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Designers of New Die Casting Machine Choose PROFIBUS for Performance and Worldwide Availability of System Components

PROFIBUS, in combination with precision hydraulic motion control, offers flexible, high-performance machine design that meets a wide range of customer needs. THT Presses, Inc. of Dayton, OH, the world's leading manufacturer of electric motor rotor casting machines, proved this is in a recent design of a new 1000-ton vertical die cast machine used to produce high integrity aluminum castings.

Several factors led to the choice of PROFIBUS as the fieldbus communications backbone for the system: compatible parts availability worldwide, high bandwidth, deterministic communications and flexible system interfaces to a wide array of system components as specified by THT's customers. The new machine is rated on clamp tonnage (1000 tons) and shot force (1000 tons). The largest casting that can be made is approximately 100 pounds. In the machine, aluminum in a semi-solid state is injected into the die from a 20-inch diameter shot sleeve that is mechanically coupled to a 26-inch diameter (bore) hydraulic cylinder with a 6-inch stroke.

By providing a PROFIBUS interface between the system PLC, motion controller and peripheral devices, machine operators can set process setpoints from their HMI and gather data in a PC-based data acquisition system.

The THT machine designers like PROFIBUS because of its support for high, 12Mbits-per-second throughput, which supports realtime data acquisition from a wide array of system devices. In one customer installation, PROFIBUS provides a connection to a Fanuc 6-axis robot that sprays release agent on the die, shot sleeve and piston. PROFIBUS also interfaces to a hot-oil system built in Austria (for heating the die) that sends temperature setpoints and diagnostics information to the system controller. The machine also provides a PROFIBUS connection to a Fonderex vacuum unit used to evacuate the die prior to metal injection.

Each installation uses a different combination of add-on components. THT gives its customers freedom in specifying components from different vendors, particularly in the PLC that performs system control. Since most PLC vendors provide PROFIBUS interfaces for their controllers, THT has found that interfacing is straightforward.

An optional electronic control system for "shot-end" control offers THT's customers the ability to enhance the quality of castings produced by the new vertical die casting machine. Normally, the hydraulic cylinder that injects the "shot" of hot metal into the die is controlled by an open-loop speed controller. With open-loop control, the rate and duration of shot stroke can vary slightly from casting to casting, which can cause minute differences in internal composition between castings produced. With the new shot control option, however, the THT machine developers can profile the shot stroke more actively, controlling the exact position and velocity of the hydraulic cylinder throughout the injection cycle. This ensures uniformity and high quality output from the machine, which can be maintained even as environmental conditions change.

The THT engineers also wanted the ability, in future enhancements of the control system, to control the pressure exerted on the shot load as well, as a means of further improving productivity of the machine. To perform the closed loop hydraulic control of the shot cylinder, the THT designers needed a motion controller that: 1) interfaces directly to PROFIBUS; 2) supports position and pressure control of hydraulics, interfacing directly to proportional servo valves; 3) is easy to program and tune; and 4) offers a good cost/performance tradeoff. To fill these requirements,

THT selected an RMC70 Series motion controller from Delta Computer Systems, Inc. (Vancouver, WA). THT first heard about the Delta through hydraulics distributor Triad Technologies, Inc. (Englewood, OH). The RMC70 Series is optimized for systems that need only one or two axes of precision motion control. The controller family is supported by RMC70Tools, a software package that provides graphical tools to help designers develop control profiles and tune control loop parameters.

The THT engineers used RMC70Tools to do the initial design and tuning and will be using it to further optimize the motion in the future. The new machine design didn't come without challenges. This machine was the first time that THT designers had used an advanced, two-stage cartridge valve (supplied by Bosch Rexroth) that combines a 50mm cartridge valve and a D03- size servo performance "pilot valve." Designers solved the challenge of coordinating operation of the two valves by using the Delta RMC70 to control the pilot valve.

By combining PROFIBUS communications and Delta's precision hydraulic motion controller, THT is confident that the new machine will produce castings that are more uniform and that have a shorter process development cycle than produced by previous die casting machines.

[Reprinted from Connection, the newsletter of the PROFIBUS Interface Center in Johnson City, TN.]