Basic Sensors and Principles

Transcription

1 Sensors Lecture 5

2 2 Basic Sensors and Principles

3 Transducer, Sensor, and Actuator Transducer: A device that converts energy from one form to another. Transducer can act as a sensor or actuator. Sensor: converts a physical parameter to an electrical output (a type of transducer, e.g. a microphone) Actuator: converts an electrical signal to a physical output (opposite of a sensor, e.g. a speaker)

4 Transducer Systems Sensors Actuators Interface Circuits Control and Processing Circuits Power Supply I/O Channel /USER 4

5 Classification of Transducers Transducers On The Basis of principle Used Active/Passive Primary/Secondary Analogue/Digital 5 Capacitive Inductive Resistive Piezoelectric Transducers may be classified according to their application, method of energy conversion, nature of the output signal, and so on.

6 Type of Sensors Temperature Sensors: Thermistors, thermocouples Displacement Sensors: resistance, inductance, capacitance, piezoelectric Electromagnetic radiation Sensors: Thermal and photon detectors

7 Measurements Techniques Before we start. Something about measurements techniques

8 Measurement Techniques: Small fractional changes Many sensors vary their electrical characteristic by a small fraction over the range over which they are used thermister - resistance varies by 4%/C Temperature varies by 1mA/K (i.e K = 3% change in current) V s Wheatstone bridge R 3 R R 3 R 1 V G x1 - + R x R 2 R x R 2

9 Temperature Measurements There are two main types: contact and noncontact temperature sensors. Contact sensors include thermocouples and thermistors that touch the object they are to measure, and noncontact sensors measure the thermal radiation a heat source releases to determine its temperature. The latter group measures temperature from a distance and often are used in hazardous environments.

10 Sensor Types: Temperature Measurement The human body temperature is a good indicator of the health and physiological performance of different parts of the body. Temperature indicates: Shock by measuring the big-toe temperature Infection by measuring skin temperature Arthritis by measuring temperature at the joint Body temperature during surgery Infant body temperature inside incubators Temperature sensors type 10 RTD (Resistor Temperature Detector ) Thermocouples Thermistors Radiation and fiber-optic detectors (non-contact type)

11 RTD Sensors What is an RTD? Resistance Temperature Detector Operation depends on inherent characteristic of metal (Platinum usually): electrical resistance to current flow changes when a metal undergoes a change in temperature. If we can measure the resistance in the metal, we know the temperature! Platinum resistance changes with temperature Wire-wound sensing element Thin-film sensing element on ceramic substrate 11 Two common types of RTD elements:

12 RTD Sensors Why is Platinum used? It is the most stable & near linear resistance versus temperature function when compared to other metals like Nickel & Balco (Nickel-Iron Alloy) 12

13 Question the resistance of the sensor at 0 C the resistance of the sensor at 100 C What does it mean Pt100, = ? Pt = Platinum = 0 deg C the probe will read 100 ohms. at 100 deg C, it will read ohms. Each resistance versus temperature relation for an RTD is qualified by a term known as alpha. Alpha is the slope of the resistance between 0 C and 100 C. This is also referred to as the temperature coefficient of resistance, with the most common being What would be the resistance at 20C? 13

14 RTD 1. They have very poor thermal sensitivity, that is a change in temperature only produces a very small output change for example, 1Ω/ o C or even less. 2. The more common types of RTD s are made from platinum and are called Platinum Resistance Thermometer or PRT s with the most commonly available of them all the Pt100 sensor, which has a standard resistance value of 100Ω at 0 o C. The downside is that Platinum is expensive and one of the main disadvantages of this type of device is its cost. 3. Because the RTD is a resistive device, we need to pass a current through them and monitor the resulting voltage. However, any variation in resistance due to self heat of the resistive wires as the current flows through it, I 2 R, causes an error in the readings.

15 RTD To avoid selfheating the RTD is usually connected into a Whetstone Bridge network which has additional connecting wires for lead-compensation and/or connection to a constant current source. Two types of Whetstone Bridge networks are follow: 2-wire: Lowest cost -- rarely used due to high error from lead wire resistance 3-wire: Good balance of cost and performance. Good lead wire compensation. 15

16 RTD Circuits 2 wire; Because the RTD is a resistive device, you must drive a current through the device and monitor the resulting voltage. However, any resistance in the lead wires that connect your measurement system to the RTD will add error to your readings. Es is the supply voltage; Eo is the output voltage; R1, R2, and R3 are fixed resistors; and RT is the RTD. In this uncompensated circuit, lead resistance L1 and L2 add directly to RT.

17 In this circuit there are three leads coming from the RTD instead of two. L1 and L3 carry the measuring current while L2 acts only as a potential lead. No current flows through it while the bridge is in balance. Since L1 and L3 are in separate arms of the bridge, resistance is canceled. RTD Circuits 3 wire; In 3 wire configuration you can compensate for the lead resistances. In this bridge configuration, the effects of L1 and L3 cancel each other out because they are located in opposite arms of the bridge. Lead resistance L2 does not add significant error because little current flows through it.

18 RTD Advantages Most stable over time Most accurate Most repeatable temperature measurement Very resistant to contamination/ corrosion of the RTD element Disadvantages High cost Slowest response time Low sensitivity to small temperature changes Sensitive to vibration (strains the platinum element wire) Decalibration if used beyond sensor s temperature ratings Somewhat fragile

19 Thermocouples A thermocouple is a device consisting of two dissimilar conductors or semiconductors that contact each other at one or more points. A thermocouple produces a voltage when the temperature of one of the contact points differs from the temperature of another, in a process known as the thermoelectric effect.

20 Thermocouples The conversion of temperature difference to electric current and vice-versa is termed as thermoelectric effect. When the two junctions of a thermocouple are maintained at different temperatures, then a current starts flowing through the loop known as thermo electric current. The potential difference between the junctions is called thermo electric emf. There are three major effects involved in a thermocouple circuit: the Seebeck, Peltier, and Thomson effects. 21

21 Seebeck Effect The Seebeck effect describes the voltage or electromotive force (EMF) induced by the temperature difference (gradient) along the wire due to diffusion. The change in material EMF with respect to a change in temperature is called the Seebeck coefficient. V- Voltage difference between two dissimilar metals a- Seebeck coefficient T h - T c - Temperature difference between hot and cold junctions 22

22 Seebeck Effect Seebeck observed that when two dissimilar metal wires are formed into a closed loop and its two junctions are held at different temperatures, it has the ability to deflect a galvanometer needle. The operation of a thermocouple is based on the different Seebeck coefficients of the dissimilar metals. If the two metals of the thermocouple were alike, or had the same Seebeck coefficient, the net emf produced at its measuring point would be zero.

23 Peltier effect The reverse of the Seebeck effect is also possible: by passing a current through two junctions, you can create a temperature difference. Peltier effect describes the temperature difference generated by EMF and is the reverse of Seebeck effect. 24

24 Thomson Effect Thomson effect is related to the emf that develops between two parts of the single metal when they are at different temperature. Thus thomson effect is the absorption or evolution of heat along a conductor when current passes through it when one end of the conductor is hot and another is cold. The heat is proportional to both the electric current and the temperature gradient. 25

25 Thermocouples Advantages Simple, Rugged High temperature operation Low cost No resistance lead wire problems Point temperature sensing Fastest response to temperature changes (=1ms) Disadvantages Least stable, least repeatable Low sensitivity to small temperature changes Extension wire must be of the same thermocouple type Wire may pick up radiated electrical noise if not shielded Lowest accuracy 26

26 Thermoelectric Sensitivity The three most common thermocouple materials used above for general temperature measurement are Iron-Constantan (Type J), Copper-Constantan (Type T), and Nickel-Chromium (Type K). The output voltage from a thermocouple is very small, only a few millivolts (mv) for a 10 o C change in temperature difference and because of this small voltage output some form of amplification is generally required. 27

27 Thermistors Thermistors are semiconductors made of ceramic materials whose resistance decreases as temperature increases or vice a versa. Advantages Small in size (0.5 mm in diameter) Large sensitivity to temperature changes (-3 to -5% / o C) Temperature differences in the same organ Excellent long-term stability characteristics (R=0.2% /year) Disadvantages 28 Nonlinear Self heating Limited range

28 Thermistors THERMal resistors Thermistors are made of semiconductor materials (metallic compounds including oxides such as manganese, copper, cobalt, and nickel, as well as single-crystal semiconductors silicon and germanium). Contrast <<--->> Common carbon resistors, made from carbon powder mixed with a phenolic binder glue. Leads, coated Glass encased Surface mount

29 Thermistors Assume a simple linear relationship between resistance and temperature for the following discussion: ΔR = k ΔT where ΔR = change in resistance ΔT = change in temperature k = first-order temperature coefficient of resistance

30 Thermistors Thermistors can be classified into two types depending on the sign of k. If k is positive, the resistance increases with increasing temperature, and the device is called a positive temperature coefficient (PTC) thermistor, Posistor. If k is negative, the resistance decreases with increasing temperature, and the device is called a negative temperature coefficient (NTC) thermistor.

31 Thermistors Thermistor-choice is based on the nominal resistance you want at the operating temperature range, on the size, and on the time constant. Time constants are about 5-10 seconds.

32 Thermistors

33 A little easier to read Thermocouple RTD Voltage (mv) Temperature ( C) Resistance (Ω) Thermistor Temperature ( C) Resistance (KΩ) Temperature ( C)

34 Self heating of thermistor 1.The voltage source is turned on, producing a current through the series combination of resistors, R t and R a. 2.The current flowing through the thermistor generates some heat because thermistor dissipates electrical power. 3.The heat causes a temperature rise in the thermistor. 4.The temperature rise in the thermistor causes the resistance of the thermistor to decrease (NTC). 5.The decrease in resistance causes and increase in current through the thermistor. 6.The increased current through the thermistor generates more heat. 7. The additional heat raises the temperature even higher.

35 Self heating of thermistor That means that the temperature it measures is not the surrounding temperature, but one that is higher. This phenomenon is called self heating. When using thermistor circuits, you want to minimize selfheating. You do that by minimizing the current through the thermistor. Given a choice, choose the situation where the thermistor has the smallest amount of current flowing through it.

36 Circuit Connections of Thermistors Bridge Connection to measure voltage R 1 R 3 V v a v b R 2 R t Amplifier Connection to measure currents 37

37 Resistance ratio, R/R 25º C Thermistors Resistance Relationship between Resistance and Temperature at zero-power resistance of thermistor. R R e [ ( T0 T )/ TT 0 ] t 0 = material constant for thermistor, K (2500 to 5000 K) T o = standard reference temperature, K T o = K = 20C = 68F R t drt dt (% / K) 2 T is a nonlinear function of temperature (a) Typical thermistor zero-power resistance ratio-temperature characteristics for various materials. (a) Temperature, C

38 Temperature Sensor Options Resistance Temperature Detectors (RTDs) Platinum, Nickel, Copper metals are typically used positive temperature coefficients Thermistors ( thermally sensitive resistor ) formed from semiconductor materials, not metals often composite of ceramic and metallic oxide (Mn, Co, Cu Fe) typically have negative temperature coefficients Thermocouples based on dissimilar metals at diff. temps

39 Semiconductors Temperature Sensor Are small and result from the fact that semiconductor diodes have voltage-current characteristics that are temperature sensitive. Temperature measurement ranges that are small compared to thermocouples and RTDs, but can be quite accurate and inexpensive.

40 Thermal Sensor Vendors Minco Pyrotek Omega Watlow Texas Instrument National Semiconductor Maxim

41 Determining Factors Low Power Serial Interface Small Accurate Wide temperature range Extras I 2 C Interface Temperature Alarms

42 Texas Instrument Specs I 2 C Interface -55º to 125ºC range ±1º accuracy (±3º max) ±0.0625ºC resolution 2.7 to 5.5 operating voltage 45 to 75 µa operating current, 0.1 to 1µA shutdown current 40ms/320ms conversion rate(9/12 bit) 25/3 conversions per second (9/12 bit) Online sample request 6 pin SO23 package Needs 400kHz clock for I 2 C Interface TMP 100/101

43 Maxim MAX6625/MAX6626 I 2 C Interface -55º to 125ºC range ±1º accuracy (±2º max) ±5/0.0625ºC resolution(625/626) 3.0 to 5.5 operating voltage 250µA to 1mA operating current, 1µA shutdown current 133ms conversion rate Online sample request 6 pin SO23 package

44 National Semiconductor I 2 C Interface -55º to 125ºC range ±2/ ±1º accuracy 9 bits/ 12 bits or ±0.0625ºC resolution 3/3.3 to 5.5 operating voltage 0.25 to 0.5 µa operating current, 4/5µA shutdown current 100ms/400ms conversion rate(9/12 bit) Online sample request 8 pin SOP package Needs 400kHz clock for I 2 C Interface LM75/LM76

45 Temperature measurement: An electronic medical thermometer Use thermistor to measure temperature in range ºC. Thermistor changes its resistance on temperature change. The change in resistances measured and digitally displayed on thermometer in terms of temperature.

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48 An electronic medical thermometer: Block Diagram

49 parameters < 0.1 ºC sensitivity required 8-bit ADC/ 12 bit is also in use An electronic medical thermometer Typical Thermistor R S =10kW at 35 ºC Temperature coefficient (suppose ) 4% per ºC At 40 ºC R S =10k x =12.166kW v0v ma x temperature Approximately linear 35 ºC to 40 ºC V S Voltage at 40 ºC: V max =0.549V V V max min V Variation is 0.05/0.549=9% 9% of ADC dynamic range is used 23 counts for 5 ºC =0.22 ºC/count S 10 k sensor V o Vo R S R 1 V ref ADC R s R S V S A DC outpu t

50 Or in graphical format only levels are unused: Levels are in use For this application voltage across thermister is high ADC has high input impedance ADC is close to thermister Thermister voltage can be applied directly to ADC without amplification But the poor use of the ADC dynamic range means that the required resolution (0.1 ºC ) is not achieved... v0v ma x temperature V S sensor V 10 k R S o V o R ADC R s R S V S A DC outpu t

51 A solution is to subtract the bias voltage with a differential amplifier. The voltage range is unchanged, but the voltage at the ADC input at the minimum temperature of 35 ºC is now zero volts Using a differential amplifier with a gain of 10 with a Vref of 0.5V is now 0.49V, making good use of the ADC dynamic range V S 10k R S 10k V o V S /2 10k o R T - x10 V o 0.5V S V ref ADC V V V ' min max 10 0 R 0.49V S S T S 1 V 2 S

52 Fiber-Optic Temperature Sensors Small and compatible with biological implantation. Nonmetallic sensor so it is suitable for temperature measurements in a strong electromagnetic heating field. Gallium Arsenide (GaAs) semiconductor temperature probe. The amount of power absorbed increases with temperature 61

53 Displacement Measurements Used to measure directly and indirectly the size, shape, and position of the organs. Displacement measurements can be made using sensors designed to exhibit a resistive, inductive, capacitive or piezoelectric change as a function of changes in position. 62

54 Displacement Measurements Examples diameter of part under stress (direct) movement of a microphone diaphragm to quantify liquid movement through the heart (indirect) Primary Transducer Types (As previous slide) Resistive Sensors (Potentiometers & Strain Gages) Inductive Sensors Capacitive Sensors Piezoelectric Sensors Secondary Transducers Wheatstone Bridge Amplifiers

55 Displacement: Resistive sensors - potentiometers Measure linear and angular position Resolution a function of the wire construction Measure velocity and acceleration 2 to 500mm From 10 o to more than 50 o

56 Strain Gauges Definition: resistive element that changes resistance proportional to an applied mechanical strain

57 Strain Gauges Compression = decrease in length by L and an increase in cross sectional area. L = length Rest Condition L - L = length Compression

58 Strain Gauges Tension = increase in length by L and a decrease in cross section area. L = length Rest Condition L + L = length Tension

59 Resistance of a metallic bar is given in length and area pl R A where R = Resistance units = ohms (Ώ) ρ = resistivity constant unique to type of material used in bar units = ohm meter (Ώm) L = length in meters (m) A = Cross sectional area in meters 2 (m 2 )

60 Gauge Factor Gauge Factor (GF) = a method of comparing one transducer to a similar transducer

61 Gauge Factor GF R L R L where GF = Gauge Factor unitless ΔR = change in resistance ohms (Ώ) R = unstrained resistance ohms (Ώ) ΔL = change in length meters (m) L = unstrained length meters (m)

62 Gauge Factor R GF R L L Where ε strain which is unitless GF gives relative sensitivity of a strain gauge where the greater the change in resistance per unit length the greater the sensitivity of element and the greater the gauge factor.

63 Example of Gauge Factor Have a 20 mm length of wire used as a string gauge and has a resistance of 150 Ώ. When a force is applied in tension the resistance changes by 2Ώ and the length changes by 0.07 mm. Find the gauge factor: GF R L R L 2W 150W 0.07mm 20mm 3.71

64 Resistance of a metallic bar is given in length and area Example: find the resistance of a copper bar that has a cross sectional area of 0.5 mm 2 and a length = 250 mm note the resistivity of copper is 1.7 x 10-8 Ώm R L A Wm 250mm 1m 1000mm 1m 1000mm 8 1.7* mm W

65 Piezoresistivity Piezoresistivity = change in resistance for a given change in size and shape denoted as h Resistance in tension = R h L L A A Resistance increases in tension L = length; ΔL = change in L; ρ = resistivity A = Area; ΔA = change in A

66 Resistance in compression = R h L L A A Resistance decreases in compression L = length; ΔL = change in L; ρ = resistivity A = Area; ΔA = change in A

67 Example of Piezoresistivity Note: Change in Resistance will be approximately linear for small changes in L as long as ΔL<<L. If a force is applied where the modulus of elasticity is exceeded then the wire can become permanently damaged and then it is no longer a transducer.

68 Example of Piezoresistivity Thin wire has a length of 30 mm and a cross sectional area of 0.01 mm 2 and a resistance of 1.5Ώ. A force is applied to the wire that increases the length by 10 mm and decreases cross sectional area by mm 2 Find the change in resistance h. Note: ρ = resistivity = 5 x 10-7 Ώm

69 Example of Piezoresistivity L L R h A A 7 R h *10 Wm 1.5W h 2.74W h 1.24W 1m (30 10) mm* 1000mm 2 1m ( ) mm * 1000mm 5 2

70 Application 1: By mounting is suitably on the walls of cardiac muscle, the force of the contraction of the cardiac muscle can be measured continuously. 2: To measure blood pressure in the heart, The strain gauge is mounted on the tip of the catheter which is inserted in the heart through a vein. In front of the strain gauge there is diaphragm the deflection of which varies with blood pressure. And in turn alters the strain gauge resistance.

71 Application: Biomechanics Force Plates/Dynamometers A force is a device that measures the ground reaction forces (GRF) exerted by a subject standing (or walking) on it. Force plates are used for gait analysis, diagnosis of foot impairment, studies of balance, sports medicine, and design of medical shoes. Force plates consist of a top plate which is separated from the bottom frame by force transducers at each corner. The forces exerted on the top surface (of the plate) are transmitted through the force tri axial transducers (operating in transverse (Z), antero posterior (X) and vertical (Y) directions).

72 Application: Weighing In weighing/scaling machine there is a sensor which converts force or weight into an electrical signal. This sensor is the load cell which is classified as a force transducer. The strain gage is the heart of a load cell.

73 Application: Kidney Dialysis A typical kidney dialysis system may depend on one or several load cells to ensure that the filtration system has perfect balance and timing. The dialysis system must remove contaminated blood, clean it, and recirculate the clean, reoxygenated blood. Load cells used for this type of system are typically in-line, small, and work by monitoring flow changes by sensing the weight of a hanging bag to ensure the dialysis procedure is performed safely every time. The load cell is attached to the end of a hanging bag. The bag is connected to the dialysis machine via two flexible tubes. One tube is used for the flow inlet and the other is for discharge. Each load cell is connected to a programmable logic controller or computer to monitor the flow by weight measurement. Using the load cell information, the system automatically processes and controls the dialysis procedure while collecting data for further analysis when needed.

74 Application: Rehabilitation A load cell attached to a gripping or tension device can indicate exact changes in an affected muscle and how much progress is being made after each therapy session. This allows the therapist to customize the types of therapy to the needs of the patient. These load cells vary in size from 1 to 4 in. diameter, with measurement ranges of lb. There are many system configurations designed for this purpose, but all have two things in common: the patient exerts force against some object that is connected to the load cell, and the load cell sends the resulting measurement to a readout device or computer. The computer then converts the signal from analog to digital to produce an accurate, real-time display.

75 Types of Strain Gauges: Unbonded and Bonded Unbonded Strain Gauge : resistance element is a thin wire of special alloy stretch taut between two flexible supports which is mounted on flexible diaphram or drum head.

76 Types of Strain Gauges: Unbonded and Bonded When a Force F1 is applied to diaphram it will flex in a manner that spreads support apart causing an increase in tension and resistance that is proportional to the force applied. When a Force F2 is applied to diaphram the support ends will more close and then decrease the tension in taut wire (compression force) and decrease resistance will decrease in amount proportional to applied force

77 Types of Strain Gauges: Unbonded and Bonded Bonded Strain Gauge: made by cementing a thin wire or foil to a diaphragm therefore flexing diaphragm deforms the element causing changes in electrical resistance in same manner as unbonded strain gauge

78 Types of Strain Gauges: Unbonded and Bonded When a Force F1 is applied to diaphram it will flex in a manner that causes an increase in tension of wire then the increase in resistance is proportional to the force applied. When a Force F2 is applied to diaphram that cause a decrease the tension in taut wire (compression force) then the decrease in resistance will decrease in amount proportional to applied force

79 Comparison: Unbonded and Bonded 1. Unbonded strain gauge can be built where its linear over a wide range of applied force but they are delicate 2. Bonded strain gauge are linear over a smaller range but are more rugged Bonded strain gauges are typically used because designers prefer ruggedness.

80 Typical Configurations A R1 = SG1 R3 = SG3 ES + - C Vo D R2 = SG2 B R4 = SG4 Electrical Circuit Mechanical Configuration 4 strain gauges (SG) in Wheatstone Bridge:

81 Strain Gauge Example + Using the configuration in the previous slide where 4 strain gauges are placed in a wheatstone bridge where the bridge is balanced when no force is applied, Assume a force is applied so that R1 and R4 are in tension and R2 and R3 are in compression. Derive the equation to depict the change in voltage across the bridge and find the output voltage when each resistor is 200 Ώ, the change of resistance is 10 Ώ and the source voltage is 10 V

82 Strain Gauge Example Es + - Circuit R1 = R +h C A Eo R3= R-h R2 = R - h R4 = R +h B D Eo Eo Eo Eo Derivation: Es Es Es R2 R1 R2 R h R h R h R h R h 2R 10W 10V 200W 2R R4 R3 R4 0.5V R h R h R h 2h Es Es 2R h R

83 Piezoelectric Sensors When a pressure is applied to a polarized crystal, the resulting mechanical deformation and displacement of charges results in an electrical charge. They are sensitive to more than one physical dimension. Therefore, it sometimes becomes necessary to compensate for unwanted Effects. For instance, sophisticated pressure sensors often use acceleration compensation elements. Those compensations are based on the fact that the measuring elements may experience both, pressure and acceleration events. A second measuring unit is added to the sensor assembly that only experiences acceleration events. By carefully matching those elements, the acceleration signal Is subtracted from the combined signal of pressure and acceleration to derive the true pressure information 93

84 General Applications External (body surface) and internal (intracardiac) phonocardiography Detection of Korotkoff sounds in blood-pressure measurements Measurements of physiological accelerations Provide an estimate of energy expenditure by measuring acceleration due to human movement. 94

85 As mentioned previously, an external force cause a deformation of the crystal results in a charge which is a function of the applied force. In its operating region, a greater force will result in more surface charge. This charge results in a voltage Vo, where q is the charge resulting from a force f, and C is the capacitance of the device. V o q k kf piezoelectric constant, C/N (typically pc/n, a material property) k for Quartz = 2.3 pc/n k for barium titanate = 140 pc/n To find V o, assume system acts like a capacitor (with infinite leak resistance): 95 V o q C kf C kfx 0 A r Capacitor: C 0 r A x

86 Models of Piezoelectric Sensors Piezoelectric polymeric films, such as polyvinylidence fluoride (PVDF). Used for uneven surface and for microphone and loudspeakers. 96

87 Transfer Function of Piezoelectric Sensors View piezoelectric crystal as a charge generator: q Kx K proportionality constant x deflection R s : sensor leakage resistance C s : sensor capacitance C c : cable capacitance C a : amplifier input capacitance R a : amplifier input resistance R a 97

88 Transfer Function of Piezoelectric Sensors Convert charge generator to current generator: i i s c C q Kx i i c s dv dt i i o R R K i s dx dt dq dt Vo R K dx dt R a Vo X j K s j j j 1 R a K s = K/C, sensitivity, V/m = RC, time constant 98

89 Limitation i i s c C i i c s dv dt i i o R R K dx dt Vo R Consider the equation, it indicates that the steady state response to a constant deflection is zero. It means static displacement cant be measured by piezoelectric material. This can be visualized by considering step displacement function (next slide)

90 Voltage-output response of a piezoelectric sensor to a step displacement x. Decay due to the finite internal resistance of the PZT When a force is applied to the sensor at t =0, the sensor output (Kx/C)start decaying with time, due to limited internal resistance. When force is released the restoration occurs equally and opposite direction. This cause a sudden drop in charge and undershoot occur. The decay and undershoot can be minimized by increasing the time constant =RC. 100

91 Piezo Electric Pulse Transducer Piezo-electric element convert force applied to the active surface of the transducer from the finger blood pressure pulse, into an electrical signal. Detects expansion and contraction of the finger circumference, due to changes in blood pressure Typical output is mv but can reach as high as 500 mv

92 Piezo Electric Pulse Transducer Finger pulse transducer is a very sensitive instrument. Even slight movements by the volunteer can result in noisy recordings. The subjects should keep their hand as still as possible between stimuli.