Patent Publication Number: US-7594833-B2

Title: Boat propulsion apparatus

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a boat propulsion apparatus and more particularly, to a boat propulsion apparatus of a motor-driven type. 
   2. Description of the Related Art 
   Conventional apparatuses of this kind are disclosed in U.S. Pat. No. 5,340,342 and U.S. Pat. No. 6,832,939. 
   In U.S. Pat. No. 5,340,342, an apparatus which can switch forward and reverse modes solely by a turning operation of a lever is disclosed. 
   On the other hand, in U.S. Pat. No. 6,832,939, an apparatus performing a so-called reverse lock is disclosed. In this conventional apparatus, forward and reverse modes can be switched by a selector switch, an electromagnetic solenoid is interlocked with a setting of the reverse mode by the switch, and a bounce up of a main body of a propulsion apparatus in a direction of a tilt up is prevented. 
   According to U.S. Pat. No. 5,340,342, the reverse lock is not interlocked with an operation of the lever. Therefore, another means needs to be provided in order to perform the reverse lock. Consequently, an operation and a structure become complicated. 
   On the other hand, according to U.S. Pat. No. 6,832,939, the reverse lock can be interlocked with a setting of the reverse mode by the switch. Another means, however, needs to be provided in order to set an amount of an output in each operation mode. Consequently, an operation and a structure become complicated. 
   SUMMARY OF THE INVENTION 
   In order to solve the problems described above, preferred embodiments of the present invention provide a boat propulsion apparatus in which an operation mode can be switched to a reverse mode and an amount of an output can be set by an easy operation without a complicated a structure and further in which a main body of a propulsion apparatus can be locked to a hull. 
   A boat propulsion apparatus according to a first preferred embodiment of the present invention includes: a main body mounted on a boat; and a locking mechanism arranged to prevent a swing of the main body of the propulsion apparatus in a direction of a tilt up; the main body of the propulsion apparatus has an electric motor, an indicating mechanism which is rotatable and which can indicate, by a position thereof, a type and an amount of an output of an operation mode having at least a forward mode and a reverse mode, a detecting mechanism arranged to detect an indication by the indicating mechanism, a first controller arranged to control an operation mode and an output of the electric motor depending on an output of the detecting mechanism, and a second controller arranged to control the locking mechanism depending on an indication provided by the indicating mechanism, a range of a turn of the indicating mechanism includes at least a forward mode area indicating the forward mode and a reverse mode area indicating a reverse mode, and the first controller switches the operation mode of the electric motor to the reverse mode and the second controller locks the main body of the propulsion apparatus to the hull by operating the locking mechanism depending on an operation for turning the indicating mechanism to the reverse mode area while the main body of the propulsion apparatus is mounted on the hull. 
   The operation mode preferably further includes a stop mode, a stop mode area indicating the stop mode has a predefined range including a neutral position located between the forward mode area and the reverse mode area in the range of a turning motion of the indicating mechanism, and the second controller locks the main body of the propulsion apparatus to the hull by operating the locking mechanism when the indicating mechanism is turned in a direction of the reverse mode area beyond a predefined position of the stop mode area. 
   The first controller preferably switches the electric motor to the reverse mode, and then the second controller locks the main body of the propulsion apparatus to the hull by operating the lock mechanism when the indicating mechanism is turned from the forward mode area to the reverse mode area. 
   The detecting mechanism preferably includes a potentiometer arranged to detect a position indicated by the indicating mechanism, and the first controller controls the electric motor depending on an output of the potentiometer. 
   The main body of the propulsion apparatus preferably further includes a housing for storing the potentiometer. 
   A stop mode sensor is preferably arranged to detect whether or not the indicating mechanism is indicating the stop mode, and the first controller preferably controls the electric motor further with reference to an output of the stop mode sensor. 
   The locking mechanism preferably includes an engaging section provided to the main body of the propulsion system and a catch section with which the engaging section can be engaged in order to lock the main body of the propulsion apparatus to the hull. 
   The second controller preferably includes a push section which can operate the engaging section in order to engage or release the engaging section in relation to the catch section and a control mechanism which mechanically controls an operation of the push section depending on an amount of a turn of the indicating mechanism. 
   The second controller preferably includes a push section preferably including an electromagnetic actuator which can operate the engaging section in order to engage or release the engaging section in relation to the catch section, a potentiometer arranged to detect a position indicated by the indicating mechanism, and a control section which controls the push section depending on an output of the potentiometer. 
   The engaging section preferably includes a lever provided on a side opposite to the hull in a vicinity of the main body of the propulsion apparatus and an arm which extends from the lever to a side of the hull and which can be engaged with the catch section tiltably provided to the main body of the propulsion apparatus, and the push section is disposed operably in relation to the lever. 
   An operation mode is preferably switched to the reverse mode and an amount of an output thereof is set solely by one operation for turning the indicating mechanism from the forward mode area to the reverse mode area, and, in addition to this, the main body of the propulsion apparatus is locked to the hull by the locking mechanism. Accordingly, another mechanism or another process is not necessary for setting the reverse lock or an output of an operation mode, and the reverse mode and an amount of an output thereof can be set with a simple structure and by a simple operation. In addition, a swinging movement of the main body of the propulsion apparatus in a direction of a tilt up can be prevented by the reverse lock. 
   The main body of the propulsion apparatus preferably is locked to the hull by the locking mechanism when the indicating mechanism is turned from the forward mode area to the predefined position of the stop mode area. Consequently, if the predefined position of the stop mode area, which is a position for enabling the reverse lock, is closer to the reverse mode area than to the neutral position, an operation mode can be switched to the reverse mode immediately after the reverse lock is enabled in a case in which the operation mode is switched from the forward mode to the stop mode and further to the reverse mode. On the other hand, a smooth operation is performed without unnecessarily enabling the reverse lock in a case in which the operation mode is switched from the forward mode to the stop mode and further to the forward mode. 
   When the indicating mechanism is turned from the forward mode area to the reverse mode area, the electric motor is switched to the reverse mode first, and the main body of the propulsion apparatus is locked to the hull by the locking mechanism afterwards. As a result, the reverse lock can be surely enabled only when the reverse mode is enabled. Therefore, the reverse lock is not mistakenly enabled. It is preferable that the reverse lock is enabled after the electric motor is switched to the reverse mode and before propulsive force is generated. 
   A position indicated by the indicating mechanism and a state of an operation for turning the indicating mechanism can be easily detected by the potentiometer. Therefore, the electric motor can be easily controlled by the first controller. 
   The potentiometer can be protected and deterioration and breakage of the potentiometer can be prevented by housing the potentiometer in the housing. Therefore, performance of the potentiometer can be maintained. 
   A detecting error of the potentiometer can be corrected by controlling the electric motor further with reference to an output of the stop mode sensor having high detecting accuracy. Therefore, the electric motor can be accurately controlled. In addition, an open circuit in the potentiometer can be detected. 
   If the catch section is provided to a structure on a side of the hull such as the hull and a member fixed on the hull (a bracket, for example), for example, the reverse lock can be easily enabled solely by engaging the engaging section provided to the main body of the propulsion apparatus with the catch section. 
   An operation of the push section can be mechanically controlled by the control mechanism. Therefore, electrical elements are not necessary for controlling the push section and the reverse lock. 
   An operation of the push section including the electromagnetic actuator can be controlled depending on an output of the potentiometer. Therefore, the push section and the reverse lock are easily controlled. 
   The push section preferably operates the lever provided on a side opposite to the hull in a vicinity of the main body of the propulsion apparatus. Therefore, the arm can be easily tilted and engaged with the catch section by the principle of lever. Thus, the reverse lock is smoothly enabled and disabled. In addition, a large free space is provided on the side opposite to the hull in the vicinity of the main body of the propulsion apparatus. Therefore, the lever can be long, and the lever can be operated by the push section even in a state in which a rudder is turned. 
   According to preferred embodiments of the present invention, another mechanism or another process is not necessary for setting the reverse lock or an output of an operation mode, and the reverse mode and an amount of an output thereof can be set with a simple structure and by a simple operation. In addition, a swing of the main body of the propulsion apparatus in a direction of a tilt up can be prevented by the reverse lock. 
   Other features, elements, steps, characteristics and advantages of the present invention will become more apparent from the following detailed description of preferred embodiments of the present invention with reference to the attached drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  shows a view illustrating a boat propulsion apparatus according to a preferred embodiment of the present invention. 
       FIG. 2  shows a perspective view illustrating a major link structure of the preferred embodiment in  FIG. 1 . 
       FIG. 3  shows a perspective view illustrating a major link structure of the preferred embodiment in  FIG. 1 . 
       FIG. 4  shows a perspective view illustrating a vicinity of locking mechanism of the preferred embodiment in  FIG. 1 . 
       FIG. 5A  shows a view illustrating a control mechanism at a time when a reverse lock is disabled. 
       FIG. 5B  shows a view illustrating the control mechanism at a time when the reverse lock is enabled. 
       FIG. 6  is a block diagram illustrating an electric constitution according to a preferred embodiment of the present invention. 
       FIGS. 7A-7E  show a graph illustrating outputs of each section or the like in relation to a turn of a throttle grip. 
       FIG. 8  shows a perspective view illustrating a major link structure of another preferred embodiment of the present invention. 
       FIG. 9  shows a view illustrating a case in which a preferred embodiment of the present invention is applied to a fishing boat. 
       FIG. 10  shows a view illustrating a case in which a preferred embodiment of the present invention is applied to a pontoon boat. 
   

   DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
   Preferred embodiments according to the present invention will be described hereinafter with reference to accompanying drawings. 
     FIG. 1  shows a boat propulsion apparatus  10  of a motor-driven type according to a preferred embodiment of the present invention. The boat propulsion apparatus  10  may be constituted as an outboard motor or as a part of a boat. 
   The boat propulsion apparatus  10  includes a main body  12  of the propulsion apparatus. The main body  12  of the propulsion apparatus includes a housing  14  having an upper housing  14   a  and a lower housing  14   b . An electric motor  16  is provided in the upper housing  14   a , and a drive shaft  20  is connected to a rotor  18  of the electric motor  16 . The drive shaft  20  is provided in a range from the upper housing  14   a  to the lower housing  14   b , and the drive shaft  20  is connected to a propeller shaft  24  via a bevel gear  22 . A propeller  26  is connected to an end of the propeller shaft  24 . A rotational direction of the propeller  26  is determined by a rotational direction of the electric motor  16 . 
   In addition, a controller  28  and a battery  30  are provided in the upper housing  14   a , and one end of a steering rod  32  is attached to a side section of the upper housing  14   a . The steering rod  32  extends generally in the horizontal direction, and a direction of the main body  12  of the propulsion apparatus is changed by swinging the steering rod  32  in the width direction. Thus, steering operation of a hull  84  (described below) can be performed. 
   As shown further in  FIG. 2  to  FIG. 4 , a link shaft  34  extending in an axial direction is provided in the steering rod  32 , and a throttle grip  36  connected to the link shaft  34  is provided on a second end of the steering rod  32 . An operation mode of the electric motor  16  and an amount of an output thereof can be adjusted by turning the throttle grip  36  in a circumferential direction. 
   A pulley  38  is attached to a first end of the link shaft  34 , and the pulley  38  and a pulley  40  in the upper housing  14   a  are connected by two cables  42 . The pulley  40  is turned when the pulley  38  turns. 
   A rotational shaft  44  is attached to the pulley  40 , and a potentiometer  46  is provided at an end of the rotational shaft  44 . An amount of rotation of the pulleys  40  and  38  and the throttle grip  36  and a position indicated by the throttle grip  36  can be detected by the potentiometer  46 . An output of the potentiometer  46  is given to the controller  28 . 
   In addition, a cam plate  48  preferably in the shape of a disk is attached to the rotational shaft  44  at a position between the pulley  40  and the potentiometer  46 . The cam plate  48  is turned when the pulley  40  turns. 
   As shown further in  FIGS. 5A and 5B , a cutout  50  used for detecting the stop mode is formed at the outer circumference of the cam plate  48 . In addition, grooves  52   a  and  52   b  in a shape of an arc passing through the cam plate  48  are continuously formed in the cam plate  48  via a groove  52   c . The grooves  52   a  and  52   c  are closer to the center of the cam plate  48  than the groove  50   b  is. 
   Further, a stop mode sensor  54  is disposed in a vicinity of the outer circumference of the cam plate  48  in order to detect the cutout  50 . In addition, an arm  58  generally in a shape of a strip of paper supported by a support shaft  56  is disposed in a vicinity of a major surface on a side of the potentiometer  46  of the cam plate  48 . 
   A collar  60  is attached to a side closer to an end of the arm  58 , and the collar  60  is inserted into the grooves  52   a  to  52   c  of the cam plate  48 . 
   In addition, a spring member (not shown) is attached to an end section  62  of the arm  58 , and the arm  58  is constantly biased in an upper direction by the spring member. As a result, a relative slide of the collar  60  in relation to the grooves  52   a  to  52   c  becomes smooth, and traceability becomes accurate. Further, a feeling of a click at a time when the collar  60  enters the groove  52   c  of the cam plate  48  is improved. Therefore, it is easy for a user to know the neutral position, and a feeling of operation is improved. 
   Moreover, a push section  64  in a shape of a rod is attached to a base end of the arm  58 . 
   As shown in  FIG. 1 , a mount (swivel bracket)  66  supporting the lower housing  14   b  is formed in an upper section of a side surface of the lower housing  14   b . The mount  66  is connected tiltably in the vertical direction to a pair of brackets (clamp brackets)  68  disposed on both sides thereof via a tilt shaft  70 . The brackets  68  have a clamp  72 . In addition, a stopper (not shown) with which the mount  66  is in contact in a state in which the main body  12  of the propulsion apparatus is tilted down to a lower limit position (the state shown in  FIG. 1 ) is provided to the brackets  68 . 
   In addition, an engaging section  74  generally in a shape of a letter Π operated by the push section  64  is tiltably supported on an outer circumference in a position closer to a top of the lower housing  14   b . As shown further in  FIG. 2  to  FIG. 4 , the engaging section  74  curves along an outer circumference of the lower housing  14   b  and includes a lever  76  in a shape of a board provided on a side opposite to the hull  84  (described below) and an arm  78  extending from the lever  76 . The arm  78  is connected tiltably in the vertical direction to the lower housing  14   b  by a support shaft  80 , and a side of an end section of the arm  78  is constantly biased in a lower direction by a spring member not shown in the drawing. On the other hand, a catch rod  82  with which an end section of the arm  78  is engaged is attached to the brackets  68 . 
   Further, the boat propulsion apparatus  10  is mounted on the hull  84  by attaching the brackets  68  to a transom  86  of the hull  84  and by fastening the clamp  72 . 
   In the constitution described above, when the cam plate  48  turns synchronously with a turn of the throttle grip  36 , the collar  60  relatively slides from the grooves  52   a  to  52   c . Because the height of the groove  52   a  and the height of the groove  52   b  are different, the collar  60  moves up and down. As the collar  60  moves up and down, the arm  58  swings on the support shaft  56 , and consequently the push section  64  moves up and down and operates the lever  76 . For example, as shown in  FIG. 5A , when the collar  60  is at the position of the groove  52   a , the push section  64  is lowered. Accordingly, the arm  78  is released from the catch rod  82 . Therefore, the reverse lock is disabled. On the other hand, as shown in  FIG. 5B , when the collar  60  is at the position of the groove  52   b , the push section  64  is raised. Accordingly, the arm  78  is engaged with the catch rod  82 . Therefore, the reverse lock is enabled. The position of insertion of the collar  60  passes the groove  52   c  when the position of insertion of the collar  60  is moving from the grooves  52   a  to  52   b  or in the opposite direction. The fact that the reverse lock is enabled or disabled is known by a user because the position of insertion of the collar  60  passes the groove  52   c . Therefore, operability is further enhanced. 
   An electric constitution of the boat propulsion apparatus  10  will be described hereinafter with reference to  FIG. 6 . 
   An output from the potentiometer  46  and the stop mode sensor  54  is given to the controller  28 . In addition, the battery  30  is connected to the controller  28  via a relay  88 , and a switching operation of the relay  88  is controlled by a main switch  90 . As shown in  FIG. 1 , the main switch  90  is provided on a steering rod  34 . 
   In addition, the controller  28  controls a motor driver  92  and thereby the electric motor  16  depending on an output of the potentiometer  46 . Moreover, a display section  94  on which information on steering or the like is displayed is controlled by the controller  28 . As shown in  FIG. 1 , the display section  94  is provided on the upper housing  14   a.    
   Moreover,  FIGS. 7A to 7E  show outputs of the potentiometer  46 , outputs of the stop mode sensor  54 , electric currents of the motor, and operations of the reverse lock in relation to a turn of the throttle grip  36 . 
   As shown in  FIG. 7A , if the throttle grip  36  is turned counterclockwise from the state of the stop mode (the state of neutral), the forward mode is set. On the other hand, if the throttle grip  36  is turned clockwise from the state of the stop mode, the reverse mode is set. In this preferred embodiment, the throttle grip  36  can be turned in a range between about ±80 degrees at the maximum, where the stop mode area indicating the stop mode is in a range between about ±15 degrees including the neutral position, the forward mode area indicating the forward mode is in a range between about −15 degrees and about −80 degrees, and the reverse mode area indicating the reverse mode is in a range between about 15 degrees and about 80 degrees. 
   As shown in  FIG. 7B , outputs of the potentiometer  46  vary linearly according to a position indicated by the throttle grip  36 . Accordingly, the position indicated by the throttle grip  36  can be detected depending on an output of the potentiometer  46 . 
   An output of the stop mode sensor  54  is at a high level when the throttle grip  36  indicates the stop mode. As understood from  FIG. 7C , a period in which the stop mode sensor  54  is at the high level is slightly shorter than the stop mode area of the throttle grip  36 . As a result, when an output of the stop mode sensor  54  is at the high level, the fact that the throttle grip  36  is indicating the stop mode is detected with high accuracy. The period of the high level of the stop mode sensor  54  and a timing thereof are determined by a position of the stop mode sensor  54 , a length of the cutout  50 , and the like. The controller  28  controls the electric motor  16  not only depending on an output of the potentiometer  46  but also with reference to an output of the stop mode sensor  54 . Accordingly, even if there is an error in detection by the potentiometer  46 , the controller  28  can recognize an indication of the stop mode by the throttle grip  36  with high accuracy by referring to an output of the stop mode sensor  54 . 
   As shown in  FIG. 7B , electric currents for the motor flowing in the electric motor  16  are controlled depending on a position of the throttle grip  36 . In other words, an operation mode of the electric motor  16  is controlled by the controller  28  depending on the position of the throttle grip  36 . When the throttle grip  36  is indicating the stop mode, the electric current for the motor is zero. Accordingly, the electric motor  16  stops. 
   Operations of the reverse lock are shown in  FIG. 7E . When the throttle grip  36  is turned approximately 10 degrees clockwise from the neutral position, the reverse lock is enabled. Specifically, when the throttle grip  36  is turned approximately 10 degrees clockwise from the neutral position, the collar  60  is at a position of the groove  52   b  of the cam plate  48 . Accordingly, the push section  64  is raised, and a side of an end of the arm  78  is engaged with the catch rod  82 . Thus, the reverse lock is enabled. 
   In the present preferred embodiment, the locking mechanism preferably includes the engaging section  74 , the support shaft  80 , the catch rod  82  as the catch section, and the spring member (not shown). The indicating mechanism preferably includes the throttle grip  36 . The detecting mechanism preferably includes the potentiometer  46 . The first controller preferably includes the controller  28 . The second controller preferably includes the push section  64  and a control mechanism including the cam plate  48 , the support shaft  56 , the arm  58 , the collar  60 , and the spring member (not shown). In addition, the transmission mechanism preferably includes the link shaft  34 , the pulleys  38  and  40 , the cables  42 , and the rotational shaft  44 . 
   According to the boat propulsion apparatus  10 , an operation mode is switched to the reverse mode and an amount of an output thereof is set by the controller  28  solely by one operation for turning the throttle grip  36  from the forward mode area to the reverse mode area. Moreover, the push section  64  is raised, and the engaging section  74  is engaged with the catch rod  82 . Therefore, the main body  12  of the propulsion apparatus is locked to the hull  84 . Accordingly, another mechanism or another process is not necessary for setting the reverse lock or an output of an operation mode, and the reverse mode and an amount of an output thereof can be set with a simple structure and by a simple operation. In addition, a swinging motion of the main body  12  of the propulsion apparatus and the boat propulsion apparatus  10  in a direction of a tilt up can be prevented by the reverse lock. 
   In addition, when the throttle grip  36  is turned from the forward mode area to a predefined position of the stop mode area, the push section  64  is raised, and the engaging section  74  is engaged with the catch rod  82 . Therefore, the main body of the propulsion apparatus is locked to the hull. Consequently, because the predefined position of the stop mode area, which is a position for enabling the reverse lock, is closer to the reverse mode area than to the neutral position (a position approximately 10 degrees from the neutral position in a direction of a clockwise turn), an operation mode can be switched to the reverse mode immediately after the reverse lock is enabled in a case in which the operation mode is switched from the forward mode to the stop mode and further to the reverse mode. On the other hand, a smooth operation is performed without unnecessarily enabling the reverse lock in case that the operation mode is switched from the forward mode to the stop mode and further to the forward mode. 
   Moreover, a position indicated by the throttle grip  36  and a state of an operation of turning the throttle grip  36  can be easily detected by the potentiometer  46 . Therefore, the electric motor  16  can be easily controlled by the controller  28 . 
   The potentiometer  46  can be protected, and deterioration and breakage of the potentiometer  46  can be prevented by housing the potentiometer  46  in the housing  14 . Therefore, performance of the potentiometer  46  can be maintained. 
   In addition, a detecting error of the potentiometer  46  can be corrected by controlling the electric motor  16  further with reference to an output of the stop mode sensor  54  having high detection accuracy. Therefore, the electric motor  16  can be accurately controlled. In addition, an open circuit in the potentiometer  46  can be detected by using the stop mode sensor  54 . 
   Moreover, the reverse lock can be easily enabled solely by mounting the catch rod  82  on the brackets  68  fixed on the hull  84  and by engaging the engaging section  74  provided to the main body  12  of the propulsion apparatus with the catch rod  82 . The catch section may be provided directly to the hull  84 . 
   In addition, an operation of the push section  64  can be mechanically controlled by the control mechanism. Therefore, electrical elements are not necessary for controlling the push section  64  and the reverse lock. 
   Moreover, the push section  64  operates in relation to the lever  76  provided on a side opposite to the hull  84  in a vicinity of the main body  12  of the propulsion apparatus. Therefore, the arm  78  can be easily tilted and engaged with the catch rod  82  by the principle of lever. Thus, the reverse lock is smoothly enabled and disabled. In addition, a large free space is provided on the side opposite to the hull  84  in the vicinity of the main body  12  of the propulsion apparatus. Therefore, the lever  76  can be long, and the lever  76  can be operated by the push section  64  in a state in which a rudder is turned. 
   In the present preferred embodiment, the reverse mode is preferably set after the reverse lock is enabled in the stop mode. The present invention, however, is not limited to the preferred embodiment above. 
   The reverse lock may be enabled after the throttle grip  36  is turned to the reverse mode area and also the electric motor  16  is switched to the reverse mode. This operation is easily achieved, for example, by elongating the groove  52   a  of the cam plate  48 . 
   In this case, the reverse lock can be surely enabled only when the reverse mode is enabled. Therefore, the reverse lock is not mistakenly enabled. 
   In addition, as understood from  FIG. 6  and  FIG. 8 , the second controller may be constituted by including a push section  96  formed with an electromagnetic actuator, the potentiometer  46 , and the controller  28  as a control section which controls the push section  96  depending on an output of the potentiometer  46 . 
   In this case, an operation of the push section  96  including the electromagnetic actuator can be controlled by the controller  28  already described above depending on an output of the potentiometer  46  already described above. Therefore, the push section  96  and the reverse lock are controlled and constituted easily. 
   In this case, a cam plate  48   a  is used by the stop mode sensor  54  in order to detect the stop mode. 
   The boat propulsion apparatus  10  can be used for a fishing boat as shown in  FIG. 9 . 
   In addition, various preferred embodiments of the present invention can be applied to a boat propulsion apparatus  10   a  which can be used for a pontoon boat as shown in  FIG. 10 , for example. The boat propulsion apparatus  10   a  is operated by remote control with a throttle lever  100  as the indicating mechanism via a wire cable  98 . The boat propulsion apparatus  10   a  may be operated by wireless remote control without using the wire cable  98 . 
   In addition, the indicating mechanism may be of a pedal type which can be the operation by foot. 
   Moreover, an optical position detection sensor and a magnetic sensor may be used in place of the potentiometer  46 . 
   While preferred embodiments of the present invention have been described above, it is to be understood that variations and modifications will be apparent to those skilled in the art without departing the scope and spirit of the present invention. The scope of the present invention, therefore, is to be determined solely by the following claims.