Variable output devices provide an accurate position indication of a valve or control rod.

EO 1.3DESCRIBE the following variable output position indicators to include basic construction and theory of operation.

a. Potentiometer

b. Linear variable differential transformers (LVDT)

Potentiometer

Potentiometer valve position indicators (Figure 5) 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.

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Figure 5 Potentiometer Valve Position Indicator

Potentiometer valve position indicator failures are normally electrical in nature. An electrical short or open will cause the indication to fail at one extreme or the other. If an increase or decrease in the potentiometer resistance occurs, erratic indicated valve position occurs.

Linear Variable Differential Transformers (LVDT)

A device which provides accurate position indication throughout the range of valve or control rod travel is a linear variable differential transformer (LVDT), illustrated in Figure 6. 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. Figure 6 illustrates a valve whose position is indicated by an LVDT. If the open and shut position is all that is desired, two small secondary coils could be utilized at each end of the extension's travel.

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Figure 6 Linear Variable Differential Transformer

LVDTs are extremely reliable. As a rule, failures are limited to rare electrical faults which cause erratic or erroneous indications. An open primary winding will cause the indication to fail to some predetermined value equal to zero differential voltage. This normally corresponds to mid-stroke of the valve. A failure of either secondary winding will cause the output to indicate either full open or full closed.

Summary

Variable output position indicators are summarized below.

Variable Position Indicator Summary

Potentiometer valve position indicators use 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 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.