Note: Descriptions are shown in the official language in which they were submitted.
CA 02323247 2000-10-12
Daniel A. Schoch
DISPLACEMENT BASED DYNAMIC LOAD MONITOR
BACKGROUND OF THE INVENTION
1. Field of the invention.
The present invention relates generally to an apparatus and
method for monitoring press force severity and press load.
Specifically, the present invention relates to a method and
apparatus for monitoring dynamic press load without the use of a
contact force sensor.
2. Description of the related art.
Mechanical presses of the type performing stamping and
drawing operations employ a conventional construction which
includes a frame structure having a crown and a bed and which
supports a slide in a manner enabling reciprocating movement
toward and away from the bed. These press machines are widely
used for a variety of workpiece operations employing a large
selection of die sets with the press machine varying considerably
in size and available tonnage depending upon its intended use.
A press applies force to a workpiece so that the workpiece
(i.e. stock material) acquires the desired geometry corresponding
to the die set being utilized. Systems for monitoring press
operating reliability assist the press owner in evaluating the
impact of certain die/load applications on the reliability of the
press being monitored. Conventional monitoring systems include
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systems which utilize contact load sensors to monitor the peak
load being developed within certain components of the press
machine during a slide stroke of the press. Known methods of
monitoring peak loads utilize an electrical resistance or
piezoresistive strain gage or other transducer which is mounted
on the press and which voltage change due to resistive change
indirectly measures a value of applied load. Monitoring load
exerted on load bearing members during a slide stroke of a
mechanical press allows press and die applications to be adjusted
when monitored peak load values are outside an acceptable range.
What is needed in the art is an apparatus and method to
compute the load on a press without utilizing a contacting load
sensor.
SUMMARY OF THE INVENTION
The present invention provides a method and apparatus for
the identification of dynamic load on a mechanical press which
does not require a contact load sensor.
More specifically, the method and apparatus of the present
invention continually computes a theoretical no load slide
displacement curve while also creating an actual slide
displacement curve during a load condition of the mechanical
press. The apparatus and method of the current invention then
employs a curve matching technique to superimpose these two
curves so that values of dynamic deflection at different points
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in the slide path may be computed. Values of dynamic deflection
are then utilized in conjunction with a constant corresponding to
the static stiffness of the press to calculate load on the press.
The invention, in one form thereof, comprises a method of
generating a theoretical slide displacement curve for a
mechanical press. This method includes the steps of: providing
an equation that can be utilized to calculate slide displacement
as a function of press speed and which includes variables tp
account for press parameters which effect slide displacement;
providing a computational device; determining the speed of the
press; determining the aforementioned equation variables;
communicating the equation, the speed of the press and the
equation variables to the computational device; calculating the
theoretical distance above bottom dead center for each increment
of a slide stroke; and plotting the calculated distance above
bottom dead center values vs. time. The step of determining the
equation variables can further include the steps of: determining
the appropriate variable corresponding to the press drive
mechanism of the mechanical press, determining the appropriate
variable corresponding to the connecting rod length of the
mechanical press, determining the appropriate variable
corresponding to the stroke length of the mechanical press, and
determining the appropriate variable corresponding to the bearing
size of the mechanical press.
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The invention, in another form thereof, comprises a speed
sensor for sensing a value of press speed, input means for
inputting a plurality of variables corresponding to
characteristics of the monitored press, computer storage means
for storing an equation which can be used for generating the
theoretical slide displacement curve, and a computer processor
means for generating the theoretical slide displacement curve.
In this form of the invention, the computer processor means are
communicatively connected to the sensor means, the input means
and the storage means. The equation utilizes the plurality of
variables corresponding to characteristics of the press and the
value of press speed to generate the theoretical slide
displacement curve. The plurality of variables input via the
input means can include a value of connecting rod length, a value
of stroke length, a value of drive type, and a value of bearing
size.
The invention, in another form thereof, comprises a method
of monitoring performance parameters for a mechanical or
hydraulic press. This method includes the steps of: generating
a theoretical no load slide displacement curve, generating an
actual slide displacement curve during a load condition of the
press, determining the contact point on the actual slide
displacement curve which corresponds to the slide contacting the
stock material, establishing a start point on the slide
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downstroke between top dead center and the contact point,
establishing an end point on the slide upstroke between top dead
center and the contact point, identifying the points on the
theoretical slide displacement curve corresponding to the start
point and the end point, identifying the points on the actual
slide displacement curve corresponding to the start point and the
end point, superimposing the identified start points on the
theoretical and actual slide displacement curves, and
superimposing the identified end points on the theoretical and
actual slide displacement curves. In this form of the invention,
the step of generating a theoretical no load slide displacement
curve may further comprise the steps of: providing an equation
that can be utilized to calculate slide displacement as a
function of press speed which equation includes variables
corresponding to press drive mechanism, connecting rod length,
stroke length and bearing size; determining the speed of the
press; determining the appropriate variable corresponding to the
press drive mechanism of the mechanical press; determining the
appropriate variable corresponding to the connecting rod length
of the mechanical press; determining the appropriate variable
corresponding to the stroke length of the mechanical press;
determining the appropriate variable corresponding to the bearing
size of the press; providing a computational device;
communicating the equation, the speed of the press and the
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equation variables to the computational device; calculating the
theoretical distance above bottom dead center for each time
increment of a slide stroke; and plotting the calculated distance
above bottom dead center values for each time increment vs. time.
The step of generating an actual slide displacement curve during
a load condition of the press can be accomplished by monitoring
the displacement of the slide of the press with either a contact
or a non-contact displacement sensor and plotting the monitored
slide displacement vs. crank angle or time. A first inflection
point corresponds to the point at which the slide contacts the
stock material (i.e. the contact point).
The invention, in another form thereof, comprises a method
of monitoring performance parameters for a mechanical press.
This method includes the steps of: generating a theoretical no
load slide displacement curve, generating an actual slide
displacement curve during a load condition of the press,
determining the contact point on the actual slide displacement
curve which corresponds to the slide contacting the stock
material, establishing a start point on the slide downstroke
between top dead center and the contact point, establishing an
end point on the slide upstroke between top dead center and the
contact point, identifying the points on the theoretical slide
displacement curve corresponding to the start point and the end
point, identifying the points on the actual slide displacement
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curve corresponding to the start point and the end point,
superimposing the identified start points on the theoretical and
actual slide displacement curves, and superimposing the
identified end points on the theoretical and actual slide
displacement curves. In this form of the invention, the method
of monitoring performance parameters for a mechanical press
further comprises the steps of: calculating the distance between
the theoretical slide displacement curve and the actual slide
displacement curve at a plurality of increments on the slide
upstroke between the contact point and the end point, calculating
the sum of the distances between the theoretical slide
displacement curve and the actual slide displacement curve at
each increment, shifting the actual slide displacement curve,
calculating the sum of the distances between the theoretical
slide displacement curve and the actual slide displacement curve
at each increment, and repeating the shifting and calculating
steps until the sum of the distances between the theoretical
slide displacement curve and the actual slide displacement curve
at each increment reaches a minimum value.
The invention, in another form thereof, comprises a method
of monitoring performance parameters for a mechanical press.
This method includes the steps of: generating a theoretical no
load slide displacement curve, generating an actual slide
displacement curve during a load condition of the press,
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determining the contact point on the actual slide displacement
curve which corresponds to the slide contacting the stock
material, establishing a start point on the slide downstroke
between top dead center and the contact point, establishing an
end point on the slide upstroke between top dead center and the
contact point, identifying the points on the theoretical slide
displacement curve corresponding to the start point and the end
point, identifying the points on the actual slide displacement
curve corresponding to the start point and the end point,
superimposing the identified start points on the theoretical and
actual slide displacement curves, and superimposing the
identified end points on the theoretical and actual slide
displacement curves. In this form of the invention, the method
of monitoring performance parameters for a mechanical press
further comprises the steps of: determining a value of dynamic
deflection, determining the value of static stiffness for the
press being monitored, providing a computational device,
communicating the value of dynamic deflection and the value of
static stiffness to the computational device, and calculating
load on the press at any point in time by multiplying the value
of dynamic deflection by the value of static stiffness. The step
of determining a value of dynamic deflection includes measuring
the distance along the ordinate between the theoretical no load
slide displacement curve and the actual slide displacement curve
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to determine a difference in displacement between these two
curves. After a value of dynamic deflection is determined, this
value of dynamic deflection may be utilized to calculate load on
the press for any time increment of a slide stroke. The
calculated load for individual time increments may then be
plotted vs. time to establish a load curve for an entire pressing
cycle of the press.
The invention, in another form thereof, comprises a method
of monitoring load on a mechanical press without using a contact
load sensor. This method includes the steps of: determining a
value of dynamic deflection, determining the value of static
stiffness for the press being monitored, providing a
computational device, communicating the value of dynamic
deflection and the value of static stiffness to the computational
device, and calculating load on the press at any point in time by
multiplying the value of dynamic deflection by the value of
static stiffness. The step of determining a value of dynamic
deflection can further include the steps of: generating a
theoretical no load value of slide displacement, generating an
actual load value of slide displacement corresponding to the
theoretical no load value of slide displacement, computing the
difference between the theoretical no load value and the actual
load value of slide displacement, and establishing the difference
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between the theoretical no load value and the actual load value
of slide displacement as the value of dynamic deflection.
The invention, in another form thereof, comprises a method
of monitoring load on a mechanical press without using a contact
load sensor. This method includes the steps of: determining a
value of dynamic deflection, determining the value of static
stiffness for the press being monitored, providing a
computational device, communicating the value of dynamic
deflection and the value of static stiffness to the computational
device, and calculating load on the press at any point in time by
multiplying the value of dynamic deflection by the value of
static stiffness. The method of monitoring load on a mechanical
press without using a contact load sensor in this embodiment of
the current invention further comprises the steps of:
determining a plurality of values of dynamic deflection at
increments of the entire slide stroke, calculating a plurality of
load values corresponding to the plurality of dynamic deflection
values, and generating a plot of load vs. time for a slide stroke
of the press.
The invention, in another form thereof, comprises a speed
sensor for sensing a value of press speed, input means for
inputting a plurality of variables corresponding to
characteristics of the press, storage means for storing an
equation which can be used for generating a theoretical slide
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displacement curve, a computational device for generating a
theoretical slide displacement curve, and a non-contact
displacement sensor for sensing slide displacement during an
actual load condition of the press. The equation stored in the
storage means utilizes a plurality of variables corresponding to
characteristics of the press and the value of press speed sensed
by the sensor means to generate a theoretical slide displacement
curve. The computational device is communicatively connected to
the sensor means, the input means and the storage means so that
the computational device may utilize the equation and its
variables to generate a theoretical slide displacement curve.
The computational device may further be utilized to plot sensed
slide displacement from the non-contact displacement sensor vs. a
count quantity. The computational device may further be utilized
to match an actual load slide displacement curve generated by
plotting the output of the non-contact displacement sensor for a
slide stroke to the theoretical slide displacement curve. In an
effort to match the theoretical slide displacement curve and the
actual applied load displacement curve, the computational device
can be utilized to determine the contact point on the actual
slide displacement curve which corresponds to the slide
contacting the stock material. The computational device may
further be utilized to establish a start point and an end point
on the slide downstroke between top dead center and the contact
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point and the slide upstroke between top dead center and the
contact point, respectively. The computational device may then
be utilized to identify the start point and the end point on both
the theoretical slide displacement curve and on the actual slide
displacement curve and to superimpose the identified start points
and end points so that the theoretical and actual slide
displacement curves can be compared to obtain indicators of press
performance. In this form of the invention, the computational
device may be, for example, a microprocessor. The count quantity
against which the slide displacement is plotted can be, for
example, a measure of time or crank angle.
The invention, in another form thereof, comprises a speed
sensor for sensing the speed of a mechanical press, a non-contact
displacement sensor for sensing slide displacement during an
actual load condition of the press, input means for inputting a
plurality of variables corresponding to characteristics of the
press, and a computational device for computing a value of load
on the press at any point of the slide stroke. The computational
device is communicatively connected to the speed sensor, the non-
contact displacement sensor and the input means. The
computational device is utilized to compute a theoretical no load
value of slide displacement and to compute a value of dynamic
deflection by computing the difference between the theoretical no
load value and the corresponding actual load value of slide
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displacement sensed during an actual load condition of the press.
The computational device then multiplies the thusly determined
value of dynamic deflection by the value of static stiffness for
the mechanical press to determine a value of load on the press at
a point of the slide stroke. The input means may be utilized for
inputting variables including: a value of static stiffness
corresponding to the press being monitored; an equation for
generating theoretical slide displacement values which includes
variables corresponding to press drive mechanism, connecting rod
length, stroke length and bearing size; a value of connecting rod
length; a value of stroke length; a value of drive type; and a
value of bearing size.
An advantage of the present invention is the ability to
accurately match a theoretical no load slide displacement curve
for a mechanical press with an actual applied load slide
displacement curve for a mechanical press.
Another advantage of the present invention is the ability to
compute load on a mechanical press without utilizing a contact
load sensor.
A further advantage of the present invention is the ability
to graph load as a function of time so that it may be utilized to
monitor the operational condition of a mechanical press.
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BRIEF DESCRIPTION OF THE DRAWINGS
The above-mentioned and other features and advantages of
this invention, and the manner of attaining them, will become
more apparent and the invention will be better understood by
reference to the following description of an embodiment of the
invention taken in conjunction with the accompanying drawings,
wherein:
Fig. 1 is a schematic representation of an embodiment of the
load computing apparatus;
Fig. 2 is an elevational view of a typical press which is
the subject of load monitoring;
Fig. 3 is a graphical representation of load vs. time
measurements for different press applications;
Fig. 4 is a graphical representation of an actual slide
displacement curve and a theoretical no load slide displacement
curve;
Fig. 5A is a graphical representation of a theoretical no
load slide displacement curve superimposed with an actual slide
displacement curve and a corresponding force curve representing a
graph of the load experienced during a slide stroke of a
mechanical press; and
Fig. 5B is a graphical representation of a theoretical no
load slide displacement curve superimposed with an actual slide
displacement curve.
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Corresponding reference characters indicate corresponding
parts throughout the several views. The exemplification set out
herein illustrates one preferred embodiment of the invention, in
one form, and such exemplification is not to be construed as
limiting the scope of the invention in any manner.
DETAILED DESCRIPTION OF THE INVENTION
Referring now to the drawings and particularly to Fig. 2,
there is depicted a typical press 22 having a bed 20 with a
bolster 24. Attached vertically to bed 20 are uprights 26 which
support crown 28. Above crown 28 and attached thereto is press
motor 34. Slide 30 is operatively connected so that during
operation, press motor 34 causes slide 30 to reciprocate in
rectilinear fashion toward and away from bed 20. Tooling 32 is
operatively connected to slide 30. Leg members 50 are formed as
an extension of bed 20 and are generally mounted to shop floor 52
by means of shock absorbing pads 54.
Generally, the present invention utilizes a computational
device to continually compute a theoretical no load slide
displacement curve as well as to continually plot an actual slide
displacement curve. The computational device is further used to
employ a curve matching technique to match these two curves so
that operational parameters of a mechanical press may be
determined. Particularly, this information is utilized to
compute a value of load on the press.
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Fig. 1 illustrates one embodiment of the invention wherein a
computational device 12 receives sensed position values from non-
contact displacement sensor 14. Non-contact displacement sensor
14 can be, for example, a hall effect sensor. Computational
device 12 further receives a value of press speed (spm) from
speed sensor 16. Storage means 18 stores an equation which
includes variables corresponding to press parameters which effect
slide displacement such as possibly including the speed of the
press and variables associated with the geometry of the press.
Storage means 18 is communicatively connected to computational
device 12. Input means 10 are utilized to input press parameters
corresponding to the geometry of the press and may additionally
be utilized to input the equation for determining a theoretical
slide displacement curve. Computational device 12 receives input
from input means 10, non-contact displacement sensor 14, speed
sensor 16 and storage means 18 and utilizes this information to
continually generate, during press operation, a theoretical no
load slide displacement curve and an actual slide displacement
curve. These two curves are superimposed one on the other so
that a comparison between the curves may be made to obtain
operational parameters corresponding to the operating state of
the press being monitored. Input means 10 may additionally be
utilized to input a value of static stiffness corresponding to
the press being monitored. Computational device 12 may utilize
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this value in conjunction with a value of dynamic deflection to
compute load at any point of the slide stroke of the press being
monitored.
During press operation, non-contact displacement sensor 14
continually monitors and communicates slide displacement values
to computational device 12. Similarly, speed sensor 16
continually monitors and communicates press speed values to
computational device 12. Prior to press monitoring, an equation
for theoretically calculating slide displacement as a function of
press speed is input into storage means 18. Prior to monitoring,
input means 10 are utilized to enter press variables
corresponding to the geometry of the press as well as a value of
static stiffness (Ketatic~ which has been empirically determined
for the press being monitored.
Computational device 12 continually utilizes speed values
derived from speed sensor 16 in conjunction with the equation
contained in storage means 18 and the press variables input
through input means 10 to generate a theoretical no load slide
displacement curve. Fig. 4 depicts such a generated theoretical
no load slide displacement curve.
Computational device 12 continually receives slide
displacement values from non-contact displacement sensor 14 and
plots an actual slide displacement curve. Such an actual slide
displacement curve is depicted in Fig. 4. Computational device
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12 continually computes both a theoretical slide displacement
curve and an actual slide displacement curve during operation of
the press being monitored. Computational device 12 then employs
a curve matching technique to superimpose these two curves in an
effort to obtain operational parameters of the press being
monitored.
To match the actual slide displacement curve and the
theoretical no load slide displacement curve, computational
device 12 first identifies start point 56 and end point 58 on
both of these curves. Start point 56 is a point on the
downstroke and is chosen as a point on the slide path between
contact point 60 (i.e. where the slide contacts the stock
material) and top dead center. Similarly, end point 58 is chosen
as a point on the slide upstroke between the contact point and
top dead center. To superimpose the actual slide displacement
curve and the theoretical no load slide displacement curve,
computational device 12 matches start points 56 and end points
58. After these two points have been matched, computational
device 12 utilizes a fine tuning method which shifts the actual
slide displacement curve until the sum of the incremental
distances between the actual slide displacement curve and the
theoretical no load slide displacement curve above the contact
point on the upstroke of the slide are minimized. Fig. 5B
illustrates curves matched using this method. In this way, a
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value of load on the press may be continually computed during
press operation so that a load vs. time curve may be generated.
Fig. 3 graphically depicts four load vs. time curves for
different press applications. As depicted in Fig. 3, different
press applications may have the same peak compressive load (L1)
and yet have very different impulse energy values. The value of
utilizing impulse energy as an indicator of press performance is
outlined in pending U.S. Provisional Patent Application Serial
No., 60/15*9,818 the disclosure of which is herein explicitly
incorporated by reference. Since impulse energy provides a
reliable indicator of press operating condition, it is
advantageous that the current invention can continually compute
values of load during press operation. Fig. 5A graphically
depicts a superimposed actual slide displacement curve with a
theoretical no load slide displacement curve as well as a force
vs. slide position curve generated by the method and apparatus of
the current invention. Computational device 12 may be
communicatively connected to a visual display device, an alert
signal, press shutoff signal or a digital storage device which
will store historical data for the press being monitored.
Computational device 12 may further be connected to a modem or
otherwise to a remote source where press operational condition
may be usefully communicated.
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While this invention has been described as having a
preferred design, the present invention can be further modified
within the spirit and scope of this disclosure. This application
is therefore intended to cover any variations, uses, or
adaptations of the invention using its general principles.
Further, this application is intended to cover such departures
from the present disclosure as come within known or customary
practice in the art to which this invention pertains and which
fall within the limits of the appended claims.
