1. Field of the Invention
This invention relates to electromagnetic drive devices of the moving coil type of which the direction of action is made to differ depending upon the direction of current flow, and more particularly to cameras with such a moving coil rendering it possible to control the operation of a focus adjusting mechanism and an exposure mechanism.
2. Description of the Prior Art
At the present time, state-of-the-art cameras are provided with a coil drivingly connected to the shutter blades and positioned in a magnetic field so that upon current supply to this coil, it is driven to move by electromagnetic force according to Fleming's left hand law so that the shutter blades are opened and closed. This is the so-called moving coil type electromagnetic drive device. However, there is not yet known a device which is made to also serve to control the initiation and termination of an operation of an automatic focus adjusting mechanism (hereinafter described as an "AF" mechanism). This is because in the moving coil type electromatic drive device, it is a prerequisite that the ordinary penlight (size AA) battery be used as the common electrical energy source. Thus, the device built into the camera cannot provide a driving torque large enough to effect functions other than opening and closing of the shutter blades, for example, actuation of an AF mechanism, while still preserving the reliability and accuracy of shutter control. Thus, if at the time of opening and closing of the shutter blades, the coil is subjected to an external interference, motion of the coil will become unstable, thereby creating a problem in that the accuracy of exposure control is reduced.
On the other hand, it is known to provide other types of electromagnetic drive devices different from the moving coil type, for example, one in which the operation of the focus adjusting mechanism and the opening and closing operation of the shutter blades is controlled by using only one solenoid as disclosed in U.S. Pat. No. 4,167,316, issued Sept. 11, 1979. This device operates in such a manner that, as the plunger of the solenoid is drivingly connected to the shutter blades, when the solenoid is first energized, the plunger is attracted to move from the initial position, whereby operation of the focus adjusting mechanism is initiated. Then when the aforesaid energization is terminated, as the plunger moves under the action of a return spring, the operation of the focus adjusting mechanism is terminated, and the shutter blades start to open. After that when the solenoid is energized again, as the plunger is attracted, the shutter blades are closed.
Since, however, the electromagnetic drive device in the form of the solenoid when energized only moves the plunger in one direction, the use of the common solenoid in selectively controlling the operations of the focus adjusting mechanism and the shutter mechanism leads to the existence of the drawback that its arrangement becomes very complicated. Also, in the aforesaid example of the prior art arrangement in which when the solenoid is de-energized, the plunger moves by spring forces to effect simultaneous stopping of the focus adjusting operation and an initiation of the opening of the shutter blades, it is impossible to control the individual operations independently of each other. Therefore, there is no opportunity for permitting a timer mechanism to be operated as the necessity arises after the completion of the focus adjusting, and thereby the initiation of the shutter operation cannot be deferred, and an exposure with a self-timer cannot be made.
In short, where the electromagnetic drive device for controlling both the focus adjusting operation and the shutter operation is constructed in the form of a solenoid arranged upon energization to move the plunger in only one direction, the complexity of the structure of the camera is increased, and the capabilities of the camera are reduced.