Page 334 - Programmable Logic Controllers, Fifth Edition - Mobile version

– Calculates error, sum of error, rate of change
of error 14.5 Motion Control
– Calculates desired power with PID equations A motion control system provides precise positioning,
– Updates control output velocity, and torque control for a wide range of motion
applications. PLCs are ideally suited for both linear
Fully Automatic or Intelligent and rotary motion control applications. Pick and Place

• This method is also known in the industry as fuzzy machines are used in the consumer products industry
logic control. for a wide variety of product transfer applications. The
• The controller uses artificial intelligence to readjust machine takes a product from one point to another. One
PID tuning parameters continually as necessary. example is the transfer of a product to a moving conveyor
belt as illustrated in Figure 14-21.
• Rather than calculating an output with a formula, A basic PLC motion control system consists of a con-
the fuzzy logic controller evaluates rules. The first troller, a motion module, a servo drive, one or more motors
step is to “fuzzify” the error and change-in-error with encoders, and the machinery being controlled. Each
from continuous variables into linguistic variables, motor controlled in the system is referred to as an axis
like “negative large” or “positive small.” Simple of motion. Figure 14-22 illustrates a bottle-filling motion
if-then rules are evaluated to develop an output. control process. This application requires two axes of
The resulting output must be de-fuzzified into a motion: the motor operating the bottle filler mechanism
continuous variable such as valve position. and the motor controlling the conveyor speed. The role of

The PID programmable controller output instruction each control component can be summarized as follows:
uses closed-loop control to automatically control physical Programmable Logic Controller
properties such as temperature, pressure, liquid level, or
flow rate of process loops. Figure 14-20 shows the PID • The controller stores and executes the user program
output instruction and setup screen associated with the that controls the process.
Allen-Bradley SLC 500 instruction set. The PID instruc- • This program includes motion instructions that
tion is straightforward: it takes one input and controls one control axis movements.
output. Normally, the PID instruction is placed on a rung • When the controller encounters a motion
without conditional logic. The output remains at its last instruction it calculates the motion commands
value when the rung goes false. A summary of the basic for the axis.
information that is entered into the instruction is as follows: • A motion command represents the desired position,
Control Block—File that stores the data required to velocity, or torque of the servo motor at the particu-
operate the instruction. lar time the calculations take place.
Process Variable—The element address that stores
the process input value.
Control Variable—The element address that stores
the output of the PID instruction.
Setup Screen—Instruction on which you can double-
click to bring up a display that prompts you for other
parameters you must enter to fully program the PID
instruction.

PID
PID
Control block
Process variable
Control variable
Control block length
Setup screen

Figure 14-20 PID output instruction and setup screen.
Source: Image Courtesy of Rockwell Automation, Inc. Figure 14-21 Pick and Place machine.

Process Control, Network Systems, and SCADA Chapter 14 315

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