MIL-HDBK-1003/7
Section 11.
CONTROL AND INSTRUMENTATION
Types of Controls and Control Systems. There are basically three types of
11.1
industrial instrumentation systems for power plant control: analog, microprocessor, and
computer.
Analog. Analog control is the representation of numerical quantities by means
11.1.1
of physical variables such as current, air pressure, voltage, rotation, resistance,
electromagnetic field (EMF), etc. Analog control over the last 30 years has consisted
primarily of two types:
a) Pneumatic - the use of air pressure (or other gases occasionally)
as the power source to represent numerical values.
b) Electronic - the use of current, voltage, resistance, EMF etc. as
the power source to represent numerical values.
Microprocessor-Based Control Stations. These are a digital stand alone single
11.1.2
controller type, or a split-architecture control system offering powerful, configurable
control capability on a modular basis. These units can accept standard analog
electronic inputs plus digital inputs and give analog outputs plus digital outputs. By
connection to a data highway for communication, other operator interfaces are easily
added.
Computer - Direct Digital Control (DDC) or Supervisory Control (SC). DDC
11.1.3
control can perform all of the control functions, operator displays and graphics,
reports and calculations for efficiency and controller tuning, or a computer can be used
as a supervisory control for analog control system, microprocessor based control units,
or as a data logger with graphic displays. Choice of analog versus microprocessor based
control units or computer (DDC) (SC) should be based on relative cost and future
requirements. Consideration should be given to the ability to readily interface to or
add to a utilities energy management system or other computer networks.
Pneumatic Control Systems. Pneumatic control systems should only be
11.1.4
considered when adding to an existing power plant already equipped with pneumatic
control instruments.
11.1.5
Types of Control Systems Available.
a)
On-off controls.
b)
Single point positioning system.
c)
Parallel positioning system.
d)
Parallel metering system.
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