As industrial automation technology continues to evolve and improve, the issue of Ethernet-based instrumentation and controls remains of great interest to many. Currently, a number of vendors support an Ethernet-based connection for sensor or device data. For example, several flow meters on the market already offer either an embedded Ethernet card or an add-on communication module in order to provide multivariable outputs, configuration data, and maintenance information via standard Ethernet networks. A range of gate and valve actuator manufacturers offer Ethernet communication packages for end-device control and monitoring. Many chemical analyzers, especially those with multisensor connections, provide Ethernet options in order to aggregate the data and serve it up to PLCs and SCADA systems. All of these devices utilize the standard twisted-pair CAT5/CAT6 Ethernet technology common to office environments. With that said, there are some drawbacks to using this medium for industrial applications.
First, this Ethernet standard is limited to 100 meter cable length. In an office environment with short runs and multiple switches or routers throughout the facility, this distance limitation isn’t a problem. Network devices can be used to extend the reach of a particular node. But in an industrial environment, especially one such as a water or wastewater facility that might span large distances for its treatment process, this maximum distance can be an extremely limiting factor. For example, an Ethernet cable running from a control room PLC enclosure down a tunnel to a rail-mounted gas flow transmitter next to a digester is likely out of the question.
Secondly, there is the issue of power. Whereas a 2-wire, 4-20mA sensor can draw power over the signal conductors in order to operate, standard Ethernet carries no such capability. Most Ethernet-capable instrumentation today requires external power, typically in the form of either a 120VAC or 24VDC supply. This power need adds cost and complexity that may not be a problem with a single sensor, but grows exponentially across an entire system if Ethernet is desired for dozens of devices. With that said, Ethernet has evolved over the years to incorporate the PoE (Power over Ethernet) standard which allows certain devices to draw power over one of the unused pairs of Ethernet cable conductors in the cable (at least in 10-100 Mbit/s applications). And in some instances, such as with radios or camera systems, that is a viable solution even for an industrial environment. However, there is one problem with using Power over Ethernet that distinguishes industrial environments from office environments: hazardous locations.
Thus, the third primary issue is that of intrinsic safety. In order to monitor a location where combustible gases, vapors, dusts, or fibers may be present, intrinsically safe (IS) circuits are the typical solution implemented. As a general rule of thumb, these circuits are limited to no more than 30VDC and 4W of power, which is to ensure that a hazardous area (such as a lift station or raw sewage screening building) is not subjected to an electrical circuit that could spark or ignite the environment. Yet, Power over Ethernet technology utilizes a 44-57 VDC range, making it incompatible with such hazardous locations.
This brings us to the latest development in the world of industrial Ethernet: “Ethernet-APL (Advanced Physical Layer).” As an offshoot of IEEE’s 802.3cg-2019 (10BASE-T1L) standard, Ethernet-APL offers characteristics that make it an intriguing solution to the aforementioned issues. Utilizing a single pair of twisted wires, the 10BASE-T1L standard is capable of 10 Mbit/s data transmission over roughly 1000 meters, making it appealing for monitoring field instrumentation at a distance. In addition, the Ethernet-APL standard allows for power delivery to devices of up to 60W, depending on the type of cable used, which streamlines installation and maintenance. And if that weren’t enough, a low-power, intrinsically safe option allows for power delivery limited to 500 mW, making it suitable for connectivity to instrumentation in classified locations. Of course, as an Ethernet standard, Ethernet-APL is also inherently compatible with many of the well-known protocols in use today, such as Ethernet/IP, ProfiNet, and OPC-UA.
All of this to say, Ethernet-APL is set to be a contender for both brownfield and greenfield projects in the near future. For decades, the 4-20mA signal has been the industry standard for instrumentation, but as process control systems become more sophisticated, implementing fully digital networks, business intelligence, and the convergence of IT and OT, there could be a new king of the hill. Keep an eye out for Ethernet-APL.
Sources:
https://www.controlglobal.com/articles/2020/advanced-physical-layer-standard-to-make-field-level-Ethernet-a-reality
https://training.ti.com/long-distance-connectivity-10base-t1l-single-pair-Ethernet-phy-demo
https://us.profinet.com/digital/apl
https://www.profibus.com/technology/industrie-40/profinet-over-apl