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Variable Gain Servo Network

IP.com Disclosure Number: IPCOM000079005D
Original Publication Date: 1973-Apr-01
Included in the Prior Art Database: 2005-Feb-26
Document File: 3 page(s) / 55K

Publishing Venue

IBM

Related People

Buhler, OR: AUTHOR [+4]

Abstract

This servomechanism varies the gain of the closed-loop control system as the parameters of the controlled environment, in this case a reel-to-reel magnetic tape transport, change. In a precision reel-to-reel transport, proper cooperation between the magnetic tape and the read/write head requires that the tape's acceleration/deceleration, speed, instantaneous position and tension parameters be accurately and continuously controlled throughout the entire length of the tape.

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Variable Gain Servo Network

This servomechanism varies the gain of the closed-loop control system as the parameters of the controlled environment, in this case a reel-to-reel magnetic tape transport, change. In a precision reel-to-reel transport, proper cooperation between the magnetic tape and the read/write head requires that the tape's acceleration/deceleration, speed, instantaneous position and tension parameters be accurately and continuously controlled throughout the entire length of the tape.

This servo network provides two separate servos, an EOT (end-of-tape) servo and a BOT (beginning-of-tape) servo, for each of the two reel motors. Each of these two servos compares the actual value of the tape parameters being controlled to a reference or command value for that parameter, for example speed and tension, to thereby originate an error signal. Ideally, the reference value is calculated to achieve the desired tape speed and tension with no error. However, should an error exist, the above-mentioned comparison and the resulting error signal provides the necessary servo corrective action.

The EOT and BOT servos are controlled by the various reference and error signals to provide output voltages, based upon the assumption that the tape is just beginning its travel from the supply reel to the take-up reel (BOT), and that the tape has ended its travel from the supply reel to the take-up reel (EOT), respectively. In actual practice, however, the BOT and EOT conditions cannot occur simultaneously, and in fact the tape position is usually somewhere between these two extremes. Therefore, the outputs of the EOT and BOT servos are applied to a weighting network which blends or weights these two signals, in accordance with the actual position of the tape/head interface relative to the two reels. For example, when the tape is at the BOT position, the weighting is such that the full output of the BOT servo is used to energize its motor, and the EOT servo output is not used to any degree.

A refinement of the above-described variable gain servo network provides a number of servos, including an EOT and a BOT servo, with additional servos constructed to energize the two reel motors for various tape zones between the BOT/EOT extremes, wherein the outputs of all servos, including the BOT and the EOT servos, are dynamically blended in accordance with the actual tape/head position.

Fig. 1 shows an idealized reel-to-reel tape transport having a supply reel 10 and its motor 11, a take-up reel 12 and its motor 13, and a read/write head 14. Tape speed/tension/position sensor 15 is associated with the length of unbuff...