Page 146 - Programmable Logic Controllers, Fifth Edition

6.13 Instrumentation
Instrumentation is the use of measuring instruments to
monitor and control a process. It involves the design and
calibration of systems used to measure, record, and control
industrial process variables. These variables may include
pressure, temperature, flow rate, weight, and chemical
consistency. An instrument is a device that measures
and/or acts to control any kind of physical process and
may include flow devices, level devices, thermocouples,
and pressure switches.
Every instrument has at least one input and one output.

• For a pressure sensor, the input could be some fluid
pressure and the output a 4- to 20-mA current signal.
• For a loop indicator, the input could be a 4- to 20-mA Figure 6-76 Process parameters display.
current signal and the output an electronic display. Source: Courtesy of Siemens.
• For a variable-speed motor drive, the input could be
an electronic signal and the output electric power to the instrument response against those known pressure
the motor. quantities. Smart instruments (Figure 6-75) that contain
microprocessors have built-in diagnostic ability, greater
To calibrate an instrument means to check, and if accuracy, and the ability to communicate digitally with
necessary adjust, its response so the output accurately host devices for reporting of various parameters.
corresponds to its input throughout a specified range. In- The PLC’s role as part of an industrial instrumentation
strument calibration involves exposure of the instrument system is to receive, process, and send signals from input
to an actual input stimulus of precisely known quantity. and to output devices. With the use of programming soft-
For a pressure gauge, this would mean subjecting the pres- ware the PLC can control, monitor, and display all the
sure instrument to known fluid pressures and comparing parameters associated with a given process (Figure 6-76).

Pressure Pressure Analog Analog-to- Digital Micro- Digital Digital-to- Analog
analog
digital
input sensor converter processor converter output
4-20 mA
Figure 6-75 Smart instruments.
Source: Photo courtesy Emerson.

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