Tuning plot generated by RMCTools software
synchronised. Some of the more complex saw lines can involve several dozen motion axes that need to be controlled. Individual machine stations, can involve as few as two axes (as is the case with the log rotator pictured), whereas other stations, such as the curve saw, can employ between five and eight axes. Besides higher yields, another
advantage of tight synchronisation of the motion is the ability for the HewSaw machines to be physically oriented in a straight line, using much less floor space than other saw lines. This makes it easier to keep track of the wood and reduces facilities and maintenance costs. How does a manufacturer such
as Veisto select motion controllers for use in their machines? “We have several criteria for selecting motion controllers,” Petri Palo, Veisto Automation Engineer said. “One of the main ones is user friendliness from a design perspective. Because we use so many of them in different ways, the controllers must be easy to use, and the vendor’s customer support must be very responsive and efficient.” From a technical perspective, the motion controllers must excel at closed-loop control of fluid power systems. Hydraulics is the
power of choice for applications such as the HewSaw machines that involve positioning and moving heavy loads. It’s not enough to simply move heavy axes, however. In order to extract maximum productivity out of a hydraulically-operated machine, the axes must be moved with the precision provided with closed loop control. Beyond closed-loop control, implementing a highly productive and compact integrated saw line requires the synchronisation and gearing of multiple simultaneous operations. All of the machine’s motion must be coordinated in order to allow the speed of operation of the line to be varied to handle different grades of input logs (lines processing soft wood can run faster than those processing hardwood) without creating bottlenecks or machine maintenance problems. “We looked at different
programmable electronic motion controllers and decided to standardise throughout our product lines on the RMC family of motion controllers from Delta Computer Systems,” Palo said. “The different models offer a range of performance and capacity, with a common easy-to- use development environment.” For machine stations that
30 International Forest Industries | FEBRUARY/MARCH 2012
involve only one or two motion axes, HewSaw uses the RMC75. For machines that control more axes, the company uses the RMC150. To provide supervisory control of the entire line, HewSaw systems use a Siemens PLC. The PLC communicates with the motion controllers over PROFIBUS, an internationally-recognised industrial fieldbus standard. The block diagram pictured is of a typical HewSaw machine station, in this case, the curve saw. The motion of all five axes need to be coordinated in order to ensure that the saw blades precisely follow the curvature of the log. Information on the positions for each axis is passed from the log scanner/optimiser to the PLC. Those values are then converted into a sequence of instructions for the motion controller. Since each log is different, the exact operations performed by the motion controller will differ from log-to-log. Part of the machine
development process is tuning the motion to optimise how the machine works. To simplify this task, Delta Computer Systems motion controllers are provided with software called RMCTools that enables the user to graphically display the motion of any of the system’s axes versus time.
The graphs compare the actual motion of the system with the system’s optimal target motion, and when the motion is tuned perfectly, the actual and target motion curves overlap. To simplify the tuning process, RMCTools includes an automated Tuning Wizard that allows designers to tweak control loop gains via a slider bar on the screen. “Delta’s software tools make design optimization very straightforward,” Palo said. “The plot manager graphs really speed up the tuning process.” The new mill products from HewSaw have seen success across the globe. Installations process materials as diverse as eucalyptus in Australia, fir and spruce in Canada and pine and spruce in the company’s Scandinavian home country. The company has gained a reputation as one of the most technologically advanced sawmill equipment providers. “We owe a lot of our technical success to the support that we’ve gotten from Delta and its Scandinavian Distributor SKS Control Oy,” concludes Palo. “It’s a long way from Delta’s headquarters in Vancouver, Washington to Finland, but they respond as though they were located next door to us.” IFI
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