Commercial beam-type balance, modified and outfitted with electronic controls and digital readout, can be remotely controlled for use in high radiation environments. This allows accurate weighing of breeder-reactor fuel pieces when they are radioactively hot

Eggenberger, D. N.

Shuck, A. B.

English

NASA Technical Reports Server

July 1969	 Brief 69-10242 ICY
	
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Remote Balance Weighs Accurately Amid High Radiation 
In Cave —.4.-- In Console 
S tepp ing	 I 
Motor	 Precision
Weight n
Sector	 Relays 
SERVO	 Lamps	 Photocells	 Steppin
20.1	 Counter 
to
=^	 i ; , — OVDc DD 
IA..I! LVDT 
..1Tri.aInr1e 
The problem: 
Accurate weighing of breeder-reactor fuel pieces 
when they are- radioactively hot. All handling and 
weighing must be done in a shielded cave by remote 
control. The balance must be reliable and rugged 
because remote-handling and decontamination pro-
cedures would cause repairs to be time-consuming and 
costly. 
The solution: - 
A commercial beam-type balance, modified and out-
fitted with electronic controls and digital readout, can 
be remotely controlled for use in high-radiation en-
vironments (1). The balance has a poise weight posi-
tioned by a precision screw that is driven by a stepping 
motor. The remote readout is coupled to a mechanical 
counter driven by another stepping motor that oper-
ates synchronously with the balance motor. The 
balance is accurate within 2 g between 100 and 18,000 
g: it has operated for more than 1 year at gamma 
radiation levels as high as lOb r/hour. 
How it's done: 
The radiation precludes use of most organic ma-
terials and glasses; the balance is almost entirely of 
metal, except for ceramic and formvar wire coatings,
asbestos sheathing of wires, and radiation-resistant 
lubricant. The low-humidity atmosphere precludes 
use of graphite as a lubricant. 
The weight measured by the balance is determined 
by the number of turns of the precision screw from its 
zero-weight position (Fig. I); the number is found by 
counting of the steps taken by the stepping motor. The 
count is an algebraic sum, backward steps being sub-
tracted from the count. A second stepping motor, in 
the readout console, is wired in parallel with the 
balance motor, and the two make precisely the same 
steps. By appropriate gearing, a bidirectional mechan-
ical counter, driven by the second stepping motor, 
then indicates the net number of forward steps taken 
by the balance motor. 
The position of the balance beam is sensed by a 
linear variable-differential transformer (LVDT) 
whose output is nominally zero at balance. The am-
plitude of the LVDT a-c signal varies nearly linearly 
with the amount of beam deflection either way from 
its balance position, but the signal phase on one side 
of balance is 180 from that on the other. The signal 
is amplified and then demodulated to give a positive 
output potential on one side of balance and a negative 
potential on the other.	 - 
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The demodulated potential is led into a servo ampli-
fier driving a bidirectional servo motor, whose direc-
tion of rotation then depends upon which side of 
balance the beam is located. Its speed is approximately 
proportional to the amount of beam deflection and 
therefore it stops at balance. The servo motor turns a 
two-bladed sector disk that interrupts the light to two 
photocells. The photocells operate two mercury relays 
that switch currents to the stepping motors. 
This system has several advantages for reliability. 
The speed of the stepping motor decreases as the 
weight beam approaches balance; thus overshoot and 
time for balancing are reduced. Also there is no am-
biguity in direction of turning of the stepping motors, 
since there is no drift that would cause the LVDT 
crossover and the motor-directional signal to become 
displaced and make the motors turn in the wrong 
direction over a range of weigh-beam deflection. 
Moreover, the system is self-checking for possible 
missed counts in case the two stepping motors should 
not move in synchrony. 
Reference: 
I. D. N. Eggenberger and A. B. Shuck, Nuclear 
Insir. Methods 57, 89 (1967).
Note: 
Inquiries may be directed to: 
Office of Industrial Cooperation 
Argonne National Laboratory 
9700 South Cass Avenue 
Argonne, Illinois 60439 
Reference: B69-10242 
Source: D. N. Eggenberger
Electronics Division
A. B. Shuck
Metallurgy Division
(ARG-10387) 
Patent status: 
Inquiries concerning rights for commercial use of 
this innovation may be made to: 
Mr. George H. Lee, Chief 
Chicago Patent Group 
U.S. Atomic Energy Commission 
Chicago Operations Office 
9800 South Cass Avenue 
Argonne, Illinois 60439 
Brief 69-10242	 Category 05


Remote balance weighs accurately amid high radiation

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