Automating Manufacturing Processes with Ethernet-Enabled I/O Modules, Linux
By: Rob Reilly
Wednesday, October 9, 2002 08:19:15 PM EST
URL: http://www.linuxplanet.com/linuxplanet/reports/4487/1/
What's A PLC?
Traditional control of industrial processes has been carried out by small special purpose-built microprocessor based systems called PLCs, also known as programmable logic controllers. They have very fast cycle times and essentially run in an endless loop looking at inputs, making decisions and controlling outputs. PLCs are programmed with an industry standard language known as ladder logic. They can respond, in real time, to hundreds of inputs and can control all types of motors, relays and actuators. PLCs are the quick thinking brains of the industrial automation world.
As the old hot rodding saying goes, "Speed costs money, how fast do you want to spend?" PLC systems are plenty fast and plenty pricey. In the case of PLCs all that speed also needs to be protected from the industrial environment. The control computer (PLC) and I/O modules have always been located physically near the industrial machine in a metal cabinet, right out there on the shop floor. Putting the cabinet next to the machine being controlled made sense because all the switches, sensors, motors and relays had to be connected by heavy wires back to the I/O modules.
But what happens when you're going to design a manufacturing process that doesn't need insane real-time performance? Also, what if you want to collect a whole lot of data, do some calculations on that data, and then automatically send the results (charts and graphs) to various people in your organization, automatically? Finally, what if you wanted to have the control computer (desktop PC) located in your programmer's office and out of that industrial shop floor environment?
New Ethernet I/O Module Meets Linux
That's where Woodhead Connectivity in Northbrook, Illinois comes in. They've developed an Ethernet-enabled I/O module that can be mounted directly on a machine and controlled from any PC or PLC using standard ladder logic programming techniques. They also make industrial strength cabling, connectors and ruggedized ethernet switches that can also mount right on a machine. Their latest project used the I/O modules tied to a Linux box that was tasked with programming, monitoring and data collection. The basic discreet 8 input/output port Ethernet enabled module from Woodhead costs $350. Prototyping a new project with these I/O modules, Ethernet, Linux and some Open Source software seems like it would be feasible for just about anyone.
System integrator Centech Automation in Palatine, Illinois teamed up with Woodhead Connectivity to deliver an electropolishing line to one of Centech's industrial clients.
The system the team designed was an electropolishing line that used a motor driven lift arm on a moving gantry for parts transfer from processing tank to processing tank. The motor control and positioning of the system was controlled over ethernet using the Ethernet/IP control protocol. The sensing and actuation was controlled by a collection of proximity and limit switches, solenoids and motor drives through Woodhead Connectivity I/O modules located on the machine.
While other operating systems could have been used for the control computer, the team chose a Linux box running Red Hat 7.2, mainly due to cost and reliability factors. When you're delivering a $100,000 industrial system the components must be cost effective and rugged. Linux has proven to be easy to set up, stable and able to support remote administration. Since Linux is multi-user, technicians can log in to the box for reprogramming, maintanence or upgrading from just about any place that has an Ethernet connection.
Software And Automation
Dan Naughton, Woodhead Connectivity's Engineering Manager said, "TCP/IP socket performance [with Linux] was better than the Windows 95/98 operating system during our testing." "Also, touchscreen support for Elo Touch monitors and additional drivers for network cards and video adapters were included in the Redhat 7.2 Distribution," Naughton added.
Naughton said they had troubles with the NT version, getting the dreaded blue screen after simply trying to change the network card. "Linux is very stable," he concluded.
For automation software on the Linux box, the team turned to the OpenController.org project. OpenController.org had a ladder logic design program, as well as a runtime control engine on the Linux box that runs the electropolishing machine. The GUI portion of both the LadderDesigner and the runtime control engine were written using wxWindows, an Open Source C++ GUI framework for cross-platform compatibility.
"We originally tried Embedded uClinux on the I/O modules themselves, but ran into implementation problems," Naughton said. "The kernel was not stable and we eventually abandoned it," he further commented.
Naughton and the team are the usual technology enthusiasts employing Linux servers at work and of course, at home. They have worked with embedded Linux, RTLinux and other operating systems.
There are other control programming methods for creating instruction sets for PLCs, but Naughton said that ladder logic was the standard for industrial control. "Around 98% of industrial automation is PLC ladder logic," Naughton explained.
The working electropolishing machine took 10 weeks to build. The team had worked with the OpenController.org project for 6 to 9 months prior to starting the electropolishing machine project. The runtime program ended up being about 400 lines of ladder logic and 1500 lines of mnemonics. Mnemonics are the actual instructions that are executed on the PLC to control the I/O module. The OpenController.org project will be posting the actual ladder logic program file use for this machine on the project website.
Future Automation
Naughton said that Woodhead Connectivity will be expanding their line of I/O modules in the future. A large portion of their current business is supplying ruggedized cables, connectors and ethernet switches. Some of their products are even submersible, which makes for a pretty tough machine.
The industrial automation crowd has been talking about ethernet connectivity for a long time. Put the control computer in an enviromentally safe area and leave the I/O equipment out near the machine, connected by ethernet. For various reasons the industry has been a little slow to embrace the the new model, possibly due to installed base versus revenue issues.
Woodhead's products coupled with Linux, ethernet and Open Source control applications might be just what you are looking for the next time you need to automate one of your factory processes.
Rob Reilly is a senior technology consultant, whose work includes Linux, business integration, innovation training and automotive design. He frequently writes and speaks about these and other subjects. He has 16 years experience in the high technology, manufacturing and the utilities industries. He is always 'on-the-lookout' for stories and projects that focus on Linux, business and the cutting edge. Send him a note or visit his web site.