智能傳感器在4至20毫安控制回路

　　本文著眼于將傳感器集成到工業(yè)自動(dòng)化應(yīng)用中的4到20 mA控制回路和現(xiàn)場(chǎng)總線實(shí)現(xiàn)中。將智能添加到傳感器允許更多的控制和選項(xiàng)的封閉或開(kāi)放的控制回路與更緊的延遲，本文著眼于不同的方法來(lái)實(shí)現(xiàn)一個(gè)智能傳感器控制回路?？雌饋?lái)那是控制回路如德克薩斯儀器xtr117收發(fā)器和Avago公司的hcpl-4200光耦優(yōu)化設(shè)備，以及控制器，對(duì)這些控制回路，如從模擬設(shè)備的aduc816傳感器優(yōu)化。

　　許多傳感器是專門為工業(yè)過(guò)程控制系統(tǒng)中常見(jiàn)的4到20 mA控制回路而設(shè)計(jì)的。

　　一個(gè)數(shù)字到模擬轉(zhuǎn)換器（DAC）和一個(gè)優(yōu)化控制器的結(jié)合，加上清晰定義的4到20 mA范圍，提供了一個(gè)成本效益和可靠的傳感器網(wǎng)絡(luò)。使用一個(gè)非零輸出電流（4毫安）的零點(diǎn)有兩個(gè)關(guān)鍵優(yōu)勢(shì)。它允許在發(fā)射機(jī)上開(kāi)路檢測(cè)，如果電流要求小于4毫安，則向傳感器提供功率。通過(guò)仔細(xì)的系統(tǒng)設(shè)計(jì)，這使得傳感器和數(shù)據(jù)采集設(shè)備可以直接由網(wǎng)絡(luò)供電，可以大大簡(jiǎn)化工業(yè)自動(dòng)化設(shè)備的發(fā)展。

　　Figure 1 shows a 4 to 20 mA output smart sensor that is completely powered by the loop power supply. In order for this to work， the sum total of all the circuits under loop power can be no more than 4 mA. The heart of the circuit is the AD421 loop-powered 16-bit DAC from Analog Devices. The internal 4 to 20 mA DAC current， as well as the rest of the return current from the AD421 and the other circuits under loop power， flows through the RSENSE resistor. The sensing circuit compensates for the additional return current and ensures that the actual loop return current corresponds to that required by the digital code applied to the DAC through the microcontroller. The sensor output is digitized by a sigma-delta ADC.