This article looks at the integration of sensors into 4 to 20 mA control loops and field bus implementations in industrial automation applications. Adding intelligence to the sensor allows more control and options for closed or open control loops with tighter latencies, and the article looks at the different approaches to implementing a smart sensor control loop. It looks at devices that are optimized for the control loops such as the Texas Instruments XTR117 transceiver and the HCPL-4200 opt coupler from Avago Technologies, as well as controllers that are optimized for sensors on these control loops such as the ADuC816 from Analog Devices.
Many sensors are designed specifically for the 4 to 20 mA control loops that are commonly found in industrial process control systems.
A combination of a digital to analog converter (DAC) and an optimized controller, coupled with the clearly defined 4 to 20 mA range, provides a cost effective and reliable sensor network. Using a non-zero output current (4 mA) for the zero point has two key advantages. It allows open-circuit detection at the transmitter, and provides power to the sensor if the current requirement is less than 4 mA.
Read More: Smart Sensors in 4 to 20 mA Control Loops