Handbook for Sound Engineers

1050 Chapter 28

device. The resulting output voltage or current provides
an electrical signal proportional to tension that can be
used to control the spooling motor drive voltage.

Numerous types of sensing devices have been
employed by various manufacturers; including photo-
cells, photo-potentiometers, rotary potentiometers, and
Hall effect devices. Noncontacting photosensors and
Hall devices have demonstrated longer lives than rotary
potentiometers with sliding mechanical contacts.

Rather than directly measuring the tape tension, the
open loop tension servos sense other parameters such as
the rotation rate of the spooling motors and infer or
calculate the amount of drive to the spooling motor
necessary to achieve the desired tape tension. For
example, the rotational velocities of the spooling motors
can be measured with dc tachometers attached to the
spooling motor shafts. The tape speed can be derived
from the frequency of the tachometer pulses coming
from the capstan servo motor. Dividing the tape speed
by the reel rotation rate yields a value proportional to

the tape pack radius. Tape tension times tape pack

radius is equal to the required motor torque. Therefore,

(28-3)

If the calculation is executed with analog circuits, the

multiplication is easily implemented by a potentiometer,

but the division requires an analog multiplier/divider

integrated circuit.

The same results can be achieved with a read-only

memory lookup table that is programmed with the

correct motor voltages for various combinations of

speed and pack diameter.

MCI tape transports using the open loop tension

control method described above provided very accurate

tension control over a wide range of speeds and reel

diameters. The major shortcoming is that the calculated

method cannot detect tension abnormalities due to bent

reels, motor problems, or changes in friction.

A further benefit of the diameter calculations is the

ability to anticipate the end of a reel of tape. Both unin-

tentional unthreading of the machine and abusive

high-speed unthreading can thus be avoided.

The MCI tension control worked very well because

an integrated circuit performed the analog division with

high precision. Other tension controls, such as the

Tentrol for Ampex transports, substituted a pair of

adjustment potentiometers for the analog divider. The

resulting straight-line approximation of the division

process was not as accurate as the MCI method, but it

was much better than the 2:1 tension change without

any sensing and control.

28.2.4 Tape Guiding

For proper recording and playback of a magnetic

recording to occur, the tape must move over the heads in

a very precise path. This tape path should be the natural

path that the tape would follow without any external

vertical constraints. The purpose of the guiding system

is not only to protect the tape and to overcome the slight

reel-to-reel variations in tape such as twists and bends

due to tape-manufacturing tolerances but not to force the

tape to perform any unnatural acts. Any such use of

brute force will lead to tape damage, excessive guide

wear, and/or instabilities and jumping of the tape.

The tape guiding system deals with five aspects of

the tape motion—height, azimuth, zenith, wrap, and

rack—with primary concern for the motion of the tape

at the heads. Each aspect is in turn composed of two

Figure 28-11. Closed and open loop tension servos.

Spooling motor

Capstan motor

Tachometer

Tachometer

Reel pack

radius

Reel speed estimater Tape speed

Motor

drive

amplifier

Tape

A. Open loop.

Tape

Motor

drive

amplifier

Difference

detector

Spooling motor

Reference

tension

Actual

tension

B. Closed loop.

Capstan motor

Vmotor t ension adjustment factor

speedtape

= uspeed------------------------------------rotational