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[Instrumentation Table of Contents]
[Sensors and Transducers Suppiers Menu]
As described within Potentiometer Position Indicator
and Linear
Variable Differential Transformer ( LVDT ) or Linear Voltage
Displacement Transducer, position detection devices
provide a method to determine the position of a valve or
control rod. The four types of position indicators discussed
within Instrumentation are limit switches, reed switches,
potentiometer position indicators, and LVDTs. Reed and limit
switches act as ON/OFF indicators to provide open and closed
indications and control functions. Reed switches can also be
used to provide coarse, incremental position
indication.
Potentiometer and LVDT position indicators
provide accurate indication of valve and rod position
throughout their travel. In some applications, LVDTs can be
used to indicate open and closed positions when small
secondary windings are used at either end of the valve stem
stroke. The indicating and control circuitry provides for
remote indication of position and/or various control
functions. Position indications vary from simple indications
such as a light to meter indications showing exact position.
Control functions are usually in the form of
interlocks. Pump isolation valves are sometimes interlocked
with the pump. In some applications, these interlocks act to
prevent a pump from being started with the valves shut. The
pump/valve interlocks can also be used to automatically turn
off the pump if one of its isolation valves go shut or to
open a discharge valve at some time interval after the pump
starts.
Valves are sometimes interlocked with each
other. In some systems, two valves may be interlocked to
prevent multiple valves from being opened at the same
time. This feature is used to prevent undesirable system
flow-paths. Control system interlocks are normally used to
prevent outward or inward motion of certain elements unless
certain conditions are met.
Position Indicators
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