Instrument Devices – Pressure Measurement and Control

Electrical signals are used in the majority of systems to obtain:

• Economically better
• Reliable
• Accurate
• Versatile
• Require field devices that send very low voltage and low amperage electrical signals (3 – 15 mA) to the centralized control circuitry

In most cases, input transducers exhibit variations in their resistance. These resistance variations are typically converted into electrical signals, specifically voltage signals. The voltage signal is then directed to other sections of the circuit, such as the input of a transistor switch. This enables further processing and utilization of the obtained signals within the circuitry. Variable resistance can be converted to variable voltage, performed by a simple circuit called a voltage divider.

Electro-mechanical transducers

Electro-mechanical transducers are utilized for fluid pressure measurement and rely on mechanical pressure sensing elements, which include Bellows, diaphragms, and Bourdon tubes.

Transducers are categorized based on their electrical connections and principles of operation. The various classifications include:

• Voltage divider transducers
• Variable resistance transducers
• Variable inductance transducers
• Linear-Variable Differential Transformer (LVDT) transducers
• Variable reluctance transducers, also known as magnetic pickup transducers
• Variable capacitance transducers
• Strain gauges
• Piezoelectric transducers

Voltage Divider Transducers

Voltage divider transducers use a Bellows or diaphragm mechanism to reposition a moving contact arm on a potentiometer. This movement causes a change in the resistance ratio and subsequently results in the measurement of the input signal.

[latex]V_o = \frac{{R2 \times V_s}}{{R1 + R2}}[/latex]

• The resistance values of R1 and R2 determine the potential outcomes in a circuit setup where they are connected in series across a supply voltage (Vs).
• This voltage is distributed between the two resistances, resulting in an output voltage (Vo) across R2.
• The behavior of Vo depends on the relative sizes of R1 and R2.
• When R2 is significantly smaller than R1, Vo is diminished due to the majority of the voltage drop occurring across R1.
• In the case where R2 is approximately equal to R1, Vo amounts to roughly half of Vs.
• Conversely, if R2 is considerably larger than R1, Vo becomes elevated since most of the voltage drop now happens across R2.

Variable Resistance Transducers

Variable resistance transducers utilize a pressure-sensing element to change the transducer’s electrical resistance.

Variable Inductance Transducers

Variable inductance transducers use the pressure-sensing element to reposition a magnetic armature, and this alters the inductance of the surrounding coil.

Advantages:

• They do not possess any moving electrical contacts that could wear out over time.
• There is no occurrence of signal distortion caused by friction between contact points.

Linear Variable Differential Transformer

The linear variable differential transformer (LVDT), operates by utilizing the extension shaft or control rod as a movable core within a transformer setup. The movable core moves along the primary and secondary windings of the transformer, resulting in variations in the inductance between the two windings. These variations, in turn, generate an output voltage that is directly proportional to the position of the valve or control rod extension being measured.

Coil Assembly

The coil assembly refers to the fixed component of the position sensor.

When an alternating current is applied to the primary coil, it generates opposing voltages in the secondary coils. The combined output of these secondary coils is zero when the movable core is positioned centrally, achieving a balanced or null state.

The polarity of the LVDT’s output signal is determined by the direction of movement away from the balanced position. By externally measuring the polarity and voltage, one can accurately indicate the extent of core displacement and the corresponding fluid pressure.

The LVDT consists of two secondary coils that are wound in series but in opposite directions, leading to a series-opposed configuration. This arrangement causes the two signals generated on each secondary coil to be out of phase by 180 degrees. As a result, the phase of the output signal serves as a determinant for the direction, amplitude, and distance of the measured quantity.

To learn more about the operation of RDP LVDT, you can click on this link to visit the webpage provided by RDP Group.

Tubular Differential Transformer

The linear variable differential transformer holds an advantage over other pressure transducers due to its ability to generate an output signal that exhibits a perfectly linear relationship with the measured variable. This characteristic reduces the requirement for signal conditioning before utilizing the signal to activate an indicating device.

Variable Reluctance Transducer

The VRT is a transducer that utilizes the principle of transformer action to induce voltage into the device through motion.

Variable Capacitance Transducer

Advantages:

• Minimal parts are involved in their construction.
• Cost-effective option.
• High sensitivity to changes in the measured variable.

Disadvantages:

• Instruments receiving their signal are often more expensive compared to other transducers due to the complexity of the required measurement bridge circuit.
• Unsuitable for applications involving very long lead wires, as the signal tends to become unreliable.

Capacitive Position Sensor

Capacitive sensors are employed for position measurement. The moving part of the sensor is depicted in a darker shade of grey, while the lines represent the distance between the two components.

The roughness of the target surface downgrades the resolution and linearity.

The curved surfaces lead to an averaged distance measurement.

Strain Gauges

In fluid pressure transducers, strain gauges are used to quantify the deflection experienced by a pressure-sensing element.

In a strain gauge, the resistance of the wire changes with elongation and compression. This change occurs because the resistance is directly proportional to the length of the conductor and inversely proportional to its cross-sectional area when the temperature remains constant.

The relationship can be expressed as:

[latex]R = kL / A[/latex]