Telegraphy – Systems – Position coordinate determination for writing
Reexamination Certificate
1998-09-10
2001-07-24
Shankar, Vijay (Department: 2673)
Telegraphy
Systems
Position coordinate determination for writing
C178S019030, C178S019040
Reexamination Certificate
active
06265676
ABSTRACT:
FIELD AND BACKGROUND OF THE INVENTION
The present invention relates to digitizer systems and, in particular, it concerns processing algorithms for ultrasound time-of-flight digitizer systems and systems employing such algorithms.
It is know to employ ultrasound time-of-flight measurements to measure distances in a two- or three-dimensional digitizer system. Such systems employ one transmitter which transmits ultrasound pulses which are received by receivers at a number of positions. Alternative configurations employ multiple transmitters with a single receiver. In either case, by measuring the time-of-flight of the pulses from the transmitter to the receiver, the distance between them can be calculated. Synchronization of the transmitter and receiver may be achieved either by a hard wired connection or by a wireless electromagnetic link.
Identification of the beginning of each ultrasound pulse received is non trivial. The reaction characteristic of transducer elements generally used to generate the pulses, together with dispersion of the signal during transit, cause amplitude. As a result, a threshold level set to reliably distinguish the pulse signals over background noise may be triggered at different stages of the pulse. Depending on the transmitter-receiver distance and various environmental conditions, the threshold may be exceeded sometimes during the first cycle, sometimes during the second, and sometimes during the third, leading to considerable imprecision (see FIGS.
1
and
2
). For a typical operating frequency of about 40 kHz and taking the speed of sound to be 330 m/s, each cycle corresponds to a distance of about 8 mm. Such a range of error is unacceptable for typical applications such as digitizers for writing implements, computer mice and the like.
Conventional ultrasound digitizer systems are also somewhat inflexible in their hardware configurations. Typically, a predefined receiver arrangement is produced for each given application. The electronic components are then centralized in a control box. Such arrangements leave little or no flexibility to adapt systems to applications with larger dimensions.
There is therefore a need for systems and methods for accurately tracking variations in distance calculated from time-of-flight measurements of a sequence of pulses of a pressure wave from a transmitter to a receiver. It would also be highly advantageous to provide a modular receiver system in which the number of receiver units may be increased to cover any desired area.
SUMMARY OF THE INVENTION
The present invention is a processing algorithm for ultrasound time-of-flight digitizer systems, and a system employing such an algorithm.
According to the teachings of the present invention there is provided, a method for tracking variations in distance D calculated from time-of-flight measurements of a sequence of pulses of a pressure wave oscillation from a transmitter to a receiver, the pressure wave oscillation having a given wavelength and wave period, the method comprising: (a) identifying a state of synchronous operation by obtaining at least two time-of-flight measurements derived from successive pressure wave pulses which satisfy given synchronicity criteria; (b) monitoring successive time-of-flight measurements to identify a shifted time-of-flight measurement which varies by at least half of the wave period from a predicted time-of-flight value calculated from a number of preceding time-of-flight measurements; (c) identifying a shift factor corresponding to an integer multiple of the wave period by which the shifted time-of-flight measurement must be corrected to obtain a corrected time-of-flight measurement falling within half of the wave period from the predicted time-of-flight value; and (d) correcting the distance D calculated from the pressure wave wavelength.
According to a further feature of the present invention, the time-of-flight measurements are made by a technique configured to identify a predetermined points within a cycle, such as by identifying a first zero crossing of a received signal after the signal has exceeded a given threshold value.
According to a further feature of the present invention, a shifted time-of-flight measurement for which the shift factor exceeds a predetermined maximum value, typically of less than 3, is discarded.
According to a further feature of the present invention, the corrected time-of-flight measurement is employed as previous time-of-flight measurement for the step of monitoring performed on a subsequent time-of-flight measurement.
According to a further feature of the present invention, the state of synchronous operation is identified by obtaining at least three time-of-flight measurements derived from successive pressure wave pulses for which successive time-of-flight measurements vary by less than half of the wave period.
According to a further feature of the present invention, the state of synchronous operation is identified by obtaining at least three time-of-flight measurements derived from successive pressure wave pulses which vary substantially linearly.
According to a further feature of the present invention, the predicted time-of-flight value is calculated by geometrical extrapolation from at least two previous time-of-flight measurements, and preferably by extrapolation of a second order polynomial fitting the previous three time-of-flight measurements.
According to a further feature of the present invention, at least one supplementary shift test is performed, the step of correcting being performed, selectively in response to the supplementary shift test.
The supplementary shift test may include determining an order in which a positive and negative signal amplitude threshold are exceeded, or may include: (a) determining at least one peak signal amplitude occurring after a signal amplitude threshold is exceeded; and (b) calculating whether the peak signal amplitude differs from that of a corresponding peak signal amplitude from a previous pulse by more than a predefined ratio.
According to a further feature of the present invention, the transmitter is associated with a drawing implement which includes a contact switch for identifying operative contact between the drawing implement and a surface, the sequence of pulses being initiated in response to identification of the operative contact, the method further comprising continuing transmission of the sequence of pulses for a given delay period, typically at least about ½ seconds, after the contact switch has ceased to indicate operative contact so as to preserve the state of synchronous operation during intermittent contact.
There is also provided according to the teachings of the present invention, a system for processing timing information and a received signal corresponding to a sequence of pulses of a pressure wave oscillation received by a receiver to track variations in a distance D calculated from time-to-flight measurements of the pulses, the pressure wave oscillation having a given wavelength and wave period, the system comprising: (a) a signal processor for processing the received signal to derive an effective time-of-arrival for each pulse; (b) a timing module associated with the signal processor, the timing module being configured to derive a time-of-flight for each pulse from the timing information and the effective time-of-arrival: (c) a synchronous operation module associated with the timing module and configured to analyze the time-of-flight to identify a state of synchronous operation when at lease two successive pressure wave pulses satisfy predefined synchronicity criteria; (d) a monitoring module associated with the timing module and configured to monitor successive time-of-flight measurements to identify a shifted time-of-flight measurement which varies by at least half of the wave period from a predicted time-of-flight value calculated from a number of preceding time-of-flight measurements; (e) a shift factor module associated with the monitoring module and configured to identify a shift factor corresponding to an integer multi
Serber Ron
Shenholz Gideon
Zloter Zahi
Electronics For Imaging, Inc.
Glenn Michael A.
Shankar Vijay
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