Altitude correcting vacuum system

A blower motor providing vacuum as, for example, for a magnetic tape handler, is programmed to provide a constant vacuum in an environment of changing altitudes. The blower motor is controlled by a pressure transducer which speeds up the motor as the altitude is increased and slows down the motor as the altitude is decreased.

PRIOR ART 
Tape handlers in magnetic tape memory systems for computers and the like 
employ reels of magnetic tape which is passed rapidly over a 
record/play-back magnetic head. Such a system requires sudden starting, 
stopping and reversals of tape travel. The faster these changes in tape 
speed and direction, the more efficient is the use of the tape since 
recording and play-back cannot take place during the slow down and speed 
up intervals required to safely start and stop the tape. The tape is moved 
past the record/play-back head by means of a light weight capstan driven 
by a fast programmable light weight motor. This combination provides the 
fast starts, stops and constant running speed of the tape during the 
record and play-back cycles of operation. 
However, in order to provide a high capacity memory capability in these 
magnetic tape handlers, large reels capable of storing hundreds of feet of 
tape are used before and after the capstan. These large reels are 
relatively heavy and require relatively large driving motors. In order to 
be able to use reasonable size motors, their slower starting and stopping 
times when compared with the capstan are accommodated by means of 
substantial slack loops before and after the capstan. These loops are 
maintained under low tension by means of vacuum tanks, one before and one 
after the capstan. Dynamically, when the tape is being moved at a constant 
speed, a degree of vacuum is maintained which will keep the average loops 
of tape within the vacuum tanks within a predetermined area. During 
stopping and starting deceleration and acceleration, these loops are 
pulled into much shorter loops or dropped into much larger loops. The 
relative acceleration and deceleration of the capstan and tape reels, and 
the degree of maintaining vacuum determine the required capacity of these 
vacuum tanks since during these transient periods the tape must not be 
pulled completely out of a tank or allowed to enter fully into a tank. 
PRESENT INVENTION 
A multistage blower is used to provide the vacuum in the prior art tape 
handling system as described above. The description sounds as though the 
vacuum draws the tape into the vacuum chamber. Actually atmospheric 
pressure which is greater than the vacuum pressure pushes the tape into 
the vacuum chambers. More precisely, the difference between the 
atmospheric pressure and the vacuum pressure is what does the pushing. A 
typical tape handler is designed to provide the required pressure 
differential at normal atmospheric pressure i.e. at or near sea level 
pressure. 
However, if the conventional tape handler is operated above or below normal 
atmospheric pressure i.e. substantially above or below sea level, the 
typical vacuum system malfunctions either pulling tape too far in to the 
tanks or not far enough. 
In accordance with the present invention, both the machine vacuum pressure 
and the atmospheric pressure are sensed. These two sensed valves properly 
weighted are used to increase or decrease the machine vacuum as by 
speeding up or slowing down the machine vacuum blower in order to maintain 
a predetermined pattern of pressure differential at all altitudes. 
There are two preferred ways in which the blower motor speed is controlled 
in accordance with combined atmospheric and blower pressure. According to 
one method, the sensed pressures are combined, compared with a reference 
in a summing amplifier and the resulting error voltage is applied to a 
voltage to frequency converter the output of which is applied to a power 
amplifier which, in turn, drives a variable speed A.C. blower motor. 
According to the other method the error voltage from the summing amplifier 
is amplifed and applied to a variable speed D.C. blower motor. Both of 
these methods are essentially degenerative feed-back system due to the 
sensing of the machine vacuum.

FIG. 1 is a view in perspective of a prior art tape handler in which a 
panel and cabinet 1, carry two tape reels 2 and 3 rotated by suitable 
motors, not shown. The magnetic tape 3 passes from supply reel 2 to 
take-up reel 3. Its path is over idler 6 into the top section of vacuum 
tank 4-5, over idlers 7 and 8, across record/play-back head 9, over idler 
9 to drive capstan 10 rotated by a suitable motor, not shown, over idler 
11 into the lower end of vacuum tank 4-5, and over idler 12 to take-up 
reel 3. Vacuum is supplied to vacuum chamber 4-5 by means of suitable 
openings 13 and a suitable blower, not shown. It is conventional in prior 
art tape handlers to provide a multiple stage blower driven by a constant 
speed motor to provide vacuum which may then be adjusted to a suitable 
constant vacuum by means of a suitable control valve. 
FIG. 2 is a block diagram of one of the two preferred form of the present 
invention which provides automatic vacuum control for changes in 
atmosphere pressure as, for example, changes due to changes in altitude. A 
multistage blower 14 supplies machine vacuum over closed tube 15 and 
exhausts sucked out air over pipe 16. Closed tube 15 is to be coupled to 
the vacuum openings in the tape handler as openings 13 of FIG. 1. In order 
to control the speed of blower 14 as a function of the altitude and/or 
atmospheric pressure, the machine vacuum is sensed by means of a pressure 
transducer 18 coupled to the machine vacuum over closed tube 17 and 
atmospheric pressure is sensed by a pressure transducer 20 coupled to the 
atmosphere over pipe 21. These two sensors provide D.C. output voltages 
which are proportional to their sensed pressures. Thus, if the output of 
transducer 18 is subtracted from the output of transducer 20, the 
resultant voltage will be proportional to the difference between the 
atmospheric pressure and the machine vacuum pressure. These outputs are 
conducted over leads 19 and 22 and combined over lead 23 to summing 
amplifier 24 where they are compared with a reference voltage 25 thereby 
providing an error voltage at output terminal 26. When this error voltage 
approaches zero, the differential pressure generated voltages will 
approach the reference voltage. 
In order to control the machine vacuum pressure at a constant difference in 
comparison with the atmospheric pressure, the error voltage at ouput 26 is 
applied to voltage to frequency converter 28 over lead 27, the controlled 
frequency from converter 28 is applied over lead 29 to power amplifier 30 
which in turn drives blower AC. motor 33 over leads 31 and 32. The blower 
speed thus controlled provides a machine vacuum at a pressure which is a 
predetermined fixed difference below atmospheric pressure. If the 
difference drops due to increased altitude, the error voltage increases, 
the motor driven frequency increases and the blower speeds up to restore 
the predetermined difference in pressure. The system is, in effect, a 
feedback system. 
FIG. 3 is a block diagram of the second preferred form of the present 
invention. The pressure sensing and summing are the same as shown in FIG. 
2 and described above. The difference is the way in which the error 
voltage from summing amplifier 24 is converted to a controlled blower 
speed. In FIG. 3 the error voltage at output 26 of summing amplifier 24 is 
applied over lead 27 to a D.C. power amplifier 35 which in turn drives 
D.C. motor 38 over leads 37. The D.c. motor drives blower 14 over shaft 
39. Since the D.C. motor speed is proportional to its applied D.C. voltage 
it will speed up increasing the blower speed and hence the machine vacuum 
until the predetermined pressure difference is restored or maintained. 
While pressure transducers 18 and 20 as shown in FIGS. 2 and 3 are 
described as analog devices, digital transducers may also be used for one 
or both. Where the summing amplifiers require analog input signals, the 
digital outputs of the digital transducers are merely converted to analog 
siganls by means of suitable digital to analog converters. 
While only two preferred forms of the present invention have been shown and 
described, modifications are possible within the spirit and scope of the 
invention as set forth in particular in the appended claims.