Seminar Report - Capacitive Sensors


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Seminar Report - Capacitive Sensors

  1. 1. Seminar ReportOnCAPACITIVE SENSORSSubmitted byStudent name: Tarun NekkantiRoll No: 142Section: CIn partial fulfillment of the requirements for the award of the degree ofBACHELOR OF ENGINEERINGINELECTRONICS AND COMMUNICATION ENGINEERINGDEPARTMENT OF ELECTRONICS AND COMMUNICATION ENGINEERINGMANIPAL INSTITUTE OF TECHNOLOGY(A Constituent College of Manipal University)MANIPAL – 576104, KARNATAKA, INDIA13 October 2010
  2. 2. SensorsDefinitionA sensor; is a device that measures a physical quantity and converts it into a signal which can beread by an observer or by an instrument. For example, a mercury-in-glass thermometer convertsthe measured temperature into expansion and contraction of a liquid which can be read on acalibrated glass tube. A thermocouple converts temperature to an output voltage which can beread by a voltmeter. For accuracy, most sensors are calibrated against known standards.DescriptionA sensor is a device which receives and responds to a signal. A sensors sensitivity indicates howmuch the sensors output changes when the measured quantity changes. For instance, if themercury in a thermometer moves 1 cm when the temperature changes by 1 °C, the sensitivity is1 cm/°C (it is basically the slope Dy/Dx assuming a linear characteristic). Sensors that measurevery small changes must have very high sensitivities. Sensors also have an impact on what theymeasure; for instance, a room temperature thermometer inserted into a hot cup of liquid cools theliquid while the liquid heats the thermometer. Sensors need to be designed to have a small effecton what is measured; making the sensor smaller often improves this and may introduce otheradvantages.Characteristics of a good sensorA good sensor obeys the following rules:Is sensitive to the measured propertyIs insensitive to any other property likely to be encountered in its applicationDoes not influence the measured property.Linearity & SensitivityIdeal sensors are designed to be linear or linear to some simple mathematical function of themeasurement, typically logarithmic. The output signal of such a sensor is linearly proportional tothe value or simple function of the measured property.The sensitivity is then defined as the ratio between output signal and measured property. Forexample, if a sensor measures temperature and has a voltage output, the sensitivity is a constant
  3. 3. with the unit [V/K]; this sensor is linear because the ratio is constant at all points ofmeasurement.ResolutionThe resolution of a sensor is the smallest change it can detect in the quantity that it is measuring.Often in a digital display, the least significant digit will fluctuate, indicating that changes of thatmagnitude are only just resolved. The resolution is related to the precision with which themeasurement is made. For example, a scanning tunneling probe (a fine tip near a surface collectsan electron tunneling current) can resolve atoms and molecules.Capacitive Displacement SensorsAn OverviewCapacitive sensors are noncontact devices capable of high-resolution measurement of theposition and/or change of position of any conductive target. The nanometer resolution of high-performance sensors makes them indispensible in todays nanotechnology world. Capacitivesensing can also be used to measure the position or other properties of nonconductive targets.A Capacitive Sensor Measurement SystemCapacitive sensor dimensional measurement requires three basic components:a probe that uses changes in capacitance to sense changes in distance to the target,driver electronics to convert these changes in capacitance into voltage changes,a device to indicate and/or record the resulting voltage change.Each of these components is a critical part in providing reliable, accurate measurements. Theprobe geometry, sensing area size, and mechanical construction affect range, accuracy, andstability. A probe requires a driver to provide the changing electric field that is used to sense thecapacitance. The performance of the driver electronics is a primary factor in determining theresolution of the system; they must be carefully designed for a high-preformance application.The voltage measuring device is the final link in the system. Oscilloscopes, voltmeters and dataacquisition systems must be properly selected for the application.What is Capacitance?Capacitance describes how the space between two conductors affects an electric field betweenthem. If two metal plates are placed with a gap between them and a voltage is applied to one of
  4. 4. the plates, an electric field will exist between the plates. This electric field is the result of thedifference between electric charges that are stored on the surfaces of the plates. Capacitancerefers to the ―capacity‖ of the two plates to hold this charge. A large capacitance has the capacityto hold more charge than a small capacitance. The amount of existing charge determines howmuch current must be used to change the voltage on the plate. It’s like trying to change the waterlevel by one inch in a barrel compared to a coffee cup. It takes a lot of water to move the levelone inch in the barrel, but in a coffee cup it takes very little water. The difference is theircapacity.When using a capacitive sensor, the sensing surface of the probe is the electrified plate and whatyou’re measuring (the target) is the other plate (we’ll talk about measuring non-conductivetargets later). The driver electronics continually change the voltage on the sensing surface. Thisis called the excitation voltage. The amount of current required to change the voltage is measuredby the circuit and indicates the amount of capacitance between the probe and the target. Or,conversely, a fixed amount of current is pumped into and out of the probe and the resultingvoltage change is measured.How Capacitance Relates to DistanceCapacitance is determined by Area, Gap, and Dielectric (the material in the gap). Capacitanceincreases when Area or Dielectric increase, and capacitance decreases when the Gap increases.The capacitance between two plates is determined by three things:Size of the plates: capacitance increases as the plate size increasesGap Size: capacitance decreases as the gap increasesMaterial between the plates (the dielectric):Dielectric material will cause the capacitance to increase or decrease depending on thematerialArea and Dielectric are held constant for ordinary capacitive sensing so only the Gap can changethe capacitance.
  5. 5. In ordinary capacitive sensing the size of the sensor, the size of the target, and the dielectricmaterial (air) remain constant. The only variable is the gap size. Based on this assumption, driverelectronics assume that all changes in capacitance are a result of a change in gap size. Theelectronics are calibrated to output specific voltage changes for corresponding changes incapacitance. These voltages are scaled to represent specific changes in gap size. The amount ofvoltage change for a given amount of gap change is called the sensitivity. A common sensitivitysetting is 1.0V/100µm. That means that for every 100µm change in the gap, the output voltagechanges exactly 1.0V. With this calibration, a +2V change in the output means that the target hasmoved 200µm closer to the probe.Focusing the Electric FieldProbes use a guard to focus the electric field.When a voltage is applied to a conductor, an electric field is emitted from every surface. Foraccurate gauging, the electric field from a capacitive sensor needs to be contained within thespace between the probe’s sensing area and the target. If the electric field is allowed to spread toother items or other areas on the target, then a change in the position of the other item will bemeasured as a change in the position of the target. To prevent this from happening, a techniquecalled guarding is used. To create a guarded probe, the back and sides of the sensing area aresurrounded by another conductor that is kept at the same voltage as the sensing area itself. Whenthe excitation voltage is applied to the sensing area, a separate circuit applies the exact samevoltage to the guard. Because there is no difference in voltage between the sensing area and theguard, there is no electric field between them to cause current flow. Any conductors beside orbehind the probe form an electric field with the guard instead of the sensing area. Only theunguarded front of the sensing area is allowed to form an electric field to the target.Effects of Target SizeThe target size is a primary consideration when selecting a probe for a specific application.When the sensor’s electric field is focused by guarding, it creates a field that is a projection ofthe sensor size and shape. The minimum target diameter for standard calibration is 30% of thediameter of the sensing area. The further the probe is from the target, the larger the minimumtarget size
  6. 6. Range of MeasurementThe range in which a capacitive sensor is useful is a function of the area of the sensing surface.The greater the area, the larger the range. The driver electronics are designed for a certainamount of capacitance at the sensor. Therefore, a smaller sensor must be considerably closer tothe target to achieve the desired amount of capacitance. The electronics are adjustable duringcalibration, but there is a limit to the range of adjustment.In general, the maximum gap at which a probe is useful is approximately 40% of the sensingsurface diameter. Standard calibrations usually keep the gap considerably less than that.Multiple Channel SensingFrequently, a target is measured simultaneously by multiple probes. Because the systemmeasures a changing electric field, the excitation voltage for each probe must be synchronized orthe probes would interfere with each other. If they were not synchronized, one probe would betrying to increase the electric field while another was trying to decrease it thereby giving a falsereading.Driver electronics can be configured as masters or slaves. The master sets the synchronizationfor the slaves in multiple channel systems.Effects of Target MaterialThe electric field from the probe sensing area is seeking a conductive surface. For this reason,capacitive sensors are not affected by the target material provided that it is a conductor. Becausethe electric field from the sensor stops at the surface of the conductor, target thickness does notaffect the measurement.Surface finish can affect the measurement. Capacitive sensors will measure the average positionof the target surface within the spot size of the sensor.Measuring Non-ConductorsNonconductors can be measured by passing the electric field through them to a stationaryconductive target behind.
  7. 7. Capacitive sensors are most often used to measure the change in position of a conductive target.But capacitive sensors can be very effective in measuring presence, density, thickness, andlocation of non-conductors as well. Non-conductive materials like plastic have a differentdielectric constant than air. The dielectric constant determines how a non-conductive materialaffects capacitance between two conductors. By inserting a non-conductive material in the gapbetween the probe and a stationary reference target, the capacitance will change in relationship tothe thickness, density, or location of the material.Fringing can be used to measure nonconductive targets without a conductive background target.Sometimes it’s not feasible to have a reference target in front of the probe. If the material has ahigh dielectric constant and a large sensor is used, measurements can still be made by atechnique called fringing. If there is no conductive surface directly in front of the probe, thesensor’s electric field will wrap back to the shell of the probe itself. This is called a fringe field.If a non-conductive material is brought in proximity to the probe, its dielectric will change thefringe field and this can be used to measure the non-conductive material.Strategies for maximizing effectiveness and minimizing errorMaximizing Accuracy: Target SizeSmall targets make measurement accuracy sensitive to small probe position errors.Unless otherwise specified, factory calibrations are done with a flat conductive target that isconsiderably larger than the sensor area. A system calibrated in this way will give accurateresults when measuring a flat target more than 30% larger than the sensing area. If the target areais too small, the electric field will begin to wrap around the sides of the target. In this case, theelectric field extends farther than it did in calibration and will measure the target as farther away.
  8. 8. This means that the probe must be closer to the target for the same zero point. Because thisdistance differs from the original calibration, error will be introduced. Error is also createdbecause the probe is no longer measuring a flat surface.An additional problem of an undersized target is that the system becomes sensitive to X and Ylocation of the probe relative to the target. Without changing the gap, the output will changesignificantly if the probe is moved left or right because less of the electric field is going to thecenter of the target and more is going around to the sides.Maximizing Accuracy: Target ShapeCurved targets change the shape of the electric field, affecting accuracy.Target shape is also a consideration. Since the capacitive sensors are calibrated to a flat target,measuring a target with a curved surface will cause errors. Because the sensor will measure theaverage distance to the target, the gap at zero volts will be different than when the system wascalibrated. Errors will also be introduced because of the different behavior of the electric fieldwith the curved surface. In cases where a non-flat target must be measured, the system can befactory calibrated to the final target shape. Alternatively, when flat calibrations are used withcurved surfaces, multipliers can be provided to correct the measurement value.Maximizing Accuracy: Surface FinishIrregular surface finish can cause different measurements as the target moves parallel to theprobe face.When the target surface is not perfectly smooth, the capacitive sensor will average over the areacovered by the spot size of the sensor. The measurement value can change as the sensor is movedacross the surface due to a change in the average location of the surface. The magnitude of thiserror depends on the nature and symmetry of the surface irregularities.
  9. 9. Maximizing Accuracy: ParallelismDuring calibration, the surface of the sensor is parallel to the target surface. If the probe or targetis tilted any significant amount, the shape of the spot where the field hits the target elongates andchanges the interaction of the field between the probe and target. Because of the differentbehavior of the electric field, measurement errors will be introduced. Parallelism must beconsidered when designing a fixture for the measurement.Parameters used to determine the quality of the sensorSensitivity ErrorSensitivity Error - The slope of the actual measurements deviates from the ideal slope.A sensor’s sensitivity is set during calibration. When sensitivity deviates from the ideal valuethis is called sensitivity error, gain error, or scaling error. Since sensitivity is the slope of a line,sensitivity error is usually presented as a percentage of slope; comparing the ideal slope with theactual slope.Offset ErrorOffset Error - A constant value is added to all measurements.Offset error occurs when a constant value is added to the output voltage of the system.Capacitive sensor systems are usually ―zeroed‖ during setup, eliminating any offset deviationsfrom the original calibration. However, should the offset error change after the system is zeroed,error will be introduced into the measurement. Temperature change is the primary factor in offseterror.
  10. 10. Linearity ErrorLinearity Error - Measurement data is not on a straight line.Sensitivity can vary slightly between any two points of data. This variation is called linearityerror. The linearity specification is the measurement of how far the output varies from a straightline.To calculate the linearity error, calibration data is compared to the straight line that would best fitthe points. This straight reference line is calculated from the calibration data using a techniquecalled least squares fitting. The amount of error at the point on the calibration curve that isfurthest away from this ideal line is the linearity error. Linearity error is usually expressed interms of percent of full scale. If the error at the worst point was 0.001mm and the full scale rangeof the calibration was 1mm, the linearity error would be 0.1%.Error BandGap(mm)ExpectedValue(VDC)ActualValue(VDC)ErrorBand(mm)0.50 -10.000 -9.800 -0.0100.75 -5.000 -4.900 -0.0051.00 0.000 0.000 0.0001.25 5.000 5.000 0.0001.50 10.000 10.100 0.005Error Band- the worst case deviation of the measured values from the expected values in acalibration chart. In this case, the total error is -0.010mm.
  11. 11. Error band accounts for the combination of linearity and sensitivity errors. It is the measurementof the worst case absolute error in the calibrated range. The total error is calculated by comparingthe output voltages at specific gaps to their expected value. The worst case error from thiscomparison is listed as the capacitive sensor system’s total error.High-Performance Capacitive SensorsIt is important to distinguish between "high-performance" sensors and inexpensive sensors.Simple capacitive sensors, such as those used in inexpensive proximity switches or elevatortouch switches, are simple devices and in their most basic form could be designed in a highschool electronics class. Proximity type sensors are tremendously useful in automationapplications and many commercially available models are well made, but they are not suited toprecision metrology applications.In contrast, capacitive sensors for use in precision displacement measurement and metrologyapplications use complex electronic designs to execute complex mathematical algorithms. Unlikeinexpensive sensors, these high-performance sensors have outputs which are very linear, stablewith temperature, and able to resolve incredibly small changes in capacitance resulting in highresolution measurements of less than one nanometer.AdvantagesCompared to other noncontact sensing technologies such as optical, laser, eddy-current, andinductive, high-performance capacitive sensors have some distinct advantages.Higher resolutions including sub nanometer resolutionsNot sensitive to material changes: Capacitive sensors respond equally to all conductorsInexpensive compared to laser interferometers.Capacitive sensors are not a good choice in these conditions:Dirty or wet environment (eddy-current sensors are ideal).Large gap between sensor and target is required (optical and laser are better).ApplicationsCapacitive sensors are useful in any application requiring the measurement or monitoring of theposition of a conductive target.Position Measurement/Sensing
  12. 12. Capacitive sensors are basically position measuring devices. The outputsalways indicate the size of the gap between the sensors sensing surfaceand the target. When the probe is stationary, any changes in the outputare directly interpreted as changes in position of the target. This is usefulin:Automation requiring precise locationSemiconductor processingFinal assembly of precision equipment such as disk drivesPrecision stage positioningDynamic MotionMeasuring the dynamics of a continuously moving target, such as arotating spindle or vibrating element, requires some form of noncontactmeasurement. Capacitive sensors are ideal when the environment is cleanand the motions are small, requiring high-resolution.Disk drive spindlesHigh-speed drill spindlesUltrasonic weldersVibration measurementsThickness MeasurementMeasuring material thickness in a noncontact fashion is a commonapplication for capacitive sensors. The most useful application is a two-channel differential system in which a separate sensor is used for eachside of the piece being measured. Details on thickness measurementswith capacitive sensors are available in the Conductive MaterialThickness Measurement with Capacitive Sensors Application Note.Capacitive sensor technology is used for thickness measurement in theseapplications:Silicon wafer thicknessBrake rotor thickness
  13. 13. Disk drive platter thicknessNonconductive ThicknessCapacitive sensors are sensitive to nonconductive materials which areplaced between the probes sensing area and a grounded back target. Ifthe gap between the sensor and the back target is stable, changes in thesensor output are indicative of changes in thickness, density, orcomposition of the material in the gap. This is used for measurements inthese applications:Label positioning during applicationLabel countingGlue detectionGlue thicknessAssembly testingAssembly testingCapacitive sensors have a much higher sensitivity to conductors than tononconductors. For this reason, they can be used to detect thepresence/absence of metallic subassemblies in completed assemblies. Anexample is a connector assembly requiring an internal metallic snap ringwhich is not visible in the final assembly. Online capacitive sensing candetect the defective part and signal the system to remove it from the line.Capacitive Sensing in HIDCapacitance sensors detect a change in capacitance when something or someone approaches ortouches the sensor. Capacitive sensing as a human interface device (HID) technology, forexample to replace the computer mouse, is becoming increasingly popular. Capacitive sensorsare used in devices such as laptop track-pads, MP3 players, computer monitors, cell phones andothers. More and more engineers choose capacitive sensors for their flexibility, unique human-
  14. 14. device interface and cost reduction over mechanical switches. Capacitive touch sensors havebecome a predominant feature in a large number of mobile devices and MP3 players.Capacitive sensors detect anything which is conductive or having dielectric properties. Whilecapacitive sensing applications can replace mechanical buttons with capacitive alternatives, othertechnologies such as multi-touch and gesture-based touchscreens are also premised on capacitivesensing.Working PrincipleA basic sensor includes a receiver and a transmitter, each of which consists of metal tracesformed on layers of a printed-circuit board (PCB). As shown in Figure 1, the AD714x has an on-chip excitation source, which is connected to the transmitter trace of the sensor. Between thereceiver and the transmitter trace, an electric field is formed. Most of the field is concentratedbetween the two layers of the sensor PCB. However, a fringe electric field extends from thetransmitter, out of the PCB, and terminates back at the receiver. The field strength at the receiveris measured by the on-chip sigma-delta capacitance-to-digital converter. The electricalenvironment changes when a human hand invades the fringe field, with a portion of the electricfield being shunted to ground instead of terminating at the receiver. The resultant decrease incapacitance—on the order of femtofarads as compared to picofarads for the bulk of the electricfield—is detected by the converter.In general, there are three parts to the capacitance-sensing solutions:
  15. 15. The driver IC, which provides the excitation, the capacitance-to-digital converter, andcompensation circuitry to ensure accurate results in all environments.The sensor—a PCB with a pattern of traces, such as buttons, scroll bars, scroll wheels, orsome combination. The traces can be copper, carbon, or silver, while the PCB can beFR4, flex, PET, or ITO.Software on the host microcontroller to implement the serial interface and the devicesetup, as well as the interrupt service routine. For high-resolution sensors such as scrollbars and wheels, the host runs a software algorithm to achieve high resolution output. Nosoftware is required for buttons.AD714x Driver ICsThese capacitance-to-digital converters are designed specifically for capacitance sensing inhuman-interface applications. The core of the devices is a 16-bit sigma-delta capacitance-to-digital converter (CDC), which converts the capacitive input signals (routed by a switch matrix)into digital values. The result of the conversion is stored in on-chip registers. The on-chipexcitation source is a 250-kHz square wave.
  16. 16. These devices interface with up to 14 external capacitance sensors, arranged as buttons, bars,wheels, or a combination of sensor types. The external sensors consist of electrodes on a 2- or 4-layer PCB that interfaces directly with the IC.The devices can be set up to interface with any set of input sensors by programming the on-chipregisters. The registers can also be programmed to control features such as averaging and offsetadjustment for each of the external sensors. An on-chip sequencer controls how each of thecapacitance inputs is polled.The AD714x also include on-chip digital logic and 528 words of RAM that are used forenvironmental compensation. Humidity, temperature, and other environmental factors can affectthe operation of capacitance sensors; so, transparently to the user, the devices performcontinuous calibration to compensate for these effects, giving error-free results at all times.One of the key features of the AD714x is sensitivity control, which imparts a different sensitivitysetting to each sensor, controlling how soft or hard the user’s touch must be to activate thesensor. These independent settings for activation thresholds, which determine when a sensor isactive, are vital when considering the operation of different-size sensors. Take, for example, anapplication that has a large, 10-mm-diameter button, and a small, 5-mm-diameter button. Theuser expects both to activate with same touch pressure, but capacitance is related to sensor area,so a smaller sensor needs a harder touch to activate it. The end user should not have to press onebutton harder than another for the same effect, so having independent sensitivity settings for eachsensor solves this problem.Different Shapes & Sizes in capacitance sensingAs noted earlier, the sensor traces can be any number of different shapes and sizes. Buttons,wheels, scroll-bar, joypad, and touchpad shapes can be laid out as traces on the sensor PCB.Figure below shows a selection of capacitance sensor layouts.Button
  17. 17. 8-WaySwitchSliderWheelKeypadTouchpadSelection of capacitance sensors
  18. 18. Many options for implementing the user interface are available to the designer, ranging fromsimply replacing mechanical buttons with capacitive button sensors to eliminating buttons byusing a joypad with eight output positions, or a scroll wheel that gives 128 output positions.The number of sensors that can be implemented using a single device depends on the type ofsensors required. The AD7142 has 14 capacitance input pins and 12 conversion channels. TheAD7143 has eight capacitance inputs and eight conversion channels. The table below shows thenumber of input pins and conversion stages required for each sensor type. Any number ofsensors can be combined, up to the limit established by the number of available inputs andchannels.Sensor TypeNumber of CIN inputsrequiredNumber of conversion channelsrequiredButton 1 1 (0.5 for differential operation)8-Way Switch 4—top, bottom, left, and right 3Slider 8—1 per segment 8—1 per segmentWheel 8—1 per segment 8—1 per segmentKeypadTouchpad1 per row, 1 per column 1 per row, 1 per columnMeasurements are taken on all connected sensors sequentially—in a ―round-robin‖ fashion. Allsensors can be measured within 36 ms, though, allowing essentially simultaneous detection ofeach sensor’s status—as it would take a very fast user to activate or deactivate a sensor within 40ms.AdvantagesCapacitance sensors are more reliable than mechanical sensors—for a number of reasons. Thereare no moving parts, so there is no wear and tear on the sensor, which is protected by coveringmaterial, for example, the plastic cover of an MP3 player. Humans are never in direct contactwith the sensor, so it can be sealed away from dirt or spillages. This makes capacitance sensorsespecially suitable for devices that need to be cleaned regularly—as the sensor will not bedamaged by harsh abrasive cleaning agents—and for hand-held devices, where the likelihood ofaccidental spillages (e.g., coffee) is not negligible.
  19. 19. ConclusionCapacitance sensors are an emerging technology for human-machine interfaces and are rapidlybecoming the preferred technology over a range of different products and devices. Capacitancesensors enable innovative yet easy-to-use interfaces for a wide range of portable and consumerproducts. Easy to design, they use standard PCB manufacturing techniques and are more reliablethan mechanical switches. They give the industrial designer freedom to focus on styling,knowing that capacitance sensors can be relied upon to give a high-performance interface thatwill fit the design.References:•••••