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Transducers and Sensors I. Friday, March 18, 2005. Learning Objectives. Know each element and its primary function in a measurement system Differentiate between a sensor (detector) and a transducer Identify signal domains as electrical or nonelectrical
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Transducers and Sensors I Friday, March 18, 2005 ISAT 253
Learning Objectives • Know each element and its primary function in a measurement system • Differentiate between a sensor (detector) and a transducer • Identify signal domains as electrical or nonelectrical • Give an example of a common sensor/transducer system and describe how it works • Give examples of several sensor/transducers and the chemical and physical principles on which they are based • Be able to calculate resistance, stress, strain, and other common sensor measurements ISAT 253
Key Concepts List Transducer Sensor Thermocouple RTD Thermistor Strain gage Photoconductive cells Photodiode ISAT 253
Definitions Transducer: a device that changes (transduces) the signal to or from an electrical domain as a voltage or current. Sensor: also called detector, a device that senses a physical or chemical stimulus and converts it into a signal that can be electrical, mechanical, or optical. ISAT 253
The Sensors We’re Born With! • stimulus and signal? • sensitivity? • detection limit? ISAT 253
The Sensors We’ll Study • Optical sensors • Thermal sensors • Chemical sensors • Strain gage sensors ISAT 253
Sensors and Data Domains • Dunn identifies data domains as: • Chemical • Electrical • Magnetic • Mechanical • Radiant • Thermal • Need to add: • Social ISAT 253
Optical Sensors • respond to ultraviolet, visible, and infrared light • non-electrical sensor: photographic film • photon transducers • Photoconductive cells • Photodiodes ISAT 253
hf semi- conductor I Vout Photoconductive Light Detectors Photoconductive light detectors are made from semiconductor materials (commonly, Pb and Cd sulfides or selenides). The figure shows a piece of semiconductor with two wires attached.
hf semi- conductor I Vout Photoconductive Light Detectors If photons have a high enough energy (Ephoton= hf), they will release electrons from their chemical bonds, making them free to move. This will reduce the resistivity (r) of the semiconductor.
hf semi- conductor I Vout Photoconductive Light Detectors Since Rsemiconductor = rl/A , the resistance of the semiconductor sample will decrease when r decreases.
Photoconductive Light Detectors A typical photoconductive cell: (a) cutaway view (b) symbol.
Photoconductive Light Detectors Characteristics of a typical CdSe photoconductive cell: (a) resistance versus illuminance, and (b) spectral response.
Applications for Photoconductive cells • Light meter in camera • Daylight sensor • Elevator safety stop • Garage door safety stop • Lots of other applications – look for them! ISAT 253
Photodiodes: Junctions and Depletion Layers “reverse bias” Apply a voltage to the pn junction to cause a depletion layer to form Electrical current ONLY flows when light strikes the diode with sufficient energy to release electrons in the depletion layer ISAT 253
Photodiodes vs Photoconductive Cells • Photodiodes convert light energy (infrared, visible, or ultraviolet) into electrical energy. • Photoconductive light detectors do not. This is because: • Photodiodes have a built-in electric field that pushes around the charges that are released from their chemical bonds by light. • Photoconductive light detectors do not.
anode (metal) antireflection coating p-type silicon p-n junction n-type silicon cathode (metal) Photodiodes • The structure of a silicon photodiode (cross section, not to scale).
Can manufacture in arrays for imaging applications + p V I - n (a) (b) Photodiodes • (a) Typical silicon photodiodes, and (b) symbol.
Electric Current Produced Optical Power Photodiodes • Photodiodes are usually characterized by giving their responsivity. This quantity is also sometimes called sensitivity. • The responsivity gives the ratio of the electric current produced by the photodiode to the optical power incident on the device. • Responsivity = Â= Rl=
Photodiode Spectral Responsivity 0.7 0.6 0.5 0.4 Responsivity (A/W) 0.3 0.2 0.1 0 300 400 500 600 700 800 900 1000 1100 1200 Wavelength (nm) Photodiodes • The responsivity of a photodiode depends on wavelength. • This dependence is shown for a typical silicon photodiode.
B: CdS photoconductive cell D: CdSe photoconductive cell F: Silicon photodiode G: PbS photoconductive cell H: Thermocouple Which sensor is best for ultraviolet light? Which one gives best response to 1100 nm wavelength light?
Photodiodes : An Application • A square array of InSb photodiodes is shown. The array consists 128 ´ 128 (= 16,384) photodiodes. The individual photodiodes are too small to be seen in this figure. Since these photodiodes can detect wavelengths up to 5400 nm = 5.4 mm, such an array can be used as an infrared-sensitive camera for night vision. Arrays of light detectors (photodiodes, photoconductors, or photocapacitors) can be made from a variety of materials. Silicon camera arrays consisting of 4096 ´ 4096 = 16.8 ´ 106 detectors, or more, are available.
Photodiodes: Another Application Light source Smoke detector Photodiode location Images from howstuffworks.com ISAT 253