For the encoders, especially the sine-cosine magnetic ones, a new method to measure absolute angular position is proposed in the paper. In the method, the code disc of the encoder has only two circle tracks and each one was divided intoNand (N-1) equal code cells. The cell angles, changing from 0° to 360° between any two neighboring code cells, are defined to represent any position on the code disc. The position value of the same point can be represented by different cell angle values of different tracks and the absolute angular position of the point can be obtained by the difference value between the cell angle value of the outer track and the inner one. To validate the correctness of the method theoretically, the derivation process of the method was provided. An electromagnetic encoder system was designed and the experimental platform was established to test the method. The experimental results indicate that the electromagnetic encoder can measure the absolute angular position. Besides, it shows that the method is easy to be realized in algorithm and can reduce computational complexity and decrease dimension of the encoder.

Dandelion seeds can stably diffuse owing to the dominant drag rather than the lift-based mechanism of the streamlined leaves of the plant, where this is known to favor their long-distance dispersal with the steady vortex attached. However, the generation mechanism of the vortex and the aerodynamic force exerted on the seeds through multiple filaments remain unknown. Clarifying these subjects may help realize the optimal performance of porous structures under different flight conditions. This study conducts numerical simulations to illuminate the influence of gaps and the Reynolds number (Re) on the wake structures and consequent drag force of dandelion seeds. We fabricate the seeds into circular disks composed of evenly distributed square cylinders placed in a vertical flow field with Re of 100 and 400, with the porosity of the pappus (ε) ranging from 0.887 to 0.964. We explain the geometric properties of the attached, steady vortex rings and clarify their generation mechanism, i.e., the base bleed and convection effects competed with vorticity generation, based on which the gaps are confirmed to delay chaotic vortices from occurring compared with the solid case. The weakened leeward pressure is critical for the increase in the drag coefficient to reach the peak level. The enhanced drag coefficient is several times higher than that in the solid case, endowing the seeds with a high loading capacity, and the porosity corresponding to its peak is beneficial for the structural design. These conclusions provide positive insights into the design of ventilated aircrafts with optimal long-distance dispersal performance.

In order to solve the inevitable instability problem of dandelion-like unpowered aircraft, transient simulations are carried out for the transition to turbulence of the wake from stable to unstable stage under different Reynolds numbers, and the vortex mode characteristics presented by the vortex structure as the Reynolds number increases are identified. Furthermore, by monitoring the velocity and lift and drag coefficients at different positions in dandelion flow field, the spectral characteristics of flow velocity and aerodynamic coefficients in the turbulent phase triggered by extreme aerodynamic environment are revealed. Compared with solid disk, it is found that the bandgap structure is more conducive to expanding the stable flight threshold of the dandelion-like aircraft, that is, the canopy configuration has weakening effect on airflow fluctuations. In addition, an evaluation method for unsteady boundary Reynolds number and load capacity of a dandelion-like aircraft is presented. The proposed method aims to plan a reasonable number of filaments (or porosity) for further designing dandelion-like aircraft with flight stability and high load capacity under aerodynamic conditions at different Reynolds numbers.

Hysteresis poses a significant challenge for control of smart material actuators. If unaccommodated, the hysteresis can result in oscillation, poor tracking performance, and potential instability when the actuators are incorporated in control design. To overcome these problems, a fundamental idea in coping with hysteresis is inverse compensation based on the Preisach model. In this paper, we address systematically the problem of Preisach model inversion and its properties, employing the technique of three-step composition mapping and geometric interpretation of the Preisach model. A Preisach right inverse is achieved via the iterative algorithm proposed, which possesses same properties with the Preisach model. Finally, comparative experiments are performed on a piezoelectric stack actuator (PEA) to test the efficacy of the compensation scheme based on the Preisach right inverse.