DYNAMIC LOCAL-E - THE GRADE INDICATING PROPERTY
Mechanical stress grading is a mechanical process that involves the measurement of a grade indicating property. The grade indicating property is evaluated by the machine using a predetermined programme of grade indicating property settings, called thresholds, resulting in stress grades being assigned to the timber by the grading machine.
In mechanical stress grading the primary grade indicating property is the local modulus of elasticity (local-E). This property is measured with the timber loaded as a plank at various points along each piece of timber and over a relatively short span. The span is usually between 3 and 4 feet (about 900 mm to 1200 mm).
Machines of this type have been used for over 40 years and have proven to be both reliable and efficient in operation. The success of this type of machine is partly due to the transparency of the way it works and it's ease of application as a grade sorter. Anyone with basic engineering skills can understand the principle behind the machine. The actual mechanism for measuring local-E in the grading machine itself can be quite complex but all of the commercial machine types work using the basic principle. The success is also due to the fact that the machines evaluate stiffness directly and with stiffness being an important structural parameter (in addition to strength), it goes a long way in justifying the use of such a machine as a grade sorter. Other approaches to timber grading are far more complex and require much more information to be generated from a number of sources before an attempt can be made to estimate the very property that the MSG machine measures naturally.
The number of active thresholds used during each grading run is usually 1, 2, 3 or 4 grades. Tadpole accommodates up to a maximum of four grades. A secondary threshold value, for average local-E, is also provided for each grade. During the grading process itself individual grades can be combined in order to boost the stiffness and strength of the lower grade. Tadpole also accommodates the situation where an intermediate grade (say grade level 2) is one primary grade and a combination of the material in grade level 1 and the material in grade level 3 makes up a second primary grade. Material in grade level 4 can then be a third primary grade.
THE IMPORTANCE OF LOCAL-E IN THE CONTROL PROCESS
From a control point of view, and taking into consideration the previous discussion, we can understand the importance of the local-E measurements taken by the grading machine.
Local-E defines the stress grade of the timber at least from the point of view of the grading machine. Therefore the overall grading accuracy of any MSG system is greatly affected by how accurately we can determine the local-E in the grading machine itself. As a key part of the control system we therefore need to determine how accurately the local-E is being measured.
Note: we call the local-E measured by the grading machine the dynamic local-E because it's measured at speed. Complimentary to this is the static local-E which is the local-E measured in a static testing machine.
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