BERUEHRUNGSEMPFINDLICHER POSITIONSKODIERER UNTER VERWENDUNG EINER PLATTE VON SCHICHTFOERMIGER STRUKTUR

1436688 Position determination CANADIAN PATENTS & DEVELOPMENT Ltd 28 June 1974 [19 July 1973] 28914/74 Heading H4D An apparatus for determining the position co-ordinates of a finger or passive stylus placed on a sheet, by timing surface acoustic wave pulses propagated over the surface of the sheet and reflected from the finger or stylus, the sheet has attached or bonded to it a layer of material capable of absorbing bulk acoustic waves which would otherwise interfere with the determination. Embodiments described use glass, fused quartz or metal for the sheet, and a plastics material or metal-powder-loaded epoxy resin as the absorbing layer. The layer may be sandwiched between two glass sheets. Fig. 5 shows transducer arrays 11a, 11b, 12a, 12b at the sheet edges and coupled through multiplexing switches 17 to a pulser 16 and receiver 18 so that the arrays can be used sequentially to avoid mutual interference. The gain of receiver amplifier 19 is swept to compensate for rangedependent attenuation of received signals. Pulse-echo times, determined by gating clock pulses from oscillator 24 to up-down counter 30, are routed to a computer via X and Y coordinate registers 25, 26. Control logic 27 enables selection of a continuous mode, in which co-ordinate data are determined repetitively while stylus contact is maintained, or a discrete mode in which the stylus must be lifted and repositioned to initiate each determination.