1. Field of the Invention
This invention relates to a coordinate input apparatus for detecting the coordinates of a designated point from a vibration transmission time on a vibration transmission plate, and a method for controlling such an apparatus.
2. Description of the Related Art
Coordinate input apparatuses are known in which a vibration is input onto a vibration transmission plate by a vibrating pen incorporating a piezoelectric element or the like, and the coordinates of a point where the vibration is input are detected by a plurality of sensors provided on the vibration transmission plate.
In coordinate calculation of the coordinate input apparatus for detecting vibration, a method is generally known in which the transmission velocity of a vibration wave transmitted through the vibration transmission plate is obtained in advance, the obtained velocity is used as a constant, and the coordinates of the input point are obtained from the distance from the vibrating pen to each of the vibration sensors obtained by performing calculations, such as multiplication of a vibration transmission time from the vibrating pen to the vibration sensor by the constant. Particularly when using a Lamb wave (plate wave) as the vibration wave transmitted through the vibration transmission plate, the transmission velocity is conventionally obtained from the detected value of the frequency of the vibration and a specific value of the thickness of the plate utilizing the fact that the transmission velocity of the Lamb wave depends on the thickness of the plate and the frequency, when the transmission velocity is not actually measured.
More specifically, as a conventional example, Japanese Patent Laid-Open Application (Kokai) No. 7-160407 (1995) proposes the following approach. That is, based on the discovery of the fact that the group velocity is obtained as a function of only the thickness of the plate, in the setting of the sound velocity of a Lamb wave, first, the frequency is measured from the phase synchronism of a detection signal, and the phase velocity is derived from the measured value and the thickness of the plate. The group velocity is derived only from the thickness of the plate. This proposal aims at improvement in accuracy in position detection, as well as improvement in reliability, in mass production capability, and the like.
However, as for the transmission velocity in the conventional coordinate input apparatus for detecting vibration, the value is obtained for each of the vibration sensors to provide the constant for calculating the coordinates, or the mean value of the obtained values for the respective sensors is used as the constant, or the value obtained for a representative vibration sensor is used as the constant for all of the vibration sensors. As a result, the following problems arise.
First, in the approach in which the value is obtained for each of the vibration sensors to provide the constant for calculating the coordinates, there is a problem in mass production capability in industrialization. That is, measuring and calculating operations corresponding to the number of the vibration sensors are required, thereby increasing the number of processes, and a burden on a calculation circuit for calculating the coordinates. In the approach in which only one constant is used for all of the vibration sensors, no problem arises when the vibration transmission plate is made of a homogeneous medium. However, when the vibration transmission plate has anisotropy in the vibration transmission velocity depending on the direction of the transmission of vibration, the problem that accuracy in the detection of the coordinates is degraded arises. A description will now be provided of the case of using the vibration transmission plate having anisotropy with reference to FIGS. 14 and 15.
FIG. 14 is a schematic diagram illustrating the configuration of a vibration transmission plate and vibration sensors in a typical conventional coordinate input apparatus for detecting vibration. This vibration transmission plate has anisotropy in the vibration transmission velocity depending on the direction of the transmission of vibration. The vibration sensors are represented by A, A', B and B' as shown in FIG. 14.
FIG. 15 illustrates an error .DELTA.L in the distance from each of the sensors to the vibrating pen obtained from a delay time in the conventional coordinate input apparatus for detecting vibration. The error is calculated by using the mean value of the vibration transmission velocity values of the respective sensors as a constant. Since the object of this description is to show problems in the conventional approach illustrating the results of calculation of the distance, the detail of the calculation method will be described later in embodiments of the present invention. The error .DELTA.L shown in FIG. 15 is measured for each of the vibration sensors by scanning the vibration transmission plate in a diagonal direction with the vibrating pen. In FIG. 15, the abscissa represents the distance from the vibrating pen, and the ordinater represents the error .DELTA.L.
The central position of the scanning distance is used as a reference point, where the error is adjusted to be zero. Acordingly, in FIG. 15, the error is small at central portions. As is apparent from FIG. 15, the error AL has maximum values at the shortest point and the longest point from the vibrating pen separated from the reference point. The tendency of variations differ, i.e., is opposite, between a pair of vibration sensors A and A', and a pair of vibration sensors B and B', because of the following reason. That is, since the vibration transmission plate has anisotropy as described above, the vibration transmission velocity differs between two directions each indicated by a two-headed arrow, so that the relationship between the distance and the vibration transmission time differs, while the distance is calculated by using the mean value of the vibration transmission velocity values of the respective sensors as a constant.
For the purpose of comparison, FIG. 16 illustrates the relationship between the error .DELTA.L and the distance from each of the vibrating sensors calculated under the same conditions as in the case of FIG. 15 in a vibration transmission plate having no anisotropy in which the vibration transmission velocity does not change depending on the direction of the transmission of vibration. It can be understood from FIG. 16 that, when no anisotropy is present, no problem arises even if the distance is calculated by using the mean value of the vibration transmission velocity values of the respective sensors as a constant.