The tree-based routing topology is widely used in various Wireless Sensor Networks (WSNs) to provide periodic data gathering. Sensors gather data and then report to the sink after data aggregation. Since sensors are usually energy limited in WSNs, it is always important to conserve node energy and extend the network lifetime that defined as the time elapsed since the network starts until the first sensor runs out of energy. Extensive research attempts to solve it as a load balancing problem aiming at balancing energy depletion among sensors. This paper studies load balancing on the tree-based data gathering networks (i.e. data gathering trees). We find that, in a Degree-constrained Data Gathering Tree (DcDGT), when the tree's height is large enough, the data traffic and load of a node are mainly determined by the distributions of its descendant nodes whose distances to it are within a certain number of hops. Based on this finding, we propose a load estimation model and further give a distributed load-balanced DcDGT construction algorithm. Simulation results demonstrate that our algorithm effectively prolongs the network lifetime and outperforms existing approaches.

Sensing coverage in wireless sensor network is one of the vital performance metrics which can arise in all design stages and usually considered together with some other performance metrics, such as connectivity or energy consumption. Whatever the metrics, the fundamental problem is to know at least how many sensors are needed to maintain both sensing coverage and network connectivity. In this paper, we proposed a percolation-based coverage & connectivity combined model (PCCC) to obtain the critical density at which the network abruptly becomes covered/connected. The PCCC is based on directional sensor network in which sensors are assigned a determined sense direction with the angular interval varies from 0 to 2π. Besides, we also discussed the coverage and connectivity together as a whole under the model proposed. It is worth mentioning that the theoretical analysis as well as simulation results of relationship between critical density and transmitting gives insights into the practical design of directional sensor network.

In this paper, techniques of force-feedback control are applied to the hydrodynamic study of a laboratory robotic fish. The experimental apparatus which allows a robotic model to accelerate from rest to a steady speed under self-propelled conditions is clearly described. In the current apparatus, the robotic fish is mounted on a servo guide rail system and the towing speed is not preset but determined by the measured force acting on the body of the fish. Such an apparatus enables the simultaneous measurement of power consumption, thrust efficiency and speed of a robotic model obtained under self-propelled conditions. The thrust efficiency of the robotic fish can be estimated based on a 2-D vortex ring force estimation method. By comparing the thrust performance of carangiform body-shaped robotic swimmer with different typical BCF (body and caudal fin ) swimming modes, i.e. anguilliform, carangiform and thunniform, we show that the robotic swimming fish with the thunniform kinematic movement not only reaches a higher steady swimming speed but is also more efficient than the other two modes However, in the start phase, using the anguilliform kinematic movement, the robotic swimmer accelerates faster among all kinematic movements. Ultimately, we found that the robotic fish always produce a double-row wake structure no matter which swimming mode used.

This paper describes the design of a biomimetic inspired fish swimming like UUV based on 2-D ocillating foil machanism which plays guiding role in biomimetic propulsion research field, the relation between vorticity control and motion control parameters for 2-D ocillating foil has been built, a fuzzy logic vorticity controller is designed to achieve straight line maneuverable swimming and thrust efficiency can be ensured at the same time. Experimental result of field test shows that an obiviously improved vehicle performance is obtained. Meanwhile, burst-and-coast manevering is found to be a more effective swimming mode for the vihicle while compared with steady swimming.