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, ice detection, shaft angle, linear position, and balances), switches (lamp dimmer, keyswitch, limit switch), communications (wireless datacom), and computer graphics input (x-y tablet)1)). Other pages that may also refer to capacitive sensing include accelerometer, hygrometer, inclinometer, and microphone.
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 dielectric. When a potential, that is a voltage, is applied to these conductors, opposing charges build up on the surface of these spaced conductive plates and an electric field is produced that stores electrical energy. Capacitance is the ratio of the charge over the voltage.
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 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, 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 capacitance 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 can generally be divided into three categories, based on their mode of operation: Load mode, transmit mode, and shunt mode 2). Another distinction can be made between capacitive sensors that are designed for contact and ones that are not.
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 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 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 “plates” of the capacitor. The body/object is very close to the electrodes in this mode, and becomes effectively an extension of them.
A familiar loaded capacitive sensor is used in Max Matthew's Radio Baton. In this instrument, two transmitting batons are directly connected to oscillators. Four receive electrodes embedded in a large plate acquire and demodulate the signal. Each of the receive electrodes is patterned such that it varies in surface area across either the x or y dimension of the plate, allowing 2D position to be sensed. By comparing the signals from all four electrodes, it is also possible to sense the height of the batons over the plate.
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/absorbs the electrical field. The current measured at the second electrode then correlates to the electrical field over the cross-section of the body. The difference between Shunt and Transmit mode is that the distance to the second electrode is known in Shunt mode, and thus allows more meaningful measurements.
Sensing touch, as with smart phones and tablet computers, is generally done using one of two variations on Load Mode technology: surface capacitance or projected capacitance.
Surface capacitance puts a small voltage on to the touch surface, and then measures the capacitance at each corner of the surface. When the user touches the surface, a capacitor is created, and due to the resistance of the surface, the capacitance will be different at each corner: this difference can be used to calculate the location of the touch. This method is simple, cheap, and durable, but lacks resolution and can be subject to false signals.
Projected capacitance uses a grid of electrodes, which gives much greater resolution. This can be done with a single electrode at each point on the grid, or with an electrode for each row and column. The prior method, called mutual capacitance, allows for multi-touch operation, whereas the latter method, called self-capacitance, gives strong signals, but cannot sense multiple touches.
Outside of these standard techniques, the SoundPlane5)) uses a novel method for adding force / depth sensing to capacitive touch surfaces. Each square of the SoundPlane's grid is an exant capacitor, with two plates and a small rubber dielectric between them. Thus, a touch compresses the dielectric and changes the capacitance accordingly, as opposed to create a capacitor with each touch.
- 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
- Distance: distance increases the chance of interference and noise, and can lead to very weak signals as the electric fields decrease over 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.
Capacitive sensing can be used to measure a wide variety of physical phenomena. According to Baxter6)), in industrial and consumer products, capacitive sensing is used for:
- Proximity sensing for industrial machine shut-off
- Touch sensing / button replacements (isometric buttons on microwave ovens, stovetops etc., rotary input like on the iPod)
- 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/Personnel detection
- Capacitor/Condenser microphones
- Liquid level sensing
- Tilt/Rotation sensing
- X-Y Input (Tablets/Laptop computer touch pads, Smart Phones)
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-Stick7) by Malloch and Wanderley. Others have built novel woodwind controllers8), have detected fret presses on guitars9), have constructed multi-touch surfaces for musical interaction10)), and have made a chair that tracks the user's position 11) using capacitive sensing.
The most popular current use of capacitive touch sensing is in smartphone and tablet devices like Apple's iPad and iPhone , Microsoft's Surface, and various devices using Google's Android OS, such as Google's Nexus 7 and Samsung's Galaxy S3. Examples of music applications on these platforms range from simple pianos to complex granular synthesis to highly customizable control surfaces.
An easy-to-understand example is provided by the 12) . According to the example, one way to sense the external capacitance on an electrode with a microcontroller is to:
- 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 CapSense library.
- 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.
Quantum Research Group / Atmel QProx QT113
Description: Charge-Transfer Touch Sensor
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
Quantum Research Group / Atmel QProx QT1081
Description: 8 Key Charge-Transfer QTouch Sensor IC
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.
- Microchip Application Notes on Capacitive Sensing: