POSITION DETERMINING APPARATUS AND METHOD

Abstract: An apparatus and method are disclosed for determining the position of a movable element (150). An elongated housing (110) is provided for positioning generally adjacent an edge of an area (10) in which the position of the movable element is to be determined. The housing (110) has a base portion (111) which contains a pair of spaced apart transducers (20) and (30) that are mounted in the surface of the base portion and face said area (10). An upper body portion (112) of the housing is disposed above the base portion (111) and protrudes in a cantilevered fashion toward said area (10), so that the transducers (20 and 30) are recessed from said area (10) beneath the protruding upper body portion (112) of the housing. Circuitry is provided for determining the position of the movable element from the respective transit times of energy propagating in either direction between the movable element (150) and the pair of transducers (20 and 30). At least a portion of the circuitry is contained within the upper body portion (112). An additional transducer is mounted in the recessed region beneath the protruding upper body portion (112), for speed of sound determining purposes. Also disclosed are techniques for increasing the accuracy of the position determination, data validation and screening the position determination data.

Description: Description POSITION.DETERMINING APPARATUS AND METHOD FIELD OF THE INVENTION This invention relates to graphical data apparatus and, more particularly, to an apparatus and method for determining the position of a movable element in a data space. BACKGROUND OF THE INVENTION Graphical digitizers are conventionally used to input graphical coordinate information, or the like, to a companion system. In a graphical digitizer, wave energy is typically passed between a movable element (such as a stylus or cursor) and transducers located at fixed reference locations. The transit time of the wave energy traveling (in either direction) between the movable element and the reference locations is used in determining the position of the movable element, typically in terms of digital coordinates. A type of graphical digitizer manufactured and sold by the assignee hereof, Science Accessories Corporation, measures the transit time of acoustic or sonic energy propagating through air. One model of such type of digitizer, called a "GRAPHBAR", employs a pair of "point" microphones, having generally circular receptivity patterns, mounted in spaced relation in an elongated generally rectangular housing. The housing or "bar" can be conveniently moved to a position adjacent an area in which the position of a movable element, containing a sound source, is to be digitized. The transit time of sound traveling from the source to each microphone is used, in conjunction with the speed of sound in air and known geometrical relationships, to compute the position of the movable element. In the described type of digitizer there is a region adjacent the location of the transducers, sometimes referred to as a "dead space", where it is difficult to determine the position of the movable element with sufficient accuracy. This is illustrated in conjunction with Fig. 1 which shows a sonic digitizer that includes a bar 90 in which are mounted a pair of spaced apart microphones 51 and 52. The microphones are mounted near opposite ends of the bar and facing the area 10 to be digitized. [The size and shape of the area 10 is somewhat arbitrary and depends, inter alia, upon the necessary accuracy of the digitizer readings. ] The x and y directions are as shown by the axes 59 in the diagram. Consider the points 1 and 2, which are a distance d apart and at respective distances L and L 2 from microphone 51, and the points 3 and 4 which are also a distance d apart and at respective distances L 3 and L 4 from microphone 51. Geometrical considerations dictate that the difference L 4-L3, will be greater than the distance L 2 -L 1 . This makes it more difficult to accurately determine the y coordinate location of points near the bar 50. Therefore, a "dead space" 11 [whose specific size and shape are determined by desired accuracy] is typically marked off and not used as part of the area in which the position of the movable element is to be accurately located. The "dead space" can be employed for a function such as menu selection, if needed, which does not require high accuracy in two dimensions. In addition to the "dead space" being wasted in many applications, the need to mark it off is a nuisance, and the risk of inaccurate measurements in the "dead space", if the movable element enters this area, is problematic. It is among the objects of the present invention to reduce the problems associated with digitizer "dead space" and to generally improve the efficiency and compactness of digitizer equipment. Graphical digitizer equipments, like most measuring equipments, are susceptible to errors caused by noise and other factors. For example, through-the-air sonic digitizers of the type described above are susceptible to extraneous acoustic noise in the environment, and also to multipath echoes of the sound energy employed by the digitizer equipment itself. Electronic interference or other intermittent phenomena can also lead to substantial digitizing errors. Since digitizers are typically utilized to measure and store data points at a relatively high acquisition rate, and since the acquired data is often immediately used by a companion system, the occurrence of occasional inaccurate coordinate measurements, even grossly inaccurate ones, may not be recognized at all, or until they cause a problem in subsequent processing. The outputting of even occasional incorrect coordinate data can be particularly undesirable for certain applications. Further, when subsequent processing involves averaging of acquired data points, a few grossly inaccurate measurements can result in substantial errors in averaged data that would otherwise be quite acceptable. It is among the further objects of the present invention to reduce or eliminate these type of problems in graphical data digitizers and particularly, although not necessarily, in sonic graphical digitizers. The accurate determination of the transit time of the acoustic energy between the transmitter and receiver locations is critical to an accurate determination of the position of the movable element. Typically, a timer is provided for each receiver. All of the timers are started when the acoustic energy is transmitted from the transmitter. As the sound is received at each receiver, the timer associated with that receiver is stopped. The transit times to each receiver can ' then be computed from the time that elapsed on each timer. Typically, each timer is a digital counter which counts pulses from a digital clock generator, and the arrival of acoustic wave energy at each microphone is determined by continuously comparing the microphone output (e.g. an amplified and filtered version thereof) to a predetermined threshold level. When the threshold level is exceeded, the associated counter is turned off. In the described type of system, a good source of acoustic wave energy pulses is a spark gap which is energized by triggering a circuit that delivers voltage pulses to a pair of closely spaced electrodes which comprise the spark gap. The trigger pulse for this circuitry is also conventionally utilized to initiate the timer or timers that are employed to measure the transit time of the acoustic wave energy over an unknown distance to be determined. [As noted above, the timers are subsequently terminated when the acoustic wave energy is received at one or more respective receivers. The measured elapsed time can be used for determination of distance or, for pilot purposes, by determination of the velocity of sound in air when the transmitter to receiver distance is known.] The spark does not occur immediately upon application of the trigger signal to the spark generation circuitry, so the timer(s) may be initiated somewhat prematurely, resulting in an incorrect elapsed time measurement. This would not necessarily be problematic if one could determine the precise time relationship between application of the trigger pulse and occurrence of the spark, since suitable correction could then be applied to the measured elapsed time. Applicant has found, however, that such solution is generally not adequate, since the time between the trigger and the actual spark can vary considerably. There is a build-up time of the voltage across the electrodes before a spark is produced (generally, at the output of the a transformer which is part of the spark generation circuitry) . The build-up may not be the same for each spark to be generated and, also, the voltage at which a spark is produced can vary over the life of the electrode pair, and can also vary for different electrode pairs. This means that the timing error will tend to vary and cannot be readily accounted for by adding a predetermined timing correction. It is among the further objects of the present invention to provide solution to the timing accuracy problem as set forth. SUMMARY OF THE INVENTION An aspect of the present invention is directed to an apparatus for determining the position of a movable element. In accordance with an embodiment of the invention, an elongated housing is provided for positioning generally adjacent an edge of an area in which the position of the movable element is to be determined. The housing has a base portion which contains a pair of spaced-apart transducers that are mounted in the surface of the base portion and face said area. An upper body portion of the housing is disposed above the base portion and protrudes in cantilevered fashion toward said area, so that the transducers are recessed from said area beneath the protruding upper body portion of the housing. Means are provided for determining the position of the movable element from the respective transit times of energy propagating in either direction between the movable element and the pair of transducers. In a form of the disclosed embodiment, the position- determining means comprises electronic circuitry, and at least a portion of the circuitry is contained within the upper body portion. Also, in a form of the disclosed embodiment, an additional transducer is mounted in the recessed region beneath the protruding upper body portion, for pilot purposes. Means are provided for determining the transit time of energy propagating in either direction between the additional transducer and at least one of said pair of spaced-apart transducers. In this form of the invention, the means for determining the position of the movable element is responsive to both the respective transit times of energy propagating in either direction between the movable element and said pair of spaced-apart transducers and the transit time of energy propagating in either direction between the additional transducer and said at least one of said pair of spaced-apart transducers. The configuration set forth has the advantage of making more efficient use of the "dead space" described above, and of providing a more compact digitizer equipment. Also, the configuration of the invention provides an advantageous location for a pilot transducer. An aspect of the present invention is directed to a method and apparatus for more accurately determining the transit time of acoustic energy travel between a transmitter location and a receiver location. In a disclosed embodiment, an electrode pair spark gap is provided at the transmitter location, and an acoustic receiver is provided at the receiver location. The spark-gap is energized to produce a spark by coupling an electrical potential across the electrode pair. Means are provided for sensing the generation of a spark at the spark gap, and for generating an initializing signal in response thereto. A timer is initialized in response to the initializing signal. Means are provided for detecting, at the receiver location, the receipt of acoustic energy from the spark, and for generating a terminating signal in response thereto. The timer is terminated in response to the terminating signal, and the time measured by the timer is indicative of the transit time of acoustic energy travel between the transmitter and receiver locations. In a preferred embodiment of this form of the invention, the means for sensing the generation of a spark at the spark-gap is operative to sense a current coupled to the electrode pair. In this embodiment, the means for energizing the spark gap includes: a transformer having primary and secondary windings,-the electrode pair being coupled across the secondary winding; a capacitor coupled cross the secondary winding; and means for applying a voltage pulse to the primary winding. Also in this embodiment, the means for sensing a current coupled to the electrode pair comprises a transformer coupled to a conductor which couples one of the electrodes of the electrode pair to the secondary winding. This aspect of the present invention has application to any technique or apparatus wherein it is desirable to determine, accurately and consistently, the transit time of acoustic energy generated at a spark-gap; for example, two- dimensional acoustic digitizers, three-dimensional acoustic digitizers, and one-dimensional acoustic distance or velocity determination systems. A further aspect of the present invention employs data validation and screening based on the proximity of sequentially measured data points. Another form of this aspect of the invention detects noisy conditions manifested in a pilot signal measurement, and discards data taken just after (and/or, if desired, just before) detection of the condition. In an illustrated embodiment of this aspect of the invention, the position of a moveable element is determined in two dimensions, but it will be understood that this aspect of the invention has application to position determination in one, two, or three dimensions, which utilizes any desired ' number or configuration of transmitters and receivers. Further features and advantages of the invention will become more readily apparent from the following detailed description when taken in conjunction with the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a diagram illustrating why the accuracy of position determination is relatively low in a particular area for a certain type of existing digitizer equipment. Fig. 2 is an elevational perspective view of an apparatus in accordance with an embodiment of the invention. Fig. 3 is schematic diagram, partially in block form, of prior art circuitry which can be utilized in conjunction with an embodiment of the invention. Fig. 4 is an end perspective view of an apparatus in accordance with an embodiment of the invention. Fig. 5 is a side and bottom perspective view of an apparatus in accordance with an embodiment of the invention. Fig. 6 is an end view of another form of an apparatus in accordance with an embodiment of the invention. Fig. 7 is a schematic diagram of the spark generation and sensing circuitry in accordance with an embodiment of the apparatus of the invention and which can be employed in practicing an embodiment of the method of the invention. Fig. 8, which includes Fig.s 8A and 8B placed one below another, is a flow diagram of a routine for programming a processor in accordance with practicing an embodiment of the invention. DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to Fig. 2, there is shown an apparatus in accordance with an embodiment of a first aspect of the invention for determining the position of an element movable in a region located to one side of the apparatus 100 and preferably, although not necessarily, within the dashed region 10. The apparatus incudes an elongated housing 110 which is positioned generally adjacent an edge of the region in which the position of a movable element 150 is to be determined. The housing 110 has a base portion 111 which contains a pair of spaced apart transducers 20 and 30 that are mounted in a surface 118 of the base that faces the region 10. The housing 110 has an upper body portion 112 which is disposed above the base portion and which protrudes, in cantilevered fashion, toward the region 10, so that the transducers 20, 30 are recessed, with respect to the region 10, beneath the cantilevered upper body portion 112 of the housing 110. In the present embodiment, the front surface 113 of the upper body portion 112 is approximately above the edge of the region 10. In this manner, if desired, the upper body portion can serve to prevent a suitably configured movable element 150, such as a stylus, pen, finger, cursor, or the like, from entering the area 11 beneath the protruding upper body portion 112. As described in the background portion hereof, the area directly adjacent the transducers in an existing bar-type digitizer is generally not utilized since the determination of position in this area may lack sufficient accuracy. The present invention has the advantage of eliminating the need for marking off a "dead space" adjacent the digitizer apparatus, as well as preventing unintended positioning of the movable element in the "dead space". Further toward this end, and as shown in Fig. 4, a suitable sound-transmitting screen 140 can be provided to enclose part or all of the region beneath the upper body portion 112. The screen may be, for example, of plastic mesh, and will preferably permit free circulation of air. A further advantage of the present invention is that the upper body portion can, if desired, be utilized to contain a portion of the electronics used in the position determination function. In this manner, the so-called dead space is not wasted, and the housing 110 can have a compact configuration. In one form of the embodiment of Fig. 2, the transducers 20 and 30 are acoustic receivers, such as point microphones, and the movable element 150 is a stylus (or cursor puck, or other suitable device), which contains a transducer for producing acoustic wave energy. Techniques for determining the position of a movable element sound emitter with respect to a pair of receivers, such as point microphones, are well known in the art. Briefly, however, and as illustrated in Fig. 3, the travel time duration is determined by circuitry 40, shown for convenience in dashed line to the rear of bar 90, which comprises a left counter 42, associated with the left microphone 20, a right counter 43 associated with the right microphone 30, a clock 44, and a spark generator circuit 41. Coincident with generation of the spark at movable element 150, the counters 42 and 43 are enabled by a gating signal from the spark generator circuit to begin counting pulses from clock 44. Upon initial reception of the sound wavefront, the microphones 20 and 30, which generally receive the wavefront at different times, produce output voltages which are coupled to high gain band pass amplifiers 21 and 31, respectively. The spark shock wave produces a fast rise time electrical impulse upon impinging on the microphone surface, and the band pass amplifiers allow only the fast rise time portion of the electrical pulse to pass while blocking out noise signals outside the band. To insure rapid operation, the amplifiers include threshold discriminators which provide an output pulse with a steep leading edge in response to the input thereto exceeding a predetermined level. The amplifier outputs are operative to disable the counters 42 and 43 and also to read out the respective counts which are indicative of the travel times between the sound source on the movable element and the microphones. The respective distances can then be computed, in known manner, by multiplying the travel times by the velocity of sound in air. This can be implemented, for example, by computing module or processor 200, or by any suitable dedicated or general purpose processor. Fig. 5 illustrates an embodiment of a further feature of the first aspect of the invention which utilizes a third fixed transducer 123 mounted on the housing 110 for the purpose of obtaining a velocity-representative signal that is used in deriving more accurate digitizer position determinations. As is well known in the art, the speed of sound through air varies substantially with the temperature of the air, and acoustic digitizers can utilize a measurement between fixed distances, sometimes called a pilot measurement, to obtain temperature compensated digitizer outputs. The fixed distance can be obtained, for example, by placing the movable element at a known position before taking a pilot measurement. This has the disadvantage of requiring a time-consuming manual operation. Also, since subsequent measurements are taken after significant time has passed, changes in conditions can occur, which would reduce the effectiveness of the pilot measurement. Another known technique is to utilize a second sound source (or receiver, if the fixed transducers are sources) which is at a fixed position with respect to the receivers. However, this gives rise to the problem of where to place the further fixed transducer so that it will not be obtrusive, and so that it will not interfere in any way with operation of the movable element of digitizer. In the Fig. 5 embodiment, a further fixed transducer 123 is positioned on the bottom wall of the protruding upper body portion 112, and this solves the aforementioned problems while, again, making use of the so-called "dead space" and not interfering with normal digitizer operation. In operation (see also Fig. 3), a spark gap can be provided as transducer 123. A spark generator (such as 41) can energize the spark gap 123 and clock pulses (such as from clock 44) are counted by a counter (e.g. 43 coupled to microphone 30) until the counter is disabled by arrival of the sound wavefront at microphone 30. The count represents the transit time of the sound wavefront. The speed of sound in the present air environment can then be computed by dividing the known distance (between fixed source 123 and microphone 30) by the obtained transit time. This speed of sound can then be utilized in the above-referenced distance computations for the movable element. The pilot measurements can also be used for determining the validity of position measurement data, as will be described hereinbelow. It will be understood that the pilot measurements can be made as frequently as desired. The embodiment of Fig. 2 was illustrated in terms of a position determining apparatus in which the movable element includes a sound source, and the transducers 20 and 30 are sound receivers. It will be understood, however, that, if desired, either or both of the transducers 20 and/or 30 can be utilized to transmit acoustic energy. In such case, the movable element can be utilized as a receiver, thereby reversing the mode of operation which was first described. As is known in the art, the transmitters can be sequentially energized, and the distance between each transmitter and the receiver in the movable element can be computed in the manner previously described. From this information, and known trigonometric relationships, the position of the movable element can be determined. In a still further variation, the movable element can be a passive reflector of acoustic energy. In this regard, see, for example, U.S. Patent No.s 4,012,588 or 4,124,838, assigned to the same assignee as the present application. In such case, one or both of the transducers 30, 40 could be used as a transmitter as well as a receiver. If desired, a separate transmitter can also be employed. It will also be understood that the pilot transducer 123 can be a receiver when the transducers 20 and/or 30 are transmitters. Further, the shape contours of the housing and the protruding upper body portion can be varied to some degree while retaining the indicated advantages of the invention. Also, additional structure can be provided for support, balance, or other purposes. For example, in Fig. 6 a base panel 105 is provided and, if desired, the front thereof can be used for a menu selection function. Fig. 7 illustrates an embodiment of the spark generator circuit 41 as improved in accordance with an aspect of the present invention, and which can be used in practicing an embodiment of a method in accordance with the invention. In the embodiment of Fig. 7, a supply voltage, V s , is utilized to charge a capacitor C via a resistor R χ . The capacitor has a discharge path through the primary winding of a transformer T and a silicon controlled rectifier (labeled SCR), when the SCR is conductive. As in known in the art, trigger pulses are applied, at appropriate times, to the trigger the gate electrode g of the SCR to render the SCR conductive and cause a pulse of relatively high voltage across the transformer secondary winding. When the SCR turns off, the capacitor can again be charged and awaits the next trigger pulse. The circuit, as just described, is known in the art, and it can be noted that prior art systems typically also utilize the trigger signal, or a signal derived therefrom, to initialize the counters 42 and 43, as first described above. In the present embodiment, the secondary winding of the transformer T χ is coupled, via a filter 210 and cable 220, to a spark gap electrode pair 225 which is illustrated as being at the tip of a stylus 150 (as in Fig.s 2 and 3). The filter 210 comprises series resistors R 2 and R 3 , and a capacitor C 2 in parallel with the spark gap. In this embodiment, the current to the spark gap is sensed, without conductive coupling, by utilizing a transformer 250. In an operating embodiment hereof, a twin-hole balun core was employed for this purpose. One of the conductors that is coupled to cable 220 is passed through a hole of the balun core 250. A further conductor 260 is passed through the other hole of the balun core. One end of conductor 260 is coupled to ground reference potential, and the other end is coupled, via a diode Ω 1 and a resistor R 4 , to the gate electrode of a field-effect transistor Q 1 . The gate electrode of Q 1 is also coupled, via resistor R s , to ground reference potential. In the present embodiment, the drain electrode of Q 1 is coupled to a positive bias voltage V + via a resistor R 6 , and the source electrode of Q. is coupled to ground reference potential. An output 270, which is taken at the drain electrode of transistor Q , is coupled to the enable inputs of counters 42 and 43 (as in Fig. 3). In operation, the network comprised of R_, R 3 and C 2 forms low pass filter 210, which limits the transformer secondary current at breakdown [i.e., when there is arcing across the spark gap electrode pair] . C 2 discharges very quickly when the arc is initiated, and a very short steep current pulse flows from C 2 into the cable 220 at the onset of the arc. The magnitude of the pulse depends on the value of C 2 , the breakdown voltage, and the speed of breakdown. The occurrence of this current pulse indicates, with good precision, the time at which the arc occurs and it is sensed, in the present embodiment, to develop a signal that is consistently related to the time of onset of the acoustic wave energy caused by the spark. The current pulse in the conductor passing through transformer 250 induces a corresponding pulse in conductor 260. This signal, applied to the gate electrode of Q χ , turns Q 1 on and causes the output voltage at 270 to go from V + to substantially ground reference potential for as long as Q 1 is on. [Of course, if a positive-going signal rather than a negative-going signal is desired for enabling the clocks 42, 43, the output on 270 can be suitably converted, or a suitable circuit which directly generates a positive-going signal can be employed.] The very short pulse from the secondary of transformer 250 charges the electrode capacitances of the field-effect transistor Q 1 which "stretch" the output while discharging through R 5 . [If these capacitances are not adequate to sufficiently "stretch" the pulse, a small capacitance can be added between ground reference and the junction of D 1 and R 4 . This will, however, subject the diode to a higher reverse voltage at the end of the pulse.] The series resistor, R 4 , limits the peak charging current and prevents the high peak voltage from appearing at the gate electrode. The diode D 1 may be, for example, a Schottky-barrier diode with a fast reverse recovery time. It will be understood that other suitable circuits could be used for detecting the spark onset. In an aspect of the invention to be described next, two types of digitizer data validity determinations are used together, although it will be understood that either form can be used to advantage without the other. Referring to Fig. 8, there is shown a flow diagram of a routine for control of the processor 200 in accordance with an embodiment of the invention. The processor may comprise, for example, an IBM-PS2, together with conventional associated memory, timing, input/output and display functions (not shown) . The diamond 802 represents inquiry as to whether the data validity mode is active. If not, the routine is exited. The routine can then be re-entered, such as after a suitable interrupt resulting from an operator selection or periodic inquiry from another routine. It is assumed that the current routine will generally be implemented when a continuous data type of mode is being used at the digitizer, although it will be understood that the routine can be employed in conjunction with any data mode. The routine is illustrated as being entered via a suitable interrupt (e.g., when a continuous run mode is entered), or via operator control. If the data valid test mode is operative, inquiry is made (diamond 805) as to whether the mode was just rendered operative. If so, " the block 808 is entered, this block representing the clearing of registers used for temporary storage of data points in conjunction with the data validity determinations and the clearing of any previously set flags. An initializing status flag is then set, as represented by the block 810. The decision diamond 813 is then entered (and is also entered from the "no" output branch of diamond 805), and inquiry is made as to whether the data currently being sought is pilot data or movable element data (i.e., stylus, cursor, or any movable element data) . [For ease of explanation, movable element data will be referred to as stylus data for this part of the description.] In the present illustrated embodiment, it is assumed that pilot and stylus data are alternately obtained, under control of processor 200, such as by alternately energizing the stylus 150 and the pilot transducer 123 of Fig. 5. Also, in the illustrated embodiment it is assumed that no stylus data point will be deemed valid until both types of validity checks described herein, viz. validity based on the pilot data and validity based on the relative positions of adjacent stylus data, are satisfied. If pilot data is currently being sought, the data is awaited (diamond 815, loop 816, or arrows 817 if interrupted and returned during the wait) and, when the measurement data has been received the counter time of the counter used for the pilot (43 in Fig. 2) is tested (diamond 820) to determine whether it is in a predetermined acceptable range. The range of travel times may be determined, for example, from the travel time of sound over the fixed pilot distance at the lowest and highest expected operating air temperatures, for a 'through-the-air digitizer. If the pilot measurement is within acceptable range, a pilot acceptability indicator bit is set high (block 825), whereas if the measurement is outside the range, the pilot acceptability bit is set low (block 827). The diamond 813 is then re-entered. If the inquiry of diamond 813 indicates that stylus measurement data is currently being sought, the data is awaited, as represented by diamond 830, loop 831, and arrows 832 which, again, indicate that interrupts can be used for performance of other functions during the wait. When the stylus measurement data arrives, the counts of counters 42 and 43 are used, in known fashion, to compute the stylus position, as represented by the block 835. Inquiry is then made (diamond 837) as to whether the initial status flag is set. If so, inquiry is made (diamond 840) as to whether the pilot acceptability bit is high. If so, the data coordinate values are stored in a previous point register (block 842), the initial status flag is turned off (block 845), and the diamond 813 is re-entered. If, however, the pilot acceptability bit is low, the computed stylus position is not stored, and the diamond 813 is re-entered directly. [Thus, in the absence of an acceptable pilot,the initial status is maintained, and no data will be read-out or stored for comparison. ] In the situation where the initial status flag is not set (the "no" branch of diamond 837), inquiry is again made (diamond 850) as to the pilot acceptability bit status. If the pilot acceptability bit is low, the previous point register is cleared (block 852), the initial status flag is set (block 853), and the diamond 813 is re-entered. [Thus, in the absence of an acceptable pilot, the initial status is re-established and no data will be read-out or stored for comparison. ] If the pilot acceptability bit is high, the block 860 is entered, this block representing the computation of the distance between the current stylus data point and the previous data point stored in the previous point register. If the current point coordinates are (x c , y ) and the previous point coordinates are (x , y ) , then the computed distance will be D = [(x c - x p ) 2 +(y c - y p 2 )] 1/2 . [For a three dimensional system, there will be a similar z term.] Inquiry is then made (diamond 862) as to whether the distance D is less than or equal, to the maximum acceptable movement distance, designated D Q . The acceptable region around the prior point (i.e., a circle of radius D o for this embodiment) can be preselected and/or made operator adjustable, and depends on the maximum expected stylus movement in the time interval between sequentially adjacent stylus data points. If D is within acceptable range, the current point is read-out (and/or placed on a valid data list), as represented by the block 865. Also, the current data point replaces the one stored in the previous point register (block 867). If D is not within the acceptable range, the previous point register is cleared (block 870), the initial status flag is set (block 873), and diamond 813 is re-entered. Thus, after an invalid data point, the validation process starts from scratch in the present embodiment. The structures of Fig.s 1-6 illustrated in terms of a position determining apparatus in which the movable element includes a sound source, and the transducers 20 and 30 are sound receivers. It will be understood, however, that, if desired, either or both of the transducers 20 and/or 30 can be utilized to' transmit acoustic energy. In such case, the movable element can be utilized as a receiver, thereby reversing the mode of operation which was first described. As is known in the art, the transmitters can be sequentially energized, and the distance between each transmitter and the receiver in the movable element can be computed in the manner previously described. From this information, and known trigonometric relationships, the position of the movable element can be determined. In a still further variation, the movable element can be a passive reflector of acoustic energy. In such case, one or both of the transducers 30, 40 could be used as a transmitter as well as a receiver. If desired, a separate transmitter can also be employed. It will also be understood that the pilot transducer 123 can be a receiver when the transducers 20 and/or 30 are transmitters. The invention is applicable to all of these situations, as well as to other types of digitizers. In the just illustrated embodiment, current points are compared with previously acquired points, and either outputted or not. However, if desired, after comparison against a current point, a previously stored point can be read out or outputted, or a combination of both techniques could be utilized. Also, it will understood that comparison of three or more points could also be utilized to localize the moveable element and determine the region of acceptability. If desired, the allowable region in which adjacent points should be located can be non-symmetrical. For example, if the direction of motion is computed from previous points, more leeway could be permitted for the next point in the general direction of motion, consistent with the physics of hand movement. Further along these lines, the velocity (rate of change of position) of motion could also be used in determining the acceptable region for an adjacent point. Also, it will be understood that while the disclosed embodiment uses a computation of distance moved based on computed coordinates, the sequentially obtained slant ranges could be directly compared, if desired. Further, it will be understood that suitable indices can be associated with received points to keep track of points which are read out and/or those which are screened out. Visual and/or audio indicators can also be generated whenever invalid data is detected.

Claims: CLAIMS : 1. Apparatus for determining the position of a movable element, comprising: an elongated housing for positioning generally adjacent an edge of an area in which the position of the movable element is to be determined, said housing having a base portion which contains a pair of spaced-apart transducers that are mounted in the surface of said base portion and face said area, said housing having an upper body portion which is disposed above said base portion and which protrudes in cantilevered fashion toward said area, so that said transducers are recessed from said area beneath the protruding upper body portion of said housing; and means for determining the position of said movable element from the transit times of energy propagating in either direction between said movable element and said transducers. 2. Apparatus as defined by claim 1, wherein said energy is acoustic energy. 3. Apparatus as defined by claim 2, wherein said position-determining means comprises electronic circuitry, at least a portion of which is contained within said upper body portion. 4. Apparatus as defined by claim 2 or 3, further comprising a sound-transmitting screen on the lower front of said housing and covering at least part of the recessed portion thereof. 5. Apparatus as defined by claim 2 or 3, wherein said movable element includes a sound emitter, said transducers are sound receivers, and said means for determining the position of said movable element is operative to compute the transit time of acoustic energy propagating from said sound emitter to said sound receivers. 6. Apparatus as defined by claim 2 or 3, further comprising an additional transducer mounted in the recessed region beneath said protruding upper body portion; means for determining the transit time of energy propagating in either direction between said additional transducer and at least one of said pair of spaced-apart transducers; said means for determining the position of said movable element being responsive to both the respective transit times of energy propagating in either direction between said movable element and said pair of spaced-apart transducers and the transit time of energy propagating in either direction between said additional transducer and said at least one of said pair of spaced-apart transducers. 7. A method for determining the transit time of acoustic energy travel between a transmitter location and a receiver location, comprising the steps of: providing an electrode pair spark gap at the transmitter location; providing an acoustic receiver at the receiver location; energizing the spark gap to produce a spark by coupling an electrical potential across said electrode pair; sensing the generation of a spark at the spark gap, and generating an initializing signal in response thereto; initializing a timer in response to said initializing signal; detecting, at the receiver location, the receipt of acoustic energy from the spark, and generating a terminating signal in response thereto; and terminating the timer in response to said terminating signal; whereby the time measured by the timer is indicative of the transit time of acoustic energy travel between said transmitter and receiver locations. 8. The method as defined by claim 7, wherein said step of initializing a timer comprises initializing a digital counter which counts clock pulses, said measured time being a digital count. 9. The method as defined by claim 7 or 8, wherein said step of sensing the generation of a spark at the spark gap comprises sensing a current coupled to the electrode pair, said initializing signal being generated in response to the sensed current. 10. A method for determining the position of a movable element in a data region, comprising the steps of: providing an electrode pair spark gap at a transmitter location; providing a plurality of acoustic receivers at respective receiver locations; energizing the spark gap to produce a spark by coupling an electrical potential across said electrode pair; sensing the generation of a spark at the spark gap, and generating an initializing signal in response thereto; providing a plurality of timers associated with respective ones of the acoustic receivers; initializing all of said timers in response to said initializing signal; detecting the receipt of acoustic energy from the spark at each of the acoustic receivers, and generating respective terminating signals in response thereto; terminating the timers in response to the respective terminating signals; and computing the position of the movable element from the times measured by the timers. 11. The method as defined by claim 10, wherein said step of initializing all timers comprises initializing respective digital counters, each of which counts clock pulses, said measured times being digital counts. 12. The method as defined by claim 10 or 11, wherein said step of sensing the generation of a spark at the spark gap comprises sensing a current coupled to the electrode pair, said initializing signal being generated in response to the sensed current. 13. Apparatus for determining the transit time of acoustic energy travel between a transmitter location and a receiver location, comprising: an electrode pair spark gap at the transmitter location; an acoustic receiver at the receiver location; means for energizing the spark gap to produce a spark by coupling an electrical potential across said electrode pair; means for sensing the generation of a spark at the spark gap, and for generating an initializing signal in response thereto; a timer which is initialized in response to said initializing signal; and means for detecting the receipt of acoustic energy from the spark at the receiver location and for generating a terminating signal in response thereto, the timer being terminated in response to said terminating signal; whereby the time measured by the timer is indicative of the transit time of acoustic energy travel between said transmitter and receiver locations. 14. Apparatus as defined by claim 13, wherein said timer comprises a digital counter which counts clock pulses. 15. Apparatus as defined by claim 13 or 14, herein said sensing means includes means for sensing a current coupled to the electrode pair. 16. Apparatus as defined by claim 13 or 14, wherein said means for energizing the spark gap comprises: a transformer having primary and secondary windings, said electrode pair being coupled across said secondary winding; a capacitor coupled across said secondary winding; and means for applying a voltage pulse to said primary winding. 17. Apparatus for obtaining and outputting the position of a moveable element in a data space, comprising: an acoustic digitizer subsystem for determining the position of said movable element by measuring the transit time of acoustic wave energy traveling, in either direction, between said movable element and a plurality of known locations, said subsystem including a pair of transducers which are spaced a known distance apart, and means for obtaining a pilot value for the velocity of sound in air from the transit time of acoustic wave energy traveling between said pair of transducers; said subsystem being operative to produce position measurement values representative of the position of said movable element as a function said transit times and said pilot velocity value; means for determining whether said pilot velocity value is within a predetermined acceptable range, and for generating a pilot acceptability indication from said determination; and means for outputting said position measurement values; and means responsive to said pilot acceptability indication; for disabling the outputting of said position measurement values when said pilot velocity value is outside the acceptable range. 18. Apparatus as defined by claim 17, wherein said subsystem is operative to alternately obtain pilot velocity values and position measurement values. 19. Apparatus as defined by claim 17 or 18, wherein said means for disabling the outputting of said position measurement values when said pilot velocity value is outside the acceptability range is operative to continue said disabling function until an acceptable pilot velocity value is received. 20. Apparatus as defined by claim 17 or 18, further comprising: means for computing the distance between determined time sequential positions of the moveable element; and means for disabling the outputting of said position measurement values when said computed distance is outside a predetermined range. 21. Apparatus as defined by claim 17 or 18, wherein said acoustic digitizer subsystem includes an acoustic source on said movable element and acoustic receivers at said known locations. 22. Apparatus for obtaining and outputting the position of a moveable element in a data space, comprising: an acoustic digitizer subsystem for determining the position of said movable element by measuring the transit time of acoustic wave energy traveling, in either direction, between said movable element and a plurality of known locations; means for outputting said position measurement values; means for computing the distance between determined time sequential positions of the moveable element; and means for disabling the outputting of said position measurement values when said computed distance is outside a predetermined range. 23. For use in conjunction with an acoustic digitizer subsystem for determining the position of a movable element in a data space by measuring the transit time of acoustic wave energy traveling, in either direction, between said movable element and a plurality of known locations, said subsystem including a pair of transducers which are spaced a known distance apart, and means for obtaining a pilot value for the velocity of sound in air from the transit time of acoustic wave energy traveling between said pair of transducers; said subsystem being operative to produce position measurement values representative of the position of said movable element as a function of said transit times and said pilot velocity value; a method for improving operation of said subsystem, comprising the steps of: determining whether said pilot velocity value is within a predetermined acceptable range, and generating a pilot acceptability indication from said determination; outputting said position measurement values; and disabling, in response to the pilot acceptability indication, the outputting of said position measurement values when said pilot velocity value is outside the acceptable range. 24. The method as defined by claim 23, wherein said disabling of the outputting of said position measurement values when said pilot velocity value is outside the acceptability range continues until an acceptable pilot velocity value is received. 25. The method as defined by claim 23 or 24, further comprising the steps of: computing the distance between determined time sequential positions of the movable element; and disabling the outputting of said position measurement values when said computed distance is outside a predetermined range. 26. For use in conjunction with an acoustic digitizer subsystem for determining the position of a movable element by measuring the transit time of acoustic wave energy traveling, in either direction, between said movable element and a plurality of known locations; a method for improving operation of said subsystem, comprising the steps of: outputting said position measurement values; computing the distance between determined time sequential positions of the movable element; and disabling the outputting of said position measurement values when said computed distance is outside a predetermined range.

Inventors: BUTLER ROBERT, SCHORUM STANLEY, SINDEBAND SEYMOUR J, BUTLER, ROBERT, SCHORUM, STANLEY, SINDEBAND, SEYMOUR, J.
Applicants: SCIENCE ACCESSORIES CORP, SCIENCE ACCESSORIES CORP.
Application year: 1991
Main classification: G08C21
IPC codes: G01B21, G06F3, G08C21