Machine designers are using sophisticated motion controllers to improve quality and reduce waste in production processes. For instance, molding machines now rely on controllers that simultaneously handle both the pressure and position of hydraulic presses to maximize output and ensure uniform material compaction. Likewise, the articulating gang saws used in sawmills have hydraulic positioning arms that are driven by motion controllers to cut logs to increase the amount of usable timber.
Such fluid power applications place special requirements on motion control systems. It's important for designers to do their homework to make sure they choose controllers that are up to the task. Here is some advice to help in that selection process.
Basically, there are three types of motion controllers: standalone control systems; general-purpose programmable motion control computers, typically available as cards that plug into PCs; and motion controllers with pre-programmed functions and flexible bus interfaces.
Each choice has distinct advantages. Standalone controllers make it easy to implement basic motion functionality but are sometimes difficult to modify. Programmable controllers allow almost any type of control scheme to be implemented but require the machine designer to handle programming and design maintenance. Pre-programmed controllers are often a good compromise because they offer a mix of flexibility and ease-of-use.
A motion controller for proportional valves can yield smooth valve control, reduce the potential for fluid leaks, and extend machine life. If the application requires that the controller transition from control based on position information to control based on pressure, the unit must be able to interface to position and pressure sensors. Controllers that provide direct transducer interfaces may offer the best cost/performance by eliminating the need for separate interfacing modules that can impose performance bottlenecks. Motion controllers that interface directly to the system's PLC or human-machine interface make the system simpler to design and contribute to higher system performance and lower hardware costs. And, motion controllers that interface to a standard fieldbus free the designer from a single vendor's control system offerings.
Technical assistance for this article was provided by Peter Nachtwey, president of Delta Computer Systems Inc., a member of the National Fluid Power Association.
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