Analysis Used in Submerged Body Research 409 
Alternating voltage from the gage passes to the balance and sensitivity control box, 
which contains a silicon diode bridge as well as other adjustments and refinements that are 
described in more detail later. The voltage is rectified by the diode bridge to produce full- 
wave rectified direct-current voltage. The total rectified voltage obtained across both coils 
is constant and is used as a reference voltage. Polarity is established by making one side 
of the line positive and the other negative. The voltage measured between each coil 
changes, however, when the gage core is displaced; the voltage across one coil increases 
while the other decreases an equal amount so that the reference voltage always remains con- 
stant. This is analogous to a three-wire system in which the voltage across the outside 
lines remains constant but the voltage from one side to the common is made variable. The 
feedback potentiometer, which is on the shaft of the digital indicator, is wired similarly; the 
voltage across the end terminals is the reference voltage and the common is attached to the 
potentiometer slider. When the gage core is at electrical center, the potentiometer slider is 
at midposition. When the gage core is displaced, an error signal results. 
The error signal is fed through a mode selector switch (static or dynamic; digital or 
recorder) to a chopper servo-amplifier similar to that contained in the Brown recorder manu- 
factured by Minneapolis-Honeywell Company. The chopper converts the direct-current error 
signal into 60-cycle alternating current. The resulting signal is amplified to drive a servo 
motor which in turn drives the potentiometer slider until the error signal is reduced to zero 
and a null-balance established. 
The digital indicator, shown in Fig. 23, is the active part of the feedback loop. The 
assembly is made up largely of commercially obtainable components. Beginning from the 
left, it may be seen that there is a digitizer with a detent unit, servo motor, speed reducer, 
and potentiometer. The components are aligned axially and are connected together with 
Oldham couplings to minimize binding. The Metron speed reducer, is an antibacklash plane- 
tary gear box with a reduction of 21 to 1. It is inserted between the servo motor and 10-turn 
Heliopot (+0.05 linearity) so that the range of the system is +5 turns on the potentiometer, 
with a little to spare. The digitizer is connected directly to the through-shaft of the servo 
motor. The digitizer is a unit manufactured by the Dayton Instrument Company. It is essen- 
tially a four-digit mechanical counter and plus-minus wheel, equipped with electrical con- 
tacts. Eleven wires are brought out of each decade, one for each unit and one common. 
These wires lead to the programmer and scanner and then to the readout equipment. The 
detent unit, which is integral with the digitizer, consists of a solenoid-operated star wheel. 
The star wheel is used to center the unit decade on a contact, when command for readout is 
given the solenoid is energized and the digit wheels are placed in contact with the electrical 
brushes. 
The digital indicator operates in two modes: balancing and readout. In the balancing 
mode, it is part of the feedback loop, as explained earlier. In the readout mode, the servo 
motor is automatically stopped and the digitizer serves as a memory which stores the last 
reading obtained. 
The scanner and programmer unit is the brain of the readout system. It serves two func- 
tions: first, introducing predetermined data such as run number, body angle, and control 
surface angle and secondly, scanning and sequencing the data actively obtained during the 
test. When a test run is made, the digital indicators are allowed to settle out at an approxi- 
mately fixed reading. At command, the servo motors are automatically stopped and the scan- 
ner unit scans each channel, decomplimenting if necessary, and feeds the readings in cor- 
rect sequence, one digit at a time, to the solenoid-operated IBM electric typewriter. The 
typewriter tabulates the data on a form specially prepared for the purpose, as shown by the 
reduced sample given in Fig. 24. 
