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+ | ====== Capacitive ====== | ||
+ | Capacitive sensors are widely used for a large variety of functions, among them **proximity sensing** (personnel detection, light switching, vehicle detection), **measurement** (flow, pressure, liquid level, spacing, scanned multiplate sensor, thickness measurement, | ||
+ | |||
+ | |||
+ | ===== Introduction to Capacitance | ||
+ | |||
+ | A capacitor is formed when any two conductors are separated by some distance. The basic idea for capacitive sensing builds on the model of an ideal capacitor, typically consisting of two plates of area S separated by distance D filled with a [[wp> | ||
+ | |||
+ | {{ : | ||
+ | |||
+ | Where C is the capacitance in Farads, Q is the charge in Coulombs, and V is voltage in Volts. | ||
+ | |||
+ | Capacitance is inversely proportional to the distance between the plates, varies proportionally with the area of the plates, and is also dependent on the properties of the substance between the plates (the dielectric). Different materials have different dielectric constants, expressed relative to E0 the electrical permitivity of vacuum. For Example, window glass has a relative dielectric constant of 7, while acrylic has 2-3, and air has an ER of about 1. | ||
+ | |||
+ | {{ : | ||
+ | |||
+ | 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, | ||
+ | |||
+ | |||
+ | ===== Capacitive sensors ===== | ||
+ | |||
+ | Capacitive sensors can generally be divided into three categories, based on their mode of operation: //Load mode//, //transmit mode//, and //shunt mode// ((Paradiso, J. and N. Gershenfeld. 1997. {{http:// | ||
+ | |||
+ | ==== Load Mode (or " | ||
+ | |||
+ | An unloaded capacitive 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 [[instruments: | ||
+ | |||
+ | ==== Transmit Mode (or " | ||
+ | |||
+ | 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 the two " | ||
+ | |||
+ | {{: | ||
+ | |||
+ | A familiar loaded capacitive sensor is used in [[http:// | ||
+ | |||
+ | ==== Shunt Mode Sensing ==== | ||
+ | |||
+ | Shunt mode capacitive sensing is very similar to Transmit mode, in that an expected capacitive load is present between a transmit electrode and a receive electrode. In this case, however, the body of the performer is not connected to the transmit electrode, and effectively screens/ | ||
+ | |||
+ | {{: | ||
+ | |||
+ | ==== Touch Sensing ==== | ||
+ | |||
+ | Sensing touch, as with smart phones and tablet computers, is generally done using one of two variations on Load Mode technology: | ||
+ | |||
+ | Surface capacitance puts a small voltage on to the touch surface, and then measures the capacitance at each corner of the surface. | ||
+ | |||
+ | Projected capacitance uses a grid of electrodes, which gives much greater resolution. | ||
+ | |||
+ | Outside of these standard techniques, the SoundPlane((Jones, | ||
+ | |||
+ | |||
+ | ===== Potential Issues ===== | ||
+ | |||
+ | * Electrode shape, area, material and spacing are important design variables. | ||
+ | * Overlap and underlap or electrodes in the case of moving parts. | ||
+ | * Electrode leads | ||
+ | * Dielectric thickness and material: glass vs. acrylic for example. | ||
+ | * Interference from nearby bodies | ||
+ | * Noisiness | ||
+ | * Distance: | ||
+ | * Water/other spills: as water has a very high relative dielectric constant (ER=80), elaborate strategies have been developed to deal with water over the sensor, because it easily interferes with normal operation. For example, the capacitive buttons replacements on a glass-ceramic stovetop must not be accidentally triggered by water spills. To achieve this, the timing characteristics of the sensed capacitance are analyzed. | ||
+ | |||
+ | ===== Industrial Applications ===== | ||
+ | |||
+ | Capacitive sensing can be used to measure a wide variety of physical phenomena. According to Baxter((Baxter, | ||
+ | |||
+ | * Proximity sensing for industrial machine shut-off | ||
+ | * Touch sensing / button replacements (isometric buttons on microwave ovens, stovetops etc., rotary input like on the iPod) | ||
+ | * Micrometers | ||
+ | * Material properties - most famous is probably the Stud Sensor patented in the seventies, that allows you to sense studs, water and mains lines in walls. | ||
+ | * Vehicle/ | ||
+ | * Capacitor/ | ||
+ | * Liquid level sensing | ||
+ | * Tilt/ | ||
+ | * Accelerometers | ||
+ | * X-Y Input (Tablets/ | ||
+ | |||
+ | ===== Musical Applications ===== | ||
+ | |||
+ | Arguably the most famous musical instrument using capacitive sensing is the Theremin invented in 1920 by Léon Theremin in Russia. Also not to be missed here is the T-Stick((Joseph Malloch and Marcelo M. Wanderley. 2007. {{http:// | ||
+ | Others have built novel woodwind controllers((Stephen Hughes, Cormac Cannon, and Sile Ó Modhráin. 2004. {{http:// | ||
+ | |||
+ | The most popular current use of capacitive touch sensing is in smartphone and tablet devices like Apple' | ||
+ | |||
+ | ===== Example Implementation ===== | ||
+ | |||
+ | An easy-to-understand example is provided by the ((Microchip Application Note 1101 {{http:// | ||
+ | . According to the example, one way to sense the external capacitance on an electrode with a microcontroller is to: | ||
+ | * Loop: | ||
+ | * Charge the pad while V < C1 | ||
+ | * Discharge it while V > C2 | ||
+ | * Count the number of charge cycles in a given time frame with an analog comparator. | ||
+ | * Compare this number to a moving average of the idle value for this measurement. | ||
+ | |||
+ | {{: | ||
+ | |||
+ | An easy to use example for the Arduino environment is the {{http:// | ||
+ | |||
+ | ===== Media ===== | ||
+ | |||
+ | * [[http:// | ||
+ | |||
+ | ===== Pros/Cons ===== | ||
+ | |||
+ | * No force for use needed, better responsiveness than resistive touch sensing - no decompression of resistive material. Obviously, this also eliminates wear and tear, and works through solid protective surfaces, such as a glass-ceramic stovetop. | ||
+ | * However, the interacting object/body must be a dielectric or conductor - using an iPhone with gloves does not work for example. | ||
+ | |||
+ | ===== Devices | ||
+ | |||
+ | {{template> | ||
+ | |company=Quantum Research Group / Atmel | ||
+ | |model=QProx QT113 | ||
+ | |sources=[[http:// | ||
+ | |description=Charge-Transfer Touch Sensor | ||
+ | |datasheet=[[http:// | ||
+ | |resources= | ||
+ | |notes=The QT113 charge-transfer (“QT’”) touch sensor is a self-contained digital IC capable of detecting near-proximity or touch. It | ||
+ | will project a proximity sense field through air, and any dielectric like glass, plastic, stone, ceramic, and most kinds of wood. It | ||
+ | can also turn small metal-bearing objects into intrinsic sensors, making them responsive to proximity or touch. This capability | ||
+ | coupled with its ability to self calibrate continuously can lead to entirely new product concepts. | ||
+ | It is designed specifically for human interfaces, like control panels, appliances, toys, lighting controls, or anywhere a | ||
+ | mechanical switch or button may be found; it may also be used for some material sensing and control applications provided | ||
+ | that the presence duration of objects does not exceed the recalibration timeout interval. | ||
+ | Power consumption is only 600µA in most applications. In most cases the power supply need only be minimally regulated, for | ||
+ | example by Zener diodes or an inexpensive 3-terminal regulator. The QT113 requires only a common inexpensive capacitor | ||
+ | in order to function. | ||
+ | The QT113’s RISC core employs signal processing techniques pioneered by Quantum; these are specifically designed to | ||
+ | make the device survive real-world challenges, such as ‘stuck sensor’ conditions and signal drift. | ||
+ | The option-selectable toggle mode permits on/off touch control, for example for light switch replacement. The | ||
+ | Quantum-pioneered HeartBeat™ signal is also included, allowing a microcontroller to monitor the health of the QT113 | ||
+ | continuously if desired. | ||
+ | |||
+ | |variants= QT113DG - QT113ISG | ||
+ | }} | ||
+ | |||
+ | {{template> | ||
+ | |company=Quantum Research Group / Atmel | ||
+ | |model=QProx QT1081 | ||
+ | |sources=[[http:// | ||
+ | |description=8 Key Charge-Transfer QTouch Sensor IC | ||
+ | |datasheet=[[http:// | ||
+ | |resources= | ||
+ | |notes=QTouch™ technology is a type of patented charge-transfer sensing | ||
+ | method well known for its robust, stable, EMC-resistant characteristics. | ||
+ | It is the only all-digital capacitive sensing technology in the market | ||
+ | today. QTouch™ sensors employ a single reference capacitor tied to two pins | ||
+ | of the chip for each sensing key; a signal trace leads from one of the | ||
+ | pins to the sensing electrode which forms the key. The sensing | ||
+ | electrode can be a simple solid shape such as a rectangle or circle. An | ||
+ | LED can be placed near or inside the solid circle for illumination. | ||
+ | |variants= | ||
+ | }} | ||
+ | |||
+ | {{template> | ||
+ | |company=Motorola | ||
+ | |model=KIT33794DWBEVM | ||
+ | |sources=[[http:// | ||
+ | |description=Electric Field Imaging Device - non contact sensing (evaluation kit) | ||
+ | |datasheet=[[http:// | ||
+ | |resources= | ||
+ | |notes=Supports up to 9 electrodes and 2 references | ||
+ | |variants= | ||
+ | }} | ||
+ | |||
+ | ===== External Links ===== | ||
+ | * [[http:// | ||
+ | * [[http:// | ||
+ | * [[http:// | ||
+ | * [[http:// | ||
+ | * Microchip Application Notes on Capacitive Sensing: | ||
+ | * [[http:// | ||
+ | * [[http:// | ||
+ | * [[http:// | ||
+ | * [[http:// | ||
+ | |||
+ | {{tag> | ||
+ | |||
+ | ===== References ===== |