Pressure Transducer

Pressure Transducer - Motorola

Pressure sensor applications include flow (HVAC), height of a column of liquid, altitude, depth of a submerged object, position, sound (dbspl), barometric pressure, map, pressure drop, vacuum, volumetric displacement, and weight.

A transducer is simply a device (or medium) that converts energy from one form to another. The term is generally applied to devices that take physical phenomenon (pressure, temperature, humidity, flow, etc.) and convert it to an electrical signal.

Pressure transducers/sensors use a wide range of operating principles including:

1. Motion transducers use a bellows or Bourdon tube to convert pressure to an output. In one common type, the LVDT, an inductive member is driven into or out of a coil. It contains numerous pivots and linkages, making it nonlinear and susceptible to wear and vibration, but it has the advantage of inherently high output.

2. Pressure potentiometers have characteristics similar to those of LVDTs. In this case, a wiper is driven across a resistive coil, with output determined by wiper position. Compared to an LVDT, it has the added disadvantage of coil wear. If continuously operated in about the same pressure range, it may suddenly short out or produce severely nonlinear output. These sensors are rather inexpensive.

3. Silicon or "chip" transducers are widely used in high-volume applications. There are two types of silicon pressure sensors, capacitive and piezoresistive. Capacitive devices are much more stable, sensitive, and temperature resistant. Piezoresistive types are easier to make and cost less and therefore dominate the market.

4. Capacitance transducers use a flexing diaphragm to produce capacitance changes proportional to applied pressure. Because of their low price, a common application of these devices is in automobiles. One drawback is at normal hydraulic pressure their operation dictates a large diaphragm making them better suited to low-pressure systems.

5. Piezoresistive Sensors are available in both gage and absolute versions. The sensor typically consists of a Wheatstone bridge etched on a silicon diaphragm which outputs a voltage that is proportional to pressure.

6. Electropneumatic transducers are used to provide regulated air pressures for the control of process systems. Typically, electropneumatic transducers are of three basic types: voice-coil beam, voice-coil beam dampened by an oil dashpot, and torque motor.
   • Voice-coil beam transducers use a nozzle/flapper arrangement to convert a small mechanical motion into a proportional pneumatic signal.

   • Damped transducers operate in a similar manner except that the arm controlling flapper position is attached to a float suspended in silicone oil.

   • Torque-motor transducers also have similar operating principles, except that a conventional torque motor replaces the voice-coil beam arrangement to position the flapper.