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We report a novel electrostatic self-excited resonator driven by DC voltage that achieves variable velocity-position characteristics via applying the pre-tension/pre-compression constraint. The resonator consists of a simply supported micro-beam, two plate electrodes, and two adjustable constraint bases, and it can be under pre-compression or pre-tension constraint by adjusting the distance L between two constraint bases (when beam length l > L, the resonator is under pre-compression and when l < L, it is under pre-tension). The oscillating velocity of the beam reaches the maximum value in the position around electrodes under the pre-compression constraint and reaches the maximum value in the middle position between two electrodes under the pre-tension condition. By changing the constraint of the microbeam, the position of the maximum velocity output of the oscillating beam can be controlled. The electrostatic self-excited resonator with a simple constraint structure under DC voltage has great potential in the field of propulsion of micro-robots, such as active rotation control of flapping wings.

It is challenging for microrobots to fly using onboard energy due to the relatively heavy power supplies and inefficient actuators that are limited to a small size (mass less than 1000 mg and wingspan less than 100 mm). Inspired by the movement of jellyfish in nature (Physophora hydrostatica), herein, a microscale aerial vehicle FlyJelly is introduced, powered by onboard energy to achieve untethered flight. FlyJelly uses a balloon filled with helium to overcome self‐gravity and an actuator powered by electrostatic power to generate thrust. The balloon, heat sealed with two gold‐covered Mylar films, weighs 95.56 mg, provides 257.9 mg of buoyancy when filled with helium, and stores 19.457 × 10−3 J of energy when powered by 2400 V of direct current. The electrostatic actuator is composed of multiflapping units arranged radially and symmetrically, consuming only 0.3370 mW of power but generating a thrust of 0.2271 mN to drive the vehicle for flight. Benefiting from a design of distributed propulsion, FlyJelly is highly reliable and continues to work well even after the actuator is damaged. The milligram‐level weight, untethered flight with onboard energy, and high reliability make the prototype suitable for development as a long‐term remote exploration and search‐and‐rescue mission.

：A new elastic-plastic model for contact of rough surfaces is presented. The model based on the traditional contact theory of Hertz and the presented fractal contact models is developed. Research the regularity of the contact condition of micro-bulge, considered on the multi contact asperity in different fractal parameters in the first detail. Then, to modify the contact areas by considering the multi-scale levels and a new way use to calculate the distribution function. The model developed evaluates that the multi-scale parameter will influence the values of critical contact parameters, the deformation of contact area will be the conditions of elastic, elastic-plastic and plastic successively and the effect factor has relationship with G and D. Besides, by considering the effect factor and using new distribution function, the new model gains the more precise results than the model of GW and MB. It is show that the predict values from present model is fitting closer than the values from MB model and GW model to the results of Bhushan' experimental observation. The comparison results strongly indicate that the new model is more reasonable in describing the contact behavior between rough surfaces.

Limited by the surface-to-volume ratio of structural materials, it is a great challenge for the millimetre-sized actuator to achieve high output performance. Traditional rigid actuators can achieve higher vibration frequency...

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A new fractal wear model is proposed. The new model is based on the multi-scale fractal contact model. Studying the elastoplastic and plastic state of contact asperities, the new wear model is established. The relationship between the amount of contact surface wear and the fractal dimension D, the characteristic coefficient G and the material property constant C is studied. Besides, wear coefficient of the rough contact surface is predicted. It can be seen from the results that the influence of the scale effect on contact surface must be considered, because of the asperities with the same deformation ?? will be in different contact state. The relationship between amount of wear in the new model and amount of wear in existing model has the same trend. When D = 1.45, the amount of wear is the lowest. With the increasing of G reflecting the roughness of contact surface, the wear amount is increasing. The material constant also has a certain influence on the wear amount, and the material with high hardness has low wear. The contributions of the amount of asperity wear (elastoplastic and plastic) have the same effect to the total wear amount. Besides, the result of new fractal wear coefficient and the result of classical have the same trend, however, the new wear model is more precise than the classical wear model and closer to the actual state.