To solve the problems of the environmental factor detection and data management in the methane fermentation, this paper proposes a design based on ZigBee and PDA technology. Sensor node with CC2530 as the core can implement detection of oxidation-reduction potential, PH value, temperature, and liquid level. Coordinator node adopts ARM11 and is designed to be a PDA form, which is flexible and easy to use. On the basis of the analysis in ZStack protocol stack, the sensor signal acquisition program is accomplished. Customize wince6.0 embedded operating systems, and transplant an embedded SQlite database for powerful data management features. By using serial communication and multi-thread communication strategies, the Zigbee network data acquisition, acquisition cycle settings, power mode settings, and time synchronous adjustment can be accomplished. As the result of the test, the average packet loss rate of the network transmission is 0.375%. The data comparison charts of sensor nodes with standard instrument detected in continuous 24h are essentially coincident, and it indicates that the sensor circuit has been designed properly, and ZigBee network data transmission is stability, so it can meet the actual requirements of applications. Meanwhile, the system is easy to extend for different scale methane engineering application.

Hidden Markov model (HMM) for the diagnosis of incipient faults in analog circuits is presented. Firstly, output voltage signals under faulty conditions are obtained with simulation. Subsequently, output voltages corresponding to the test frequencies are extracted from the response of analog circuits. Finally, the output voltage vectors are used to form the observation sequences, which are sent to training HMM to accomplish the diagnosis of the incipient faults. The performance of the proposed method is tested, and it indicates that the method is effective and has better recognition capability than the popularly used back-propagation (BP) network.