4~20mA电流论文

摘要

摘要

工业上利用传感器测量到的各种非电量信号通常都需要转换成标准模拟量电信号才能传输到几百米外的控制室或显示设备上。由于电压信号远距离传送时易受干扰，且会产生压降，所以远距离传送时通常采用电流信号。其中使用最广泛的是4~20mA标准模拟量电流信号，可以使用双绞线进行远距离传送。

随着微处理器的应用推广，现场测量越来越多地采用智能化仪表。为了使现场智能仪表和控制室系统间进行数字通信（可以实现对测量系统进行在线调整、数据读取等功能），需要在此4~20mA模拟信号上叠加上数字信号进行双向的数字通信，以共用通道的方式来传送数字信号。本设计利用4~20mA电流源芯片AD421和调制解调器芯片A5191HRT实现这一双向通信过程，半双工工作，信息接收与处理接口以MSP430F149单片机为例，如用其它的微处理器将之替换即可。

本文系统的介绍了实现此过程的电路设计过程，包括单片机的最小系统的设计，4~20mA电流信号传送电路的设计，加载FSK信号的电路设计，以及实现FSK信号反向传送的电路的设计。

关键词： 4~20mA电流信号 FSK信号 数据双向传送半双工工作方式

ABSTRACT

ABSTRACT

In industry，we use sensors to measure a variety of non-power signals. In order to transmit these signals to the control room or the display devices several hundred meters away， these non-power signals are often needed to be converted into standard analog signals. Since that the voltage signal is susceptible to disturbances and generates pressure drop，we use current signal in long distance transmission circuit，and the most widely used standard analog electrical signal is the current signal of 4~20mA. Twisted-pair cables are most popular transmission medium.

With the application and popularization of microprocessors, field measurements are increasingly adopting intelligent instrumentation.In order to make digital communications between intelligent instrument and the control room systems to realize on-line adjustment of the measurement system and the data reading function，we need to load digital signal on the communication channel of 4~20mA analog signal to realize the aim of two-way digital communication. In this design，the principle of HART is used. The current source chip AD421 and the modem chip A5191HRT are used to realize the two-way communication process. To conserve power the receive circuits are disabled during transmit operation and vice versa. This provides the half-duplex operation in HART communications. The entire circuit is controlled by the MSP430F149.

This paper describes the process of the implementation of the circuit， including the design of the microcomputer system and peripheral circuits minimum， the design of 4~20mA current signal transmission circuit，the design of loading the FSK signal and the design of the implementation of FSK reverse transmission circuit.

Keywords: 4~20mA current signal FSK signal

two-way communication the half-duplex operation

目录 i

目录

第一章绪论 ....................................................................................................................... 1

1.1 设计背景 ........................................................................................................... 1 1.2 研究内容与框架 ............................................................................................... 1 第二章 4~20mA电流传送通道电路设计 ................................................................... 3

2.1 4~20mA电流概述 ............................................................................................ 3 2.2 环路供电型4~20mA DAC—AD421 ............................................................... 3

2.2.1 AD421概述 ............................................................................................ 3 2.2.2 AD421电路描述 .................................................................................... 5 2.3 AD421外围电路连接图设计 .......................................................................... 7 2.4 AD421数字接口电路设计 .............................................................................. 8 2.5 4~20mA电流接收端电路设计 ........................................................................ 9 第三章 FSK信号加载电路设计 ................................................................................ 11

3.1 HART协议概述 ............................................................................................. 11 3.2 FSK（频移键控）信号概述 ......................................................................... 12 3.3 HART调制解调器A5191HRT ..................................................................... 13 3.4 A5191HRT外围电路设计 ............................................................................. 17 3.5 MSP430F149控制A5191HRT电路图设计 ................................................. 17 3.6 4~20mA传送通道上加载FSK信号电路设计 ............................................ 18 第四章半双工工作方式及交流驱动电路实现 ............................................................ 21

4.1 电路半双工工作方式实现 ............................................................................. 21 4.2 交流电流驱动电路设计 ................................................................................. 24 第五章 MSP430单片机最小系统设计 ..................................................................... 25

5.1 MSP430F149单片机概述 ............................................................................. 25 5.2 MSP430F149的最小系统设计 ..................................................................... 26

5.2.1 电源 ....................................................................................................... 27 5.2.2 复位电路 ............................................................................................... 27 5.2.3 晶体电路 ............................................................................................... 28 5.2.4 简易仿真器JTAG下载线 ................................................................... 29

ii 目录

5.3 MSP430单片机通用同步/异步串行接口..................................................... 29 5.4 ADC模块概述 ............................................................................................... 30 第六章总结 ..................................................................................................................... 33

6.1 设计的成果 ..................................................................................................... 33 6.2 电路中可能的不足 ......................................................................................... 34 致谢 ................................................................................................................................. 35 参考文献 ......................................................................................................................... 37 附录A ............................................................................................................................. 39 附录B ............................................................................................................................. 40 附录C ............................................................................................................................. 41 附录D ............................................................................................................................. 42