1044 Chapter 28
correct speed. Various implementations use drums that
range from over 2 inches in diameter to tiny shafts less
than 0.1 inch in diameter. In general, the larger the
drum, the more accurate the tape speed control. The
very small spindles are usually employed at the very
slow tape speed found with compact cassettes and
consumer videocassettes.
The rotating drum is called a capstan, named after a
device used on sailing ships to pull in cables and
hawsers, and the clamping device is called a pinch roller.
The simplest capstan is the shaft at the end of a motor.
The diameter of the shaft is chosen so that the shaft’s
circumference will move at the desired linear tape
velocity when the motor is spinning at operating speed.
The actual linear velocity of the capstan surface is
slightly lower than the tape’s speed. The effective speed
of the tape is measured at the neutral axis of the tape,
about ½ of the tape thickness into the tape for large
capstans, but dropping down to about of the tape
thickness for small capstans. Remember that the total
thickness of a tape is the sum of the backing and coating
thicknesses. A nominal 1.5 mil tape is really about
2 mils thick—1.5 mils of backing substrate and 0.5 mils
of oxide coating.
Other designs use a capstan/flywheel assembly that
is driven by belts or rubber-tired idlers that engage the
primary drive motor’s shaft. The resulting reduction in
rotational speed permits the use of a larger capstan
diameter. A good example with a belt reduction is the
3M Isoloop™ tape transport. At 15 in/s the capstan
motor spins at 30 rev/s, but the large capstan turns only
2½ rev/s. The 12:1 speed reduction permits large diame-
ters on all drive surfaces.
A flywheel in normally employed on the capstan’s
shaft to smooth out any small speed variations. The
effectiveness of the flywheel increases directly with
increased flywheel moment of inertia, but inversely
with the square of the diameter of the capstan. The large
capstan diameter of the 3M transport required a
flywheel weighing 6 pounds!
Any rotational speed disturbances in the capstan will
show up as linear speed variation in the recording tape.
This means that the capstan must spin at an absolutely
constant speed. The simplest constant speed device is a
hysteresis synchronous motor. Synchronous indicates
that the motor runs at a speed that is locked to the
frequency of the voltage driving the motor, similar to a
clock motor (before battery operated clocks). The motor
contains a pair of windings for each operating speed.
The two windings are physically offset by ¼ of the
distance the motor rotates during one cycle of the drive
voltage. One of the windings in each pair is connected
directly to the power source. The second winding is
connected with a large capacitor in series with the
winding to shift the phase of the current and the
resulting magnetic field in the second winding approxi-
mately 90° with respect to the main winding. The phys-
ical and electrical shifts work together to create a
rotating magnetic field.
Two-speed hysteresis synchronous motors are
common in tape recorders, and a few three-speed
motors are also used. The motor must be designed for
the intended operating frequency of 50 Hz or 60 Hz and
the phase-shifting capacitor chosen for the appropriate
frequency.
Although the hysteresis synchronous motor is an
economical solution, it has major shortcomings. First,
the speed of the motor is only as good as the stability of
the frequency driving the motor. We assume that ac
source is stable. However the power companies only
guarantee a certain number of cycles each day. At any
given time the frequency of the grid at any location may
be slightly high or low depending how much adjustment
is needed to bring the daily total of cycles into compli-
ance. Sometimes, however, it is desirable to run at other
than nominal speed for special effects or pitch correc-
tion. This Variable Speed Oscillator (VSO) operation
requires a versatile power source for the capstan motor
that can be shifted in frequency. If the frequency is
shifted very far from the nominal frequency, the
phase-shift capacitor will no longer provide a true 90°
of phase shift. The motor will begin to vibrate, the
power of the motor will decline, and the motor’s
temperature may rise. The maximum practical speed
shift is then less than 15%.
A third problem is that the selection of speeds is quite
limited. For 60 Hz operation, there can be motor speed
pairs of 3600/1800, 1800/900, 1200/600, and
900/450 rpm. If the shaft of the capstan motor is used as
the actual drive surface, the desired tape speeds will
determine the diameter of the shaft. Slow tape speeds
require very small capstan diameters. The resulting small
contact area can create speed errors due to slippage.
All these problems can be avoided by substituting a
servo-controlled motor for the hysteresis synchronous
motor. A servo-controlled motor utilizes a speed-
measuring device on the capstan in the form of a
high-resolution optical or magnetic tachometer. This
tachometer may provide as many as 1200 speed samples
per revolution of the motor, a rate high enough to detect
not only overall average speed, but even very small
speed transients due to imperfections in other compo-
nents in the tape path. By comparing the speed sensed
by the tachometer to a high-accuracy reference derived(^1) » 3