Notes on Transducers

8/9/2019 Notes on Transducers

1/8

2010

Mahmoud Hassan Kamel

Observed by: Dr. Amer

3/26/2010

Notes on Transducers


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ByMahmoud Hassan Kamel

3rd

year

Dept. of Electrical Engineering communications and electronics, University of Minia, Egypt

[email protected]

Observed by : Dr.Amer
mailto:[email protected]:[email protected]


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1. IntroductionFor any closed loop industrial control system, there are three steps applying the input (reference value) ,control the

process and feedback (negative feedback) that measuring the output and applying it back to the input stage.

Figure (1.1): Simple control system.

Figure (1.2): Typical industrial process control loop.

This paper is concerned with the measuring process and techniques used in this stage.The definition of transducers and sensorsA transducer is a device that obtains information in the form of one or more physical quantities and converts this into

an electrical output signal. Transducers consist of two principle parts, a primary measuring element referred to as a

sensor, and a transmitter unit responsible for producing an electrical output that has some known relationship to the

physical measurement as the basic components. In more sophisticated units, a third element may be introduced which

is quite often microprocessor based. This is introduced between the sensor and the transmitter part of the unit and has

amongst other things, the function of linearizing and ranging thetransducer to the required operational parameters .Listing of common measured variables:

Temperature

Pressure Flow rate Composition Liquid level.

Input

stage

Process

Control

Output

Measuring


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2. Temperature2.1 ThermocouplesThermocouples cover a range of temperatures, from262 to +2760 C and are manufactured in many materials, arerelatively cheap, have many physical forms, all of which make them a highly versatile device. Thermocouples suffer

from two major problems that cause errors when applying them to the process control environment.

The first is the small voltages generated by them, for example a 1 C temperature change on a platinumthermocouple results in an output change of only 5.8 V = (5.8 106 V).

The second is their non-linearity, requiring polynomial conversion; look up tables or related calibration to beapplied to the signaling and controlling unit.A thermocouple could be considered as a heat-operated battery, consisting of two different types of homogeneous (of

the same kind and nature) metal or alloy wires joined together at one end of the measuring point and connected

usually via special compensating cable, to some form of measuring instrument. At the point of connection to the

measuring device a second junction is formed, called the reference or cold junction, which completes the circuit.A

place to find one is in a natural gas furnace in a home similar to that shown in Figure 2. It controls the pilot light for

the burners in the furnace. The thermocouple is a closed tube system that contains a gas. The gas expands as it is

heated and expands a diaphragm at the end of the tube that is in the gas control module. The system works as follows:

A button on the pilot light gas control module is pressed to open valve A to initially allow gas to flow to light the pilot

light. The expanded diaphragm of the thermocouple system controls valve A; therefore, the button for the pilot light

must be held until the thermocouple is heated by the pilot light so that the gas expands and expands the diaphragm.

The expanded diaphragm holds valve A open; therefore, the pilot light button can be released because the pilot lightheating the thermocouple keeps the gas expanded. Since the pilot light is burning, any demand for heat from the

thermostat will light the burners and the house is heated until the demand by the thermostat is met.

Figure (2.1): A bimetal thermocouple

Figure (2.2): A residential furnace pilot light control


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2.2 Thermistorsdevices that change their electrical resistance in relation to their temperature. They typically consist of a combination

of two or three metal oxides that are sintered in a ceramic base material and have lead wires soldered to a

semiconductor wafer or chip, which are covered with epoxy or glass. Thermistors are available in two different types:

positive temperature coefficient (PTC) and negative temperature coefficient (NTC). PTC devices exhibit a positive

change or increase in resistance as temperature rises, while NTC devices exhibit a negative change or decrease in

resistance when temperature increases. The change in resistance of NTC devices is typically quite large, providing a

high degree of sensitivity. They also have the advantage of being available in extremely small configurations for

extremely rapid thermal response. In addition to metal oxide technology, PTC devices can also be produced usingconductive polymers. These devices make use of a phase change in the material to provide a rapid increase in

electrical resistance. This allows for their use in protection against excessive electrical current as well as excessive

temperature. Like RTDs, thermistors resistance value is specified with a plus -or-minus tolerance at a particulartemperature. Thermistors are usually specified at 25C. Thermistors resistance can be made virtually linear usingsupport circuitry such as a Wheatstone bridge. The resistance can then be interpreted using look-up tables to perform a

switching function or to drive a meter. They can also be used in liquid level sensing applications. RTDs (resistive

temperature devices), like thermistors, employ a change in electrical resistance to measure or control temperature.

RTDs consist of a sensing element, connection wires between the element and measurement instrument, and a support

for positioning the element in the process. The metal sensing element is an electrical resistor that changes resistance

with temperature. The element usually contains a coil of wire or conductive film with conductors etched or cut into it.

It is usually housed in ceramic and sealed with ceramic cement or glass. The sensing element should be positioned

where it can reach process temperature quickly. Wire wound devices should be adequately secured in high vibrationand shock applications. Extension wires between the element and instrument allow resistance to be measured from

great distances.

Flexible wire wound and etched foil RTDs are available in various standard configurations. Typically a Kapton,

silicone rubber, Mylar or clear polyester dielectric material is used for electrical insulation. They can be mounted on

curved or irregular surfaces using pressure sensitive adhesives, thermally conductive glues, silicone tape, or

mechanical clamps. This type of configuration is far superior for monitoring a large area such as the outside diameter

of a pipe or tank. They can also be integrated into a flexible heater circuit for optimum control.


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3. PressurePressure is probably the second most commonly used and important measurement in process control. The most

familiar pressure measuring devices are manometers and dial gauges, but these require a manual operator. For use in

process control, a pressure measuring device needs a pressure transmitter that will produce an output signal for

transmission, e.g. an electric current proportional to the pressure being measured. A transmitter typically that produces

an output of a 420 mA signal is rugged and can be used in flammable or hazardous service.

There are seven principle methods of electronically measuring pressure for use in process control and each of these is

listed and described under its numeric heading, in principle detail below:

1. Strain gauge (bonded or unbonded wire or foil, bonded or diffused semiconductor)2. Capacitive3. Potentiometric4. Resonant wire5. Piezoelectric6. Magnetic (inductive and reluctive)7. Optical8. Piezoresistive Diaphragm

Let's have a deeper look at piezoelectric.

3.1 Piezoresistive DiaphragmModern day semiconductor technology has been applied to the design and manufacturing of pressure sensors. A

descriptive diagram is shown in Figure 3. T he thin diaphragm is micromachined from a silicon substrate on which a

high-resistivity epitaxial layer has been deposited. The position of the diaphragm and its thickness on and in the

substrate is defined using typical semiconductor techniquesform a silicon dioxide on the surface, coat it with

photoresist, expose the photoresist with ultraviolet light through a mask to define the diaphragm area, and etch away

the oxide and silicon to the correct depth for the thin diaphragm. The assembly is then packaged to allow pressure to

deflect the diaphragm. Using integrated circuit metallization techniques, the thin diaphragm, which changes resistance

as it deflects, is connected into a Wheatstone bridge circuit as shown in Figure 4. This provides a very sensitive,

temperature compensated, measuring circuit.

Micromachined silicon resistorFigure(3.1):


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Figure (3.2): Wheatstone bridge

4. Others4.1 phototransistorsA phototransistor, a transistor designed to be activated by light, has the same basic operation as the NPN and PNP

transistor except it has no base connection. Its wide base junction is left exposed to light. Phototransistors are most

sensitive to infrared light. The symbols and voltages are shown in Figure4.1.a. Light rays that impact the base-emitter

junction effectively produce base current that activates the phototransistor. Through transistor action a larger collector

current is produced. As shown by the characteristic curves of Figure4.1.b, more light intensity produces more collector

current.

(a) (b)Figure (4.1): (a)Phototransistor symbol and operation (b)Characteristic curves.

References W. Altmann, "Practical Process Control for Engineers and Technicians", Newens, 2005. J.luecke, "Analog and Digital Circuits for Electronic Control System Applications", Newens, Elsevier Inc, 2005. J.s.Wilson,edited,"Sensor Technology Hansdbook",Newens, Elsevier Inc, 2005.

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Notes on Transducers