No abstract

Optically trapped nanospheres are demonstrated capable of atto-Newton force sensing, where its experiment need to control the modulation voltages of the laser power according to the positions of the trapped nanosphere in high speed. In this paper, position fluctuations of the trapping nanospheres due to thermal motions are simulated using Monte-Carlo method and finite difference method. Equal-scale amplified transformations of those position sequences are generated as the discrete voltage signal of the virtual position detector. A high-speed digital incremental PID(Proportion-Integration-Differentiation) control system is developed by a low-cost FPGA circuit system, which can generate feedback voltage signals correspondingly. The responsive signal frequency is up to 1MHz with a time delay of 0.3μs and quite high amplitude stability. It is validated to integrate the virtual position detector and the PID feedback system into a low-cost semi-physical system, which can test various feedback cooling mechanisms for the complex systems of optical tweezers in vacuum. It will be a further step relative to the pure simulations in digital computers and provide references for the development of optical tweezers in vacuum.

Predicting net river fluxes is important to promote good water quality, maritime transport, and water exchange in estuaries. However, few studies have observed and evaluated net water fluxes to estuaries under complex conditions. This study used advanced survey techniques to obtain high-frequency monitoring data of cross-sectional current velocity, water level, and salinity in the Liao River Estuary (LRE) from 2017 to 2020. The net water flux into the sea was computed based on field data and the impacts of the rubber dam and sea ice cover on water flux and salinity processes were analyzed in the study region. In the Liao River Station (LRS), the fluctuations of water level and discharge were not obvious in winter due to the sea ice cover. There were significant seasonal and inter-annual changes in water fluxes due to variability in river discharge and tidal oscillations. The results also showed that the net water flux into the sea from the LRS was positive in wet season, and greater during ebb tides than flood tides. The net water fluxes in the normal and dry seasons were mostly negative due to the influence of tides, indicating that the annual runoff from the Liao River fluctuated greatly throughout the year. The water flux in the LRS was more suitable for representing water flux into the sea than the Liujianfang Hydrometric Station (LHS) in the LRE. The impacts of the rubber dam and Panshan Sluice on water fluxes to the sea were both significant. Lower salinity in the study area coincided mostly with height water fluxes to the sea and periods when the rubber dam was raised. This study results provide us new insights to measure the water flux into sea under the condition of ice cover in the tidal reach of estuary and the method can be used for water flux observation for other estuaries.

The calibration accuracy of inertial measurement unit (IMU) is one of the important factors affecting the positioning accuracy of strapdown inertial navigation system (SINS). A 24-sequence calibration rotation scheme is proposed in this paper, which improves the observability and observability degree compared with the traditional 18-sequence scheme, and makes the calibration results more accurate. Simulation results show that the calibration results of 24-sequence scheme are closer to the actual value than those of 18-sequence scheme.

Vacuum optical trap is a technology that captures and manipulates particles in a near-vacuum environment by means of laser mechanical effects. It is an ideal platform for precision measurement with high quality factor. Due to its low environmental damping, vacuum optical traps require feedback cooling to reduce the equivalent temperature of particle centroid motion to prevent particle escape. Parametric feedback cooling is a mainstream solution to achieve stable trapping of particles in optical traps. Monte Carlo method and finite difference method were used to simulate the displacement fluctuation of the microsphere without feedback, and the FPGA circuit was used to output control signals with different waveforms to test the feedback cooling effect. Under the premise of reaching mK cooling level, square wave control signals have advantages in fast feedback cooling, and sine wave control signals have advantages in maintaining phase matching.