This invention relates to magnetic tape information storage and retrieval apparatus, and particularly, to such apparatus having members movable for manipulating the tape for processing.
Magnetic tape cassettes, for the type having two reels lying in the same plane on parallel axes, are convenient for packaging, shipping and storing tape. The cassette usually can be loaded into an elevator of an operating device, such as a tape transport, which lowers the cassette for engagement of the tape therein to automatically "thread" it into an operating mode within the transport for processing. After the processing, the tape thereby must be "unthreaded" to return it into the cassette for removal of the cassette from the tape transport.
During movement of tape transport members, such as the elevator and tape guides and capstans which carry the tape, it is very desirable to minimize the time of travel of the member so that the tape can be loaded and unloaded as quickly as possible. Conventional devices simply move the members until they contact a mechanical stop. Completed travel is sensed by operation of limit switches. When such members are driven at a high velocity, there is a substantial impact force when the end stop is reached. This becomes particularly significant during the threading operation during which a tape guide moves tape for processing. Tape transports also typically include a tape tension sensing arm which is swingable through a range of movement indicative of the tension on the tape. A supply reel is controlled, ideally, to supply the tape at a rate which maintains the tension at a desired level. If the tape is accelerated too quickly, before the supply reel can release sufficient tape, the tension arm swings to a limited position, resulting in stretching and even breakage of the tape. Once the tape guide achieves a high speed, the supply reel reaches a speed of operation which provides the desired tension on the tape during travel of the member. However, such systems do not have an infinite response bandwidth, and are not able to adjust to changes in velocity instantly. Thus, when the end stop is suddenly reached, the supply reel lets out excess tape, causing a sudden decrease in the amount of tension on the tape. Slack develops in the tape and the tension arm reaches a maximum position beyond which it cannot adjust for the excess tape. The resulting uncontrolled unravelling of tape inside the tape transport can result in substantial damage to the tape.
Apparatus for controlling movement of tape transport members is subject to constantly changing operating conditions. For example, the control circuit may be subject to noise and drifting of component values. Further there may be changes in the mechanical functioning of the devices, such as changes in the friction between moving parts. These changes produce erroneous readings and varying operating performances which make the sensed information different than the actual. It is therefore desirable to have controllers which can adapt to such changing factors.