Importance

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Abstract

The disclosure is directed to an apparatus for determining position in a data space. A generally flat tablet is provided, the tablet being formed of a solid acoustically-conductive material. A movable element, such as a stylus, is adapted to be manually movable over the tablet surface, the movable element having a tapered tip which contacts the surface of the tablet. An acoustic transducer is coupled to the tip of the movable element. Fixed transducer means are coupled to the tablet. Means are provided for energizing acoustic energy to propagate in either direction between the transducer of the movable element and the fixed transducer means. The midrange frequency of the energy is selected such that the wavelength of the energy mode propagating parallel to the tablet surface is a function of the tablet thickness. Means are provided for determining the time of propagation of the acoustic energy travelling, in either direction, between the transducer of the movable element and the fixed transducer means. Each time of propagation is indicative of a coordinate position of the movable element. In the preferred embodiment, the tablet is formed of plastic and the fixed transducer means are strips of piezoelectric plastic. In this embodiment, the tip of the stylus is horn-shaped and the material of the tip has an acoustic impedance which is between the acoustic impedance of the piezoelectric transducer and the acoustic impedance of the tablet material. This further facilitates coupling of acoustic energy between the stylus tip and the tablet. In a form of the disclosure, means are provided for determining the amplitude of the acoustic energy which has propagated through the tablet. The pressure of the stylus tip on the tablet controls the amplitude of transmitted or received (as the case may be) acoustic energy. Accordingly, the stylus pressure can be determined from the measured amplitude. The writing pressure can be used to convey information such as the thickness of lines being hand drawn. Adjustment of the amplitude based on the distance travelled through the tablet is achieved using a time-gain compensation technique.

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First Claim