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Course: Sensors and Actuators

Course Code: 21EC35

Course Co-ordinator: Vishalakshi Patil

Module-3

• Measurement of Temperature: RTD, Thermistor, Thermocouple, Laws of thermocouple, Thermopile, AD590.
• Measurement of force & torque: Introduction , Force measuring sensor-Load cells-column types devices, proving rings, cantilever beam, pressductor. Hydraulic load cell, Electronic weighing system.

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• Measurement of Temperature:

What is RTD (Resistance Temperature Detector)?

A Resistance Temperature Detector (also known as a Resistance Thermometer or RTD) is an electronic device used to determine the temperature by measuring the resistance of an electrical wire. This wire is referred to as a temperature sensor.

If we want to measure temperature with high accuracy, an RTD is the ideal solution, as it has good linear characteristics over a wide range of temperatures. Other common electronics devices used to measure temperature include a thermocouple or a thermistor.

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Resistance type temperature sensors

Resistance type temperature sensors

• The application of a dc potential across the metallic element results in a directional flow of the electrons.
• When the metal is heated, the temperature rises and the mean free path length between collisions decreases due to the increase in the amplitude of oscillation, resulting in an increase in the electrical resistance.
• In general the resistivity of metals increases with an increase in temperature(i.e. The temperature coefficient is positive), whereas in some semiconductors the resistance decreases with an increase in temperature temperature(i.e. The temperature coefficient is negative). Such variations in resistance are measured precisely with suitable electrical circuits.

• The resistance thermometer based on the above phenomenon is one of the most accurately reproducible temperature sensing device.
• The resistance thermometer is applicable for measurements of small temperature differences as well as for wide ranges of temperature.
• Within narrow ranges of temperature, the temperature coefficient of resistance is constant. The resistance value Rt at any temperature T can be expressed as

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• Where R_o is the resistance of the conductor at temperature T_o, a is a constant standing for the temperature coefficient of resistance for that material and t is the difference in temperature.
• For larger temperature ranges, the resistance value follows more accurately the polynomial relation.

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Where a and b are constant related to the material.

• The metals that exhibit good sensitivity and reproducibility for temperature measurement purposes are copper, nickel and platinum.

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Platinum resistance thermometer:

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Construction of platinum resistance thermometer elements

• Figure shows the common open-wire element in which the platinum wire is wound in the form of a free spiral or held in place by an insulated Carrier, such as silica or ceramic. The diameter of the wire varies from 0.02 to 0.2mm.
• For many applications, the resistance wire is enclosed in a protective tube made of glass, quartz, porcelain or metal for protection from mechanical damages and chemical reactions as shown in fig.

Resistance thermometer circuits

An assumption made in the above equations is that R_T and R_C are constant during the time required for making the two balances.

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Thermistor

• It is semiconductor devices which behave as thermal resistors having a high negative temperature coefficient of resistance.

Thermocouples

• Thermocouples are perhaps the most commonly used electrical devices for temperature measurement.
• The sensing is based on the principle that a current flows in a closed circuit made up of two dissimilar metals if the junctions of the two metals are kept at different temperature.
• In each lead, the concentration of valence electrons is proportional to the temperature and at the point of contact , the electrons diffuse through the boundary layer between the two leads, resulting in one lead becoming positive and the other becoming negative. Thus the emf generated is proportional to the temperature difference in a predictable manner. This phenomenon is known as seebeck effect.
• The seeback coefficient or sensitivity S of thermal element is given by S=elt=a+bt+ct²

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Force-measuring sensor-Load cells�Load Cells: Application of force will produce some deformation in specially design elements and this deformation may be measured using various types of sensors. Such elements are referred as load cells.�Load cell is an electromechanical sensor employed to measure static and dynamic forces.�Load cells exist in both compression and tension forms.�

Shear-type load cell

• A unique method of measuring the load with the output immune to side loads and bending moments is based on the measurement of shear components.
• The load cell is basically a double cantilever beam supported at the both ends as shown in fig., wherein strain gauges bonded at 45 ^o are employed to measure shear stresses developed on the cross members when the load is applied.

Pressductor

• The pressductor is basically a magneto-elastic type force transducer, wherein the permeability of a magnetic material is altered by the mechanical stress developed in it.
• The device consists of a number of laminated sheets of special magnetic material bonded together to form a transducer body, on which primary and secondary coils are wound and positioned perpendicular to each other and at an angle of 45^o to the direction of mechanical force as shown in fig

• When the primary is excited with an alternating current, no magnetic induction is produced in the secondary coil under the no load condition.
• On the application of mechanical force on the transducer, the permeability in the direction of the force is reduced resulting in a change in shape of the magnetic field. Some of the flux lines now cut the secondary windings, thus inducing a voltage in the secondary.
• The output voltage is proportional to the applied force.