Chapter 04D - Pressure Sensors

Chapter 04D - Pressure Sensors - PRESSURE SENSOR Can be...

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Unformatted text preview: PRESSURE SENSOR Can be divided into three types: 1. Deflection type 2. Strain gauge type g g yp 3. Piezoelectric type Dr. Abdirahman 111 CHAPTER 4 Page 59 of Typical Pressure Detector System yp y Dr. Abdirahman 111 CHAPTER 4 Page 60 of 1. DEFLECTION TYPE PRESSURE SENSOR This sensor uses an elastic material to convert pressure to displacement e.g. stainless steel, brass. The displacement will be proportionate to the value of pressure exerted. Suitable to be used in an automatic control system. The main element used is in the shape of Bourdon tube, bellow or diaphragm. The secondary element is the element that will convert the displacement to electrical signals where the displacement can be detected through resistivity change, inductance or capacitance. Dr. Abdirahman 111 CHAPTER 4 Page 61 of The Main Typical Element Used In A Deflection Type Pressure Sensor Dr. Abdirahman 111 CHAPTER 4 Page 62 of Basic Form of Mechanical B i F fM h i l Pressure Sensors Dr. Abdirahman 111 CHAPTER 4 Page 63 of Example : i. Bellow-resistance pressure sensor The pressure is proportionate to the resistivity. The resistance change is detected by displacement of sliding contact in the resistance element. ii. ii Bellow-inductance pressure sensor The pressure is proportionate to the inductance change which is detected from the displacement of the core in the wire coil. Dr. Abdirahman 111 CHAPTER 4 Page 64 of The core movement will produce AC signal output which will give the value and direction of inductance. LVDT (linear variable differential transformer) (li i bl diff i l f ) demodulator is used to convert the AC output to DC. iii. Diaphragm-capacitance pressure sensor Diaphragm capacitance The pressure is proportionate to the capacitance change at the output through dielectric change. Pressure from the sensor element causes the diaphragm to move towards the plate and produces g dielectric change. Dr. Abdirahman 111 CHAPTER 4 Page 65 of Bellows C lib t d spring B ll Calibrated i Sliding contact Resistance R Output Signal O l A Bellows Resistance Pressure Sensor Dr. Abdirahman 111 CHAPTER 4 Page 66 of Bellows B ll Core C Output Signal O i l A Bellows Inductance Pressure Sensor Dr. Abdirahman 111 CHAPTER 4 Page 67 of A Diaphragm Capacitance Pressure Sensor p g p Dr. Abdirahman 111 CHAPTER 4 Page 68 of Resistance Type Capacitance Type Dr. Abdirahman 111 CHAPTER 4 Page 69 of 2. STRAIN GAUGE PRESSURE SENSOR Strain gauge is a type of resistive S f transduction. Pressure measurement is obtained from displacement of elastic element. Pressure is measured through force that is exerted on the di h d h diaphragm where the f h h force will be detected by the strain gauge and resistance change will be produced. Wheatstone Bridge circuit is used to detect the change in pressure and an amplifier is used to amplify the small output signals signals. Dr. Abdirahman 111 CHAPTER 4 Page 70 of Strain Gauge Pressure Sensor Dr. Abdirahman 111 CHAPTER 4 Page 71 of 3. PIZOELECTRIC PRESSURE SENSOR This sensor consists of a piezoelectric crystal (made from quartz) which functions as a forcesensitive voltage source where the piezoelectric material will be in between two plates (see the following figure). Pressure exerted on the crystal surface is proportionate to the voltage produced by the ti t t th lt g d d b th crystal. This sensor does not require any voltage supply. l This sensor is suitable for fast changing pressure measurement (dynamic pressure). Dr. Abdirahman 111 CHAPTER 4 Page 72 of Mechanical Stress Piezoelectric Material Output Voltage Plate Mechanical Stress Piezoelectric Pressure Sensor Dr. Abdirahman 111 CHAPTER 4 Page 73 of Piezoelectric Pressure Sensor Dr. Abdirahman 111 CHAPTER 4 Page 74 of ...
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This note was uploaded on 05/09/2010 for the course CHE 3022 taught by Professor Abdirahman during the Fall '07 term at National Central University.

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