Capacitive sensing
Encyclopedia
In electrical engineering
, 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 trackpads, digital audio players, computer display
s, mobile phone
s, mobile devices and others. More and more design engineers are selecting capacitive sensors for their versatility, reliability and robustness, unique human-device interface and cost reduction over mechanical switches.
Capacitive sensors detect anything which is conductive or has a dielectric
different than that of air. While capacitive sensing applications can replace mechanical buttons with capacitive alternatives, other technologies such as multi-touch
and gesture-based touchscreen
s are also premised on capacitive sensing.
(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). The 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
, the type of ground plane used is very important. Since the parasitic capacitance
of the sensor is related to the electric field
'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
range, what radio frequencies are present and how the user will interact with the interface.
There are two types of capacitive sensing system: mutual capacitance, where the object (finger, conductive stylus) alters the mutual coupling between row and column electrodes, which are scanned sequentially; and self- or absolute 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.
is applied to the conductive layer, resulting in a uniform electrostatic field. When a conductor
, such as a human finger, touches the uncoated surface, a capacitor
is dynamically formed. Due to 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
as measured from the four corners of the panel; the larger the change in capacitance, the closer the touch is to that corner. As it has no moving parts, it is moderately durable. But it has limited resolution, is prone to false signals from parasitic capacitive coupling
, and needs calibration
during manufacture. It is therefore most often used in simple applications such as industrial controls and kiosks
.
the conductive layer. An X-Y grid
is formed either by etching one layer to form a grid pattern of electrode
s, or by etching two separate, perpendicular layers of conductive material with parallel lines or tracks to form the grid; comparable to the pixel
grid found in many liquid crystal display
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. Due to the top layer of a PCT being 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
devices that require signature capture. Gloved fingers may or 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 due to the 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
at each intersection of each row and each column. A 12-by-16 array, for example, would have 192 independent capacitors. A voltage
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
operation where multiple fingers, palms or stylus 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, the capacitive load of a finger is measured on each column or row electrode by a current meter. This method produces a stronger signal than mutual capacitance, but it is unable to resolve accurately more than one finger, which results in "ghosting", or misplaced location sensing.
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
or LC circuit
. Basically the technique works by charging the unknown capacitance with a known current. (The equation of state for a capacitor is i = C dv/dt. This means that the capacitance equals the current divided by the rate of change of voltage across the capacitor.) 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
of the oscillator circuit.
The primary source of error in capacitance measurements is stray capacitance, which if not guarded against, may fluctuate between 10 pF to 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 as possible.
Another measurement technique is to apply a fixed-frequency AC-voltage signal across a capacitive divider. 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 measured by an AC voltmeter. More accurate instruments may use a capacitance bridge configuration, similar to a wheatstone bridge
. The capacitance bridge helps to compensate for any variability that may exist in the applied signal.
s.
A standard stylus cannot be used for capacitive sensing unless it is tipped with some form of conductive material, such as anti-static conductive foam. However, capacitive styli—different from standard styli
—can be used as well as finger input on capacitive screens. Capacitive touchscreens are more expensive to manufacture and offer a significantly lesser degree of accuracy than resistive touchscreen
s. Some cannot be used with gloves, and can fail to sense correctly with even a small amount of water on the screen.
Power supplies with high electronic noise
can reduce accuracy.
that works on capacitive touchscreen
s primarily designed for fingers, as on iPhone
and most Android devices. They are different from standard styli designed for resistive touchscreens.
According to a report by ABI Research, styli are especially needed in China for handwriting recognition because of the nature of its writing system.
Electrical engineering
Electrical engineering is a field of engineering that generally deals with the study and application of electricity, electronics and electromagnetism. The field first became an identifiable occupation in the late nineteenth century after commercialization of the electric telegraph and electrical...
, capacitive sensing is a technology based on capacitive coupling
Capacitive coupling
In electronics, capacitive coupling is the transfer of energy within an electrical network by means of the capacitance between circuit nodes. This coupling can have an intentional or accidental effect...
that is used in many different types of sensors, including those to detect and measure: proximity, position or displacement
Capacitive displacement sensor
Capacitive displacement sensors “are non-contact devices capable of high-resolution measurement of the position and/or change of position of any conductive target”. They are also able to measure the thickness or density of non-conductive materials...
, humidity
Hygrometer
A hygrometer is an instrument used for measuring the moisture content in the environmental air, or humidity. Most measurement devices usually rely on measurements of some other quantity such as temperature, pressure, mass or a mechanical or electrical change in a substance as moisture is absorbed...
, fluid level, and acceleration
Accelerometer
An accelerometer is a device that measures proper acceleration, also called the four-acceleration. This is not necessarily the same as the coordinate acceleration , but is rather the type of acceleration associated with the phenomenon of weight experienced by a test mass that resides in the frame...
. Capacitive sensing as a human interface device
Human interface device
A human interface device or HID is a type of computer device that interacts directly with, and most often takes input from, humans and may deliver output to humans. The term "HID" most commonly refers to the USB-HID specification. The term was coined by Mike Van Flandern of Microsoft when he...
(HID) technology, for example to replace the computer mouse, is growing increasingly popular.
Capacitive touch sensors are used in many devices such as laptop trackpads, digital audio players, computer display
Computer display
A monitor or display is an electronic visual display for computers. The monitor comprises the display device, circuitry, and an enclosure...
s, mobile phone
Mobile phone
A mobile phone is a device which can make and receive telephone calls over a radio link whilst moving around a wide geographic area. It does so by connecting to a cellular network provided by a mobile network operator...
s, mobile devices and others. More and more design engineers are selecting capacitive sensors for their versatility, reliability and robustness, unique human-device interface and cost reduction over mechanical switches.
Capacitive sensors detect anything which is conductive or has a dielectric
Dielectric
A dielectric is an electrical insulator that can be polarized by an applied electric field. When a dielectric is placed in an electric field, electric charges do not flow through the material, as in a conductor, but only slightly shift from their average equilibrium positions causing dielectric...
different than that of air. While capacitive sensing applications can replace mechanical buttons with capacitive alternatives, other technologies such as multi-touch
Multi-touch
In computing, multi-touch refers to a touch sensing surface's ability to recognize the presence of two or more points of contact with the surface...
and gesture-based touchscreen
Touchscreen
A touchscreen is an electronic visual display that can detect the presence and location of a touch within the display area. The term generally refers to touching the display of the device with a finger or hand. Touchscreens can also sense other passive objects, such as a stylus...
s are also premised on capacitive sensing.
Sensor design
Capacitive sensors can be constructed from many different media, such as copper, Indium tin oxideIndium tin oxide
Indium tin oxide is a solid solution of indium oxide and tin oxide , typically 90% In2O3, 10% SnO2 by weight. It is transparent and colorless in thin layers while in bulk form it is yellowish to grey...
(ITO) and printed ink. Copper capacitive sensors can be implemented on standard FR4
FR-4
FR-4 is a grade designation assigned to glass-reinforced epoxy laminate sheets, tubes, rods and printed circuit boards . FR-4 is a composite material composed of woven fiberglass cloth with an epoxy resin binder that is flame resistant .FR-4 glass epoxy is a popular and versatile high-pressure...
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). The 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
Ground plane
In electrical engineering, a ground plane is an electrically conductive surface.-Radio antenna theory :In telecommunication, a ground plane structure or relationship exists between the antenna and another object, where the only structure of the object is a structure which permits the antenna to...
, the type of ground plane used is very important. Since the parasitic capacitance
Parasitic capacitance
In electrical circuits, parasitic capacitance, stray capacitance or, when relevant, self-capacitance , is an unavoidable and usually unwanted capacitance that exists between the parts of an electronic component or circuit simply because of their proximity to each other...
of the sensor is related to the electric field
Electric field
In physics, an electric field surrounds electrically charged particles and time-varying magnetic fields. The electric field depicts the force exerted on other electrically charged objects by the electrically charged particle the field is surrounding...
'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
Operating temperature
An operating temperature is the temperature at which an electrical or mechanical device operates. The device will operate effectively within a specified temperature range which varies based on the device function and application context, and ranges from the minimum operating temperature to the...
range, what radio frequencies are present and how the user will interact with the interface.
There are two types of capacitive sensing system: mutual capacitance, where the object (finger, conductive stylus) alters the mutual coupling between row and column electrodes, which are scanned sequentially; and self- or absolute 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 a conductive layer. A small voltageVoltage
Voltage, otherwise known as electrical potential difference or electric tension is the difference in electric potential between two points — or the difference in electric potential energy per unit charge between two points...
is applied to the conductive layer, resulting in a uniform electrostatic field. When a conductor
Electrical conductor
In physics and electrical engineering, a conductor is a material which contains movable electric charges. In metallic conductors such as copper or aluminum, the movable charged particles are electrons...
, such as a human finger, touches the uncoated surface, a capacitor
Capacitor
A capacitor is a passive two-terminal electrical component used to store energy in an electric field. The forms of practical capacitors vary widely, but all contain at least two electrical conductors separated by a dielectric ; for example, one common construction consists of metal foils separated...
is dynamically formed. Due to the sheet resistance of the surface, each corner is measured to have a different effective capacitance. The sensor's controller
Controller (computing)
In computing and especially in computer hardware, controller is a chip, an expansion card, or a stand-alone device that interfaces with a peripheral device...
can determine the location of the touch indirectly from the change in the capacitance
Capacitance
In electromagnetism and electronics, capacitance is the ability of a capacitor to store energy in an electric field. Capacitance is also a measure of the amount of electric potential energy stored for a given electric potential. A common form of energy storage device is a parallel-plate capacitor...
as measured from the four corners of the panel; the larger the change in capacitance, the closer the touch is to that corner. As it has no moving parts, it is moderately durable. But it has limited resolution, is prone to false signals from parasitic capacitive coupling
Capacitive coupling
In electronics, capacitive coupling is the transfer of energy within an electrical network by means of the capacitance between circuit nodes. This coupling can have an intentional or accidental effect...
, and needs calibration
Calibration
Calibration is a comparison between measurements – one of known magnitude or correctness made or set with one device and another measurement made in as similar a way as possible with a second device....
during manufacture. It is therefore most often used in simple applications such as industrial controls and kiosks
Interactive kiosk
An Interactive kiosk is a computer terminal featuring specialized hardware and software designed within a public exhibit that provides access to information and applications for communication, commerce, entertainment, and education....
.
Projected capacitance
Projected capacitive touch (PCT) technology is a capacitive technology which allows more accurate and flexible operation, by etchingEtching (microfabrication)
Etching is used in microfabrication to chemically remove layers from the surface of a wafer during manufacturing. Etching is a critically important process module, and every wafer undergoes many etching steps before it is complete....
the conductive layer. An X-Y grid
Cartesian coordinate system
A Cartesian coordinate system specifies each point uniquely in a plane by a pair of numerical coordinates, which are the signed distances from the point to two fixed perpendicular directed lines, measured in the same unit of length...
is formed either by etching one layer to form a grid pattern of electrode
Electrode
An electrode is an electrical conductor used to make contact with a nonmetallic part of a circuit...
s, or by etching two separate, perpendicular layers of conductive material with parallel lines or tracks to form the grid; comparable to the pixel
Pixel
In digital imaging, a pixel, or pel, is a single point in a raster image, or the smallest addressable screen element in a display device; it is the smallest unit of picture that can be represented or controlled....
grid found in many liquid crystal display
Liquid crystal display
A liquid crystal display is a flat panel display, electronic visual display, or video display that uses the light modulating properties of liquid crystals . LCs do not emit light directly....
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. Due to the top layer of a PCT being 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
Point of sale
Point of sale or checkout is the location where a transaction occurs...
devices that require signature capture. Gloved fingers may or 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 due to the moisture from fingertips can also be a problem. There are two types of PCT: self capacitance, and mutual capacitance.
Mutual capacitance
Mutual capacitive sensors have a capacitor
Capacitor
A capacitor is a passive two-terminal electrical component used to store energy in an electric field. The forms of practical capacitors vary widely, but all contain at least two electrical conductors separated by a dielectric ; for example, one common construction consists of metal foils separated...
at each intersection of each row and each column. A 12-by-16 array, for example, would have 192 independent capacitors. A voltage
Voltage
Voltage, otherwise known as electrical potential difference or electric tension is the difference in electric potential between two points — or the difference in electric potential energy per unit charge between two points...
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
Multi-touch
In computing, multi-touch refers to a touch sensing surface's ability to recognize the presence of two or more points of contact with the surface...
operation where multiple fingers, palms or stylus can be accurately tracked at the same time.
Self-capacitance
Self-capacitance sensors can have the same X-Y grid as mutual capacitance sensors, but the columns and rows operate independently. With self-capacitance, the capacitive load of a finger is measured on each column or row electrode by a current meter. This method produces a stronger signal than mutual capacitance, 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.The design of a simple capacitance meter is often based on a relaxation oscillator
Relaxation oscillator
A relaxation oscillator is an oscillator based upon the behavior of a physical system's return to equilibrium after being disturbed. That is, a dynamical system within the oscillator continuously dissipates its internal energy...
. The capacitance to be sensed forms a portion of the oscillator's RC circuit
RC circuit
A resistor–capacitor circuit ', or RC filter or RC network, is an electric circuit composed of resistors and capacitors driven by a voltage or current source...
or LC circuit
LC circuit
An LC circuit, also called a resonant circuit or tuned circuit, consists of an inductor, represented by the letter L, and a capacitor, represented by the letter C...
. Basically the technique works by charging the unknown capacitance with a known current. (The equation of state for a capacitor is i = C dv/dt. This means that the capacitance equals the current divided by the rate of change of voltage across the capacitor.) 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
Time constant
In physics and engineering, the time constant, usually denoted by the Greek letter \tau , is the risetime characterizing the response to a time-varying input of a first-order, linear time-invariant system.Concretely, a first-order LTI system is a system that can be modeled by a single first order...
of the oscillator circuit.
The primary source of error in capacitance measurements is stray capacitance, which if not guarded against, may fluctuate between 10 pF to 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 as possible.
Another measurement technique is to apply a fixed-frequency AC-voltage signal across a capacitive divider. 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 measured by an AC voltmeter. More accurate instruments may use a capacitance bridge configuration, similar to a wheatstone bridge
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. Its operation is similar to the original potentiometer. It was invented by Samuel Hunter Christie in 1833 and...
. The capacitance bridge helps to compensate for any variability that may exist in the applied signal.
Comparison with other touchscreen technologies
Since capacitive screens respond to only materials which are conductive (human finger used most commonly), they can be cleaned with cloths with no accidental command input. Capacitive touchscreens are more responsive than resistive touchscreenResistive touchscreen
In electrical engineering, resistive touchscreens are touch-sensitive computer displays composed of two flexible sheets coated with a resistive material and separated by an air gap or microdots. There are two different types of metallic layers. The first one is Matrix, which is a striped electrodes...
s.
A standard stylus cannot be used for capacitive sensing unless it is tipped with some form of conductive material, such as anti-static conductive foam. However, capacitive styli—different from standard styli
Stylus (computing)
In computing, a stylus is a small pen-shaped instrument that is used to input commands to a computer screen, mobile device or graphics tablet...
—can be used as well as finger input on capacitive screens. Capacitive touchscreens are more expensive to manufacture and offer a significantly lesser degree of accuracy than resistive touchscreen
Resistive touchscreen
In electrical engineering, resistive touchscreens are touch-sensitive computer displays composed of two flexible sheets coated with a resistive material and separated by an air gap or microdots. There are two different types of metallic layers. The first one is Matrix, which is a striped electrodes...
s. Some cannot be used with gloves, and can fail to sense correctly with even a small amount of water on the screen.
Power supplies with high electronic noise
Noise (electronics)
Electronic noise is a random fluctuation in an electrical signal, a characteristic of all electronic circuits. Noise generated by electronic devices varies greatly, as it can be produced by several different effects...
can reduce accuracy.
Capacitive stylus
A Capacitive stylus is a special type of stylusStylus (computing)
In computing, a stylus is a small pen-shaped instrument that is used to input commands to a computer screen, mobile device or graphics tablet...
that works on capacitive touchscreen
Touchscreen
A touchscreen is an electronic visual display that can detect the presence and location of a touch within the display area. The term generally refers to touching the display of the device with a finger or hand. Touchscreens can also sense other passive objects, such as a stylus...
s primarily designed for fingers, as on iPhone
IPhone
The iPhone is a line of Internet and multimedia-enabled smartphones marketed by Apple Inc. The first iPhone was unveiled by Steve Jobs, then CEO of Apple, on January 9, 2007, and released on June 29, 2007...
and most Android devices. They are different from standard styli designed for resistive touchscreens.
According to a report by ABI Research, styli are especially needed in China for handwriting recognition because of the nature of its writing system.
See also
- TouchscreenTouchscreenA touchscreen is an electronic visual display that can detect the presence and location of a touch within the display area. The term generally refers to touching the display of the device with a finger or hand. Touchscreens can also sense other passive objects, such as a stylus...
- Resistive touchscreenResistive touchscreenIn electrical engineering, resistive touchscreens are touch-sensitive computer displays composed of two flexible sheets coated with a resistive material and separated by an air gap or microdots. There are two different types of metallic layers. The first one is Matrix, which is a striped electrodes...
- Capacitive displacement sensorCapacitive displacement sensorCapacitive displacement sensors “are non-contact devices capable of high-resolution measurement of the position and/or change of position of any conductive target”. They are also able to measure the thickness or density of non-conductive materials...
- Body capacitanceBody capacitanceBody capacitance is a biological property of the human body that makes it a good capacitor and able to store charge. Every person has a characteristic called capacitance. Capacitance is an electronics term that has units of measure called farads. Capacitors are electronic components that store...
External links
- Build A Touch-Sensor Solution For Wet Environments
- Capacitive Touch Sensing Design
- Capacitive sensor theory - How Capacitive Sensors Work and How to Use Them Effectively
- Apple: Accessibility Solutions for iPhone, iPad, and iPod touch
- CNET News: HTC patents stylus for capacitive screens
- Dagi's capacitive stylus (video)
- GottaBeMobile: iPad Stylus Query (comparative review)
- The New York Times: Q&A: Can a Stylus Work on an iPhone?
- MSP430 Capacitive Touch Microcontroller from Texas Instruments