1. Field of the Invention
The invention relates to apparatus for eliminating slack in driven cables and, in particular, to apparatus for eliminating slack in motorized cables for raising and lowering autoclave and sterilizer doors.
2. Description of the Invention Background
There exists a wide variety of cable assisted devices adapted to open and close doors. In particular, devices such as the one described in U.S. Pat. No. 1,358,859 employ cables and pulleys to assist in the opening and closing of doors that are heavy and cumbersome to operate. In that device, a counterweight is attached to the free end of the cable in an effort to counteract the weight of the door so that it can be lifted more easily. The counterweight also serves to remove the slack from the cable to thereby prevent the cable from tracking off the attending pulleys.
Cable assisted devices are also used to effect the opening and closing of chamber doors on autoclaves and sterilizers used in the medical industry. In those types of applications, the chamber door is usually arranged to slide vertically within corresponding guide tracks that are mounted to the open end of the chamber. The door is suspended by a cable and pulley system that controls the movement of the door relative to the end of the chamber. Typically, one end of the cable is attached to a take-up drum that is rotatably driven by a reversible gear motor; the other end of the cable is connected to a counter weight that assists the door opening process. To close the door, the motorized take-up drum is rotated to accumulate the cable thereon. This action causes the door to slide vertically upward to a closed position. The door is opened by simply reversing the rotation of the take-up drum which permits the cable to slowly unwind therefrom. The weight of the door along with the counterweight that is attached to the free end of the cable keep the cable taut as it unwinds from the drum.
The door is lowered in this manner until it reaches a fully opened position. A stop switch arranged to detect when the door has reached the open position causes the gear motor to be de-energized. However, due to the inertia of the gear motor arrangement, the take-up drum typically continues to rotate for a short time after the gear motor has been de-energized. That additional rotation permits an additional length of cable to unwind from the take-up drum causing the cable to become slackened on the attending pulleys. Often, depending upon the amount of cable slack introduced, the cable is permitted to track off the attending pulleys causing malfunction of the door drive system. This problem has heretofore been typically alleviated by employing an expensive electro-mechanical brake system to stop the rotation of the take-up drum as soon as the gear motor is de-energized by the stop switch. Thus, the need exists for a method of eliminating the cable slack without the use of expensive electrically controlled components such as electro-mechanical brakes.
Another problem typically encountered with prior cable systems is that a gear motor having high starting torque capabilities must be used. More specifically, the prior cable and gear motor arrangements cause the gear motor to encounter a load corresponding to the weight of the door and the inertia of the system immediately upon being activated. As such, a gear motor that is capable of developing the starting torque necessary to immediately overcome the weight of the door and the inertia associated with the cable controlled system must be used. It will be appreciated that gear motors having that kind of starting torque capabilities are larger and more expensive than similar gear motors having smaller starting torque capabilities. Thus, the need exists for a simple cable operated door system wherein a smaller and less expensive gear motor can be utilized.
Yet another problem commonly encountered with prior cable controlled door operating systems is the inability to automatically monitor the door's progress while it is being opened. Should the door become jammed or obstructed while it is being opened, the operator may not discover the problem until excess cable has been partially or completely unwound from the take-up drum causing damage and malfunction. As such, there is a need for a monitoring and controlling apparatus for cable-controlled doors wherein the gear motor is automatically stopped without operator intervention when the door becomes jammed while opening and wherein the operator is provided with timely notice of the problem.