The dissertation discusses a magnetic microsensor system for displacement measurement
comprising microcoils, which are realized as microtransformers. The primary windings of the
microtransformers are excited with an AC source with a frequency of several MHz and a
current of several mA, thus making the microsystem inductive. First, relevant inductive linear displacement measurement solutions found in the literature are introduced, focusing on integrated devices. The available literature has shown that the integrated microinductors in displacement measurement applications are commonly fabricated during additional post
processing steps of the integrated circuit fabrication. We focus on the demonstration of the
feasibility of a monolithic integrated microsystem for linear displacement measurement with
microtransformers, produced in internal metal layers of an integrated circuit, and fabricated
using a completely conventional 350-nanometer commercial microtechnological process,
along with corresponding circuits for the processing of the microtransformers’ output signals. The major advantages of such system are its cost-effectiveness due to its straightforward fabrication and the absence of the need for an external field generator, such as permanent magnets at Hall Effect encoders and a light source at optical encoders.
A model electric circuit of a microtransformer is presented. Such model is not sufficient to
account for the effect of a measurement scale, which is used for the incremental displacement measurement. Therefore, the finite element method is used to model the effect of the measurement scale, placed over the microtransformer. The effects of material and geometric properties of the scale on the secondary induced voltage are demonstrated. The differential output voltage of a microtransformer pair was further analyzed as a mathematical or highlevel model in Matlab/Simulink environment, where the demodulation methods of the AC signal, modulated with the target position, were studied. It is shown that the signal is modulated by a combination of the amplitude and phase modulation. As the most straightforward demodulation method, synchronous demodulation was selected. Since the targets with different properties introduce different amplitude-phase characteristics into the microtransformers’ output voltage, the minimal distortion and the related optimal phase of the mixing signal used for the demodulation are dependent on the specific properties of the target.
The described method allows for the investigation of this dependence, and the presented
models enable the investigation of the effects of the measurement scale and the electronics
comprising the measurement channel on the nonlinearity of the position signal. The described design methodology for microsystems comprising microtransformers was practically evaluated on multiple microsystems. The dissertation describes one microsystem in more detail, employing a fully-differential measurement channel and a mixer with a Gilbert cell. A prototype microsystem was fabricated, demonstrating the sensitivity of 0.99 V/mm with a copper target and approximate microsystem-target distance of 200-250 μm. Finally, based on findings provided by simulations and measurements, an improved measurement channel with better resolution is proposed.
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