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SENSORS FOR BIOMEDICALAPPLICATIONEngr Hinesh Kumar Lecturer O Differentiate between the terms Sensor Transducer Actuator . Active and Passive Transducers Sensors Sensors used in Biomedical Instruments Sensor Error Sources Sensor Terminology The Wheatstone Bridge. Displacement Transducers Resistive Inductive or Capacitive type Temperature Transducers Thermocouples Thermistors PN Junctions Piezoelectric Transducers. Definitions Transducer A transducer is a device whichconverts energy from one form to Sensor .A sensor is a device which converts aphysical parameter to an electrical ActuatorAn actuator is a device which convertsan electrical energy to a mechanical or. Active sensors Active sensors generate electricaloutput directly in response to anapplied stimulation or measurand An active sensor doesn t require an.external voltage source to produceelectrical output Example Solar Cell PiezoelectricMaterial Thermocouple etc PASSIVE SENSORS. Passive sensors produce a change in somepassive electrical quantity such ascapacitance resistance or inductance inresponse to an applied stimulus ormeasurand . Therefore a passive sensor does requirean external ac or dc voltage source inorder to convert passive electrical quantitysuch as capacitance resistance orinductance in to electrical output. Example Photo Diode Thermistor Strain Gauge etc Examples of Sensors used inBiomedical Instruments Sensors are now available to measure.many parameters of clinical andlaboratory interest Some types of sensors aresummarized in the Table below Sensors in Medical Instruments. Example of sensors used in typicalmedical instruments Sensor Error Sources Sensors like all other devices sustain certain The error is defined as the difference between.the measured value and the true value Sensor errors an be break into five basiccategories 1 Insertion Error2 Application Error.3 Characteristic Error4 Dynamic Error5 Environmental Error Sensor Error Sources1 Insertion Errors.The insertion errors occur during theact of inserting the sensor into thesystem being measured 2 Application ErrorsApplication errors are caused by the.3 Characteristic ErrorsThe characteristic errors are inherent inthe device itself i e the differencebetween the ideal characteristic transferfunction of the device and the actual.characteristic This form of error may include a dc off set value a false pressure head anincorrect slope or a slope that is notperfectly linear . 4 Dynamic Errors Many sensors are characterized andcalibrated in a static condition i e with aninput parameter that is either static orquasi static . Many sensors are heavily damped so thatthey will not respond to rapid changes inthe input parameter Dynamic errors include response time amplitude distortion and phase distortion . 5 Environmental Errors These errors are derived from theenvironment in which the sensor is used They most often include temperature butmay also include vibration shock altitude .chemical exposure or other factors These factor most often affect thecharacteristic errors of the sensor so areoften combined with that category inpractical application . Sensor Terminology1 Sensitivity2 Sensitivity Error4 Dynamic Range5 Precision.6 Resolution7 Accuracy9 Linearity10 Hysteresis11 Response time.12 Dynamic linearity13 Transfer function15 Bandwidth Sensor Terminology1 Sensitivity. The sensitivity of the sensor is defined as theslope of the output characteristic curve Y X More generally the minimum input of physicalparameter that will create a detectable output In some sensor the sensitivity is defined as the.input parameter change required to produce astandardized output change In others it is defined as an output voltagechange for a given change in input parameter Sensor Terminology. Sensor Terminology4 Dynamic Range The dynamic range is the total range ofthe sensor from minimum to maximum 5 Precision. The precision refers to the degree ofreproducibility of a measurement 6 Resolution The resolution is define as the smallestdetectable incremental change of input.parameter that can be detected in the output Sensor Terminology7 Accuracy The accuracy of the sensor is themaximum difference that will exist.between the actual value which must bemeasured by a primary or good secondarystandard and the indicated value at theoutput of the sensor Sensor Terminology. The offset error of a transducer is defined asthe output that will exist when it should be zero Alternatively the difference between the actualoutput value and the specified output valueunder some particular set of conditions .9 Linearity The linearity of the transducer is an expressionof the extent to which the actual measuredcurve of a sensor departs from the ideal curve Sensor Terminology.Ideal versus measured curve showinglinearity error Sensor Terminology10 Hysteresis A transducer should be capable of.following the changes of the inputparameter regardless in which directionthe change is made hysteresis is themeasure of this property Sensor Terminology.11 Response Time Sensors do not change output stateimmediately when an input parameterchange occur Rather it will change to thenew state over a period of time called the.response time The response time can be defined as thetime required for a sensor output to changefrom its previous state to a final settledvalue within a tolerance band of the correct.new value Sensor Terminology12 Dynamic Linearity The dynamic linearity of the sensor is ameasure of its ability to follow rapid.changes in the input parameter Amplitude distortion characteristics phase distortion characteristics andresponse time are important indetermining dynamic linearity . Sensor Terminology13 Transfer Function The functional relationship betweenphysical input signal and electrical output Almost all type of sensors produce some.output noise in addition to the output The noise of the sensor limits theperformance of the system Most common types of noise are 50 Hz Sensor Terminology.15 Bandwidth All sensors have finite response timesto an instantaneous change in physical In addition many sensors have decaytimes which would represent the time.after a step change in physical signal forthe sensor output to decay to its original The reciprocal of these times correspondto the upper and lower cutoff frequencies The Wheatstone Bridge. Many biomedical passive transducers sensors are usedin a circuit configuration called a Wheatstone The Wheatstone bridge circuit is ideal for measuringsmall changes in resistance The Wheatstone bridge can be viewed as two resistor.voltage dividersWheatstone Bridge connected in parallel withBridgeWheatstone the voltagesourceCircuitE Circuit Redrawn for.Simplify Analysis The Wheatstone BridgeThe output voltage E0 is the difference betweenthe two ground referenced potentials EC and EDproduced by the two voltage divider networks .Where EC and ED can be calculated as So the output can be calculated as Example A Wheatstone bridge isexcited by a 12 v dc source andcontains the following resistances R1 .1 2 k R2 3 k R3 2 2 k and R4 5 k Find the output voltage E0 Null Condition The null condition in a Wheatstone bridge circuit exists when theoutput voltage E0 is zero . The equation of Wheatstone bridge is The null condition exists when either the excitation sourcevoltage E must be zero or the expression inside bracket s mustbe equal to zero So the null condition occurs when and . Therefore the ratio of two equals are Replacing voltages with the equivalent current and resistance So the null condition in a Wheatstone bridgecircuit occurs whenExample Show that the null condition exists in a.Wheatstone bridge consisting of the following resistances R1 2 k R2 1 k R3 10 k and R4 5 k Note that it is not necessary for the resistances to be equal forthe null condition only that the ratios of the two half bridgevoltage dividers must be equal . Since both sides of the equation evaluate to the same quantity we may conclude that the bridge is in the null condition A bridge in the null condition is said to be balanced Strain Gauge Strain gauges are displacement type transducers.that measure changes in the length of an object asa result of an applied force A strain gauge is a resistive element that producesa change in its resistance proportional to an appliedmechanical strain . A strain is a force applied in either compression apush along the axis to word the center or tension a pull along the axis away from the center The piezoresistive effect describes change inthe electrical resistivity of a semiconductor.when mechanical stress force is applied Mechanism for PiezoresistivityFigure a shows a small metallic barwith no force applied It will have a length L and a cross .sectional area A Changes in length are given by Land changes in area are given by A Figure b shows the result of applying acompression force to the ends of the bar . The length reduces to L L and thecross sectional area increases to A Figure c shows the result of applying atension force of the same magnitude to The length increases to L L and.the cross sectional area reduces to A Strain Gauge Resistance The resistance of a metallic bar is given in terms of thelength and cross sectional area in the expression as is the resistivity constant of the material in ohm meter.L is the length in meters m A is the cross sectional area in square meters m 2 The above equation shows that the resistance isdirectly proportional to the length and inverselyproportional to the square of the cross sectional area . Strain Gauge Strain GaugePiezoresistivity The change of resistance with changes in size and shape issome called piezoresistivity . The resistance of the bar will become R h in tension The resistance of the bar will become R h in compression Where the h is change in resistance Examine the equation of strain gauge it is found thatchanges in both length and cross sectional area tend to.increase the resistance in tension and decrease theresistance in compression The resistances after force is applied are in tension The resistances after force is applied are in compression Strain Gauge.Example A thin constantan wire stretched taut has alength of 30 mm and a cross sectional area of 0 01 mm2 The resistance is 1 5 The force applied to the wire isincreased so that the length further increases by 10 mmand the cross sectional area decreases by 0 0027 mm2 .Find the change in resistance h where the resistivity ofconstantan is approximately 5 x 10 7 m Gauge Factor GF The fractional change in resistance R R divided by the fractional change in length L .L is called the gauge factor GF The gauge factor GF is a unit less number The gauge factor provides sensitivityinformation on the expected change inresistance for a given change in the length of a.strain gauge The gauge factor varies with temperature andthe type of material Therefore it is important to select a materialwith a high gauge factor and small.temperature coefficient For a common metal wire strain gauge madeof constantan GF is approximately equal to Semiconductor strain gauges made of siliconhave a GF about 70 to 100 times higher and.are therefore much more sensitive thanmetallic wire strain gauges The gauge factor GF for a strain gauge transducer isa means of comparing it with other semiconductortransducers . The definition of gauge factor is GF is the gauge factor dimensionless R is the change in resistance in ohms R is the unstrained resistance in ohms L is the change in length in meters m .L is the length in meters m Example A 20 mm length of wire used as a strain gauge exhibits aresistance of 150 When a force is applied in tension the resistancechanges by 2 and the length changes by 0 07 mm Find the gaugefactor GF . The gauge factor gives us a means for evaluating the relativeMany biomedical passive transducers/sensors are used in a circuit configuration called a Wheatstone bridge. The Wheatstone bridge circuit is ideal for measuring small changes in resistance. The Wheatstone bridge can be viewed as two resistor voltage dividers connected in parallel with the voltage source . E.