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:

http://www.embedded.com/design/connectivity/4401019/1/Capacitive-sensing-for-advanced-user-interfaces
2. Larry K. Baxter (1996). Capacitive Sensors. John Wiley and Sons. p. 138. ISBN 978-0-7803-5351-0.

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.