An Automatic Feedback System for Resonant Column Testing

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Courtesy of ASTM International

A test method and electronic circuitry is described that greatly simplifies measurement of resonant frequency in the resonant column test. It utilizes a feedback system to detect resonance automatically without requiring manual adjustment and also eliminates the need for a sine wave audio oscillator or oscilloscope. A constant and controllable strain level in the soil sample is ensured, irrespective of driving frequency or sample condition, by providing a voltage-limiting circuit within the system. The design described will automatically track and display variations in resonant frequency as a result of changes in applied cell pressure and/or strain level, etc.

In addition, double integration allows direct digital output of strain level to be continuously displayed. An alternative (and very rapid) method of measuring damping is also suggested.

Preliminary results are presented for tests performed in this way. An integrated package is shown that replaces the discrete components by a single unit.

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