1). Gold or aluminium wires are welded to the aluminium contacts
on the chip and to the glass feed-through, pins of the header.
2). TAB (Tape Automated Bonding). The contacts on the chip
have a gold dot.
A pretinned felxible printed circuit is directly soldered to these
gold dots and the other end to a PC-board, or the header.
Pressure transducers are pressure measuring instruments,
ready to use. It is an OEM transducer with pressure port,
integrated compensation resistors and a cable or connector.
Low cost sensors are devices where the they are exposed to
the media without protection.
The glass feed-through and the silicon cell is mounted in a
plastic housing with pressure ports for positive and negative
pressure. The silicon sensor with the TAB print
is fixed between two plastic mouldings with pressure ports.
The silicon sensor is bonded to a brass pressure port. The
contacts are made either by gold wires to soldering pins, or
by TAB flexible printed circuit.
To communicate the amount of pressure requires the use of an agreed on scale and convention. Just as the conventions and units for temperature are referred to as Fahrenheit, Centigrade or Kelvin; industry uses PSID, PSIA, PSISG and PSIVG notation for pressure. PSI (Pounds per Square Inch) is the pressure unit while the last letter(s) refers to the convention used to measure the pressure. You would use PA( Pascels) units instead of PSI in the metric system.
By nature, pressure is the difference between two points or ports. Pressure between two ports (or pipe connections) is differential pressure. A designation of 10 PSID means 10 Pound per Square Inch Differential.
The two voltage out points from the bridge circuit are fed to an amplifier that changes the very small voltage into a 0-5V signal or most commonly to a 4-20 mA signal. This signal is fed out the cable (sometimes along with a vent tube) which finishes our voyage.
The output of a Strain gauge is sometimes referred to as ratiometric to the supply voltage. The term ratiometric means that the output varies as a ratio of the the supply voltage. An example helps make all things things clear.
· Let us assume that With a 1/2 full scale pressure input the out put is x volts, if we double the supply voltage, the voltage output will now be 2X.
By varying the supply voltage we have a handy way to scale the output to what ever units we want. Changing the value of the reference element resistor gives us a way to offset the zero point. Between the two adjustments we can scale and offset the output to what ever we want. In real word practice the output voltage and currents are kept low to minimize thermal error so the output is usually followed with an amplifier stage.
Pressure transducers used in industry today take on a rather typical appearance. A round, tubular stainless steel body with a pipe fitting on one end and a cable coming out of the other end. Stainless is used because of its high strength and resistance to corrosion.
With the steam age came the demand for pressure measuring
instruments. Bourdon tubes or bellows, where mechanical
displacements were transferred to an indicating pointer were
the first pressure instruments, and are still inuse today.
Pressure metrology is the technology of transducing pressure
into an electrical quantity. Normally, a diaphragm construction
is used with strain gauges either bonded to , or diffused into it,
acting as resistive elements. Under the pressure-induced strain,
the resistive values change.
Sensors are used in everyday life. Applications include automobiles, machines, aerospace, medicine, industry and robotics.
Technological progress allows more and more sensors to be manufactured on the microscopic scale as microsensors using MEMS technology. In most cases a microsensor reaches a significantly higher speed and sensitivity compared with macroscopic approaches. See also MEMS sensor generations.
