1、Modern SensorsLecture 4X. WuA Review of Lecture 3Background on electrical measurement of sensor outputsResistiveVoltage dividerBridge circuitCapacitivePure resistive load resistorInductiveTemperature Effect and CompensationProvide an overview of piezoresistive devices. Some examples are worked out u

2、sing this sensing techniqueA Review of Lecture 3Strain gaugeStrainPoissons ratioGauge factorMercury tube/metal wireResistance change 1%-0.001%Various bridge circuitsTemperature EffectLecture 4: Basic IntentOverview of the use of capacitance measurements in sensors Describe the fundamentals of accele

3、rometers. Capacitance measuring systems, Limiting factors of the measurement, and obtainable performance levels. Fundamentals of accelerometer operation, including The relationship between the mechanical characteristics of the sensor and its performance, The limitations of the performance of most ac

4、celerometers. Capacitive SensingThe capacitance C=Q/V If the capacitance is large, more charge is needed to establish a given voltage difference. In practice, capacitance between two objects can be measured experimentally. Predicting the capacitance between a pair of arbitrary objects is very compli

5、cated,to know the electric field throughout the space between the objects. The field distribution is affected by the charge distribution, which is, in turn, affected by the field distribution. Iterative analytical techniques are generally required, and accurate calculations are very costly. One appl

6、ication: proximity sensingCapacitors with Simple Geometry Parallel platesElectrodes with area 10mm x 10mm,separation 1m. C 1000 pF,which isnt very big but still about the biggest you would ever expect to find in a real sensor !More generally, capacitive sensors have capacitance closer to 100 pF or l

7、ess. Change in Capacitance due to the Lateral Movement The capacitance signal changes linearly with displacement. To implement such a sensor, it is necessary to guarantee that the lateral motion does not also affect the separation between the electrodes, d.Difficult to use for measurement of very sm

8、all lateral displacements, A 1um lateral displacement would cause only 10 PPM change in the capacitance of the capacitor geometry worked out earlier. Lateral displacement capacitive transducersUseful for many applicationsRotary capacitive transducers for positioningHigh precision monitoring systemMi

9、litary application140 8 mV/degrees of shaft rotationManufacturer: Bently, USA Lateral displacement capacitive transducersCapacitance Change v.s. Plate Separation change in capacitance isnt obviously linear, but for small changes in separation, If the initial separation a few microns, a 1% change in

10、the capacitance displacement of a few tens of nanometers, Such a measurement should be considered well within the capabilities of capacitive sensing. Capacitance Change v.s. Plate SeparationNice features associated with such a measurement include good sensitivity to very small deflections no natural

11、 sensitivity to temperature. Precision fabrication is required, since it is necessary to produce electrodes which are very close to one another and highly parallelCapacitive sensing is generally used for situations in which a precision measurement is required, and the expense associated with the sen

12、sor fabrication is acceptable. Differential Capacitor One technique for reducing the effect of the nonlinearity : differential capacitorThe circuit is set up to measure the difference between the two capacitances,the nonlinearity associated with the term 2/d2 is subtracted away, and the first nonlin

13、earity appears as a cubic term 3/d3 , substantially smaller than the squared term. LinearityWhy do we care so much about linearity in capacitive sensors?Generally, capacitive measuring techniques are only applied in cases where precision measurement is necessaryOtherwise, a strain gauge based measur

14、ement would suffice. One example of such a measurement is the measurement of acceleration for inertial navigation applications. A common problem in navigation situations is due to vibrations of the vehicle. What is navigation?In geomatics engineering sense, navigation is understood as (quasi-) conti

15、nuous positioning of a moving objectModern navigation makes use of the so-called hybrid (integrated) navigation systems, two or more electronic sensing devices (sensors) are used together to collect the information necessary to find the position of the object. These systems can then be installed on-

16、board vehicles, ships, aircraft, or missiles. Some of the sensors that are being part of such systems are: Inertial Navigation Systems (INS), radio-navigation aids (LORAN, GPS, etc.), Doppler Velocity Sensors (DVS), laser-ranging devices, barometric altitude-meters, etc Inertia Navigation System (IN

17、S)Three main forces that an INS has to take into account are: (a) Gravitational acting down; (b) Centrifugal due to Earths rotation and sensed by gyros a radial force acting outward from the object, unlike centripetal that acts toward the object; and (c) Coriolis force in the direction of the moveme

18、nt, coming from compound acceleration of coriolis (in navigation: Coriolis correction of the sensed acceleration) : ac = 2v,Nonlinearity ProblemIn inertial navigation, offset errors in the output of the accelerometer accumulate as errors in position as t2: If an accelerometer with a small nonlineari

19、ty in the form of a term 2:in a situation which includes a vibration, there will be a displacement of the form sin(t). There will be a term in the output of the sensor of the form Vibration Rectification This expression includes an oscillating term a static term. Generally, this phenomenon is referr

20、ed to as vibration rectification - the process of generating a dc offset signal from a vibration signal. As described above, inertial navigation is one application which is particularly concerned about such phenomena, and so cancellation of nonlinearities in capacitive sensing is very important for

21、such applications. Capacitance Measurement: bridge circuitCapacitance Measurement: Switched Capacitance ConverterSwitched Cap Circuit:the use of a square-wave oscillation and FET switches sample and rectify a waveform in order to convert a capacitance to a voltage. Such circuits are ing very common

22、because it is a simple matter to design an entire circuit of this type as an integrated circuit on a single silicon chip. Sensors using cap measurementPressure sensors, accelerometers, position detectors, level sensors, A good way to measure displacement. If implemented carefully, very small displac

23、ements may be measured. Best suited to applications which require better performance than can be obtained from a strain gauge, and where the added cost of the capacitance detection is allowed. Advantage and disadvantagethat it is not directly sensitive to temperature. However, the output of a capaci

24、tive transducer is not immediately linear. If linearity is important, differential capacitance schemes are advisable. Accelerometer overviewAccelerometers: devices that produce voltage signals in proportion to the acceleration experienced. Techniques for converting an acceleration to an electrical S

25、pring-mass+cap measurementPotentiometric Variable Reluctance PiezoelectricGeneral AccelerometerThe most general way: suspend a mass on a linear spring from a frame which surrounds the mass, When the frame is shaken, it begins to move, pulling the mass along with it. If the mass is to undergo the sam

26、e acceleration as the frame, there needs to be a force exerted on the mass, which will lead to an elongation of the spring. We can use any of a number of displacement transducers (such as a capacitive transducer) to measure this deflection. General Accelerometer: Oscillatory forceimpose an accelerat

27、ion by forcing X to take the form:If we assume all the time varying quantities also oscillate, General Accelerometer: Amplitude Response of Vibration-measuring Instruments If b = 0 (no damping), signal at the resonance can lead to infinitely large signals, generally impose finite damping on the syst

28、em. If 0, then For high frequency signals, during which the mass remains stationary, and the accelerometer frame shakes around it. The displacement is the same size as the motion of the frame. This mode of operation is generally referred to as seismometer mode. Seismometers are instruments which att

29、empt to measure ground motion, rather than ground acceleration. An ExampleEXAMPLE An accelerometer has a seismic mass of 0.05 kg and a spring constant of 3.0 X 103 N/m Maximum mass displacement is 0.02 m (before the mass hits the stops). Calculate (a) the maximum measurable acceleration in g, and (b

30、) the natural frequency.SolutionWe find the maximum acceleration when the maximum displacement occursa. b. The natural frequency Accelerometer Selection based on ApplicationsApplicationsSteady-State Acceleration Vibration ShockSteady-state AccelerationSteady-Statea measure of acceleration that may v

31、ary in time but that is nonperiodic. the stop-go motion of an automobile is an example of a steady-state acceleration. we select a sensor having adequate range to cover expected acceleration magnitudesa natural frequency sufficiently high that its period is shorter than the characteristic time span

32、over which the measured acceleration changes. By using electronic integrators, the basic accelerometer can provide both velocity (first integration) and position (second integration) information.Steady-state Acceleration: an exampleAn accelerometer outputs 14 mV per g. Design a signal-conditioning s

33、ystem that provides a velocity signal scaled at 0.25 volt for every m/s, and determine the gain of the system and the feedback resistance ratio.SolutionWe chose T = RC = 1 so that the integrator output is scaled at We pick R = 1 M and C = 1mF and make R2 = 175 k R1 = 1 k VibrationThe application of

34、accelerometers for vibration first requires that the applied frequency is less than the natural frequency of the accelerometer. Second, one must be sure the stated range of acceleration measured will never exceed that of the specification for the device. This assurance must come from a consideration

35、 of the following equation under circumstances of maximum frequency and vibration displacement. ShockThe primary elements of importance in shock measurements are that the device have a natural frequency that is greater than 1 kHz and a range typically greater than 500 g. The primary accelerometer th

36、at can satisfy these requirements is the piezoelectric type COLD ATOMS : Atomic InterferometerThe atom as a measuring deviceFor about twenty years, the development of laser techniques for manipulating atoms has made it possible to determine more easily the wave nature of atoms and has yielded a whol

37、e range of applicable tools for these atomic waves. know how to make mirrors, beam splitters, diffraction arrays, lenses and all sorts of other tools for developing operational instruments for atomic optics. Unlike photons, atoms interact a lot with their environment. Even though they are electrical

38、ly neutral, atoms have electric and magnetic dipolar moments, which make them sensitive to external electric and magnetic fields.Atoms also have mass, which enables them to interact with the gravitational field, just as any other body with mass. Their high thermal agitation speed (several hundreds of metres or even kilometres per second) general