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Capacitive sensor


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Capacitive sensor

  2. 2. IntroductionLevel sensors can detect the level of substances that flow, including liquids, slurries, granularmaterials, and powders. The level measurement can be either continuous or point values.Continuous level sensors measure level within a specified range and determine the exactamount of substance in a certain place, while point-level sensors only indicate whether thesubstance is above or below the sensing point. Generally the latter detect levels that areexcessively high or low.Currently we are focusing on liquid level capacitive sensor.Description and Figure Capacitive SensorThe first reference to capacitive is found in Nature 1907, but the peneration today is only afew percent of all sensor types. Capacitive sensors can directly sense a variety of thing suchas motion, chemical composition, electric field and indirectly can senses other variableswhich can converted into motion or dielectric constant such as pressure, accelaration, fluidlevel and fluid composition.Capacitive sensors use the electrical property of capacitance to make measurements.Capacitance is a property that exists between any two conductive surfaces within somereasonable proximity. Changes in the distance between the surfaces changes thecapacitance. It is this change of capacitance that capacitive sensors use to indicate changesin position of a target. High-performance displacement sensors use small sensing surfacesand as result are positioned close to the targets (0.25-2 mm).
  3. 3. Capacitance Liquid LevelThey are built with conductive sensing electrodes in a dielectric, with excitation voltages onthe order of 5V and detection circuits which turn a capacitance variation into a voltage,frequency, or pulse width variation. This technology is low cost and stability with simpleconditioning circuits. Often, the offset and gain adjustments needed the most sensor type arenot required.Capacitance and Distance Figure 1Applying a voltage to conductive objects causes positive and negative chargesto collect on eachobject.This creates an electric fieldin the space between the objects.Noncontact capacitive sensors work by measuring changes in an electrical property calledcapacitance. Capacitance describes how two conductive objects with a space between themrespond to a voltage difference applied to them. When a voltage is applied to the conductors,an electric field is created between them causing positive and negative charges to collect oneach object (Fig. 1). If the polarity of the voltage is reversed, the charges will also reverse.
  4. 4. Figure 2Applying an alternating voltage causes the charges to move back and forth between the objects,creating an alternating current which is detected by the sensor.Capacitive sensors use an alternating voltage which causes the charges to continuallyreverse their positions. The moving of the charges creates an alternating electric currentwhich is detected by the sensor (Fig. 2). The amount of current flow is determined by thecapacitance, and the capacitance is determined by the area and proximity of the conductiveobjects. Larger and closer objects cause greater current than smaller and more distantobjects. The capacitance is also affected by the type of nonconductive material in the gapbetween the objects. Figure 3Capacitance is determined by Area, Distance, and Dielectric (the material between the conductors).Capacitance increases when Area or Dielectric increase, and capacitance decreases when theDistance increases.Technically, the capacitance is directly proportional to the surface area of the objects and thedielectric constant of the material between them, and inversely proportional to the distancebetween them (Fig. 3).In typical capacitive sensing applications, the probe or sensor is one of the conductiveobjects and the target object is the other. The sizes of the sensor and the target areassumed to be constant as is the material between them. Therefore, any change incapacitance is a result of a change in the distance between the probe and the target. Theelectronics are calibrated to generate specific voltage changes for corresponding changes incapacitance. These voltages are scaled to represent specific changes in distance. Theamount of voltage change for a given amount of distance change is called the sensitivity. Acommon sensitivity setting is 1.0V/100µm. That means that for every 100µm change indistance, the output voltage changes exactly 1.0V. With this calibration, a +2V change in the
  5. 5. output means that the target has moved 200µm closer to the probe.Electric Field Figure 4 Capacitive sensor probe components Figure 5 Cutaway view showing an unguarded sensing area electric field Figure 6 Cutaway showing the guard field shaping the sensing area electric fieldWhen a voltage is applied to a conductor, the electric field emanates from every surface. In acapacitive sensor, the sensing voltage is applied to the Sensing Area of the probe (Figs. 4,5).
  6. 6. For accurate measurements, the electric field from the sensing area needs to be containedwithin the space between the probe 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.A technique called “guarding is used to prevent this from happening. To create a guard, theback and sides of the sensing area are surrounded by another conductor that is kept at thesame voltage as the sensing area itself (Fig. 4, 6).When the 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 andthe guard, there is no electric field between them. Any other conductors beside or behind theprobe form an electric field with the guard instead of the sensing area. Only the unguardedfront of the sensing area is allowed to form an electric field with the target.The target size is a primary consideration when selecting a probe for a specific application.When the sensing electric field is focused by guarding, it creates a slightly conical field that isa projection of the sensing area. The minimum target diameter for standard calibration is30% of the diameter of the sensing area. The further the probe is from the target, the largerthe minimum target size.In general, the maximum gap at which a probe is useful is approximately 40% of thesensor diameter. Standard calibrations usually keep the gap considerably less thanthat.The range in which a probe is useful is a function of the size of the sensing area. The greaterthe area, the larger the range. The driver electronics are designed for a certain amount ofcapacitance at the probe. Therefore, a smaller probe must be considerably closer to thetarget 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 atwhich a probe is useful is approximately 40% of the sensing area diameter. Standardcalibrations usually keep the gap considerably less than that.Using multiple probes on the same target requires that the excitation voltages besynchronized. This is accomplished by configuring one driver as a master and othersas slaves. Frequently, a target is measured simultaneously by multiple probes. Because the
  7. 7. system measures a changing electric field, the excitation voltage for each probe must besynchronized or the probes would interfere with each other. If they were not synchronized,one probe would be trying to increase the electric field while another was trying to decrease itthereby giving a false reading.Driver electronics can be configured as masters or slaves. The master sets thesynchronization for the slaves in multiple channel systems.Capacitive sensors measure all conductors: brass, steel, aluminium, or even salt-water, asthe same. The sensing electric field is seeking a conductive surface. Provided that the targetis a conductor, capacitive sensors are not affected by the specific target material. Becausethe sensing electric field stops at the surface of the conductor, target thickness does notaffect the measurement. .Measuring Non-Conductors Figure 7 Non-conductors can be measured by passing the electric field through them to a stationary conductive target behind. Figure 8 Without a conductive target behind, a fringe field can form through a nearby non-conductor allowing the non-conductor to be sensed
  8. 8. Capacitive sensors are most often used to measure the change in position of a conductivetarget. But capacitive sensors can be very effective in measuring presence, density,thickness, and location of non-conductors as well. Non-conductive materials like plastic havea different dielectric constant than air. The dielectric constant determines how a non-conductive material affects capacitance between two conductors. When a non-conductor isinserted between the probe and a stationary reference target, the sensing field passesthrough the material to the grounded target (Fig. 7). The presence of the non-conductivematerial changes the dielectric and therefore changes the capacitance. The capacitance willchange in relationship to the thickness or density of the material.It is not always feasible to have a reference target in front of the probe. Measurements maystill be possible by a technique called fringing. If there is no conductive reference directly infront of the probe, the sensing electric field will wrap back to the body of the probe itself. Thisis called a fringe field. If a non-conductive material is brought in proximity to the probe, itsdielectric will change the fringe field; this can be used to sense the non-conductive material.The sensitivity of the sensor to the non-conductive target is directly proportional to thedielectric constant of the material.Compared to other noncontact sensing technologies such as optical, laser, eddy-current, andinductive, high-performance capacitive sensors have some distinct advantages. Higher resolutions including subnanometer resolutions Not sensitive to material changes: Capacitive sensors respond equally to all conductors Less expensive and much smaller than laser interferometers.Meanwhile, this capacitive technology is sensitive to humidity and needs unstable, highimpedance circuits. In fact, as the dielectric constant of humid is only a few ppm higher thatdry air, humidity itself is not a problem. Very high impedance circuit needed. The capacitivesensor are rugged as any other sensor type. Its can not tolerate immersion or condensinghumidity, but a few circuits can.Capacitive sensors are not 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)
  9. 9. The design usually consider following steps: Design eletcrode plates to measure the desired variable. Maximize capacitance with large area, close spaced plates. Surround this sensor with appropriate guard or shield electrodes to handle stray capacitance and crosstalk from other circuits. Calculate sensor capacitance, stray capacitance and output signal swing. Specify tranfer function like Eo = C (area-linear), Eo = 1/C (spacing-linear) . Use two balanced capacitors for high capacitors for high accuracy with transfer function like C1/C2 or (C1-C2)/(C1+C2) . Choose an excitation frequency high enough for low noise.As excitation frequency increases, external and circuit generated noise decreases. Design circuit to meet accuracy specifications and provide immunity to environment challenges.
  10. 10. CIRCUITRY AND EQUATIONDevices that have an output in the form of a change in capacitance include a capacitive levelgauge, capacitive displacement sensor, capacitive moisture meter and capacitivehygrometer. Capacitance is measured in unit of Farad (F). Like inductance, capacitance canbe measured accurately by an AC bridge circuit and various type of Capacitance Bridge isavailable commercially.As an example, dry leather has a loss tangent of 0.045, but with a relative humidity of 15%the loss tangent increases to 1.4--possibly a good hygrometer. Aviation gas at 100 octaneexhibits a loss tangent at 1 kHz of 0.0001, but at 91 octane loss tangent increases to 0.0004.Water has a high K (80) and a loss tangent which peaks at low frequencies and again at1010 Hz. With this high dielectric activity, the loss tangent or the dielectric constant of watercan be used to detect the moisture content of materials.Another characteristic of capacitor dielectrics which may have some use in detecting materialproperties is dielectric absorption. It is measured by charging a capacitor, discharging for 10s, and measuring the charge which reappears after 15 min. A relatively low-quality dielectriclike metalized paper has a dielectric absorption of 10%. Approximate method of measuring inductance
  11. 11. Approximate method of measuring capacitanceAs figure shown above, it consists of connecting the unknown capacitor in series with aknown resistance in a circuit excited at a known frequency. An AC voltmeter is used tomeasure the voltage drop across both the resistor and the capacitor. The capacitance valueis then given by:Where VrandVcare the voltage measured across the resistance and capacitance respectively,f is the excitation frequency and R is the known resistance.For non-conducting substance (less than 0.1µmho/cm3), two bare metal capacitor plates inthe form of concentric cylinder are immersed in the substances as shown below.The substances behave as a dielectric between the plates according to the depth of thesubstances. For concentric cylinder plates of radius a and b (b>a) and total height L, thedepth of the substances, h, is related to the measured capacitance C by:where
  12. 12. The value of the capacitance depends on the permittivity of the liquids and that of the gas orair above it. The total permittivity changes depending on the liquid level (for non0conductingliquid application). In the case of conducting substance, the same measurement techniquesare applied but the capacitors plates are encapsulate in an insulating materials. Therelationship between C and h has to modify to allow for the dielectric effect of the insulator.In a parallel plate condenser which has identical plates each of the area, A (cm3) separatedby a distance, d (cm) and insulating medium with dielectric constant K (K=1 for air) betweenthem, the expression for the capacitance is given by: orWhere;A=area of plates in square inchesD=distance between the plates in inchesK=dielectric constant of materialFrom the equation we can observe that the capacitance varies directly with the dielectricconstant which is turn varies directly with the liquid level between the plates.But as the plate spacing increases relative to area, more flux lines connect from the edgesand backs of the plates and the measured capacitance can be much larger thancalculated.Some other simple geometries are C = 35.4  10-12r  d
  13. 13. C = 55.6  10-12r  dAPPLICATIONSCapacitive sensing is a technology based on capacitive coupling that is used in manydifferent types of sensors, including those to detect and measure: proximity, position ordisplacement, humidity, fluid level, and acceleration. Capacitive sensing as a humaninterface device (HID) technology, for example to replace the computer mouse, is growingincreasingly popular. Capacitive touch sensors are used in many devices such as laptoptrack pads, digital audio players, computer displays, mobile phones, mobile devices andothers. More and more design engineers are selecting capacitive sensors for their versatility,reliability and robustness, unique human-device interface and cost reduction overmechanical switches.Capacitive sensors detect anything which is conductive or has a dielectric different than thatof air. While capacitive sensing applications can replace mechanical buttons with capacitivealternatives, other technologies such as multi-touch and gesture-based touchscreens arealso premised on capacitive sensing.These are some of the applications of capacitivesensors that show its wide range of uses; Fingerprintdetectors and infrared detectors: capacitive technologies which displacing piezoresistance in silicon implementations ofaccelerometers and pressure sensorsare appearing on silicon with sensor dimensionsin the microns and electrode capacitance of 10 fF, with resolution to 5 aF (10-18 F) .
  14. 14. Oil refineries: Capacitive sensors used measure the percentage of water in oil.Grain storage facilities: measure the moisture content of wheat.Motion detectors: can detect 10-14 m displacements with good stability, highspeed,and wide extremes of environment, and capacitive sensors with largeelectrodes can detect an automobile and measure its speed.Laptop computers: thetwo-dimensional cursor controluse capacitive sensors whiletransparent capacitive sensors on computer monitors.Ice detector: Airplane wing icing can be detected using insulated metal strips in wingleading edges.
  15. 15. Thickness measurement: Two plates in contact with an insulator will measuretheinsulator thickness if its dielectric constant is known, or the dielectric constant ifthethickness is known. wafer thickness sensor 1Lamp dimmer switch: The common metal-plate soft-touch lamp dimmer uses 60Hzexcitation and senses the capacitance to a human body.
  16. 16. Limit switch: detect the proximity of a metal machine componentas an increase incapacitance, or the proximity of a plastic component by virtue of its increaseddielectric constant over air.Liquid level: Capacitive liquid level detectors sense the liquid level in a reservoirbymeasuring changes in capacitance between conducting plates which areimmersed inthe liquid, or applied to the outside of a non-conducting tank.