MIL-HDBK-1003/7
b)
Cascade control.
c)
Auto-selector control.
d)
Nonlinear control.
These units can be analog type and either electronic or pneumatic; however,
additional pieces of instrumentation may be required to actually do the above types of
control. With the new microprocessor-based control stations they can perform most of
the types of control listed above without additional devices and still provide 18 or so
control algorithms. For standard controller action see Figure 41. For a controller to
operate correctly, the controller must be properly tuned. Each controller must be tuned
for its control loop--seldom are two alike. This can be time consuming, but it is
necessary to have correct controller response to process changes. Some new
microprocessor controllers have "Self Tuning" capability. This is a major addition for
exact tuning of the control parameters to match the process dynamics.
The new microprocessor-based controllers should be given first consideration
as they are less expensive, state of the art, and can perform many more functions than a
standard analog controller without additional pieces of equipment. These new units
should also be considered for retrofit work as they can easily replace old electronic
and pneumatic analog controllers and other devices. Some of the new microprocessor-
based controllers are configured from push buttons located on the controller face plate.
Special calibrators or mini-computers are not required to configure these stand alone
controllers.
Operators. Operators are used to drive (move) final control elements such as
11.11
control valves and dampers from one position to another. The general types of operators
are pneumatic diaphragm or piston, electric, and electro-hydraulic.
Pneumatic Operators. Pneumatic operators including I/P converters are the
11.11.1
least expensive when compared to electric or electric-hydraulic types. Operators (final
control elements) for major control loops should be pneumatic unless normally furnished
otherwise on equipment as standard by a manufacturer.
Electric Operators. Electric operators are used prevalently in heating,
11.11.2
ventilating, and air conditioning systems.
150
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