Counter EMF effects in Resonant Column Testing

by Yu-Hsing Wang, Giovanni Cascante, J. Carlos Santamarina
Year: 2003


Wang, Y. H., Santamarina, J. C., and Cascante, G. (2003). "Counter EMF effects in Resonant Column Testing." ASTM Geotechnical Testing Journal, Vol. 26, No. 4, pp. 410-420


​The standard magnet-coil driving system in resonant column devices provides the required cyclic excitation; however, it inherently produces a counter electromotive force that opposes the motion. In this study, the resonant column is modeled as an electro-mechanical system to quantitatively examine the counter electromotive effect and to explore its effect on resonant frequency and damping ratio computed from voltagebased measurements. The model is verified with two independent sets of experiments. Experimental and analytical results show that the measurement bias is more pronounced on the damping ratio than on the resonant frequency, the damping bias is not a device constant but varies with frequency, and the error is particularly relevant in low-loss and low-stiffness specimens (such as dry sands at low confinement). The electro-mechanical model permits developing device-specific correction charts that can be used to reexamine previously published damping ratio data gathered with voltage-based resonant column procedures.


resonant column testing EMF effect magnet-coil driving system