Decentralized hydraulics boost sawmill output

March 17, 2005
A new type of servoactuator may change the way machine builders view hydraulics.

The Rexroth Integrated Axis Controller is part of a highly automated sawmill system introduced by S^derhamn Eriksson that positions, clamps, and saws up to 20 logs/min.

The IAC subassembly includes a temperature and vibration-hardened microcontroller, control valve, position transducer, and hydraulic cylinder.

The Integrated Axis Controller (IAC) from Bosch Rexroth, Lohr am Main, Germany (, lets OEMs easily integrate sophisticated and highly accurate position-control capabilities into their designs without any in-depth knowledge of hydraulics.

The pretested subassembly consists of a temperature and vibration-resistant microcontroller built into a fast-acting control valve, along with a position transducer and hydraulic cylinder. It communicates to higher-level PLCs, motion controllers, or CNCs via fieldbuses such as Profibus and CANopen.

According to company officials, the IAC opens up the advantages of decentralized control, including modular subassemblies, faster commissioning, ready access to diagnostic data, and enhanced productivity.

The wood-processing industry is one of the first to take advantage of this technology, with the aim of increasing the amount of usable lumber obtained from logs. "The emphasis today is on making equipment more productive," says Jan Ake Wiklund, design manager for Söderhamn Eriksson AB, Söderhamn, Sweden (, a leading supplier of sawmill machinery. Yield is critical when converting tree trunks into planks and boards because the raw material costs about 80% of the finished product, he explains. "A 1% increase in yield can make all the difference for a medium-size sawmill, potentially increasing profits by 100,000 Euros per year," says Wiklund.

Because no two logs are exactly the same, a laser scanner measures trunks and passes the details to a master computer. Knowing the size and shape of the log, specialized software calculates the cutting profile that maximizes output.

Curve saws, also called sweep saws, do the actual cutting. "Curve saws do not simply cut a straight line through a log, but can follow the outer dimensions or even a specially calculated profile to cut boards in the most efficient way," says Wolfram Ulrich, head of Bosch-Rexroth's Wood & Paper Industry Application Center.

Depending on the shape of the individual trunk, curved paths generate up to 20% more finished wood, says Ulrich. The curved beams and planks are straightened during drying. Cutting lengthwise along the annual rings also reduces the degree of cross-grain cuts, resulting in more-homogenous lumber with higher load-bearing capacity, he adds.

The IAC-R integrated axis controller is particularly suited for positioning the blades, says Ulrich. Onboard electronics close the drive-axis position loop in the valve, with motion-control capabilities right in the drive. After receiving target-value data from the machine controller via a fieldbus, the IAC-R automatically moves the saw to the required position. Acceleration and speed settings are stored in the IAC-R, so network delays or the speed of the main controller are not an issue. Together with the proven dynamiccontrol valve, the system ensures highly accurate cuts.

In addition to better yields, the integrated drive brings a number of machine-design benefits. "It's compact, so we save space," says Ulrich. It uses fewer components and less wiring. And it withstands the rough environment seen in the lumber industry, including exposure to dust and wood chips, vibration, shock, and extreme fluctuations in temperature.

The integrated axis controller is part of a revolution currently taking place in hydraulics, says Johannes Grobe, head of Product Management Industrial Controls at Bosch Rexroth. Electrohydraulic axes with fieldbus capability have reached new dimensions in terms of dynamic capabilities and offer accuracies to ±1 µm, he explains.

The IAC axis controllers operate both in open and closed-loop modes. Differential-pressure measurement opens up the possibility of volume flow control without a pressure balance because the required valve characteristics are stored in tabular form in the on-board electronics. Position/pressure and position/force controls are also possible with soft transitions between the two modes.

One considerable advantage compared with control-cabinet electronics is substantially less wiring. It also requires fewer components compared with conventional hydraulic systems, which speeds commissioning and startup, Grobe stresses.

And the fieldbus link opens up the advantages of continual data access for diagnostics and maintenance, says Grobe. As with electromechanical axes, status information, target values, and parameter changes can be exchanged between the main controller and the intelligent axis, or called up via the Internet. This also applies to program or sequence changes, and quality data. The possibility of modifying motion sequences and parameters via the fieldbus increases system flexibility, which can considerably reduce the total cost of ownership, he says.

Developments such as the IAC have led Bosch Rexroth to modify its business model, emphasizes Grobe. The company no longer simply supplies components which users must assemble. Instead it supplies complete "plug-and-run" electrohydraulic systems and, in doing so, takes responsibility from the machine manufacturer for the subassembly. With the aid of special simulation programs, the axes can be designed based on critical parameters such as speed, path, dynamics, or positioning accuracy as well as the control and fieldbus system used.

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