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electrical engineering Electrical engineering is an engineering discipline concerned with the study, design, and application of equipment, devices, and systems that use electricity, electronics, and electromagnetism. It emerged as an identifiable occupation in the l ...
, capacitive sensing (sometimes capacitance sensing) is a technology, based on
capacitive coupling Capacitive coupling (electronics), coupling is the transfer of energy within an electrical network or between distant networks by means of displacement current between circuit(s) node (circuits) , nodes, induced by the electric field. This coup ...
, that can detect and measure anything that is conductive or has a
dielectric constant The relative permittivity (in older texts, dielectric constant) is the permittivity of a material expressed as a ratio with the electric permittivity of a vacuum. A dielectric is an insulating material, and the dielectric constant of an insul ...
different from air. Many types of
sensors A sensor is often defined as a device that receives and responds to a signal or stimulus. The stimulus is the quantity, property, or condition that is sensed and converted into electrical signal. In the broadest definition, a sensor is a devi ...
use capacitive sensing, including sensors to detect and measure proximity, pressure, position and displacement,
force In physics, a force is an influence that can cause an Physical object, object to change its velocity unless counterbalanced by other forces. In mechanics, force makes ideas like 'pushing' or 'pulling' mathematically precise. Because the Magnitu ...
,
humidity Humidity is the concentration of water vapor present in the air. Water vapor, the gaseous state of water, is generally invisible to the human eye. Humidity indicates the likelihood for precipitation (meteorology), precipitation, dew, or fog t ...
, fluid level, and
acceleration In mechanics, acceleration is the Rate (mathematics), rate of change of the velocity of an object with respect to time. Acceleration is one of several components of kinematics, the study of motion. Accelerations are Euclidean vector, vector ...
. Human interface devices based on capacitive sensing, such as
touchpad A touchpad or trackpad is a type of pointing device. Its largest component is a tactile sensor: an electronic device with a flat surface, that detects the motion and position of a user's fingers, and translates them to 2D motion, to control a Cu ...
s, can be used in place of a
computer mouse A computer mouse (plural mice; also mouses) is a hand-held pointing device that detects Plane (mathematics), two-dimensional motion relative to a surface. This motion is typically translated into the motion of the Cursor (user interface)#Po ...
.
Digital audio player A portable media player (PMP) or digital audio player (DAP) is a portable consumer electronics device capable of storing and playing digital media such as audio, images, and video files. Normally they refer to small, battery-powered devices ...
s,
mobile phone A mobile phone or cell phone is a portable telephone that allows users to make and receive calls over a radio frequency link while moving within a designated telephone service area, unlike fixed-location phones ( landline phones). This rad ...
s, and
tablet computer A tablet computer, commonly shortened to tablet, is a mobile device, typically with a mobile operating system and touchscreen display processing circuitry, and a rechargeable battery in a single, thin and flat package. Tablets, being computers ...
s will sometimes use capacitive sensing
touchscreen A touchscreen (or touch screen) is a type of electronic visual display, display that can detect touch input from a user. It consists of both an input device (a touch panel) and an output device (a visual display). The touch panel is typically l ...
s as input devices. Capacitive sensors can also replace mechanical buttons. A capacitive touchscreen typically consists of a capacitive touch
sensor A sensor is often defined as a device that receives and responds to a signal or stimulus. The stimulus is the quantity, property, or condition that is sensed and converted into electrical signal. In the broadest definition, a sensor is a devi ...
along with at least two complementary
metal–oxide–semiconductor upright=1.3, Two power MOSFETs in amperes">A in the ''on'' state, dissipating up to about 100 watt">W and controlling a load of over 2000 W. A matchstick is pictured for scale. In electronics, the metal–oxide–semiconductor field- ...
(
CMOS Complementary metal–oxide–semiconductor (CMOS, pronounced "sea-moss ", , ) is a type of MOSFET, metal–oxide–semiconductor field-effect transistor (MOSFET) semiconductor device fabrication, fabrication process that uses complementary an ...
)
integrated circuit An integrated circuit (IC), also known as a microchip or simply chip, is a set of electronic circuits, consisting of various electronic components (such as transistors, resistors, and capacitors) and their interconnections. These components a ...
(IC) chips, an
application-specific integrated circuit An application-specific integrated circuit (ASIC ) is an integrated circuit (IC) chip customized for a particular use, rather than intended for general-purpose use, such as a chip designed to run in a digital voice recorder or a high-efficienc ...
(ASIC) controller and a
digital signal processor A digital signal processor (DSP) is a specialized microprocessor chip, with its architecture optimized for the operational needs of digital signal processing. DSPs are fabricated on metal–oxide–semiconductor (MOS) integrated circuit chips. ...
(DSP). Capacitive sensing is commonly used for mobile
multi-touch In computing, multi-touch is technology that enables a surface (a touchpad or touchscreen) to recognize the presence of more than one somatosensory system, point of contact with the surface at the same time. The origins of multitouch began at CE ...
displays, popularized by
Apple An apple is a round, edible fruit produced by an apple tree (''Malus'' spp.). Fruit trees of the orchard or domestic apple (''Malus domestica''), the most widely grown in the genus, are agriculture, cultivated worldwide. The tree originated ...
's
iPhone The iPhone is a line of smartphones developed and marketed by Apple that run iOS, the company's own mobile operating system. The first-generation iPhone was announced by then–Apple CEO and co-founder Steve Jobs on January 9, 2007, at ...
in 2007.


Design

Capacitive sensors are constructed from many different media, such as copper,
indium tin oxide Indium tin oxide (ITO) is a ternary composition of indium, tin and oxygen in varying proportions. Depending on the oxygen content, it can be described as either a ceramic or an alloy. Indium tin oxide is typically encountered as an oxygen-saturate ...
(ITO) and printed ink. Copper capacitive sensors can be implemented on standard FR4 PCBs as well as on flexible material. ITO allows the capacitive sensor to be up to 90% transparent (for one layer solutions, such as touch phone screens). Size and spacing of the capacitive sensor are both very important to the sensor's performance. In addition to the size of the sensor, and its spacing relative to the
ground plane In electrical engineering, a ground plane is an electrically conductive surface, usually connected to electrical ground. Ground planes are typically made of copper or aluminum, and they are often located on the bottom of printed circuit boards ...
, the type of ground plane used is very important. Since the
parasitic capacitance Parasitic capacitance or stray capacitance is the unavoidable and usually unwanted capacitance that exists between the parts of an electronic component or circuit simply because of their proximity to each other. When two electrical conductors a ...
of the sensor is related to the
electric field An electric field (sometimes called E-field) is a field (physics), physical field that surrounds electrically charged particles such as electrons. In classical electromagnetism, the electric field of a single charge (or group of charges) descri ...
's (E-field) path to ground, it is important to choose a ground plane that limits the concentration of E-field lines with no conductive object present. Designing a capacitance sensing system requires first picking the type of sensing material (FR4, Flex, ITO, etc.). One also needs to understand the environment the device will operate in, such as the full
operating temperature An operating temperature is the allowable temperature range of the local ambient environment at which an electrical or mechanical device operates. The device will operate effectively within a specified temperature range which varies based on the de ...
range, what radio frequencies are present and how the user will interact with the interface. Tools such as CapExt, ANSYS Q3D Extractor, and solutions from FastFieldSolvers can be employed to optimize designs by enhancing sensitivity, accurately modeling electromagnetic fields, and improving performance across varying environmental conditions. There are two types of capacitive sensing systems: # mutual capacitance, where the object (finger, capacitive stylus) alters the mutual coupling between row and column electrodes, which are scanned sequentially; and # self-capacitance, where the object (such as a finger) loads the sensor or increases the parasitic capacitance to ground. In both cases, the difference of a preceding absolute position from the present absolute position yields the relative motion of the object or finger during that time. The technologies are elaborated in the following section.


Surface capacitance

In this basic technology, only one side of the insulator is coated with conductive material. A small
voltage Voltage, also known as (electrical) potential difference, electric pressure, or electric tension, is the difference in electric potential between two points. In a Electrostatics, static electric field, it corresponds to the Work (electrical), ...
is applied to this layer, resulting in a uniform electrostatic field. When a conductor, such as a human finger, touches the uncoated surface, a
capacitor In electrical engineering, a capacitor is a device that stores electrical energy by accumulating electric charges on two closely spaced surfaces that are insulated from each other. The capacitor was originally known as the condenser, a term st ...
is dynamically formed. Because of the sheet resistance of the surface, each corner is measured to have a different effective capacitance. The sensor's controller can determine the location of the touch indirectly from the change in the
capacitance Capacitance is the ability of an object to store electric charge. It is measured by the change in charge in response to a difference in electric potential, expressed as the ratio of those quantities. Commonly recognized are two closely related ...
as measured from the four corners of the panel: the larger the change in capacitance, the closer the touch is to that corner. With no moving parts, it is moderately durable, but has low resolution, is prone to false signals from parasitic
capacitive coupling Capacitive coupling (electronics), coupling is the transfer of energy within an electrical network or between distant networks by means of displacement current between circuit(s) node (circuits) , nodes, induced by the electric field. This coup ...
, and needs
calibration In measurement technology and metrology, calibration is the comparison of measurement values delivered by a device under test with those of a calibration standard of known accuracy. Such a standard could be another measurement device of known ...
during manufacture. Therefore, it is most often used in simple applications such as industrial controls and
interactive kiosk An interactive kiosk is a computer terminal featuring specialized hardware and software that provides access to information and applications for communication, commerce, entertainment, or education. By 2010, the largest bill pay kiosk network ...
s.


Projected capacitance

Projected capacitance touch (PCT) technology is a capacitive technology which allows more accurate and flexible operation, by
etching Etching is traditionally the process of using strong acid or mordant to cut into the unprotected parts of a metal surface to create a design in intaglio (incised) in the metal. In modern manufacturing, other chemicals may be used on other type ...
the conductive layer. An X-Y grid is formed either by etching one layer to form a grid pattern of
electrode An electrode is an electrical conductor used to make contact with a nonmetallic part of a circuit (e.g. a semiconductor, an electrolyte, a vacuum or a gas). In electrochemical cells, electrodes are essential parts that can consist of a varie ...
s, or by etching two separate, parallel layers of conductive material with perpendicular lines or tracks to form the grid; comparable to the
pixel In digital imaging, a pixel (abbreviated px), pel, or picture element is the smallest addressable element in a Raster graphics, raster image, or the smallest addressable element in a dot matrix display device. In most digital display devices, p ...
grid found in many
liquid crystal display A liquid-crystal display (LCD) is a flat-panel display or other Electro-optic modulator, electronically modulated optical device that uses the light-modulating properties of liquid crystals combined with polarizers to display information. Liq ...
s (LCD). The greater resolution of PCT allows operation with no direct contact, such that the conducting layers can be coated with further protective insulating layers, and operate even under screen protectors, or behind weather and vandal-proof glass. Because the top layer of a PCT is glass, PCT is a more robust solution versus resistive touch technology. Depending on the implementation, an active or passive stylus can be used instead of or in addition to a finger. This is common with
point of sale The point of sale (POS) or point of purchase (POP) is the time and place at which a retail transaction is completed. At the point of sale, the merchant calculates the amount owed by the customer, indicates that amount, may prepare an invoice f ...
devices that require signature capture. Gloved fingers may not be sensed, depending on the implementation and gain settings. Conductive smudges and similar interference on the panel surface can interfere with the performance. Such conductive smudges come mostly from sticky or sweaty finger tips, especially in high humidity environments. Collected dust, which adheres to the screen because of moisture from fingertips can also be a problem. There are two types of PCT: self capacitance, and mutual capacitance. ''Mutual capacitive'' sensors have a
capacitor In electrical engineering, a capacitor is a device that stores electrical energy by accumulating electric charges on two closely spaced surfaces that are insulated from each other. The capacitor was originally known as the condenser, a term st ...
at each intersection of each row and each column. A 12-by-16 array, for example, would have 192 independent capacitors. A
voltage Voltage, also known as (electrical) potential difference, electric pressure, or electric tension, is the difference in electric potential between two points. In a Electrostatics, static electric field, it corresponds to the Work (electrical), ...
is applied to the rows or columns. Bringing a finger or conductive stylus near the surface of the sensor changes the local electric field which reduces the mutual capacitance. The capacitance change at every individual point on the grid can be measured to accurately determine the touch location by measuring the voltage in the other axis. Mutual capacitance allows
multi-touch In computing, multi-touch is technology that enables a surface (a touchpad or touchscreen) to recognize the presence of more than one somatosensory system, point of contact with the surface at the same time. The origins of multitouch began at CE ...
operation where multiple fingers, palms or styli can be accurately tracked at the same time. ''Self-capacitance'' sensors can have the same X-Y grid as mutual capacitance sensors, but the columns and rows operate independently. With self-capacitance, current senses the capacitive load of a finger on each column or row. This produces a stronger signal than mutual capacitance sensing, but it is unable to resolve accurately more than one finger, which results in "ghosting", or misplaced location sensing.


Circuit design

Capacitance is typically measured indirectly, by using it to control the frequency of an oscillator, or to vary the level of coupling (or attenuation) of an AC signal. Basically the technique works by charging the unknown capacitance with a known current, since rearranging the current–voltage relation for a capacitor, I(t) = C\frac \, , allows determining the capacitance from the instantaneous current divided by the rate of change of voltage across the capacitor: C = \frac \, . That can be integrated over a charging time period from t_0 to t_1 to be expressed in integral form as:C = \frac \, .


Types


Step response

For a simple example of the above equation, if the charging current is constant and the starting voltage V(t_0) is 0 V, then the capacitance is simply the value of that constant current multiplied by the charging time duration (t_1-t_0) and divided by the final voltage V(t_1) \, . Either this charging time or voltage can be a predetermined constant. For instance, if measuring after a constant amount of time, then the capacitance can be determined using only the final voltage. Alternatively if using a fixed threshold voltage, then instead only need to measure the charging time duration to reach that voltage threshold. This step response measurement can be continually repeated (e.g. by using a
square wave Square wave may refer to: *Square wave (waveform) A square wave is a non-sinusoidal waveform, non-sinusoidal periodic waveform in which the amplitude alternates at a steady frequency between fixed minimum and maximum values, with the same ...
). For an example capacitive sense IC,
Texas Instruments Texas Instruments Incorporated (TI) is an American multinational semiconductor company headquartered in Dallas, Texas. It is one of the top 10 semiconductor companies worldwide based on sales volume. The company's focus is on developing analog ...
's FDC1004 applies a 25-kHz step waveform to charge up an electrode, and after a defined amount of time, converts the analog voltage representing that charge into a digital value of capacitance using a built-in
analog-to-digital converter In electronics, an analog-to-digital converter (ADC, A/D, or A-to-D) is a system that converts an analog signal, such as a sound picked up by a microphone or light entering a digital camera, into a Digital signal (signal processing), digi ...
(ADC).


Relaxation oscillator

The design of a simple capacitance meter is often based on a relaxation oscillator. The capacitance to be sensed forms a portion of the oscillator's
RC circuit A resistor–capacitor circuit (RC circuit), or RC filter or RC network, is an electric circuit composed of resistors and capacitors. It may be driven by a voltage source, voltage or current source and these will produce different responses. A fi ...
or LC circuit. The capacitance can be calculated by measuring the charging time required to reach the threshold voltage (of the relaxation oscillator), or equivalently, by measuring the oscillator's frequency. Both of these are proportional to the RC (or LC)
time constant In physics and engineering, the time constant, usually denoted by the Greek language, Greek letter (tau), is the parameter characterizing the response to a step input of a first-order, LTI system theory, linear time-invariant (LTI) system.Concre ...
of the oscillator circuit.


Voltage divider

Another measurement technique is to apply a fixed-frequency AC-voltage signal across a capacitive divider (a
voltage divider In electronics, a voltage divider (also known as a potential divider) is a passive linear circuit that produces an output voltage (''V''out) that is a fraction of its input voltage (''V''in). Voltage division is the result of distributing the i ...
that uses capacitors instead of resistors). This consists of two capacitors in series, one of a known value and the other of an unknown value. An output signal is then taken from across one of the capacitors. The value of the unknown capacitor can be found from the ratio of capacitances, which equals the ratio of the output/input signal amplitudes, as could be measured by an AC voltmeter.


Bridge configuration

More accurate instruments may use a capacitance bridge configuration, similar to a
Wheatstone bridge A Wheatstone bridge is an electrical circuit used to measure an unknown electrical resistance by balancing two legs of a bridge circuit, one leg of which includes the unknown component. The primary benefit of the circuit is its ability to prov ...
. The capacitance bridge helps to compensate for any variability that may exist in the applied signal.


Charge transfer

While not specific to capacitive sensing, charge transfer uses a
switched capacitor A switched capacitor (SC) is an electronic circuit that implements a function by moving charges into and out of capacitors when electronic switches are opened and closed. Usually, non-overlapping clock signals are used to control the switches ...
network to accumulate charge onto an integrating capacitor over a series of discrete steps, to produce an accurate sum of all the individual charge contributors.


Delta-sigma

Delta-sigma modulation Delta-sigma (ΔΣ; or sigma-delta, ΣΔ) modulation is an oversampling method for encoding signals into low bit depth digital signals at a very high sample-frequency as part of the process of delta-sigma analog-to-digital converters (A ...
can also measure capacitance instead of voltage.


Errors

The primary source of error in capacitance measurements is stray capacitance, which if not guarded against, may fluctuate between roughly 10 pF and 10 nF. The stray capacitance can be held relatively constant by shielding the (high impedance) capacitance signal and then connecting the shield to (a low impedance) ground reference. Also, to minimize the unwanted effects of stray capacitance, it is good practice to locate the sensing electronics as near the sensor electrodes as possible.


Comparison with other touchscreen technologies

Capacitive touchscreens are more responsive than
resistive touchscreen A resistive touchscreen is a type of touchscreen, touch-sensitive display that works by detecting pressure applied to the screen. It is composed of two flexible sheets coated with a resistive material and separated by an air gap or microdots. ...
s (which react to any object since no capacitance is needed), but less accurate. However, projective capacitance improves a touchscreen's accuracy as it forms a triangulated grid around the point of touch. A standard
stylus A stylus is a writing utensil or tool for scribing or marking into softer materials. Different styluses were used to write in cuneiform by pressing into wet clay, and to scribe or carve into a wax tablet. Very hard styluses are also used to En ...
cannot be used for capacitive sensing, but special capacitive styluses, which are conductive, exist for the purpose. One can even make a capacitive stylus by wrapping conductive material, such as anti-static conductive film, around a standard stylus or by rolling the film into a tube. Until recently, capacitive touchscreens were more expensive to manufacture than
resistive touchscreen A resistive touchscreen is a type of touchscreen, touch-sensitive display that works by detecting pressure applied to the screen. It is composed of two flexible sheets coated with a resistive material and separated by an air gap or microdots. ...
s. Not any more (see touchscreen#Construction). Some cannot be used with gloves and can fail to sense correctly with even a small amount of water on the screen. Mutual capacitive sensors can provide a two-dimensional image of the changes in the electric field. Using this image, a range of applications have been proposed. Authenticating users, estimating the orientation of fingers touching the screen and differentiating between fingers and palms become possible. While capacitive sensors are used for the touchscreens of most smartphones, the capacitive image is typically not exposed to the application layer. Power supplies with a high level of electronic
noise Noise is sound, chiefly unwanted, unintentional, or harmful sound considered unpleasant, loud, or disruptive to mental or hearing faculties. From a physics standpoint, there is no distinction between noise and desired sound, as both are vibrat ...
can reduce accuracy.


Pen computing

Many
stylus A stylus is a writing utensil or tool for scribing or marking into softer materials. Different styluses were used to write in cuneiform by pressing into wet clay, and to scribe or carve into a wax tablet. Very hard styluses are also used to En ...
designs for resistive touchscreens will not register on capacitive sensors because they are not conductive. Styluses that work on capacitive touchscreens primarily designed for fingers are required to simulate the difference in dielectric offered by a human finger.


See also

*
List of touch-solution manufacturers A list is a set of discrete items of information collected and set forth in some format for utility, entertainment, or other purposes. A list may be memorialized in any number of ways, including existing only in the mind of the list-maker, but ...
*
Theremin The theremin (; originally known as the ætherphone, etherphone, thereminophone or termenvox/thereminvox) is an electronic musical instrument controlled without physical contact by the performer (who is known as a thereminist). It is named aft ...


References

{{Reflist


External links


Part 1: Fundamentals of Projected-Capacitive Touch Technology, Geoff Walker, June 2014


Pointing devices User interface techniques Computing input devices