Tracking and characterizing knocks atop large interactive displays

Abstract

Purpose

To describe the development of and experience with a system that tracks the position of knocks and taps atop a large sheet of glass for use as an inexpensive retrofit that can make large windows into interactive interfaces.

Design/methodology/approach

The structural‐acoustic wavefront coming from the impact is simultaneously recorded by four contact piezoelectric pickups mounted near the sheet's corners. A digital signal processor extracts relevant characteristics from these signals, such as amplitudes, frequency components, and differential timings, which are used to estimate the location of the hit and derive other parameters, including a degree of confidence in the position accuracy, the strike intensity and the nature of each hit (e.g. knuckle knock, metal tap, or fist bang – our system responds to any kind of impact). A set of heuristically‐guided rules are employed to compare the waveforms recorded by different sensors and determine the differential timing.

Findings

Across sensitive areas ranging up to 2×2 m, we have obtained position resolutions of σ=2.5 cm for 1/4 in. tempered glass and σ=3‐4 cm for 1 cm thick shatterproof glass. Our system delivers 65 ms latency, hence is essentially real time. The system has been installed in several public settings, and has proven to be very robust.

Research limitations/implications

Suggestions are given for doing everything in software and not using the DSP. Analytical compensation of the dispersion would probably yield better precision but require more computation time.

Practical implications

As this system requires only simple hardware, it needs no special adaptation of the glass pane, and allows all tracking transducers to be mounted on the inner surface, hence it is quite easy and inexpensive to deploy as a retrofit to existing windows. This opens many applications, such as an interactive storefront, with content controlled by knocks on the display window, an interactive museum display case, or a vending machine where one can select by tapping directly above the desired item.

Originality/value

As large displays become less costly and more common, systems like these can make them interactive. This paper details our approach.

Citation

Paradiso, J.A. and King Leo, C. (2005), "Tracking and characterizing knocks atop large interactive displays", Sensor Review, Vol. 25 No. 2, pp. 134-143. https://doi.org/10.1108/02602280510585727