This invention relates to a camera with a shutter the blades of which are actuated by electromagnetic force.
Many kinds of shutters which are actuated by electromagnetic force have been proposed. This type of shutter not only has a simple mechanism but also makes the whole mechanism of a camera simpler because a shutter charge operation is not necessary, whereas the conventional shutter is charged by the force generated by another mechanism such as the film winding mechanism and the like.
For that reason, there is a general tendency that the electromagnetic actuated shutter is widely applied to a popular camera as well as a high-grade camera.
In FIG. 5, an example of a monostable high speed control solenoid used as an actuator of this type of shutter, is shown. This solenoid is composed of the permanent magnet 30D, the coil 30C adjacent to the magnet, the movable iron core 30A which is set in a bobbin of the coil 30C being able to oscillate, and the movable member 30B provided to the tip of the movable iron core 30A.
When the coil 30C is not electrified, the movable iron core 30A is placed at the first position, shown in the drawing, being attracted by the permanent magnet 30D. When the coil 30C is electrified, the electromagnetic force generated by the coil, acts in two directions according to the direction of current. One is the same direction as the attraction force of the permanent magnet 30D and the other is the opposite direction to the attraction force of the permanent magnet 30D. In the latter case, when the current exceeds a certain level, drive force exceeding the attraction force of the permanent magnet 30D is generated by the coil 30C. Therefore, the movable member 30B moves counterclockwise, rocking on the shaft 30F which is the fulcrum, and reaches the second position opposite to the first position, wherein the first and second positions are shown in FIG. 5.
When the current supply to the coil 30C is cut off, the moving member 30B is returned to the first position. Hereinafter, the explanation will be limited to the case in which the electromagnetic force generated by the coil 30C, acts only in the opposite direction to the attraction force of the permanent magnet 30D.
Accordingly, when the coil 30C is electrified, the shutter blades can be opened and closed by means of fitting the pin 30E mounted on the movable member 30B to elongated holes of the shutter blades. It can be understood that the exposure time can be controlled by the time period while the coil 30C is electrified.
It is clear that the structure of a camera becomes simpler compared with conventional cameras when a solenoid is used as an actuator of a shutter.
Since the operation of this type of solenoid is reliable and rapid compared with conventional ones, it can be expected that a shutter in which this type of solenoid is used will be activated reliably. It can be also expected that the shutter's opening and closing speed is faster than conventional ones and the shutter efficiency is superior.
Especially when the shutter blades are activated directly by the actuator and the shutter blades are made of synthetic resin, the load on the solenoid becomes very small. As a result, the shutter opening and closing speed becomes still faster.
Recently, many cameras combine the diaphragm with the shutter in order to minimize the body size, reduce the cost, and simplify the mechanism.
When a photograph is taken by a camera with this type of shutter mechanism in the high luminance level in which exposure control is considered to be difficult, in many cases a photographic film is given a prescribed exposure by the following shutter operation; the shutter is partially opened to reach the prescribed aperture size before being fully opened, and then the shutter is shifted to the closing motion to give a photographic film the prescribed exposure.
When a monostable high speed solenoid is applied to the shutter described above, there is a problem; since the shutter opening and closing speed is fast, as mentioned before, the time which is necessary for the shutter to reach the prescribed aperture size is short, and the exposure fluctuates remarkably according to fluctuations of the solenoid electrifying time. This means that the exposure is affected by a fluctuation of the power source and environmental conditions such as a variation in temperature and humidity, which causes a variation of the frictional resistance of the shutter blades.
Accordingly, it is very difficult to accurately control exposure by adjusting the actuator solenoid electrifying time in the case of a shutter in which the monostable high speed control solenoid is used.
In order to solve this problem, the shutter opening and closing speed may be reduced. The mechanical brake method or the mechanical speed control method is generally used to attain this object.
The mechanical method is described as follows. A part of the energy supplied to the actuator is consumed by the above-described methods in order to reduce and stabilize the shutter opening and closing speed.
Another method is disclosed in Japanese Patent Publication Open to Public inspection No. 113130/1986 the details of which are as follows; information about the condition of the opened shutter is detected and fed back in order to establish a relation between the time and the shutter aperture size, and the characteristics of the time and the shutter aperture size are made adjustable.
Since this method is a feedback control, it possesses the advantage of being able to stabilize its efficiency even if disturbed by fluctuations of power source voltage or environmental conditions.
As described above, when a monostable high speed solenoid is used for the actuator of a camera shutter, it is very difficult to obtain exposure accuracy by controlling the actuator electrifying time.
There are methods to solve this problem such as the mechanical brake control method or the mechanical speed control method. However, these methods consume part of the energy to be supplied to the actuator. Therefore, all of the energy necessary to drive the apparatus must be sufficiently large.
On the other hand, the camera has been developed in pursuit of small size and light weight. For that reason, a small actuator has been used for the shutter device and a small power source has been used for the camera; therefore, it is difficult to obtain sufficient energy to activate the shutter device.
As for the mechanical brake control method or the mechanical speed control method, the device for them must be installed in a camera. As a result, a simple shutter device becomes too complicated to apply these mechanical methods to a camera. For these reasons, there are many problems to apply this mechanical speed control method to a camera.
As for the method to make the characteristics of the shutter opening motion adjustable by feeding back the information about the condition of the opened shutter, it is not reliable in view of the response speed of the system, including the feedback system, since the object to control is a high speed phenomenon. Furthermore, the shutter opening condition detecting means and the control means to make the characteristics of the shutter opening motion adjustable, must be installed in a camera. Accordingly, the shutter device becomes complicated and the cost tends to become high.
Sufficiently high accuracy can not be obtained by the shutter device in which a monostable high speed control solenoid is utilized.
The electromagnetic shutter device is characterized in that; the device is simple; energy loss is little; the shutter blades can be activated at a high speed; and sufficiently high exposure accuracy can be obtained. The electromagnetic shutter device for a camera has the structure as follows; when the solenoid is not electrified, the movable iron core of the actuator is held by a spring at a stable first position; when the solenoid is electrified, the movable iron core of the actuator moves to the second position resisting the force of the spring; the movable iron core restricting member is provided which moves between the blocking position and the withdrawn position, wherein the movable iron core restricting member blocks the movable iron core at the blocking position and it does not block the movable iron core at the withdrawn position; the movable iron core restricting member drive means is installed which moves the movable iron core between the blocking position and the withdrawn position, and stops the movable iron core at each position; and the shutter is opened and closed by the above-described actuator.
The plunger type solenoid is widely used as the above-described movable iron core restricting member drive means. The movable iron core of the plunger type solenoid is relatively heavy in the same way as the movable iron core of the monostable solenoid. For that reason, when the camera is subjected to strong vibration or shock such that a photographer drops the camera to the floor or knocks it, the movable iron core restricting member and the moving member of the actuator cause malfunction by their own inertia and interfere with each other. In other words, they are locked and can not move.
FIG. 6 shows the condition that the actuator is locked. The restricting member 40 follows the movement of the movable iron core 50B of the plunger solenoid 50 by the action of the spring 43. The figure shows the condition when the restricting member 40 follows the movable iron core 50B as the core falls instantaneously with any shock or vibration, and is withdrawn from the locus of the movable iron core 30A; the movable iron core 30A is rotated and the moving member 30B and the restricting member 40 interfere each other; and they are locked with each other being pressed by the spring 43.
This lock condition will continue unless the plunger solenoid 50A is electrified and the movable iron core 50B is attracted and lowered. While the lock condition continues, the shutter blade 23 is opened and the photographic film continues to be exposed.
In the present invention, the arrangement of the plunger solenoid 50 was studied dynamically in order to avoid the above-described trouble caused by shock and vibration given to the camera. One of the objects of the invention is to provide a camera with a electromagnetic shutter in which an actuator such as a monostable high speed control solenoid does not interfere with another electromagnetic drive member such as a plunger solenoid during the operation.