Position sensors

Capacitive sensors

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

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

How does capacitive sensor detect touch?

Figure 3

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.

Sources:

1. http://www.embedded.com/design/connectivity/4401019/1/Capacitive-sensing-for-advanced-user-interfaces
2. 2. Larry K. Baxter (1996). Capacitive Sensors. John Wiley and Sons. p. 138. ISBN978-0-7803-5351-0.
3. http://www.capacitive-sensing.com/ More about capacitive sensors. Math and applications.
4. http://www.analog.com/library/analogdialogue/archives/40-10/cap_sensors.html

Interface electronic circuits

AD7142:  PROGRAMMABLE CONTROLLER FOR CAPACITANCE TOUCH SENSORS

AD7142 is connected to the sensor with excitation source and CIN wires. Sensor itself is integrated to the circuit board.

Figure 1

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

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

What is the electrical input impedance of the sensor?

Maximum Output Load 250 pF(Capacitance load on source to ground).

On which parameters does it depend on? e.g. frequency, temperature, etc.

Capacitance sensor output levels are sensitive to temperature, humidity, and in some cases, dirt.

 Does the signal need amplification?

Epic Figure 4

Does the sensor need excitation current or voltage? How much?

It uses 250kHZ excitation source, but there is no mention of current or voltage.

Figure 5

What other analog electronics are needed for the signal conditioning?

It has two DACs(Digital-to-analog converter) to null any capacitance sensor offsets.

How the signal is converted to digital format?

It uses capacitance-to-digital(CDC) converter which is Σ-Δ architecture with 16-bit resolution. It has a      sampling frequency of 250 kHz.

Does the A/D conversion happen already in the sensor or is it typically converted in PC or embedded system?

It happens on the circuit board. It is possible to program two DACs.

How much there is noise? And what kind of?

It has parasitic capacitance of 40 pF.

How is the sensor calibrated?

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

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.

Sources

1. http://www.analog.com/en/analog-to-digital-converters/capacitance-to-digital-converters/ad7142/products/product.html AD7142, PROGRAMMABLE CONTROLLER FOR CAPACITANCE TOUCH SENSORS
2. http://www.analog.com/static/imported-files/data_sheets/AD7142.pdf AD7142 datasheet

Acoustic camera

What is acoustic camera?

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

Ultrasonic sensors emit a high frequency sound waves and evaluet the echo which is received back by the sensor. The principle in ultrasonic sensors is pretty much the same as on radars and sonars, after sending the sound wave, the sensor calculates the time it takes between sending the signal and receiving the echo to determine how far the object is from the source. Capacitive sensors are also called as transceivers/transducers, and this is because the sensor sends and receives at the same time.

Figure 2

The formula for calculating how far the object is, is really simple, basically you just measure the time it takes for the pulse to travel, multiply it with the speed of sound which is 341m/s in air, and divide the whole thing with 2, since the pulse travels from the sensor to the object and back.

Figure 3

One of the drawbacks is that the ultrasonic sensor is that there is no way of telling the difference between small and large objects because the pulse that's emitted is cone shaped, because of the shape, an echo will be returned by all objects the pulse comes into contact with. One way to work around this is to use rotating sensor or multiple sensors to find out the shape and objects size.

Figure 4 (Ultrasonic pulse from the transducer sensor)

In industry, ultrasonic sensors are used  to detect movement of targets and to measure the distance to targets in many automated factories. As for example, they can be used to sense the edge of material as part of a web guiding system.

Sources:

1. http://en.wikipedia.org/wiki/Ultrasonic_sensor
2. http://www.ehow.com/how-does_4947693_ultrasonic-sensors-work.html?ref=Track2&utm_source=ask
3. http://www.education.rec.ri.cmu.edu/products/nxt_video_trainer2/resources/helpers/nxt_sensors/ultrasonic.html

Position sensors characteristics comparison

Electret microphone

Capacitive sensors 

Touch Switch D5C

D5C sensor is mainly used in the industry.

Figure 1

In figure 1 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.

Below is all the information I could find about D5C sensor.

D5C_Datasheet.pdf