An Electromagnetically Actuated Oscillating Sphere Used as a Viscosity Sensor

We present a novel viscosity sensor principle which utilizes an oscillating metal sphere attached to a wire acting as mechanical spring. The oscillation is electromagnetically excited using four actuation coils. The arrangement of the coils allows a linear oscillation in one plane but also a circular motion if the actuation coils are driven accordingly. The latter causes a constant speed of the sphere and thus, a stationary flow around the sphere, which allows probing a different rheological regime compared with the oscillation mode featuring continuous acceleration and deceleration. For viscosity measurements, the drag force acting on the sphere is the crucial parameter in the sensing approach. The calculation of this force requires modeling of the magnetic field generated by the actuation currents in the coils. To verify the model for the magnetic field and the forces, a COMSOL simulation was performed and the results were compared with those obtained with the model. Finally, the approach was verified by means of measurements using different viscous liquids (viscosity standard oils and water glycerol mixtures).