TYPES OF PRESSURE TRANSDUCERS FOR BIOMEDICAL APPLICATION.pptx

content describes the types of pressure transducers used in biomedical application

1. PRESSURE TRANSDUCERS

2. Pressure Transducer Pressure transducers are converting pressure into measurable electrical signals. The fundamental principle involves applying the pressure to be measured onto a flexible diaphragm. The diaphragm, typically a thin, flat, circular plate, deforms in response to the applied pressure. Deformation of the diaphragm is then translated into an electrical signal for measurement. Common diaphragm materials include stainless steel, phosphor bronze, and beryllium copper.

3. Diagphragm

4. P ressure transducers make use of the diaphragm are of the following types: Capacitance manometer—in which the diaphragm forms one plate of a capacitor. Differential transformer—where the diaphragm is attached to the core of a differential transformer. Strain gauge—where the strain gauge bridge is attached to the diaphragm.

5. LVDT A Linear Variable Differential Transformer (LVDT) is an electrical transducer or sensor used for the measurement of physical quantities like displacement, force, pressure, acceleration, etc. It works on the principle of mutual inductance, that generates an electrical signal proportional to the physical quantity applied which is to be measured. Let us see the various applications of LVDT.

6. LVDT Principle It works on the principle of mutual inductance ( When changing current in one coil induces an EMF in the other ), It generates an electrical signal proportional to the physical quantity applied which is to be measured.

7. LVDT PRESSURE TRANSDUCER

8. Construction An LVDT consists of one primary winding to which an ac supply voltage is given and two secondary windings. The two secondary windings are connected in series of opposite connections so that a differential output voltage can be obtained. Between the primary and two secondary windings, a core is placed Depending on the position of the core, more or less output will be generated in the two secondary windings. Now connect pressure sensing elements, to the core of LVDT such that the output displacement resulting from this transducer by the applied pressure will be transmitted to the core of LVDT

9. LVDT

10. 3D View

11. Case 0: Let the emf induced in the two secondary windings are e s1  and e s2  respectively. Under the normal condition, when there is no displacement is applied to the movable core. An equal amount of voltages will be induced in the two secondary windings. Since the secondaries are connected in series opposition the net output voltage will be zero.

12. Case 1 When the pressure to be measured is applied to the diaphragm , the diaphragm deflects (expands upwards) and this movement is given to the rod of the core of LVDT. With this, the core will be lifted up and more voltage will be induced in the upper part of the secondary winding 1. ( e s1  > e s2 ). Therefore, a positive voltage is generated at the output (i.e., E o  = e s1  - e s2 ). This positive output will indicate applied pressure.

13. Case 2 when the pressure is decreased the diaphragm contracts (comes downwards) and the core comes down from the null position causing more voltage to induce in secondary winding 2 , which is denoted by e s2 T he net output voltage is negative which indicates the decrease in pressure ( e s1  < e s2 ). Therefore, a negative voltage is generated at the output (i.e., E o  = e s1  - e s2 ). This output will indicate applied pressure.

14. LVDT

15. Advantages of LVDT Pressure Transducer Sensitivity is very high Rugged construction It has low hysteresis It has very good repeatability

16. Disadvantages of LVDT Pressure Transducer It has poor accuracy The performance of this transducer is affected by temperature change

17. Strain Gauge Transducer A strain gauge pressure sensor is a device that measures pressure by converting the mechanical deformation experienced by a material into an electrical signal. This type of sensor is widely used across various industries and applications, including automotive, aerospace, medical, and industrial automation.

18. Strain Gauge At the core of a strain gauge pressure sensor is a strain gauge It is a thin, flexible material The strain gauge is typically made from a metal, such as constantan or nichrome, or a semiconductor material, like polysilicon or amorphous metal. Its electrical resistance when it undergoes mechanical deformation. It is bonded to a diaphragm or another structure that deforms when subjected to pressure .

19. Contd … As the diaphragm experiences pressure, it deforms and causes the strain gauge to change its shape as well. This change in shape leads to a change in electrical resistance, which can be measured using a Wheatstone bridge circuit. The output voltage from the Wheatstone bridge is proportional to the amount of strain experienced by the gauge, and thus to the pressure applied to the diaphragm.

20. Strain Gauge

21. Strain Gauge- Pressure Transducer

22. Pressure Transducer This type of pressure transducer utilizes a foil or silicon strain gauge (arranged as a Wheatstone bridge) attached to the surface of the diaphragm, on the opposite side of the media. When a change in pressure of the media occurs, this will result in deformation of the elastic material, thereby also changing the resistance of the strain gauge. This resistance change is converted into an electrical signal, which is then amplified and conditioned to provide transducer-voltage or transmitter-current output.

23. Types of Strain Gauges Electrical resistance strain gauges are mainly classified into two types. They are, Unbonded strain gauges Bonded strain gauges

24. Unbonded Strain Gauge In an unbounded strain gauge, the strain gauge is not directly bonded to the surface which is subjected to stress I t consists of resistance wire stretched between frames P and Q with the help of insulated pins . These two frames are movable with respect to each other, and this arrangement can be connected in one of the arms of Wheatstone's bridge .

25. Unbounded Strain Gauge

26. Bonded strain gauges Bonded strain gauges are directly placed or bonded on the surface of the device or component which is subjected to stress. In bonded type, the strain gauge is directly pasted on the surface of the structure under test. To paste the strain gauge on the structure, adhesives are used which are responsible for transmitting the strain from the structure to the gauge wires.

27. Bounded Strain Gauge

28. Gauge Factor Gauge Factor  is defined as the ratio of relative change in electrical resistance to the mechanical strain. Here relative change in resistance is defined as the ratio of change in resistance produced due to strain to its original resistance (without strained). It is also referred as Strain factor of a strain gauge. According to the definition, when applied pressure in a solid bar of length (L) produces change in length (ΔL) which results in gauge resistance change (ΔR) from its original resistance (R), Strain is defined as a change in length to actual length

29. Resistance The resistivity of a material is defined as the magnitude of electric current across it

30. Wheatstone Bridge Circuit In order to measure strain with a bonded resistance strain gauge, i t is connected to wheatstone Bridge circuit. The change in resistance is proportional to applied strain It is measured using wheatstone bridge circuit

31. Contd.. A Wheatstone bridge is a divided bridge circuit used for the measurement of static or dynamic electrical resistance. The output voltage of the Wheatstone bridge is expressed in millivolts output per volt input. 

32. Advantages High accuracy:  These sensors can achieve high levels of accuracy, typically within 0.1% to 0.5% of the full-scale pressure range. This allows for precise pressure measurements in critical applications. Wide pressure range:  Strain gauge pressure sensors can measure pressures ranging from very low to extremely high levels, making them suitable for diverse applications. Good temperature stability:  With proper compensation techniques, these sensors can maintain accuracy over a wide temperature range, allowing for their use in environments with significant temperature fluctuations. Long-term stability:  Strain gauge pressure sensors exhibit excellent long-term stability, ensuring consistent performance over time.

33. Disadvantages Cost:  The complexity of the sensor design and the need for precise manufacturing techniques can lead to higher costs compared to other pressure sensor types. Vulnerability to electromagnetic interference (EMI):  The electrical signal generated by the strain gauge can be susceptible to interference from external electromagnetic sources. This can be mitigated through proper shielding and grounding techniques. Low signal output:  The output signal from a strain gauge pressure sensor is typically in the millivolt range, requiring additional signal conditioning and amplification to be usable.

34. Capacitive Transducer A capacitive transducer has a static plate and a deflected flexible diaphragm with a dielectric in between. When a pressure is exerted to the outer side of the diaphragm the distance between the diaphragm and the static plate changes. This produces a capacitance which is measured using an capacitance bridge. .

35. Capacitive Transducer A metallic diaphragm enclosed in an airtight container It moves to the left when pressure is applied to the chamber and to the right when vacuum is applied. This diaphragm is used as one plate of a  variable capacitor . The pressure applied to the unit is determined by Its distance from the stationary plate to its left The monitor indicates the pressure equivalent of the unit’s capacitance by measuring the capacitor’s reactance to the ac source voltage.

36. Capacitance Transducer

37. Capacitance Measurement From this equation, it is seen that capacitance increases ( i ) if the effective area of the plate is increased, and (ii) if the material has a high dielectric constant. The capacitance is reduced if the spacing between the plates is increased.

38. Advantages and Disadvantages Advantages It produces an accurate frequency response to both static and dynamic measurements. Disadvantages An increase or decrease in temperature to a high level will change the accuracy of the device. As the lead is lengthy it can cause errors or distortion in signals .

39. Piezoelectric Transducer Type Piezoelectric pressure sensors convert mechanical pressure into electrical signals, It offering high sensitivity, wide range, and durability for diverse applications The term “piezoelectric” is derived from the Greek words “piezo,” meaning pressure, and “electric,” referring to electricity.

40. Principle When piezoelectric material is placed under mechanical stress Material undergoes deformation Electric charge accumulates in a solid material in response to a applied mechanical stress It causes a shifting of the positive and negative charge centers in the material takes place, which then results in an external electrical field. In verse piezoelectric effect When an electrical field applied to a material It either stretches or compresses the piezoelectric material. Converts electrical signal in to mechanical signal

41. Piezoelectric Pressure Sensors Piezoelectric materials exhibit a specific property where they produce an electric charge when subjected to mechanical stress, such as pressure or force.   When pressure is applied to the sensor, the piezoelectric material deforms, causing a voltage difference across the electrodes. This voltage can then be measured and correlated to the applied pressure.

42. Piezoelectric materials The most commonly produced piezoelectric ceramics are  Lead Zirconate Titanate barium titanate lead titanate Rochelle Salt Quartz

43. Advantages of Piezoelectric Material High sensitivity:  Piezoelectric materials can generate significant electrical signals in response to small mechanical forces, allowing for the detection of minute pressure changes. Wide dynamic range:  These sensors can measure pressures over a wide range, from a few millibars to several hundred atmospheres, making them suitable for numerous applications. Fast response time:  Due to their low mass and high stiffness, piezoelectric sensors can respond to pressure changes rapidly, making them ideal for dynamic and transient pressure measurements. Excellent stability:  Piezoelectric materials are inherently stable and do not exhibit significant drift over time, ensuring long-term measurement accuracy. Resistance to harsh environments:  Piezoelectric sensors can operate in extreme temperatures, pressures, and corrosive environments, making them suitable for use in challenging conditions.