Articles | Volume 5, issue 2
Regular research article
22 Dec 2016
Regular research article |  | 22 Dec 2016

Active magnetic levitation and 3-D position measurement for a ball viscometer

Friedrich Feichtinger, Stefan Clara, Alexander O. Niedermayer, Thomas Voglhuber-Brunnmaier, and Bernhard Jakoby

Abstract. We present a new technique for 3-D position sensing and active magnetic levitation of a steel ball for use in a levitating ball viscometer. In order to achieve a stable levitation, a very sensitive positioning measurement system is mandatory. For this task the differential transformer principle was chosen to realize a 3-D position measurement. This leads to a purely magnetic sensor and actuator system without the need for other transducer types such as optical readout. The actuation utilizes power efficient switch-mode electronic circuitry which opens the possibility of upscaling the device, if demanded, for future applications. It is shown that this switch-mode actuation can be combined directly with the position measurement when special switching patterns are applied. A position resolution of  ∼  100 µm in all three axial directions at a sample rate of 476.19 Hz is achieved. For viscosity sensing, the steel ball is magnetically driven to orbital movements of variable revolution frequency of up to 2.5 Hz within a fluid chamber. The frequency response is analyzed and related to the shear viscosity of the fluid under test. As a proof of concept, measurements in various viscous liquids were performed with the prototype, showing promising results in the range of 1–10 mPa s. The principle may also be of interest for applications beyond viscosity sensing, such as fluid mixers, or as actuators in microfluidic devices.

Short summary
We present a new technique which can be used in devices measuring the viscosity of a liquid. To this end, a steel ball is submerged in the liquid and levitated by magnetic forces. The ball's position is measured and controlled to keep the ball in a stable levitated position. The ball is then actuated to perform a circular motion through the liquid. This motion is measured and can be used to draw conclusions about the liquid's viscosity.