Abstract:
To prevent rebounding motion of sectors caused in stopping the sectors and achieve small-sized formation of a shutter, there is provided a shutter for a camera including sectors for opening and closing a shutter opening by reciprocal and pivotal movement, a motor for reciprocally and pivotally moving the sectors by forwardly and reversely rotating a rotor of the motor by applying current to a coil in forward and reverse directions, a control circuit for controlling current applied to the coil for exposure control in accordance with a brightness of an object to be photographed and a memory circuit for outputting data used for applying current to the control circuit in correspondence with operational characteristics of the motor and the sectors. The memory circuit is provided with current data for applying current to the coil for rotating the rotor in a direction reverse to a rebounding direction of the sectors to prevent a rebounding motion of the sectors.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a shutter for a camera having a motor for effecting opening and closing of a set of shutter sectors, and more particularly to a shutter for a camera in which forward and reverse pulses are applied to the motor to prevent rebounding motion of the sectors. 
     In a shutter for a camera having a motor or actuator for rotating a rotor formed of a permanent magnet by conducting electricity to a coil and opening and closing a shutter opening by operating a plurality of sectors driven by the motor, it is well known that at a position at which the sectors are fully opened or a position at which the sectors are completely closed, the actual positions of the sectors differ from desired positions thereof due to the inertia of the moving parts including the rotor, the sectors, and the like. This is commonly referred to as the bounding or rebounding phenomenon. The drawbacks associated with rebounding motion include the possibility that portions of the sectors remain covering the shutter opening after the sectors are intended to be fully opened, or re-exposure may be caused after the sectors are intended to be fully closed. 
     Conventionally, these drawbacks have been resolved by opening the sectors beyond the shutter opening or by closing the sectors to a greater degree than otherwise necessary to avoid the influence of rebounding of the sectors. 
     According to one such conventional method for overcoming the rebounding phenomenon, an outer diameter of the shutter is increased by a given amount to avoid rebounding of the sectors. However, this creates the problem that a small-sized shutter opening becomes difficult to obtain. Further, even if the sectors have been fully opened, the timing at which the sectors are to be shifted to a closing operation may nonetheless be affected by the rebounding phenomenon and a normal exposure operation may be hampered. 
     SUMMARY OF THE INVENTION 
     In order to resolve the above-described problems, after finishing a shutter opening operation and a shutter closing operation of the sectors, current is applied to a coil of the motor for a short period of time to rotate a rotor of the motor in a direction reverse to a rebounding direction of the sectors and rotation of the rotor is decelerated to reduce the rebounding motion of the sectors as much as possible. 
     According to an aspect of the invention, there is provided a shutter for a camera comprising a shutter opening, sectors for opening and closing the shutter opening by reciprocal pivotal movement, a motor for reciprocally and pivotally moving the sectors by forwardly and reversely rotating a rotor by application of forward and reverse currents to a coil disposed around the rotor, a memory circuit for storing electrical conduction data used for exciting the coil and corresponding to operational characteristics of the motor and the sectors, and a control circuit for controlling the application of current to the coil in accordance with the electrical conduction data and the brightness of an object to be photographed and executing exposure control in accordance with the brightness of the object, wherein the electrical conduction data comprises driving pulses for driving the sectors in a forward and reverse directions and brake pulse data used for generating braking pulses for exerting a force on the sectors in a direction opposite a direction of a rebounding motion of the sectors near or after completion of opening and closing operations of the sectors, and the control circuit outputs a control signal for controlling the application of electricity to the coil be reading specific electrical conduction data from the memory circuit in accordance with the brightness of the object to be photographed. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a plan view showing an initial state of a shutter for a camera according to an embodiment of the present invention; 
     FIG. 2 is a block diagram of the shutter for a camera according to the embodiment of the invention; and 
     FIGS.  3 ( a ) and  3 ( b ) are timing diagrams showing a relationship between opening and closing operations of the shutter and the application of current to a coil of the motor in the embodiment of the invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     An explanation will be given of an embodiment of the invention with reference to the attached drawings. In FIG. 1, a stator  9 , a rotor  10  and-a coil L constitute a swing motor M which is an actuator. The stator  9  is provided with a coil bobbin L 1  wound with the coil L and by conducting electricity to the coil L, an N pole and an S pole are selectively generated at pole portions  9   a  and  9   b  of the stator  9 . By changing the direction of conducting electricity to the coil, the rotor  10  can be rotated both in a forward (clockwise) direction and a reverse (counterclockwise) direction. 
     A set of shutter sectors  23  and  24  driven by rotation of the swing motor M are arranged symmetrically relative to a shutter opening  2 . The sector  24  is pivotably supported on a shaft  25  projected from a base plate (not illustrated), and the sector  23  is supported pivotably on a shaft  22  projected from the base plate. A sector opening-and-closing lever  20  is pivotably supported on the shaft  22  as projected from the upper surface of the sector  23 . A spring  20   d  is provided at an arm portion  20   b  of the sector opening-and-closing lever  20  and urges the sector opening-and-closing lever  20  in the clockwise direction as viewed in FIG.  1 . The sector opening-and-closing lever  20  is brought into contact with a pin  21  provided at the base plate to thereby restrict further rotation in the clockwise direction. A pin  20   a  provided at one end of the sector opening-and-closing lever  20  is engaged with the sector  23  and is further engaged with a groove portion  24   a  of the sector  24  to thereby drive the sectors  23  and  24 . A fork portion  20   c  is provided at other end (lower side of the drawing) of the sector opening-and-closing lever  20  and engages with a pin  11   b  of a rotor operating lever  11 . 
     The rotor operating lever  11  has a hole portion  11   a  fixedly press-fitted to a rotating shaft  10   a  of the rotor  10  and the rotor operating lever  11  moves integrally with the rotor  10 . The pin  11   b  provided at one end of the rotor operating lever  11  slidably engages with the fork portion  20   c  of the sector opening-and-closing lever  20 . Therefore, when the rotor  10  is rotated or angularly displaced in the clockwise (or counterclockwise) direction, the rotor operating lever  11  is also pivotally moved or angularly displaced in the clockwise (or counterclockwise) direction whereupon the sector opening-and-closing lever  20  is pivotally moved or angularly displaced in the counterclockwise (or clockwise) direction by rotor operating lever  11  to thereby operate the sectors  23  and  24 . 
     Referring to FIG. 2, the rotation or angular displacement (e.g., pivotal movement) of the rotor  10  is carried out by a motor drive unit  8  provided in a camera main body (not shown). The rotor drive unit  8  pivotally or angularly drives the rotor  10  by receiving electrical conduction data from a control circuit  7 . The control circuit  7  reads information of film sensitivity from an ISO information reading circuit  27 , information of brightness of an object to be photographed from a light measuring unit  26  and, depending on the sophistication of the shutter, other information affecting the exposure value, determines an exposure amount, and reads the electricity conduction data of an electricity conduction time period and an electricity conduction direction for operating the rotor  10  in correspondence with the exposure amount from a memory circuit  5 . Details of the content of the electricity conduction data will be described later. 
     An explanation will be given of operation of such a motor-controlled shutter device. FIG. 1 shows a state before operating the shutter. In this state, when a release button  6  of the camera shown in FIG. 2 is initially operated, the exposure positions of the sectors  23  and  24  are determined by the control circuit  7  according to brightness information of the object to be photographed read from the light measuring unit  26 , film sensitivity information read from the ISO information reading circuit  27 , zoom information, and the like shown in FIG.  2 . Thereafter, the electricity conduction data for driving the sectors  23  and  24  to the desired exposure positions is read from the memory circuit  5 . In order to drive the sectors  23  and  24  to the desired exposure positions, electricity is conducted to the coil L shown in FIG. 1 by the control circuit  7  via the motor drive unit  8 . 
     By way of example, by conducting electricity to the coil L, firstly, an S pole is generated at the pole portion  9   a  of the stator  9  and an N pole is generated at the pole portion  9   b  and the rotor  10  is rotated or angularly displaced in the clockwise direction. By the rotational movement of the rotor  10 , the rotor operating lever  11  which is fixedly press-fitted to the rotor  10  is also pivotally moved in the clockwise direction on the shaft  11   a  thereof and accordingly, by pivotally moving the rotor-operated lever pin  11   b  in the clockwise direction, the sector opening-and-closing lever  20  is pivoted in the counterclockwise direction. In accordance therewith, the sectors  23  and  24  are pivotally moved in directions opposed to each other via the sector opening-and-closing lever pin  20   a  provided on the sector opening-and-closing lever  20  and the exposure opening starts to form. 
     The sectors  23  and  24  are pivotally moved to predetermined exposure positions determined by the control circuit  7  in accordance with brightness information of an object to be photographed obtained from the light measuring unit  26  shown in FIG. 2, the ISO information  27  of film sensitivity, and the like. Thereafter, electricity is conducted to the coil L in the reverse direction, the rotor is rotated in the reverse direction (operation in the counterclockwise direction), the exposure is finished and the sectors  23  and  24  return to the state shown in FIG.  1 . 
     In this case, the electrical conduction data which is used to control the motor drive unit  8  in accordance with an exposure amount determined at the control circuit  7  is output from the memory circuit  5 . That is, the electrical conduction data takes into consideration inertia of the rotor  10 , the sector opening-and-closing lever  20  and the sectors  23  and  24 . Further, the electrical conduction data is data for conducting electricity alternately in the forward and reverse directions such that electricity is conducted in the reverse direction immediately before the sectors  23  and  24  reach the fully opened positions to prevent the rebounding motion caused by inertia from being brought about in the sectors  23  and  24  after the sectors  23  and  24  have reached the fully opened positions and thereafter, electricity is alternately applied in the forward and reverse directions to stabilize movement of the sectors. 
     An explanation will be given next of the operation of the shutter with reference to timing diagrams shown in FIGS.  3 ( a ) and  3 ( b ). FIGS.  3 ( a ) and  3 ( b ) explain electricity conduction data applied to the coil L in correspondence with the opening motion of the sectors  23  and  24 . FIG.  3 ( b ) shows a state of conducting electricity to the coil L, the abscissa shows an elapsed time period and the ordinate shows an electricity conduction direction, respectively. The elapsed time period is made to correspond in FIG.  3 ( a ) and FIG.  3 ( b ). 
     As shown by FIGS.  3 ( a ) and  3 ( b ), when electricity starts conducting to the coil L to move the rotor  10  in the forward direction at time ti by application of a forward driving pulse, the rotor  10  is rotated in the clockwise direction and operates to open the sectors  23  and  24  as mentioned above (period T 1 ). Further, at the end of the shutter opening operation, at time t 2  immediately before the sectors  23  and  24  arrive at fully opened positions (period T 2 ), a reverse braking pulse is provided to the coil L and electricity is conducted in the reverse direction for a short period of time by application of the short reverse braking pulse. Therefore, although the sectors  23  and  24  would ordinarily overrun, as shown by a dotted line in FIG.  3 ( a ), this is prevented by applying electricity to the coil L in the reverse direction so that a reverse magnetic field is generated at the magnetic poles  9   a  and  9   b  of the stator  9  and the reverse magnetic field operates to forcibly brake the rotor  10 . 
     Therefore, the sectors  23  and  24  do not overrun significantly, and the path of the sectors is represented by a bold line. Thereafter, similarly, in compliance with the direction of motion of the sectors  23  and  24 , forward and reverse pulses of opposite polarity are provided as shown by FIG.  3 ( b ) at times t 3  and t 4  to operate the rotor  10  in directions respectively reverse to the direction of motion of the rotor  10  and electricity is applied alternately in the forward and reverse directions for short periods of time. Thereafter, at time t 4 -t 5 , no energy is applied to the rotor  10  to maintain the sectors  23  and  24  in the fully opened position. The times t 2 , t 3  and t 4  are determined based on the respective shutter type and can be set substantially uniformly and therefore, the electricity conduction data of the memory circuit  5  can be specified for respective shutter types. 
     Further, when the sectors  23  and  24  finish a closing operation (period T 3 ) in response to application of a reverse driving pulse at time t 5 , similar rebounding motion is brought about since the rotor  10  and the sectors  23  and  24  are stopped rapidly. Hence, according to the invention, at time t 6  immediately before the closing operation (period T 3 ) has completely finished in response to application of electricity to the coil L in the reverse direction, a forward braking pulse is provided and electricity is applied in the forward or regular direction for a short period of time. Thereafter, depending upon the direction of motion of the sectors  23  and  24 , reverse and forward pulses of opposite polarity are applied at times t 7  and t 8  to operate the rotor  10  in directions respectively reverse to the direction of motion of the rotor  10  and electricity is alternately applied in the reverse and forward directions for short periods of time as shown by FIG.  3 ( b ). Further, although the sectors  23  and  24  would have otherwise operated as shown by a dotted line in FIG.  3 ( b ), the sectors  23  and  24  are operated in a modified manner to avoid rebounding as shown by a solid line. 
     Although according to the above-described embodiment, a swing motor is used for the actuator, any motor capable of reciprocally and pivotally moving may be used, such as a step motor, an ultrasonic motor, or the like. Further, although electricity is conducted to rotate the rotor in the direction reverse to the rebounding direction at time t 2 , t 3  and t 4  in opening the sectors and at time t 6 , t 7 , t 8  and t 9  in closing the sectors, there frequently arises a problem due to the fact that the sectors rebound and return in a direction reverse to a progressing direction. Accordingly, there may be provided only braking pulses for conducting electricity in the progressing direction of the sectors which is carried out at time t 3 , t 7  and t 9 . 
     The shutter according to the invention is constructed as described above and accordingly, additional space to accommodate for extra motion of the sectors due to rebounding of the sectors during shutter opening and closing operations is not needed, and the shutter can thus be reduced in size and cost. 
     Further, since motion of the sectors is stabilized at an early stage, in shifting to regular electricity conduction for finishing a shutter opening operation, the sectors can be shifted to the closing operation at an accurate timing with no influence of rebounding of the sectors and high accuracy exposure control can be achieved.