Measurement modes of the response time of a magneto-rheological fluid (MRF) for changing magnetic flux density |
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Authors: | Hans Martin Laun Claus Gabriel |
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Institution: | (1) Polymer Physics GKP-G 201, BASF Aktiengesellschaft, Carl Bosch Strasse, 67056 Ludwigshafen, Germany |
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Abstract: | The response of a magneto-rheological fluid (MRF) to a change of magnetic flux density is investigated by using a commercial
plate–plate magneto-rheometer MCR501 (Anton Paar GmbH) at constant shear rate. The instrument was modified to allow an online
determination of the transient flux density in the MRF. Both current and voltage imposition to the magneto-cell were applied
by using a power operational amplifier to drive the electromagnet. Assuming a Maxwell behavior with switching time λ and a linear increase in shear stress with flux density, analytic relations for the transient shear stress are derived for
sinusoidal and single exponential flux densities vs time. True switching times of a few milliseconds are only obtained if
the low pass filter in the original MCR501 torque signal is surpassed by a firmware allowing a sampling rate of 0.1 ms. For
a sinusoidal flux density, the switching time is derived from the modulation depth of the shear stress. An upper bound of
λ < 3 ms for a flux density of 0.8 T was found. For step coil current imposition of 1 T magnitude, switching times of 2.8 ms
(start-up) and 1.8 ms (shutdown) allowed to fit the transient torque signal more than 2/3 of the total change. Finally, the
effect of a sigmoidal characteristic on the switching time determination is addressed.
This paper was presented at Annual European Rheology Conference (AERC) held in Hersonisos, Crete, Greece, April 27-29, 2006. |
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Keywords: | Magneto-rheological fluid (MRF) Switching time Transient magnetic flux density Steady shear Plate– plate geometry Measurement modes Effect of nonlinear characteristic |
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