Designing Distributed 24-bit Data Acquisition System Based On CANBus

The distributed 24-bit data acquisition system (DAS) based on CANBus integrates the high precision data acquisition technology and advanced CANBus technology. It can realize the distributed DAS with multi-channel and high precision. And it can be applied not only on the signal acquisition of temperature, humidity, pressure, machine vibration in the general industry measurement but also on the fileds of the modern seismic exploration and electric method of the geophysics, blood analysis and cardiac diagnosis of the modern medicine, environment inspecting and monitor and so on. With the standard interface, this intelligent and networked high precision data acquisition system, which has wide application prospects, is the main direction of the current instrument. This article briefly introduced the development of the field bus technology, pointed out the importance of the field bus technology in the automatization system in future, and emphasized the distributed controlling system based on field bus technology is the main direction. The purpose and the significance of the research of this system are offered after the analysis of the market demand of the high precision data collection system, and architecture of the software, hardware are also offered.The hardware of the system contains three parts. They are 24-bit data acquisition substation, CAN repeater and CAN adapter used for PC. Paper pointed out the core problem is the precision of the 24-bit data acquisition substation.Data acquisition substation is designed with two kinds of 24-bit A/D converters, CS5396 and ADS1252, for different applications. The CS5396 is used to sample the AC signal of 1~2 channels and 1Hz~40kHz, while the ADS1252 is used to sample the signal of 1~8 channels and DC~10kHz. The paper described the design process of the two data acquisition substation, summarized the common designs of the sensor interface, preamplifier circuit, analog programmable low-pass filter circuit, analog programmable high-pass filter circuit, power filter circuit, single-ended to differential circuit and the multi-function outer trigger I/O circuit, in addition to the design method whose core is the microcontroller, large volume SRAM and CPLD. Some key technologies such as the output data format transforming of the CS5396, the power filter in the high precision data acquisition system, the synchronization of multiple ADS1252, the storage and the decoding of theconverted data and the placement and routing of the printed circuit board have been discussed.The design of the power supply directly influences the technical features of the system. After consulting a large amount of domestic and international design materials, the thesis summed up the design method of general high precision analog-digital mixed instruments, the power filter circuit and the overall placement of the system printed circuit board with the author's own practical experience.Paper analyzed principle and design requirement of CAN repeater, designed CAN repeater hardware and proved the feasibility of the design through logic simulation; It proposed the design of CAN interface adapter through LPT port with EPP/ECP protocol, which is useful for Notebook to be center controller, monitor or system backup host. The paper has also designed the circuit of the CAN interface adapter and analyzed the advantages of the two independent CAN interface in one CAN interface adapter.. It has also explained how to prolong communication distance using double CAN interface with a high speed through the example of the 24-bit top-layer seismic instrument. Software is one of the most important part for the intelligent instrument system. In this system, the 24-bit data acquisition substation, CAN interface adapter and super-station have independent CPU. Systematic communication and data collection are realized through modularized program. The thesis has analyzed the structures of system software and offered the software flows the data acquisition substation, CAN interface adapter and super-station. It...