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BACKGROUND OF THE INVENTION
This application relates in general to systems for assembling components
and in particular to a system for sensing and forming leads of components.
This application also relates to optical systems for illuminating objects
and the sensing of objects.
Industrial robots are increasingly being used for assembly work in
manufacturing. In one area of assembly work, leads of electronic
components are to be inserted into holes on printed circuit boards. When
the leads are separated spatially by the correct distances so that they
can fit simultaneously into the holes, the leads are said to be in the
"drop-in" condition. Frequently, however, the leads are not in the drop-in
condition for a wide variety of reasons. The leads may have been bent
during handling, shipping or storage or they may not have been formed
properly in the first place. If the leads are not in their proper drop-in
condition, it will generally be difficult for a robot to insert the leads
of the component into holes on printed circuit boards.
A number of solutions to the above-described problem have been previously
proposed. In "Vision System Aligns Leads for Automatic Component
Insertion" by Asano et al. in Assembly Automation, February 1983, pp.
32-35 the solution proposed is to cut the leads to different lengths. The
longer lead is then aligned with its corresponding hole and inserted
therein. A shorter lead is then aligned with its corresponding hole for
insertion. If the spacing between the two leads inserted is not the same
as the spacing between their corresponding holes, one of the two leads may
be bent while the shorter lead is being aligned with its hole. Essentially
the same solution has been proposed by Klass in "Circuit Board Assembly
Plant to Test Systems" in Aviation Week & Space Technology, Aug. 2, 1982,
pp. 66-68, by Sanderson and Perry in "Sensor-Based Robotic Assembly
Systems: Research and Applications in Electronic Manufacturing",
Proceedings of the IEEE, Vol 71, No. 7, July 1983, pages 856-871 and by
Murai et al in "Automatic Insertion of Electronic Components by Optical
Detection of Lead Positions", Fourth International Conference on Assembly
Automation, pages 390-399.
The above described approach is not entirely satisfactory since it may not
be suitable for components with many leads. Furthermore, it takes more
effort to prepare these staggered leads. These leads remain staggered
after insertion on the other side of the printed circuit board, so that
they are non-uniform in appearance and cause quality soldering more
difficult.
Another popular approach uses special fixtures into which a robot inserts
the leads. All the leads are then bent to one side and then another. After
the leads are released, most leads will move back by short distances
towards the original positions. Such motion of the leads is known as
"spring-back". If the leads have been bent beyond their elastic limit or
yield points, they will stop short of moving back to their original
positions. Distances for which the leads will move back are determined by
many factors referred to below as the spring-back characteristics of the
leads. Thus, even though all the leads have been bent to the same
positions on one side and then the other, they may move back by different
distances so that the lead spacings are still different from those in the
drop-in condition.
Yet another conventional approach is to use a specialized gripper which
pushes leads inward from an initially outward sprung condition. Where a
large number of different types of components must be inserted, a large
number of special fingers and grippers may have to be designed for their
insertion which may be difficult and uneconomical.
None of the above-described approaches are entirely satisfactory. It is
therefore desirable to provide a system for sensing and forming leads so
that the leads are in the drop-in condition.
In order to detect the positions of the lead tips, appropriate lighting is
necessary so that the cross section of the lead tips reflect light
uniformly and a clear visual image can be obtained. In "Automatic
Insertion of Electronic Components by Optical Detection of Lead Positions"
by Murai et al., Fourth International Conference on Assembly Automation, a
circular fluorescent lamp is proposed as the light source for illuminating
lead tips. Even when such a source is used, it may be difficult to detect
the positions of the lead tips. Thus, as described by Murai et al. in the
above article, the body of the electronic component may come out as
background in a photograph, particularly when the component body is highly
reflective of light. To screen out the noise caused by this background
Murai et al. developed an algorithm for obtaining the optimum threshold
level for light sensing. Thus, while a circular lamp may be advantageous
for some reasons, it has disadvantages since it also illuminates the body
of the electronic component which causes background noise. It is therefore
desirable to provide other illuminating devices to alleviate such
difficulties.
SUMMARY OF THE INVENTION
This invention is based on the recognition that the positions of the leads
can be matched to the positions of the holes to discover if any leads need
to be bent in a process referred to below as fine-forming. Thus, one
aspect of the invention is directed towards an apparatus for determining
whether any leads of a first component having two or more leads need to be
bent in order to fit into predetermined holes in a second component. The
apparatus includes means for sensing the positions of the tips of the
leads of the first component. The apparatus further includes means for
comparing the positions of the leads to the positions of the predetermined
holes and for determining from the comparison which leads, if any, need to
be bent in order for all the leads of the first component to fit
simultaneously into the holes.
After establishing that one or more leads of the first component need to be
bent, such lead or leads must then be bent so that all the leads will fit
simultaneously into the holes. Thus, another aspect of the invention is
directed towards an apparatus for bending a lead of the first component,
said component having at least one other lead. The apparatus includes
means for detecting the position of a portion of the lead to be bent, and
for generating an output signal to indicate the position of the lead
portion. The apparatus further includes a controller means for producing a
control signal in response to the detecting means output signal and a
bending means for bending the lead in response to the control signal so
that, after bending, all the leads of the first component can
simultaneously fit into the predetermined holes.
Another aspect of the invention is based on the observation that the
spring-back characteristic of each lead to be bent can be measured in the
bending process itself, so that the proper amount of bending can be
applied to assure that the final position of the leads is within a given
tolerance of its drop-in position after the bending process has been
completed. The spring-back characteristics of a lead can be sensed by
detecting both the force on and the displacement of the lead caused by the
bending during spring-back. Thus, another aspect of the invention is
directed towards an apparatus for bending a lead of a component comprising
a bending means for bending the lead of the component in response to a
control signal. The apparatus further includes detecting means for
detecting the force on the lead by the bending means and its displacement
and producing an output signal, and a controller means for producing the
control signal in response to the detecting means output signal for
controlling the bending means.
Still another aspect of the invention is directed to a device for
illuminating a region, such as the region of the lead tips. The device
comprises means for providing light in substantially isotropic directions
towards said region from locations substantially coplanar with the region
so that only the portion of an object in or near the plane of the region
and the locations is illuminated without illuminating the remaining
portion of the object.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a perspective view of a system for detecting the positions of the
leads on an electrical component, for fine-forming the leads where
necessary and for inserting the leads into a printed circuit board to
illustrate the preferred embodiment of the invention.
FIG. 2 is a flow chart to illustrate the steps for controlling the system
of FIG. 1.
FIG. 3 is a flow chart illustrating an algorithm for identifying which
leads, if any, need bending to illustrate the preferred embodiment of the
invention.
FIGS. 4A-4D are schematic diagrams which together with the flow chart of
FIG. 3, illustrate an algorithm for finding which leads need bending.
FIG. 5 is a simplified perspective view of a portion of the system of FIG.
1 to illustrate the fine-forming of a lead.
FIG. 6 is a graph of the force exerted on the lead by a bending means as a
function of the displacement of the lead to illustrate the steps for
bending the lead to a desired target position.
FIGS. 7A, 7B together comprise a flow chart showing the steps of
fine-forming a lead to illustrate the preferred embodiment of the
invention.
FIG. 8 is a simplified perspective view of an illumination system suitable
for use in the system of FIG. 1 in which background optical noise is
reduced to illustrate the preferred embodiment of the invention.
FIG. 9 is a perspective view of an illumination system suitable for use in
the system of FIG. 1 to illustrate an alternative embodiment to that shown
in FIG. 8.
FIG. 10 is a cross-sectional view along the line 10--10 in FIG. 9.
FIG. 11 is a schematic view of a simple telecentric system to illustrate
the invention.
FIG. 12 is a schematic view of a compound telecentric system to illustrate
the preferred embodiment of the invention.
DETAILED DESCRIPTION OF THE INVENTION
FIG. 1 is a perspective view of a system for detecting the positions of the
leads on an electrical component, for fine-forming (defined below) the
leads where necessary and for inserting the leads into a printed circuit
board to illustrate the preferred embodiment of the invention. As shown in
FIG. 1, system 20 comprises a robot 22 having a robot arm 24 with fingers
26 for handling and gripping electrical component 28 with leads 30.
Fingers 26 of robot 22 first pick up a component 28 from a feeder 32 which
may be located on a cassette 34. As shown in FIG. 1, feeder 32 is such
that the components therein are all located in substantially known
orientations so that when the fingers 26 of the robot grip the body of a
component, the orientation of the component is known with respect to the
frame of reference of the robot. While this is the case in the preferred
embodiment, it will be understood that other feeders may be used which do
not cause the components to be in a known orientation with respect to the
robot. Existing image processing methods can then be applied to detect the
orientation of the components with respect to the robot.
Leads 30 of the component 28 have been preformed. In other words, leads 30
have been cut and or bent in such a manner that they are either in the
drop-in condition or close to it. If the leads are already in the drop-in
condition, no further work needs be done and the robot may simply insert
them into the printed circuit board. If they are not, then further work
will need to be done on them in a process referred to below as
fine-forming.
The positions of the lead tips of leads 30 are sensed using an optical
sensing system comprising a light source 42, a light detector such as a
camera 44 and a second optical illumination and detection system 50. The
positions of the holes 52 on boards 40 into which leads 30 are to be
inserted are sensed by means of an optical sensor such as a camera 54.
While it is preferable for the tips of the leads to be coplanar, they do
not need to be. The same is true for the holes on each board. In the
preferred embodiment holes 52 are illuminated by a source (not shown in
FIG. 1) above board 40 on the opposite side of camera 54. The positions of
the lead tips sensed by detectors 44 and system 50 and the positions of
the holes 52 sensed by camera 54 are all supplied to controller 60, where
the positions of the lead tips are compared to those of holes 52. From the
comparison controller 60 determines whether the tips of leads 30 can be
brought to match exactly holes 52; if they do, the leads are then ready to
be inserted into holes 52 without any need for bending the leads either
before or during the insertion process. When in such condition, the leads
are in a condition known as the drop-in condition. Frequently, however,
the leads may be bent during storage or handling or they may not have been
formed properly in the first place. For these reasons, one or more of
leads 30 may have to be bent in order for their tips to be in the drop-in
condition. Once the leads are in the drop-in condition, the insertion
process can be done automatically and conveniently by the robot.
Where one or more of the leads 30 need to be bent to achieve the drop-in
condition, controller 60 compares the positions of their tips to the
positions of holes 52 sensed by detector 44, system 50 and camera 54 to
determine which lead or leads need to be bent, the distances for which
they must be bent and the directions for bending to cause leads 30 to be
in the drop-in condition. After the controller determines which leads need
to be bent, and the distances and directions for bending such leads,
controller 60 causes robot 22 to move component 28 towards a bending and
sensing mechanism 70 comprising a bending member 72, and a force sensor 74
to which the bending member is attached. The force sensor is in turn
attached to a X-Y table for moving the force sensor and bending member in
two mutually perpendicular directions. As shown in FIG. 1 the X-Y table
comprises base 76 attached to two cross-slides 78 for moving the bending
member 72 and force sensor 74 in two perpendicular horizontal directions
X, Y. It will be understood, however, that other conventional
constructions for the X-Y table may be used and are still within the scope
of the invention. The movement of the cross-slides 78 is controlled by a
motor 82 which is in turn controlled by the controller 60. Controller 60
causes motor 82 to drive cross-slides 78 so that the bending member 72 is
moved in the X and Y directions for the desired distances for bending
leads 30.
When leads 30 are in or are rendered in the drop-in condition, robot 22
translates and rotates component 28 to a position above board 40 until the
tips of leads 30 are above holes 52. The robot then inserts leads 30 into
their corresponding holes and the insertion process is then completed.
Boards 40 are fed by a mechanism (not shown in FIG. 1) on to two rails 84,
86. The mechanism moves each board 40 until it is above camera 54 for
detecting the position of holes 521. After all components have been
inserted into board 40, the board is moved away and the next printed
circuit board is moved to take its place so that the process can be
repeated.
The operation of system 20, including robot 22, is controlled by controller
60. Some of the connections between other components of system 20 and
controller 60 have been omitted or truncated to simplify FIG. 1. FIG. 2 is
a flow chart to illustrate the steps by which controller 60 controls
system 20. The system is first initialized (block 100). If there is an
unloaded board system 20 waits until the board is loaded onto rails 84, 86
(diamond 102, block 104). The loading can be performed either manually or
automatically in a conventional manner. If there is no unloaded board the
system simply loops upon itself and waits until an unloaded board is
available. In such manner a board 40 is loaded onto rails 84, 86 above
camera 54; which is referred to in FIG. 2 as the current board.
Controller 60 then checks to see if a component remains to be loaded into
the circuit board (block 106). If a component still has to be loaded,
controller 60 causes robot 22 to acquire a component from feeder 32 by
means of fingers 26 (block 108). Controller 60 then causes robot 22 to
move the component 28 to a position between the illumination source 42 and
detector 44 and causes the illumination source 42 to illuminate the
component. The illumination source (not shown in FIG. 1) for illuminating
board 40 is also turned on. The approximate height of the tips of leads 30
below fingers 26 of the robot are detected by detector 44. The positions
of holes 52 are also detected by sensing system 54 (block 110). Robot 22
moves the component 28 into an illuminating and sensing system 50 for
detecting the positions of the tips of leads 30 (block 112). Controller 60
then compares the positions of the lead tips to the positions of the holes
52 to determine whether the leads are in the drop-in condition. If they
are, then fine-forming can be avoided; if not, fine-forming is then
performed (diamond 114, block 116). In either case leads 30 are then in
the drop-in condition and controller 60 instructs robot 22 to insert leads
30 into holes 52 (block 118). Excess protruding portions of the leads are
then clinched and the insertion of all leads are then confirmed in a
conventional manner (blocks 120, 122). The assembly process is then
completed. The operation of robot 22 and the transformation between
different reference frames of the robot and those of the optical and force
sensing components of system 20 is conventional. An explanation of robot
operation and frame of reference transformation may be found, for example,
in Robot Manipulators: Mathematics, Programming and Control, by Richard P.
Paul, The MIT Press, Cambridge, Mass., 1981.
The manner in which the positions of lead tips and board holes are compared
for determining which leads need to be fine-formed will now be described
in reference to FIGS. 3, 4A-4D. As will be clear from the description
below, the process of finding which leads need to be fine-formed also
determines the distances and directions for bending such leads so that all
the leads are in the drop-in condition.
As described above in reference to FIGS. 1 and 2 the positions of holes 52
are sensed by means of system 54 and the positions of the lead tips are
sensed by means of system 50. These same steps are referred to in FIG. 3
by the same block numbers (blocks 110, 112) as in FIG. 2. Where the
positions of the holes are not sensed but originate by computer aided
design (CAD) such positions are stored in controller 60 or supplied to it.
The positions of the holes and the positions of the lead tips are then
compared to determine if any of the leads need to be fine-formed. In the
comparison, if the distances between holes and between lead tips are large
compared to the internal dimensions of the holes and lead tips, the
internal dimensions of the tips and holes may be ignored and the holes and
lead tips simply treated as points in the comparison. In some cases, the
internal dimensions of the holes and lead tips may be comparable to such
distances. For example, the holes may be of the order of 20 mils whereas
the distances between holes are of the order of 100 mils. In such
circumstances it is necessary to compute the centers of the holes and lead
tips (block 114). The centers of the holes and lead tips are then compared
for determining if any leads need to be fine-formed.
A conventional method for finding the centers of holes and lead tips may be
used to determine the centers of holes and lead tips. Thus, where the
sensing systems 50, 54 each comprises a television camera, the images of
the lead tips and holes are detected in terms of light intensities at
different pixels of the camera. First, pixels with light intensities
between certain preset intensity levels are identified. In one
conventional method known as the connectivity method, each pixel
identified is given a unit weight and the center of mass of the set of
pixel points are then computed as described by A. Rosenfeld and A. C. Kak
Digital Picture Processing, 2nd Section, Academy Press, New York 1982,
vol. 2, pp. 241-242. In another conventional method known as the boundary
method, the pixels detecting light intensities between two intensity
levels are enclosed by a chain-encoded polygon. The center of mass of the
polygon is then found by a conventional method as described in "Computer
Processing of Line-Drawing Images" by Herbert Freeman in Computing
Surveys, Vol. 6, No. 1, March 1974 pp. 57-97.
In the description below, the terms "hole centers" and "lead tip centers"
may be point positions computed in accordance with any one of the accepted
conventional methods, such as the two methods referenced above. In the
description to follow, where the internal dimensions of the holes and lead
tips are significant compared to the distances between holes and lead
tips, the distance between two holes will mean the distance between the
centers of such holes and the distance between two lead tips will mean the
distance between the centers of such tips.
The algorithms for comparing the lead tips to the holes for determining
which leads need to be bent can be best understood by imagining a manual
process for accomplishing the same task. Thus, in trying to identify which
leads need to be fine-formed, an observer must first try to identify the
hole corresponding to a particular lead so that the lead can be inserted
into such hole. Thus, in block 116, controller 60 matches each lead tip
with a hole using a conventional correspondence algorithm. One such
algorithm, the Hungarian method for the assignment problem, is described
for example by C. H. Papadimitrion and K. Steiglitz in Combinatorial
Optimization: Algorithms and Complexity, Prentice-Hall, Englewood Cliffs,
N.J., 1982.
Using such conventional method, a one to one correspondence is set up
between an unordered list of holes and an unordered list of leads. The
center of mass of the list of holes is found and the center of mass of the
list of lead tips is also found. Each list is then normalized by
translating the centers of mass to the origin of a reference frame. The
principal axis of each list of holes or lead tips is found as described by
Rosenfeld and Kak in their book referenced above. The list of holes are
then rotated until the list of holes have the same principal axis as the
list of lead tips. A weighted bipartite graph is constructed whose
vertices are the list of hole and lead tip centers and whose edges each
traveling between a hole center and a lead center. Each edge has a weight
which is proportional to the negative distance between the hole and the
lead for such edge. The different steps described so far for matching a
lead tip to a hole may be referred to as normalization.
In the weighted bipartite graph, each lead tip may be matched with any one
of the holes. Each lead tip is matched with a hole so that the sum of the
edges between the matched lead tips and holes is maximum. In other words,
each lead tip is matched to a hole such that the sum of the distances
between the holes and lead tips is minimized. Such matching may be
performed in a conventional manner, for example, by applying the
above-referenced Hungarian method. Intuitively, this means that each lead
tip is matched to a hole such that the lead tips can be matched to the
holes by traveling the smallest distances. While the above described
algorithm for matching a lead tip to a hole may be preferable, other
correspondence algorithms may be used and are within the scope of the
invention.
The steps to find which leads, if any, need to be bent will now be
described in reference to blocks 118-124 of FIG. 3 and FIGS. 4A-4D.
According to block 118, an association graph is constructed. The manner
for constructing an association graph will now be described in reference
to FIGS. 4A-4C. For the purpose of discussion, assume that five leads
L1-L5 of FIG. 4B are to be inserted into five holes H1-H5 of FIG. 4A. A
comparison to the five leads to the five holes will indicate that the
leads L1-L3 are in position to fit into holes H1-H3 without bending, and
only leads L4, L5 will need to be bent. A correspondence algorithm is
applied to identify each lead tip with a hole. Thus applying the
correspondence algorithm described above, each lead tip Li will be
identified with the hole Hi, i=1 . . . , 5. Each pair of corresponding
hole and lead tip (Hi, Li) form the ith node Ni of the association graph.
The tolerances will normally depend on the internal dimensions of the
holes and leads. Thus, node N3 is the pair (H3, L3). The distance between
each pair of holes is then compared to the distance between each pair of
corresponding lead tips. If the two distances are equal within certain
preset tolerances, a line is then drawn between the corresponding two
nodes of the association graph. Thus, since the holes H1-H3 match lead
tips L1-L3, the distance between any two of the three holes will be equal
to, within certain tolerances, the distance between the two corresponding
tips. Hence lines are to be drawn between the three notes N1, N2, N3. The
distance between holes H2, H4, however, is different from the distance
between lead tips L2, L4. For this reason no line can be drawn between the
nodes N2, N4. The distance between holes H3, H5 is equal within certain
tolerance the distance between tips L3, L5. For this reason a line can be
drawn between nodes N3, N5. Applying the same principle to all holes and
tips, the association graph of FIG. 4C is obtained.
A clique of an association graph is a group or clique of nodes, provided
that each node in the clique is connected to every other node in the
clique by a line. In the association graph of FIG. 4C, for example, a
clique comprises the node N1, N2, N3. While there are cliques having two
nodes such as the clique comprising nodes N1 and N3, such cliques are
smaller than the clique comprising the three nodes N1-N3. The addition of
any other node to the three nodes will cause at least two of the nodes in
the four nodes to be unconnected therebetween. That is, the clique is
maximal. Furthermore, there are no other cliques with more nodes. Thus,
the clique comprising nodes N1-N3 is then the largest or the maximum
clique. In some cases, particularly where many lead tips and holes are
involved, there may be more than one maximum cliques with the same number
of nodes. When a particular lead tip is in a maximum clique, this means
that the distance between it and other lead tips also in the clique are
equal, within tolerances, to those between corresponding holes. Thus,
these leads are in position to fit into the corresponding holes without
fine-forming. Therefore, the lead tips not in a maximum clique are the
ones needing fine-forming. Nodes N4, N5 corresponding to lead tips L4, L5
are not in the maximum clique. Therefore it is determined that the leads
having tips L4, L5 will need to be fine-formed. This matches the intuitive
comparison of the holes and leads in FIGS. 4A, 4B described above.
Since the leads have been preformed, in most cases, only a small proportion
of leads among all the leads of a component needs to be fine-formed. For
this reason, it is usually faster and thus more economical to first
determine the leads that need no fine-forming by a method such as finding
the maximum clique described above. The remaining leads then are the ones
needing fine-forming. It is, of course, possible to first find instead the
leads that need fine-forming by an alternative conventional algorithm;
such processes are also within the scope of the invention.
If the five leads having tips L1-L5 are in the drop-in condition, it will
be noted that the maximum clique will include all the lead tip centers.
Thus, if all lead tip centers are in the maximum clique found, all the
leads are in the drop-in condition and fine-forming can be avoided. If at
least one lead tip is not in the clique, it indicates that at least one
lead will need to be bent so that fine-forming cannot be avoided.
Maximal cliques have been used in conventional methods for matching machine
parts. Thus, Robert C. Bolles describes one such technique in "Robust
Feature Matching Through Maximal Cliques", Proc. SPIE Technical Symposium
on Imaging and Assembly, Washington, D.C., April 1979, pp. 1-9.
Next, controller 60 constructs a transformation for transforming all hole
positions in the maximum clique into the corresponding lead tips (block
120, FIG. 3). Again in reference the FIGS. 4A, 4B, controller 60 finds a
transformation which causes holes H1-H3 to coincide with lead tips L1-L3.
These transformed hole positions in the frame of reference of the lead
tips are shown in FIG. 4D and labeled H1'-H3'. The transformation so
constructed is also used to transform holes not in the clique. In
reference to FIGS. 4A, 4B, 4D, holes H4, H5 are transformed into positions
H4', H5' in FIG. 4D. In other words the five holes are transformed into
the frame of reference of the lead tips. The vectors 126, 128 of FIG. 4D
are then the displacement vectors indicating the distances and directions
for fine-forming the leads for tips L4, L5. In other words if the lead
with tip L4 is bent until the tip center coincides with the transformed
hole center position H4', and the lead with tip center L5 bent until the
tip center coincides with H5', all the leads will then be in the drop-in
condition.
The process above described in reference to FIGS. 3 and 4 gives the answer
to the question in diamond 114 in FIG. 2. If all the lead tips are in the
maximum clique then fine-forming can be avoided and robot 22 of FIG. 1
brings the component to a position above holes 52 and proceeds to insert
the component on to the board 40 as described. Where fine-forming cannot
be avoided robot 22 brings the component 28 towards the bending and
sensing system 70 for fine-forming the leads.
The process of fine-forming the leads will now be described in reference to
FIGS. 5, 6, 7A and 7B. As described above in reference to FIGS. 3, 4A-4D,
controller 60 determines the displacement factors 126, 128 for
fine-forming the leads with tips L4, L5. Thus, controller 60 has stored in
its memory the distances and directions for bending each lead that needs
to be fine-formed. For simplicity, component 28 is shown to have only
three leads with only one lead needing fine-forming. It will be
understood, of course, that the process described below may be used to
fine-form more than one lead in the same manner.
Instead of bending a lead along the direction indicated by the displacement
vector, such as vector 126, it is possible to bend the lead first along
the direction 126', and subsequently along the direction 126" for such
distances that the three sides 126, 126' 126" form a right angle triangle
as shown in FIG. 4D. The consecutive bendings are found to cause the final
position of the lead tip center L4 to be the same as if it had been bent
once along vector 126. Hence, the robot may accomplish the bending in two
fixed X, Y directions irrespective of the orientations of the displacement
vectors 126, 128. This may eliminate certain robot maneuvers which may be
advantageous.
Controller 60 causes robot 22 to move component 28 with respect to bending
member 72 so that member 72 is in the proper position for bending lead 30a
that needs fine-forming. Frequently, however, leads have elastic and
plastic properties so that after they are bent, they will spring-back for
a distance after the bending member 72 is removed, so that even after
bending, the three leads of component 28 will not be in the drop-in
condition. The position 30b in dotted lines in FIG. 5 is the desired
position of lead 30a in order that the three leads of component 28 be in
the drop-in condition.
Thus, if bending member 72 bends lead 30a until it is in position 30b,
after member 72 is removed, lead 30a will spring-back for a short distance
so that it is no longer in position 30b. In many cases the leads will
spring-back for a sufficient distance so that it is no longer in a preset
tolerance limit for insertion into corresponding holes on the printed
circuit board 40. This invention is also based on the recognition that the
leads may be bent successively until it is within the tolerance limit of
the desired position.
This invention is also based on the observation that, irrespective of the
distance for which lead 30a is bent, the spring-back characteristics of a
lead remains nearly the same even if successively bent. This is
illustrated for example in FIG. 6. FIG. 6 is a graph illustrating the
spring-back characteristics in terms of the force exerted by member 72 on
lead 30a as a function of the displacement on the tip of the lead from its
initial unbent position. As shown in FIG. 6, lead 30a is bent two times,
first from point A to point B. When released gradually, the lead
spring-backs to point C. The lead is then again bent from point C to point
D and again released, allowing it to spring-back to point E. As
illustrated in FIG. 6, the shape of the force versus displacement
spring-back curve or characteristic from B to C is nearly the same shape
as that portion of the spring-back curve from D to E that is below the
force valve at B. Thus, if the force versus displacement spring-back curve
from B to C is recorded by controller 60, the same shaped spring-back
curve can be constructed to through the target point F and suitably
extrapolated (curve 226). Upon re-application of force at C, force and
displacement are continuously sampled and measured until intersection with
curve 226 at point D is achieved. Force is then released allowing the lead
to return to point E. If the distance between E and F is less than the | | |
