Using a PC for a machine motion controller offers the advantage of leveraging state-of-the-art processing capabilities at competitive prices. But, when the PC uses the Windows operating system (OS) there are issues.
By itself, the PC platform with a Windows OS has limitations in its control of time-critical processes. Windows is not deterministic. Without determinism, there's no guarantee that the system will respond to stimuli in exactly the same way during each machine cycle, and as a result the machine will not perform reliably.
SEE ALSO: Should We Switch to PC-Based Control?
That's the issue system integrator Integrated Industrial Technologies (I²T) of Pittsburgh faced with the design of its PC-based control system. With its software and industry expertise, I²T, has developed intuitive upgrades for many industrial applications, including milling, grinding and gantry robots.
Its new control system design was to be based around an industrial PC as a drop-in replacement into an existing HMI panel cutout. Intuitive user interfaces built on the Windows platform would eliminate the costly need to retrain operators. System changeover time was to be minimal, and I²T's customers' existing drive and motor investments had to be preserved.
I²T has a long history with PLC, CNC, HMI and proprietary control systems. Our breadth of experience has exposed us to a wide range of devices with hardware incompatibilities, communication differences and other factors that make it challenging to deliver a system that doesn't feel thrown together. This is especially true on retrofit solutions when the customer desires to keep some and replace other components on the machine.
We began using early versions of PC-based control systems in a similar fashion as any other controller. It became apparent how much easier it was to integrate other systems into the PC. The architecture of the PC provides many customization options. One can install software packages, add cards and communicate with external devices (USB/Ethernet) very easily. Communicating to a camera could be as simple as opening an Ethernet socket. Exchanging I/O with a legacy PLC could be accomplished by adding a PCI card to the motherboard. All of the development could center on the PC box. By using the PC solution, I²T engineers can divide the development efforts into manageable subsystems and use their software expertise to merge them into a final solution.
Benefits of COTS
A major benefit of development on the PC is the ability to use commercial off-the-shelf (COTS) technologies. The user interfaces can be developed in modern, commercially available environments such as Microsoft's Visual Studio. No longer are integrators restricted to working with proprietary software tools written by programmers, who might be excellent at motion control, but are not proficient at development tool design. I²T's user interface packages can take advantage of modern programming practices and features, such as databases, web and animations, years ahead of when they would appear in a proprietary HMI. As new development technologies emerged, I²T was able to transition its core knowledge and take advantage of the latest and greatest features, including C++ to C# and MFC to WPF.
One of I²T's retrofit customers, Atlas Industries in Fremont, Ohio, is a producer of crankshafts that are precision machined from castings, forgings and billets with overall lengths up to 240 inches. Atlas had used proprietary controllers in its milling machines, which were nearing the end of their useful lives and had become costly to maintain. The upgrade control system that Atlas would turn to — based on I²T's approach — needed to insure that real-time control functions were supported while a new PC-based operator interface was added.
"The milling tolerances that we maintain are critical to our operation," explains Bob Stein, Atlas Industries' electrical engineer. "A tight tolerance during the mill makes it easier to maintain tolerances in later processes such as grinding and polishing, and we deliver an overall better quality part. To stay ahead of our competition, we needed reliable control equipment."
Atlas wanted to upgrade its machines to use a common control platform with an operator interface that is easy to use and consistent between its machines, explains Jim Clark, Atlas Industries' vice president. "The machines we employed used proprietary CNC-based control systems from the original manufacturer. These older systems were limited in functionality and hard to maintain,” he says. "It was often very difficult to acquire spare parts for them. However, the hardware bases and mechanical components were in excellent — or at least salvageable — shape. We wanted to modernize our fleet of milling machines to increase the machine capabilities and reduce their maintenance costs."