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Research of Network Instrument Node Based on the CAN Bus

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Tutor: ZhanMing
School: Northeastern University
Course: Detection Technology and Automation
Keywords: multi-channel,variable gain,AD7683,PIC24HJ128GP506,CAN,protocol of application l
CLC: TP273
Type: Master's thesis
Year:  2009
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Abstract:
The emergence and development of Fieldbus have contributed to a new revolution in the instrument industry, as new products, network instruments is playing more and more important role in the Industrial Control field, therefore, the study of network instruments has become a hot spot currently. I have done some research on CAN communications based on multi-channel temperature acquisition circuit designed by myself in order to make the module have a CAN network communication feature. So the main work of this design is divided into two parts:one task is to design a multi-channel temperature acquisition circuit, and another task is to carry out CAN communication network research.Multi-channel temperature acquisition circuit is divided into two parts:motherboard and acquisition board. Motherboard is mainly responsible for data processing and communication functions of CAN network, and acquisition board is responsible for multi-channel thermocouple signal acquisition. The benefit of doing so is to facilitate development of acquisition of other analog modules. Because the motherboard is a generic module, we can only design acquisition board to complete the design of the other analog acquisition modules. The controller uses PIC24HJ128GP506 produced by MICROCHIP, and it contains CAN controller that can use efficient DMA access, which can save CPU resources. CAN transceiver uses MCP2551. ISO7221 is used in separation between the CAN communication and MCU. Separation among multi-channels uses photoMOS relay AQW214. After single-channel thermocouple signal voltage is uplifted by 30mV reference, they together with 30mV reference voltage and ground are connected to the multi-way switch ADG1204. With multiway switch successively toggling each way at regular intervals, we can sample these signals. Then we can obtain the true signal value by processing these datas in the MCU. Signal amplification is divided into two-stage, and in the second-level digital potentiometer AD5245 is used to change gains. ADC uses 16-bit high-speed A/D with SPI-compatible interface. The node has the function of local display. The entire module uses power supply network of 24V, and isolated power supplys transformed by switching-power-supply-modules respectively power MCU circuits and communication circuits.The specific CAN application layer protocol is designed according to the characteristics of the modules. The module can achieve the following functions:its operating mode can be configured as single-channel acquisition mode or sequential multi-channel acquisition mode via CAN network, and its communication parameters and alarm parameters can be set up online.
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