1. Field of the Invention
The present invention relates to a coordinates input apparatus and, more particularly, to a coordinates input apparatus which detects coordinates of an indication point from a vibration propagation time on a vibration propagating plate and has a structure in which vibration proof materials are arranged in edge portions of the vibration propagating plate.
2. Related Background Art
Hitherto, there has been known a coordinates input apparatus in which a vibration is input to a vibration propagating plate by a vibration pen having therein a piezoelectric transducing element or the like, the input vibration is detected by a plurality of sensors attached to the vibration propagating plate, the vibration propagation times are measured, and the coordinates of the input point are detected.
In such a coordinates input apparatus, there is a construction in which the edge portions of the vibration propagating plate are supported by vibration proof materials so as to prevent a situation such that the input vibration is reflected by the edge portions of the vibration propagating plate and an error is caused in the detection by the vibration sensor due to the reflected waves.
The conventional vibration proof material has mainly been developed to prevent noises and is largely classified as a material for use as a countermeasure for the air sound and a material for use as a countermeasure for the solid sound. Therefore, in the case of using the conventional vibration proof material in the above application fields, the material as a countermeasure for the solid sound is used. As a conventional damping material as a countermeasure for the solid sound, a damping sheet for a thin plate as shown in FIG. 8A, a paint, or the like has been known.
In FIG. 8A, reference numeral 8' denotes a vibrating plate whose vibration should be damped and 7 indicates a damping sheet. The vibrating plate 8' is made of a metal plate such as thin steel plate, aluminum plate, or the like, a resin plate, a glass plate, or the like.
In such a structure, by adhering the damping sheet 7 onto the vibrating plate 8' which is vibrating, the vibration of the plate 8' is decreased by using a vibration attenuation of the damping sheet 7, so that the noises can be reduced.
As materials for commercially available damping sheets 7, there are a polyvinyl chloride resin, atactic polypropylene, polyethylene vinyl acetylate, a styrene-butadiene rubber, a silicon rubber, a cement paste, and the like. Further, the products which are obtained by adding or mixing a plasticizer, a stabilizer, a softener, metal powder of lead, iron, or the like, quarts sand, asphalt, and the like to the above materials are used. As a molding shape of those materials, there is a sponge-like porous shape.
The conventional damping sheets as mentioned above are considered to suppress the vibration of the whole plate mainly by adhering the damping sheet onto the whole plate which vibrates.
Therefore, even if the conventional damping sheet is attached to the periphery of the vibration propagating plate 8 as shown in FIG. 8B in order to suppress the reflected waves at the edge portions of the vibration propagating plate, the reflected waves cannot be sufficiently reduced.
FIG. 8B is a diagram showing the vibration propagation in the case where the damping sheet has been attached to the peripheral portion of the vibration propagating plate. FIG. 8C is a diagram of a part of a cross sectional view of FIG. 8B and shows a state of the reflected waves.
In the diagrams, reference numeral 8 denotes a vibration propagating plate; 7 the damping sheet; 3 a vibration pen as a source to apply a vibration; a wave showing a vibration which is propagated from a vibration applied point as a contact point between the vibration pen 3 and the vibration propagating plate 8; B a reflected wave at the edge surface of the vibration propagating plate; and C a reflected wave at the boundary surface in the portion where the damping sheet has been attached.
The damping effect of the conventional damping sheet 7 is large for the vibration (also including a natural vibration, i.e., resonant vibration) which occurs in the whole plate as mentioned above. However, as shown in FIG. 8B, an adequate damping effect is not derived for a vibration which has been applied to a region where no damping sheet is attached and which propagates as a progressing wave from such a region. Therefore, as shown in FIG. 8C, although the vibration is slightly attenuated in a portion D to which the damping sheet 7 has been attached, the reflected wave B at the edge surface of the vibration propagating plate cannot be sufficiently suppressed.
Further, by attaching the conventional damping sheet 7, the reflected wave C is newly generated at the interface surface where the damping sheet has been attached. Consequently, if the conventional damping sheet has merely been attached to the peripheral portion of the vibration propagating plate of the coordinates input apparatus using an elastic wave, the foregoing two reflected waves are generated and become noises when a direct wave from the vibration applying source is detected, so that a detecting precision is deteriorated.
When considering the conventional example of the vibration proof material (damping sheet) attached to the vibration propagating plate mentioned above, it is necessary to pay an attention to the following two points with respect to the positional relation between the vibration sensor (hereinafter, simply referred to as a sensor) and the vibration proof material which is attached to the peripheral portion of the vibration propagating plate in the coordinates input apparatus in which a vibration which has been input from the vibration pen is detected by a plurality of vibration sensors attached to the vibration propagating plate and the position of the vibration pen is detected from the vibration propagation times until the vibration sensors. One point relates to an influence which is exerted on the vibration propagation by the attenuation of the vibration proof material. Another point relates to an influence by the reflected wave which is generated by the surface on which the vibration proof material has been attached. The same applicant as the present invention has hitherto proposed the following apparatuses with regard to the above two points. In the "Coordinates input apparatus" disclosed in Japanese Patent Application No. 62-67856, by attaching sensors a predetermined distance away from a vibration proof material (two wavelengths) or longer, the influence exerted on the vibration propagation by the attenuation of the vibration proof material is reduced. In the "Coordinates input apparatus" disclosed in Japanese Patent Application No. 61-251598, by attaching sensors so as to be away from a vibration proof material by a predetermined distance (reflection interference distance) or longer, the influence by the reflected wave which is generated by the vibration proof material attaching surface is reduced. Further, in the "Coordinates input apparatus" disclosed in Japanese Patent Application No. 61-251599, by attaching sensors onto the vibration proof material attaching boundary surface, the influence of the reflected wave which is generated by the vibration proof material attaching surface is reduced.
In the above conventional apparatuses, however, there are drawbacks such that in the case of attaching the sensors so as to be away from the vibration proof material by a predetermined distance or longer, outer dimensions of the whole apparatus are too large, while in the case of attaching the sensors onto the vibration proof material attaching boundary surface, the vibration proof material and the sensors cannot be attached onto the same surface of the vibration propagating plate and the thickness of apparatus increases and the like.