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1、Chapter 2-Webster Basic Sensors and Principles,酒店设计 http:/,Transducer: a device that converts energy from one form to anotherSensor: converts a physical parameter to an electrical output (a type of transducer, e.g. a microphone)Actuator: converts an electrical signal to a physical output (opposite o
2、f a sensor, e.g. a speaker),Transducer, Sensor, and Actuator,Type of Sensors Displacement Sensors:resistance, inductance, capacitance, piezoelectric Temperature Sensors:Thermistors, thermocouples Electromagnetic radiation Sensors:Thermal and photon detectors,Displacement measurements can be made usi
3、ng sensors designed to exhibit a resistive, inductive, capacitive or piezoelectric change as a function of changes in position.,Displacement Measurements,Used to measure directly and indirectly the size, shape, and position of the organs.,Measure linear and angular position Resolution a function of
4、the wire construction Measure velocity and acceleration,Resistive sensors - potentiometers,2 to 500mm,10o and more,Devices designed to exhibit a change in resistance as a result of experiencing strain to measure displacement in the order of nanometer.,For a simple wire:,A change in R will result fro
5、m a change in (resistively), or a change in L or A (dimension).,The gage factor, G, is used to compare various strain-gage materials, Is Poissons ratio,Resistive sensors strain gages,Semiconductor has larger G but more sensitive to temperature,Wheatstone Bridge,vo is zero when the bridge is balanced
6、- that is when,If all resistor has initial value R0 then if R1 and R3 increase by R, and R2 and R4 decreases by R, then,Show Proof in class,Show Proof in class,Unbonded strain gage:,Error in Fig. 2.2 legend: R1 = A, R2 = B, R3 = D, R4 = C,Wheatstone Bridge,With increasing pressure, the strain on gag
7、e pair B and C is increased, while that on gage pair A and D is decreased.,A,B,C,D,Initially before any pressure R1 = R4 and R3 = R2,Bonded strain gage:,- Metallic wire, etched foil, vacuum-deposited film or semiconductor is cemented to the strained surface,- Rugged, cheap, low mass, available in ma
8、ny configurations and sizes,- To offset temperature use dummy gage wire that is exposed to temperature but not to strain,Bonded strain gage terminology:,Carrier (substrate + cover),Semiconductor Integrated Strain Gages,Pressure strain gages sensor with high sensitivity,Integrated cantilever-beam for
9、ce sensor,Figure 14.15 Isolation in a disposable blood-pressure sensor. Disposable blood pressure sensors are made of clear plastic so air bubbles are easily seen. Saline flows from an intravenous (IV) bag through the clear IV tubing and the sensor to the patient. This flushes blood out of the tip o
10、f the indwelling catheter to prevent clotting. A lever can open or close the flush valve. The silicon chip has a silicon diaphragm with a four-resistor Wheatstone bridge diffused into it. Its electrical connections are protected from the saline by a compliant silicone elastomer gel, which also provi
11、des electrical isolation. This prevents electric shock from the sensor to the patient and prevents destructive currents during defibrillation from the patient to the silicon chip.,Gel,Clear plastic,4 cm,Saline,Flush valve,IV tubing,Electrical cable,Silicon chip,To patient,Elastic-Resistance Strain G
12、ages,Filled with a conductive fluid (mercury, conductive paste, electrolyte solution. Resistance = 0.02 - 2 /cm, linear within 1% for 10% of maximal extension,Extensively used in Cardiovascular and respiratory dimensional and volume determinations.,As the tube stretches, the diameter decreases and t
13、he length increases, causing the resistance to increase,b) venous-occlusion plethysmographyc) arterial-pulse plethysmography,Inductive Sensors,Amperes Law: flow of electric current will create a magnetic field Faradays Law: a magnetic field passing through an electric circuit will create a voltage,i
14、,v,+,-,v1,v2,+,-,+,-,N1,N2,Inductive Sensors,Self-inductance Mutual inductance Differentialtransformer,n = number of turns of coil G = geometric form factor m = effective magnetic permeability of the medium,Amperes Law: flow of electric current will create a magnetic field Faradays Law: a magnetic f
15、ield passing through an electric circuit will create a voltage,LVDT : Linear variable differential transformer- full-scale displacement of 0.1 to 250 mm- 0.5-2 mV for a displacement of 0.01mm- sensitivity is much higher than that for strain gages Disadvantage requires more complex signal processing,
16、http:/ As x moves through the null position, the phase changes 180 , while the magnitude of vo is proportional to the magnitude of x. (b) An ordinary rectifier-demodulator cannot distinguish between (a) and (b), so a phase-sensitive demodulator is required.,+,_,Capacitive sensorsFor a parallel plate capacitor:,0 = dielectric constant of free space r = relative dielectric constant of the insulator A = area of each plate x = distance between plates,
