Capacitive sensing

In electrical engineering, capacitive sensing (sometimes capacitance sensing) is a technology, based on capacitive coupling, that can detect and measure anything that is conductive or has a dielectric different from air.

Many types of sensors use capacitive sensing, including sensors to detect and measure proximity, position and displacement, force, humidity, fluid level, and acceleration. Human interface devices based on capacitive sensing, such as trackpads,[1] can replace the computer mouse. Digital audio players, mobile phones, and tablet computers use capacitive sensing touchscreens as input devices.[2] Capacitive sensors can also replace mechanical buttons.

Design

Capacitive sensors are constructed from many different media, such as copper, indium tin oxide (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, 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,[3] where the object (finger, conductive stylus) alters the mutual coupling between row and column electrodes, which are scanned sequentially;[4] 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 conductive material. A small voltage is applied to this layer, resulting in a uniform electrostatic field.[5] When a conductor, such as a human finger, touches the uncoated surface, a capacitor 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 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, and needs calibration during manufacture. Therefore, it is most often used in simple applications such as industrial controls and interactive kiosks.[6]

Projected capacitance

TouchScreen projective capacitive
Schema of projected-capacitive touchscreen

Projected capacitive touch (PCT) technology is a capacitive technology which allows more accurate and flexible operation, by etching the conductive layer. An X-Y grid is formed either by etching one layer to form a grid pattern of electrodes, or by etching two separate, parallel layers of conductive material with perpendicular lines or tracks to form the grid; comparable to the pixel grid found in many liquid crystal displays (LCD).[7]

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 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 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 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.[8]

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. 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 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.

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 could be measured by an AC voltmeter. More accurate instruments may use a capacitance bridge configuration, similar to a Wheatstone bridge.[9] The capacitance bridge helps to compensate for any variability that may exist in the applied signal.111

Comparison with other touchscreen technologies

Capacitive touchscreens are more responsive than resistive touchscreens (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.[10]

A standard stylus cannot be used for capacitive sensing, but special capacitive stylus, which are conductive, exist for the purpose. One can even make a capacitive stylus by putting some form of conductive material, such as anti-static conductive foam on the tip of a standard stylus.[11] Capacitive touchscreens are more expensive to manufacture than resistive touchscreens. 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 [12][13], estimating the orientation of fingers touching the screen[14][15] and differentiating between fingers and palms[16] 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 can reduce accuracy.

Pen computing

Capacitive Stylus
Capacitive stylus

Many stylus 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 digit.[17]

See also

References

  1. ^ Larry K. Baxter (1996). Capacitive Sensors. John Wiley and Sons. p. 138. ISBN 978-0-7803-5351-0.
  2. ^ Wilson, Tracy. "HowStuffWorks "Multi-touch Systems"". Retrieved August 9, 2009.
  3. ^ US Pat No 5,305,017 5,861,875
  4. ^ e.g. U.S. Pat. No. 4,736,191
  5. ^ "Capacitive Sensor Operation and Optimization". Lionprecision.com. Retrieved 2012-06-15.
  6. ^ "Please Touch! Explore The Evolving World Of Touchscreen Technology". electronicdesign.com. Archived from the original on 2015-12-13. Retrieved 2009-09-02.
  7. ^ "Capacitive Touch (Touch Sensing Technologies — Part 2)". TouchAdvance.com. Retrieved 2011-20-2011. Check date values in: |accessdate= (help)
  8. ^ Self-Capacitive Touchscreens Explained (Sony Xperia Sola)
  9. ^ "Basic impedance measurement techniques". Newton.ex.ac.uk. Retrieved 2012-06-15.
  10. ^ "Technical Overview About Capacitive Sensing Vs. Other Touchscreen-Related Technologies". Glider Gloves. Retrieved 13 December 2015.
  11. ^ "How To Make A Free Capacitive Stylus". Pocketnow. 2010-02-24. Retrieved 2012-06-15.
  12. ^ Holz, Christian; Buthpitiya, Senaka; Knaust, Marius (2015). "Bodyprint: Biometric User Identification on Mobile Devices Using the Capacitive Touchscreen to Scan Body Parts" (PDF). Proceedings of the Conference on Human Factors in Computing Systems. doi:10.1145/2702123.2702518. Retrieved 26 March 2018.
  13. ^ Guo, Anhong; Xiao, Robert; Harrison, Chris (2015). "CapAuth: Identifying and Differentiating User Handprints on Commodity Capacitive Touchscreens" (PDF). Proceedings of the International Conference on Interactive Tabletops & Surfaces. doi:10.1145/2817721.2817722. Retrieved 26 March 2018.
  14. ^ Xiao, Robert; Schwarz, Julia; Harrison, Chris (2015). "Estimating 3D Finger Angle on Commodity Touchscreens" (PDF). Proceedings of the International Conference on Interactive Tabletops & Surfaces. doi:10.1145/2817721.2817737. Retrieved 26 March 2018.
  15. ^ Mayer, Sven; Le, Huy Viet; Henze, Niels (2017). "Estimating the Finger Orientation on Capacitive Touchscreens Using Convolutional Neural Networks" (PDF). Proceedings of the International Conference on Interactive Tabletops & Surfaces. doi:10.1145/3132272.3134130. Retrieved 26 March 2018.
  16. ^ Le, Huy Viet; Kosch, Thomas; Bader, Patrick; Mayer, Sven; Niels, Henze (2017). "PalmTouch: Using the Palm as an Additional Input Modality on Commodity Smartphones" (PDF). Proceedings of the Conference on Human Factors in Computing Systems. doi:10.1145/3173574.3173934. Retrieved 26 March 2018.
  17. ^ J.D. Biersdorfer (2009-08-19). "Q&A: Can a Stylus Work on an iPhone?". Gadgetwise.blogs.nytimes.com. Retrieved 2012-06-15.

External links

Alarm sensor

In telecommunication, the term alarm sensor has the following meanings:

1. In communications systems, a device that can sense an abnormal condition within the system and provide a signal indicating the presence or nature of the abnormality to either a local or remote alarm indicator, and (b) may detect events ranging from a simple contact opening or closure to a time-phased automatic shutdown and restart cycle.

2. In a physical security system, an approved device used to indicate a change in the physical environment of a facility or a part thereof.

3. In electronic security systems, a physical device or change/presence of any electronic signal/logic which causes trigger to electronic circuit to perform application specific operation. In electronic alarm systems the use of this trigger event done by such devices is to turn on the alarm or siren producing sound and/or perform a security calling through telephone lines.

Note: Alarm sensors may also be redundant or chained, such as when one alarm sensor is used to protect the housing, cabling, or power protected by another alarm sensor.

Source: from Federal Standard 1037C and from MIL-STD-188 and from TRISHAM Software Systems

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. Capacitive displacement sensors are used in a wide variety of applications including semiconductor processing, assembly of precision equipment such as disk drives, precision thickness measurements, machine tool metrology and assembly line testing. These types of sensors can be found in machining and manufacturing facilities around the world.

Catadioptric sensor

A catadioptric sensor is a visual sensor that contains mirrors (catoptrics) and lenses (dioptrics), a combined catadioptric system. These are panoramic sensors created by pointing a camera at a curved mirror.

Displacement receiver

A displacement receiver is a device that responds to or is sensitive to directed distance (displacement).

Examples of displacement receivers include carbon microphones, strain gauges, and pressure sensors or force sensors, which, to within an appropriate scale factor, respond to distance.

In music, certain music keyboards can be considered displacement receivers in the sense that they respond to displacement, rather than velocity (as is more commonly the case).

Examples of displacement-responding sensors include the mechanical action of tracker organs, as well as the force-sensing resistors found in music keyboards that had polyphonic aftertouch capability. Polyphonic aftertouch is no longer a feature of presently manufactured keyboards, but certain older models such as the Roland A50 featured a pressure sensing resistor, similar in principle-of-operation to a carbon microphone, in each key.

Electromechanical film

Electromechanical Film is a thin membrane whose thickness is related to an electric voltage. It can be used as a pressure sensor, microphone, or a speaker. It can also convert electrical energy to vibration, functioning as an actuator.

Humistor

A humistor is a type of variable resistor whose resistance varies based on humidity.

IPod click wheel

The iPod click wheel is the navigation component of several iPod models. It uses a combination of touch technology and traditional buttons, involving the technology of capacitive sensing, which senses the capacitance of the user's fingers. The wheel allows a user to find music, videos, photos and play games on the device. The wheel is flush on the face of the iPod and is located below the screen.

The design was first released with the iPod Mini, and was last used with the iPod Classic. It is credited to Apple's Vice President of worldwide marketing, Phil Schiller.

Infrared point sensor

An infrared point sensor is a point gas detector based on the nondispersive infrared sensor technology.

Intelligent sensor

An intelligent sensor is a sensor that takes some predefined action when it senses the appropriate input (light, heat, sound, motion, touch, etc.).

LG Rumor

LG Rumor is a series of feature phones from Sprint in the United States, manufactured by LG Electronics. Each phone is equipped with a slide-out Qwerty keyboard with the latest featured touchscreen.

List of sensors

This is a list of sensors sorted by sensor type.

Pellistor

A pellistor is a solid-state device used to detect gases which are either combustible or which have a significant difference in thermal conductivity to that of air. The word "pellistor" is a combination of pellet and resistor.

Position sensor

A position sensor is any device that permits position measurement. It can either be an absolute position sensor or a relative one (displacement sensor). Position sensors can be linear, angular, or multi-axis.

Some position sensors available today:

Capacitive transducer

Capacitive displacement sensor

Eddy-current sensor

Ultrasonic sensor

Grating sensor

Hall effect sensor

Inductive non-contact position sensors

Laser Doppler vibrometer (optical)

Linear variable differential transformer (LVDT)

Multi-axis displacement transducer

Photodiode array

Piezo-electric transducer (piezo-electric)

Potentiometer

Proximity sensor (optical)

Rotary encoder (angular)

Seismic displacement pick-up

String potentiometer (also known as string pot., string encoder, cable position transducer)

Confocal chromatic sensor

Synaptics

Synaptics is a publicly owned San Jose, California-based developer of human interface (HMI) hardware and software, including touchpads for computer laptops; touch, display driver, and fingerprint biometrics technology for smartphones; and touch, video and far-field voice technology for smart home devices and automotives. Synaptics primarily sells its products to original equipment manufacturers (OEMs) and display manufacturers.

Since its founding in 1986, the company's notable innovations include the first ever computer touchpad, touch technology for the click wheel on the classic iPod, touch sensors used in numerous Android phones, touch and display driver integrated chips (TDDI), and biometrics technology for fingerprint sensors. All touch and fingerprint technology was based on capacitive sensing up until the introduction of optical fingerprint sensing in late 2016.

Tactile sensor

A tactile sensor is a device that measures information arising from physical interaction with its environment. Tactile sensors are generally modeled after the biological sense of cutaneous touch which is capable of detecting stimuli resulting from mechanical stimulation, temperature, and pain (although pain sensing is not common in artificial tactile sensors). Tactile sensors are used in robotics, computer hardware and security systems. A common application of tactile sensors is in touchscreen devices on mobile phones and computing.

Tactile sensors may be of different types including piezoresistive, piezoelectric, capacitive and elastoresistive sensors.

Touchpad

A touchpad or trackpad is a pointing device featuring a tactile sensor, a specialized surface that can translate the motion and position of a user's fingers to a relative position on the operating system that is made output to the screen. Touchpads are a common feature of laptop computers, and are also used as a substitute for a mouse where desk space is scarce. Because they vary in size, they can also be found on personal digital assistants (PDAs) and some portable media players. Wireless touchpads are also available as detached accessories.

Velocity receiver

A velocity receiver (velocity sensor) is a sensor that responds to velocity rather than absolute position. For example, dynamic microphones are velocity receivers. Likewise, many electronic keyboards used for music are velocity sensitive, and may be said to possess a velocity receiver in each key. Most of these function by measuring the time difference between switch closures at two different positions along the travel of each key.

There are two types of velocity receivers, moving coil and piezoelectric. The former contains a coil supported by springs and a permanently fixed magnet and require no output signal amplifiers. Movement causes the coil to move relative to the magnet, which in turn generates a voltage that is proportional to the velocity of that movement.

Piezoelectric sensor velocity receivers are similar to a piezoelectric accelerometer, except that the output of the device is proportional to the velocity of the transducer. Unlike the moving coil variety, piezoelectric sensors will likely require an amplifier due to the small generated signal.

Water sensor

The Water in Fuel Sensor or WiF sensor indicates the presence of water in the fuel. It is installed in the fuel filter and when the water level in the water separator reaches the warning level, the Wif sends an electrical signal to the ECU or to dashboard (lamp).

The WiF is used especially in the Common Rail engines to avoid the Fuel injector damage.

The WiF sensor uses the difference of electric conductivity through water and diesel fuel by 2 electrodes.

First generation WiF sensors use a potting resin to isolate the electronic circuit, while the latest generation of Wif sensors (the WS3 sensor in Surface-mount technology) are made totally without leakage using an innovative co-moulding process.

The latest generation of WiF sensors have a high resistance to vibrations and to thermal excursion cycles.

The main automotive WiF designer and producer is SMP Poland.

Yaw-rate sensor

A yaw-rate sensor is a gyroscopic device that measures a vehicle’s angular velocity around its vertical axis. The angle between the vehicle's heading and vehicle actual movement direction is called slip angle, which is related to the yaw rate.

Acoustic, sound, vibration
Automotive, transportation
Chemical
Electric, magnetic, radio
Environment, weather,
moisture
Flow, fluid velocity
Ionising radiation,
subatomic particles
Navigation instruments
Position, angle,
displacement
Optical, light, imaging
Pressure
Force, density, level
Thermal, heat,
temperature
Proximity, presence
Sensor technology
Related

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