APPLICATION BULLETIN - MAY 1996 | BACK TO INDEX  

PDL drives spell end to Hydraulics

The word is being passed round in the timber industry. Electronic motor drives are IN and hydraulics are definitely OUT.

Motor drives are taking over all the functions previously controlled via hydraulics and a new era of control, accuracy, energy savings and increased profitability is opening up for timber processing plants, whatever their size and whatever the state of their current technology.

And the agreed technology leader both in the South Pacific and overseas is PDL Electronics, with its internationally-acclaimed Microdrive and Microvector series.

Neil Mythen is plant manager of the Forestry Corporation's giant Kaingaroa Processing Plant, half-an-hour's drive south of Rotorua, New Zealand and deep in the heart of the southern hemisphere's largest plantation forest. As manager, he's responsible for a $25 million investment, designed to work round the clock and maximise the value out of every log passed through the plant. And at the heart of this impressive investment are five PDL Microvector drives, controlling key features of the plant's extensive log conveyor system.

However, Neil Mythen is not just a supporter of PDL motor drives because he's running a state-of-the-art processing plant. He's a convinced supporter because, above all else, the drives have proved themselves to be the superior technology in the tough mill environment.

"AC Microvector drives have taken over a dynamic function in the industry that formerly could only be done by hydraulics," he explains. "The problem with hydraulics was that they were never able to deliver the consistent fine control over speed, dynamic stopping and starting and changes in direction that an AC Microvector drive can deliver. That level of control is vital whatever type of plant you're running, but is particularly important when you're operating the sophisticated data-collecting systems we've got here."

"When a log is being scanned to determine the optimum cuts, it's important that factors like speed and direction be completely under control. If you have to jockey the log for the purpose of laser scanning or if you're positioning it for the saws, you don't want it slipping. With traditional hydraulic technology you couldn't get the fine control necessary for a computerised measuring system and you couldn't get a spilt-second controlled stop. Hydraulics can't consistently give you completely stepless control and particularly with wet logs that can mean unwanted log movement which makes really accurate measurement and cutting impossible."

"With the Microvector drives, on the other hand, control is smooth, stepless and repeatable, time after time. Nothing seems to worry them."

The Kaingaroa Processing plant is the first of its type in the world. Although it incorporates a lot of ideas from the big timber merchandising yards of North America, it's unique in many aspects of its design and function and innovative because of its focus on extracting quality and value from every metre of timber.

The design work on the plant, which started production in May 1995, was done by Canadian electrical control engineers, MPM Engineering, who specialise in timber processing plant design back in North America. They had built their experience in the world of hydraulic control and were initially sceptical of the process control offered by the PDL drives.

The Forestry Corp team had no such reservations, having already cut their teeth on an earlier generation of PDL equipment installed in the Corporation's Waipa mill.

Andy Archer, the Kaingaroa plant's systems superintendent elaborates. "We'd already had a lot of experience with PDL motor drives at the Waipa plant, where we used PDL's ASDi drives. As far as the plant sparkies were concerned, electronic drives were the great technology of the future. I can still remember us winding up our first Microdrive to maximum Hertz - the motor was screaming!"

"After we'd installed the motor drives the first thing we noticed was the energy savings - we were using half the energy of the previous hydraulic system because the PDL Microvector drives could control the motors according to demand - and we found the old maintenance problems caused by high loadings simply disappeared."

"But the big improvement was in the area of dynamic control. Hydraulics just can't come anywhere near drives when it comes down to control and repeatability."

The giant Kaingaroa Processing Plant 60 kms out of Rotorua, New Zealand.

The results from installing PDL Microvector drives in the advanced Kaingaroa mill might have surprised the American engineers, but not the Kaingaroa team.

"The Americans were worried the AC Microvector drives wouldn't be able to perform," says Neil Mythen, "but they were over the moon with the results when they saw the drives in action and now they're convinced supporters of drive technology. As far as they're concerned, hydraulics are now a thing of the past."

We move to the soundproof process control booth presided over by grading operator, Dean Lang.

"The drives are essential to the whole grading process," he says. "They make the whole measuring system responsive to fine control."

A PDL Microvector MVi-250 (110kW) drive controls the grading conveyor from the room behind, which also houses the central processing system.

Dean Lang's hands rest on joysticks controlling conveyor speed and direction and the 4-beamed laser scanner operating on 4m tracks in front of the booth. A mirror opposite permits him to see the reverse side of the log and so input all the relevant data about the log, its quality and its economic worth.

This data all becomes the basis for the cutting decisions that will be made at the automated optimisation phase and actioned further down the conveyor track where the cut-off saws are situated.

As the mill team are at pains to point out, a log - or rather, a stem, in Kaingaroa terms (the plant takes the complete tree stem from the forest and these can weight up to 9 tonnes) - carries a lot of inertia. This is where the PDL Microvector drives come into their own, ramping down the conveyor system to a stop, starting, accelerating and decelerating in a stepless curve which keeps the log/stem completely under control.

This control at the grading stage is complemented at another essential stage of the process, the cut-off floor, which contains a similar booth to the grading section with the familiar joystick controls operating the various cut-off functions.

Here too a PDL Microvector MVi-140 (75kW) drive controls the linked conveyors in a carefully choreographed duet as first a stop rises to block the log, a clamp descends to hold it and twin tungsten-tipped high speed circular saws dip in and out, severing the stem into log sections according to the final report from the log optimisation program.

As Andy Archer explains, "We don't want the stem to come crashing into the stop. Apart from increasing wear on the equipment, that takes away the whole point of quality timber recovery, which emphasises accuracy and dynamic control. With the Microvector, the conveyor actually drives under full control up to the stop so that when the stem touches the stop, it is completely stationary. There's no crashing or jarring which you would get with hydraulics. The log can also be driven out of the precise length manually using only the Microvector i-140 as a gauge.

"Apart from the efficiency, accuracy and energy savings of the drive in conveyor and other controlled operations, there's an obvious reduction in maintenance costs due to the absence of shock loadings and so on in the conveyor drive train."

What went into the choice of PDL Electronics-supplied drives over competing brands?

"It was no contest as far as who we got the drives off." explains Neil Mythen. "They had to be PDL drives. There were several competing American drive manufacturers, but they couldn't supply vector drives. PDL could."

"Not only that - but we wanted to buy into reliable service and backup, which is what PDL Electronics have always provided. In the end. you trust the people you have dealt with reliably in the past."