Edge computing basically means taking the input/output (IO) constructs of a controller to the point of use with computational abilities. In a plant environment, that edge could mean the beginning and the end of a process.
Imagine a paper machine whose length could be in the hundreds of feet that has sensors and controls throughout the process.
Enter remote I/O, whereby the controller’s sensor and control points are taken to the area of control and tied back into the controller using communication techniques such as Ethernet/IP or Profinet. Even Wireless HART gets into the action by communicating with the controller over the air.
But is remote I/O the same as distributed I/O?
I remember back in the 1980s when a newfangled technology was being introduced by Rockwell Automation, and it was called distributed control. The devices that made up part of the network were intelligent, and these devices made decisions based on their programming.
It was a sorting application using different-colored ping pong balls. Once the color was determined, it sent a direct message to the sorting sensor. Imagine it being like an inverted Y with a diverter to direct the ball into the proper tube.
The sorting sensor detected the ball as it entered the diverter area, and knew the color therefore alerting the control element (diverter) to move one way or the other to properly divert the ball. This sorting sensor told the diverter that a ball was present, and which direction to move, and the controlling element solenoids did the rest.
There was no central PLC or controller in this system, which for that time was heresy.
The main reason for having remote or distributed I/O was to cut down on installation time and cost. Taking the I/O to the sensors and controlled elements meant less wiring but also meant that troubleshooting required a steadier hand since all the I/O lights and modules were not in the same place.
The need for software became more obvious for troubleshooting and starting up the system.
In today’s world distributed I/O is ever-present regardless of implementation of control strategy. Discrete and process both benefit from communication networks and third-party I/O suppliers.
EtherCAT, Ethernet/IP, Profinet, Wireless HART and ModbusTCP are examples of the connective tissue needed for distributed/remote I/O, which can lead to smart devices and brings us closer Rockwell’s distribute-control mindset.
IEC 61499 is very supportive of systems that have no controller. It is defined as a system-level design language for distributed measurement and control systems.
It is based on the notion that devices can house and execute event-driven function blocks from within the device itself.
I first ran into this standard in the late 1990s and in fact spoke with a pioneer at Rockwell whose name is James Christensen while I was the managing director for PLCopen. IEC 61131 was having all kinds of trouble entering the North American market, so who would have thought that another IEC standard would gain any traction? What a difference a couple of decades makes.
The idea of function blocks is definitely more accepted in this day and age. Process people have been dealing with them forever, but, with IEC 61131 software becoming more prevalent and add-on instructions in many PLC offerings becoming the norm, function blocks are a known quantity now.
The IEC 61499 function block is similar to a normal function block but with an added event input and an event output. The construct of the block allows for a state-driven algorithm that is executed using transitions.
The algorithms can be programmed in any IEC 61131 language including other function blocks and are objects from a software point of view.
From a hardware standpoint, even the Raspberry Pi can get into the action. PLCs of course are also able to support IEC 61499, but the actuators and sensors that can be connected together. True distributed procession through direct I/O will have its place in the sun in control systems of the now and in the future.
Working with Rockwell’s CompactLogix platform, which supports electronic data sheets, allows third-party vendors to provide the PLC with a catalog entry in order to support the hardware. Vision cameras and the like also are easy to implement using the electronic-data-sheet catalogs. In Rockwell’s case Ethernet/IP is the connective tissue that makes things work. Beckhoff uses EtherCAT, but the results are the same.
Remote and/or distributed I/O makes installations much easier. It reduces costs, but requires higher levels of monitoring and troubleshooting skills. With software, however, that part should be easily overcome.
