The final goal of this project is to be able to detect when a desk has been tapped and also where. The accuracy should be kept as high as possible even though a tradeoff between accuracy and sampling rate is to be expected. The hardware used for the digital signal processing will be the TMDX5535EZDSP USB Stick Development Kit from Spectrum Digital having Texas Instrument's TMS320C5535 processor. If a person is tapping a place on the desk which is closer to a specific piezo, the signal will arrive there earlier since it has a shorter distance to travel inside of the desk. By looking at the time differences to be expected, it is possible to give a rough estimate about the accuracy. Let us suppose that a signal is travelling with the speed of sound Vsound = 300m/s within the material. At a sampling rate of 48k samples per second, the time difference between two samples is Tdiff = 21 micro sec. Within that time a signal is travelling a distance of 6.3mm (Dmin). In order to be able to properly separate the tapping signals from different keys it will be necessary to have a few samples distance between the tapping signals. Since the sampling rate is so low, the minimal measurable distance Dmin is relatively high. A demonstrator keyboard cannot be too big so we are limited to a few keys for our first demonstrator. 25 centimeters seemed to us like an appropriate keyboard size, which results in around 6 usable keys if we allow a gap of 8 sample values between the different key pressing locations. A key will then be around 3 centimeters wide, which is bigger than on conventional keyboards but still good enough for a demonstrator.
Shrihari Deshmukh
Comments