Capacitive
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Summary
Introduction
A capacitor is formed when any two conductors are separated by some distance. When opposing charges build up on the surface of these spaced conductive plates, an electric field is produced, inducing a potential difference (or voltage) across the plates. Capacitance is the ratio of the charge over the voltage.
\leftC=\left\frac{q}{V}\right C=q/V
Where C is the capacitance in Farads, q is the charge in coulombs, and V is voltage in Volts. Capacitances, as one would measure them in the lab, typically vary in the order of microFarads (1µF = 10-6F) down to picoFarads (1pF = 10-12F).
Capacitance is inversely proportional to the distance between the plates, varies proportionally with the area of the plates, and is also dependant on the properties of the substance between the plates (the dielectric). Different materials have different dielectric constants.
It is important to note that capacitance can be quantified between any two objects that store charge. Objects may be separated by a great distance and still exhibit capacitance, just as a small discrete capacitor with very little space and area may exhibit the same capacitance. This is an important consideration that must be taken into effect when designing any electrical circuit. When the external capactance of a circuit is of the same order of magnitude as the internal capacitance, it is imperative to include it in the design considerations. Capacitive coupling between ambient electric fields and a supply voltage, for example, can result in voltage swings on the power supply voltage. For this reason, it is necessary to filter out any noise induced on power supply voltage caused by ambient electromagnetic fields.
Capacitive sensors
There are several different manifestations of the capacitive sensor. On the most general level, capacitive sensors fall into two categories: unloaded and loaded.
A loaded capacitive sensor is one in which a signal is capacitively coupled through an object or performer and the amplitude of the signal received varies with the distance between two "plates" of the capacitor. An unloaded capactive sensor is one in which the circuit anticipates a certain capacitive load and an external capacitance is applied, resulting in a change of total capacitance .
The Theremin is a well known electronic instrument that operates on the principle of unloaded capacitive sensing. Before a performer plays the instrument, he or she must calibrate the device to establish a reference capacitance between the performer and the instrument. As the performer plays the instrument, she varies a capacitance between her body and one of the antennae which in turn modulates an internal oscillator. This internally modulated signal is then translated into frequency and amplitude.
A familiar loaded capacitive sensor is Max Matthew's Radio Baton. In this instrument, a transmitting baton is directly connected to an oscillator. Receive electrodes acquire and demodulate the signal. Because several fixed receivers are used, the relative strengths of the acquired signals may be used to determine the position of the baton.
There are many circuits in which one might detect capactive coupling. With a little circuitry it is possible to create either type.
Another distinction can be made between capacitve sensors that are designed for contact and ones that are not. The Theremin and Radio Baton are examples of non-contact interfaces (the radio-baton player does hold the baton, but the baton need not come in contact with the receiver in order to work). Some of the more commonly known capacitive sensors are contact based. The trackpad and iPod-style jogwheel are two such examples.
Devices
Source Country Price Digikey (http://www.digikey.ca/) Canada CAN$ 3.94
[edit]Quantum Research Group QProx QT1081
8 Key Charge-Transfer QTouch Sensor IC
Variants:
Datasheet: [1] (http://www.qprox.com/assets/Downloadablefile/qt1081_1r0.04-15735.pdf)
Resources:
Notes: 32QFN IC package
Source Country Price Digikey (http://www.digikey.ca/) Canada CAN$ 91.98
[edit]Motorola KIT33794DWBEVM
Electric Field Imaging Device - non contact sensing (evaluation kit)
Variants:
Datasheet: [2] (http://rocky.digikey.com/WebLib/Motorola/Web%20Data/MC33794.pdf)
Resources:
Notes: Supports up to 9 electrodes and 2 references
Media
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External links & references
- Paradiso, J. and N. Gershenfeld. 1997. “Musical Applications of Electric Field Sensing”. Computer Music Journal, 21(2):69-89.
- Baxter, L. K. 1996. Capacitive Sensors: Design and Applications. John Wiley & Sons.
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