Combined primary and secondary log breakdown optimisation
- PDF / 941,182 Bytes
- 11 Pages / 595 x 842 pts (A4) Page_size
- 72 Downloads / 166 Views
#1999 Operational Research Society Ltd. All rights reserved. 0160-5682/99 $12.00 http://www.stockton-press.co.uk/jor
Combined primary and secondary log breakdown optimisation CL Todoroki1 and EM RoÈnnqvist2* 1
Forest Research Institute, New Zealand and 2The University of Auckland, New Zealand
At sawmills logs are converted into boards by a series of cutting operations. Primary cuts reduce logs into slabs of wood, secondary cuts produce boards. Boards incorporating natural defects such as knots (branch sections) are inferior to clear boards. The aim of the sawmill is to cut logs to produce boards of greatest value. However, when logs are pruned, knots are only exposed after primary cutting. This complicates the conversion problem. To effectively convert logs into boards the interrelated effects of the cutting phases must be recognized. In this paper linked dynamic programming formulations are developed. The inner (secondary) formulation determines the optimal sequence for cutting a slab into boards. The result is passed to the master (primary) formulation that determines the optimal sequence for cutting the log into slabs. The objective functions can be modi®ed to maximise either total value of boards (incorporating quality) or total volume. Results from simulations indicate that signi®cant increases in value are possible when quality is considered. Keywords: dynamic programming; forestry; optimisation; quality
Introduction The main operations that transform a log to boards of various qualities and sizes are shown in Figure 1. In the ®rst operation, bark is removed from the log in an attempt to maintain the saw blade and improve the quality of bark chips that can be salvaged for other uses. Following debarking, logs are loaded onto a sloping platform where they are held until they can be processed on a carriage. Once on the carriage, a log can be turned at the discretion of the sawyer. Although there are many ways to saw a log, the most common methods are: live, cant, and around sawing (Figure 2). Live sawing is the primary focus of
Figure 1
this paper. With live sawing all cuts made by the primary saw are parallel. While this is also true for cant sawing, the latter method produces a cant, shown by the hatched area in Figure 2(b). A cant is a large slab of wood that is further processed at a resaw. Resaws relieve the primary saw of additional cuts, aiding material ¯ow within a mill. In contrast, the orthogonal cuts shown by the shaded areas in Figure 2(c) are made by turning the log on the carriage.
Sequence of log processing operations.
*Correspondence: Dr CL Todoroki, Sala Street, Private Bag 3020, Rotorua, New Zealand.
Figure 2 Sawing methods.
220 Journal of the Operational Research Society Vol. 50, No. 3
Cuts made by the primary saw produce sawdust and slabs of wood. Outer slabs, characterised by one sawn surface only, are turned into pulp chips. Pulp chips are also manufactured from inner slabs that are too thin for secondary processing. Under-size slabs can be produced intentionally in an attempt t
Data Loading...
