sensor characteristics

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Information about sensor characteristics

Published on January 16, 2009

Author: vivekanandan


Unit –I Introduction : Unit –I Introduction S.Vivekanandan Contents : Contents Definition and example for Sensor and transducer General Concept and terminology of measurement system Transducer classification General input-output configuration Static characteristics of measurement system Dynamic Characteristics of measurement system Calibration and standards Error analysis in measurement system Definition of Sensor : Definition of Sensor Formal definition: “A device that receives and responds to a signal or stimulus” (American Heritage Dictionary of the English Language) Informally, a sensor is a device that takes in information from the outside world. Based on the information, the sensor creates a signal on which a system can base a decision In our case, a sensor will measure some physical quantity and convert it into some electrical signal (e.g., voltage, current) Examples: Resistive sensor, optical sensors, physical sensor chemical, Biomedical sensors etc.. Definition for Transducer : Definition for Transducer A Device which converts energy from one form to another form A Device which converts a physical quantity into an electrical quantity A Device which for the purpose of measurement converts the physical input quantity into electrical output quantity, its output-input and its output-time relationship being predictable to a known degree of accuracy at specified environmental conditions. Examples : Thermocouple, LDR, LVDT, Piezo-electric transducer, Magnetostrictive transducer, capacitive and inductive transducer Difference between Sensor and Transducer : Difference between Sensor and Transducer A sensor is a device that responds to a physical stimulus (as heat, light, sound, pressure, magnetism, or a particular motion) and transmits a resulting impulse (a signal relating to the quantity being measured). For example, certain sensors convert temperature into a change in resistance. A transducer is a device that is actuated by power from one system and supplies power usually in another form to a second system. For example a loudspeaker is a transducer that transforms electrical signals into sound energy. Often the words transducer and sensor are used synonymously. Slide 6: All sensors are Transducer But All Transducer are not Sensor Concepts of Generalized Measurement system : Concepts of Generalized Measurement system Sensor Signal Conditioner End/display Device Sensor Variable Manipulation Element Data Transmission Element Data Presentation Element Variable Conversion Element Transducer Classification -1 : Transducer Classification -1 Based on source of Energy Active Transducer: a transducer which does not require auxiliary energy source for conversion. (e.g. Thermocouple, piezoelectric, photovoltaic cell) Passive Transducer: a transducer which requires auxiliary energy source for conversion. (e.g. LDR, Strain Gauge, LVDT) Transducer Classification -2 : Transducer Classification -2 Based on Parameter of Measurement Displacement transducer - (1)linear (2)Angular Velocity transducer - (1)linear (2)Angular Acceleration transducer - (1)Linear (2)Angular Force Temperature Light Time Transducer Classification -3 : Transducer Classification -3 Based on Transduction Principle Variable Resistance Transducers Variable Inductance Transducers Variable Capacitance Transducers Piezoelectric Transducers Hall effect Transducers Magnetostrictive Transducers Eddy Current Transducers Fiber optic Transducers IC sensors General input and output configuration : General input and output configuration Measuring Instrument Desired input Interfering input Modifying input Output Example: Strain Gauge Desired Input – Strain, Interfering Input – Temperature ?R1 ?R2 Change in resistance due to strain Change in resistance due to temperature R = R + ?R Slide 12: Example: Thermistor Thermistor Temperature Light Strain Resistance Modifying Input Modifying input changes relationship between desired input and output and/or interfering input and output Characteristics of measurement system : Characteristics of measurement system Static Characteristics Involves measurement of quantities that are either constant or vary slowly with time Dynamic Characteristics Applications like aerospace and bio-logical inputs are subjected to vary with time Static Characteristics : Static Characteristics D A L R Re S T S H I P? List of Static Characteistics : List of Static Characteistics Static sensitivity Linearity Precision/Repeata-bility Accuracy Threshold Drift, Zero Drift Stability Resolution Hysteresis Range & Span Input Impedance/ Loading effect Input and Output Range : Input and Output Range Input Range the interval between the maximum and minimum admissible input range: Imax, Imin Output Range the interval between the maximum and minimum reachable output range: Omax, Omin Slide 17: Span and Zero Zero: the system output corresponding to a zero input. Slide 18: Accuracy & Error Bands Error Bands ±h manufacturer defined performance values Error bands is an indication of accuracy in terms of a statistical density function. Resolution : Resolution Resolution is the smallest detectable incremental change of input that can be detected in output signal. For any devices, their resolution is fixed. Slide 20: Sensitivity & Gain Sensitivity (Gain) is the rate of change in output corresponding to the rate of change in input dO/dI. At different range, the sensitivity may be different a device dI dO Slide 21: Repeatability Inability of a sensor to represent the same valve under identical conditions. Slide 22: Bias (offset) the residual error between the output and the true value after all possible compensations. Drift rate of change of the output with time NOT caused by input. Bias Drift True value Bias and Drift Deadband : Deadband Deadband (Dead Band) range of input in which the output remains at 0: Xd i o d Hysteresis : Hysteresis Hysteresisthe delay phenomenon in output due to energy dissipation. The actual output is either smaller or greater than the theoretical output depends on increasing or decreasing in input. Non-Linearity : Non-Linearity Dynamic Characteristics : Dynamic Characteristics Speed of Response The rapidity with which a measurement system responds to changes in the measured quantity Dynamic error Difference between the true value of the quantity changing with time and the value indicated by the measurement system Fidelity Degree of which a measurement system indicates changes in the measured quantity without any dynamic error Dynamic analysis of measurement system : Dynamic analysis of measurement system Time domain Analysis Time is used as an independent variable inputs is applied to the system and the behavior of the system is studied For the purpose of analysis and design it is necessary to assume some basic types of input signal like step, ramp, parabolic, impuls Frequency domain Analysis : Frequency is used as an independent variable Sinusoidal, cosine are input signals ZERO ORDER INSTRUMENT : ZERO ORDER INSTRUMENT ZERO ORDER SYSTEM (Example: Potentiometer) Characterized by a zero order differential equation Infinite bandwidth Instantaneous response Static gain / Sensitivity ABSENCE OF ENERGY STORAGE ELEMENT No time delay Step response Frequency response FIRST ORDER INSTRUMENT : FIRST ORDER INSTRUMENT FIRST ORDER SYSTEM (Example: Thermocouple without thermo well, mercury in glass thermometer) Characterized by a first order differential equation Static gain (which determines the dynamic response) Time Constant ( which determines the dynamic response) Settling time ( 2%, 5 % and 10 %) Slope at time t=0 will be 1/ (time constant) { based on step response of I-order instrument) Slope at time t=infinity will be zero. PRESENCE OF ONE ENERGY STORAGE ELEMENT. Contd.. : Contd.. Step Response of first order Instrument Ramp Response of first order Instrument Impulse Response of first order Instrument Terminated Ramp response of first order Instrument Frequency Response ( 3 dB bandwidth of first order instrument {0,1/Time constant} – first order instrument will behave like a low pass filter FIRST PRINCIPLES MODEL ( TIME CONSTANT AND GAIN WILL BE FUNCTION OF PARAMETERS LIKE HEAT TRANSFER AREA, HEAT TRANSFER COEFFICIENT, MASS OF THE SENSING ELEMENT ETC.) Estimation of first order parameters ( Gain and Time constant) Least Square Method (pseudo inverse) Number of Data Points (N) SECOND ORDER SYSTEM : SECOND ORDER SYSTEM SECOND ORDER SYSTEM (Example: Thermocouple with thermo well) Characterized by a second order differential equation Damping ratio ( Under damped -Galvanometers, over damped- T/C with thermo well, critically damped) Damping ratio recommended – 0.6 to 0.7 ( Linear phase characteristics and amplitude ratio being constant) Undamped Natural Frequency ( Larger natural frequency will have lower settling time) Damped Natural Frequency Rise time Peak time Settling time ( Percentage Tolerance) Percentage Overshoot SECOND ORDER SYSTEM – Cont’d : SECOND ORDER SYSTEM – Cont’d Step Response of second order under damped, critically damped and over damped systems Ramp Response of second order under damped, critically damped and over damped systems Frequency Response second order under damped, critically damped and over damped systems Slope at time t=0 will be zero for second order over damped system. Estimation of Second order parameters from step response Least Square Method ( pseudo inverse) For all second order critically damped and over damped instruments the time for 73 % recovery will be at about t = 1.3* (tau1+tau2) Higher order Instrument As the order of the instrument increases the response of the instrument will become sluggish. Calibration : Calibration CALIBRATION is the method of checking the accuracy of an instrument It is the comparison of two measurement devices or systems one of known uncertainty and other of unknown uncertainty to estimate the correct value of the unknown and its uncertainty Why Calibration? : Why Calibration? Result of Calibration enables the estimation of errors of measuring instrument or the assignment of values to mark or arbitrary scales In many Instruments, suitable adjustments (range, span) are made during calibration in order to reduce the error Calibration is an accredited laboratory enables to establish TRACEABILITY When products are exported to other countries, they insist on traceability In short calibration enhances CREDIBILITY in measurement Note: Traceability is a document able link between the accuracy of the Instrument and the highest level of standard as maintained by a national/international laboratory Calibration : Calibration Absolute Method Calibration Comparative Method Error Measurement Standards : Standards Standard is a physical representation of unit of measurement Standard is a material measure, measuring instrument, reference material or system intended to define, realize, conserve, reproduce a unit in order to transmit them to other instruments by comparison Classification of Standards : Classification of Standards Primary Standards International Standard Working Standards Secondary Standards Errors in measurement system : Errors in measurement system Gross Error Random error Limiting error Relative limiting error Known error Systematic error Instrumental error Environmental error Observational error Error Analysis : Error Analysis Arithmetic Mean Deviation Average Deviation Standard deviation Variance Median Mode Probable error Standard deviation of mean Standard deviation of Standard deviation Thank u : Thank u

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