2. first reference to capacitive is found in Nature 1907, but the
peneration today is only a few percent of all sensor types
Capacitive sensors can directly and indirectly sense a variety of
thing
Capacitive 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.
3. CAPACITANCE AND DISTANCE
FIGURE 1
Applying a voltage to conductive objects causes positive and negative charges to collect on each
object. This creates an electric field in the space between the objects.
4. 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 2
Capacitance is determined by Area, Distance, and
Dielectric (the material between the conductors).
Capacitance increases when Area or Dielectric
increase, and capacitance decreases when the Distance
increases.
Technically, the capacitance is directly proportional to the
surface area of the objects and the dielectric constant of
the material between them, and inversely proportional to
the distance between them
5.
6. 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.
7. 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
8. CIRCUITRY AND EQUATION
Devices 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.
9. Approximate method of measuring inductance Approximate method of measuring capacitance
As figure shown above, it consists of connecting the unknown capacitor in series with a
known 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.
10. 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.
11. 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
12. 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 the
dielectric constant which is turn varies directly with the liquid level between the
plates.
13. But as the plate spacing increases relative to area, more flux lines connect
from the edges and backs of the plates and the measured capacitance can be
much larger than calculated.
14. APPLICATIONS
Capacitive sensing is a technology based on capacitive coupling
that 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.
15. Fingerprint detectors and infrared detectors: capacitive technologies
which displacing piezoresistance in silicon implementations of
accelerometers and pressure sensors are appearing on silicon with
sensor dimensions in the microns and electrode capacitance of 10
fF, with resolution to 5 aF (10-18 F)
16. 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, high
speed, and wide extremes of environment, and capacitive sensors with large
electrodes can detect an automobile and measure its speed.
17. 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.
18. 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 the
liquid, or applied to the outside of a non-conducting tank.
