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1. THEORY OF OPERATION
The GK-401 Vibrating Wire Readout is used to excite and read out vibrating wire gages
which include: strain gages, crackmeters, piezometers, total pressure cells, etc. The basic
principle is that a wire that is held under tension has a specific natural or resonant
frequency of vibration which is dependent on the strain and the length of the wire. If the
wire is plucked, as in a stringed instrument, the frequency will always be the same
provided that the strain and length do not change. Vibrating wire gages have specific
fixed lengths of wire, and the change in strain is measured by measuring the change in
vibration frequency. The readout box provides the means of exciting the wire and
reading the resultant frequency.
In use, a pulse of varying frequency is generated by the Readout Box and is applied to an
electromagnetic coil assembly which is located close to the sensing wire. When a
frequency corresponding to that of the wire is generated, the wire is "plucked" and
vibrates at that frequency. The wire continues to vibrate after the "pluck" ends and a
signal, primarily the resonant frequency, is induced in the coil assembly and transmitted
to the readout where it is conditioned and displayed.
The GK-401 amplifies the return signal, converts it to a square wave and counts 255
cycles of vibration. This is then manipulated by the processor to display the required
units: period, frequency squared or microstrain. For weaker gage signals the processor
counts fewer cycles to try to obtain consistent readings during the signal decay period.
The gage factors and arithmetic functions are stored in the EPROM and can be changed
for special application by obtaining custom EPROMs from Geokon.
