POSITION DETECTOR

Abstract: A position detector comprising a plurality of magnetostriction propagating media (11) extending substantially in parallel with one another and having first coils (12) wound about the media at respective first ends thereof and second coils (16) wound around the media over respective lengths thereof. A current pulse is applied to each of the first coils to generate magnetostrictive oscillation waves in the regions of the mediae (11) around which the first coils (12) are wound. The waves propagate through the media (11) and cause an induced voltage to be generated in the respective second coils (16) at the position where position indicators generate a steady magnetic field. The position of the position indicators is determined by measuring the time that elapses between generating the oscillating waves and receiving the induced voltages. Adjacent two of the plurality of magnetostriction propagating media are respectively paired and each of the magnetostriction propagating media (11) of the respective pairs are individually wound with first coils (12) that are connected together and second coils (16) that are connected together so as to generate a magnetostrictive oscillating wave in the respective pairs. The plurality of magnetostriction propagating media (11) may be evenly spaced or may be irregularly arranged with media that separate pairs being closer together than the media that comprise a pair. In one embodiment the current pulse is applied to the second coils (16) to generate magnetostrictive oscillation waves at the position of the position indicators. In this embodiment the waves propagate through the mediae and cause an induced voltage to be generated in the respective first coil (12).

Description: I. Field of the invention The present invention relates to a position detector employed for detecting positions indicated by position indicators, utililizing a magnetostrictive oscillation wave propagated through magnetostriction media. II. Prior art From EP-0 130 578 A1 is known a position detecting apparatus, in which the propagation of a magnetostrictive wave in a magnetostrictive transmission medium is utilized for position indication. The principle of operation is based firstly on the generation of a magnetostrictive wave in the medium by applying an electric pulse current to a first coil wound around a part of the medium; secondly on the propagation of the magnetostrictive wave in the medium with a characteristic velocity, and thirdly due to the local change of the electromagnetic coupling between the propagating mechanical wave and the accompanying magnetic field in a region where an external position-indicating magnetic field is applied to the magnetostrictive medium, a voltage is induced in a second coil which is wound around the medium nearly over its whole length. Thus, by measuring the time delay between application of the electric pulse to the first coil and the occurence of an inductive voltage in the second coil, the position of the permanent magnet used as a position indicator can be determined along the length of the magnetostrictive medium, when taking into account that the progagation speed of the magnetostrictive wave is known. Apart from describing this functional principle and a simple apparatus for applying the principle this application doesn't deal with detailed problems of such kind of position detectors. EP-0 134 539 A2 describes a position detecting apparatus utilizing magnetostrictive waves propagating in bundles of magnetostrictive media which are arranged matrix-like to perform a 2-dimensional position-detection in a X- and a Y-direction. Determining the position of only a single position indicator this arrangement provides a high measurement speed because for each direction (X or Y) the first coils are commonly wound around the one end of all the respective magnetostrictive media, in order to excite a magnetostrictive wave in all media of the specific direction at the same time, and the respective second coils are connected in series to provide a composite output for all media of one direction. This solution has the disadvantage that two or more position indicators can't be distinguished in the matrix if they have the same (or near-by) X- or Y-components. The position detector according to EP-0 204 184 A 1 also makes use of the same physical effect, but the aim of this application is to improve the reliability and accuracy of the measurement by utilizing a peak hold-circuit for varying the detector threshold-level. Fig. 13 shows a position detector as has been proposed by the applicant of the present application (Japanese Patent Application No. 1983-220071; JP-A-60111918). In this position detector of the prior art, a plurality of magnetostriction propagating media 1 of an amorphous alloy or the like containing a high percentage of iron each supports a first coil 2 wound around it's one end and a second coil 4 wound around over its length for detection of the positions indicated by the position indicators 3p, 3q. The first coil 2 is connected to a pulse current generator 5 while the second coil 4 is connected to a processor 6. A biasing magnetic field generator 7 comprising a permanent magnet is provided in opposition to the end surfaces of the magnetostriction propagating media 1, adjacent to the portions of the respective magnetostriction propagating media around which said first coil 2 is wound. Said first coil 2 is crossed between each pair of adjacent magnetostriction propagating media and wound around each magnetostriction propagating medium 1 in the reverse direction with respect to the direction in which it is wound around the adjacent magnetostriction propagating medium 1. All second coils 4 are wound around the respective magnetostriction progagating media 1 in the same direction and are serially connected with adjacent second coils so that the connection polarity is successively reversed between adjacent second coils. Such arrangement is effective in reducing the undesired magnetic flux emitted externally to the system. External noise is cancelled or reduced between adjacent coils, because an external magnetic flux induces in adjacent coils a voltage with reversed polarity. Thus, the produced noise voltages are reduced or cancelled in the composite coil. Application of a pulse current from the pulse current generator 5 to the first coil 2 causes an instantaneous field variation in the first coil 2 and thereby, a magnetostrictive oscillation wave to be generated in the region of the respective magnetostriction propagating media 1 around which the first coil 2 is wound. This magnetostrictive oscillation wave is propagated longitudinally through the respective magnetostriction propagating media 1 at a propagating velocity specific to the magnetostriction propagating media 1. During this propagation conversion from mechanical energy to the corresponding magnetic energy occurs in the region of the respective magnetostriction propagating media 1 in which the magnetostrictive oscillation wave exists, according to the electromechanical coupling factor (i.e., the factor representing the conversion efficiency from the mechanical energy to the corresponding electric energy or from the electric energy to the corresponding mechanical energy) specific to said regions and, in consequence, an inductive voltage is generated in the second coil 4. Assuming that a steady magnetic field exists in a region of the magnetostriction propagating media 1, which is sufficient to increase the electromechanical coupling factor of this region, a high inductive voltage is generated in the second coil 4 when a magnetostrictive oscillation wave reaches said region. With a pair of position indicators 3p, 3q generating such a steady magnetic field, respectively, in two regions, as shown by Fig. 13, two high inductive voltages V₁, V₂ corresponding to the positions indicated by the pair of position indicators 3p, 3q are generated, as shown by Fig. 14. Detection of the positions indicated by the respective position indicators is accomplished by detecting the points in time at which the respective inductive voltages V₁, V₂ exceeded a predetermined voltage value (i.e., threshold value E). Referring to Fig. 14 of the accompanying drawings, the time duration T₂ that elapses between the point in time at which the pulse current is applied to the first coil 2 to the point in time at which the inductive voltage V₁ is detected, is substantially equal to the time duration elapsing between the point in time at which the magnetostrictive oscillation wave is generated in the region of the magnetostriction propagating media 1 around which the first coil 2 is wound to the point in time at which said magnetostrictive oscillation wave reaches the position indicated by the position indicator 3p. Accordingly, this time duration T₁ so determined may be multiplied by the propagation velocity of the magnetostrictive osciallation wave in the processor 6 to calculate the distance l₁ between the first coils 2 and the position indicator 3p, i.e., the position (coordinates) indicated by the said position indicator 3p. The distance l₂ between the first coils 2 and the position indicator 3q, i.e., the position (coordinates) indicated by the position indicator 3q may be calculated on the basis of the time duration T₂ elapsed before detection of the inductive voltage V₂. In the position detector shown by Fig. 13, the inductive voltages V₁, V₂ derived from the pair of position indicators 3p, 3q are provided as a composite output, since the second coils 4 wound around each magnetostriction propagating media 1 are connected in series. As a consequence, the composite inductive voltage sometimes may not exceed the threshold value E as best can be seen in Fig. 15 (c), when a difference between said distances l₁ and l₂ is adequately small. Specifically, the inductive voltage V₁ derived from the position indicator 3p located at the distance l₁ has a form as illustrated by Fig. 15 (a) while the inductive voltage V₂ derived from the position indicator 3q has a form as illustrated by Fig. 15 (b). The combination of these inductive voltages V₁, V₂ results in a mutual cancellation between their positive and negative components, as illustrated in Fig. 15 (c). When the distance l₁, l₂ are equal to each other, the resulting detector output erroneously appears as if a single position is indicated, in spite of the actual situation that two positions are indicated. In view of the above-mentioned disadvantage, i.e. the combinnation or composition of two inductive voltages V₁, V₂, another type of position detector has been proposed, in which both the first coils 2 and the second coils 4 are wound around the individual magnetostriction propagating media 1 so that the pulse current is separately applied to the respective first coils 2 and thereby detection of said inductive voltages as well as determination of the pertinent propagation time durations are separately achieved. With a position detector of this type, the number of the magnetostriction propagating media to be monitored inconveniently increases when the position detection device extends over a larger area and consequently the time required for determination becomes longer since the determination has to be done separately for an increased number of magnetostriction propagating media. III. Objective of the invention It is a principal objective of the present invention to provide a position detector in which the determination of the inductive voltages can be achieved within a time shorter than in the case of separately determining the inductive voltages for the individual magnetostriction propagating media and, in which it is possible to achieve position detection over a wider range. IV. Measures to solve the problem The problem is solved as described in claim 1. The dependent claims describe favourable embodiments. V. Operation Application of a pulse current to the first coils wound around one of the pairs of adjacent magnetostriction propagating media causes a magnetostrictive wave to be simultaneously generated in the pair of magnetostriction propagating media and then propagate through the media. This magnetostrictive wave causes the inductive voltage to be generated in the second coils wound around the pair of magnetostriction propagating media and such inductive voltage has the highest value at the position where the associated second coils are subjected to the steady magnetic field generated by one of the magnets of the position indicators. In view of such fact, the time duration elapsing from the point in time at which said pulse current was applied to the first coils to the point in time at which this highest inductive voltage is generated, is determined. A predetermined time after the above-mentioned operation was performed on the first pair of magnetostriction propagating media, a similar operation is repeated on the second pair of magnetostriction propagating media and so forth. By repeating a similar operation successively on the respective pairs of magnetostriction propagating media, it is possible to detect the positions indicated by the respective position indicators in a time period as short as at least one-half of the time period taken by the arrangement of prior art, e.g., the arrangement in which a plurality of magnetostriction propagating media are separately provided with first and second coils wound therearound. VI. Embodiments The present invention will now be described in detail, by way of specific embodiment and with reference to the accompanying drawings: Fig. 1 is a perspective view schematically illustrating an embodiment of the position detector constructed according to the teachings of the present invention; Fig. 2 is a diagram showing electromechanical coupling factor versus magnetic bias; Fig. 3 is a plane view illustrating a position detector for two-dimensional position detection; Fig. 4 is a sectional view taken along a line IV-IV in Fig. 3; Fig. 5 is a block diagram illustrating an electric circuit arrangement for generation of the pulse currents and evaluation of the position signals in a two-dimensional position detector according to Fig. 3; Fig. 6 is a graphic diagram showing the inductive voltage generated in a position detection coil wound around each of the magnetostriction propagating media as a function of time; Fig. 7 is a side view illustrating coils and magnetostriction propagating media arranged at regular intervals; Fig. 8 is a view similar to Fig. 7 but with the magnetostriction propagating media arranged at irregular intervals; Fig. 9 is a graphic diagram showing the variation of the maximum inductive voltage as a position indicator is moved transversely of the magnetostriction propagating media; Fig. 10 is a graphic diagram showing the variation in the inductive voltage as a function of time by changing the position of a position indicator transverse to the respective magnetostrictive medium. Fig. 11 is a position detector similar to that in Fig. 1, but illustrated is another embodiment constructed in accordance with the teachings of the present invention; Fig. 12 is a perspective view showing by way of example an eraser suitable for use with a position detector according to the present invention; Fig. 13 is a view similar to Fig. 1 but schematically illustrating a position detector of prior art; and Fig. 14 and Fig. 15 are graphic diagrams showing a variation in the inductive voltage occurring in the position detector of prior art as a function of the time, when a pair of position indicators are relatively remote from each over longitudinally of the magnetostriction propagating media as shown by Fig. 14 or when the pair of position indicators are close to each other as shown by Fig. 15. Referring to Fig. 1 which schematically illustrates an embodiment of the position detector constructed in accordance with the present invention. There are provided a suitable number of magnetostriction propagating media 11 made of material having a magnetostrictive effect and extending parallel with another. Although these magnetostriction propagating media may be of any ferromagnetic material, these magnetostriction propagating media are preferably made of material having a sufficiently high magnetostrictive effect to generate an intense magnetostrictive wave and a holding force sufficiently small that the magnetostriction propagating media are not readily magnetized when a magnet is placed adjacent thereto. As such material, amorphous alloys or the like of high iron content are preferable. The suitable amorphous alloys include Fe₆₇Co₁₈B₁₄Si₁ (atom %) and Fe₈₁B 13.5 Si 3.5 C₂ (atom %). Each of the magnetostriction propagating media 11 is long and narrow, preferably in the form of a ribbon having a rectangular cross-section or wire having a circular cross-section. When embodied in the form of ribbon, such ribbon may be conveniently several mm wide and several µm to several ten µm from the viewpoint of productivity as well as characteristics. It is practically possible to obtain such amorphous alloys as a sheet having a thickness of 20 to 50 µm, so that such sheet may be cut into desired ribbon or wire. In the present embodiment, the magnetostriction propagating media 11 of Fe₈₁B 13.5 Si 3.5 C₂ (atom %) are used in the form of ribbon which is 2 mm wide and 0.02 mm thick. Said magnetostriction propagating media 11 are provided at respective one ends with associated first coils 12 wound therearound. It should be noted that, although this embodiment employs a single turn, the number of turns may be two or more. The first coils 12 function to generate instantaneous field variations perpendicularly to a plane defined by the coils and thereby to generate a magnetostrictive wave in a region of the respective magnetostriction propagating media 11 around which said first coils are wound. The first coils 12 are connected through a first multiplexer 13 to a pulse current generator 14. Pairs of first coils 12 associated with respective pairs of adjacent magnetostriction propagating media 11 are connected in series and a pulse current is separately applied to each pair of coils 12 as selected by the first multiplexer 13. Accordingly, the magnetostriction propagating media 11 must be even-numbered. Eight magnetostriction propagating media are provided in this embodiment. The respective magnetostriction propagating media are designated by reference numerals 11a₁, 11a₂,... ..., 11d₂ wherein letters a, b, c and d represent the particular pair of media while indices 1 and 2 represent the particular magnetostriction propagating media constituting the respective pairs. Various members associated with these magnetostrictive media 11 are designated in the same manner. It should be understood that the pulse current generator 14 is adapted to supply the first coils 12 with sufficiently high pulse current to generate a magnetostrictive wave. There is provided a biasing magnetism generator 15 such as a permanent magnet opposed to the end surfaces of the magnetostriction propagating media 11 adjacent to the respective first coils 12, in order to generate biasing magnetic fields in the regions of the respective magnetostriction propagating media 11 around which the first coils 12 are wound. The field direction is arranged longitudinally along the magnetostriction propagating media 11. Generation of such biasing magnetic fields enable relatively small current to generate the magnetostrictive wave of relatively large magnitude. More specifically, the electromechanic coupling factor of the magnetostriction propagating media 11 takes the maximum value when applied with a predetermined level of the biasing magnetic field, as seen in Fig. 2. In this manner, said regions of the respective magnetostriction propagating media 11 carrying the first coils 12 may be maintained to be applied with said magnetic bias to achieve an efficient generation of the magnetostrictive wave. Second coils 16 are wound around the respective magnetostriction propagating media 11 over their longitudinal extents along which desired position detections are to be performed. Like the first coils 12, these second coils 16 also are separately provided for the respective pairs of adjacent magnetostriction propagating media 11 and the respective pairs correspond to the respective pairs of magnetostriction propagating media 11 for which the first coils 12 are separately provided. These second coils 16 are provided to detect inductive voltages generated by the magnetostrictive waves propagated through the associated magnetostriction propagating media 11 and wound therearound in the same direction so that the polarity is reversed for every pair of magnetostriction propagating media 11. These second coils 16 are connected by a second multiplexer 17 to a processor 18. The processor 18 functions to supply both the first multiplexer 13 and the second multiplexer 17 with signals, to connect the first coils 12 on the respective pair of magnetostriction propagating media 11 to the pulse current generator 14 and to connect the second coils 16 on said respective pair of magnetostriction propagating media 11 to said processor 18. Referring to Fig. 1, reference numerals 19p and 19q designate position indicators adapted to generate steady magnetic fields. Although the present embodiment utilizes bar magnets as position indicators each having a diameter of 3 mm and a length of 50 mm they may be of various configurations such as plate-like, ring-like and angular magnet or electromagnets may alternatively be used. According to the present embodiment, the positions indicated by these position indicators 19p, 19q are to be detected. The first coils 12 are carried by magnetostriction propagating media 11 in a manner as shown by Fig. 7. Specifically, the respective magnetostriction propagating media 11 are enclosed by associated cylindrical holders 20 around which said second coils 16 are wound. The first coils 12 are wound around these holders 20 or around the respective one ends of said magnetostriction propagating media 11 themselves projected from these holders 20 and, after wound by a predetermined number of turns, each of said first coils 12 has its turn-terminals 12u crossing each other on the lower side of the associated magnetostriction propagating medium 11, then connected to a print substrate 21. The first coils 12 of the associated pair of adjacent magnetostriction propagating media 11 are interconnected on this print substrate 21 and then connected to said first multiplexer 13. It should be understood that, in the embodiment shown by Fig. 7, all the individual magnetostriction propagating media 11 are arranged at equidistant intervals. Two pairs of the above-mentioned assemblies comprising the components such as the magnetostriction propagating media 11 and the first coils 12 are combined in the orthogonal relationship as seen in Fig. 3 to provide a two-dimensional position detector in which one pair is used to detect a X-position while the other pair is used to detect a Y-position. The pair of said assemblies used to detect the X-position is designated by additional reference x and the pair of said assemblies used to detect the Y-position is designated by additional reference y. In the same manner as previously mentioned with respect to Fig. 11, the particular pairs of magnetostriction propagating media 11 including the first and the second coils are designated by letters a, b, c, d, ... as the further additional references and the individual assemblies constituting the respective pairs are designated by indices 1 and 2 as still other additional references. In this manner, the respective assemblies are expressed as the magnetostriction propagating media 11xa₁, 11xa₂, ..., 11xd₂, 11ya₁, 11ya₂, ..., 11yd₂. The magnetostriction propagating media 11x and 11y combined in the orthogonal relationship are placed one on another as seen in Fig. 4, contained in a casing 22 of nonmagnetic metal and fixed in place with adhesive or the like. Although the biasing magnetic field generator 15 is shown as stationarily disposed on the bottom of the casing 22 so as to be opposed to the one end surface of the magnetostriction propagating media 11, said generator 15 may be disposed above, beneath or laterally of said magnetostriction propagating media 11. The top of the casing 22 is closed by a cover 23 made of nonmagnetic metal or synthetic resin, along the top surface of which said position indicators 19p, 19q may be moved to perform the operation of position indication. Fig. 5 is block diagram illustrating an electric circuit arrangement corresponding to the X- and Y-position detector shown by Fig. 3 and 4. Referring to Fig. 5, reference numerals 13x and 13y designate multiplexers respectively associated with the first coils 12x, 12y of X-and Y-directions, respectively; reference numerals 14x and 14y designate pulse current generators for X- and Y-directions, respectively; and reference numerals 17x and 17y designate multiplexers respectively associated with the second coils 16x, 16y of X- and Y-directions, respectively. The processor 18 comprises a microprocessor 31, multiplexers 32, 33, a monostable multivibrator (monoflop) 34, a counter 35, a clock pulse generator 36, an amplifier 37 and a comparator 38. Now operation of said X- and Y-position detector will be described on the assumption that two positions simultaneously indicated by the pair of position indicators 19p, 19q are to be detected. It is first assumed that position detector of Fig. 1 is used to detect the X-position and the position indicator 19p is positioned above the magnetostriction propagating media 11xa₁ with interposition to the cover 23 of the casing 22 at a distance of lpx in the X-direction from a center of the coil plane defined by the associated first coil 12x while the other position indicator 19q is positioned above the magnetostriction propagating medium 11xc₂ at a distance of lqx in the X-direction from a center of the coil plane defined by the associated first coil 12x. It is then assumed that the position detector of Fig. 1 is used to detect the Y-position and the position indicator 19p is positioned above the magnetostriction propagating medium 11yc₂ at a distance of lpy in the Y-direction from a center of the coil plane defined by the associated first coil 12y while the other position indicator 19q is position above the magnetostriction propagating medium 11yd₁ at a distance of lqy in the Y-direction from a center of the coil plane defined by the associated first coil 12y. Thus, it is assumed that the respective magnetostriction propagating media 11xa₁, 11yc₂, 11xc₂, 11yd₁ are engaged by respective position indicators 19p, 19q with a magnetic field sufficient to increase the electromechanical coupling factor. Upon reception of an instruction signal for start of determination as a result of operation of appropriate switching means or the like (not shown), the microprocessor 31 provides a direction selecting transfer signal to the multiplexers 32, 33 and thereby selects the pulse current generators 14x, 14y and the direction of the second coils to be measured. The microprocessor 31 also provides control signals for the first multiplexers 13x, 13y and the second multiplexers 17x, 17y to select the respective first pairs of coils of the X- and Y-direction. Thus, the first coils 12xa₁, 12xa₂ and the second coils 16xa₁, 16xa₂ of the first X-direction pair and the first coils 12ya₁, 12ya₂ and the second coils 16ya₁, 16ya₂ of the first Y-direction pair are independently selected. For example, the microprocessor 31 supplies through the multiplexer 32 a trigger pulse to the pulse current generator 14x which supplies, in turn, pulse current to the first coils 12xa₁, 12xa₂ of the X-direction. The trigger pulse is also applied through the monostable multivibrator 34 to the counter 35 which is then reset and starts to count clock pulses provided from the clock pulse generator 36. This clock pulse generator 36 has a pulse repetition frequency on the order of 100 MHz. When the pulse current generator 14x is activated and the pulse current generated is applied to the first X-direction coils 12xa₁, 12xa₂, instantaneous field variations accur in these first coils 12xa₁, 12xa₂, generating magnetostrictive waves in the regions of the magnetostriction propagating media 11xa₁, 11xa₂ around which the first coils 12xa₁, 12xa₂ are wound. A magnetic field is generated as the pulse current flows through the first coils 12. It should be noted here that the direction in which the current flows through the portion of the coil extending from the turn-terminal 12u to the print substrate 21 is reverse to the direction in which the current flows through the circular portion of the coil directly wound around the magnetostriction propagating medium 11xa₁, since the first coil 12xa₁ crosses itself at the turn-terminals 12u. Accordingly, magnetic flux developing around these portions have their directions reverse to each other so that the corresponding undesired magnetic flux emitted externally as noise advantageously is cancelled and undesirable effects are reduced. This is true also for the other first coil 12x and for the first Y-direction coils 12y. The magnetostrictive waves generated in the manner as has been mentioned above are propagated at a specific propagation velocity (approximately 5000 m/sec.) longitudinally through the respective magnetostriction propagating media 11xa₁, 11xa₂. During such propagation, if the magnetostriction propagating media 11xa₁, 11xa₂ are placed within a steady magnetic field, inductive voltages are generated in the respective second coils 16xa₁, 16xa₂ under the effect of said steady magnetic fields. The position indicator 19p is positioned at a point above the magnetostriction propagating medium 11xa₁, as has previously been assumed, and said inductive voltage is generated in the second coil 16xa₁ after a propagation time delay T px indicating the position of the position indicator 19p. This is illustrated by Fig. 6(a), in which the time t is plotted as abscissa and the variation in the inductive voltage is plotted as ordinate. As shown in Fig. 6(a), a pulse-like high inductive voltage Vpx appears when the magnetostrictive wave generated in said region of the first coil 12xa₁ after propagated through the magnetostriction propagating medium 11xa₁, reaches the position at which position indicator 19p generates a magnetic field sufficient to increase the electromechanical coupling factor of this magnetostriction propagating medium 11xa₁. The time delay T px from the instant at which the pulse current is applied to the first coil 12xa₁ to the instant at which the inductive voltage V px exceeds a predetermined threshold value E, substantially corresponds to the propagation time duration of the magnetostrictive wave through the magnetostriction propagating medium 11xa₁ from the position of the first coil 12xa₁ to the position indicated by the indicator 19p. The inductive voltage Vpx generated in said second coil 16xa₁ is supplied through the second multiplexer 17x to the amplifier 37 and, after amplification, to the comparator 38. The comparator 38 compares this inductive voltage with the threshold value E and, when the inductive voltage V px exceeds the threshold value E, supplies a stop pulse to the counter 35, causing the latter to terminate counting. In this manner, the counter 35 provides a digital value corresponding to the time duration elapsing from the instant at which the pulse current is applied to the first coil 12xa₁ until the instant at which the inductive voltage V px generated in the second coil 16xa₁ exceeds a predetermined threshold value E. This digital value also corresponds to the time duration required for propagation of the magnetostrictive wave through the magnetostriction propagating medium 11xa₁ at the predetermined propagation velocity from the po of the first coil 12xa₁ to the position indicated by the position indicator 19p, i.e. by the distance of lpx in the X-direction. The microprocessor 31 reads the count value of the counter 35 as a first X-position data X₁. After a predetermined time has elapsed, the microprocessor 31 provides the first multiplexers 13x, 13y and the second multiplexers 17x, 17y with a control signal indicating that the respective second pairs of coils shall be selected so that the first coils 12xb₁, 12xb₂ as well as the second coils 16xb₁, 16xb₂ of the second pair of X-direction are selected to be operated. At the same time, the microprocessor 31 outputs a trigger pulse to the multiplexer 32 and the monostable multivibrator 34 in the same manner as previously described with respect to the first pair of X-direction. This trigger pulse resets the counter 35 and activates the pulse current generator 14x to apply a pulse current to the first coils 12xb₁, 12xb₂ of the X-direction. In response, magnetostrictive waves are generated in the regions of the respective magnetostriction propagating media 11xb₁, 11xb₂ along which the respective first coils 12xb₁, 12xb₂ are wound. The inductive voltages generated due to these magnetostrictive waves in the respective second coils 16xb₁, 16xb₂ of the X-direction are supplied, just as in the previously mentioned case, through the second multiplexer 17x to the amplifier 37 and to the comparator 38 in which these inductive voltages are compared with the threshold value E. Because in this case none of the position indicators is placed above the magnetostriction propagating media 11xb₁, 11xb₂, no inductive voltage in excess of the threshold value E is generated, and hence no stop pulse required to obtain the digital value is produced. The similar operation may be successively performed on the respective pairs of the first coils 12x and the second coils 16x to achieve desired position detection for every pair of the magnetostriction propagating media 11x. When the first coils 12xc₁, 12xc₂ and the second coils 16xc₁, 16xc₂ associated with the pair including the magnetostriction propagating medium 11xc₂, indicated in this example by the position indicator 19q, are selected, an inductive voltage Vqx exceeding the threshold value E is generated in the second coil 16xc₂, as shown in Fig. 6(b). Counter 35 provides a digital value corresponding to the time duration Tqx elapsing from the instant at which the first coils 12xc₁, 12xc₂ are applied with the pulse current to the instant at which the magnetostrictive wave generated thereby reaches the position indicated by the position indicator 19q and an inductive voltage pulse Vqx in excess of the threshold value E is detected, i.e., the distance lqx in X-direction. Said digital value provided by the counter 35 is read by the microprocessor 31 as a second X-position data X₂. Upon completion of the position detecting operation with respect to the X-direction, the microprocessor 31 provides a Y-direction selecting transfer signal to the multiplexers 32, 33 and at the same time selects the pulse current generator 14y and the second multiplexer 17y. The microprocessor 31 successively provides a control signal selecting the respective pairs of coils of X- and Y-directions by switching the first multiplexers 13x, 13y and the second multiplexers 17x, 17y. Furthermore, the microprocessor 31 successively provides a trigger pulse to the multiplexer 32 and the monostable multivibrator 34. In this way, the position detection may be achieved for every pair of magnetostriction propagating media 11ya₁, 11ya₂ ...., 11yd₁, 11yd₂ just as in the case of the X-direction position detection. When the first coils 12yc₁, 12yc₂ and the second coils 16yc₁, 16yc₂ associated with the pair including the magnetostriction propagating medium 11yc₂ are selected and when the first coils 12yd₁, 12yd₂ and the second coils 16yd₁, 16yd₂ associated with the pair including the magnetostriction propagating medium 11yd₁ are selected, the inductive voltages in excess of the threshold value E are detected in the respective second coils 16yc₂, 16yd₁ and the digital values corresponding to the distances lpy, lqy in the Y-direction are provided by the counter 35. The microprocessor 31 reads these digital values provided by the counter 35 as the first and second Y-position data Y₁ and Y₂. The X-position data X₁, X₂ and the Y-position data Y₁, Y₂ obtained in this manner in the form of digital values are temporarily stored in the microprocessor 31 so that they may be outputted to a computer or the like if necessary. As long as the determination start instructing signal continues to be applied to the microprocessor 31, said operation of determination is repeated and the determined value is correspondingly updated. Upon reception of a data input instructing signal generated by operation of appropriate switch means or the like(not shown), the microprocessor 31 outputs said X- and Y-position data X₁, X₂ and Y₁, Y₂ in the form of digital values at the current instants as the input values are acquired. Such operation may be repeated to obtain the position-data successively as indicated by the position indicators. With the present embodiment, the position indicated by the position indicator 19p or 19q is detected by each set of paired magnetostriction propagating media 11. Therefore, the inductive voltages are generated in at least a pair of the second coils 16 wound around the associated pair of adjacent magnetostriction propagating media 11. Assuming that the position indicator 19 is moved transversely of the magnetostriction propagating media 11, said inductive voltage will have its maximum value vary as illustrated by Fig. 9, in which a cross-section of the arrangement of the magnetostriction propagating media 11 is depicted on the abscissa and the maximum value of the inductive voltage is indicated on the ordinate. Circles drawn by broken lines along the abscissa represent the respective magnetostriction propagating media 11xa₁, 11xa₂, 11xb₁, 11xb₂ arranged at equidistant intervals and the corresponding maximum value of the inductive voltage varies as shown by the curves plotted by broken lines. When the position indicator 19 is located substantially in the middle between the paired magnetostriction propagating media 11xa₁, 11xa₂ (or 11xb₁ 11xb₂), high inductive voltages are generated in both the magnetostriction propagating media 11xa₁, 11xa₂. These inductive voltages are combined into a peak amplitude which is the highest of those occurring in all cases. Deviation of the position indicator 19 from said middle position towards any of the paired magnetostriction propagating media results in a higher inductive voltage being generated in the magnetostriction propagating medium 11 closer to the position indicator 19 while a lower inductive voltage being generated in the magnetostriction propagating medium 11 remote from the position indicator 19, so that the maximum value of the combined inductive voltages decreases as the deviation from the middle position increases. When the position indicator 19 is located intermediately of the magnetostriction propagating media 11xa₂, 11xb₁ which are adjacent to each other but respectively belong to different pairs, the maximum value of the inductive voltage further decreases. This maximum value at a middle position between these magnetostriction propagating media 11xa₂, 11xb₁ is indicated as Vm in Fig. 9. Accordingly, the threshold value E has to be set to a level lower than Vm when the magnetostriction propagating media 11 are arranged at equidistant intervals. The threshold value E set to such lower level may lead to a malfunction, since inductive noise voltages which should be suppressed tend to be detected. When the difference between the highest and lowest inductive voltages that must both be detected is unacceptably large, the instant at which the high inductive voltage exceeds the threshold value E and the instant at which the low inductive voltage exceeds said threshold value deviate from each other, as shown in Fig. 10, even when the position indicator 19 is held on the same position. More specifically, the high inductive voltage indicated by solid line is detected at T₁ to exceed the threshold value E while the low inductive voltage indicated by broken line is detected at T₂ to exceed said threshold value E. Thus, the detection points deviate with respect to each other. To overcome this problem, it is important to increase the threshold value E and to reduce the difference between the high and low inductive voltages. When the magnetostriction propagating media 11 are arranged at equidistant intervals, the intervals may be made closer in order to overcome this problem. However, this would result in a increased number of the magnetostriction propagating media 11 to be arranged within a range over which positions must be detected. In view of such inconvenience, a spacing between the magnetostriction propagating media 11xa₁, 11xa₂ belonging to the same pair is made larger than a spacing between the magnetostriction propagating media 11xa₂, 11xb₁ which are adjacent to each other but belong to the different pairs. The maximum value of the inductive voltage versus the arrangement of the magnetostriction propagating media 11xa₁, 11xa₂, 11xb₁, 11xb₂ in this case is illustrated by solid lines in Fig. 9. As shown, the spacing between the magnetostriction propagating media 11xa₁, 11xa₂ belonging to the same pair is enlarged while the spacing between the magnetostriction propagating media 11xa₂, 11xb₁ which are adjacent to each other but belong to different pairs is reduced with respect to the case in which all the individual magnetostriction propagating media 11 are arranged at equidistant intervals. Thus, the variation of the maximum value of the inductive voltage is reduced. Additionally, when the position indicator 19 is located in a middle position between the magnetostriction propagating media 11xa₂, 11xb₁ being adjacent to each other but belonging to different pairs, the position indicator 19 is closer to these magnetostriction propagating media 11 than when all the individual magnetostriction propagating media 11 are arranged at equidistant intervals. As a result, the generated inductive voltage is correspondingly increased and this permits the threshold value E to be set to a sufficiently high level to avoid any malfunction. It has been found, that with the magnetostriction propagating media 11 being equidistant arranged, the position can be detected substantially with a desired accuracy when said magnetostriction propagating media are arranged at intervals of approximately 21 mm. With the magnetostriction propagating media 11 being arranged at irregular intervals, it has been found that the spacing between the magnetostriction propagating media 11 belonging to the same pair may be set to approximately 32 mm while the spacing between the adjacent magnetostriction propagating media 11 belonging to the different pairs may be set to approximately 10 mm to detect the positions with a desired accuracy. With the position detector comprising eight magnetostriction propagating media 11, the distance between the outermost magnetostriction propagating media 11 is approximately 147 mm when these magnetostriction propagating media are arranged at regular intervals and said distance is approximately 158 mm slightly larger than in the former case. Accordingly, the number of magnetostriction propagating media 11 may be reduced with the position detector comprising the magnetostriction propagating media 11 arranged at irregular intervals with respect to the position detector comprising the magnetostriction propagating media 11 arranged at regular intervals for a given range of detection. Another embodiment of the position detector constructed in accordance with the teachings of the present invention is shown in Fig. 11. In this embodiment, the first coils 12 and the second coils 16 are further divided into several groups, respectively, so that the first coils 12 and the second coils 16 associated with a certain pair belonging to each group may be simultaneously selected to perform the desired position detection. Referring to Fig. 11, reference numerals 13a, 13b designate first multiplexers and reference numerals 17a, 17b designate second multiplexers. The first multiplexer 13a is provided to connect selectively the first coil pair 12a or 12b to a pulse current generator 14 while the first multiplexer 13b is provided to connect selectively the first coil pair 12c or 12d to said pulse current generator 14. The second multiplexer 17a is provided to connect selectively the second coil pair 16a or 16b to a processor 18 while the second multiplexer 17b is provided to connect selectively the second coil pair 16c or 16d to said processor 18. The respective multiplexers 13a, 13b, 17a, 17b are supplied from the processor 18 with the same selection signals so that the position detection may be simultaneously achieved on the pair of magnetostriction propagating media 11a, 11b and the pair of magnetostriction propagating media 11c, 11d. According to the construction of this embodiment, it is possible to perform the position detection simultaneously on two pairs of magnetostriction propagating media 11 and, therefore, the time period required to complete the position detected on all the magnetostriction propagating media 11 can be one half or shorter relative to the case in which the first coils and the second coils are not divided into the several groups. Although the present invention has been described hereinabove as the first coils 12 being used to generate the magnetostrictive wave and the second coils 16 being used to detect the inductive voltage, the reversed arrangement is also possible such that the second coils 16 are used to generate the magnetostrictive wave while the first coils 12 are used to detect the inductive voltage. In such a case, said magnetostrictive wave is generated immediately beneath the position indicator 19 and the inductive voltage generated when this magnetostrictive wave reaches the associated first coil 12 is detected. When position detection is performed according to this embodiment, no more than one position indicator may be simultaneously located above the magnetostriction propagating media 11 belonging to the same pair. This requirement is easily met when the positions indicated by a pair of position indicators 25p, 25q are always spaced from each other by a predetermined distance as in an eraser 24 used for erasure of an electronic blackboard or the like, shown in Fig. 12, are to be detected. Specifically, the spacing between the magnetostriction propagating media 11 paired to be used for the position detection is appropriately selected with respect to said predetermined distance between the position indicators 25p, 25q so that the pair of position indicators 25p, 25q cannot be located simultaneously above the magnetostriction propagating media 11 of the same pair. VII. Effect of invention As will be apparent from the foregoing description, the position detector constructed according to the present invention enables the position detection to be performed simultaneously on each pair of adjacent magnetostriction propagating media and thereby enables the position detection to be achieved in one half or shorter time relative to the case in which the position detection is performed on the individual magnetostriction propagating media. Within the same time period, it is possible for the present invention to achieve the position detection over a wider range. Furthermore, the multiplexers of this invention may have fewer channels or choices than in the position detector of prior art adapted for position detection based on the individual magnetostriction propagating media. The number of parts may be correspondingly reduced, making a cost-efficient position detector available. Additionally, the feature of the present invention such that the spacing between the magnetostriction propagating media belonging to the same pair is dimensioned larger than that between the magnetostriction propagating media which are adjacent to each other but belong to the different pairs, makes it possible to minimize malfunctions and thereby makes it possible to reduce the number of magnetostriction propagating media with respect to the position detector comprising the magnetostriction propagating media arranged at regular intervals. In this manner, this feature also makes a cost-efficient position detector available.

Claims: A position detector comprising a plurality of magnetostriction propagating media (11) extending substantially in parallel with one another, first coils (12) wound around the respective one ends of said plurality of magnetostriction propagating media (11), and second coils (16) wound around respective extents of said plurality of magnetostriction propagating media (11), position indicators (19) adapted to generate steady magnetic fields; wherein a pulse current is applied to first coils (12) or second coils (16) to generate a magnetostrictive oscillation wave in regions of the magnetostriction propagating media (11) around which the first coils (12) are wound or at positions indicated by the position indicators (19), then measuring a propagation time interval through the media (11) elapsing from the point in time at which said magnetostrictive oscillation wave has been generated to a point in time at which said magnetostritive oscillation wave reaches the regions indicated by the position indicators (19) or the regions of the magnetostriction propagating media (11) around which the first coils (12) are wound, by detecting an inductive voltage generated in the respective other of said first coils (12) or said second coils (16) after arrival of the wave, and on the basis of said measured time interval, the positions are determined, said position detector being characterized by that adjacent two of said plurality of magnetostriction media (11) are respectively paired, said respective pairs of magnetostriction propagating media (11) are individually wound with said first coils (12) and said second coils (16) so as to generate said magnetostrictive oscillation wave in said respective pairs, and the positions indicated by said position indicators (19) are detected. A position detector as recited in claim (1), wherein said plurality of magnetostriction media (11) are arranged at regular intervals. A position detector as recited in claims (1), wherein each of said paired magnetostriction propagating media (11) is spaced from the adjacent paired magnetostriction propagating media (11) by an interval larger than that by which the individual magnetostriction propagating media (11) of the respective pairs are spaced from each other. A position detector as recited in claim (1), wherein said first coils (12) have a plurality of turns. A position detector as recited in claim (1), wherein said first coils (12) are crossed at their turn terminals associated with the respective magnetostriction propagating media (11) so that the electric current flowing through said first coils (12) are reversed in its direction at said turn terminals between the portions of said first coils (12) wound around the respective magnetostriction propagating media (11) and the remaining portions thereof. A position detector as recited in claim (1), wherein said plurality of magnetostriction propagating media (11) are divided into a plurality of groups and the adjacent magnetostriction propagating media (11) are respectively paired within each of said groups. A position detector as recited in claim (1), wherein said plurality of magnetostriction propagating media (11) are arranged in a single orientation so as to detect one-dimensional position. A position detector as recited in claim (1), wherein said plurality of magnetostriction propagating media (11) are arranged in orthogonal two directions so as to detect two-dimensional position.

Inventors: MURAKAMI, AZUMA, TAGUCHI, YOSHINORI, TAKAHASHI, KENICHI, TANAKA, TAKESHI, TOMOFUJI, YOSHIAKI
Applicants: KABUSHIKI KAISHA WACOM
Application year: 1988
Main classification: G01D5
IPC codes: G01B7, G01D5, G06F3, G01B17