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%.

VT-HACD controllers permit high-speed, closed-loop control of hydraulic axes with precision to fractions of a millimeter. The unit is preprogrammed for a variety of servo and proportional valves. It includes 32 predefined function blocks for sophisticated control with algorithms written for hydraulic axes. A serial PC interface permits programming via Windows-based Bodac software that can be downloaded free from the Internet. Screens guide users through details such as sensor calibration and generating application-specific command values.

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