Electronic speed control design maintains magnetic tape in close synchronism at the airborne and ground stationed devices. Use of the servo system during the record and reproduce modes results in the minimum amount of frequency distortion and flutter

Bentley, R.

Couchman, R.

English

NASA Technical Reports Server

December 1967	 Brief 67-10595 
NASA TECH BRIEF 
NASA Tech Briefs are issued to summarize specific innovations derived from the U.S. space program, 
to encourage their commercial application. Copies are available to the public at 15 cents each 
from the Clearinghouse for Federal Scientific and Technical Information, Springfield, Virginia 22151. 
Conceptual Servo Technique for Controlling Tape Drivers 
The problem: 
Quality performance in a magnetic tape system re-
quires that the tape be driven in quite close synchron-
ism at the airborne and ground stationed devices. 
Long term speed variation or instantaneous speed per-
turbations must be avoided in the airborne system 
lest frequency distortion and flutter exceed ground-
based system tolerance. 
The solution: 
An electronic speed control designed to maintain 
long term speed accuracy in tape transport systems 
within ±5 microseconds at a tape speed of 30 inches 
per second and to control flutter to within 0.8% rms 
from dc to 5 kHz. 
A tachometer, which is an integral part of the motor 
assembly, provides an output signal that is fed to a 
tachometer amplifier and then to a frequency dis-
criminator. The output of the frequency discriminator 
is a voltage level • that is inversely proportional to 
the input frequency. The output of the frequency dis- 
criminator drives an operational amplifier that pro-
vides the proper loop gain and compensation required 
for loop stability. The output of the operational am-
plifier is then fed to the motor driver which raises 
the loop power level to a point high enough to pro-
vide the required motor torque. 
The phase detector loop is used 'to compare the 
phase of a reference frequency recorded on the tape 
to the same phase and frequency of a precise reference 
oscillator. The output of the phase detector is a volt-
age proportional to the phase difference between the 
signal recorded on the tape and the output from the 
precision reference oscillator. This phase error voltage 
is applied to the loop amplifier to further correct 
the tape speed. 
The recorder/ reproducer system is provided with a 
tape which has a precision 30 kHz reference signal 
recorded on the reference track. During the record
mode the recorder is phase locked to this previously 
recorded signal. The tachometer loop is used as a 
back-up system during periods when loss of signal 
due to tape dropouts occurs. At the same time a 
precision 30 kHz signal is being recorded on a second 
reference track to ensure that the servo system will 
correct flutter introduced during the record mode 
when the unit is in the reproduce mode. During the 
reproduce mode the recorder is phase locked to the 
second reference track with the tachometer loop 
again being used as a back-up 'system during periods 
of loss of signal due to dropouts. 
Use of the servo system during the record and 
reproduce modes results in the minimum amount of 
frequency distortion and flutter. The time displace-
ment error will then be held to less than one period 
of the reference oscillator frequency. 
Notes: 
I. The approach described would permit the design 
of a multispeed high-performance recorder/repro-
ducer system, where the tape speed would be 
electronically selected. This system would do away 
with belts, gears and clutches or other mechanical 
speed changing devices. Speed selection could 
then be accomplished by changing circuit constants 
in the servo system electronics. 
2. This development is in conceptual stage only, and, 
as of date of publication of this Tech Brief, neither 
a model nor prototype has been constructed. 
Patent status: 
No patent action is contemplated by NASA. 
Source: R. Couchman and R. Bentley 
of Kinelogic Corporation 
under contract to 
Marshall Space Flight Center 
(MFS-12955) 
('iteonrv 0 1 
_.
This document was prepared under the sponsorship of the National 
Aeronautics and Space Administration. Neither the United States 
Government nor any person acting on behalf of the United States
Government assumes any liability resulting from the use of the 
information contained in this document, or warrants that such use 
will be free from privately owned rights.
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Conceptual servo technique for controlling tape drivers

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