Browse Prior Art Database

Capstan Servo Control

IP.com Disclosure Number: IPCOM000079000D
Original Publication Date: 1973-Apr-01
Included in the Prior Art Database: 2005-Feb-26
Document File: 3 page(s) / 67K

Publishing Venue

IBM

Related People

Larson, DD: AUTHOR [+2]

Abstract

This apparatus insures that the capstan of a magnetic tape unit will stop the tape with the read/write head positioned at an optimum position in the interblock gap (IBG), in order to achieve both minimum access time on the next capstan movement command and minimum size IBG.

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Capstan Servo Control

This apparatus insures that the capstan of a magnetic tape unit will stop the tape with the read/write head positioned at an optimum position in the interblock gap (IBG), in order to achieve both minimum access time on the next capstan movement command and minimum size IBG.

When the capstan receives a command to stop, deceleration of the magnetic tape begins. If a capstan movement reinstruct to move tape in the same direction is thereafter received within a very short time interval, the capstan motor is immediately reenergized to continue such tape movement. However, if such a reinstruct calls for tape movement in the opposite direction, or if the reinstruct is not received until after this short time interval has expired, but before the capstan has actually stopped, the capstan servo measures the distance to the stop condition and uses this parameter, in combination with the specific reinstruct, to "look-up" an optimum stop position for the capstan from a table or memory which has been constructed to optimize the subsequent capstan acceleration. In order to achieve a minimum size IBG, this optimum stop position is usually behind the capstan's first stop position and, therefore, the capstan "hitches" or moves the opposite direction to this optimum point. Once the capstan is in position at the optimum position, as selected from memory, the capstan motor begins execution of the new movement instruction.

If no reinstruct has been received by the time the capstan first stops the tape, a "look-up" is executed based upon the assumption that the prior capstan movement command will be repeated, and a hitch is executed based upon this assumption.

Fig. 1 shows a typical single capstan magnetic tape unit. This tape unit includes a bidirectional capstan 10 for moving tape 11 forward (FWD) or backward (BKWD), under the control of an electrical capstan motor and a closed- loop servomechanism 12. As tape moves in a forward direction, head 13 can be electronically controlled to either read or write. When tape is moving backward, the head can only read. The capstan servo is controlled by stop command 14 whenever a tape movement task has been completed. A new task is signalled by start command 15. This new task (reinstruct command) may take a number of forms and can follow a number of different previous tasks (previous command). Some of the task sequences are shown in the table of Fig. 1.

Fig. 2 shows the capstan servo in greater detail. A closed-loop velocity servo 16 compares actual motor velocity feedback 17 to a reference signal, representing the desired steady state velocity. By means of this comparison, servo 16 maintains the capstan velocity constant during read and write operations. A closed-loop position servo 18 controls tape movement to the look- up new starting position for the capstan on...