Sorting plant control and choosing bins

Dimension sorting plants must meet the capacity requirements of the sawmill so that green sorting does not create a bottleneck in the production process.

Methods of feeding timber for sorting

Stack feed

In this method, the centre pieces and boards are transferred from the sawline to a single lamella conveyor from where the goods are dropped on to a collecting conveyor. The adjustable photo cell helps to collect a stack of timber of a predetermined height on the collecting conveyor, after which the conveyor is run forward by the next lamella conveyor.

If the sorting line processes timber at the same speed as the sawline, the pieces of timber will not stack up. If the sorting process stops, the sawline can continue sawing until all the lamella conveyors before the unscrambler are full. In this situation, the conveyors that are situated before the unscrambler are called buffer conveyors.

The step feeder and unscrambler clear the stack. The step feeder breaks the stack into smaller piles and the lugs of the unscrambler take them apart and form a mat where the pieces of timber no longer overlap. There can even be two consecutive unscramblers to ensure that an even carpet of timber is formed.

Synchronised feeding

In synchronised feeding, the centre pieces and boards are loaded directly from the sawline and edging line cassette conveyors on to the sorting line lug conveyor. The cassette feeding method is more prone to disruptions but it is also gentler because the timber is not dropped and there is no stack to clear.

The edging line transfers the pieces one by one, but the log center pieces (‘exlog’) must be separated from the other pieces before loading on to the sorting conveyor can be done.

Some sawmills have separate sorting lines for center pieces and boards, and these lines can use different feeding methods.

Loading

The purpose of the loader is to load each piece of timber one by one so that they land in between the lugs of the sorting conveyor. Each loader requires the timber to be lying flat in one layer before it reaches the loader. This is ensured by manual control if necessary. The automated control system monitors the pieces using several photo cells. The tightness and width of the carpet are adjusted by several consecutive frequency converter-controlled feeder chains.

Loader types include:

1. disc loader (< 100pcs/min)
2. loader with ‘shark fin’ lugs and width adjustment. Works best when one width is processed at a time, e.g. in dry sorting (< 180pcs/ min)
3. Turboloader with thickness detection (<150pcs/min) (E.g. models by Heinola Sawmill Machinery, Jartek Invest Oy or Springer AG)
4. ‘Catch and release’ loader (< 250pcs/min) (e.g. TongLoader by Framtec AB and Lug Loader by Comact)

Scanning

The timber is scanned so that it can be directed to the correct bin according to certain properties. In Finland, only one species is processed at a time because different species are already separated in the forest or at the log sorting station at the latest.

The properties scanned in green sorting are:

1. Thickness, width and length
2. Geometric shape, profile: wane, dimensional accuracy, crookedness, warpedness and checks
3. External quality grade, scanning one, two, three or four sides: knots, their size and type, rot defects, blue stain and insect damage
4. In future, these might include internal quality control by X-ray in the same way as Xray technology is already used in log sorting

Automated quality scanning can be replaced or supplemented by visual quality control. However, automated sorting has become the main method of sorting in Finland.

Cutting

Dimension sorting plants use two different cutting methods:

1. One cutting method aims to cut the timber into its final length at the green sorting stage which maximises the amount of green chips and reduces the amount of material that ends up in the stick-stacking and drying stages.
2. The other cutting method is dimension sorting which only cuts the wane pieces that do not even qualify for the lowest grade. In this method, the final grading is done at the dry sorting stage. This way, defects can be evaluated all at once and there is no risk of the quality of a piece of timber graded at the dimension sorting stage deteriorating at some later stage of the process.

Manual pulling table

Here, the operator manually pulls a piece of timber with a bad top, for example, towards the top cutting blade. As this is physically demanding work, the maximum speed must not exceed 60pcs/min.

Stopper lug and one-blade cutter

Alignment rollers guide the timber ends against a stopper lug that rises in front of it. The stopper lugs are typically placed every 300mm. The cutting blade remains in place.

The automated control system chooses the stopper lug to be used. The speed can be no more than 120pcs/min. The maximum processing speed is affected by the length of the stopper lug group and the maximum transfer distance for a short piece of timber to a stopper lug.

Servostopper and trimmer

A servo stopper places the timber into the position required for the butt end cutting process. The stopper runs at the same pace as the sorting conveyor lugs.

The sideways positioning of the stoppers is controlled by a hydraulic or electric servo. The piece of timber is aligned against the stopper as with the stopper lug. There can be one to three stoppers. The maximum speed with three fences is 200pcs/ min. After the servo stopper, the piece is no longer allowed to move sideways.

A trimmer typically has 21 fixed blades every 300mm. The blades are moved up and down by either a hydraulic or pneumatic system. The automated control system controls the timing of the movements.

There are also trimmers where the upward motion of the blades is mechanically aligned with the motion of the line in a pattern similar

to a sine wave. This makes the movements of the blades smoother. The timing of the control is done mechanically; the automated control system controls a single blade.

After cutting, the timber is aligned once more, usually at the butt end, and transferred to the rake conveyor (vertical bins) or divider conveyor (horizontal trays).

Automatic monitoring and alarms

The automated control system monitors the system in order to avoid any mechanical damage. The vertical pieces can be detected by the photo cell that monitors for pieces that are too high.

The diagonal pieces are detected by the photo cell monitoring for diagonal pieces. Rake conveyors have inductive guards. Photo cells are also used to monitor the bins for overfilling.

Monitoring the filling of the bins

Sorting into bins and monitoring the filling of the bins is automated at several different stages. The filling of vertical bins is monitored in three different ways, all of which are still in use.

1. Automatic counting
2. Automatic bin-specific photocell
3. Automatic laser distance meter

Automatic counting

The automated control system counts the pieces dropped into a bin without any bin-specific measuring but using bin selection, conveyor memory and dropping control. The bin bottom is lowered by one step each time the number of pieces set for the step has been dropped into the bin.

If the pieces in the bin become tangled, they will take up more space. This way, the bin can become excessively full. Overfilling is monitored separately by a long-range transmitter/receiver photocell pair that travels across the entire bin row. If the monitoring system detects a piece over the threshold, the line is stopped and an overfilling alarm is activated.

Automatic bin-specific photocell

The filling of bins is monitored by a bin-specific photocell. The upper surface of each bin has a transmitter/receiver photocell pair that specifies the actual level of the timber in the bin.

If the photocell can detect a piece for a certain period of time the bottom of the bin is lowered one step. This method ensures that the standard dropping height is maintained regardless of how the bin is actually being filled. The downside is that the photocell easily becomes dusty and needs to be cleaned regularly.

Automatic laser distance meter

A long-range distance laser meter is placed above the row of bins to monitor whether the bins are full or not. In order to shorten the measured distance, laser meters are placed at both ends of the row. The automated control system monitors the distance from the laser meter to its nearest detected object.

If the distance remains the same for a set period of time, the automated control system refers to the pre-programmed bin distance table to see which bin is concerned and drops that bin one step lower. The downside of this is that the laser alignment needs to be precise. These laser distance meters were first introduced in Finnish sawmills in 2010.

Moreover, the bottoms of vertical bins are monitored by inductive limits. The minimum requirement is to reach the ‘full’ limit. However, you can also have an ‘upper’ limit and an ‘open’ limit. The limit can also be detected via the cogs of a chain sprocket, if there is one. The position of the bin bottom can also be measured using a laser distance meter.