ALBERT

Description: This paper presents an extended predictive trajectory control scheme combined with an inner torque ripple minimization considering the current-, flux-linkage-, and voltage-planes of permanent magnet synchronous machines. The extension of a fundamental machine model with flux-linkage harmonics allows the calculation of the inner torque ripple and enables its minimization. For this, the control is divided in two cases: (1) The dynamic operation or large signal behavior which uses the maximal torque gradient for the trajectory strategy during each control period for fastest dynamic operation, and (2) The stationary operation or small signal behavior, utilizing a real time capable polynomial approximation of the rotor position dependent torque hyperbolas (iso-torque curves) of permanent magnet synchronous machines for the ideal torque to current reference values. Since dynamic and steady-state operation is covered, torque to current look-up tables, such as maximum torque per ampere (MTPA)/maximum torque per volt/voltage (MTPV) look-up tables, are not required anymore. The introduced, new control approach is implemented in Matlab/Simulink based on finite element analysis and measured data. Furthermore, test-bench implementations based on measurement data are presented to show the real-time capability and precision.

Description: Seismic monitoring of the cryosphere is mostly done with land seismometers on the surface of ice masses. Seismic monitoring beneath sea ice at the bottom of ice-covered oceans has hardly been attempted, because ocean bottom seismometers (OBS) are difficult to recover in perennial sea ice. As a result, for example the tectonic activity of the Arctic mid-ocean ridge system is poorly known. Recently, ambient seismic noise in long-term seismic records proved a useful tool to monitor the state of the sea ice cover. From September 2018 - September 2019, we deployed a trial network of 4 broadband OBS in Arctic sea ice. OBS were positioned at distances of 10 km at a water depth of 4 km on eastern Gakkel Ridge. Station noise levels from power spectral density analysis are considerably lower for sub-ice stations than for OBS in the Greenland Sea and are comparable to those of Arctic land stations. The network was designed to record local earthquakes along Gakkel Ridge, but it also yielded valuable data on the sub-ice ambient seismic noise in the Arctic Ocean. Spectrograms covering the entire deployment reveal pronounced seasonality in different frequency bands: Above 5 Hz, noise levels increase when sea ice cover is present. In addition, anthropogenic noise is prominently seen. The secondary microseismic noise peak has two clearly separable components with opposite seasonal evolution. Microseisms at 3-10 s periods relate to swell events outside the Arctic Ocean with a higher incidence of such events during winter time. In contrast, secondary microseisms originating in the Arctic Ocean peak in September during the annual sea ice minimum. Their periods increase from 0.5 s to 5 s as the fetch area for wave evolution increases from June to September.

Description: Studies of ambient seismic noise have proved a powerful tool to monitor the sea state in oceans, track large storms or even to conclude on the state of the sea ice covering the polar oceans. However, most studies of ambient noise use coastal seismic stations far from the source areas of seismic noise. Seasonal records of ambient seismic noise from the bottom of the polar oceans do not yet exist. In a pilot experiment from September 2018 to September 2019, we deployed four broadband ocean bottom seismometers at eastern Gakkel Ridge, Arctic Ocean, at water depths of about 4000 m underneath perennial sea ice. Only in August and September, the marginal ice zone of the Laptev Sea extended to the OBS position. Spectrograms show the seasonal variations of the ambient seismic noise. Long-period double-frequency microseisms are slightly stronger in winter time. Their source is outside the Arctic Ocean. Short-period double-frequency microseisms are seasonally modulated and appear when the Laptev Shelf area becomes ice-free, suggesting that deep water conditions are not necessary to produce mid-ocean microseism. The longest periods of this noise band increase as the fetch area for swell generation increases. At high frequencies (6-50Hz), considerable noise is present, mostly as short, distinct noise bands in winter time. We associate these signals to noise generated by the sea ice. To analyze the seasonality of the noise sources, we extracted the spectral power in various frequency bands for the entire year and compared it with variations of the significant wave height from Wave Watch III hindcast models and of ice concentration and drift from satellite data. This comparison revealed that sea ice-related noise decays suddenly in late May while sea ice concentration is still 100%, suggesting that the physical properties of the sea ice change at this time prior to break-up. Likewise, sea ice only gradually develops its noise-generating capabilities after the freezing period, when compression of ice floes contributes to their thickening. During autumn, several swell events cause large-amplitude short-period double-frequency microseisms and simultaneously high-frequency noise although ice-noise is otherwise not present in this season. Ice concentration decreases following the swell events, showing the impact of swell on the state of the sea ice during the freezing season.