1. FTIR multi-touch screen
P12302
FTIR multi-touch screen /
Brian Rella(EE)
Andrew Middleton (EE)
Hyun Jin Kim (EE)
Gerald Garavuso (Team Guide)
Mark Baily (Assistance)
Alex Coleman (Assistance)
Objective
The objective of this project is to design and implement a photodiode array for use in an FTIR multi-touch device. The design must also incorporate all the necessary circuitry to convert the data collected (IR light striking the photodiodes) into a digital matrix and to send the data to a PC via a USB connection.
The main concern will be to increase the SNR in order to improve reliability and performance by increasing the immunity to ambient IR light sources. To achieve this, one or more techniques will be implemented.
Requirement
1. 8x8 implementation of passive pixel array
2. An embedded signal designed to differentiate
the touch signal from background noise
3. Minimum of 5mm interpolated touch precision
4. Detect 6 simultaneous finger placements
5. Complete scan of array within 10ms
6. Improvement of Signal to Noise Ratio
7. Product shall communicate over a USB connection
Specification
Testing method
The circuit will be comprised on a photodiode, infrared LED, and a resistor and assembled on a breadboard.  A varying frequency sinusoidal input signal will be fed to the LED through the use of a waveform generator.  The output from the photodiode will be observed with an oscilloscope and the waveform will be captured for each frequency input. The photodiode will be directly angled toward the photodiode.  The setup will be placed inside a dark enclosed box to reduce as much background infrared radiation (ambient light) as possible. The outputted signal will be used for harmonic Fourier analysis.
Result
Low Level (Florescent) IR Noise
-The low level IR from the florescent room lighting was used. The data array was converted to greyscale for visual comparison.
Reference frame (smooth) signal frame (smooth) Double Correlated frame (smooth)
Testing Configuration
SNR improvement was tested using double correlated sampling. Known levels of IR Noise were introduced and filtered from a known IR Signal. The Noise was increased to a maximum in order to test the limits of capabilities of the project.
Medium Level (Normal) IR Noise
-A grow light was introduced to the setup to increase the level of IR Noise. A sheet of paper was held between the grow light and the photodiode array, to create a more common level of natural IR background noise. There was approximately a 2 feet distance between the grow light and the array.
Reference frame (smooth) signal frame (smooth) Double Correlated frame (smooth)
High Level (Saturated) IR Noise
-The sheet of paper was removed so that the grow light saturated the photodiode array, simulating maximum background IR noise.
-The physical IR LED blocked the grow light from saturating photodiodes in the line of sight, which enables the correlated double sampling to be effective. This is comparable to the effects of a finger touch in an FTIR touch screen configuration.
PCB layout
Reference frame (smooth) signal frame (smooth) Double Correlated frame (smooth)
Discussion - SNR Improvement
-The detection of the IR LED Signal despite any the range of IR Noise was achieved, through correlated double sampling.
-The detection of the signal during a saturated level was achieved unlike in an FTIR configuration, where signal detection is impossible.