JOHNS HOPKINS UNIV SILVER SPRING MD APPLIED PHYSICS LAB
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The servo voltmeter is an instrument with capabilities of measuring any DC voltage between the limits of zero and one hundred plus or minus .01 volts at any point. The input impedance is in the order of magnitude of two megohms. The servo voltmeter is basically very simple in that it is merely the summing of the unknown voltage, DC f, and a portion of a standard DC voltage of opposite polarity, called reference voltage, picked off the variable arm of a potentiometer called reference voltage pot or follow up pot. The result of this sum, the error voltage, is then fed to a servo amplifier whose output in turn operates a servo motor which drives, through a suitable gear train, a direct reading dial and the reference voltage potentiometer arm. As the output of the reference voltage pot approaches DC f the error voltage decreases, and when the output of the reference voltage pot equals DC f, the error voltage is zero. At this point of zero error voltage the system is nulled at the point corresponding to the value of DC f which can then be read off the dial. In order that the polarity of the reference voltage be opposite to that of DC f, both plus and minus are available, and an automatic, self-synchronizing selector chooses the correct polarity for an arbitrary input.
- Test Facilities, Equipment and Methods