By
Rod Nelson. 2001
Nelson Bros. Engineering
advocates lineally scanned board edgers, so we are
repeatedly asked why we think lineal is better than the more dominant
transverse scanned board edgers. Our initial response
is: Simplicity--Accuracy--Lower initial cost--Lower maintenance cost.
If this were
true, one would think that mills would only buy lineally scanned systems; this
is the case for many new mills, but is rarely the case for upgrades in existing
mills. It is NBE's contention that there is never a valid reason to buy a board
or gang edger with a transverse scanner and a positioning infeed
table.
The most common
arguments against lineal scanning are:
* It won't fit in the mill.
* It is not fast enough.
* It is not accurate.
* It is too expensive.
There are plenty
of board edger configurations to consider:
SCANNERS
* Top only transverse scanner
* Top and bottom transverse scanner
* Transverse scanner with point lasers, point laser plus light curtain or line
lasers
* Single zone lineal scanner
* Multizone lineal scanners
* Lineal scan zones with 2, 3 or 4 heads.
INFEED
TABLES
* Hand fed infeed table
* Linebar infeed
table
* Centering infeed table
* Positioning infeed table
EDGERS
* Conventional edger
* Slewing edger
* Flying edger
OUTFEED
PICKERS
* Individually positioned finger picker
* Stacked pop-up finger picker
* Belt picker
How do you
decide?
Compare a single
zone lineally scanned system with a typical transverse scanned system.
First, a
transverse scanned system measures the board on a scan deck as the board
travels laterally toward the infeed table. The board
is positioned over a feed chain to the desired skew angle and offset.
The board is
lowered onto the feed chain, then carried thru the sawbox. Before the board enters the sawbox
the saws are positioned to make the desired board sizes.
A lineal system
scans the board after it is traveling lengthwise on a scan belt heading toward
the edger. The saws or the saw arbor is set at an angle (usually in the range
of +/- 2 ), then when the board enters the saws the saws are slewed left or
right to make the desired skewed solution. The infeed
for a lineal system can be as simple as a lug chain pushing the board onto a rollcase. The goal of the infeed
is to keep the boards "kind of" straight and appropriately gapped.
The major differences are in the scanner, infeed
table, feedworks, sawbox
and picker. The decks leading to the infeed table and
the outfeed after the picker system are the same for
either configuration.
Scanner
The transverse
scanner must span the maximum board length. For a simple 20 ft. top only
system, the scanner would consist of ten 2 ft. long scan heads and a cabinet
with scanner electronics.
The lineal
scanner must span the board width (18-30 in.) and usually consists of two scan
heads and a scanner cabinet with terminals and a network hub.
Both systems have
the same computer configuration. Typical costs for an NBE scanner and
optimizer: 20 ft. top only transverse--$150,000; 2 head lineal-- $75,000 (all
figures in U.S. dollars).
Infeed Table
The transverse system
needs a positioning table to accurately position the board as directed by the
optimizer. A 20 ft. table has four or five positioners.
The two common positioning techniques are side clamping or top/bottom clamping
the board, then positioning the clamps. Each positioner
usually has three to five air actuators to help complicate matters. The table
usually has plates that lower the board on the chain and overhead pressrolls to keep it from moving around too much. The feedchain has to serpentine around all the positioning
devices and has to track straight. The need for speed and accuracy results in
complicated positioning tables that are a maintenance nightmare.
The lineal system
can be as simple as a lug pushing the board onto a canted rollcase,
which runs the board to linebar, then onto a scan
belt. The lineal infeed table does not need to
accurately position the board, since the scanning is done later. To obtain
better gap control, some systems push the board onto a straight rollcase with overhead pressrolls
to accelerate the board onto the belt. The simplicity of the
lineal infeed guarantees less maintenance and
downtime than with a positioning table.
Typical costs:
positioning table--$150,000; lineal infeed
table--$35,000.
Feedworks
The transverse system is
closely coupled to the edger. The scanner feedchain
usually extends 1 or 2 ft. past the zero lumber line, then
it is into the sawbox.
The lineal system
requires a scan belt to carry the board thru the scanner and into the sawbox. The length of this belt must be about two board
lengths plus one second travel. For a 20 ft. system running 900 FPM, the length
would be 55 ft. The belt must track correctly from the scan zone to the sawbox.
Typical costs:
positioning system--zero; scan belt--$30,000
Sawbox
The lineal system
requires a rotation of the saw guides or the saw arbor and guide assembly. Both
systems require saw positioners, guides, lube, etc.
Typical costs: 4
saw conventional box--$130,000; 4 saw slewing saw box--$150,000.
Outfeed Picker
The finger picker
with individually, positionable fingers is the most
popular configuration on new systems. The finger picker with stacked pop-up
fingers is harder to maintain and usually still requires a positioner
to fine-tune the stack. Finger pickers usually require one more board length of
runway behind the edger. When that length is not available, the mill can resort
to a belt picker. Belt pickers are more expensive and require more maintenance.
A lineal system
does require a lineally positioned picker. Since nearly all new pickers are positionable, there is no additional
hardware costs. When a lineal edger slews in the cut, the resulting
boards are slightly skewed on the outfeed belt. On
finger picker systems, the fingers must be slewed, just like the saws were
slewed. This keeps the center of the finger in the center of the board along
the board's entire length. For belt pickers, the belts are positioned at the
board's center, one-third of the length from the lead end.
The main problem
with all picker systems is that the lead end of the boards occasionally move left
or right as the boards are sawn. NBE is developing a scanner that finds the
lead ends then adjusts the pickers as necessary.
Costs: The
additional programming for the pickers on a lineal board edger costs $1.98.
Controls
The complication
of the positioning infeed table adds costs to the
operators console and the amount of I/O in the PLC. Typically each servo axis
adds $2,000 and positioning tables usually have four or five servo positioners.
Typical costs:
PLC system and console for transverse system--$65,000; PLC system and console
for lineal systems--$55,000.
Startups
The simplicity of
the lineal system is best illustrated by the calibration procedure during
installation. For the lineal system you align the saws to machine centerline, linebar or piano wire.
You calibrate the
scanner to a linebar of fixed reference. After the
belt is tracking, you run some boards, then adjust the
saws (typically less than +/-0.10 in.) to make the cuts match the optimized
solution. I had the first belt scanned system running in three hours after
arriving on site with scanners in my car trunk. Everyone was amazed.
Transverse
systems need to have the positioning table calibrated in addition to the
scanner and saws. All the scanner installation and machine calibration activity
requires the system to be locked out, thus interfering with the PLC controls
startup. It normally takes two days before you get a good optimized board.
Complication
takes time and time costs money: seven day startup of transverse
system--$8,000; five day startup for lineal system--$6,000.
Accuracy
The main
advantage of the lineal system is cut accuracy. The goal is
to measure the board, find the most value board or combination of boards then
make the cuts at the desired positions. Whatever system has the best
scan accuracy and least cut positioning uncertainty is the ultimate winner.
That and piece count are what give you the payback, the ROI and all those other
accounting terms we use to measure success.
The advantages of
a lineal scanner over the transverse scanner are data density and board
stability. Transverse scanners typically collect 20 to 40 thickness
measurements per inch across the face of the board at each laser position. When
these scanners get to the wane, the data density decreases and when the
steepness angle of the wane exceeds 75 , they usually
quit getting data. They collect 100 to 200 data points on a consistent 5 in.
face, but at the edges, where data is important, they quit getting data. To
mask this problem, most transverse scanner systems add a bottom scanner or a
light curtain to at least get the width right. This also adds $50,000 to the
$150,000 top only scanner/optimizer and makes it even more complicated.
The scan heads on
a lineally scan system can be directed at the edges. This gets better data
density where it is needed and less data density on the sawn face.
Also, transverse
systems are plagued with chain vibration, surging encoders and board vibration,
all of which decrease system accuracy. The lineal scanner has none of these
problems.
Cut
Uncertainty
Assuming a good
scan and that the optimizer comes up with the best solution, the board still
has to be presented to saws correctly. For a transverse scanner system, the board
must be positioned on the feed chain at the correct angle and correct offset.
To do this, there must be a positioning table. I like to describe a positioning
table as a mechanism that randomly prevents boards from getting sawn correctly.
Here's why: The
board must first be registered against a reference. For a top/bottom clamping
system the reference is usually the last hook stop or a stop pin. The optimizer
guesses how the board will settle against this reference, then
clamps the board accordingly. If the board does not settle against the
reference exactly as predicted by the optimizer, you get positioning
uncertainty. For side clamping systems, the reference is one of the clamp
faces. This has even greater positioning uncertainty, since the edge crush is
impossible to guess. Now the board has to be positioned, held by overhead press
rolls, released by the clamps, lowered onto a chain going 1000 FPM.
Each step and
device adds positioning uncertainty. To make matters worse, we want this entire
cycle to take 1.5 seconds. The best maintained top/bottom clamping tables have
a +/- 0.10 in. positioning uncertainty. Typical positioning tables have a +/-
0.20 in. uncertainty.
I can not
emphasize enough how difficult it is to install, maintain and troubleshoot a
positioning table. Just ask mills that have done both. Once you have done a
lineal system, you will never choose a positioning table. The board is already
stable on the belt with a lineal system. The belt must track correctly to the sawbox. The only added uncertainty in a lineal saw box is
the timing of saw slewing. This requires the detection of the lead end of the
board and tracking the board to the saws. An abnormally large slew rate of 2
in. over 10 ft. of length would introduce a cut position error of +/- 0.02 in.
if the lead end of the board is misjudged by 1 in. of travel. Since most boards
require a smaller slew rate, the uncertainty due to slewing is usually
negligible.
Calibration
The typical
transverse scanner requires both a thickness and an alignment calibration. The
calibration bar is normally an 2x4 aluminum tube that
is longer than the scanner zone. Two people are needed to handle the
calibration bar. The thickness calibration can be done statically, but the
alignment calibrate requires feeding the bar thru the scanner. The calibration
process is complicated in that it is calibrating from 80 to 300 lasers plus 500
to 1000 light curtain sensors and requires stable transport thru the scanner.
Safe scanner
calibration requires locking out the system to get the calibration bar,
unlocking the system to run the calibration piece, then
locking out the system again to store the calibration bar.
The positioning
table must also be calibrated. Each servo positioner
must be calibrated to a fixed reference and the reference stop for top/bottom
clamping systems must likewise be calibrated. This always requires getting into
unsafe and awkward locations.
The lineal
scanner calibration is a simple static process. The belt is stopped,
the 20 in. to 30 in. calibration piece is slid against a reference of
mounted-on tooling pins. The calibration process is calibrating two or three
scan heads each with one camera and one laser.
The saw
calibration is the same for all types of systems, except the lineal system
includes a rotation positioner.
After all the
scanner, infeed and saw calibrations, you then need
to do dynamic calibration. This is needed because there are too many things
that influence the cut positions that can not easily be calibrated. Dynamic
calibration consists of comparing the optimizer solutions to the final cuts.
Simple offset calibration is easy. You look at 10 boards and if the cuts appear
to be too far to the right you adjust the saws to left. You normally will be
checking width sizes at the same time.
What if there is
skewing error? First they are less obvious and if you have a positioning table
the cause will likely be one of the four positioners,
so the error will only show up on particular lengths.
Maintenance
The transverse
scanner has 20 ft. of windows to keep clean. When a light curtain and/or bottom
scanner is used a pressurized enclosure and blower is needed. Cleaning requires
a lockout procedure and climbing into awkward and unsafe areas.
The lineal scan
heads are to the side of the scan belt. They have two windows per head that can
be accessed from the walkway.
A positioning
table has four to five positioners, 20 to 30 air
actuators, a serpentine chain, four to five pressrolls,
seven to 12 sensors and a lube system. Because of the need for speed, many
actuators have flow controls. It is reasonable to believe that there is always
something not working correctly on a positioning table. No one wants to work on
a positioning table, but every weekend someone is working on a positioning
table.
The lineal system
requires a rollcase and a 50 ft. scan belt. Need I
say more.
The sawbox for a lineal system needs to handle slewing. This is
accomplished by rotating the saw guides or rotating the entire saw arbor and guide
assemble. Both require some additional maintenance over the traditional sawbox.
Actually saw
rotation is not always needed. We have done two lineal systems with centering
tables that have conventional guided saw sawboxes.
With a centering infeed table, most of the recovery
gain is realized with slew rates less than 1 in. in 10 ft. (i.e. one-half
degree of saw rotation). These systems are slewing up to 3/4 in. in 10 ft. with
minor increase in saw maintenance. On a centered system, most of the one board
decisions do not require any slewing. Typically, the saws will slew on one out
of 10 boards. But, a centering table can be as difficult to maintain as a
positioning table.
Troubleshooting
The
troubleshooting procedure is another revealing comparison. What do you do if
the mill manager says, "It just isn't cutting right"? First you
compare the board plots with the actual boards. This will let you know if you
have a scanner problem.
Second you
compare the optimizer solution with the board exiting the sawbox.
If they are not the same solution, you have a data sequencing problem. For
transverse systems, sequencing can get screwed up on the scan deck or at the
positioning table and at the edger infeed. For a
lineal system, there is rarely more than one board between the scanner and the sawbox, so sequencing problems are rarer.
Third, you check
if the cut locations forecast by the optimizer match the actual cuts. On a
lineal system the problem can be due to: poor scanner calibration; poor belt
tracking; improper saw slewing.
For a transverse
system the problem can be due to:
* Poor scanner calibration
* Poor scan due to board vibration or motion in scan zone
* Poor registration at infeed at one or more
stops
* Poor positioning at one or more positioners
* Poor infeed sequencing due to one or more air
actuators
* Board skidding left or right on a not-so-sharp chain
* Misaligned pressroll or feed chain
The complicated
positioning table leads to a common troubleshooting ailment. The millwrights
blame it on the scanner, the electricians blame it on calibration, the QC
people blame in on the mechanical items. The end result is nothing gets fixed.
Future
If cost, simplicity, etc. are not enough to convince you that lineal is best, then consider the future. Future scanners will incorporate biological scanning such as Xray or high resolution color scanning. The cost of these scanners will prohibit their use on transverse systems for many years, while lineal biological scanning is already being used in planer mill application. It will not be long before split, rot and bark detection is added to lineal sawmill scanners.