GROUP : EE2403Afaculty of electrical engineering UiTm shah alam
first reference to capacitive is found in Nature 1907, but the peneration today is only a few percent of all sensor typesCapacitive sensors can directly and indirectly sense a variety of thingCapacitive sensors use the electrical property of capacitance to make measurements. exists between any two conductive surfaces within some reasonable proximity. Changes in the distance between the surfaces changes the capacitance.
CAPACITANCE AND DISTANCE FIGURE 1Applying a voltage to conductive objects causes positive and negative charges to collect on eachobject. This creates an electric field in the space between the objects.
Applying an alternating voltage causes the charges to move back and forth between the objects, creating an alternating current which is detected by the sensor. FIGURE 2Capacitance is determined by Area, Distance, andDielectric (the material between the conductors).Capacitance increases when Area or Dielectricincrease, and capacitance decreases when the Distanceincreases.Technically, the capacitance is directly proportional to thesurface area of the objects and the dielectric constant ofthe material between them, and inversely proportional tothe distance between them
low cost and stability with simple conditioning circuits. the offset and gain adjusments needed the most sensor type are not required. 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.
sensitive to humidity and needs unstable, high impedance circuits. rugged as any other sensor type, its can not tolerate immersion or condensing humidity Dirty or wet environment Large gap between sensor and target is required
CIRCUITRY AND EQUATIONDevices that have an output in the form of a change in capacitance include a capacitive level gauge, capacitive displacement sensor, capacitive moisture meter and capacitive hygrometer. Capacitance is measured in unit of Farad(F). Like inductance, capacitance can be measured accurately by an AC bridge circuit and various type of Capacitance Bridge is available commercially.
Approximate method of measuring inductance 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 to measure the voltage drop across both the resistor and the capacitor. The capacitance value is then given by: Where Vr and Vc are 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 0.1µ/cm3), two bare metal capacitor plates in the 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 the substances.
For concentric cylinder plates of radius a and b (b>a) and total height L, the depth of the substances, h, is related to the measured capacitance C by: where
In a parallel plate condenser which has identical plates each of the area, A (cm3) separated by a distance, d (cm) and insulating medium with dielectric constant K (K=1 for air) between them, the expression for the capacitance is given by:From the equation we can observe that the capacitance varies directly with thedielectric constant which is turn varies directly with the liquid level between the plates.
But as the plate spacing increases relative to area, more flux lines connectfrom the edges and backs of the plates and the measured capacitance can be much larger than calculated.
APPLICATIONS Capacitive sensing is a technology based on capacitive couplingthat is used in many different types of sensors, including those to detect and measure: proximity, position or displacement, humidity, fluid level, and acceleration. Capacitive sensing as a human interface device (HID) technology, for example to replace the computer mouse, is growing increasingly popular. Capacitive touch sensors are used in many devices such as laptop track pads, digital audio players, computer displays, mobile phones, mobile devices and others.
Fingerprint detectors and infrared detectors: capacitive technologies which displacing piezoresistance in silicon implementations ofaccelerometers and pressure sensors are appearing on silicon withsensor dimensions in the microns and electrode capacitance of 10 fF, with resolution to 5 aF (10-18 F)
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 large electrodes can detect an automobile and measure its speed.
Laptop computers: the two-dimensional cursor control use capacitive sensors while transparent capacitive sensors on computer monitors. Ice detector: Airplane wing icing can be detected using insulated metal strips in wing leading edges.
Liquid level: Capacitive liquid level detectors sense the liquid level in a reservoir by measuring changes in capacitance between conducting plates which are immersed in theliquid, or applied to the outside of a non-conducting tank.