gtu topic 3_basic transducer principles

1. COMPILED BY: PROF G B RATHOD ET department-BVM College, Email: [email protected] BASIC TRANSDUCER PRINCIPLES

2. OUTLINES THE TRANSDUCERS AND TRANDSDUCTION PRINCIPALES ACTIVE TRANSDUCERS PASSIVE TRANSDUCERS TRANSDUCERS AND BIOMEDICAL APPLICATIONS OUTCOMES REFERENCES QUESTIONS 7/13/20152 BVM,ET DEPARTMENT, BMI

3. THE TRANSDUCERS AND TRANDSDUCTION PRINCIPALES A variable is any quantity whose value changes with time. A variable associated with physiological process of the body is called a physiological variable. A transducer is required to convert each variable into an electrical signal which can be amplified or otherwise processed and converted into some form of display. The device that performs the conversion of one form of variable into another is called a transducer. Here we discuss the transducer which is having input some other quantity and output will be electrical quantity. 7/13/20153 BVM,ET DEPARTMENT, BMI

4. THE TRANSDUCERS AND TRANDSDUCTION PRINCIPALES Two different principles are involved in the process of converting nonelectrical variables into electrical signals. One of these is energy conversion: Transducers based on this principle is called active transducers. Second of these is control of an excitation voltage or modulation of a carrier signal. Transducers based on this principle is called a passive transducers. 7/13/20154 BVM,ET DEPARTMENT, BMI

5. ACTIVE TRANSDUCERS 7/13/20155 BVM,ET DEPARTMENT, BMI

6. ACTIVE TRANSDUCERS Some conversion principles are describe here Magnetic Induction: if an electrical conductor is moved in a magnetic field in such a way that the magnetic flux through the conductor is changed, a voltage is induced which is proportional to the rate of change of magnetic flux. Basically two basic configuration are used using this concept: one is linear motion and the other is rotary motion. The applications are heart sound microphones, pulse transducers and electromagnetic blood flow meters. 7/13/20156 BVM,ET DEPARTMENT, BMI

9. ACTIVE TRANSDUCERS The natural materials in which this piezoelectric effect can be observed are primarily slices from crystals of quartz(SiO2) or Rochelle salt(sodium potassium tartrate). The piezoelectric process is reversible. If an electric field is applied to a slab of material that has piezoelectric properties, it changes its dimensions. We will see equivalent circuit of the piezoelectric transducer connected to an amplifier. The piezoelectric principal is also used in ultrasonic instruments. 7/13/20159 BVM,ET DEPARTMENT, BMI

10. ACTIVE TRANSDUCERS Equivalent circuit 7/13/201510 BVM,ET DEPARTMENT, BMI

12. ACTIVE TRANSDUCERS The Thermoelectric effect: If two wires of dissimilar metals (e.g., iron and copper) are connected so that they form a closed conductive loop, a voltage can be observed at any point of interruption of the loop which is proportional to the difference in temperature between the two junction between the metals. The sensitivity of a thermocouple is small and amounts to only 40 microvolts per degree Celsius. (microV/oC) for copper-constantan. 7/13/201512 BVM,ET DEPARTMENT, BMI

14. ACTIVE TRANSDUCERS The use of thermoelectric effect to convert from thermal to electrical energy is called the seeback effect. In reverse direction it is called the Peltier effect.(its used to cool parts of instruments. E.g. a microscopic stage) The photoelectric Effect: The selenium cell, has been used to measure the intensity of light in photographic exposure meters or the light absorption of chemical solution. The silicon photoelectric cell(solar cell), has a much higher efficiency than the selenium cell. 7/13/201514 BVM,ET DEPARTMENT, BMI

17. PASSIVE TRANSDUCERS 7/13/2015BVM,ET DEPARTMENT, BMI17 Passive transducers utilize the principle of controlling a dc excitation voltage or an Ac carrier signal. There are only three passive circuit elements that can be utilized as a passive transducers. Resistors, capacitors, and Inductors. Passive Transducers Using Resistive Elements: Special linear potentiometer can be used to convert displacement into a resistance change.

18. PASSIVE TRANSDUCERS 7/13/2015BVM,ET DEPARTMENT, BMI18

20. PASSIVE TRANSDUCERS 7/13/2015BVM,ET DEPARTMENT, BMI20 Some passive resistive based transducers are : Photo resistive cells, photo diode, photo emissive cell(either vacuum or gas filled) Most popular is strain gage. The principle of strain gauge is as follow: We know Gage factor The Gage factor of metal is 2 and for silicon is 120. L R A / / R R G L L

22. PASSIVE TRANSDUCERS 7/13/2015BVM,ET DEPARTMENT, BMI22 Mercury strain gage: Resistive material consists of a column of mercury enclosed in a piece of silicon rubber tubing. Used in physiological variable measurement( diameter of blood vessels). First time introduced by Whitney thats why this gage some times called a Whitney gages. When replacing metallic train gage: there are two types. (1) Unbonded strain gage , (2) Bonded strain gage(foil gage, semiconductor strain gages)

25. PASSIVE TRANSDUCERS 7/13/2015BVM,ET DEPARTMENT, BMI25 Passive transducer using inductive elements. In inductive transducer the core is a permanent magnet which when moved induces a voltage in the coil. In this passive transducer the core is made of a soft magnetic material which changes the inductance of the coil when it is moved inside. Another type of passive inductive based is variable reluctance transducer where core remain stationary but the air gape is varied and ultimately permeability is varied. E.g., LVDT

28. PASSIVE TRANSDUCERS 7/13/2015BVM,ET DEPARTMENT, BMI28 Passive transducer using Capacitive elements. The capacitance of the capacitor can be changed by varying the physical dimension of the plate structure or by varying the dielectric contestant of the medium between the plates. E.g., the capacitance plethysmography. Passive Transducers Using Active Circuit Elements. Transistor , Photoelectric transducers

29. TRANSDUCERS FOR BIOMEDICAL APPLICATIONS 7/13/2015BVM,ET DEPARTMENT, BMI29

30. TRANSDUCERS FOR BIOMEDICAL APPLICATIONS 7/13/2015BVM,ET DEPARTMENT, BMI30 Force Transducers: A design element frequently used for the conversion of physical variable is the force summing member. By using that summing of force vector are summing and been zero for the spring thats why the name given is force summing member. Using this element we can design the force based transducers for various applications. All force transducer should be isometric.( no change in dimension) All displacement transducer should be isotonic.( No resistance change in displacement)

35. TRANSDUCERS FOR BIOMEDICAL APPLICATIONS 7/13/2015BVM,ET DEPARTMENT, BMI35 Transducers for Displacement, velocity and Acceleration dD V dt 2 2 dV d D A dt dt V Adt 2 ( )D Vdt A dt

36. TRANSDUCERS FOR BIOMEDICAL APPLICATIONS 7/13/2015BVM,ET DEPARTMENT, BMI36 Pressure Transducers: Pressure transducers are closely related to force transducers. Force summing members used in pressure transducers are shown in figure. We can use the strain gage also for designing of such pressure transducers. Diaphragm type pressure transducers can be designed for a wide range of operating pressures, depending on the diameter and stiffness of the diaphragm, bourdon tube transducers are usually used for high pressure ranges. In differential pressure transducers the two pressure are applied to opposite side of the diaphragm.

38. TRANSDUCERS FOR BIOMEDICAL APPLICATIONS 7/13/2015BVM,ET DEPARTMENT, BMI38 Flow Transducers: For fluids and gases flow rate measurements, the methods are described in upcoming topics. Transducers with Digital Output: ADC can be used to convert analog signal to digital output for analog transducers. For digital output, specially design encoding disks are to be used in the process of the conversion from the transducers circuit. Usually photo diodes or photo transistors related circuit are used for the digital out put data.

40. Outcomes 7/13/2015BVM,ET DEPARTMENT, BMI40 will get more ideas about the transducers and its biomedical applications. Basically understand the working principles of various transducers and artifacts related to each principles with proper designing concepts. The proper knowledge about ,which principle should be apply for which physiological measurements ?

41. Reference 7/13/2015BVM,ET DEPARTMENT, BMI41 Book: Biomedical instrumentation and measurements ,by L. Cromwell, F .Weibell, and E. Pfeiffer. PHI publication 2nd Edition

42. Questions.???? 7/13/2015BVM,ET DEPARTMENT, BMI42 Thank you

gtu topic 3_basic transducer principles

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