Page 709 - Mechatronics with Experiments

CHAPTER 9

PROGRAMMABLE LOGIC

CONTROLLERS

9.1 INTRODUCTION

The programmable logic controller (PLC) is the de-facto standard computer platform used in
industrial control, factory automation, automated machine and process control applications.
PLCs were developed as a result of a need in the automotive industry. In the early 1960s,
the General Motors (GM) Corporation stated that the factory automation systems based
on hard-wired relay logic panels were not flexible enough for the changing needs of the
industry. When a new car model required a difference sequence of control logic, re-wiring
the control panel was taking too much time and the response time of the company was
slow for new car models. With hard-wired relay logic panels, if there is a need to change
the automation logic and functions of a line, the logic wiring between the input and output
signals in the panel had to be physically changed. This is a time consuming and costly
process. It was requested in a design specification that a general purpose wiring of all I/O
devices be brought to a panel, but the logical relationship between the I/O be defined in
software instead of being hard-wired. In other words, it was desired that the logic be soft-
wired instead of hard-wired. That was the beginning of PLCs. PLCs play a very important
role in the automated factories of the industrial world. PLCs replaced the hard-wired relay
logic panels. The current trend is large scale networking via the internet between PLCs
at the factory and enterprise level computers which may be physically distributed all over
the world.
The physical shape of PLCs made by many different companies all have the same
form: it is rack mounted with standard size slots to plug in I/O interface units (Figure 9.1).
A typical PLC rack starts with a power supply and a CPU module plugged into a backplane
of an interface bus. A rack may have a different number of slots,such as 4 slots, 7 slots,
10 slots, 15 slots and so on. A PLC may support multiple I/O racks (main rack that has
the CPU and expansion I/O racks). Typical software development tools for a PLC include
a notebook PC, a serial or ethernet communication interface and cable, and a software
development environment for that particular PLC (Figure 9.2). Depending on the I/O
capabilities, CPU speed and program functions, PLCs are categorized into three to four
major sizes (Figure 9.1). Any of the I/O interface modules can be plugged into the slots.
All of the interface and unit power lines are provided by the snap-on connection to the rack.
Typical I/O units supported by virtually every PLC platform include digital input and output
modules, analog input and output modules, a high speed counter and timer module, serial
communication module, communication network interface modules (i.e., DeviceNet, CAN,
ProfiBus), a servo motor control module, and a stepper motor control module. For a given
application, the necessary I/O units are selected and inserted into the slots. Furthermore, if

Mechatronics with Experiments, Second Edition. Sabri Cetinkunt.
© 2015 John Wiley & Sons, Ltd. Published 2015 by John Wiley & Sons, Ltd.
Companion Website: www.wiley.com/go/cetinkunt/mechatronics

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