Patent Publication Number: US-7582821-B2

Title: Electronic musical instrument keyboard apparatus

Description:
BACKGROUND 
   The present invention relates generally to keyboard apparatus for electronic musical instruments, such as electronic organs and electronic pianos. 
   With acoustic pianos, there is produced a key touch feeling called “let-off feeling” that, when a key has been depressed (i.e., static load has been applied to the key), the key touch temporarily becomes heavy (i.e., reactive force to the depressed key increases) partway through the key depression and then rapidly becomes light (i.e., the reactive force to the depressed key decreases) as the key is further depressed. Various schemes have been proposed to allow an electronic musical instrument keyboard apparatus to emulate such a let-off feeling. Japanese Patent No. 3458400, for example, discloses an electronic musical instrument keyboard apparatus, which includes hammers each not only pivoting in interlocked relation to movement of a corresponding key but also giving a feeling of mass during the key depression, and in which the rear end of the hammer contacts a roller that is supported by a resilient member. Immediately before the end of the key depression, the rear end of the hammer engages (or contacts) with the roller so that the reactive force to the depressed key increases by resistance of the resilient member. Upon end of the key depression, the rear end of the hammer disengages from the roller so that the reactive force to the depressed key rapidly decreases. In the aforementioned manner, a let-off feeling can be emulated. 
   In the aforementioned conventional keyboard apparatus, there can be achieved a left-off feeling, but, even at the time of key release, the reactive force to the depressed key would increase due to the engagement (or contact) between the hammer and the roller. Such increase of the reactive force at the time of the key release would retard a returning velocity of the key and thus adversely influence a successive key depression performance of the keyboard apparatus. 
   SUMMARY OF THE INVENTION 
   In view of the foregoing, it is an object of the present invention to provide an improved electronic musical instrument keyboard apparatus capable of achieving a let-off feeling while at the same time securing a good successive key depression performance. 
   In order to accomplish the above-mentioned object, the present invention provides an improved electronic musical instrument keyboard apparatus, which comprises: a depressable and releasable key; a key frame disposed beneath the key for supporting the key in such a manner that the key is pivotable with a front end of the key swinging vertically; a key urging mechanism assembled to the key frame for normally urging upwardly the front end of the key and limiting the front end to a predetermined height position; a movable (displaceable) member provided in the key urging mechanism and movable in interlocked relation to the key; a load member that imparts a load to pivoting movement of said key via the movable member; an actuator that drives the load member; a key position detection section that detects a pivoting position of the key responsive to depression and release operation of the key; and a load control section that performs driving control on the actuator in accordance with the pivoting position of the key detected by the key position detection section to impart a load to the pivoting movement of the key, in accordance with the detected pivoting position of the key, in such a manner that the load to be imparted by the load member in a depression stroke of the key is greater than the load to be imparted by the load member in a release stroke of the key. 
   In this case, the movable member may be in the form of a mass body having an elongated shape, movable in interlocked relation to the pivoting movement of the key and normally urging the front end of the key upwardly, and the load control section may cause the load member to engage (or contact) with the mass body in the depression stroke and terminates the engagement (or contact) of the load member with the mass body in the release stroke. Further, the actuator may be, for example, in the form of an electric actuator employing a super magnetostrictive device not only capable of providing a relatively great driving force with a low voltage but also having a quick response speed. 
   With the load control section performing driving control on the actuator, in accordance with the pivoting position of the key detected by the key position detection section, and changing the state of engagement of the load member with the movable member, in accordance with the detected pivoting position of the key, in such a manner that the load to be imparted in the depression stroke of the key is greater than the load to be imparted in the release stroke of the key, the present invention can increase a reactive force to depression of the key and decrease the reactive force during release of the key. As a result, the present invention can achieve not only a let-off feeling but also a good successive key depression performance. 
   The movable member and load member employed in the aforementioned invention may be replaced with a load member engageable with the key to impart a load to the pivoting movement of the key, and the load control section may perform driving control on the actuator in accordance with the pivoting position of the key detected by the key position detection section to change the state of engagement of the load member with the key, in accordance with the detected pivoting position of the key, in such a manner that the load to be imparted by the load member in the depression stroke of the key is greater than the load to be imparted by the load member in the release stroke of the key. In this case, the load control section may cause the load member to engage with the key in the depression stroke and terminate the engagement of the load member with the key in the release stroke. With such arrangements too, the present invention can increase the reactive force to depression of the key and decrease the reactive force during release of the key. As a result, the present invention can achieve not only a let-off feeling but also a good successive key depression performance. 
   The electronic musical instrument keyboard apparatus of the present invention may further comprise a depressing velocity detection section that detects a depressing velocity of the key, in which case the load control section may perform driving control on the actuator so that the force of the engagement of the load member or key with the movable member decreases as the detected depressing velocity increases. With such arrangements, the present invention can give a human player a massive key touch feeling in response to slow key depression and a light key touch feeling in response to rapid key depression. As a result, the key touch feeling can be even further improved. 
   The electronic musical instrument keyboard apparatus of the present invention may also be constructed so that the force of the engagement of the load member with the movable member or key decreases as the tone pitch corresponding to the key increases. In this case, the driving force to be applied to the actuator by the load control section may be set at different values in advance on a key-by-key or key-range-by-key-range basis, or the force of the engagement of the load member with the movable member or key may be set at different values in advance on the key-by-key or key-range-by-key-range basis. With such arrangements, the present invention can give the human player a massive key touch feeling in response to depression of a key in a low-pitch key range and a light key touch feeling in response to depression of a key in a high-pitch key range. As a result, the key touch feeling can be even further improved. 
   The following will describe embodiments of the present invention, but it should be appreciated that the present invention is not limited to the described embodiments and various modifications of the invention are possible without departing from the basic principles. The scope of the present invention is therefore to be determined solely by the appended claims. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For better understanding of the objects and other features of the present invention, its preferred embodiments will be described hereinbelow in greater detail with reference to the accompanying drawings, in which: 
       FIG. 1  is a plan view of an electronic musical instrument to which are applied keyboard apparatus according to first to third embodiments of the present invention; 
       FIG. 2A  is a vertical sectional view of the keyboard apparatus according to the first embodiment of the present invention, which shows a state where all keys are in a released position; 
       FIG. 2B  is a vertical sectional view of the keyboard apparatus according to the first embodiment of the present invention, which shows a state where one of the keys is in a depressed position; 
       FIG. 3  is a detailed sectional view showing a construction of a key switch shown in  FIGS. 2A and 2B ; 
       FIGS. 4A-4I  are views showing positional relationship between a mass body of a pivot lever and a load member during depression/release operation of a key; 
       FIG. 5  is a vertical sectional view of the electronic musical instrument keyboard apparatus according to the second embodiment of the present invention, which shows a state where all of the keys are in a released position; 
       FIGS. 6A-6C  are views showing positional relationship between the mass body of the pivot lever and the load member during depression/release operation of a key; 
       FIG. 7  is a vertical sectional view of the electronic musical instrument keyboard apparatus according to the third embodiment of the present invention, which shows a state where all of the keys are in a released position; 
       FIG. 8  is a block diagram of an electric control unit according to the fourth embodiment of the present invention; 
       FIG. 9  is a graph showing variation characteristics of driving forces corresponding to various key depressing velocities; and 
       FIG. 10  is a graph showing variation characteristics of driving forces corresponding to various tone pitches. 
   

   DETAILED DESCRIPTION 
   a. First Embodiment 
     FIG. 1  is a plan view showing an electronic musical instrument and a keyboard apparatus according to a first embodiment of the present invention employed in the electronic musical instrument. The electronic musical instrument includes a plurality of panel switchers PSW for selecting a desired operation style. The electronic musical instrument includes, on an upper front surface portion thereof, a keyboard apparatus having a plurality of white keys  10  and black keys  10  arranged in a horizontal left-right direction of the musical instrument. 
     FIGS. 2A and 2B  are vertical sectional views of the keyboard apparatus. More specifically,  FIG. 2A  shows a state where all of the keys  10  are in a released or non-depressed position, and  FIG. 2B  shows a state where one of the keys  10  is in a depressed position. The plurality of keys  10  are assembled to a key frame  20  formed integrally of synthetic resin, and the key frame  20  is fixed to and located over a support table  30 . A plurality of pivot levels  40 , constituting key biasing mechanisms corresponding to the keys  10 , are assembled to the key frame  20  under the corresponding keys  10 . 
   Each of the keys  10  is integrally formed of synthetic resin into a downwardly-opening U or channel sectional shape. Rear end portion  11  of each of the keys  10  is pivotably fitted in a forwardly-opening recess formed in a rear end portion  21  of the key frame  20 . Each of the keys  10  is pivotably supported on the key frame  20  in such a manner that its front end portion  12  can vertically swing with side surfaces of the rear end portion  11  as a pivot point. Key guide  22  projecting upward from a front-end horizontal portion of the key frame  20  enters the front end portion  12  of the key  10  from below. During depression of the key, the front end portion  12  of the key  10  is displaced vertically while being guided by the key guide  22 . Driving portion  13  is formed integrally with the underside of a front region near the front end portion  12  of the key  10  and extends vertically downward from the underside. The driving portion  13  has a U or channel horizontal sectional shape opening rearwardly and has a closed lower end. 
   The pivot lever  40  comprises a resin-made lever base section  41  and a metal-made mass body  42  as a movable or displaceable member. The lever base section  41 , which is molded into an elongated and flat plate shape, extends in a front-rear-direction of the keyboard apparatus and is located under a front portion of the corresponding key  10  with its plate surface oriented generally vertically. The lever base section  41  has a recessed portion  41   a  formed in the lower surface of its longitudinally-middle region and having an axis line lying in the horizontal left-right direction (or key-arranged direction) of the keyboard, and the recessed portion  41   a  has a greater thickness in the axial direction. The recessed portion  41   a  opens obliquely forwardly and downwardly and engages with a pivot support portion  23   a  that is provided at the upper end of a slanting plate  23  extending obliquely rearwardly and upwardly from a front lower end position of the key frame  20 . The pivot support portion  23   a  extends in the horizontal left-right direction (i.e., key-arranged direction) of the keyboard. The lever base section  41  is normally urged forward by a leaf spring  43  that constitutes a key urging mechanism supported on the rear end portion  11  of the key  10 . In this manner, the pivot lever  40  is vertically pivotably supported on the key frame  20 . 
   Front end portion of the lever base section  41  is vertically bifurcated into a pair of upper and lower leg portions  41   b  and  41   c  vertically spaced from each other by a predetermined space, and the upper leg portion  41   b  has a smaller length than the lower leg portion  41   c . Lower end wall portion of the driving portion  13  of the key  10  is located between and engages with the upper and lower leg portions  41   b  and  41   c . Thus, as the key  10  is released from the depressed position, a front end portion of the pivot lever  40  is displaced upward due to the weight of the lever  40 , so that a front end portion of the key  10  too is displaced upward. On the other hand, as the key  10  is depressed, the lower end surface of the lower end wall portion of the driving portion  13  presses the upper surface of the lower leg portion  41   c , so that the front end portion of the pivot lever  40  is displaced downward. 
   Downwardly-projecting switch driving portion  41   d  is formed on the underside of the lever base section  41  between the recessed portion  41   a  and the upper leg portion  41   b . The switch driving portion  41   d  is opposed to a key switch  52  provided on a printed circuit board  52  via a window  23   b  formed through the slanting plate  23 , and this key switch  52  constitutes a key position detection means. Such key switches  52  are provided in corresponding relation to the keys  10  and arranged in the horizontal left-right direction (i.e., key-arranged direction) of the keyboard. 
   As shown in enlarged scale in  FIG. 3 , each of the key switches  52  comprises first to third switches  52   a ,  52   b  and  52   c  arrayed in the front-rear direction of the keyboard. The first to third switches  52   a ,  52   b  and  52   c , each of which is formed into a semispherical shape (or bowl shape) having an inner space, are provide on a switch member formed of a resilient substance, such as rubber or silicon, elongated in the left-right direction of the keyboard. Further, each of the first to third switches  52   a ,  52   b  and  52   c  has a cylindrical columnar portion formed integrally on, and extending downward from, a central inner surface portion thereof. Electric contact is provided on the lower end surface of the downwardly-projecting cylindrical columnar portion of each of the first to third switches  52   a ,  52   b  and  52   c , and each of the electric contacts is opposed to two electric contacts provided on the printed circuit board  51  in corresponding relation to thereto. As the switch driving portion  41   d  moves downward in response to depression operation of the key  10 , the first to third switches  52   a ,  52   b  and  52   c  are brought into contact with (or turn on) the corresponding electric contacts on the printed circuit board  51 . Further, the cylindrical columnar portions of the first to third switches  52   a ,  52   b  and  52   c  have different lengths that become sequentially smaller in the order of mentioning. As the key  10  is depressed, the first, second and third switches  52   a ,  52   b  and  52   c  sequentially turn ON in the order of mentioning. As the key  10  is released, the third, second and first switches  52   c ,  52   b  and  52   a  turn OFF in the order of mentioning. 
   The mass body  42  of the pivot lever  40  is in the form of a rod, which is assembled integrally to the lever base section  41  by outsert-molding the lever base section  41  onto a front outer peripheral portion of the pivot lever  40 . The mass body  42  has a rear folded-back portion  42   a . The folded-back portion  42   a  differs in length among the mass bodies  42  corresponding to the plurality of keys  10 , so that the mass bodies  42  differ in weight from one another. More specifically, for both the white keys  10  and the black keys  10 , the folded-back portions  42   a  have lengths that gradually decrease, on a key-by-key or key-range-by-key-range basis, in a direction from the lowest-pitch key to the highest-pitch key, so that the mass bodies  42  for the keys of lower pitches or pitch ranges have greater weights, i.e. greater rotational moments. Further, for each pair of adjoining white and black keys  10 , the length of the folded-back portion  42   a  of the black key  10  is set smaller than that of the white key  10  to avoid a difference that would occur in reactive force to key depression due to a difference in key depression position. 
   Further, an elongated lower limit stopper  53  made of an impact absorbing substance, such as felt, is secured to the upper surface of the support table  30  located on a rear end portion of the key frame  20 , and the lower-limit stopper  53  extends in the horizontal left-right direction of the keyboard. The lower limit stopper  53  functions to limit downward displacement or movement of a rear end portion of the pivot lever  40 , to thereby limit upward displacement of a front end portion of the key  10  when the key  10  is released from the depressed position. Further, an elongated upper limit stopper  54  made of an impact absorbing substance, such as felt, is secured to the lower surface of an upper surface plate  24  located on a rear end portion of the key frame  20 . The upper limit stopper  54  is vertically spaced apart from the lower limit stopper  53  by a predetermined distance and extends in the left-right direction of the keyboard. The upper limit stopper  54  functions to limit upward displacement of the rear end portion of the pivot lever  40 , to thereby limit downward displacement of the front end portion of the key  10  when the key  10  is depressed. The lower limit stopper  53  and upper limit stopper  54  both constitute the key urging mechanism. Proximity sensor (proximity switch)  55  is provided forwardly of the upper limit stopper  54  and in opposed relation to the mass body  42 . The proximity sensor  55  is a sensor for detecting when the mass body  42  is in abutting contact with or located proximal to the upper limit stopper  54 , using electromagnetic induction, electrostatic capacitance, ultrasonic sound wave, photo-electric effect, magnetic change, or the like. The proximity sensor  55  constitutes a key position detection means. 
   Driving unit  60  is assembled to the key frame  20  behind the keys  10 . The driving unit  60  includes a support plate  61  bent into a generally hook-like shape and fixed to the key frame  20 . Actuators  63 , accommodated in a case  62  fixed to the support plate  61 , are secured to the support plate  61  in corresponding relation to the keys  10 . Driving rod  63   a  is normally urged leftward by a built-in spring. Each of the actuator  63 , which is controlled electrically, displaces, by application of a voltage, the driving rod  63   a  in a rightward direction in the figure, to reciprocate a load member  64  fixed to the distal end of the driving rod  63   a . The actuator  63  is preferably in the form of an electric actuator employing a super magnetostrictive device not only capable of providing a relatively great driving force with a low voltage but also having a quick response speed, although various other types of actuators, such as an electromagnetic solenoid, may be used as long as driving of the actuator can be electrically controlled. The load member  64  is molded of a resilient substance into a generally cylindrical columnar shape with a semispherical distal end. When the load member  64  is in a leftward projecting position as shown  FIG. 2A , it engages (or contacts) with a rear end portion of the mass body  42 , functioning as the movable or displaceable member, to impart a load to the pivoting movement of the mass body  42  and hence the key  10 . This load can be adjusted by adjustment of any of the projecting amount, shape, substance, etc. of the load member  64 . 
   The key switches  52  and proximity sensors  55  provided in corresponding relation to the keys  10  are connected to a load control circuit  70 . The load control circuit  70  electrically controls the driving of each of the actuators  63  to cause the load member  64  to engage (or contact) with the mass body  42  as the displaceable member, to thereby impart a load to the pivoting movement of the key  10 . The load control circuit  70  includes a microcomputer comprising a CPU, ROM, RAM, etc., and a drive circuit for outputting a driving signal to each of the actuators  63  in accordance with an instruction given by the microcomputer. More specifically, the load control circuit  70  is responsive to detection, by the proximity sensor  55 , of proximity of the mass body  42  to output a driving voltage to the actuator  63  corresponding to the detected mass body  42  and then pull or retract the driving rod  63   a  in the rightward direction of the figure against the biasing force of the spring. Further, the load control circuit  70  detects a change from an ON state to an OFF state of the first switch  52   a , in response to which it cancels or terminates the driving force to the actuator  63  corresponding to the detected first switch  52   a  to thereby cause the driving rod  63   a  to project in the leftward direction of the figure by the biasing force of the spring. 
   Signals from the first to third switches  52   a - 52   c  of each of the key switches  52  corresponding to the keys  10  are also supplied to a not-shown tone signal generation circuit. Upon detection of a change from the OFF state to the ON state of the third switch  52   c , the tone signal generation circuit starts generating a tone signal of a tone pitch corresponding to the third switch  52   c  having changed to the ON state. Further, upon detection of a change from the ON state to the OFF state of the first switch  52   a , the tone signal generation circuit starts attenuating the tone signal of the tone pitch corresponding to the first switch  52   a  having changed to the OFF state, and then ends the generation of the tone signal. Furthermore, the tone signal generation circuit inputs tone signals from the first and second switches  52   a  and  52   b  of each of the key switches  52  corresponding to the keys  10  and a key depressing velocity per key by measuring a length of time from a time point when the first switch  52   a  changes from the ON state to the OFF state to a time point when the second switch  52   b  changes from the OFF state to the ON state. The thus-detected key depressing velocity is used to control a tone volume and color of a tone signal to be generated. 
   Next, a description will be given about behavior of the keyboard apparatus constructed as above according to the first embodiment of the present invention. For each key  10  which is not being depressed (i.e., is in the non-depressed position), the rear end portion of the mass body  42  of the corresponding pivot lever  40  is in abutment against the lower limit stopper  53  by its own weight. Once the key  10  is depressed by a human player in this state, the key  10  starts pivoting in a counterclockwise direction of  FIG. 2A  about the rear end portion  11  against the weight of the mass body  42  of the pivot lever  40 . Once the key  10  is depressed to a predetermined depth, the rear end portion of the mass body  42  abuts against the upper limit stopper  54  as shown in  FIG. 2B , so that further downward displacement of the front end portion of the key  10  is prevented. At that time, as the pivot lever  40  pivots, the downwardly-extending switch driving portion  41   d  presses the key switch  52 , so that the first, second and third switches  52   a ,  52   b  and  52   c  sequentially turn on the order of mentioning. Then, once the key  10  is released, the pivot lever  40  pivots in a clockwise direction of  FIG. 2B  about the pivot support portion  23   a  because of the weight of the mass body  42  of the pivot lever  40  until the rear end portion of the mass body  42  abuts against the lower limit stopper  53 , so that the mass body  42  returns to its original position. The tone signal generation circuit controls the tone volume and color of a tone signal to be generated, in accordance with the detected key depressing velocity. 
   During such depression/release operation of the key  10 , the load control circuit  70  performs control on the actuator  63  in accordance with the pivoting movement of the mass body  42 , i.e. pivoting position of the key  10 . When the first switch  52   a  of the key switch  52  is in the OFF state, the load control circuit  70  imparts no driving force to the actuator  63 . In this state, the driving rod  63   a  of the actuator  63  is kept in the leftward projecting position as shown in  FIG. 4A . As the key  10  and pivot lever  40  pivot in response to key depression operation so that the rear end portion of the mass body  42  is displaced upward as shown in  FIGS. 4B and 4D , the rear end portion of the mass body  42  contacts a front end portion of the load member  64  to deform the load member  64 . Then, as the rear end portion of the mass body  42  further moves upward, it gets over the front end portion of the load member  64  as shown in  FIG. 4E . In this case, a force by the engagement (or contact) between the rear end portion of the mass body  42  and the load member  64 , i.e. reactive force (resilient force) resultant from the deformation of the front end portion of the load member  64 , acts as a load to the key depression operation by the human player. This load acts in such a manner that the key touch temporarily becomes heavy (i.e., reactive force to the depressed key increases). Then, as the key is further depressed, the engagement (or contact) between the rear end portion of the mass body  42  and the load member  64  is canceled and the load member  64  rapidly becomes light (i.e., the reactive force to the depressed key decreases), so that the human player can enjoy a let-off feeling. 
   As the key is depressed even further, the upper surface of the rear end portion of the mass body  42  abuts against the upper limit stopper  54 , so that the pivot lever  40  stops pivoting. Immediately before the end of the pivoting movement of the pivot lever  40 , the third switch  52   c  of the key switch  52  changes from the OFF state to the ON state, so that generation of the tone signal is started. The first and second switches  52   a  and  52   b  of the key switch  52  both change from the OFF state to the ON state by the time the rear end portion of the mass body  42  abuts against the load member  64 . 
   When the upper surface of the rear end portion of the mass body  42  has abutted against the upper limit stopper  54 , the proximity sensor  55  detects proximity of the mass body  42 , and the load control circuit  70  energizes and drives the actuator  63 . Thus, the actuator  63  retracts the driving rod  63   a  in the rightward direction as shown in  FIG. 4F  against the biasing force of the spring. Then, once the depressed key  10  is released in the above-mentioned state, the rear end portion of the mass body  42  is displaced downward, as shown in  FIG. 4F  to  FIGS. 4G and 4H , as the key  10  and pivot lever  40  pivot upward. Because the load member  64  has been retracted rightward, the rear end portion of the mass body  42  moves downward without contacting the front end portion of the load member  64 . Then, the lower surface of the rear end portion of the mass body  42  abuts against the lower limit stopper  53 , so that the key  10  and pivot lever  40  stop pivoting upward. In such a release stroke of the key  10 , no load is imparted from the load member  64  to the pivoting movement of the key  10  and pivot lever  40  because the front end portion of the load member  64  does not engage (or contact) with the load member  64 . Thus, a velocity at which the key  10  returns to the original position can be accelerated, with the result that operation for successively depressing the key  10  can be performed appropriately as desired; namely, successive depression performance of the key  10  can be effectively enhanced. 
   Immediately before the lower surface of the rear end portion of the mass body  42  abuts against the lower limit stopper  53  as noted above, the first switch  52   a  of the key switch  52  changes from the ON state to the OFF state. In response to the ON-to-OFF state change of the first switch  52   a , the load control circuit  70  terminates the driving of the actuator  63 . Thus, as shown in  FIGS. 4I and 4A , the driving rod  63   a  of the actuator  63  again projects leftward by virtue of the biasing force of the spring. Then, the aforementioned behavior is repeated once the key  10  is again depressed and released by the human player. 
   If the same key  10  has been again depressed during the release stroke before the first switch  52   a  changes from the ON state to the OFF state, i.e. the same key  10  has been depressed successively more rapidly than in the aforementioned successive depression, the driving rod  63   a  is kept by the actuator  63  in the rightward-retracted position as shown in  FIG. 4H . Thus, during the upward displacement of the mass body  42  caused by the key depression operation, the rear end portion of the mass body  42  moves downward without contacting the load member  64 , and thus, no load is imparted from the load member  64  to the pivoting movement of the key  10  and pivot lever  40 . Therefore, during the rapid successive depression operation of the key  10 , the load member  64  does not engage (or contact) with the rear end portion of the mass body  42  in the key depression stroke too. Consequently, during the rapid successive depression operation of the key  10 , the human player can perform depression and release operation of the key  10  as desired with no load applied from the load member  64  to the key  10 , which can significantly facilitate a performance involving rapid successive depression operation of the key  10 . 
   In the first embodiment, as set forth above, the driving rod  63   a  of the actuator  63  is normally urged by the spring in the leftward projecting position so that, in the release stroke of the key  10 , the actuator  63  is driven to retract the driving rod  63   a  rightward against the biasing force of the spring. In a modification, however, the driving rod  63   a  of the actuator  63  may be normally urged by the spring to in a rightward projecting position so that, in the depression stroke of the key  10 , the actuator  63  is driven to retract the driving rod  63   a  leftward to cause the rear end portion of the mass body  42  and the load member  64  to engage (or contact) with each other. In such a case, the load control circuit  70  drives the actuator  63  to cause the driving rod  63   a  to project leftward when the first switch  52   a  of the key switch  52  has changed from the OFF state to the ON state, taking electric power consumption into account. Then, once the rear end portion of the mass body  42  reaches the predetermined position proximal to the upper limit stopper  54 , the load control circuit  70  terminates the driving of the actuator  63  so as to retract the driving rod  63   a  rightward by the biasing force of the spring. 
   In the aforementioned modification of the first embodiment too, a left-off feeling can be given, during the depression stroke of the key  10 , to the key depression operation of the human player by the load member  64  engaging with the rear end portion of the mass body  42 . Further, during the release stroke of the key  10 , the load member  64  is disengaged from the rear end portion of the mass body  42  so that the returning velocity of the key  10  can be accelerated and thus a good successive depression performance can be maintained as desired. Furthermore, in this modification too, if the same key  10  has been again depressed before the first switch  52   a  changes from the ON state to the OFF state, i.e. the same key  10  has been depressed successively more rapidly than in the aforementioned successive depression, the driving rod  63   a  of the actuator  63  can continue to be is kept by the spring in the rightward retracted position. Namely, in this case, the load control circuit  70  does not detect a change from the OFF state to the ON state of the first switch  52   a  and does not drive the actuator  63 . Thus, in this modification too, during the rapid successive depression operation of the key  10 , the load member  64  does not engage (or contact) with the rear end portion of the mass body  42  in the key depression stroke too, and thus, a performance involving rapid successive depression operation of the key  10  can be executed with ease. 
   b. Second Embodiment 
   Next, a description will be given about the electronic musical instrument keyboard apparatus according to the second embodiment of the present invention, which employs a modified load mechanism for giving a load to the pivoting movement of the key  10  and pivot lever  40 .  FIG. 5  is a vertical sectional view of the electronic musical instrument keyboard apparatus according to the second embodiment of the present invention. This keyboard apparatus includes a load member  65  that is driven by the driving unit  60  to give a load to the pivoting movement of the key  10  and pivot lever  40 . The load member  65  is formed integrally of synthetic resin into a hook-like or L shape having vertical and horizontal portions  65   a  and  65   b  extending substantially at right angles to each other. The load member  65  is rotatably supported at an intermediate region of the vertical portion  65   a  on a support section  66 , fixed to the support table  30 , via a pin  66   a . The horizontal portion  65   b  of the load member  65  has a distant end portion projecting into an upper area where the front end portion of the mass member  42  passes. Only a front end portion of the horizontal portion  65   b  or the whole of the load member  65  may be formed of a resilient substance. The load member  65  is normally urged in a counterclockwise direction in the figure by a built-in weight or spring (not shown). Pressing member  63   b  fixed to the distal end of the driving rod  63   a  of the actuator  63  is held in abutment against the rear surface of a lower end portion of the vertical portion  65   a  of the load member  65 . The pressing member  63   b  is formed of resin integrally with the driving rod  63   a  and has a distal end portion formed into a semispherical shape. The pressing member  63   b  may be formed of an elastic substance, such as rubber or elastomer, into a cylindrical columnar shape with a semispherical distal end. 
   The actuator  63  in the second embodiment is constructed similarly to the actuator  63  in the first embodiment, but, in the second embodiment, the driving rod  63   a  in this actuator  63  is normally urged by the built-in spring in a leftward retracted position. In this state, the rear end of the mass body  42  contacts (i.e., engages with) the front end of the horizontal portion  65   b  of the load member  65  as the mass body  42  is displaced between the lower limit stopper  53  and the upper limit stopper  54  by the pivoting movement of the key  10  and pivot lever  40 . Once the actuator  63  is driven via the load control circuit  70 , it causes the driving rod  63   a  to project in the leftward direction of the figure. Thus, the load member  65  pivots in the clockwise direction, so that the distal end portion of the horizontal portion  65   b  is displaced rearward. In this state, the rear end of the mass body  42  does not contact (i.e., does not engage with) the front end of the horizontal portion  65   b  of the load member  65  even if the mass body  42  is displaced between the lower limit stopper  53  and the upper limit stopper  54  by the pivoting movement of the key  10  and pivot lever  40 . The actuator  63  is accommodated in the case  62  as in the above-described first embodiment, and the case  62  is fixed to the support table  30 . 
   The load control circuit  70  starts driving the actuator  63  when the proximity sensor  55  has detected proximity of the mass body  42 , and terminates the driving of the actuator  63  when the first switch  52   a  of the key switch  52  has changed from the ON state to the OFF state. The other structural arrangements of the second embodiment are similar to those of the first embodiment. Note that, in the second embodiment, the intensity of the load to the pivoting movement of the key  10  and pivot lever  40  is adjustable by adjusting any of the rotational amount, shape, substance, etc. of the load member  65 . 
   Next, a description will be given about behavior of the keyboard apparatus constructed as above according to the second embodiment of the present invention. Tone signal generation and termination of the tone signal generation responsive to depression/release operation of the key  10  is similar to that in the above-described first embodiment. In the second embodiment, during depression/release operation of the key  10 , the load member  65  pivots about the pin  66   a  in response to the pivoting movement of the pivot lever  40  and driving of the actuator  63 . When the key  10  is in the released position and the rear end portion of the mass body  42  is located over the lower limit stopper  53 , the actuator  63  is in the non-driven state, so that the driving rod  53  is kept in the rightward retracted position, the pressing member  63   b  is kept in abutment against the rear surface of the lower end portion of the vertical portion  65   a  of the load member  65 , and the front end of the horizontal portion  65   b  is kept in the forward (leftward in the figure) projecting position. When the key  10  is depressed in such a state, the rear end portion of the mass body  42  is displaced upward by the pivoting movement of the key  10  and pivot lever  40  as shown in  FIG. 6B , so that the rear end portion of the mass body  42  engages (or contacts) with the front end portion of the horizontal portion  65   b  of the load member  65  and the load member  65  pivots in the clockwise direction against the biasing force of the weight or spring. 
   Then, as the rear end portion of the mass body  42  further moves upward, it gets over the front end portion of the horizontal portion  65   b  of the load member  65  and abuts against the upper limit stopper  54 , as shown in  FIG. 6C . Once the rear end portion of the mass body  42  gets over the front end portion of the horizontal portion  65   b  of the load member  65 , the load member  65  returns to the original position (i.e., position shown in  FIG. 6A ) by virtue of the biasing force of the weight or spring. While the rear end portion of the mass body  42  is in engagement with the front end portion of the horizontal portion  65   b  of the load member  65 , a force causing the load member  65  to pivot in the clockwise direction serves as a load to key depression operation of the human player during the depression stroke. Further, if the distal end of the horizontal portion  65   b  of the load member  65  or the whole of the load member  65  is formed of an elastic substance, a force caused by deformation of the elastic substance as well as the force causing the load member  65  to pivot in the clockwise direction serves as a load to key depression operation of the human player. Such a load acts in such a manner that the key touch temporarily becomes heavy (i.e., reactive force to the depressed key increases) partway through the key depression. Then, as the key is further depressed, the engagement between the rear end portion of the mass body  42  and the horizontal portion  65   b  of the load member  65  is canceled and the key touch rapidly becomes light (i.e., the reactive force to the depressed key rapidly decreases), so that the human player can enjoy a let-off feeling. 
   When the upper surface of the rear end portion of the mass body  42  has abutted against the upper limit stopper  54 , the proximity sensor  55  detects proximity of the mass body  42 , and the load control circuit  70  energizes and drives the actuator  63 . The actuator  63  causes the driving rod  63   a  to project in the leftward direction as shown in  FIG. 6C  against the biasing force of the spring. Thus, the load member  65  pivots in the clockwise direction so that the front end of the horizontal portion  65   b  of the load member  65  moves rightward. Then, once the depressed key  10  is released in this state, the rear end portion of the mass body  42  is displaced downward as the key  10  and pivot lever  40  pivot upward. Because the front end of the horizontal portion  65   b  of the load member  65  has been retracted rightward, the rear end portion of the mass body  42  moves downward without contacting the front end portion of the load member&#39;s horizontal portion  65   b . Then, the lower surface of the rear end portion of the mass body  42  abuts against the lower limit stopper  53 , so that the key  10  and pivot lever  40  stop pivoting upward as shown in  FIG. 6A . During the release stroke of the key  10 , no load is imparted from the load member  65  to the pivoting movement of the key  10  and pivot lever  40  because the rear end portion of the mass body  42  does not engage (or contact) with the horizontal portion  65   b  of the load member  65 . Thus, the velocity at which the key  10  returns to the original position can be accelerated, with the result that operation for successively depressing the key  10  can be performed more appropriately; namely, the successive depression performance of the key  10  can be effectively enhanced. 
   Immediately before the lower surface of the rear end portion of the mass body  42  abuts against the lower limit stopper  53  as noted above, the first switch  52   a  of the key switch  52  changes from the ON state to the OFF state. In response to the ON-to-OFF state change of the first switch  52   a , the load control circuit  70  terminates the driving of the actuator  63 . Thus, as shown in  FIG. 6A , the driving rod  63   a  of the actuator  63  is again retracted rightward. Then, the aforementioned behavior is repeated once the key  10  is again depressed and released by the human player. 
   If the same key  10  has been again depressed before the first switch  52   a  changes from the ON state to the OFF state, i.e. the same key  10  has been depressed successively more rapidly than in the aforementioned successive depression, the driving rod  63   a  is kept by the actuator  63  in the leftward projecting position as shown in  FIG. 6C . Thus, during the upward displacement of the rear end portion of the mass body  42  caused by the key depression operation, the rear end portion of the mass body  42  moves without contacting the horizontal portion  65   b  of the load member  65 , and thus, no load is imparted from the load member  65  to the pivoting movement of the key  10  and pivot lever  40 . Therefore, during the rapid successive depression operation of the key  10 , the load member  65  does not engage (or contact) with the rear end portion of the mass body  42  in the key depression stroke too. Consequently, during the rapid successive depression operation of the key  10 , the human player can perform depression and release operation of the key  10  with no load applied from the load member  65  to the key  10 , which can facilitate a performance involving rapid successive depression operation of the key  10 . 
   In the second embodiment, as set forth above, the driving rod  63   a  of the actuator  63  is normally urged by the spring in the rightward retracted position so that, in the release stroke of the key  10 , the actuator  63  is driven to cause the driving rod  63   a  to project leftward against the biasing force of the spring. In a modification, however, the driving rod  63   a  of the actuator  63  may be normally urged by the spring in a leftward projecting position so that, in the depression stroke of the key  10 , the actuator  63  is driven to retract the driving rod  63   a  rightward to cause the rear end portion of the mass body  42  and the horizontal portion  65   b  of the load member  65  to engage (or contact) with each other. In such a case, the load control circuit  70  drives the actuator  63  to cause the driving rod  63   a  to project rightward when the first switch  52   a  of the key switch  52  has changed from the OFF state to the ON state, taking electric power consumption into account. Then, once the rear end portion of the mass body  42  reaches the predetermined position proximal to the upper limit stopper  54 , the load control circuit  70  terminates the driving of the actuator  63  so as to cause the driving rod  63   a  to project leftward by the biasing force of the spring. 
   In the aforementioned modification of the second embodiment too, a left-off feeling can be given, during the depression stroke of the key  10 , to the key depression operation of the human player by the horizontal portion  65   b  of the load member  65  engaging with the rear end portion of the mass body  42 . Further, during the release stroke of the key  10 , the horizontal portion  65   b  of the load member  65  is disengaged from the rear end portion of the mass body  42  so that the returning velocity of the key  10  can be accelerated and thus a good successive depression performance can be maintained. Furthermore, in this modification too, if the same key  10  has been again depressed before the first switch  52   a  changes from the ON state to the OFF state, i.e. the same key  10  has been depressed successively more rapidly than in the aforementioned successive depression, the driving rod  63   a  of the actuator  63  can continue to be is kept by the spring in the leftward projecting position. Namely, in this case, the load control circuit  70  does not detect a change from the OFF state to the ON state of the first switch  52   a  and does not drive the actuator  63 . Thus, in this modification too, during the rapid successive depression operation of the key  10 , the horizontal portion  65   b  of the load member  65  does not engage (or contact) with the rear end portion of the mass body  42  during the key depression stroke too, and thus, a performance involving rapid successive depression operation of the key  10  can be executed with ease. 
   b. Third Embodiment 
   Next, a description will be given about the electronic musical instrument keyboard apparatus according to the third embodiment of the present invention, which is constructed to give a load to the pivoting movement of the key  10  and pivot lever  40 .  FIG. 7  is a vertical sectional view of the electronic musical instrument keyboard apparatus according to the third embodiment of the present invention. This keyboard apparatus includes, in place of the support table  30  employed in the first and second embodiments, a bottom plate  31  elongated in the left-right direction of the keyboard apparatus and formed by processing wood. Front plate  32  elongated in the left-right direction of the keyboard apparatus is fixed to the front longitudinal end edge of the bottom plate  31  and extends vertically upward from the front longitudinal end edge of the bottom plate  31 , and a metal back surface panel  33  elongated in the left-right direction of the keyboard apparatus is fixed to the rear upper surface of the bottom plate  31  and extends vertically upward from the rear upper surface of the bottom plate  31 . The key frame  20  in the third embodiment has a different shape from that employed in the first and second embodiments, and the key  10  is pivotably supported by the key frame  20  in a space surrounded by the bottom plate  31 , front plate  32  and back surface panel  33 . 
   Key support member  25  is fixed to the upper surface of a rear portion of the key frame  20 , and this key support member  25  supports the key  10  in such a manner that the key  10  is pivotable about the axis of a pin  25   a  at a rear end portion of the key support member  25 ; the key support member  25  permits vertical pivoting movement of the key  10 . The third embodiment of the keyboard apparatus, shaped differently from the above-described first and second embodiments, also includes the pivot lever  40  for normally urging the front end portion of the key  10  upward by the weight of the lever  40  and for limiting the front end portion of the key  10  to predetermined upper and lower positions. In this case too, the pivot lever  40  includes a lever base section  44  and mass body  45 . The lever base section  44  is formed of synthetic resin and supported at a rear end portion on a lever support section  26  provided on the underside of the key frame  20  in such a manner that it is pivotable about the axis of a pin  26   a . In this case too, the lever base section  44  has a pair of upper and lower leg portions  44   a  and  44   b  at its front end. The upper leg portion  44   a  has a smaller length than the lower leg portion  44   b . Lower end wall portion of the driving portion  13  of the key  10  is located between and engages with the upper and lower leg portions  44   a  and  44   b . Thus, as the key  10  is released from the depressed position, the front end portion of the pivot lever  40  is displaced upward due to the weight of the lever  40 , so that the front end portion of the key  10  is displaced upward. As the key  10  is depressed, on the other hand, the lower end surface of the lower end wall portion of the driving portion  13  presses the upper surface of the leg portion  44   b , and the front end portion of the pivot lever  40  is displaced downward. 
   The mass body  45  is in the form of a metal rod and fixed at its front end portion to the lever base section  44 , and a resin-made stopper member  45   a  is integrally fixed to a rear end portion of the mass body  45 . Similarly to the mass body  42  in the first and second embodiments, the mass body  45  urges the pivot lever  40  in the clockwise direction by its own weight. As the key  10  is released from the depressed position, the stopper member  45   a  abuts against the lower limit stopper  53  to limit clockwise pivoting movement of the pivot lever  40 . As the key  10  is depressed, on the other hand, the stopper member  45   a  abuts against the upper limit stopper  54  to limit counterclockwise pivoting movement of the pivot lever  40 . In the third embodiment too, the mass bodies  45  or stopper members  45   a  corresponding to the keys  10  have weights differing on the key-by-key or key-range-by-key-range basis so that the key depression touch becomes heavier for the keys  10  of lower pitches or lower pitch ranges. 
   Further, in the third embodiment, the key switch  52  is provided on the upper surface of a middle region, in the front-rear direction of the keyboard apparatus, of the key frame  20  and is depressed by a switch driving portion  14  provided on the underside of the key  10 . The key switch  52  comprises first, second and third switches  52   a ,  52   b  and  52   c  as in the first and second embodiments. As the key  10  is depressed, the first, second and third switches  52   a ,  52   b  and  52   c  sequentially turn ON in the order of mentioning, while, as the key  10  is released, the third, second and first switches  52   c ,  52   b  and  52   a  turned OFF in the order of mentioning. 
   The driving unit  60  is provided in opposed relation to the rear end surface of the key  10 . The driving unit  60  includes the actuators  63  accommodated in the case  62  fixed to the key frame  20  and back surface panel  33 . Each of the actuator  63  is constructed similarly to that in the first and second embodiments, and the driving rod  63   a  retractably projects out of the case  62  toward the rear end surface of the key  10 . In this case, the driving rod  63   a  is normally urged by the built-in spring in the leftward direction of the figure, and a load member  67  is held in a leftward projecting position when the actuator  63  is in the non-driven state. Once driven, the actuator  63  retracts the driving rod  63   a  rightwardly. 
   The load member  67  is fixed to the distal end of the driving rod  63   a . The load member  67  is formed of an elastic substance, such as rubber or elastomer, and its front end surface  67   a  is recessed to have an arcuate sectional shape (as viewed transversely to the axis of the load member  67 ) and opposed to the rear end surface  15  of the key  10 . The rear end surface  15  of the key  10  is formed convexly to have an arcuate sectional shape. When the actuator  63  is in the non-driven state, the front end surface  67   a  of the load member  67  is held in abutting engagement (or contact) with the rear end surface  15  of the key  10 , and as the key  10  is depressed, the load member  67  gives a load to the pivoting movement of the key  10  and pivot lever  40  by a frictional force between the front end surface  67   a  and the rear end surface  15 . When the actuator  63  is driven, the load member  67  is retracted rightward, so that the front end surface  67   a  of the load member  67  disengages from the rear end surface  15  of the key  10 . The front end surface  67   a  of the load member  67  and the rear end surface  15  of the key  10  may be formed into a non-arcuate sectional shape, and the intensity of the load to be imparted to the pivoting movement of the key  10  and pivot lever  40  is adjustable by adjusting any of the driving force of the actuator  63 , substance of the load member  67 , shapes of the front end surface  67   a  of the load member  67  and the rear end surface  15  of the key  10 , etc. 
   Further, the third embodiment of the keyboard apparatus includes, in addition to the proximity sensor  55  for detecting proximity of the mass body  45  during upward displacement of the mass body  45 , a proximity sensor  56  fixed on the upper surface of the bottom plate  31  for detecting proximity of the mass body  45  during downward displacement of the mass body  45 . The proximity sensor  56  is constructed similarly to the proximity sensor  55 , and it detects proximity of the stopper member  45   a  (mass body  45 ) when or immediately before the stopper member  45   a  abuts against the lower limit stopper  53 . Once the proximity sensor  55  detects proximity of the mass body  45  as the mass body  45  approaches the lower limit stopper  54 , the load control circuit  70  drives the actuator  63  to retract the load member  67  in the rightward direction of  FIG. 7 . Further, once the proximity sensor  56  detects proximity of the mass body  45 , i.e. shifts from the non-mass-body-proximity-detecting state to the mass-body-proximity-detecting state, the load control circuit  70  terminates the driving of the actuator  63  to thereby cause the load member  67  project in the leftward direction of  FIG. 7 . 
   Next, a description will be given about behavior of the keyboard apparatus constructed as above according to the third embodiment of the present invention. Tone signal generation and termination of the tone signal generation responsive to depression/release operation of the key  10  is similar to that in the above-described first and second embodiments. In the third embodiment, during depression/release operation of the key  10 , the pivot lever  40  pivots to function as a reactive force to key depression. Further, in response to depression/release operation of the key  10 , the load member  67  moves leftward and rightward in response to the driving of the actuator  63 . When the key  10  is in the released position and the stopper member  45   a  of the mass body  45  is located over the lower limit stopper  53 , the actuator  63  is in the non-driven state so that the driving rod  53  is kept in the leftward projecting position. 
   When the key  10  is depressed to pivot counterclockwise with the key&#39;s rear end moving downward in the aforementioned state, a load is imparted to the depression operation of the key  10  because the front end surface of the load member  67  is in engagement (or contact) with the rear end surface of the key  10 . As the mass body  45  and stopper member  45   a  move upward, in response to the key depression operation, to approach the proximity sensor  55 , the proximity sensor  55  detects proximity of the mass body  45  and stopper member  45   a . In response to the detection by the proximity sensor  55 , the load control circuit  70  retracts the actuator  63  in the rightward direction of the figure. Consequently, the load having been imparted by the load member  67  to the depressed key  10  is removed so that the key touch rapidly becomes light (i.e., the reactive force to the depressed rapidly key decreases), and thus, the human player can enjoy a let-off feeling. After that, the upward movement of the mass body  45  and stopper member  45   a  is terminated by the stopper member  45   a  abutting against the upper limit stopper  54 . 
   Then, once the depressed key  10  is released, the mass body  45  and stopper member  45   a  are displaced downward as the key  10  and pivot lever  40  pivot. Because the front end surface  67   a  of the load member  67  is located out of contact with the rear end surface  15  of the key  10 , the key  10  pivots in the clockwise direction without contacting the front end surface  67   a  of the load member  67 , and the lower surface of the stopper member  45   a  of the mass body  45  abuts against the lower limit stopper  53 , so that the key  10  and pivot lever  40  stop pivoting. Thus, the returning velocity of the key  10  can be accelerated, which allows the key  10  to be successively depressed appropriately as desired and thus achieve a good successive depression performance of the keyboard apparatus. Further, during the release stroke, the proximity sensor  56  detects proximity of the mass body  45  and stopper member  45   a  as the mass body  45  and stopper member  45   a  approach the sensor  56 . In response to the detection by the proximity sensor  56 , the load control circuit  70  terminates the driving of the actuator  63 . Thus, the driving rod  63   a  of the actuator  63  is again pushed out leftward, so that the front end surface  67   a  of the load member  67  again engages (or contacts) with the rear end surface  15  of the key  10 . Then, the aforementioned behavior is repeated once the key  10  is again depressed and released by the human player in such a state. 
   During the release stroke, on the other hand, the actuator  63  is kept in the driven state, the load member  67  is kept retracted in the rightward direction of the figure and the front end surface  67   a  of the load member  67  is kept out of engagement (or contact) with each other, before the proximity sensor  56  detects proximity of the mass body  45  and stopper member  45   a . Thus, if the same key  10  has been again depressed before the proximity sensor  56  detects proximity of the mass body  45  and stopper member  45   a , i.e. the same key  10  has been depressed successively more rapidly than in the aforementioned successive depression, the rear portion of the mass body  45  moves upward, in response to the key depression operation, without the front end surface  67   a  of the load member  67  engaging with the rear end surface  15  of the key  10 . Thus, when rapid successive depression operation of the key  10  is to be performed, the human player can depress and release the key  10  with no load imparted from the load member  67  to the key  10 . As a result, a performance involving rapid successive depression operation of the key  10  can be executed with ease. 
   In a modification of the above-described third embodiment, the driving, by the load control circuit  70 , of the actuator  63  may be controlled using the key switch  52  in place of the proximity sensor  56 . In this case, the load control circuit  70  may terminate the driving of the actuator  63  in response to a change from the ON state to the OFF state of the first switch  52   a  of the key switch  52 . In this way, the modification can achieve the same advantageous results as the above-described third embodiment. 
   In the third embodiment, as set forth above, the driving rod  63   a  of the actuator  63  is normally urged by the spring to in the leftward projecting position so that, in the release stroke of the key  10 , the actuator  63  is driven to retract the driving rod  63   a  rightward against the biasing force of the spring. In a modification, however, the driving rod  63   a  of the actuator  63  may be normally urged by the spring in the to rightward retracted position so that, in the depression stroke of the key  10 , the actuator  63  is driven to cause the driving rod  63   a  to project leftward and thereby cause the front end surface  67   a  of the load member  67  to engage (or contact) with the rear end surface  15  of the key  10 . In such a case, the load control circuit  70  drives the actuator  63  to cause the driving rod  63   a  to project leftward when the proximity sensor  56  shifts from the state where it is detecting proximity of the mass body  45  and stopper member  45   a  to the state where it is no more detecting proximity of the mass body  45  and stopper member  45   a , taking electric power consumption into account. Then, once the rear end portion of the mass body  42  reaches the predetermined position proximal to the upper limit stopper  54 , the load control circuit  70  terminates the driving of the actuator  63  so as to retract the driving rod  63   a  rightward by the biasing force of the spring. 
   In the aforementioned modification of the third embodiment too, a left-off feeling can be given, during the depression stroke of the key  10 , to the key depression operation of the human player by the front end surface  67   a  of the load member  67  engaging with the rear end surface  15  of the key  10 . Further, during the release stroke of the key  10 , the front end surface  67   a  of the load member  67  disengages from the rear end surface  15  of the key  10 , so that the returning velocity of the key  10  can be accelerated and thus a good successive depression performance can be maintained. Furthermore, in this modification too, if the same key  10  has been again depressed before the proximity sensor  56  detects proximity of the mass body  45  and stopper member  45   a , i.e. the same key  10  has been depressed successively more rapidly than in the aforementioned successive depression, the driving rod  63   a  of the actuator  63  is kept by the spring in the rightward retracted position. Namely, in this case, the load control circuit  70  does not perform the driving control on the actuator  63  in response to a change from the proximity-detecting state to the non-proximity-detecting state of the proximity sensor  56 . Thus, in this modification too, during the rapid successive depression operation of the key  10 , the front end surface  67   a  of the load member  67  engaging with the rear end surface  15  of the key  10  does not engage (or contact) with the rear end surface  15  of the key  10 , and thus, a performance involving rapid successive depression operation of the key  10  can be executed with ease. 
   In such a modification of the above-described third embodiment too, the termination of the driving, by the load control circuit  70 , of the actuator  63  may be controlled using the key switch  52  in place of the proximity sensor  56 . In this case, the load control circuit  70  may start the driving control of the actuator  63  in response to a change from the ON state to the OFF state of the first switch  52   a  of the key switch  52 . In this way, the modification can achieve the same advantageous results as the above-described third embodiment. 
   In further modifications of the first and second embodiments, the driving control of the actuator  63  by the load control circuit  70  using the key switch  52  may be performed by the load control circuit  70  using the proximity sensor  56  of the third embodiment. In such a case, the proximity sensor  56  in the third embodiment is used to detect proximity, to the lower stopper  53 , of the mass body  42 . In the first and second embodiments and modifications thereof, the load control circuit  70  may replace the driving control of the actuator  63  responsive to a change from the OFF state to the ON state of the first switch  52   a  with the driving control of the actuator  63  responsive to a change from the mass-body-proximity detecting state to the non-mass-body-proximity detecting state of the proximity sensor  56 . Alternatively, the load control circuit  70  may replace such driving control of the actuator  63  responsive to a change from the ON state to the OFF state of the first switch  52   a  with the driving control responsive to a change from the non-mass-body-proximity detecting state to the mass-body-proximity detecting state of the proximity sensor  56 . 
   d. Fourth Embodiment 
   Next, a description will be given about an electronic musical instrument keyboard apparatus according to a fourth embodiment of the present invention. Briefly stated, the fourth embodiment is applicable to any one of the first, second and third embodiments and modifications thereof and arranged to perform control for changing a load to be imparted by the load member  64 ,  65  or  67  in accordance with a key depressing velocity and tone pitch. In this case, the manner in which the driving control of the actuator  63  is to be performed by the load control apparatus  70  differs among the first, second and third embodiments and modifications thereof. Only a portion of the driving control common to the first, second and third embodiments and modifications will first be explained first, and details of the application of the driving control, differing among to the first, second and third embodiments and modifications, will be later discussed. 
     FIG. 8  is a block diagram of an electric control unit which is common to (i.e., sharable among) the first, second, third embodiments and modifications thereof. To the load control circuit  70  are connected a key touch detection section  71 , key-touch-correspondent driving force determination section  72  and tone-pitch-dependent driving force determination section  73 . These key touch detection section  71 , key-touch-correspondent driving force determination section  72  and tone-pitch-dependent driving force determination section  73  are implemented by a computer comprising a CPU, ROM, RAM, etc, and have the following functions performed through execution of software programs. The key touch detection section  71 , detects depressing velocities of a plurality of keys  10 , inputs signals from the first and second switches  52   a  and  52   b  of the key switches  52  corresponding to the keys  10 . Then, the key touch detection section  71  measures a length of time from a time point when the first switch  52   a  has changed from the OFF state to the ON state to a time point when the second switch  52   b  changes from the OFF state to the ON state, to thereby detect a depressing velocity per key  10 . 
   The key-touch-correspondent driving force determination section  72  refers to a key touch-driving force table stored in the ROM, to determine a driving force of the actuator  63  corresponding to the key depressing velocity detected by the key touch detection section  71 . As indicated by a solid line A (or solid line B) in  FIG. 9 , the key touch-driving force table has stored therein driving forces that increase or decrease as the key depressing velocity increases. The key-touch-correspondent driving force determination section  72  outputs to the load control circuit  70  a signal indicative of a driving force that increase or decrease as the key depressing velocity increases or decreases. The tone-pitch-dependent driving force determination section  73  inputs signals from the first switches  52   a  of the key switches  52  corresponding to the keys  10  and refers to a tone pitch-driving force table stored in the ROM to determine a driving force of the actuator  63  in accordance with a tone pitch of the key  10  of which the first switch  52   a  has changed from the OFF state to the ON state. As indicated by a solid line A (or solid line B) in  FIG. 10 , the tone pitch-driving force table has stored therein driving forces that increase or decrease as the tone pitch increases or decreases. The tone-pitch-dependent driving force determination section  73  outputs to the load control circuit  70  a signal indicative of a driving force that increase or decrease as the tone pitch increases. The solid lines A and B of  FIGS. 9 and 10  differ among the applications of the control according to the fourth embodiment to be explained below. 
   d1. Application to the First Embodiment: 
   In the first embodiment, the load member  64  is kept in the leftward projecting position by the biasing force of the spring while the actuator  63  is in the non-driven state, as shown in  FIG. 2A . While the actuator  63  is driven, the load member  64  is kept in the rightward retracted position against the biasing force of the spring. Once the proximity sensor  55  detects proximity of the mass body  42 , the load control circuit  70  drives the actuator  63  to retract the load member  64  rightward. Once the first switch  52   a  of the key switch  52  changes from the ON state to the OFF state, the load control circuit  70  terminates the driving of the actuator  63 . 
   In such an application to the first embodiment, the key-touch-correspondent driving force determination section  72  employs the characteristic indicated by the solid line A of  FIG. 9  to determine a driving force that increases as the key depressing velocity increases, and the tone-pitch-dependent driving force determination section  73  employs the characteristic indicated by the solid line A of  FIG. 10  to determine a driving force that increases as the tone pitch increases. Further, the load control circuit  70  adds together the thus-determined two driving forces and then drives the actuator immediately after the second switch  52   b  of the key switch  52  has changed from the OFF state to the ON state, i.e. upon detection of the key depressing velocity, so that the added driving force (i.e., driving force equal to the sum of the determined two driving forces) is produced. Note that the driving force thus generated by the actuator  63  is smaller than the driving force with which the load member  64  is retracted rightward by the actuator  63  being driven upon detection, by the proximity sensor  55 , of proximity of the mass body  42 . The driving of the actuator  63  responsive to the detection by the proximity sensor  55  and termination of the driving of the actuator  63  responsive to the change from the OFF state to the ON state of the first switch  52   a  is similar to that in the above-described first embodiment. 
   When the actuator  63  is driven with the smaller driving force, the load member  64  is retracted rightward because of balance between the biasing force of the built-in spring and the driving force, but the rear end portion of the mass body  42  engages (or contacts) with the load member  64  as the mass body  42  moves upward. Because the added driving force increases as the key depressing velocity and tone pitch increase, the rightward retracted amount of the load member  64  too increases as the key depressing velocity and tone pitch increase. Thus, the amount of the engagement between the rear end portion of the mass body  42  and the load member  64  during the upward movement of the mass body  42  decreases as the key depressing velocity and tone pitch increase. The load imparted from the load member  64  to the key  10  and pivot lever  40  in the key depression stroke decreases as the key depressing velocity and tone pitch increase. Consequently, with this application of the control to the first embodiment, it is possible to give the human player a massive key touch in response to slow key depression and a light key touch in response to rapid key depression. Further, it is possible to give the human player a massive key touch in response to depression of a key in a low pitch range and light key touch in response to depression of a key in a high pitch range. As a result, the key touch feeling can be even further improved. 
   d2. Application to the Modification of the First Embodiment 
   In the modification of the first embodiment, the load member  64  is kept in the rightward retracted position by the biasing force of the spring while the actuator  63  is in the non-driven state. While the actuator  63  is driven, the load member  64  is kept in the leftward projecting position against the biasing force of the spring. Once the first switch  52   a  of the key switch  52  changes from the OFF state to the ON state, the load control circuit  70  drives the actuator  63  to cause the driving rod  63   a  to project leftward. Further, once the rear end portion of the mass body  42  reaches the predetermined position proximal to the upper limit stopper  54 , the load control circuit  70  terminates the driving of the actuator  63  so as to retract the driving rod  63   a  rightward by the biasing force of the spring. 
   In such an application of the control to the modification of the first embodiment, the key-touch-correspondent driving force determination section  72  employs the characteristic indicated by the solid line B of  FIG. 9  to determine a driving force that decreases as the key depressing velocity increases, and the tone-pitch-dependent driving force determination section  73  employs the characteristic indicated by the solid line B of  FIG. 10  to determine a driving force that decreases as the tone pitch increases. Further, the load control circuit  70  adds together the thus-determined two driving forces and then drives the actuator  63  immediately after the second switch  52   b  of the key switch  52  has changed from the OFF state to the ON state, i.e. upon detection of the key depressing velocity, so that the added driving force is produced. Then, the load member  64  projects leftward and stops at a position where the driving force of the actuator  63  and the biasing force of the built-in spring balance. Note that, at the time of the upward movement of the mass member  42 , the position where the load member  64  stops is where the rear end portion of the mass body  42  and the load member  64  engage (or contact) with each other. The driving of the actuator  63  responsive to the change from the OFF state to the ON state of the first switch  52   a  may be or may not be omitted as necessary. The driving termination of the actuator  63  responsive to the detection by the proximity sensor  55  is similar to that in the above-described modification of the first embodiment. 
   Because the added driving force, equal to the sum of the driving forces determined in accordance with the key depressing key and tone pitch, decreases as the key depressing velocity and tone pitch increase, the leftward projecting amount of the load member  64  too decreases as the key depressing velocity and tone pitch increase. Thus, the amount of the engagement (or contact) between the rear end portion of the mass body  42  and the load member  64  during the upward movement of the mass body  42  decreases as the key depressing velocity and tone pitch increase. The load imparted from the load member  64  to the key  10  and pivot lever  40  during the key depression stroke decreases as the key depressing velocity and tone pitch increase. Consequently, with this application to the modification of the first embodiment too, it is possible to even further improve the key touch feeling. 
   d3. Application to the Second Embodiment 
   In the above-described second embodiment, the driving rod  63   a  is kept in the rightward retracted position and the load member  65  is normally urged in the counterclockwise direction by the biasing force of the spring while the actuator  63  is in the non-driven state, as shown in  FIG. 6 . When the actuator  63  is driven, the driving rod  63   a  projects leftward so that the load member  65  pivots in the clockwise direction against the biasing force of the spring or weight. Once the proximity sensor  55  detects proximity of the mass body  42 , the load control circuit  70  drives the actuator  63  to cause the load member  64  to pivot in the clockwise direction rightward. Once the first switch  52   a  of the key switch  52  changes from the ON state to the OFF state, the load control circuit  70  terminates the driving of the actuator  63 . 
   In such an application to the second embodiment, the key-touch-correspondent driving force determination section  72  employs the characteristic indicated by the solid line A of  FIG. 9  to determine a driving force that increases as the key depressing velocity increases, and the tone-pitch-dependent driving force determination section  73  employs the characteristic indicated by the solid line A of  FIG. 10  to determine a driving force that increases as the tone pitch increases. Further, the load control circuit  70  adds together the thus-determined two driving forces and then drives the actuator  63  immediately after the second switch  52   b  of the key switch  52  has changed from the OFF state to the ON state, i.e. upon detection of the key depressing velocity, so that the added driving force (i.e., driving force equal to the sum of the determined two driving forces) is produced. Note that the driving force thus generated by the actuator  63  is smaller than the driving force with which the load member  65  is caused to pivot clockwise by the actuator  63  being driven upon detection, by the proximity sensor  55 , of proximity of the mass body  42 . The driving of the actuator  63  responsive to the detection by the proximity sensor  55  and termination of the driving of the actuator  63  responsive to the change from the OFF state to the ON state of the first switch  52   a  is similar to that in the above-described second embodiment. 
   When the actuator  63  is driven with the smaller driving force, the load member  64  pivots clockwise by virtue of balance between the driving force and the biasing force of the built-in spring or weight, but the rear end portion of the mass body  42  engages (or contacts) with the load member  65  as the mass body  42  moves upward. Because the added driving force increases as the key depressing velocity and tone pitch increase, the amount of the clockwise pivoting movement of the load member  65  too increases as the key depressing velocity and tone pitch increase. Thus, the amount of the engagement (or contact) between the rear end portion of the mass body  42  and the load member  65  during the upward movement of the mass body  42  decreases as the key depressing velocity and tone pitch increase. The load imparted from the load member  65  to the key  10  and pivot lever  40  in the key depression stroke decreases as the key depressing velocity and tone pitch increase. Consequently, with this application to the second embodiment too, the key touch feeling can be even further improved. 
   d4. Application to the Modification of the Second Embodiment 
   In the above-described modification of the second embodiment, the driving rod  63   a  is kept in the leftward projecting position by the biasing force of the spring and the load member  65  is in the clockwise pivoting position against the biasing force of the spring or weight while the actuator  63  is in the non-driven state. When the actuator  63  is driven, the driving rod  63   a  is retracted rightward, and the load member  65  pivots counterclockwise by the biasing force of the spring or weight. Once the first switch  52   a  of the key switch  52  changes from the OFF state to the ON state, the load control circuit  70  drives the actuator  63  to retract the driving rod  63   a  rightward and thereby causes the load member  65  to pivot counterclockwise. Further, once the sensor  55  detects that the rear end portion of the mass body  42  has reached the predetermined position proximal to the upper limit stopper  54 , the load control circuit  70  terminates the driving of the actuator  63  so as to cause the driving rod  63   a  to project leftward by the biasing force of the spring. 
   In such an application to the modification of the second embodiment, the key-touch-correspondent driving force determination section  72  employs the characteristic indicated by the solid line B of  FIG. 9  to determine a driving force that decreases as the key depressing velocity increases, and the tone-pitch-dependent driving force determination section  73  employs the characteristic indicated by the solid line B of  FIG. 10  to determine a driving force that decreases as the tone pitch increases. Further, the load control circuit  70  adds together the thus-determined two driving forces and then drives the actuator  63  immediately after the second switch  52   b  of the key switch  52  has changed from the OFF state to the ON state, i.e. upon detection of the key depressing velocity, so that the added driving force is produced. Then, the driving rod  63  is retracted rightward, and the load member  65  pivots counterclockwise and stops at a position where the driving force of the actuator  63  and the biasing force of the built-in spring balance. Note that, at the time of the upward movement of the mass member  42 , the position where the load member  65  stops is where the rear end portion of the mass body  42  and the horizontal portion  65   b  of the load member  64  engage (or contact) with each other. The driving of the actuator  63  responsive to the change from the OFF state to the ON state of the first switch  52   a  may be or may not be omitted as necessary. The driving termination of the actuator  63  responsive to the detection by the proximity sensor  55  is similar to that in the above-described modification of the second embodiment. 
   Because the added driving force, equal to the sum of the driving forces determined in accordance with the key depressing key and tone pitch, decreases as the key depressing velocity and tone pitch increase, the amount of the pivoting movement in the counterclockwise direction of the load member  65  too decreases as the key depressing velocity and tone pitch increase. Thus, the amount of the engagement (or contact) between the rear end portion of the mass body  42  and the load member  64  during the upward movement of the mass body  42  decreases as the key depressing velocity and tone pitch increase. The load imparted from the load member  64  to the key  10  and pivot lever  40  during the key depression stroke decreases as the key depressing velocity and tone pitch increase. Consequently, with this application to the modification of the second embodiment too, it is possible to even further improve the key touch feeling. 
   d5. Application to the Third Embodiment 
   In the above-described third embodiment, the driving rod  63   a  is normally urged in the leftward direction by the biasing force of the spring and the load member  67  is kept in the leftward projecting position while the actuator  63  is in the non-driven state, as shown in  FIG. 7 . When the actuator  63  is driven, the driving rod  63   a  is retracted rightward so that the load member  67  moves rightward. Once the proximity sensor  55  detects proximity of the mass body  42 , the load control circuit  70  drives the actuator  63  to retract the load member  67  rightward. Further, once the proximity sensor  56  changes from the non-mass-body-proximity detecting state to the mass-body-proximity detecting state, the load control circuit  70  terminates the driving of the actuator  63  to cause the load member  67  to project leftward. 
   In such an application to the third embodiment, the key-touch-correspondent driving force determination section  72  employs the characteristic indicated by the solid line A of  FIG. 9  to determine a driving force that increases as the key depressing velocity increases, and the tone-pitch-dependent driving force determination section  73  employs the characteristic indicated by the solid line A of  FIG. 10  to determine a driving force that increases as the tone pitch increases. Further, the load control circuit  70  adds together the thus-determined two driving forces and then drives the actuator  63  immediately after the second switch  52   b  of the key switch  52  has changed from the OFF state to the ON state, i.e. upon detection of the key depressing velocity, so that the added driving force (i.e., driving force equal to the sum of the determined two driving forces) is produced. Note that the driving force thus generated by the actuator  63  is smaller than the driving force with which the load member  67  is retracted rightward by the actuator  63  being driven upon detection, by the proximity sensor  55 , of proximity of the mass body  42 . The driving of the actuator  63  responsive to the detection by the proximity sensor  55  and termination of the driving of the actuator  63  responsive to the detection by the proximity sensor  56  is similar to that in the above-described third embodiment. 
   When the actuator  63  is driven with the smaller driving force, the front end surface  67   a  of the load member  67  engages (or contacts) with the rear end surface  15  of the mass body  42  as the mass body  42  moves upward. Because the added driving force increases as the key depressing velocity and tone pitch increase, the amount of the rightward retraction of the load member  67  too increases as the key depressing velocity and tone pitch increase. Thus, the amount of the engagement (or contact/friction) between the rear end portion of the mass body  42  and the horizontal portion of the load member  67  during the upward movement of the mass body  42  decreases as the key depressing velocity and tone pitch increase. The load imparted from the load member  67  to the key  10  and pivot lever  40  during the key depression stroke decreases as the key depressing velocity and tone pitch increase. Consequently, with this application to the third embodiment too, the key touch feeling can be even further improved. 
   d6. Application to the Modification of the Third Embodiment 
   In the above-described modification of the third embodiment, the driving rod  63   a  is normally urged in the rightward direction by the biasing force of the spring and the load member  67  is kept in the rightward retracted position while the actuator  63  is in the non-driven state. When the actuator  63  is driven, the driving rod  63   a  projects leftward so that the load member  67  moves leftward. Once the proximity sensor  56  changes from the state where it is detecting proximity of the mass body  45  and stopper member  45   a  to the state where it is no more detecting proximity of the mass body  45  and stopper member  45   a , the load control circuit  70  drives the actuator  63  to cause the driving rod  63   a  to project leftward. Further, once the proximity sensor  55  detects that the rear end portion of the mass body  42  has reached the predetermined position proximal to the upper limit stopper  54 , the load control circuit  70  terminates the driving of the actuator  63  so as to retract the driving rod  63   a  rightward by the biasing force of the spring. 
   In such an application to the modification of the third embodiment, the key-touch-correspondent driving force determination section  72  employs the characteristic indicated by the solid line B of  FIG. 9  to determine a driving force that decreases as the key depressing velocity increases, and the tone-pitch-dependent driving force determination section  73  employs the characteristic indicated by the solid line B of  FIG. 10  to determine a driving force that decreases as the tone pitch increases. Further, the load control circuit  70  adds together the thus-determined two driving forces and then drives the actuator  63  immediately after the second switch  52   b  of the key switch  52  has changed from the OFF state to the ON state, i.e. upon detection of the key depressing velocity, so that the added driving force is produced. Then, the driving rod  63  projects leftward and stops at a position where the driving force of the actuator  63  and the biasing force of the built-in spring balance. In this state, whereas the front end surface  67   a  of the load member  67  is abutting against the rear end surface  15  of the key  10 , the abutting or pressing force of the front end surface  67   a  of the load member  67  is smaller than that in the above-described modification of the third embodiment. The driving of the actuator  63  responsive to the detection by the proximity sensor  56  may be or may not be omitted as necessary. The driving termination of the actuator  63  responsive to the detection by the proximity sensor  55  is similar to that in the above-described modification of the third embodiment. 
   Because the added driving force, equal to the sum of the driving forces determined in accordance with the key depressing key and tone pitch, decreases as the key depressing velocity and tone pitch increase, so that the amount of the counterclockwise direction of the load member  65  decreases as the key depressing velocity and tone pitch increase. Thus, the amount of the engagement (or contact/friction) of the front end surface  67   a  of the load member  67  with the rear end surface  15  of the key  10  in the key depression stroke decreases as the key depressing velocity and tone pitch increase. Thus, the load imparted from the load member  67  to the key  10  and pivot lever  40  in the key depression stroke decreases as the key depressing velocity and tone pitch increase. Consequently, with this application to the modification of the third embodiment too, it is possible to even further improve the key touch feeling. 
   d7. Modification of the Fourth Embodiment 
   In the above-described fourth embodiment, the load control circuit  70  is constructed to add together the driving forces determined by the key-touch-correspondent driving force determination section  72  and tone-pitch-dependent driving force determination section  73  and perform the driving control of the actuator  63  in accordance with a control signal indicative of the sum of the two driving forces. Alternatively, the load control circuit  70  may be constructed to multiply together the driving forces determined by the key-touch-correspondent driving force determination section  72  and tone-pitch-dependent driving force determination section  73  and perform the driving control of the actuator  63  in accordance with a control signal indicative of the product of the two driving forces. What matters here is to allow the driving forces, determined by the key-touch-correspondent driving force determination section  72  and tone-pitch-dependent driving force determination section  73 , to be used in the driving control of the actuator  63 . 
   Further, whereas the fourth embodiment has been described as constructed to vary the driving force continuously in accordance with variation in the key depressing velocity, the driving force may be varied in a stepwise fashion in accordance with variation in the key depressing velocity. Furthermore, the driving force may be varied in a stepwise fashion in accordance with variation in the tone pitch. Furthermore, relationship between the key depressing velocity and the driving force and relationship between the tone pitch and the driving force may be defined using respective predetermined functions. 
   Furthermore, in the driving force control responsive to the tone pitch, i.e. the control for imparting a load to the depressed key in accordance with the tone pitch, the load to be imparted to the depressed key may be changed through a given mechanical mechanism without the style of the driving control of the actuator  63  being changed in accordance with the tone pitch. Namely, in the above-described first embodiment, the load to be imparted to the depressed key by the load member  64  may be preset at different intensity values corresponding to various tone pitches by adjusting the projecting amount, shape, material, etc. of the load member  67  per key. In the above-described second embodiment, the load to be imparted to the depressed key by the load member  65  may be preset at different intensity values corresponding to various tone pitches by adjusting the pivoting amount, shape, material, etc. of the load member  66  per key. Further, in the above-described third embodiment, the load to be imparted to the depressed key by the load member  67  may be preset at different intensity values corresponding to various tone pitches by adjusting the shapes of the front end surface  67   a  and rear end surface  15  of the key  10 , etc. 
   Furthermore, the fourth embodiment has been described above as applying, to the first embodiment, second embodiment and modifications thereof, the control responsive to the key depressing velocity and tone pitch, only in relation to control of the actuator  63  based on operation of the key switch  52  and detection by the proximity sensor  55 . However, as set forth above in relation to the modification of the third embodiment, the control of the actuator  63  based on operation of the key switch  52  may be replaced with control based on detection by the proximity sensor  56 . Further, the control responsive to the key depressing velocity and tone pitch to be performed on the third embodiment and modification thereof has been described above only in relation to control of the actuator  63  based on detection by the proximity sensors  55  and  56 . Alternatively, as set forth above in relation to the modification of the third embodiment, the control of the actuator  63  based on detection by the proximity sensors  55  and  56  may be replaced with control based on operation of the key switch  52 . 
   Furthermore, the fourth embodiment has been described above as varying the intensity of the load, which is to be imparted from the load member  64 ,  65  or  67  to a depressed key, in accordance with the key touch and tone pitch. Alternatively, the intensity of the load to be imparted from the load member  64 ,  65  or  67  to a depressed key may be controlled in accordance with only one of the key touch and tone pitch. 
   e. Other Modification 
   The present invention should not be construed as limited to the above-described embodiments, and various modifications of the invention are also possible without departing from the purposes and basic principles of the invention. 
   The first to fourth embodiments have been described above as constructed to detect a position of the mass body  42  or  45  by means of the proximity sensors  55  and  56 . However, because the proximity sensors  55  and  56  detect pivoting positions of the key  10  and pivot lever  40 , moving positions of other portions of the key  10  and pivot lever  40  may be detected. Further, the proximity sensors  55  and  56  may be replaced with contact switches, so as to detect contact with the contact switches in place of the proximity to the proximity sensors  55  and  56 . Further, the first to the fourth embodiments have been described above as terminating the engagement (or contact) between the load member  64 ,  65  or  67  and the mass body  42  or  45  and key  10  during the key release stroke. Alternatively, the engagement (or contact) may be terminated in response to detection of a change from the ON state to the OFF state or from the OFF state to the ON state of the third switch  52   c  of the key switch  52 . 
   Further, the first to fourth embodiments have been described above as causing the front end portion of the key  10  to swing vertically with the rear end portion as the pivot point. However, the pivot point of the key  10  may be other than the rear end portion of the key  10 , such a middle portion of the key  10 . In such a case, the key switch  52  for detecting depression and release of the key  10  may be provided on a rear portion of the key  10  so that the key switch  52  is activated in response to displacement of the rear end portion of the key  10 . Further, the first to fourth embodiments have been described above as using the mass body  42  or  45  as a means for imparting a reactive force to key depression operation. Alternatively, in place of or in addition to the mass body  42  or  45 , a spring may be employed, as the means for imparting a reactive force to key depression operation, to normally urge the key  10  upwardly. 
   Furthermore, the first to fourth embodiments have been described above as detecting a key depressing velocity on the basis of outputs from the first and second switches  52   a  and  52   b  of the key switch  52 . However, the key depressing velocity may be detected in various other manners as along as a moving velocity of the key  10  or pivot lever  40  can be detected appropriately; for example, the moving velocity of the key  10  or pivot lever  40  may be detected electromagnetically by use of a coil or solenoid. In another alternative, the key depressing velocity may be detected by detecting a position of the key  10  or pivot lever  40  through electromagnetic induction, electrostatic capacitance, ultrasonic sound wave, photo-electric effect, magnetic change or the like and then differentiating the detected position. 
   Furthermore, the first and second embodiments have been described above as imparting a load to depression operation of the key  10  by engaging the load member  64  or  65  with the rear end portion of the mass body  42  of the pivot lever  40 , and the third embodiment has been described above as imparting a load to depression operation of the key  10  by engaging the load member  67  with the rear end surface of the key  10 . However, the present invention is not so limited, and the load member may be caused to engage (or contact) with any other suitable portion of the key  10  or pivot lever  40  as long as the load is imparted to the pivoting key  10  or pivot lever  40 . For example, the load member may be caused to engage (or contact) with the front end portion  12  of the key  10 , driving portion  13  of the key  10 , or lever base section  41  or  44  of the pivot lever  40 . 
   This application is based on, and claims priority to, JP PA 2007-151098 filed on 7 Jun. 2007. The disclosure of the priority applications, in its entirety, including the drawings, claims, and the specification thereof, is incorporated herein by reference.