Week 9 - Sensors Part 2.pdf

Week # 9 Sensors Part 2 ( Analog) Analog sensors can be used to measure: Distance Speed (linear or rotational) Temperature Pressure Force Any other physical quantity that varies over a range 2 Energy System Engineering 1 Accuracy How closely the sensor indicates the true quantity being measured Reso...

Week # 9 Sensors Part 2 ( Analog) Analog sensors can be used to measure: Distance Speed (linear or rotational) Temperature Pressure Force Any other physical quantity that varies over a range 2 Energy System Engineering 1 Accuracy How closely the sensor indicates the true quantity being measured Resolution What is the smallest change that can be observed 3 Repeatability How closely will the sensor measure the same quantity again Precision Refers to a group of sensors How closely does the output of one match the others in the group? Important for interchangeability 4 Energy System Engineering 2 Common temperature sensors: Thermocouple RTD Thermistor IC (Semiconductor) Temperature Sensors 5 Most widely used temperature sensor Two dissimilar metal wires joined at one or both ends Dissimilar metals produce small voltage when joined, proportional to difference in temperature at cold and hot junction Very accurate devices 6 Energy System Engineering 3 7 Resistance Temperature Device Basically a precision resistor Electrical resistivity of metals change with temperature Some metals change very predictably Platinum is most common metal used – Very linear change over very wide range 8 Energy System Engineering 4 Semiconductors with negative temperature coefficient – Resistance decreases as temperature p increases Very precise and stable Large variation for small temperature change Output is not very linear – max 300 C 9 Some semiconductors increase reverse bias current across P-N junction as temperature increases Very linear output 10 Energy System Engineering 5 Biased with a current Magnetic field will cause small current flow Used to measure: – Flow rates – Linear Displacement – Rotary displacement or velocity 11 12 Energy System Engineering 6 Used to measure force Stretching an elastic wire increases its resistance (by reducing area) Only sensitive in one direction Can be used to measure: Pressure and Acceleration Need to be temperature compensated 13 14 Energy System Engineering 7 Linear Variable Displacement Transformer Electrical transformer with three windings Movable core passes through center of transformer Output voltage is proportional to core position 15 Resolvers Rotary position transducer Also uses transformer principle Two stationary windings are 90 out of phase Rotor winding phase is proportional to the angle of the rotor 16 Energy System Engineering 8 Sensor Installation Considerations Electrical – Limit load current (between 50 – 200 mA) S – Shield sensor wiring ffrom electrical noise – Match sinking with sourcing components Mechanical – Mount horizontally if possible – Be sure it doesn doesn’tt detect its own mount or another sensor – Do not over tighten fasteners! 17 Analog Signals Two Common Types: Voltage (V) and Current (I) 18 Energy System Engineering 9 Analog SignalsSignals- Voltage 19 Analog SignalsSignals- Current 20 Energy System Engineering 10 Analog I/O Module The analog input interface module accepts an analog signal and converts it to a digital signal. The analog output module accepts a digital signal and converts it to an analog signal that operates the output. 21 Basic Analog Connections 22 Energy System Engineering 11