaling values
use the tieback as the feedforward input
take the square root of the feedforward input
add a feedforward offset
multiply the feedforward term by a constant
perform lead/lag filtering on the feedforward term
download a feedforward value from the PC processor
cascade the output of loop 1 into the set point of loop 2
decouple the VPID output of loop 1 into the feedforward input of loop
2
These features including integral term anti-reset wind-up are described in
chapter 3.
PID Algorithm
An algorithm is a step-by-step procedure. The purpose of the PID
algorithm is to maintain the process at the desired setpoint. A diagram of
PID closed loop control is shown in Figure 1.1a.
The PID module has many features. Select only those features that are
required for the particular application. Major control functions are shown
in Figure 1.1b. Refer to figure 3.15 for a detailed algorithm flow chart.
This flow chart shows the relationship of the user-selectable features.
PID modules (rev C or later) let you select the ISA or Allen-Bradley
algorithm. Refer to appendix E for a comparison of algorithm values.
Hardware Description
The PID module has four analog inputs and two analog outputs each with
12-bit binary resolution. Each input and output can be individually
selected to either the +4 to +20mA or +1 to +5V DC range without
recalibration. The selection is made with internal programming plugs.
Each loop has one process variable input, one tieback input and one
analog output associated with it. The tieback analog input is used to track
the analog output of the manual control station to permit bumpless
transfer. Each loop uses one discrete input to track the status of the
optional manual control station. A contact output is used to switch the