Sensors -measure and report the state of some variable which characterizes the process uses a physical/chemical propertyconverts it into useful signal

Temperature ?? mercury thermometers thermoelements physical property used? Seebeck effect: emf in a circuit connecting elements maintained at different temp RTD (resistance temperature detector) Thermistors (thermally sensitive resistors) resistors: dR/dT != 0 semiconductors: dR/dT != 0 Applications: chemical processes, food processing, pasteurization, many manufacturing processes, … inaccurate, but very sensitive platinum: linear over large range

Position/Displacement variable  (potentiometer) variable capacitance physical property used? variable inductance (resolver) capacitors: dC/dx != 0 Applications: ?? Faraday’s law, induced emf in coils very sensitive (sub-micron range)

Velocity dR/dx Linear Velocity physical property used? Applications: ? Faraday’s law potentiometer Linear Velocity Angular Velocity tachometer

Derivatives of velocity acceleration: a = dV/dt (accelerometer) physical property used? jerk: da/dt Applications: Machine tools, Crash sensors, … spring loaded potentiometer electronic differentiator

Presence detection Faraday’s law moving magnet type physical property used? optical sensor applications inductive Faraday’s law Photoelectric effect oscillator induced emf reverse emf damped current sensor Photoconductive cells cadmium selenide, cadmium sulfide: conductivity  incident light Photovoltaic cells

Other sensors ammeter, glavanometer current voltage physical property used? Flow rates of fluids Mass voltmeter resistance ohmmeter multimeter Bernoulli's principle ?? Force/Pressure dynamometers piezo-electric piezoelectric effect Newton’s law quartz: watches Barium titanate, lead zirconate: ultrasonic sensors, microphones

Sensor Characteristics: Static Sensitivity: change in output / change in the input Resolution: smallest amount of change in the input that can be detected and accurately indicated Linearity: (of calibration curve)(i) plot static output versus static input (ii) measure linearity of graph Drift: deviation in output value when the sensor is kept at constant input level for long time Special cases: Zero Drift, Full-scale drift Range: (upper limit – lower limit) of output (or input) Repeatability: deviation in repeated measurements of same object, from same direction Reproducibility: repeatability over long time lapses between measurements

Sensor Characteristics: Dynamic Rise time: time to pass between 10% to 90% of the steady state value (SSV) of response Delay time: time it takes to reach 50% of SSV for the first time Peak time: time it takes to reach the maximum reading for the first time Settling time: time taken to settle down to within, e.g. ± 1%, the steady state value (SSV) Percentage overshoot: (peak value - SSV) / SSV Steady-state error: deviation of the actual SSV from the desired value

Sensor Characteristics: Statistical ACCURACY: the difference between the ACTUAL and the MEASURED value measure: (mean readings – actual value) PRECISION: the variations of the MEASURED value measure: (standard deviation of readings)