WHAT IS A POSITION INDICATORS?


 INTRODUCTION

Position indicating device is used to provide remote indication of position, such as indication of the open or shut condition of a valves.




THERE ARE FOUR TYPES OF POSITION INDICATORS

  • Limit switches
  • Reed switches
  • Potentiometer
  • LVDT
Limit and Reed switches act as ON/OFF indications to provide open and closed indication and control function. Reed switches can aslo be used to provide an incremental-position indication.

Potentiometer and LVDT position indicators provide an accurate indications, such as the position of a rod throughout its travel.


LIMIT SWITCHES





  • Limit switch is a mechanical device, which can be used to determine the physical position of any part to provide full  open or full shunt indication.
  • Many limit switch are the push button variety which gives ON/OFF corresponding to the valve position.
  • Example: An extension of a valve shaft mechanically trips a limit switch as it moves from open to shunt or shunt to open. When the valve extension comes in contact with the limit switch the switch depresses to complete, or turn  "ON"  the electrical circuit.

REED SWITCHES





  • Reed switches are more reliable than limit switches. These are constructed of flexible ferrous strips (called as reeds) and are placed near the intended travel of the valve steam or control rod extension.
  • When using reed switches, the extension used is a permanent magnet. As the magnet approaches the reed switch, the switch shuts. When the magnet moves away, the reed switch opens.
  • By using a large number of  magnetic reed switches, incremental position can be measured.

POTENTIOMETER





  • Potentiometer valve position indicators provide an accurate indication of position throughout the travel of a valve or control rod.
  • The extension is physically attached to a variable resistor. As the extension moves up or down, the resistance of the attached circuit changes, changing the amount of current flow in the circuit.
  • The amount of current is proportional to the valve position.
  • Potentiometer valve positon indicators failure are normally electrical in nature.

LINEAR VARIABLE DIFFERENTIAL TRANSFORMERS (LVDT)





  • A device that provides accurate position indication throughout the range of valve or control rod travel is linear variable differential transformer.
  • Unlike the potentiometer position indicator, no physical connection to the extension is required. The extension valve shaft, or control rod, is made of a metal suitable for acting as the movable core of a transformer.
  • Moving the extension between the primary and secondary windings of a transformer causes the inductance between the two windings to vary, thereby varying  the output voltage proportional to the position of the valve or control rod extension
  • LVDT are extremely reliable failures that are limited to rare electrical faults.


POTENTIOMETER VS LVDT


  • Potentiometer valve position indicators are an extension which is physically attached to a variable resistor. As the extension moves up or down, the resistance of the attached circuit changes, changing the amount of current flow in the circuit.
  • An LVDT uses the extension shaft or control rod as a movable core of the transformer, moving the extension between the primary and secondary windings of a transformer causes the inductance between the two windings to vary, thereby varying the output voltage proportional to the position of the valve or control rod extension.

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