Sawmill System Automation Cuts Complexity and Costs

APPLICATION OVERVIEW

A long-time builder of sawmill systems has for several years favored the use of PC-based control over traditional systems with PLCs. At a recent installation of one of their complete systems, they incorporated INTERBUS along with PC-based networking and control to run a sawmill which starts with raw tree trunks at one end of the operation, and ends with finished boards and lumber at the other. One person operates the entire system from a main control desk.

CONTROL SYSTEM

Hardware
This installation consists of about 6,000 I/O points installed in de-centralized subdistribution boards connected to two PC-based INTERBUS controllers via two principal INTERBUS network lines. Frequency converters allow the conveyor system speed to be varied to match production volume. With this ability, tree trunks of up to 26 feet in length can be fed into the main sawing unit nearly back-to-back. In addition to the frequency converters and absolute rotary encoders, two subordinate PLCs are incorporated into the system. These PLCs position the 14 servo axes and route important information to the main system.

All production units are linked via INTERBUS to assure a smooth flow of product from the infrared measuring point for the tree trunks, through cutting and sawing operations to the stacking station for the resulting lumber.

Software
One of the advantages of the PC-based INTERBUS control is that individual items of information are supplied to the visualization software running at the main control desk, making it possible to optimize the timing of the wood cutting operation. The software includes individual part tracking capability and sets the feed speed individually between from about 65 to more than 260 feet per minute, depending upon the bole diameter. Cutting width can also be varied from approximately 3 to 28 inches.

RESULTS/BENEFITS SUMMARY

The linking of production units considerably reduced wiring need and resulting installation complexity since the individual stations are interconnected using only a control cable with D-Sub plugs. Decentralizing with subordinate PLCs reduced installation and commissioning times by about 50%. The company has employed this principle in numerous systems and is convinced that PC-based INTERBUS control systems represent the optimum cost effectiveness for processes of this type.

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