Cutting curves, not corners

Wolfram Ulrich
Head of Global Branch Strategies
Bosch Rexroth
Lohr am Main, Germany

Highly automated sawmills are the norm in today's lumber industry. Modern lines debark, trim, cut, and saw up to 20 logs/min — depending on the type and size — with tolerances in the tenths of millimeters.

But speed is not the only path to improving production. Wood processors increasingly need to reduce waste and make the most efficient use of raw materials. One recent innovation is curve saws, which are not restricted to straight cuts. They can also follow a trunk's outer dimensions or specially calculated profiles to cut boards more efficiently.

The aim is getting the most usable lumber from each log. Yield is critical when turning tree trunks into planks and boards because raw materials make up about 80% of cost of the finished products. Even a 1% increase in yield can substantially boost sawmill profits.

Depending on the shape of the individual trunk, curved paths carve out up to 20% more finished wood. The curved boards and timbers are then straightened during drying. Cutting lengthwise along annual rings also lessens the degree of cross-grain cuts. This, in turn, reduces internal tensions. The result is more-homogenous lumber with greater load-bearing capacity and higher commercial value.

CALCULATING CUTS
Curve-sawing technology combines developments in sensors, microprocessors, and electrohydraulics. Because no two logs are exactly the same, multilaser scanners and cameras first measure trunks and pass the digital details to a master computer — typically an industrial PC. Special software takes the size and shape and generates a 3D model of the log, and calculates the cutting profile that maximizes output.

Normally, the PC works with higher-level enterprise software. This directs the optimizing program to determine position and shape of cuts based on factors such as satisfying orders for specific sizes of lumber and boards, fluctuating prices, and so on.

Because there is little time to spare between scanning and cutting, software specialists have refined the algorithms to minimize processing time, yet produce accurate results. Major saw manufacturers, such as Coe Manufacturing and USNR, offer proprietary software, as it is now a key component of their machines. Some independent firms, such as MPM Engineering and Porter Engineering, also market software to curve-saw OEMs.

Once generated, cutting data is sent to a high-level PLC that oversees every stage of an entire saw line. The PLC, in turn, sends positioning information to the motion controller that drives the electrohydraulic axes and, in turn, sets saw-blade angles.

SETTING BLADES
Typically eight or more blades are used to make curved cuts. They are housed in what's termed a sawbox, and electrohydraulic actuators control the spacing between blades. Before curve sawing begins, the blades are positioned once for each trunk and then held in place during cutting.

All hydraulic drives involved in the cutting process must have high dynamic response, position accurately, and resist vibration and contamination from sawdust. For some axes, such as those that adjust cutting width, compact size and simple integration into the machine are additional requirements.

The drives include a high-response proportional valve and a hydraulic actuator housing a position-feedback transducer. The controller sends position commands to the actuator and ensures the cylinder rod reaches its set position at a certain time. Signals from the transducer to the controller close the positioning loop.

While cylinder size depends on the application, typical bores range from 1.5 to 3.25 in., with rod diameters of 1.0 to 1.5 in. and strokes to 24 in. Blade positioning typically takes 0.5 to 1.0 sec, with accuracy to 0.1 mm. Operating pressure is normally 1,500 to 2,000 psi, generating forces of several hundred pounds to hold blades steady during cutting.

BUILT-IN CONTROLLER
For saw-blade spacing and other relatively simple motions, the axes are more or less independent. They do not interact or synchronize with hydraulics that control the curve-sawing profile.

For that reason, a recent innovation regarding these standalone axes transfers motion-control "intelligence" from the controller to the axis itself. The PLC merely commands a position value, and the actuator internally performs all closed-loop positioning. Bosch Rexroth terms the device an integrated axis controller (IAC).

The IAC subassembly consists of a temperature and vibration-resistant microcontroller built into a fast-acting, proportional-control valve, along with a position transducer and hydraulic cylinder. This intelligent drive closes the position-control circuit of the drive axis directly within the valve electronics. It controls all hydraulic-specific motions, with acceleration and speed data stored in on-board memory.

Once the PLC specifies a setpoint by way of a fieldbus or analog interface, the IAC moves the saw to the required position independently, with no further interaction with the main controller.

The IAC simplifies machine-control architecture and demands on the main controller. It also brings a number of machine-design benefits. The modular subassembly is compact, uses fewer components and less wiring than traditional hydraulic axes, and is pretested — all of which can speed commissioning and startup. And it is built for rough environments, including exposure to dust and wood chips, vibration, shock, and extreme temperature fluctuations.

Onboard electronics also offers ready access to quality and diagnostic data, so status information and actual position values can be easily sent to the main controller. And users can download and modify motion sequences and change operating parameters via the main controller, increasing system flexibility. IACs in sawmills are currently positioning edger blades, with applications involving curve saws in the works.

COORDINATED MOTION
Unlike decentralized controls for adjusting cutting width, curve-sawing demands coordinated motion between two or more electrohydraulic actuators.

In one typical design, two main axes control blade angle. One axis moves the sawbox back and forth while a second rotates the entire saw body. Coordinating movements of both axes creates a continuous, curvilinear cut.

The closed-loop motion controller directs actuator position by modulating valve-spool position in real time, reading the actual position via a synchronous serial interface (SSI). Therefore, responsive control valves and a powerful motion controller — both tailored to the application — are critical. For example, Bosch Rexroth 4WRPEH proportional valves are widely used in sawmills because they combine high power density, good controllability, and high dynamic response.

The Size-6 valve, for instance, features a direct-operated servosolenoid with a precision-machined, servo-quality control piston and sleeve. The control solenoid uses factory-calibrated electronics and an inductive transducer to sense spool position. Hysteresis is ≤0.2% and response time to input signals is ≤10 msec. Maximum operating pressure is about 4,600 psi and maximum flow is to 10.6 gpm. Larger, pilot-operated versions of the valves are also available.

But just as important as speed and precision, the valves must satisfy requirements specific to wood processing: durability and long service life despite brutal operating conditions.

Sawmills, as one might imagine, are rough environments for machinery. Cutting large logs at high speeds induces significant machine vibrations. And dirt and sawdust play havoc with unprotected systems. The hydraulics and electronics must be built for this environment.

Most hydraulics suppliers learned long ago that standard industrial valves quickly fail under these conditions. Today, they build special ruggedized versions of standard valves. This includes using smaller electronic components, like capacitors, that are less affected by vibration, as well as special antivibration mountings for electronic components to boards, and for boards in the housing.

The 4WRPEH valves, for instance, have an IP65 rating and resist vibration to 25 g. In extreme instances, Bosch Rexroth reduces the electronics in the valve to a minimum and only uses the aluminum housing and a rugged connector for cabling. Electronics instead mount in a separate amplifier module housed in a remote enclosure. These units withstand vibrations to 40 g.

Steel spools and sleeves render the valves insensitive to typical sawmill hydraulic-fluid contamination, and operating temperature range is –4 to 122°F.

CONTROL ISSUES
The controller is also critical to curve-saw performance. Saw manufacturers sometimes use general-purpose controllers designed for electromechanical drives and adapt them to hydraulic systems. This tends to produce less-than-optimum results.

For instance, general motion controllers often feature autotuning. Users gravitate to the feature because it simplifies setup. But this can limit controller gain — and actuator speed — in order to maintain system stability. Controllers designed for the nuances of hydraulics — adjusting for cylinder differential piston areas, making valve characteristic-curve corrections, and selecting the optimal controller type, for example — offer higher gains and can improve positioning speed by up to 50%.

In some machinery in the woodworking industry high masses lead to low natural frequencies of the axis. This makes it difficult to combine high speeds with precise control. Normally, systems with low-natural-frequency axes can only be tuned to be slow and stable. Setting controller parameters for higher dynamics quickly makes the system unstable.

To counteract this problem, engineers at Bosch Rexroth recently installed a VT-HACD motion controller in a curve saw, in combination with a high-level PLC. The controller provides so-called state control, with algorithms based on feedback pressure and position in a unique control structure. This keeps the axis stable despite high speeds. The control's active-damping feature lets springy, low-natural-frequency systems operate without shock at high speeds, and with position repeatability not previously possible. Simple adjustments allow smooth, accurate acceleration and deceleration.

The motion-control hardware — controller, valve, and SSI interface — can be highly accurate. In fact, the same components, except for the transducer and low-friction cylinders, are used in machine-tool applications that have accuracies up to 1 µm. Such precision is unnecessary in woodcutting applications, though sawmill systems can offer 0.01-mm accuracy.

Today, there is another way to prevent unacceptable low natural frequencies on hydraulic axes. Specific hydraulic-simulation software can readily determine a system's natural frequency and performance limits. Rexroth's Hyvos software, for example, includes models of typical proportional valves. This tool lets leading curve-saw manufacturers optimize system performance early in the design process.

MAKE CONTACT
Bosch Rexroth, boschrexroth-us.com