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 integrated axis controller assembly is geared to harsh environments typical of the wood industry. It features robust digital electronics, a dynamic control valve proven in the wood-processing industry, and a digital position-measuring system protected inside the cylinder.

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.