Page 493 - Sensors and Control Systems in Manufacturing

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4. Addition of external mass storage for programs and data
(cassette recorder or floppy disk)
5. Connection to a printer or other peripheral device
6. Integration of an interpreter for high-level programming
language
A microcomputer system with this configuration is often referred
to as a personal computer, which is misleading terminology. Although
the microprocessor and single-board computer were initially devel-
oped for control purposes, the nomenclature directed interest toward
purely commercial applications.
The personal computer (or single-board computer) is similar to
any peripheral device that requires interfacing to communicate with
a central control unit to perform control functions. The personal com-
puter’s input and output signals must also be adapted and made
available to peripheral devices. Control technology employs a variety
of signal forms and levels (e.g., with both analog and binary signals
and voltages that may be as high as 24 V). The microcomputer oper-
ates with internal voltage levels of 0 and 5 V. Thus, interfacing must
provide voltage-level conversion and D/A and A/D conversion.
Figure 9.6 is a block diagram of a personal computer and its interfac-
ing to process computer peripherals.

9.5.1 Role of Sensors in Programmable Logic
Controllers—PLC
The programmable logic controller (PLC) is used in processes where
mainly binary signals are processed. The PLC first came into the
market at the beginning of the 1970s as a replacement for relay cir-
cuits, pioneered by Allen Bradley Company, currently Rockwell
International Corporation, USA. Since then, these systems have
been continually developed, supported by the progress in micro-
electronics. For that reason, the present systems are much more
powerful than those originally named PLC. Therefore, the terms
PLC, process computer, microcomputer, and so on, often overlaps
one another. Technically, these systems are often indistinguishable.
In addition, there are often many similarities with regard to their
functions. Figure 9.7 shows the functional relations between the
components of a PLC.
The input signals supplied by the sensors are passed onto the
central control unit via an input module. The signals generated by
the central control unit are prepared by the output modules and
passed on to the actuators. The program is drawn up by using an
external programming device and transferred into the program
memory (input of the program varies, however, according to the
PLC).