Note: Descriptions are shown in the official language in which they were submitted.
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TITLE OF INVENTION: METHOD FOR VARIABLY CONTROLLING WORK
FEED RATE FOR CUTTING WOOD, METAL AND
OTHER MATERIALS
INVENTOR: H. REID SMITH, GENERAL DELIVERY, KOOSKIA, IDAHO
83539
D E S C R I P T I O N
BACKGROUND OF THE INVENTION
Technical Field. This invention relates to circular saw
and bandsaw machines, and incorporates a method of variably
controlling the rate at which work is fed into the saw blade
based upon performance of the saw blade within predetermined
levels of stability.
Background: Circular saw and bandsaw machines have long
been used as economical means for cutting wood, metal and
other materials. In recognition of the high costs for raw
material and labor, automatic/computer control of work feed
rates and sawing accuracy becomes of paramount importance.
Optimized automatic control of work feed rates and saw blade
stability keeps material and production costs down. The use
of thinner saw blades and smaller rough sawn dimension sizes
can conserve natural resources in the wood products industry,
and reduce material waste in all industries which use circular
saw and bandsaw machines in the manufacturing process. In the
lumber industry, current production methods result in a larger
than necessary amount of waste in order to manufacture
finished dimension lumber. Reduction of this waste requires
the solution of several technical problems.
The first problem is the rate at which the work is fed
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into the saw blade. The work is either manually or
automatically fed into these sawing machines. In manually fed
machines, the operator listens to the sound of the saw blade
and varies the feed rate by judgment, frequently not realizing
that the saw blade is being overfed. Sawing machines having
automatic feed systems, use predetermined feed rates as a
function of thickness of work (depth of cut). These
predetermined feed rates do not consider any variable except
depth of cut. This automatic feed system permits overfeeding
and in some cases underfeeding of the saw machine, resulting
in erratic lateral movement of the saw blade and loss of
control of the saw line. Slower than optimum feed rates are
required to compensate for the variable densities of wood
encountered from summer to winter, density changes within the
same log, partially frozen logs, and the sharpness of the saw
blade cutting teeth. The variables all require on-line
adjustments to work feed rates.
The second problem is the target size of the rough sawn
work, which must be maintained large enough so that finished
lumber is not undersized. This excess material, which is
later removed to produce finish dimension lumber, represents
waste. Uncontrolled lateral deviations in the saw lines
during the cutting operation require larger rough sawn target
sizes. These saw blade movements have several causes: mis-
alignment of saw blade guides, normal saw blade tooth wear,
bending or uneven dulling of saw teeth, and knots in the saw
log. These typical conditions can cause lateral instability
of the saw blade, with resultant deviations of the saw line.
The third problem is offsetting of the saw blade from the
desired saw line. If the saw teeth are dulled by sand, gravel
or other foreign objects embedded within the material being
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cut, offsetting the saw line from minor deviation of 0.005 to
a major of 0.080 of an inch. When a saw blade runs in an
offset condition and encounters a knot, or is substantially
over-fed, it is possible for the saw blade to run completely
out of the work. This ruins the saw blade tension, requiring
hours of bench work to bring the saw back into proper tension
so that it will again cut straight and accurately. If there
are large embedded rocks, or the tree had been spiked by an
environmental terrorist, the saw blade could also
disintegrate, destroying itself and surrounding equipment,
thus requiring down time to repair the damage. The safety of
personnel is also placed in jeopardy if the saw blade
disintegrates.
The fourth problem is the width of the saw cut, or kerf.
Reduction of the saw blade gauge/thickness, and of side
clearance, (the distance the tooth extends beyond the side of
the saw blade body), decreases the width of the kerf. Heavier
gauges and larger side clearance are currently used to protect
the saw from the instability effects of excessive feed rates.
The fifth problem encompasses other considerations that
directly affect optimized cutting efficiency, such as saw
blade design, saw blade strain, and guide pressure.
At the present time, these considerations are being
addressed by using larger rough sawn lumber target sizes,
thicker saw blades, and larger kerf dimensions. Numerous
attempts through the years have been made to solve these
problems, with varied success.
1. Saw blade strain devices such as weight and lever or
high strain pneumatic systems have improved saw blade
performance. Some improvement in saw blade stability has been
obtained, and higher feed rates achieved.
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2. The use of pressure guides provides an additional
increment of saw blade stability. These devices are commonly
used in the wood products industry.
3. A control system utilizing the saw blade sagging
angle in the direction of the work feed was the basis for U.S.
Patent No. 4,437,367, which was issued to Karl Hauser. This
system works well in small bandsaw machines, but will not
function adequately with the larger bandmill machines which
have wider saw blades. This patent applies to bandsaw
machines that hinge and move to the work in lieu of the work
feed system common in larger sawing machines in which the work
is fed into the machine.
4. Utilizing the pressure imposed by the work on the
back of the saw blade to control work feed rates, as in U.S.
Patent No. 3,680,417 issued to John R. Wells, has merit when
using small band mill machines which use throw-away saw
blades. Large band saw machines have blade widths exceeding
two inches, and the same problems exist with this patent as
with the saw blade sagging angle control system.
5. Utilizing a control system as shown in my U.S. Patent
No. 4,644,832, which uses a mean or averaged signal
proportional to the lateral position of the saw blade for
slowing down the depth of cut entry speed. This patent allows
for the work entry feed speeds to be set higher than normal
and the control logic to use "slow down steps" to reduce work
feed speed in the event of unacceptable lateral movement of
the saw blade. However, using this system, once a slow-down
step has been made, the speed remains slowed down for the
entire length of a log or cant being sawn, which lowers
production output. This prior art does not address variable
conditions within the same work piece, such as a log wherein
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the sawing conditions may vary significantly from one end to
another, for example, from the butt end to the top end, as
they relate to material density, sap rings, moisture content
and even temperature.
In addition, my prior U.S. patent No. 4,644,832, does not
take into consideration changes in zero reference signals
which can change, in a 2-hour period, as much as .020 inches
as a result of wear during cutting.
Finally, this prior art patent utilizes only processed
signals, and in the event of significant lateral deviation of
the saw blade, the prior art system does not react quick
enough to prevent damage to the saw blade and/or the work
piece because of the processing time required to condition the
signals.
Accordingly, it is an object of this invention to provide
a saw control system which is responsive to changing cutting
condition so as to optimize saw cutting conditions.
DISCLOSURE OF INVENTION
These objects are achieved in a control system which
continuously monitors the lateral displacement of the sawing
blade from its designed straight line position by means of a
sensor which is installed at a fixed known position relative
to the blade. The signal is processed into an averaged signal
proportional relative to a zero reference point, and thus
proportional to the lateral motion of the blade for any given
period of time. This signal is then compared to a plurality
of reference signals to monitor the lateral position of the
blade, and for purposes of adjusting the feed rate at which
the material being cut is presented to the saw, or the saw is
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presented to the work material, as the case may be. The
reference signals are provided in pairs, with the first being
those proportional to a predetermined acceptable lateral
motion range, followed by a second and third range reference
signals. Since every sawing application is different, the set
points for the reference levels are empirically determined for
the particular application.
In the alternative, the set points can be determined by
using mathematical formulas or fuzzy logic wherein the set
points are determined in relationship to the number of times
each set point is violated, so that a percentage or
mathematical formula sets the violation level so that the
number of violations matches the formula.
The work feed motor has a variable feed rate capability.
An empirically determined table assigns to each depth of cut a
thickness designation and an initial entry feed rate
assignment. These entry feed rate assignments are either
determined empirically or derived from existing tables
published for most particular saw blade configurations. The
greater the depth of cut, the slower the initial entry feed
rate will be.
Prior to the material being cut being presented to the saw
blade by the carriage or handling machine, it is first passed
through a depth of cut thickness measurement device, where the
thickness is measured and compared to the entry feed rate
table and the appropriate initial entry feed rate is selected.
As the material engages the saw blade, the lateral
position of the saw blade is continuously monitored by a
sensor and compared to the acceptable lateral motion reference
signals. As long as the blade motion signal remains within
the first, or acceptable lateral motion set points, to the
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work feed motor will be continuously increased at a rate of
acceleration proportional to how close the saw blade is to
it's zero reference point. Maximum acceleration of the feed
rate and work feed motor will occur when the lateral position
of the saw blade is close to it's zero reference point, and
the rate of acceleration will decrease as the lateral
displacement of the saw blade approaches it's first set
points. Once the saw blade is displaced to a position between
the first and second reference points, or deadband, then no
further acceleration of the feed rate occurs. If the lateral
position of the saw blade moves beyond the second reference
range, the controller will signal the work feed motor to begin
to slowly drop its operating speed. If the lateral position
of the saw blade moves further outside of the second reference
range the rate at which the work feed motor is being slowed
will increase.
If the raw wave signal from the sensor indicates that the
saw blade has moved laterally past a third reference point,
then the controller will signal the work feed motor to
immediately drop in feed rate to a much slower speed relative
to the depth of cut being sawn.
In a like manner, the controller can be used to speed up
or down the blade drive motor and thus provide adjustments for
tip speed for the blade.
The position of the saw blade is also monitored, using the
sensor. During those periods of time when the saw blade is
not cutting, these readings are used to periodically reset the
initial zero reference point so as to compensate for changes
in position resulting from wear on the pressure guide blocks.
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BRIEF DESCRIPTION OF THE DRAWINGS
Fig. 1 is a sectional side view of a typical prior art
band saw blade;
Fig. 2 is a sectional top view of the band saw blade taken
along plane 2-2;
Fig. 3 is a schematic representation of a typical band saw
showing the sensor position;
Fig. 4 is a schematic representation of a typical circular
saw blade configuration showing motion sensor position;
Fig. 5 is a schematic representation of a lateral motion,
raw wave signal for a band saw blade when not engaged in
cutting;
Fig. 6 is a schematic representation of a lateral motion
raw wave signal for a band saw engaged in cutting;
Fig. 7 is a schematic representational block diagram of
the control system;
Fig. 8 is a first schematic representation of the
acceptable band width, and the first, second and third
reference signals and various acceleration and deceleration
rates for control signal for the work feed motor.
Fig. 9 is a second schematic representation of the
acceptable band width, and the first, second and third
reference signals and various acceleration and deceleration
rates for control signal for the work feed motor.
Fig. 10 is a graphical representation of the
interrelationship between lateral displacement of the saw
blade and variable work feed rate.
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BEST MODE FOR CARRING OUT INVENTION
It is an object of the present invention to optimize
production of cutting products. To do this, it is necessary
to understand the saw blade cutting process and to adjust saw
blade rim speed and/or feed speed to ensure that optimal
straight line cuts are achieved at the fastest possible rate.
Reference is made to prior art Figs. 1 and 2. Prior art Fig.
1 discloses a section of a band saw cutting blade 30 which is
formed of body 32 and teeth 34. The dynamics of band saw
cutting will be described in this preferred embodiment in the
context of a band saw cutting dimensional lumber from a log or
cant. However, it should be understood that the dynamics of
cutting with other materials are essentially the same, as are
the dynamics of cutting with a radial saw, as opposed to a
band saw. The processes in the control system described
herein apply equally well to the cutting of other materials,
including metal, polymers, rods, silicates, and virtually any
material which is capable of being cut.
Again, referring to prior art Figs. 1 and 2, ideally, as
the material is being fed into the cutting teeth of the band
saw, tips 42 of teeth 34 chip away at the material being fed
into it, with the chips, which in this example are wood chips,
or sawdust, collecting in the gullet 36 which is the area
defined between adjacent teeth 34. For optimal cutting, the
teeth 34 and gullet 36 should clear or exit the material being
cut just as the gullets are nearing completely full. If the
material to be cut is fed too slowly, the gullets 36 will
remain partially empty, and the saw is capable of cutting or
biting into the material at greater distance during its pass
through the material as it is being cut. If the material to
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be cut is being fed in too fast, gullets 36 will fill before
the adjoining teeth exit the material and, as a result, the
chipped material or sawdust will be forced out of gullets 36
along side body 32 of band saw blade 30 causing frictional
heat and packing the kerf 40 with sawdust.
Although there are a number of different designs for band
saw and circular saw blades, the typical band saw blade used
in cutting dimensional lumber has, as shown in prior art Fig.
2, has swaged teeth with swaged tips 38, thus forming a wider
tip at 38 than the body 32 of the blade. The width of the cut
is called the kerf, and is shown as 40 in the prior art Fig.
2. The purpose of this is to reduce spring back of cut
material and to reduce abrasion of body 32 as it passes
through the material being cut. If the material feed rate
into the blade is too high, and gullets 36 fill, this excess
material will be pushed out along side body 32 and cause rapid
and unacceptable wear and heat deformation. Also, if the
gullets fill too early and material is forced out from the
gullets, it may be forced out along one side of the blade and
not the other, thus causing the body 32 of blade 30 to become
angled within the kerf 40, and thereby cause a deviation from
a straight line cut.
Thus, the two primary factors which must be optimized in
order to optimize the cutting process are the speed of the
blade and the rate at which material is fed into it.
These factors are strongly influenced by the conditions
and overall dimensions of the material being cut. For
example, a typical band saw may be used to cut a typical log
which is, at its base is 18" in diameter, and at its opposite
end, only 12" in diameter. The same log may be much denser at
its base than at its opposite end, dryer at one end or the
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other, or even of a different temperature from one end to the
other. The greater the diameter of the log, the greater the
depth of cut, and for a given fixed saw blade tip speed, the
log must be fed more slowly into the blade in order to
optimize cutting performance. Yet, if a constant speed is
maintained over the length of the entire log, the slower speed
required for optimal cutting at the 18" base may be much
slower than that which would be permissible for optimal
cutting at the smaller diameter, less dense opposite end.
Another set of factors which affect optimal cutting speed
is the condition or characteristics of the material being cut.
With logs, it is not uncommon, at the base of the log, to have
the grain of the wood and the sap rings angling out from the
center line or longitudinal axis of the log. Thus, as the saw
blade passes through the log parallel to the longitudinal axis
of the log, it is encountering alternating rings of wood fiber
and sap rings and, as a result, varying densities of material
to be cut. This can result in unequal forces on the swage
tips 38, which can again displace the blade from its straight-
line path, and cause an angled or non-straight cut.
In a like manner, when the saw blade encounters knots in
the log, the same conditions can occur which cause the blade
to deviate from its straight-line path.
Ultimately it is the displacement of the blade from its
straight line position which results in the non-straight cuts
and, for that reason, in the present invention and as
discussed in this preferred embodiment, it is the detecting of
the lateral displacement of the blade from its designed
straight line position which indicates that optimal cutting
conditions no longer exist.
In the preferred embodiment, as shown in Fig. 3, saw blade
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30 for band saw 44 is stretched around two opposing band saw
wheels 46, and tensioned and guided, in the cutting area, by
means of upper and lower guide blocks 48 and 50. A means for
detecting the lateral movement of the saw blade 30 is provided
in the preferred embodiment. A sensor, 12, which is, in the
preferred embodiment, is an electromagnetic proximity sensor
which senses eddy current, and thus the distance of a sensor
to a metallic object. This is used to detect the lateral
position of band saw blade 30 in the cutting area. In the
preferred embodiment, sensor 12 is an electromagnetic
proximity sensor developing a 50 MH RF envelope which is
affected by the proximity of a moving metal blade. This
proximity sensor 12 can thus generate a signal indicating the
lateral position of blade 30 relative to itself. Proximity
sensor 12 is installed in a fixed, known position and is
attached to the sawing machine frame and/or foundation. In
the preferred embodiment, sensor 12 can be attached to either
of guide blocks 48 or 50, with the preferred attachment
attached to the bottom of the upper guide block holder which
is in a position over the top of the material being cut. An
alternate position is attached to the upper side of the lower
guide block 50. In some cases, when the distance between the
guide blocks 48 and 50 is more than four times the width of
the band saw blade, having both a top and a bottom sensing
means is desirable. The sensor may also be affixed so the
distance below the guide block between the work and the edge
of the saw guide produces a number that represent a one to
five times multiple of the sensed saw blade lateral position
within the work being sawn. This ratio is the relationship
between the sensing means movement and the actual movement of
the saw within the work. In the preferred mounting, this
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requires that the top saw guide block 48 be positioned far
enough above the work so that the sensor 12 obtains superior
representation of saw blade lateral movement within the work.
The sensor means must be mounted so it senses the movement
closest to the saw teeth or just behind the gullets. In case
of a double-cut machine, a sensor is used for both front and
back of the saw blade.
Other means for detecting lateral movement of the saw
blade presently include optical sensors, sonic sensors, laser
sensors, and perhaps, even in some applications where feed
rates are much slower than in logs, mechanical and strain
sensors can be used.
In a circle saw machine application, such as that shown in
Fig. 4, the sensor 12 is attached to the machine frame,
preferably in a radial location equidistant from the entrance
and exit of and adjacent to circular saw blade 52. Sensor 12
is also placed, as with the band saw machine, not more than
one diameter of the sensor behind the gullets of the blade.
When band saw 44 is operating, and not cutting, there may
typically be a small amount of lateral movement of the blade
from its zero reference or stationary position. This
typically will be a sinusoidal type of oscillation of minimal
measurement, and is shown in representational format in Fig. 5
as sine wave 60. This type of signal from sensor 12 is
averaged and used to establish the zero reference point for
the lateral position of the blade. When the band saw is
actually cutting material, its lateral displacement and
oscillations are considerably more complex due to the various
factors discussed above. The raw wave signal, from sensor 12,
would correspond, during cutting conditions, to a path more or
less similar to representational cutting lateral displacement
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path 62, as shown in Fig. 6. This raw wave signal is sent
from sensor 12 to a means for generating a signal proportional
to the detected lateral movement of the saw blade 30, and a
zero reference point primary signal conditioning means 14 as
shown in Fig. 7, in which a proportional electrical signal
representing the reference position of the saw blade is
generated. This primary signal conditioning means 14 can be
incorporated into either sensor 12 or incorporated within the
control unit 16. The configuration may change to accommodate
advances in technology without changing the function or logic
of the invention. This signal can now be used for direct feed
rate logic control and further mathematically calculated into
mean or average position and peak-to-peak vibration, and RMS
values for additional control. These separate signals,
reduced to engineering units of measurement, are now ready to
be used in the control logic.
In the preferred embodiment, the signal conditioning means
14 is used to average the raw signal being sent by sensor 12.
There are, in this preferred embodiment, two averagings
occurring, the first being the average lateral displacement of
the absolute peak valves in both the plus and minus directions
from the zero reference point, and second, an averaging over
time of the continuous signals being received from sensor 12.
In the preferred embodiment, the time averaging occurs
between ten milliseconds to two hundred milliseconds,
depending upon the application. The purpose is to avoid
having the control system react to non-harmful transients,
such as for example, the blade encountering a small knot in
the log.
The control unit 16 can incorporate means such as a
central processing unit, computer, microprocessor or other
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programmable logic controller. The operator access unit 18
allows the operator to examine and change control parameters
and set points. Control unit 16 also serves as a means for
comparing the generating signal to a reference signal and also
as a means for generating a comparative signal as is
hereinafter described.
The control unit 16 contains several pairs of control set
points. As shown in Fig. 8, these set points are on either
side of the zero reference or center position, having both a
positive and a negative setting. The first set points, +1 and
-1, are labeled the range of acceptable lateral motion and are
followed by the next set points the +2 and -2 range, and then
followed by the +3 and -3 set points. Additional ranges can
be added as necessary. Every sawing application is different
and requires set points to be set at different levels. For
example, the +3 or -3 level can be ten to twenty times greater
than the acceptable lateral motion level.
The work feed motor 24 has an infinitely variable feed
rate over a preselected range. The feed rate is responsive and
proportional to the feed rate control signal from control unit
16. In the preferred embodiment work feed motor 24 is also
capable accelerating and decelerating at variable rates within
a selected range which is representationally shown in Fig. 8
as +1000 for the maximum rate of acceleration and -100 for
the maximum rate of deceleration. Although it should be
pointed out that preferred embodiment uses a variable rate of
acceleration and deceleration, the invention will also work
with a work feed motor 24 which accelerates and decelerates at
a fixed or preselected rate.
In the preferred embodiment, control unit 16 is used to
generate a comparative signal which is proportional to how
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close the generated signal of the detected lateral movement of
the saw blade is to the zero reference point and when the
generated signal of the detected lateral movement of the saw
blade is between the zero reference point and one or the other
of the first set points, and when the generated signal of the
detected lateral movement of the saw blade is between one pair
of the second and third set points, proportional to the
distance the generated signal of detected lateral movement
represents is to one of the third set points.
Also, as shown by line 88 in Fig. 8, the rate of
acceleration or deceleration of the feed rate is dependent
upon the lateral displacement or deviation of saw blade 30
from its centerline. The rate of acceleration and deceleration
as shown in Fig. 8 varies proportionately to the comparative
signal and relative to the amount of lateral displacement of
saw blade 30 from its zero reference point or centerline. The
rate of acceleration of work feed motor 24 is maximum when
there is no lateral displacement, and gradually decreases to
no acceleration as the lateral displacement reaches one of the
first set points. Deceleration is also proportional with
maximum deceleration of work feed motor 24 occurs when the
lateral displacement of saw blade 30 approaches one of the
third reference points and gradually reduces the rate of
deceleration as lateral displacement of saw blade 30
approaches the second reference point. There are a number of
different methods of generating the comparative signal and
establishing this proportionality ranging from the
establishment of empirical tables to the use of integral or
derivative signals derived from the lateral displacement
signals from sensor 12 and/or control unit 16.
In the alternative, the set points can be determined by
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using mathematical formulas or fuzzy logic wherein the set
points are determined in relationship to the number of times
each set point is violated so that a percentage or
mathematical formula sets the violation level so that the
number of violations matches the formula over time.
As long as saw blade 30 remains within range defined by
the first set points, +1 and -1, the control signal from
control unit 16 will keep increasing the feed rate of work
feed motor 24. It will accelerate at or near its maximum rate
as long as saw blade 30 remains at or near the centerline or
zero reference point. As the lateral displacement of saw
blade 30 nears one of the first set points, +1 or-1, the rate
of rapid acceleration decreases and stops as the lateral
displacement of saw blade 30 passes the first set point and
into the range between a first and second set point. In the
preferred embodiment, once the lateral displacement of saw
blade 30 enters into the range between the +1 to +2, or
between -1 to -2, there are no further increases in the feed
rate.
In a like manner, if the lateral displacement of saw blade
30 exceeds either of the second set points, +2 or -2, the
control signal from control unit 16 will first begin a gradual
or slow deceleration, and will increase to the maximum
possible rate of deceleration as the lateral displacement of
saw blade 30 approaches either of the third set points, +3 or
-3.
In the preferred embodiment, if the lateral displacement
of saw blade 30 as represented by the raw wave signal exceeds
a third set point, control unit 16 will stop work feed motor
24 to hopefully prevent damage to saw blade 30, and its
operator, if, for example, saw blade 30 were to encounter a
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large rock, or even a metal spike embedded in the log when it
was a standing tree by an environmental terrorist.
The cutting cycle for the work, which as the example being
used in this best mode section is a log, begins with log 28
being loaded on to a feed carriage or other material handling
device movable into and out of saw blade 30 by work feed motor
24. An empirically determined table assigns, to each depth of
cut, a thickness designation and an initial entry feed rate
assignment. These entry feed rate assignments are either
determined and set empirically or derived from existing tables
published for most particular saw blade configurations. The
greater the depth of cut, the slower the initial entry feed
rate will be. Along with the entry feed rate, the maximum
feed rate and the maximum rates of acceleration and
deceleration are also preselected, so that the greater the
depth of cut, the lower the maximum feed rate and the rates of
acceleration and deceleration, and the smaller the depth of
cut, the greater the rates of feed and acceleration and
deceleration.
Alternatively, and as some saw mill operators prefer to
do, the entry feed rate may be manually determined by the
sawyer, in which case the maximum feed rate, and rates of
acceleration and deceleration, are determined by using the
manually selected entry feed rate selected by the sawyer as
the initial entry set point for the work feed rate control
signal generated by control unit 16. It doesn't really matter
which method of determining entry speed is used since the work
feed rate will rapidly change dependent upon sawing
conditions.
In some cases where manually determined entry feed rates
are used by the sawyer, a sensor may be positioned on the
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control handle used by the sawyer to establish a high and low
threshold set point for determining the maximum feed rate
increase that will be permitted by the control system. That
is to say, if the entry rate manually determined by the sawyer
is below the set point, then one hundred per cent acceleration
as is shown by curve 88 of Fig. 8 will be permitted. However,
in the case that the sawyer sets an entry feed rate above the
threshold, only fifty per cent acceleration will be permitted,
as represented by curve 92 of Fig. 8. In other words, the
control system is configured if the sawyer selects an entry
feed rate which is higher than a predetermined set point.
In a depth of cut feed rate sawing machine application,
the normal continuous sequence of events for the controller
system as follows:
The work, which as previously stated in this description
of the Best Mode is a log or cant 28, passes through a depth
of cut thickness measurement means 70 prior to feed entry into
the saw blade so that its thickness is measured and compared
to a predetermined entry feed rate table and the appropriate
output value is sent to the work feed motor 24 to move the log
or cant into the saw blade at the appropriate predetermined
speed. The depth of cut or thickness measurement means 70 is
typically a set of optical sensors, or other devices, all of
which are well known in the art.
As log or cant 28 engages saw blade 30, the lateral
position of saw blade 30 is continuously sensed by sensor 12
and compared to the reference signals, +1 and -1, identified
as the acceptable lateral motion signals in Fig. 8. These
reference signals are proportional to the blade motion signal
that would be generated at predetermined acceptable lateral
movement of the saw blade.
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The averaged blade-motion signal is then compared to the
zero reference signal and the acceptable lateral motion or
first reference signals. If the blade motion signal remains
within the acceptable lateral motion reference signals for a
period of time, for example, twenty-five milliseconds or one
hundred fifty milliseconds, then the output to the work feed
motor 24 will be rapidly increased. This loop continues at
specified time intervals as long as the lateral position of
the saw blade, as indicated by the generated blade motion
signal, is in the acceptable lateral motion range. As the
lateral displacement of saw blade 30 approaches either of the
first reference signals, the rate of increase in speed slows
down, and as the lateral displacement of saw blade 30 crosses
either of the first set points, +1 or -1, acceleration stops,
and the control signal from control unit 16 remains constant,
thus controlling work feed motor 24 at a constant speed.
As long as the lateral position of the saw blade remains
between the acceptable lateral motion set points of +1 or -1
and the +2 or -2 range, the feed rate remains at its obtained
setting. However, it is not necessary to do this. And in a
second embodiment of the invention, in lieu of the hold
constant speed range, it is possible to immediately begin
gradual deceleration to bring the lateral displacement of saw
blade 30 back within the first reference signal. This
configuration is not preferred since it may result in control
signal hunting, and thus increases the wear on work feed motor
24, but it can be used.
If the lateral position of the saw blade, as indicated by
the generated blade motion signal crosses beyond the either +2
or -2 set points, the control unit 16 will send the
comparative signal to the work feed motor 24 to begin slowly
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dropping its operating speed. If the lateral position
deviation continues to increase, the rate at which work feed
motor 24 decelerates will increase. As the lateral deviation
of saw blade 30 begins to return towards the second set point,
rate at which the feed carriage is decelerating. This process
will continue until lateral saw blade motion drops within the
range between the first and second set points at which time
the control signal will remain constant.
In a third embodiment two different sets of control signal
acceleration tables are used. In the case of increasing feed
rate speed and increasing lateral displacement, the
acceleration table, as represented by line 88, is the same as
the for the first preferred embodiment. However, as is shown
in Fig. 9, a different acceleration table, as shown by line
90, is used for conditions when lateral displacement is
decreasing and eventually reduced to the either of the first,
+2 or -2, set points. In this embodiment, instead of holding
feed rate constant, as control is being restored to the
lateral displacement of saw blade 30, as it returns to within
the second set point, it is instead immediately increased. It
has been found in practice that immediately increasing speed
helps in recovery of saw blade 30 to within the acceptable
lateral displacement range. The reason for this is that with
saw blade 30 slowed down the gullets are being filled with
extremely fine or small wood chips and dust which can spill
out of the gullets and again laterally displace the blade. By
immediately speeding up saw blade 30, the chips in the gullet
are larger and thus do not fall out of the gullet into the
kerf .
The primary signal conditioning means 14 also monitors and
maintains the raw wave signal being provided from sensor 12,
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without averaging over time, but rather in a peak-to-peak
absolute value configuration. If, in the preferred
embodiment, the raw wave signal passes over the value of the
third, +3 or -3, reference signals, then controller 16 will
signal the work feed motor 24 an immediate stop or slow to a
very low predetermined speed. In this manner, control unit 16
adjusts speed cycle up or down at varying rates of
acceleration, but can also move from the maximum speed for any
particular depth of cut to a stop signal in one jump if the
raw wave signal exceeds the third reference value. This is
done as a safety precaution, since the raw signal can be
processed much quicker than the time averaged blade motion
signal.
In this manner, the control system operates within the
established set points to cycle the feed speed up or down
according to the lateral position of the saw blade
In a like manner, control unit 16 can be used to control
the speed, up or down, of saw blade drive motor 20 and thus
provide adjustment for the tip speed for blade 30.
Another control consideration, depending upon the
application, is the optional monitoring of both the work feed
motor load monitor 26 and saw blade motor load monitor 22 to
monitor load amps or horsepower usage. Set points for maximum
amps can then be used to either inhibit feed rate increases or
drop the feed, and in a like manner inhibit or increase blade
speed. If a chipper or Blabber is utilized in series with the
saw blade assembly, a load measurement means can also be
provided to control the speed and feed rate of these machines
in the same manner as the saw blade and feed rate motors.
Fig. 10 is a representational graph which compares lateral
displacement of saw blade 30 as shown in line 82 to time,
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given in tenths of seconds, and to feed rate, given as a
percentage of maximum feed rate for any given depth of cut,
using a stepping signal to motor 24 for work feed speed, on
line 84, and the work feed signal to motor 24 of the present
invention as shown by line 86. Fig. 10 is not necessarily
drawn to scale, but rather is meant to illustrate the
advantages of the present invention. The line 84 showing the
feed rate for stepping feed signal to motor is representative
of the feed rate control system of my United States Patent
Application Serial No. 08/569,518 filed December 8, 1995 and
entitled METHOD FOR CONTROLLING WORK FEED RATE FOR CUTTING
WOOD, METAL AND OTHER MATERIALS. At time 0.0, the log 28,
used as an example in this description of the first preferred
embodiment, initially enters the saw blade 30, which upon
entry is its zero reference set point or central line. With
the stepping motor the control system senses that lateral
blade displacement is within the acceptable range, and at time
.4 seconds the control system increases the feed rate one
step, which causes lateral displacement of saw blade 30 to
increase to between the first and second reference points, as
shown in line 84. At time 1.2 seconds, saw blade 30
encounters a hypothetical knot in log 28 and is laterally
displaced from the +1 to +2 range to between the -2 and -3
range of lateral blade displacement, thus causing the feed
rate for the stepping motor to drop an immediate three steps,
which restores lateral displacement to the range of between -1
and -2, thus causing the feed rate to remain at a constant
slower speed. As saw blade 30 continues to recover from its
laterally displaced position, beginning at 2.2 seconds, the
stepping motor of line 84 is gradually stepped up in speed
back to its original entry speed and eventually four steps
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beyond it to obtain a maximum feed rate at 3.6 seconds.
However, the control system of the present invention
reacts more quickly, as is shown by line 86. At .2 seconds,
upon entry of the log 28 into saw blade 30, since there is no
lateral saw deviation, an immediate, rapid acceleration occurs
until at .4 seconds the lateral displacement of saw blade 30
is in the +.l to +.2 range and remains there until 1.1 seconds
when it encounters the hypothetical knot. At this time, the
lateral displacement for saw blade 30 is in the -2 to -3
range, and as a result, the feed rate is rapidly decreased,
and as can be seen, as the lateral displacement begins to
recover to the -1 to -2 range, the rate of deceleration is
slowed and the work feed rate levels out at approximately the
same level as it would with the stepping signal, as shown in
lines 86 and 84.
The advantage of the present invention is that when the
lateral displacement of saw blade 30 recovers into the
acceptable range, the work feed rate is rapidly and smoothly
accelerated, and continues its rapid acceleration until the
lateral displacement approaches the +1 set point, at which
time the acceleration slows, and the feed rate gradually
approaches a constant speed when lateral displacement of saw
blade 30 exceeds the +1 reference set point at 3.6 seconds, as
shown on line 86. The net effect of the variable acceleration
and control of the present invention has a quicker reaction
time to improving sawing conditions, and thus operates at
higher speeds more of the time than would otherwise occur if a
stepping motor or other control system were used. In
practice, 5% increases in production capacity for any given
mill may be realized by use of this new system.
Another feature of the present invention, found in all
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three of the preferred embodiments, is the fact that the
proximity sensor control system can be used to control the
return rate of the carriage once a cut has been completed on
the log or its derivative cant. On the return of the
carriage, the rotating saw blade 30 travels back down along
side the kerf surface as that portion of the cant remaining on
the carriage is withdrawn from the saw. During the return
portion of the cycle, the blade hopefully travels along side
the remaining side of the kerf and does not encounter the
sides of the cut work. High speed return of the carriage
saves significant time of operation since the log or cant may
be sawn several times before it is unloaded off the carriage.
However, a high speed return is restricted without automatic
speed control by the possibility of a saw being snagged or
hooked by a sliver from the log or cant and thus the
possibility of pushing the saw blade off the wheel, which not
only wrecks the saw blade, but is a dangerous situation for
saw mill personnel. As a result, continued monitoring of
lateral displacement of saw blade 30 during the return of the
carriage provides override control of the manual feed control
for these emergencies, and is critical for safety. The
ability to control carriage speed during the return phase of
operation is a part of this control system. In the case of a
double cutting sawing machine, that is to say in a saw where
sawing also occurs as the feed carriage is returned, sawing is
controlled in both cutting directions as previously described.
As normal wear occurs on upper and lower guide blocks 48
and 50, the distance between sensor 12 and band saw blade 30
can change. This wear can easily amount to as much as ten
thousandths of an inch per hour. This type of wear can alter
the position of the zero reference set point, and, in many
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applications, can drastically affect the effectiveness of the
calibrations of the first, second and third reference points.
To automatically compensate for this wear, the zero reference
point is periodically recalibrated, and thus the values of the
acceptable lateral motion, first, second and third reference
points are updated. This is accomplished by monitoring the
position of the saw blade, using sensor 12, during those time
periods when material is not being fed into the saw and the
saw is not cutting. At the beginning of the work cycle, this
first detection of location of the saw blade, when not engaged
in sawing material, is used to establish the initial zero
reference point. Thereafter, in the preferred embodiment,
each time the saw is not working or engaged in sawing
material, its location is monitored by sensor 12 and saved in
controller 16. Averages of the stored readings are then
taken, in the preferred embodiment, every five minutes. This
average is then compared with the readings of the then current
zero reference value, and if different, the zero reference
value is reset as needed.
While there is shown and described the present preferred
embodiment of the invention, it is to be distinctly understood
that this invention is not limited thereto but may be
variously embodied to practice within the scope of the
following claims.
