Stenner is to wood saws what Hoover is to vacuum cleaners. The company's
latest fully automated machine - the PK - exploits modern ac drive
technology to the full, and offers remarkable performance improvements
compared with previous product generations
Wood cutting is divided into three main areas of activity. The first
turns trees into planks of wood or rough sawn timber; the second
processes the rough sawn timber and makes it into smaller sawn timber
elements and the final stage sees the material accurately and finely sawn
(to within+/-0.15mm) to provide high quality, finished timber products.
It has always been difficult to achieve accurate cutting and
repeatability at this third stage, but with a bit of help from Mitsubishi
Electric, Stenner believes its new PK series third order cutter has the
edge over the competition.
The PK is twice as fast as previous machines and is capable of reducing
cut waste by up to 70%, thanks to a clever bit of automation that
constantly adjusts the positions of the saw blades. The significance of
reducing cut waste is considerable. Less waste means more wood available
for making product, which translates into bigger profits. Waste
reduction, moreover, has a long term, desirable impact on the environment.
Machine control
Stenner's saws are more likely to be used in factories than timber mills,
the former presenting a cleaner, more efficient environment that demands
a sophisticated approach to machine control. Indeed, the PK's use of
modern ac drive technology has achieved remarkable performance
improvements over previous product generations.
Material is presented to the PK via a continuous driven chain feed
conveyor, the speed of which is adjusted according to the type of wood
being processed - generally 50m/s for softwoods and 20m/s for hardwoods.
Speed adjustment is automatic with precise control provided by a
Mitsubishi A500 variable speed drive (VSD). Cutting speed has been
increased dramatically due to new advances in blade technology, and in
the case of the PK, this is controlled by four A500 drives - two per
blade band and one for each of the two wheels around which the band
loops. The PK blade bands are maintained at a very high tension, which is
controlled and monitored by a Mitsubishi FX2N PLC.
The design utilises the A500´s speed matching function to synchronise
band speed. Different woods need different cutting speeds so, thanks to
the drive, the operator is able to change wood types - and hence cutting
speed - without having to stop the machine. Wood type is selected via the
machine's E910 touch key HMI; once entered, the saw automatically sets
itself for this feed condition, as well as the required cutting thickness.
The saw uses a floating datum point when it is started and then
calculates and adjusts itself to the required sawing conditions. The
position of the saw blade height is controlled using screw jacks, driven
a pulse generator connected to the FX2N´s high speed counter input.
Feedback signals from the drives are also fed to the FX2N PLC using
analogue inputs. The FX2N monitors each drive's frequency and current
consumption, aiding the detection of any problems, errors or inaccuracies
that might possible arise during operation.
Set up, starting and stopping controls (including saw speeds and width
cutting of sawn material) are all performed via the HMI. The large 10.4in
display area and good viewing angles permits the operator to see the
status of the machine from a distance. Stenner has taken full advantage
of the RS422 connection between the HMI and PLC, which allows the HMI to
be mounted and controlled remotely from the control cabinet, for greater
flexibility of operation. Using the PLC to control all aspects of 'start'
and 'stop' also ensures that the PK saw meets mandatory safety
regulations pertaining to starting and stopping procedures.
Sten