Sensors. Signal Source Sensors Transducer

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Preston Leonard
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1 Sensors Signal Source Sensors Transducer

2 Introduction Measuring System Sections Sensor-detector stage Signal conditioning stage Terminating readout stage Information I = out f ( I ) in

3 Introduction Transfer efficiency This may not be more then unity T. E. = I I out in Sensitivity η = di di out in Very often sensitivity approximates a constant.

4 The Bourdon-tube Pressure gage

5 The Bourdon-tube Pressure gage

6 The Bourdon-tube Pressure gage 1. Organe moteur, tube de Bourdon 2. Support de tube 3. Capuchon du tube 4. Secteur denté 5. Biellette 6. Engrenage 7. Aiguille 8. Cadran

7 The secondary transducer The Bourdon-tube pressure gage Mechanical elements system The primary detecting-transducing element A circular tube (approximately elliptical cross section) The flattened tube tends toward a more circular form The secondary transducer The linkage-gear arrangement As an amplifier, yielding a magnified output.

8 A Strain-Gage Load Cell

9 Stress and Strain Strain-Gage

10 Strain-Gage fundamental

11 Strain-Gage classification

12 Strain-Gage application

13 Classification of First-Stage Devices Used as detector only Used as detector and single transducer Used as detector with two transducer stages Input signal Primary Detectortransducer Intermediate Analogous signal Secondary transducer Analogous driving signal

14 Primary Detector-Transducer Mechanical Contacting spindle, pin, or finger Elastic member (Load cells, Proving ring ) Mass (Seismic mass, Pendulum, ) Thermal (Thermocouple, Bimaterial, Tgermistor, Chemical phase, ) Hydropneumatic (Static:Float,Hydrometer; Dynamic:Orifice, Venture, )

15 Primary Detector-Transducer Electrical Resistive (Contacting, Conductor ) Inductive (Variable coil dimensions, ) Capacitive (Changing air(plate) gap, ) Piezoelectric Semiconductor junction (Junction threshold voltage, Photodiode current, ) Photoelectric (Photovoltaic, Photoconductive, ) Hall Effect Most commonly used

16 Electrical elements Advantage Amplification or attenuation can be easily obtained Mass-inertia effects are minimized The effects of friction are minimized An output power of almost any magnitude can be provided Remote indication or recording is feasible The transducers can often be miniaturized

17 Variable-resistance Transducer Resistance (Ω) Relationship R = ρl A L = the length of the conductor [cm], A = cross-sectional area [cm 2 ], ρ = the resistivity of material [Ω.cm].

18 Sliding-Contact Devices Convert a mechanical displacement input into an electrical output

19 Angular-Motion Variable Wire-Wond Resistance

20 Angular-Motion Variable Resistance Carbon-Composition

21 Other Transducer Elements The resistance Strain Gage Thermistors Thermally sensitive variable resistors Resistivity values : 100~450,000 Ω.cm The thermocouple Advantage : small size, reliability, large range of usefulness,

22 Other Transducer Elements Thermistors

23 Other Transducer Elements The thermocouple

24 Other Transducer Elements The thermocouple

25 Variable-Inductance Transducer Used Method Based on the voltage output of an inductor whose inductance changes in the measured. The inductive reactance (Ω) X = 2πfL L f = the frequency of applied voltage (Hz), L = inductance (H).

26 Inductive Transducer Classification Variable self-inductance Single coil Two coil connected for inductance ratio Variable mutual inductance Simple two coil Three coil Variable reluctance Moving iron Moving coil Moving magnet

27 Simple Self-inductance Performance The mechanical input usually changes the permeance of the flux path generated by the coil. Arrangements

28 Two-coil inductance-ratio Performance transducer Two-coil self-inductance (Movement of the core or armature alters the relative inductance of the two coils)

29 Two-coil mutual-inductance Performance Arrangements The magnetic flux from a power coil is coupled to a pickup coil, which supplies the output.

30 Two-coil mutual-inductance Arrangements An electronic micrometer

31 The differential Transformer

32 The differential Transformer

33 The differential Transformer

34 The differential Transformer

35 Performance Variable-Reluctance Transducers In most cases these devices are limited to dynamic application, either period or transient, where the flux lines supplied by the magnet are cut by the turns of the coil. According to Faraday s law V dφ = n dt V = induced voltage (V) n = number of turns in coil, Φ = magnetic flux through coil (Wb)

36 A simple variable-reluctance pickup

37 Capacitive transducers The Capacitance A set of equally spaced parallel plates C = KA( N 1) d C = the capacitance (pf), K = the dielectric constant (=1 for air), A = the area of one side of one plate (in. 2 ) N = the number of plates, d = the separation of plate surfaces (in.)

38 Capacitive transducers Changing Dielectric Constant For determining level of liquid hydrogen

39 Capacitive transducers Changing Area Teeth in capacitance-type torque meter

40 Capacitive transducers Changing Distance The capacitance-type pressure pickup

41 Capacitive transducers

42 Capacitive transducers

43 Piezoelectric Sensors Piezo: meaning to press or to squeeze Piezoelectric effect Certain materials can electrical charge when subjected to mechanical strain Can change dimension when subjected to voltage Applied field Measure surface roughness, force and torque, pressure, motion, sound,

44 Piezoelectric Sensors

45 Piezoelectric Sensors Equivalent Circuit

46 Equivalent Piezoelectric Sensors

47 Equivalent Piezoelectric Sensors

48 Piezoelectric Sensors The voltage across the piezoelement at any time is Simply V0 ( t) = Q( t) C t requires a very high impedance circuit to prevent charge loss The voltage can alternatively be expressed in terms of the stress on the piezoelement V t) = Ghσ h is the thickness of the element between the electrodes and σ is the stress. G is a material constant (=0.055Vm/N for quartz in compressive stress) 0 (

49 Semiconductor Sensors Advantage Produced compact and inexpensive sensors (digital voltmeters, computer data acquisition system, readout and data-processing systems) Quite small, mechanical structures(can be etched into the device) Directly implanted with the sensor to form a transducer having onboard signal conditioning

50 Semiconductor Electrical Behavior Materials Silicon and germanium, and compounds ( gallium arsenide and cadmium sulphide) Differ from metals Relatively few free electrons are available to carry current(a positive charged hole is formed and will move in the direction oppsite the electron) The number of charge carriers is a strong function of temperature, T. number per unit volume T 3/ 2 constant exp T = n c

51 Semiconductor Electrical Behavior Resistivity of a Material Is proportional to 1/nc Resistance decreases rapidly with increasing temperature Electrical behavior Is obtained by doping it with impurity atoms (electron donor or electron acceptors) n-type : doping with donor atoms creates a predominance of negative charge carriers (electrons) p-type : doping with acceptor atoms

52 Semiconductor Sensors

53 Semiconductor Sensors Temperature Sensors Undoped Semiconductors (NTC sensors) The number of carriers increases rapidly with temperature (a negative temp. coeff. of resistance) Thermistors are based on this effect Heavily doped Semiconductors (PTC sensors) The resistance increases with temperature Strain Sensors A p-type region diffused into an n-type base functions as a resistor whose value increases strongly when it is strained (piezoresistivity)

54 Semiconductor Sensors Temperature Sensors AD590

55 pn-junction Behaves like a diode Current flows easily from the p-type to the n- type material Current flows in the opposite direction meets much greater resistance Forward-biased V is positive, the current becomes very large once the voltage reaches a threshold level Reverse-biased

56 pn-junction with applied voltage

57 pn-junction voltage-current curve i i q = I0 exp V 1 kt Reverse bias 0 I0 Vt Forward bias V

58 Photodiodes Performance Semiconductor junction are sensitive to light as well as heat Photons reaching the junction can create new pairs of electrons and holes, which then separate and flow in opposite directions The irradiating light produces an additional current Iλ, the photocurrent,iλ, is directly proportional to the intensity, H, of the incoming light (in W/m 2 )

59 Photodiodes i-v Characteristic Dark characteristic (H=0) i V H1 H2 i = I 0 exp q kt V 1 I λ H3 I λ = constant H Reverse bias: i -Iλ α -H

60 Light-Detecting Transducers Light-sensitive detectors (photosensors, photocells) Categorized : Thermal detectors, Photon detectors The photon devices respond directly to absorbed photons,either by emitting electrons from a surface or by creating additional electron-hole pairs in a semi-conductor

61 Type Photomissive or Photomultiplier Photocells (1) Symbol and Typical circuit Form of output current Relative Frequency Response Extremely fast Consist of a cathode-anode combination in an evacuated glass or quartz envelop PMT gain can be 10 3 to Bulky; requires high voltage; and has given way to solid-state devices.

62 Type Photoconductiv e or Photoresistive Photocells (2) Symbol and Typical circuit Form of output Resistance change Relative Frequency Response Slow Consist of a thin layer of material such as cadmium selenide, several of the metallic sulphides, or doped germanium. Light-sensitive resistor. Increased light intensity casses reduced resistance.

63 Photocells (3) Type Symbol and Typical circuit Form of output Relative Frequency Response Photovoltaic (solar cell) Voltage Fast Consist of a sanwich of unlike materials, such as an iron base covered with a thin layer of iron selenide. Typical open-circuit voltage, 0.45 V. In bright sunlight, 0.4 to 0.5 ma.

64 Type Photodiode (pn junction) PIN Photodiode Photocells (4) Symbol and Typical circuit Form of output Current Relative Frequency Response Very fast Primary disadvantage is low output current. Duck current very low (nanoampere range), but not zero. PIN diode has intrinsic later between P and N layers that provides response over wider range of light wavelengths. PIN is faster than PN type.

65 Photocells (5) Type Symbol and Typical circuit Form of output Relative Frequency Response Phototransistor Current Slower than photodiode Produces much higher current for given input than photodiode does because of its amplifying ability. Base lead, if accessible, is seldom used. Photodarlington much more sensitive than phototransistor, but frequency response slower then phototransistor.

66 Photosensors and Photocells Sensitive to light Photosensors may be made selectively sensitive to light,not only in the visible spectrum but also in the infrared and ultraviolet ranges. Applications Simple counting, strain measurement dewpoint controls, temperature measurement, edge and tensor controls, and digital cameras.

67 A photointerrupter (an LED light source) Mechanical interruption of the light path can be used for various purposes, such as counting, triggering, and Synchronization.

68 The Essentials of A photoisolator Used for connecting low-impedance current circuits to high-impedance voltage circuits. This isolator is also useful for providing complete electrical isolation between circuits, sometimes imperative in heath-related electronics.

69 Hall-Effect Sensors Hall effect The appearance of a transverse voltage difference on a conductor carrying a current perpendicular to a magnetic field. This voltage is directly proportional to the magnetic field strength. It is present in any conductor carrying current in a magnetic field, but it is much more pronounced in semiconductors than in metals.

70 Hall-Effect magnetic force F = qv B d B electric field F = qe = E F B

71 Hall-Effect

72 Hall-Effect Voltage magnetic force l FB B electric FB = qv d field B E FE F E E = = v qe d B = F B Hall - effect voltage V Hall = le = v d lb

73 Hall-Effect Sensor That transducers use a semiconducting material as the conductor, often in conjunction with an integrated-circuit signal conditioner. Applications Position sensors Solid-state keyboards actuators Current sensors Automotive crankshaft-timing sensors Gear-tooth sensors

74 Hall-Effect Gear-Tooth Sensor

Ultrasonic. Advantages

Ultrasonic. Advantages Ultrasonic Advantages Non-Contact: Nothing touches the target object Measures Distance: The distance to the target is measured, not just its presence Long and Short Range: Objects can be sensed from 2