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Capacitive sensors technology is based on capacitive coupling which detects human body capacitance. The capacitive touch screen consists of an electric insulator such as glass, which is plated with an electrically conductive material. When human touches the glass, it creates a change in the electric field that can be detected as capacitance change by the sensor. In Figure 1 we can see a typical capacitive sensor.
Figure 1. iPhone touchscreen sensor
A good example about capacitive sensors is the touch screen of iPhone. It has a circuitry which can detect changes at each point along the grid. Every point has its own unique signal when touched and relays that to the iPhone’s processor. This way of detecting human touch gives iPhone the ability to determine the location and movement of simultaneous touches in multiple locations on the phone’s touch screen.
Because of iPhone’s reliance of this capacitive material, iPhone’s touch screen doesn’t detect with stylus or non-conductive gloves. In Figure 2 we can see inside iPhone’s touch screen.
Figure 2. Layers of iPhone
How does capacitive sensor detect touch?
Figure 3. Pre-processing circuit for capacitance measurement
Capacitive sensor coverts measured capacitance into a digital value called Raw Counts. With Raw Counts we can detect is the finger on or near the sensor since the sensor can detect change of capacitance even if you are not touching the screen. Figure 3 is a block diagram which shows capacitive touch sensing pre-processing circuit.
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Capacitive sensors
Touch Switch D5C
Figure 1. D5C sensor
D5C sensor is mainly used in the industry.
Figure 12. Tempeture characteristics DC and AC
In figure 1 2 there is one comparison between DC and AC models. In these pictures is sensitivity when temperature is changing. In these pictures we can see comparison with DC and AC models. AC models are more sensitive but they can change less sensitive when temperature increases.
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AD7142 is connected to the sensor with excitation source and CIN wires. Sensor itself is integrated to the circuit board.
Figure 1. AD7142 circuit board
How the input signal is conditioned, e.g. how the signal from the sensor is bring up to the format compatible with the load device?
Figure 2. When is the sensor avtive and not active
The practical response from the sensor is defined by the converter’s output change when the sensor goes from inactive to active. When the sensor is not active the AD714x sensors measures the capacitance value as ambient value as seen from Figure 3. As previously explained in Position sensor page, the capacitive increases when the finger is touching screen or a button and it decrease when finger is moving away of the screen or button.
Figure 3. Ambient cpacitive value
What is the electrical input impedance of the sensor?
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Does the signal need amplification?
Epic Figure 4. Grumpy cat
Does the sensor need excitation current or voltage? How much?
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It has on-chip calibration logic to account for changes in the ambient environment. Calibration works when sensor is not touched. The calibration sequence is performed automatically and at continuous intervals. By this way the calibration ensures that there are no false or nonregistering touches on the external sensors due to different environment changes.
Figure 6. Sensor calibration
In Figure 6 we can see how exactly calibration works. It goes offline, when user touches sensor, and enables itself after user leaves sensor area and small delay.
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- http://www.analog.com/en/analog-to-digital-converters/capacitance-to-digital-converters/ad7142/products/product.html AD7142, PROGRAMMABLE CONTROLLER FOR CAPACITANCE TOUCH SENSORS
- http://www.analog.com/static/imported-files/data_sheets/AD7142.pdf AD7142 datasheet
Materials used in capacitive sensors
Capacitive sensor material have has to be conductive. It doesn't really matter from what material sensor is made, so long it's conductive. Common conductor capacitive sensing materials include copper, ITO (Indium tin oxide), glass, acrylic and silver ink. The resistance is also a big factor when choosing the correct material.
ITO for example has a high resistance, which might need a PreScaler, which is used to slow down frequency in the CSD User Module for Capsense to facilitate optimum performance. ITO is broadly used in touchscreens and handling. In manufacturing ITO has some disadvantages. Its materials are fragile and heavy, and the manufacturing process is labor intensive and expensive. Touchscreens today use 1, 2 or 3 layers of ITO depending on the specific product design considerations and the touch panel supplier technical capabilities. More information about touchscreen layers can be read below.
Manufacturing technologies
In this article, we will be studying touchscreens of iPhone and Samsung Galaxy. Newest iPhone and Samsung Galaxy touch screens have been constructed using several layers of materials. This is called 'stackup' (which can be seen in Figure 1).
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Figure 1. Typical stack sensor with multiple layer technology
The The top layer is a protective layer, which is made from glass with an anti-scratch coating, or PMMA (polymethyl methacrylate), which is also called acrylic or plexiglas. Directly underneath the surface layer is a layer of thin adhesive and then the electrically conductive layers for touch sensing. In Figure 2 we can see 3 different touchscreen examples. They show just how thick each layer actually is.
Figure 2: Example of layering and thickness differences in sensor designs
Figure 3: Typical touchscreen manufacturing process flow
In figure 3, we can see an example of a typical process flow for manufacturing ITO-based sensors. Steps include sputtering ITO powder over glass, thermal baking the ITO to its melting point and creating a conductive layer and then etching the sensing circuit topology on the conductive layer by use of laser lithography or photo. Every one of these steps adds cost as a result of materials cost, manufacturing time, and yield loss.
Sources
- 2010, Materials used for capacitive sensing http://www.cypress.com/?id=4&rID=36843
- 2013, Trevor Davis, Cypress Semiconductor, Reducing capacitive touchscreen cost in mobile phones http://www.embedded.com/design/system-integration/4407698/Reducing-capacitive-touchscreen-cost-in-mobile-phones-
Acoustic camera
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Acoustic camera is camera with an array of microphones. Array of microphones is basically a group of microphones. Acoustic illustrates the source and intensity of the sounds.
Image 1. Acoustic camera with 32 microphones.
How it works?
General
Each microphone in the array records audio stream of their own from specific position on the camera. This creates the microphones amount of streams. The position difference is stored in memory so that other microphones audio data is filtered from other microphones in the array.
Beamforming
Because of the position of the microphone in the array, there is delays in the receiving sounds. It's dependent on the distance of the sound source and the microphone. Delays are adjusted to focus one single point in space. This is called beamforming. Area that is recorded is token as an image. The focusing process is repeated for every point in area which is recorded.
Problems/Challenges
Signal processing in acoustic camera is very hardware heavy and memory heavy. Due to this usually the digital signal processing is done after the recording.
Applications?
Acoustic camera is used to search sound sources and noise reduction. It is used for instance for cars, airplanes, helicopters, trains and structures.
Companies selling acoustic cameras
- http://www.distran.ch/
- http://www.gfaitech.de/en/products/acoustic-camera
- http://www.norsonic.com/en/products/acoustic_camera/Acoustic+Camera+Nor848A.b7C_wtnQYO.ips
- http://www.acoustic-camera.com/
Sources
http://www.metsystem.hu/Companies/Fellner/Acoustic-camera/Image17.jpg (1. Image)
http://blog.kaistale.com/?p=246
http://en.wikipedia.org/wiki/Beamforming
http://en.wikipedia.org/wiki/Acoustic_camera
Ultrasonic sensor
Figure 1
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- http://en.wikipedia.org/wiki/Ultrasonic_sensor
- http://www.ehow.com/how-does_4947693_ultrasonic-sensors-work.html?ref=Track2&utm_source=ask
- http://www.education.rec.ri.cmu.edu/products/nxt_video_trainer2/resources/helpers/nxt_sensors/ultrasonic.html
Position sensors characteristics comparison
Electret microphone
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