Ask people in the forestry industry about the new training sawmill at Waiariki Polytechnic and most agree on one of the most important aspects - controllability. That's because the sawmill at the polytechnic's Timber Technology Campus is fitted out with Microvector drives supplied by PDL Electronics.
Using Microvector drives is quite a break through for the sawmilling industry in New Zealand. Traditionally the heavy log carriages were driven by winches attached to hydraulic motors. However, when Waiariki Polytechnic set out to rebuild its training sawmill after a disastrous fire, educators and industry representatives wanted the new facility to be a showcase for the best and latest in New Zealand manufacturing.
"We chose the Microvector drive system over hydraulics because we want to do research with this sawmill. We require a high degree of control over the head-rig and we can get that with these Microvector drives," says Steve Hemsley, Head of Timber Processing Operations at the TITC (Timber Technology Campus) of Waiariki Polytechnic's Faculty of Forestry.
The Microvector drives are situated in an electrical switchroom, immediately adjacent to the mill. The lack of audible noise in the new system allows the motor and winch to be situated directly under the sawmill operator.
Previously, much noisier hydraulic motors, with their accompanying oily mess, have been situated at the other end of the sawmill, away from the operator. However, there was still a much higher noise level even when the hydraulic machines were at rest.
Steve Hemsley and Tom O'Toole, the TITC's Head of Department (Sawmilling), say the positiveness of the controls on the Microvector drive is a key aspect. They say the stop-start nature of a training mill will put some extra pressure on the electronic system, as in a commercial operation logs would be milled on a relatively constant basis.
The main focus in training is on recovery, grade and yield of timer from logs. The Microvector drive helps to get a higher recovery of timber from the milling processes. "A key factor here is the controllability the operator has over the log going through the saw," says Tom O'Toole. "With the Microvector we can control the sawing operation much more. We can set the parameters and say to a trainee, for example, that we want the log cut to this depth and at this speed."
With an 8 ton carriage and 10 inch wide blade, run by a 75kW motor, the sawmill represents a challenge for trainees learning sawing skills. However, with the Microvector drive the learning process is much more controllable.
"We pushed for the Microvector drive system to get better positioning control of the log carriage," says Norm Agnew, Managing Director of Jaymor Industries (1985) Ltd, supplier of the mill's setworks, scanners, metal detector and SCADA package. Having the Microvector drives in the system allowed Jaymor to apply other forms of control to the system, based on an equation involving feed speed relative to the depth of the cut and sawing deviation.
"We couldn't do that with hydraulics - they are too mushy to control at thigh speeds. Because we have a closed loop system, we are able to accurately position the log carriage and apply travel limits. With hydraulics you can get the log carriages going extremely fast, but the trick is to stop them."
The Microvector drive allows very good control in deceleration, so that higher speeds can be maintained under control. We can accelerate to full speed in about two meters and decelerate and stop within two meters and stop within five millimeters of the physical stops."
The controllability of the system takes away the necessity of the operator to be always trying to stop in the right place and the carriage can be stopped in the right log loading position each time.
"The overall outcome is that we can reduce the cycle time and there will be less wear and tear on the equipment," says Norm Agnew.
Alan Tims, PDL's Senior Applications Engineer who worked on the project, says that when 12 ton of laden log carriage heads for the end of the building running at 180m/min, with minimal friction, the operator must have absolute confidence that the controlling equipment will stop the machine within the specifications.
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