ELECTRICAL AND ELECTRONICS COMBINATION
Below are some method which are used to convert mechanical motion into electrical signal
Electro mechanical pressure transducer converts the motion produced by mechanical sensing elements into changes in electrical signals.
1. Potentiometric transducers
2. Capacitance type
3. Strain gauge
4. Piezoelectric type
After mechanical movement by using this method we get output in electrical signal. different methods are depends on what manufacture use.
1. POTENTIOMETRIC TRANSDUCERS
The potentiometric pressure sensor provides a simple method for obtaining an electronic output from a mechanical pressure gauge. The device consists of a precision potentiometer, whose wiper arm is mechanically linked to a Bourdon or bellows element. The movement of the wiper arm across the potentiometer converts the mechanically detected sensor deflection into a resistance measurement, using a Wheatstone bridge circuit.
The mechanical nature of the linkages connecting the wiper arm to the Bourdon tube, bellows, or diaphragm element introduces unavoidable errors into this type of measurement. Potentiometric transducers can be made extremely small and installed in very tight quarters, such as inside the housing of a 4.5-in. dial pressure gauge. Provide a strong output that can be read without additional amplification. This permits them to be used in low power applications. They are also inexpensive. Potentiometric transducers can detect pressures between 5 and 10,000 psig (35 KPa to 70 MPa). Their accuracy is between 0.5% and 1% of full scale, not including drift and the effects of temperature.
2. CAPACITANCE TYPE
Capacitive pressure sensors use the electrical property of capacitance to measure the displacement of a diaphragm. The diaphragm is an elastic pressure sensor displaced in proportion to changes in pressure. It acts as one plate of a capacitor. It is a differential pressure sensor, because it is subject to atmospheric pressure. The amount by which the diaphragm moves with respect to the unknown pressure applied to it depends on its shape and construction, size, thickness, and material. Therefore the precise relationship between pressure and displacement varies with each design. The capacitance c of the capacitor is (approximately) inversely proportional to the distance d between the plates.That is,
c ∝1/d
An electrical signal is connected to the two plates of the capacitor. The change in value of the capacitance causes this electrical signal to vary. This is then conditioned and displayed on a device calibrated in terms of pressure. There are techniques other than change in capacitance used to measure the displacement of a diaphragm. For example, another design of pressure sensor uses a diaphragm made from silicon. Semiconductor strain gauges are diffused into the diaphragm. They are arranged in a form similar to the Wheatstone bridge. A stable electrical output is produced, and the arrangement of the strain gauges compensates for temperature changes. However, these types of pressure sensor are relatively expensive. Because of the sensitivity of diaphragm based pressure sensors, they are used to measure small changes in pressure. An example application is small changes in pressure in liquid or gas pipelines so that the flow rate can be measured. For better understanding you can check in video section where you can find cut view of capacitance type pressure transmitter.
3. STRAIN GAUGE
Strain gauge is used to measure strain of an object. Stress is defined as the object's internal resisting forces.
Strain is defined as the displacement and deformation that occur. Strain is defined as the amount of deformation per unit length of an object when a load is applied.
There are two types of strain gauges:
1. Bonded strain gauge
2. Unbounded strain gauge
BONDED STRAIN GAUGE
These devices represent a popular method of measuring strain. The gauge consists of a grid of very fine metallic wire, foil, or semiconductor material bonded to the strained surface or carrier matrix by a thin insulated layer of epoxy. When the carrier matrix is strained, the strain is transmitted to the grid material through the adhesive. The variations in the electrical resistance of the grid are measured as an indication of strain. The grid shape is designed to provide maximum gauge resistance while keeping both the length and width of the gauge to a minimum.
UNBOUNDED STRAIN GAUGE
A fine wire resistance strain gauge is stretched around the insulated pins. The strain gauge is connected to a wheatstone bridge. Thus measuring resistance.
4. PIEZOELECTRIC TYPE
These sensors are similar to capacitive pressure sensors in that they detect pressure changes by the displacement of a thin metal or semiconductor diaphragm. In a pressure sensor using the piezoelectric effect, the diaphragm causes a strain on the piezoelectric crystal when flexing due to pressure changes. The electric charges of opposite polarity appearing on
the faces of the crystal are proportional to this strain. The piezoelectric crystal is usually quartz. This type of pressure sensor often incorporates signal conditioning circuitry in a sealed unit, using integrated circuit technology. Piezoelectric pressure sensors operate at high temperatures and can be made small in size.
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