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By Paolo Catterina, EUROelectronics
EUROelectronics develops and sells electronic systems, including control software both for PC and PLC platforms. For several years we have focused mainly on developing electronics for diecasting machine builders. We build and install the complete electrical-electronic control cabinet including PLC and PC software for all the applications we develop.
We are a highly specialized company in this sector and work together with OMS Presse, which began building machines three years ago, after 30 years as a rebuilder. Together we produce systems and presses ranging from 200 to 2,500 tons (Figure 1).
We have delivered systems for almost all the diecasting machine builders in the world including Buehler, Idra, Italpresse, Colosio and Mueller Weingarten. Since 1980 we have deployed more than 3,000 systems all over the world, and our activity is increasing.
Recently, EUROelectronics was asked to design a closed-loop hydraulic cylinder control system for a die-casting press machine. In order to meet performance parameters, we needed to think differently about our control solution.
We previously used a Moog programmable controller with a Moog servo valve. It was difficult to program and had limited functionality. Although the system did function—we had about 15 installations in Italy and Spain—we were unsatisfied enough to look for a different solution. We had problems with the electronics and had to replace three of the controllers.
The biggest reason for a new control approach was the need for more complete control of the hydraulics by extending PID to control pressure. We also saw the need to have a more standard product and programming tool.
Why Speed Matters
Position and pressure control of a hydraulic cylinder is a common application in the industrial automation field, but the precision control of such systems traditionally presented significant challenges because of their high speeds and pressures.
In our application, the cylinder moves over a software-defined trajectory with specific velocity and acceleration profiles. We must guarantee accuracy and repeatability up to a maximum speed of 10 m/s. For reliable control of cylinder braking and acceleration, closing of the loop must be at 1 kHz processing rates (Figure 2).
In a high-speed, closed-loop control system such as this one, the quality of sensors required for pressure measurement and positioning is critical. In this case, we selected linear magnetic strip sensors for position measurement. These sensors must be highly precise and reliable, and the signal processing interface for the sensor signals must be fast and rugged.
The FPGA Alternative
We were attracted to a possible FPGA-based solution by some tests done by National Instruments in Milan. Their engineers demonstrated a deterministic cycle time under 1 msec in an easy and interesting way.
The compactness and modularity of the NI CompactRIO hardware was attractive, but the most important factor was its FPGA architecture. It seemed very quick and robust enough for the application. The high-speed press moves 0–10 m/s and therefore requires high-speed control.
We choose NI because I had been using its data acquisition cards as far back as 1993. Roughly 1,500 of the systems we have developed are PC-based platforms, about 700 with an NI acquisition card. We installed eight systems with the FPGA solution during 2006-’07, and in 2008 we have orders for 10 more systems using this approach.
PC: Not for Motion Control
We never considered a PC-based motion controller as a possible solution for this need. In our experience, the PC is the least-reliable component in the complete electronics package we deliver and we prefer more robust and dedicated devices.
We’ve developed the software on a PC using LabWindows CVI, starting from DOS version to the current Windows-based platform. I always have been happy with NI hardware and software, so I wanted to move forward with NI products.
Although our five PC software programmers use LabWindows CVI, my experience started in 1992 with a LabView DAQ application for a diecasting machine, which was sold to China that year. Although many years passed, I realized significant advantage by drawing on my experiences while developing this new application.
With the integrated FPGA on the controller, we developed a system capable of low-level customization using commercially available tools. To meet the unique requirements of the application, we implemented a highly optimized encoder interface in the FPGA to measure the cylinder position while programming the system entirely in LabView.