Page 51 - Programmable Logic Controllers, Fifth Edition

Figure 2-33 Stepper-motor.
Source: Courtesy Sherline Products.
Figure 2-35 PID module.
STEPPER-MOTOR MODULE Source: Courtesy Red Lion Controls.
The stepper-motor module provides pulse trains to a
stepper-motor translator, which enables control of a step- temperature, flow, level, or speed within set limits of a
per motor (Figure 2-33). The commands for the module specified set point.
are determined by the control program in the PLC.
MOTION AND POSITION CONTROL
BCD-OUTPUT MODULE MODULE
The BCD-output module enables a PLC to operate devices Motion and position control modules are used in applica-
that require BCD-coded signals such as seven- segment tions involving accurate high-speed machining and pack-
displays (Figure 2-34). aging operations. Intelligent position and motion control
Some special modules are referred to as intelligent I/O modules permit PLCs to control stepper and servo mo-
because they have their own microprocessors on board tors. These systems require a drive, which contains the
that can function in parallel with the PLC. These include: power electronics that translate the signals from the PLC
module into signals required by the motor (Figure 2-36).
PID MODULE
The proportional-integral-derivative (PID) module COMMUNICATION MODULES
(Figure 2-35) is used in process control applications Serial communications modules (Figure 2-37) are used to
that incorporate PID algorithms. An algorithm is a establish point-to-point connections with other intelligent
complex program based on mathematical calculations. devices for the exchange of data. Such connections are
A PID module allows process control to take place normally established with computers, operator stations,
outside the CPU. This arrangement prevents the CPU process control systems, and other PLCs. Communication
from being burdened with complex calculations. The modules allow the user to connect the PLC to high-speed
basic function of this module is to provide the control local networks that may be different from the network
action required to maintain a process variable such as communication provided with the PLC.

Outputs
Servo
Servo motor
drive
Inputs Encoder

PLC servo module
Figure 2-34 Seven-segment display.
Source: Courtesy Red Lion Controls. Figure 2-36 PLC servo module.

32 Chapter 2 PLC Hardware Components

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