INSTRUMENTATION 2 0 0 : 1 ratio of m a x i m u m to minimum speed.
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S T O U G H T O N 112 , M A S S A C H U S E T T S For further information, circle number 52 A on Readers' Service Card, page 73 A
52 A
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ANALYTICAL CHEMISTRY
, radiation beam in the phantom systematically. Movement of the probe involves the simultaneous action of two motor drives: the patrolling drive, which causes the probe to be driven horizontally across the radiation beam and is under control of the operator; and the hunting drive, which causes the probe to search vertically for the desired radiation intensity and is under the control of a servo system. The schematic in Figure 1 illustrates the principle. The operator sets the isodose selector switch at the desired radiation intensity. The hunting drive, which uses the servo balancing motor, then moves the probe vertically into the phantom until the output voltage of the probe is equal to that of the isodose selector. At this balanced condition, the vertical motion of the hunting drive should cease. But the patrolling drive moves the probe horizontally across the phantom, at the operator's command. This, of course, changes the radiation intensity on the probe and leaves the system in an unbalanced state. The servo amplifier (hunting drive) then causes the probe to be driven vertically, up or down, until the radiation intensity on the probe again equals the preset value. The chart of the electronic recorder (designed by Honeywell's Industrial Division) is driven by a receiver Selsyn. The transmitter Selsyn is mechanically coupled to the vertical motion of the radiation probe, and, therefore, sends a signal which represents the depth in the phantom at which the analysis is being made. The horizontal motion of the probe is mechanically coupled to a 40-turn Helipot that has a fixed voltage impressed across it. The output voltage from the Helipot thus represents the horizontal position of the probe. Since it is fed into the amplifier of the recorder in order to actuate the pen, the pen takes a position which represents the horizontal position of the probe within the phantom. Thus the path of the probe as it is moved through the phantom along the line of selected radiation intensity is accurately indicated by the ink line of the chart. Servo Amplifier Used The heart of the servo system is the Brown 40X servo amplifier. It was chosen for its high sensitivity, which is required at certain portions of the curves. But at other portions of the curves, the amplifier must handle extremely large input signals from the detecting element. These signals, more than 1 volt in some instances, would normally greatly overload the amplifier and seriously distort the wave form of
