CONCEPTS AND ISSUES IN MEASUREMENT & INSTRUMENTATION

CAPACITIVE TRANSDUCERSPIEZOELECTRIC AND HALL EFFECT TRANSDUCERS

Lecture 14Instructor : Dr Alivelu M ParimiLinearization by Pulse Width Modulation (PWM) Technique2

PWM is a technique in which variable capacitors are used for producing a pulse train whose duty cycle varies with displacement between the plates. Op-amps can also be used for pulse width modulation in linearization of capacitive pressure transducers. PIEZOELECTRIC transducers Piezoelectricity describes the phenomenon of generating an electric charge in a material when subjecting it to a mechanical stress (direct effect) and conversely generating a mechanical strain in response to an applied electric field (converse effect).

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Materials Many natural and man-made materials, typically in the form of crystals, ceramics, or polymers display the piezoelectric effect. Piezoelectric materials belong to a class of materials called Ferroelectrics. Piezoelectric Crystals exhibit the piezoelectric effect naturally, without any processing Piezoelectric materials are anisotropic i.e. their electrical, mechanical, and electromechanical properties differ along different directions Some piezoelectric materials under natural and synthetic crystals are given below.Quartz, Rochelle Salt, Tourmaline (Natural)Lithium Sulfate, Cadmium sulfide, Ammonium Dihydrogen Phosphate(ADP)Piezoceramic elements like Lead Zirconate Titanate (PZT); Barium Titanate Piezoelectric Polymer like; Polyvinylidene Fluoride (PVDF), Eethylenediamine tartrate (EDT)

4the vast majority of Piezoelectric sensors use PZT ceramic elements.Configurations PZT, the most commonly used material, can be formed, pressed, and cut into various complex configurations to provide many geometry and performance options. Some popular configurations are:

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Stack ActuatorsPIEZOELECTRIC ACTION6

The d and g Coefficients7The main characteristics of piezoelectric motion-to-voltage transducers can be explained by considering only one common mode of deformation say thickness expansion shown in Figure

The d and g Coefficients8

CIRCUIT ANALYSIS OF PIEZOELECTRIC TRANSDUCERwe consider piezoelectric transducers for displacement to voltage conversion. The displacement could be caused due to force, pressure, or acceleration9

The circuit diagram of piezoelectric transducer 10

The circuit diagram of piezoelectric transducer 11

Problem 12

Problem 13

Problem: assignment 14

Problem: Assignment Electrical circuit diagram of piezoelectric crystal connected with amplifier is shown in Figure. For pulse input, derive and plot the response of piezoelectric transducer.

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APPLICATIONS OF PIEZOELECTRIC TRANSDUCERSFind atleast five applications where this transducers is utilized.

Advantages & Disadvantages of Piezoelectric Transducers16HALL EFFECT SENSORSWhen a current carrying conductor is placed in a magnetic field, a voltage will be generated perpendicular to both the current and the field. The presence of this measurable transverse voltage is called the Hall effect17

HALL EFFECT SENSORS18

HALL EFFECT SENSORS19

Problem For a Hall crystal of length l, width w, and thickness d, derive expression for Hall voltage when current is in z direction, magnetic field in y direction 20

Hall Transducers Analog Output SensorsDigital Output Sensors

21Analog Output Sensors

Analog output sensors provide an output voltage that is proportional to the magnetic field to which it is exposed. Hall element is followed by amplifier. To further increase the interface flexibility of the device, an open emitter, open collector, or push-pull transistor is added to the output of the amplifier22

Digital Output Sensors

Digital output Hall sensor has an output that is just one of two states: ON or OFF. Figure illustrates a typical internally regulated digital output Hall Effect sensor. The Schmitt trigger compares the output of the differential amplifier with a preset reference. When the amplifier output exceeds the reference, the Schmitt trigger turns on. Conversely, when the output of the amplifier falls below the reference point, the output of the Schmitt trigger turns off. Hysteresis is included in the Schmitt trigger circuit for jitter-free switching23

Application The measurement of proximity, position and displacement of objects is essential in many different applications like valve position, level detection, process control, machine control, security, etc. Hall effect sensors are used to detect the proximity, presence or absence of a magnetic object. Hall-effect technology has provided solutions for reliable solid state magnetic switching and linear magnetic sensing24