The invention relates to a device for limiting the extent of movement of a moveable member and is particularly suitable in the field of focusing mechanisms for cameras, and in particular is directed to a device for cutting off a motor which drives a focusing member when the focusing member arrives at a position determined by a mechanical stop member.
In many cameras, such as television cameras, movie cameras or the like it is common to provide a motor driven objective focusing lens. Such may also appear in still cameras having automatic focusing. The lens is mounted on a moveable member, such as a rack of a rack and pinion mechanism, and, a motor is energized to move the moveable member, such as, by rotating the pinion. It is also common in such devices to provide a mechanical stop member for insuring that the moveable member is accurately stopped at a precise position for infinity object focusing. However, there are disadvantages attendent to the mechanical stop member.
When the focusing lens moveable member impinges on the stop member, an excessive force is exerted on the electric motor possibly harming or causing failure of the motor. If a battery is used as the energy source, there is the additional disadvantage of unnecessary battery discharge at this time.
One method for overcoming the latter problems is to provide a clutch mechanism between the motor and the moveable member. This has been found to be unsatisfactory because even after interruption of the movement of the moveable member, the electric motor remains rotating due to the frictional load of the clutch.
Another method is to provide a limit switch which is actuated by the moveable member upon reaching the infinite focusing position to interrupt the flow of current to the motor. This, also has not been satisfactory. For one thing, it is difficult to completely accord an actuating position to the limit switch with the infinite focusing position of the objective lens. In the case where the switch is actuated before the focusing lens arrives at the stopper, defocusing results. If the switch actuating position is located slightly behind the mechanical stop position than all of the above mentioned problems, resulting from applying current to the motor after mechanical stoppage, will be incurred.
An appreciation of the criticallity in stop position can be had when it is realized that the allowable error of position of the objective focusing lens at infinite focusing is of the order of 0.01 mm. It is desirable to have a fine tuning or correcting means for adjusting the mechanical stop position so as to accurately position same. If a limit switch is used to stop the motor, the latter would have to be separately but relatively identically adjusted causing further difficulty.