Biomedical Sensors PDF

Physiological Variables A variable associated with the physiological process of the body is called a physiological variable. Examples are body temperature, electrical activity of heart, arterial blood pressure, respiratory airflow etc. Major function of medical i...

Physiological Variables A variable associated with the physiological process of the body is called a physiological variable. Examples are body temperature, electrical activity of heart, arterial blood pressure, respiratory airflow etc. Major function of medical instrumentation is measurement of these variables. Physiological variables occur in many forms such as ionic potential, mechanical movements, hydraulic pressures, temperature variations, chemical reactions etc.  Transducers The device that performs the conversion of one form of variable into another is called transducer. Transducer is required to convert each variable into an electrical signal which can be amplified or processed and then converted to suitable display. Two principles are involved in the process of converting nonelectrical variables into electrical variables.  Transduction Principle Passive Active Transducer Transducer Transducers Transducers based on the based on the principle of principle of control of an energy excitation voltage conversion or modulation of carrier signal.  Active Transducers The most popular phenomenon involved in designing of active transducers for biomedical applications are: – Magnetic Induction – Piezoelectric Effect – Thermoelectric Effect – Photoelectric Effect Active Transducers: Magnetic Induction Operating Principle: 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. Active Transducers: Magnetic Induction This principle can be used for the measurement of linear or rotary motion. The output voltage in each case is proportional to the linear or angular velocity. The most important biomedical applications are heart sound microphones, pulse transducers and electromagnetic blood flow meters. It also plays an important role at the output of many biomedical instruments. Analog meters, pen motors in ink or thermal recorders etc. are such example. Active Transducers: Piezoelectric Effect Operating Principle: When pressure is applied to certain non conductive materials so that deformation takes place, a charge separation occurs in the materials and an electrical voltage can be measured across the material.  This effect is reversible in nature. Example: same crystal can behave like mic as well speaker. Unit 1-Physiological Transducers Active Transducers: Thermoelectric Effect Operating Principle: When two dissimilar metals such as iron and copper are joined at both ends to form a closed circuit, and one of the junctions is at a higher temperature than the other, a current is set up. The e.m.f. driving this current is called a 'thermoelectric e.m.f.', and the phenomenon is known as the thermoelectric effect. Active Transducers: Thermoelectric Effect A voltage can be observed at any point of interruption of the loop which is proportional to the difference in temperature between the two junctions. The polarity depends on which junction is warmer. The device formed in such a way is called a thermocouple. Unit 1-Physiological Transducers Active Transducers: Thermoelectric Effect Since the thermocouple measures the temperature difference rather than absolute temperature one of the junctions must be kept at known reference temperature. Reference is usually freezing point i.e. 0◦C or 32◦F. The use of thermoelectric effect to convert from thermal to electrical energy is called seebeck effect. When the flow of current causes one junction to heat and other to cool it is called peltier effect. Unit 1-Physiological Transducers Active Transducers: Photoelectric Effect The photoelectric effect is the emission of electrons when electromagnetic radiation, like light, hits a material. Electrons emitted in this manner are called photoelectrons. Selenium cell is popularly used to measure the intensity of light in photographic meters. When operated for small load resistance the current delivered is proportional to intensity of light. Unit 1-Physiological Transducers Unit 1-Physiological Transducers  Passive Transducers Passive transducers utilize the principle of controlling a DC voltage or an AC carrier signal. The transducer consists of a passive circuit element which changes its value as a function of the physical variable to be measured. There are three circuit elements that are used as passive transducers: Resistor, Capacitor and Inductor. Unit 1-Physiological Transducers Passive Transducer Using Resistive Element Any resistive element that changes its resistance as a function of a physical variable can be used as a transducer for that variable. Example: – a linear/rotary potentiometer can be used to convert linear/rotary displacement into resistance change. – Resistivity of conductive materials is a function of temperature; this property can be used in temperature transducers. Unit 1-Physiological Transducers Potentiometer can be used to convert displacement into resistance change (a) Linear displacement, (b) Rotatory displacement. Unit 1-Physiological Transducers  Passive Transducer Using Resistive Element In some semiconductor materials the conductivity is increased by light striking the material. This effect is used in photo resistive cells; a form of photoelectric transducer. Most transducers used for mechanical variables utilize a resistive element called the strain gauge. Unit 1-Physiological Transducers  Passive Transducer Using Resistive Element: Strain Gauge A strain gauge is a resistor used to measure strain on an object. When an external force is applied on an object, due to which there is a deformation occurs in the shape of the object. This deformation in the shape is both compressive or tensile is called strain, and it is measured by the strain gauge. When an object deforms within the limit of elasticity, either it becomes narrower and longer or it become shorter and broadens. As a result of it, there is a change in resistance end-to- end. The strain gauge is sensitive to that small changes occur in the geometry of an object. By measuring the change in resistance of an object, the amount of induced stress can be calculated. Unit 1-Physiological Transducers  Passive Transducer Using Resistive Element: Strain Gauge Consider a resistive element of cylindrical shape having length L and cross sectional area A. If resistivity of the material is r, its resistance is : R=r*L/A ohms If an axial force F is applied to the element causing it to stretch, its length increases by ΔL and cross sectional area decreases by ΔA. This results in an increase in resistance. Unit 1-Physiological Transducers  Passive Transducer Using Resistive Element: Strain Gauge The change in resistance normally has very small value, and to sense that small change, strain gauge has a long thin metallic strip arrange in a zigzag pattern on a non-conducting material called the carrier, as shown below, so that it can enlarge the small amount of stress in the group of parallel lines and could be measured with high accuracy. The gauge is literally glued onto the device by an adhesive. When an object shows physical deformation, its electrical resistance gets change and that change is then measured by gage. Unit 1-Physiological Transducers  Passive Transducer Using Resistive Element: Strain Gauge The ratio of the resulting resistance change ΔR/R to the change in length ΔL/L is called the Gauge factor, G. The gauge factor for metals is about 2 and for silicon is about 120. Unit 1-Physiological Transducers  Passive Transducer Using Capacitive Element The capacitive transducer contains two parallel metal plates. These plates are separated by the dielectric medium which is either air, material, gas or liquid. This transducer uses the electrical quantity of capacitance for converting the mechanical movement into an electrical signal. The capacitive transducer works on the principle of variable capacitances. The input quantity causes the change of the capacitance which is directly measured by the capacitive transducer. The capacitance of the capacitive transducer changes because of many reasons like overlapping of plates, change in distance between the plates and dielectric constant. Unit 1-Physiological Transducers  Capacitive transducer The principle of operation of capacitive transducers is based on the familiar equation of capacitance of parallel plate capacitor  d= distance between plates  A=overlapping area  0 = 8.85x10-12 F/m is the absolute permittivity,  r =dielectric constant (r =1 for air and r =3 for plastics) The capacitive transducer is work on the principle of change in capacitance which may be caused I. Change in overlapping Area (A) of capacitor plates. II. Change in distance d between the plates. III. Capacitance changes because of dielectric constant. Unit 1-Physiological Transducers 1. Transducer using the change in the Area of Plates The capacitive transducers are used for measuring the large displacement approximately from 1mm to several cms. The area of the capacitive transducer changes linearly with the capacitance and the displacement. Initially, the nonlinearity occurs in the system because of the edges. Otherwise, it gives the linear response. The equation below shows that the capacitance is directly proportional to the area of the plates. The capacitance changes correspondingly with the change in the position of the plates. x – the length of overlapping part of plates ω – the width of overlapping part of plates. Unit 1-Physiological Transducers  The sensitivity of the displacement is constant, and therefore it gives the linear relation between the capacitance and displacement. This type of capacitive transducer is suitable for the measurement of linear displacement ranging from 1mm to 10mm with accuracy is as high as 0.005% 2. Transducer using the change in the distance of plates The capacitance of the transducer is inversely proportional to the distance between the plates. The one plate of the transducer is fixed, and the other is movable. The displacement which is to be measured links to the movable plates. Unit 1-Physiological Transducers 3. Transducer using the change in the dielectric constant The initial capacitance of transducer can be given as Unit 1-Physiological Transducers Passive Transducer Using Capacitive Element (a)Capacitance changes with change in distance. (b)Capacitance changes due to change in overlap area. (c)Capacitance change due to change in Dielectric constant. Unit 1-Physiological Transducers  Passive Transducer Using Capacitive Element Advantages:- Disadvantages:- It requires an external force for The metallic parts of the transducers require operation and hence very useful for insulation. small systems. The frame of the capacitor requires earthing The capacitive transducer is very for reducing the effect of the stray magnetic sensitive. field. It gives good frequency response Sometimes the transducer shows the because of which it is used for the nonlinear behaviors because of the edge dynamic study. effect which is controlled by using the guard The transducer has high input ring. impedance hence they have a small The cable connecting across the transducer loading effect. causes an error. It requires small output power for operation. Unit 1-Physiological Transducers  Passive Transducer Using Capacitive Element The capacitive transducer uses for measurement of both the linear and angular displacement. It is extremely sensitive and used for the measurement of very small distance. It is used for the measurement of the force and pressures. The force or pressure, which is to be measured is first converted into a displacement, and then the displacement changes the capacitances of the transducer. It is used as a pressure transducer in some cases, where the dielectric constant of the transducer changes with the pressure. Unit 1-Physiological Transducers  Passive Transducer Using Inductive Element: LVDT LVDTs operate on the principle of a transformer. LVDT consists of a coil assembly and a core. The coil assembly is typically mounted to a stationary form, while the core is secured to the object whose position is being measured. The coil assembly consists of three coils of wire wound on the hollow form. A core of permeable material can slide freely through the center of the form. The inner coil is the primary, which is excited by an AC source as shown. Magnetic flux produced by the primary is coupled to the two secondary coils, inducing an Linear Variable Differential AC voltage in each coil. Transformer Unit 1-Physiological Transducers  Working of LVDT: Case 1: On applying an external force which is the displacement, if the core reminds in the null position itself without providing any movement then the voltage induced in both the secondary windings are equal which results in net output is equal to zero V1-V2=0. Unit 1-Physiological Transducers Case 2: When an external force is applied and if the steel iron core tends to move in the left hand side direction then the emf voltage induced in the secondary coil is greater when compared to the emf induced in the secondary coil 2. Therefore the net output will be V1-V2. Unit 1-Physiological Transducers  Case 3: When an external force is applied and if the steel iron core moves in the right hand side direction then the emf induced in the secondary coil 2 is greater when compared to the emf voltage induced in the secondary coil 1. The net output voltage will be V2-V1 Unit 1-Physiological Transducers Stage 1 Stage 2 Stage 3 Working of LVDT  Advantages of LVDT: 1) Infinite resolution is present in LVDT 2) High output 3) LVDT gives High sensitivity 4) Very good linearity 5) Ruggedness 6) LVDT Provides Less friction 7) Low hysteresis 8) LVDT gives Low power consumption. Unit 1-Physiological Transducers STATIC CHARACTERISTICS OF INSTRUMENTS & CALIBERATION  STATIC CHARACTERISTICS OF INSTRUMENTS The performance characteristics of an instrument are mainly divided into two categories: Static Characteristics Dynamic Characteristics  Accuracy This is the closeness with which the measuring instrument can measure the true value of the measurand under stated conditions of use, i.e. its ability to tell the truth. The accuracy of an instrument is quantified by the difference of its readings and the one given by the ultimate or primary standard. Accuracy depends on inherent limitations of instrument and shortcomings in measurement process.  REPRESENTATION OF ACCURACY For example, a 100 psi gauge with 0.1 % of FS accuracy would be accurate to ± 0.1 psi across its entire range. By convention, a gauge specified as 0.1% accuracy is implied to be 0.1% FS. When manufacturers define their accuracy as “% of reading”, they are describing the accuracy as a percentage of the reading currently displayed. For example, a gauge with 0.1 % of reading accuracy that displays a reading of 100 psi would be accurate to ± 0.1 psi at that pressure. At 50 psi, the same gauge would have an accuracy of ± 0.05 psi (twice as accurate).  PRECISION Precision is defined as the ability of instrument to reproduce a certain set of readings within given accuracy. Precision describes an instrument’s degree of random variations in its output when measuring a constant quantity. Precision depends upon repeatability. Precision is often confused with accuracy. High precision does not imply anything about measurement accuracy. ACCURACY VS PRECISION  RESOLUTION & THRESHOLD Resolution It is the minimum change or smallest increment in the measured value that can be detected with certainty by the instrument. It can be least count of instrument. Threshold Threshold is defined as the range of different input values over which there is no change in output value in starting.  RANGE and SPAN RANGE The range of an instrument defines the maximum value of a quantity that the instrument is designed to measure. The span of an instrument defines the minimum (Min) and maximum (Max) values of a quantity that the instrument is designed to measure. Span =Max-Min  sensitivity The sensitivity of an instrument is the change of output divided by the change of the measurand (the quantity being measured). As an example, consider a pressure sensor that has a measurement range of 0-100 PSI and an output range of 0-5V. Its sensitivity is.05 Volt/PSI. It can be defined as the ratio of the incremental output and the incremental input. While defining the sensitivity, we assume that the input-output characteristic of the instrument is approximately linear in that range.  HYSTERESIS Hysteresis exists not only in magnetic circuits, but in instruments also. For example, the deflection of a diaphragm type pressure gage may be different for the same pressure, but one for increasing and other for decreasing, as shown in Fig.  BACKLASH It is defined as the maximum distance or angle through which any part of mechanical system may be moved in one direction without causing motion of next part. Can be minimized if components are made to very close tolerances.  REPEATABILITY For repeatability to be established, the following conditions must be in place: the same location; the same measurement procedure; the same observer; the same measuring instrument, used under the same conditions; and repetition over a short period of time. What’s known as “the repeatability coefficient” is a measurement of precision, which denotes the absolute difference between a pair of repeated test results.  reproducibility Reproducibility, on the other hand, refers to the degree of agreement between the results of experiments conducted by different individuals, at different locations, with different instruments. Put simply, it measures our ability to replicate the findings of others.  Linearity It is defined as the ability of an instrument to reproduce its input linearly. It is a measure of the maximum deviation of the calibration points from the ideal straight line.  Drift Drift is a departure in the output of the instrument over the period of time. An instrument is said to have no drift if it produces same reading at different times for the same variation in the measured variable. Drift is unrelated to the operating conditions or load. The following factors could contribute towards the drift in the instruments: i) Wear and tear ii) Mechanical vibrations iii) Stresses developed in the parts of the instrument iv) Temperature variations v) Stray electric and magnetic fields  Continued Drift may be of any of the following types; a) Zero drift: Drift is called zero drift if the whole of instrument calibration shifts over by the same amount. It may be due to shifting of pointer or permanent set. b)Span drift: If the calibration from zero upwards changes proportionately it is called span drift. It may be due to the change in spring gradient. c) Zonal drift: When the drift occurs only over a portion of the span of the instrument it is called zonal drift.  WHAT IS CALIBRATION? Calibration is a comparison between a known measurement (the standard) and the measurement using your instrument. Calibration of your measuring instruments has two objectives. It checks the accuracy of the instrument and it determines the traceability of the measurement. In practice, calibration also includes repair of the device if it is out of calibration. A report is provided by the calibration expert, which shows the error in measurements with the measuring device before and after the calibration.  Why calibration is important? The accuracy of all measuring devices degrade over time. This is typically caused by normal wear and tear. However, changes in accuracy can also be caused by electric or mechanical shock or a hazardous manufacturing environment. Depending on the type of the instrument and the environment in which it is being used, it may degrade very quickly or over a long period of time. The bottom line is that, calibration improves the accuracy of the measuring device. Accurate measuring devices improve product quality. A measuring device should be calibrated: According to recommendation of the manufacturer. After any mechanical or electrical shock. Periodically (annually, quarterly, monthly)  Example A Starrett-Webber gauge block is used to calibrate an electronic caliper. DYNAMIC CHARACTERISTICS  DYNAMIC CHARACTERISTICS The set of criteria defined for the instruments, which changes rapidly with time, is called ‘dynamic characteristics’. The various static characteristics are: Speed of response Measuring lag Fidelity Dynamic error  DYNAMIC CHARACTERISTICS Speed of response: It is defined as the rapidity with which a measurement system responds to changes in the measured quantity. Measuring lag: It is the retardation or delay in the response of a measurement system to changes in the measured quantity. Dynamic error: It is the difference between the true value of the quantity changing with time & the value indicated by the measurement system if no static error is assumed. It is also called measurement error. Fidelity is defined as the degree to which a measurement system indicates changes in the measured quantity without any dynamic error  Time response The time response of a system is the output (response) which is function of the time, when input (excitation) is applied. Time response of a control system consists of two parts: Transient Response Steady State Response Ct Css Mathematically,  Standard Signals The measurement systems may be subjected to any type of input. To study the dynamic behavior of measurement systems, certain standard signals are employed for which the mathematical equations have been developed. These standard signals are: (i) Step input, (ii) Ramp input, (iii) Parabolic input, and (iv) Impulse input. The above signals are used for studying dynamic behavior in the time domain and the dynamic behavior of the system to any kind of inputs can be predicted by studying its response to one of the standard signals.  Test signals For analysis of time response of a control system, following input signals are used:  Order of a system The order of a dynamic system is the order of the highest derivative of its governing differential equation. Equivalently, it is the highest power of S in the denominator of its transfer function. Zero order system FIRST ORDER SYSTEM continued  RESPONSE OF FIRST ORDER SYSTEM WITH UNIT STEP INPUT continued Thank You