1. Field of the Invention
The present invention relates to a keyboard apparatus having plural keys, and more particularly to a keyboard apparatus having electronic actuators that apply a reaction force with respect to the operation of depressing the plural keys.
2. Description of the Related Art
A natural keyboard instrument such as an acoustic piano or the like is configured to generate a live sound by a hammer, which rotates with the key depression, striking a string, for example. The natural keyboard apparatus of this type has a so-called action mechanism provided between a key, which is a performance operation element, and a hammer. A player receives a unique reaction force (key damping force) from the key by this action mechanism. Specifically, a feeling of a key touch specific to the natural keyboard instrument can be obtained by providing the action mechanism.
On the other hand, in a conventional electronic keyboard instrument, such as an electronic piano, that generates an electronic sound, a mechanical structure, such as a spring or a mass body (hammer) member, that returns a key to an initial position is provided in order to simulate a touch feeling of a natural keyboard apparatus such as an acoustic piano. A player operates a key against the returning force of the spring or the mass body member when he/she depresses the key. In general, the mechanical structure of the electronic keyboard apparatus is compact and not complicated, compared to the action mechanism of the natural keyboard apparatus, so that the touch feeling of a key in the electronic keyboard apparatus is different from the touch feeling of the natural keyboard apparatus, to be strict.
In view of this, there has been proposed a keyboard apparatus in which a key is driven by driving means, such as an electromagnetic solenoid, for changing the reaction force against the key depression, in order to provide a touch feeling similar to that of a natural keyboard instrument in an electronic keyboard instrument. In this keyboard apparatus, the key is driven by the electromagnetic solenoid in such a manner that a position of a movable member of the electromagnetic solenoid is detected, and the key is driven in accordance with the detected position. The keyboard apparatus of this type is, for example, configured as illustrated in FIGS. 6A and 6B (see Japanese Unexamined Patent Application No. 2005-195619).
The keyboard apparatus described above has plural white keys 10 and black keys 11 arranged in the lateral direction, and plural actuator sections 40 and position sensor sections 50 corresponding respectively to the plural white keys 10 and black keys 11. The actuator sections 40 and the position sensor sections 50 are arranged side by side in two rows along the lateral direction of the white keys 10 and the black keys 11. Each of the actuator sections 40 is composed of an electromagnetic solenoid. Each of the position sensor sections 50 includes a reflection plate 51 and an optical sensor 52 for detecting the position of the height of a plunger (movable member) 41 in the actuator section 40. The reflection plate 51 is fixed to the plunger 41, and the light reflectance is changed along the driving direction of the plunger 41. The optical sensor 52 is composed of a light-emitting device that emits light toward the reflection plate 51, and a light-receiving device that receives light, which is reflected by the reflection plate 51, from the light-emitting device, whereby it outputs an electric signal in accordance with the quantity of light received by the light-receiving device as the position of the plunger 41.
However, in the keyboard apparatus described above, a mechanical looseness (microclearance) is caused between a bobbin 42c and the plunger 41. A friction force in the longitudinal direction is applied between the lower end of the plunger 41 and the white key 10 and the black key 11 when the key is depressed or released. Therefore, the plunger 41 clatters in the longitudinal direction Y2 by the vertical movement (pivot movement) of the white key 10 and the black key 11. In the conventional keyboard apparatus, the reflection plate 51 is fixed to the plunger 41 in such a manner that the orthogonal direction Y1 orthogonal to the surface of the reflection plate 51 and the longitudinal direction Y2 of the white key 10 and the black key 11 are parallel to each other. Therefore, when the plunger 41 clatters in the longitudinal direction Y2, the reflection plate 51 also clatters in the longitudinal direction Y2, so that the distances L1 and L2 between the reflection plate 51 and the optical sensor 52 vary as shown in FIG. 4B.
Further, the clattering direction of the plunger 41 upon the key depression and the clattering direction thereof upon the key release are different from each other, whereby the distance L1 upon the key depression and the distance L2 upon the key release are different from each other even if the plunger 41 is located at the position of the same height. Therefore, as shown in FIG. 5A, a problem arises that the voltage signal outputted from the optical sensor 52 is different between the case of the key depression and the case of the key release, i.e., a hysteresis characteristic is generated. Due to this hysteresis, the voltage signal outputted from the optical sensor 52 is different between the case of the key depression and the case of the key release, even if the plunger 41 is located at the position of the same height, thereby entailing a problem that the position of the plunger 41 cannot be specified from the voltage signal. A method of correcting the hysteresis has been considered, but a complicated process is needed for this method.