Circuit Design Of Programmable Logic Controller And Research On Temperature Measurement Technology

Programmable logic controller (PLC)-one of the three pillar industries-is highly favored with its stable, reliable, easy development and maintenance, easy network extending. In the fields of industrial control, it is playing a more and more important role. In this article, the author gives the PLC hardware design scheme based on the LPC2378, on the basis of the structure of the PLC research; and also studies the temperature measurement with thermocouples(TC) and RTD, giving the sensor non-linear calibration scheme and TC cold-junction compensation programs based on temperature field modeling methods.The Major study of the hardware circuit involves LPC2378 and its peripheral circuit design, DC-DC power supply, analog IO, digital IO, communication circuit and LED status indication circuit design. Using LPC2378 external bus the author extends SRAM chips 62LV5128, providing 512K user data storage space, and designs a power brown-out protection circuit. In the power supply design, two groups of isolated power -analog and digital-are obtained through the transformer in the use of push-pull transform structure, which not only improves the circuit reliability, but also avoids the digital circuit noise impact on the analog measurement accuracy. LPC2378 communicates with an external analog IO and digital IO through optical isolators, using the optocoupler common-mode voltage isolation capacity to effectively inhibit the electromagnetic interference to ensure the processor work normally. The design of two isolated RS-485 and one Ethernet communication interface enables PLC to meet the control needs of users with field bus technology. 46 LED is designed for status indicators, in the use of LPC2378 SPI-interface 74HC595 to drive LED.Combination of continuous non-linear AD conversion technology, the author analyses the RTD sensor non-linear causes and gives the 4th degree polynomial fitting calibration method of PT100 based on the least-squares, using the Matlab simulation. The full temperature range of fitting accuracy is better than 0.03℃. Using 6th polynomial fitting methods to solve nonlinear problems of the thermocouple sensors, the author simulates K, B, E, J, N, R, S, and T-type TCs. The full temperature range fitting accuracy is better than 0.3℃. The author proposes temperature-field modeling method to achieve TC cold-junction compensation, aiming at multi-channel TC design. By the means of temperature-field modeling, the author deals with cold-junction temperature problem by four temperature points to complete eight channel TC cold-junction compensation. According to energy conservation principles and Newton's cooling equation, the author constructs the mathematical model of two-dimensional temperature distribution, and then divides plane into grids, and converts the differential problem into numerical difference, obtain the temperature distribution on the plane using Gauss - Seidel iterative method, achieves TC cold-junction compensation. Finally, by the experimental validation of the method, the compensation accuracy is up to±0.5℃.Finally, the author tests the waveform of the programmable logic controller hardware circuit, it is proved to be the correctness of circuit design.