A Public Blockchain Consensus Mechanism for Fault-Tolerant Distributed Computing in LEO Satellite Communications

In LEO(Low Earth Orbit)satellite communication systems,the satellite network is made up of a large number of satellites,the dynamically changing network environment affects the results of distributed computing.In order to improve the fault tolerance rate,a novel public blockchain consensus mechanism that applies a distributed computing architecture in a public network is proposed.Redundant calculation of blockchain ensures the credibility of the results;and the transactions with calculation results of a task are stored distributed in sequence in Directed Acyclic Graphs(DAG).The transactions issued by nodes are connected to form a net.The net can quickly provide node reputation evaluation that does not rely on third parties.Simulations show that our proposed blockchain has the following advantages:1.The task processing speed of the blockchain can be close to that of the fastest node in the entire blockchain;2.When the tasks’arrival time intervals and demanded working nodes(WNs)meet certain conditions,the network can tolerate more than 50%of malicious devices;3.No matter the number of nodes in the blockchain is increased or reduced,the network can keep robustness by adjusting the task’s arrival time interval and demanded WNs.

Full-State-Constrained Non-Certainty-Equivalent Adaptive Control for Satellite Swarm Subject to Input Fault

Satellite swarm coordinated flight(SSCF)technology has promising applications,but its complex nature poses significant challenges for control implementation.In response,this paper proposes an easily solvable adaptive control scheme to achieve high-performance trajectory tracking of the SSCF system subject to actuator efficiency losses and external disturbances.Most existing adaptive controllers based on the certaintyequivalent(CE)principle show unpredictability and nonconvergence in their online parameter estimations.To overcome the above vulnerabilities and the difficulties caused by input failures of SSCF,this paper proposes an adaptive estimator based on scaling immersion and invariance(I&I),which reduces the computational complexity while improving the performance of the parameter estimator.Besides,a barrier Lyapunov function(BLF)is applied to satisfy both the boundedness of the system states and the singularity avoidance of the computation.It is proved that the estimator error becomes sufficiently small to converge to a specified attractive invariant manifold and the closed-loop SSCF system can obtain asymptotic stability under full-state constraints.Finally,numerical simulations are performed for comparison and analysis to verify the effectiveness and superiority of the proposed method.

Fault-tolerant control of an open-winding brushless doublyfed wind power generator system with dual three-level converter

To improve the fault redundancy capability for the high reliability requirement of a brushless doubly-fed generation system applied to large offshore wind farms,the control winding of a brushless doubly-fed reluctance generator is designed as an open-winding structure.Consequently,the two ends of the control winding are connected via dual three-phase converters for the emerging open-winding structure.Therefore,a novel fault-tolerant control strategy based on the direct power control scheme is brought to focus in this paper.Based on the direct power control(DPC)strategy,the post-fault voltage vector selection method is explained in detail according to the fault types of the dual converters.The fault-tolerant control strategy proposed enables the open-winding brushless doubly-fed reluctance generator(BDFRG)system to operate normally in one,two,or three switches fault of the converter,simultaneously achieving power tracking control.The presented results verify the feasibility and validity of the scheme proposed.

Fault tolerant control strategy for modular PWM current source inverter

In this paper,a fault-tolerant control method for an input-series output-parallel modular grid-tied pulse-width modulation(PWM)current source inverter is proposed to address the most commonly seen single symmetrical gate-commutated thyristor(SGCT)open-circuit fault problems.This method actively offsets the neutral point of the current space vector to ensure a sinusoidal output of the grid current,and it can achieve the upper limit power of the inverter under the condition of a single SGCT open-circuit fault.In addition,an active damping control method based on grid harmonic current feedback is proposed after analyzing the influence of the transformer ferromagnetic resonance caused by the neutral point offset on the power quality of the grid current.It has been demonstrated that the proposed method effectively suppresses the resonance caused by the transformer and the modified modulation,improving the grid current’s power quality.

New technique: Design and implementation of the highly-reliable, low-cost housekeeping system in the ZDPS-1A pico-satellite

The ZDPS-1A pico-satellite designed in Zhejiang University with a mass of 3.5 kg and a power consumption of less than 3.5 W is the smallest satellite in China up to now. The housekeeping system (HKS) is the core part of ZDPS-1A. The reliability of HKS has an important influence on the safety of the satellite. Traditional fault-tolerant methods do not apply to ZDPS-1A due to such pico-satellite characteristics as light weight, compactness in size, energy saving, and high integration. This paper deals with a highly-reliable, low-cost design for HKS using industrial devices. The reliable strategies of HKS include a dual modular redundancy scheme, CPU warm backup, a static triple modular redundancy scheme, and two-level watchdogs. Recursive experiments, special tests, and environmental tests show that this system meets the design target. This design has already been applied to ZDPS-1A, which was launched to execute in-orbit tasks on Sept. 22, 2010. To date, the satellite has been in a proper state for more than 15 months.