Many older process plants still use what most of us would believe to be an archaic technology, 4-20 mA control. In all the years I have been writing these articles about advanced network techniques, this is something I’ve never discussed, yet it’s the dominant standard in the process industry and believe it or not, it’s still pretty popular. Yes, 4-20 mA is ancient in today’s world of EtherNet/IP, PROFINET IO, and all the IoT communications, but it’s still widely used in a lot of water, chemical, and even pharmaceutical plants. And if you think that’s unbelievable, there are even some prehistoric systems still using its predecessor: pneumatic control.
In an analog communication system like current loop, data is transferred by varying the amount of voltage or current on the wire. For example, you could setup a system where a device puts 10 volts on the wire to represent a digital zero and 100 volts to represent a digital 1. There are many schemes to transfer digital information, but this article will focus on 4-20 mA or, as its more commonly called, current loop control.
Every current loop system has five basic components: a sensor, a transmitter, a power source, a receiver, and an electrical circuit. The sensor is the part of the system that detects a measurement value: pressure, temperature, speed, or any other measurable value. The transmitter is what converts that measured value into an analog level and transmits that analog value on the wire. The receiver detects the analog level driven by the transmitter. And the voltage source is the provider of a voltage to the system while the electrical circuit is the wiring that connects it all together.
The loop is governed by a few basic principles. One, there is only one transmitter or active device in the loop. Two, there can be multiple passive devices in the loop that sense the current in the loop. Three, there can be only a single power source. Sometimes the power source is also one of the active devices. A common early implementation of a current loop system is a chart recorder connected to a temperature sensor. The chart recorder provides the voltage to the system and records the amount of current from the sensor which it scales to record actual temperatures. It might be calibrated as 4 mA for 20 degrees and 20 mA for 100 degrees.
There are many advantages to this system. It is immune to noise, it can easily detect failed devices (0 mA indicates failure), it’s easy to understand and deploy, and it works well over long distances, though the signal does degrade over very long wire runs. Detractors point to the facts that each loop can only serve a single transmitter. You must create a loop for every signal you want to transmit, and the more loops you have, the more possibility of ground loops – the bane of my existence in my first job as a development engineer.
Why are these sensors still used? Mostly because It’s difficult to transition away from them. Any new technology in an operating plant must be phased in slowly, and that requires both duplicating the plant control systems during the transition and retraining the maintenance and operations people. Add some lost production from downtime and the cost just gets too high. What’s interesting is that this is the same reasoning that a lot of people used to avoid moving to current loop from pneumatic control in the first place.
To mitigate those costs, there are efforts to take these current loop systems and transition them in place. One such system is HART (Highway Addressable Remote Transducer). A HART system adds a digital channel to the analog current loop signal. The digital channel can be used for calibration and diagnostics but can be extended to provide additional signal data. And HART now also provides the ability to multidrop transmitters, a vast improvement in the data collection capabilities of these old plants. There is also some work being done on combining HART and current loop control with OPC UA, yielding an even better solution for remote control of these process plants.
We live in an industry where things change rather slowly, and current loop is one of those things. With all the talk about IoT, OPC UA, advanced analytics, Cloud control and all the rest, we sometimes forget that there are a lot of plants that are committed to their current control strategies and just don’t have the funding, time, or ability to use these “newfangled” systems.