Controlled pin insertion using airflow sensing and active feedback

An automatic assembly apparatus or robot holds a pin in an instrumented gripper, moves open loop to the "detection region" in which airflow forces, primarily lateral forces, in the vicinity of the hole may be dependably sensed. Relatively large lateral airflow forces in the detection region are sensed and the pin is moved toward the hole. The airflow forces exhibit an abrupt force change as the pin reaches the insertion point within the "capture region" of the hole defined by the mechanical chamfer or significantly extended as a result of the virtual chamfer of airflow through the hole. The insertion point is the point where lateral X,Y force vectors balance to a null, where Z force readings change from complex X,Y,Z force vectors to simple Z force vectors. The controller commands an open loop move to place the pin within the detection region; then commands a closed loop move, using a skirt around the pin or the pin itself as a probe, and sampling the force vectors until the insertion point is detected; then commands a move in the Z direction to put the pin into the hole. Insertion success or failure is detected by sampling the Z force vectors for blockage by the plate, or by sampling pressure as the pin plugs the hole. Retry routines follow detection of any insertion failure.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to automatic assembly techniques, and more 
particularly relates to automatic sensing of the airflow at a hole, and 
moving a pin laterally to a position within the airflow where lateral 
forces of the sensed airflow most closely approach a null over the hole, 
to position the pin at the insertion point for the hole. 
2. Description of the Prior Art 
A recurring problem in automated assembly is the classic "pin-in-hole" 
problem wherein a cylindrical pin must be inserted into a cylindrical hole 
of closely matching diameter. A traditional approach is to put chamfers on 
the hole, the pin, or both, and to use compliant fixturing for gripping 
the pin. This approach fails if the pin and hole axes are separated by 
more than the capture region of the chamfers. This approach also fails in 
the case of flexible or curved pins, such as wires or filaments. Even the 
addition of vacuum suction will not extend the capture region of the hole 
much beyond the chamfer, that is, beyond a "virtual chamfer". The addition 
of vacuum suction to the hole can increase the capture region beyond that 
defined by mechanical chamfers, but due to the mass of large pins or 
fragility of small ones it may be difficult to achieve vacuum sufficient 
to increase the capture region of the hole significantly. Moreover, the 
geometry of the insertion problem may make it difficult to create a 
pressure differential across a hole which causes aiding airflow into the 
hole, while the same geometry may easily allow the opposite differential 
which causes resisting airflow from the hole. It might be outside the 
normal precision of the robot to go directly to the insertion position in 
open loop mode. It is difficult to achieve the optical and mechanical 
precision necessary for a closed loop approach, even with a 
vision-equipped robot, because the necessary precision is difficult to 
achieve and because the robot gripper, and indeed the pin itself, may 
obscure the hole. 
The following patents are representative of the prior art: 
U.S. Pat. No. 3,667,103, Edwyn H. Petree, APATUS FOR INSERTING TERMINALS 
IN AN APERTURED PLATE, June 6, 1974, shows the use of vacuum together with 
vibration to pull pins into chamfered holes in a plate. 
U.S. Pat. No. 4,155,169, S.H. Drake et al, COMPLIANT ASSEMBLY SYSTEM 
DEVICE, May 22, 1979, shows the use of a compliant gripper to ease 
insertion of a pin into a hole. 
U.S. Pat. No. 4,485,453, Taylor, DEVICE AND METHOD FOR DETERMINING THE 
LOCATION AND ORIENTATION OF A DRILLHOLE, Nov. 27, 1984, shows the use of a 
drill bit as a probe for a robot to use in determining the location of a 
drillhole. 
German Patent No. DE28 34 6984, Petermann et al, Feb. 14, 1980, shows a 
vacuum assisted insertion technique for electrical components. 
The prior art shows the use of vacuum in pin-in-hole operations, shows the 
use of a compliant gripper in pin-in-hole operations, and shows the use of 
instrumented grippers. The prior art thus shows the use of the chamfer 
capture region to capture the pin, and shows vacuum enhancement of the 
chamfer capture region (virtual chamfer) but fails to show any automatic 
techniques in a detection region, in the outer vicinity of the hole, which 
the robot can follow to position the pin at the insertion point for the 
hole. The prior art does not show and does not suggest the inventive 
combination, which allows the robot to respond to the forces of airflow 
through a hole, using sensors closely associated with the pin, or the pin 
itself, as a probe to direct and control motion of the pin to the 
insertion point within the capture region of the hole, perform the 
insertion, and detect successful insertion. 
Nor does the prior art teach the method of positioning the pin in the 
general vicinity of the selected hole in open loop mode; searching for a 
null in lateral airflow forces in closed loop mode, to determine the 
insertion point; inserting the pin in open loop mode; and detecting 
successful insertion. 
SUMMARY OF THE INVENTION 
A part having holes for receiving pins is arranged for access by the 
insertion apparatus (e.g., an instrumented robot) with fluid flow into or 
from the holes, such as by airflow into vacuum. For each individual hole 
the fluid flow creates a detection region in which the lateral and 
vertical forces due to the hole can be detected, so that the direction to 
the hole can be derived. If the flow is from the hole this direction is 
opposite to the direction of the lateral force, whereas if the flow is 
into the hole the direction is the same as that of the lateral force. 
The robot is open-loop controlled to position the robot gripper so that the 
pin is in the outer vicinity of the hole, within the detection region of 
the lateral force vectors of airflow through the hole. The robot, acting 
upon a sensor-equipped gripper or skirt about the pin, senses lateral 
forces acting upon the pin as a probe, or very closely related to the pin. 
The robot then acts in closed-loop (feedback) mode, moving the pin toward 
the insertion point derived from sensing the lateral forces, while 
continuously sensing the lateral force vectors. The robot uses 
go-with-the-flow (or go-against) servo techniques to drive the robot 
gripper into juxtaposition with the hole (within the insertion capture 
region of the hole) as defined by a null in the lateral force vectors. 
Upon sensing of the force vector changes resulting from the null, the 
robot moves the robot gripper vertically (downward, in open-loop mode) to 
insert the pin into the hole. 
The object of the invention is to enable a manufacturing robot to place a 
pin into a hole even though the required alignments of pin and hole are 
beyond the global accuracy capability of the robot. 
A feature of the invention is a force-vector-sensing, instrumented, 
compliant gripper to hold the pin as a probe in combination with a 
servomechanism controller to guide the pin to the virtual chamfer capture 
region of the hole. 
The instrumented robot gripper produces signals related to direction and 
magnitude of the force. The controller receives the signals, calculates a 
lateral move sequence and commands the move sequence. During the move 
sequence, the controller responds to changes in the sensed lateral forces 
on the pin. At a position within the detection region at which lateral 
forces most closely approach a null, the null defines the capture region 
of the hole, which defines the insertion point. The controller determines 
from the null that the robot gripper has positioned the pin at the 
insertion point, and commands the robot to drive the pin into the hole (or 
allows the pin to be vacuum-pulled in the hole in the case of light and/or 
flexible pins or filaments). 
Another feature of the invention is the combination of a highly sensitive 
instrumented compliant gripper, to hold the pin and/or a probe skirt, with 
servo repositioning by the robot to the insertion position. For insertion, 
the instrumentation senses the lateral forces of airflow forming a capture 
region of a hole linking pressure differentials. The robot control 
computer iteratively calculates and commands a sequence of lateral moves, 
in response to iteratively sensed lateral forces to a position within the 
detection region at which lateral forces most closely approach a null. The 
null defines the capture region of the hole. The robot control computer 
determines from the null of lateral forces that the pin has been 
positioned at the insertion point, centered over the capture region formed 
by the airflow through the hole. It then commands a vertical move (or 
allows the pin to be vacuum-pulled in the hole if flow is into the hole 
and forces are sufficient.) 
An advantage of the invention is its ability to insert fragile pins or 
wires, even flexible filaments, into unchamfered holes. 
Another advantage of the invention is that it enables a manufacturing robot 
to carry out pin-in-hole operations even though pin and hole dimensions 
fall outside the open loop precision of the robot gripper. 
The apparatus and method for controlled insertion of pins into holes using 
airflow detection and active feedback offers the following significant 
advantages: 
(1) its applicability is general enough to handle situations using either 
vacuum or pressure on large or small pins which may be rigid or flexible 
or even fragile; 
(2) it is self guiding in that the hole itself supplies directional 
information to the pin; 
(3) during insertion, the high velocity air film in the annular space 
between the pin and the hole provides a lubricating function reducing the 
possibility of failure due to jamming; 
(4) using the pin itself as a probe provides a spatially minimal sensing 
arrangement; 
(5) since the detection region is typically much larger than the hole size, 
successful insertion can be initiated in a region much larger than the 
mechanical chamfer; 
(6) it overcomes many classical problems of pin in hole insertion, i.e., 
small clearance, imprecise data, and/or lack of global accuracy of robots. 
The foregoing and other objects, features and advantages of the invention 
will be apparent from the more particular description of the preferred 
embodiments of the invention, as illustrated in the accompanying drawings.

DESCRIPTION OF A PREFERRED EMBODIMENT OF THE INVENTION 
FIG. 1 shows a basic embodiment of the apparatus for inserting pins in 
holes. The purpose is to place pin 1 in hole 2 of plate 3. The problem is 
due to the following considerations: 
(1) the clearance between pin 1 and hole 2 may be small; 
(2) the locations of pin 1 and hole 2 may not be known sufficiently 
accurately for insertion, due to measurement errors or to flexing of the 
pin; 
(3) the apparatus 4 does not have sufficient global accuracy to complete 
insertion. 
A pressure differential is applied across hole 2, by vacuum or pressurized 
air on either side of plate 3, creating flow lines 7. This causes 
measurable lateral and vertical forces on pin 1 when it is within the 
extended vicinity of hole 2, which might be several diameters. 
Robot 4 includes instrumented gripper 5, which is equipped with force 
sensors 6. (These sensors are normally included within gripper 5, but for 
clarity are shown here external to the gripper.) Information from force 
sensors 6 is fed back to robot 4 for controlling repositioning of gripper 
5. Pin 1 acts as its own probe. Once placed in the extended vicinity of 
hole 2, pin 1 is influenced by airflow through hole 2, illustrated by flow 
lines 7. The portion of the extended vicinity which is unambiguously 
affected by airflow of the selected hole 2 (and not some other hole) is 
accepted as detection region 9. The airflow includes vertical (Z) and 
horizontal (X,Y) force components relative to coordinate frame 8. The 
lateral components of these force vectors are detected by force sensors 6, 
and fed back to robot controller 4 to command repositioning to place pin 1 
at the lateral force null which is directly over hole 2 and is accepted as 
defining the capture region of hole 2. This is the insertion point 10. 
Upon determining arrival of the pin 1 at the insertion point, the robot 4 
activates a Z repositioning to move the pin 1 into the hole 2. Pump 11 
supplies the appropriate fluid flow, usually vacuum, to provide fluid flow 
through hole 2 and flow force 7. 
FIG. 2 is a schematic graph of the lateral and vertical components of force 
on pin 1 due to the flow lines 7 of FIG. 1. The pressure differential 
across the hole sets up a flow field in the vicinity of the hole which 
exerts lateral and vertical forces upon the pin. Sensible forces are 
present for distances up to several diameters away from the hole center, 
defining a relatively large detection region, more than six diameters in 
the example of FIG. 2. As the pin is moved closer into alignment with the 
hole, the lateral force increases to a maximum where the pin and hole are 
partially overlapping. The lateral force reaches a null as it approaches a 
(zero) when pin 1 and hole 2 are perfectly aligned. At the same time, the 
magnitude of the vertical force increases to a maximum when perfect 
alignment is reached (see FIG. 2). As shown in curve (a) of FIG. 2, the 
magnitude of the lateral force F.sub.x,y rises from a null at perfect 
alignment (0 diameters misalignment) to a maximum adjacent the hole, then 
tapers off as distance from the hole increases. Curve (b) in FIG. 2 shows 
the magnitude of the axial force associated with the hole, assuming the 
pin 1 is quite close to the surface of the block 3. Force F.sub.z peaks at 
perfect alignment. 
Up to this point it has not been necessary to distinguish between the case 
of flow into the hole from the case of flow out of the hole. For 
clarification, flow "into" the hole is defined to be a flow which is 
caused by having a pressure greater on the same side of the hole as the 
pin than on the other side. Flow "from" the hole is defined to be a flow 
of the opposite direction. The two cases of flow are illustrated in FIGS. 
3-6. 
FIG. 3 is a schematic diagram of the case of flow into the hole and a 
misalignment of pin 1 and hole 2. Flow lines 7 cause forces on pin 1 which 
are detected by sensors 6, one each in the left and right fingers of 
gripper 5. As shown in FIG. 3 the left sensor registers a negative force 
value while the right sensor measures a corresponding positive force 
value. The robot controller uses this information to command a movement of 
the pin 1 towards the hole 2 (i.e., in the direction of the sensed force). 
In FIG. 4 the gripper 5 has moved in the direction of arrow 9 to bring the 
pin 1 into vertical alignment with hole 2. Left and right force sensors 6 
register a force value, indicating the null of lateral forces which 
distinguishes this position. FIGS. 5 and 6 are equivalent to FIGS. 3 and 
4, but are for the case of flow out of the hole. The primary difference is 
that the movement commanded must be in the direction opposite to that of 
the sensed forces. 
The success of the invention depends on the ability to measure the forces 
in the detection region of the hole 2. These forces act upon the pin 1 due 
to airflow 7 associated with the hole 2. A control system interprets these 
forces and generates appropriate motions. 
FIG. 7 shows diagrammatically how these forces are incorporated in a 
feedback loop. The detection region is a truncated cone containing the 
aperture of the hole in which the dominant forces on the pin are due to 
the primary flow; that is, the forces due to secondary induced flows are 
negligible. For flow into a hole, the angle of this cone approaches the 
half space of the plate surface and the flow is mainly laminar. For flow 
out the cone is narrower and somewhat taller, but with a dominant overall 
direction. In either case a detection region is created with a horizontal 
cross section away from the plate surface several diameters larger than 
the hole diameter and a vertical height several times the hole radius. The 
forces within this detection region are typically large enough to be 
measured by commercially available force sensors such as resistive or the 
semiconductor type strain gauges found in the IBM 7565 robot. Referring to 
FIG. 7 in conjunction with FIG. 1, the feedback loop can be discrete 
samples of iterative steps (iterative closed loop mode) or of a continuous 
nature (continuous closed loop mode). 
The command from robot controller 4 appears at node 12 and is mixed with 
sensory data on line 13, sent to compensation 14 and interpreted as a 
motion control directive at 15. 
In the iterative approach the force on the pin 1 is measured by the force 
sensors 6, and the control system 4 determines the direction to the hole 2 
from the direction of the force 7, and determines proximity to the hole 2 
from the magnitude of the force 7. The control system 4 then commands a 
move, in the correct direction, of size inversely proportional to the 
magnitude. During the move, the system tests for the null of lateral 
forces indicating arrival at the capture region, and this sequence is 
repeated if necessary. In the continuous closed loop mode approach, forces 
are continuously sensed and position and velocity corrections are made 
during the motion. Many texts on the theory of control systems discuss 
systems of this nature, and more particularly control systems in robotic 
applications similar to this are considered in Robot Motion: Planning and 
Control, edited by Michael Brady, et al., The MIT Press, Cambridge, Mass., 
1982. 
As previously discussed, there are two major cases of flow--namely flow 
into the hole and flow out of the hole. In the first case, an additional 
control strategy can be employed. Within the detection region of the hole 
a smaller region, which contains the capture region, may exist. In this 
region, which is denoted the "virtual chamfer" region, the virtual chamfer 
forces on the pin 1 are sufficient to seat and insert the pin in the hole 
2 passively. When the control determines the pin's position in the virtual 
chamfer, it can switch from force controlled mode to a passive compliant 
mode for seating and insertion of the pin. This go-with-the-flow technique 
can be most advantageous in situations requiring insertion of small or 
flexible pins or filaments. 
The control system can sense successful insertion by several strategies by 
noting: 
(1) the Z position of the gripper; 
(2) large restoring lateral forces in response to small lateral motions; 
(3) pressure change registered by an auxiliary pressure sensor. 
In the case where airflow is directed into the hole, the method and 
apparatus offers the following additional advantages: 
(1) the flow pattern is simpler and the detection region is larger; (2) the 
active go-with-the-flow control strategy in effect amplifies the airflow 
force by many times; (3) after detection of the virtual chamfer region, by 
allowing the forces due to the airflow to draw the pin 1 into the hole 2, 
or by switching from active force control to passive compliance, a virtual 
chamfer much larger than the mechanical chamfer is created. (4) it is 
applicable in the case of semiflexible or even very flexible pins or 
filaments; for example, it is useful in insertion of optical fibers or 
print wires in holes where pushing is difficult or impossible. 
FIG. 8 shows an alternative placement and type of sensors for the lateral 
forces of airflow through hole 2. The lateral airflow forces are sensed by 
hot wire anemometers (or other devices) 16 carried at the hem of skirt 17, 
so as to be very near to plate 3. The skirt 17 is relieved by one or more 
vents 18 so as not to create turbulence in the lateral airflow forces 7. 
The lateral airflow is thus sensed directly, at a position very near to 
the hole; the relationship between skirt 17 and pin 1 is concentric or 
otherwise known, so that the computer can determine the appropriate 
locations. 
SPECIFIC USAGES 
Assembly of Wire Matrix Print Heads 
Wire matrix printers have become very popular as peripherals for 
microprocessor-based small computers. A major factor in their success has 
been their relatively low cost. Fully automated assembly can drive the 
base manufacturing cost of such printers even lower. A major impediment to 
achieving fully automated assembly is the wire matrix printhead itself. 
Here, several tiny wires must be carefully threaded through a series of 
focusing plates, each having guide holes to direct the wires to a line 
focus at the printing paper. 
A typical example of a three-plate-focusing print head is seen in FIG. 9. 
It has been proposed to use robots to assemble the wires in the printhead. 
By means of programmable and hard tooling, the printhead body is oriented 
at the proper position and angle in space for insertion of wires, one at a 
time. A difficulty with such a scheme is the extremely close tolerances 
which are involved, and the delicacy of the wires. The holes in the body 
must be perfectly aligned, and cannot be much larger than the diameter of 
the wires. Lead-in chamfers on the holes help, but are limited by the 
close proximity of adjacent holes. Finally, if the wires themselves are 
bent even slightly, they will not pass through holes in all focussing 
plates, or will be incorrectly aligned, and may even pass through the 
wrong holes. 
A pressure differential across the hole of nearly one atmosphere 
(1.013.times.10.sup.6 dynes/cm.sup.2) is easy to provide and makes it easy 
to sense changes as they occur. Assuming the wire diameter to be 0.25 
millimeter, the maximum force generated on a print wire in the hole is 
approximately 500 dynes. As wire 1 approaches the hole 2 with some 
misalignment, a fraction of this maximum force exerts a measurable lateral 
component in the detection region. This force is sufficient to servo the 
wire with a go-with-the-flow strategy to a smaller capture region for 
insertion. If, for example, 30% of the force acts laterally, then the 
sideways force on pin 1 is about 150 dynes at the pin 1 approaches the 
hole. The pin tip will respond to this force if it is held in a compliant 
manner, or if pin 1 is sufficiently flexible to bend slightly. 
It is difficult to analyze the problem's dynamics well enough to predict 
the size and shape of the detection region and the virtual chamfer, but it 
is a multiple of the actual chamfer, depending upon pin variables and the 
sensitivity of the force sensors. 
The presence of more than one hole further complicates the analysis of the 
situation due to the presence of competing forces, but the planar regions 
for a particular hole are still a multiple of the hole diameter. 
Experimentation with a stage similar to the second stage 21 of FIG. 9, but 
in an unconstrained configuration, has shown detection and capture regions 
similar to regions 9 and 30 of FIG. 10. 
Several strategies are possible for providing the necessary airflow through 
the holes. One possibility is a fixture for sequentially evacuating 
chambers as shown in FIG. 11. Each plate 20, 21, 22 is equipped with a 
vacuum-source-equipped cover 23, 24, 25 which forms a progressive sequence 
of vacuum chambers, cover 25 defining the highest vacuum of the three 
chambers. 
FIG. 12 shows a second method to mount a nine-wire printhead assembly on an 
indexable fixture with nine angular positions (one position for each of 
the nine holes), using a small movable vacuum suction line 26. Suction 
line 26 moves in and out in a linear fashion to cover each of the nine 
holes separately. A successful wire insertion can be detected by noting 
the slight change in pressure which occurs when the wire blocks off the 
hole, using a simple pressure operated microswitch (not shown). 
Aircraft Skin Riveting 
Another specific usage of the invention is in placement of rivets in sheet 
metal, such as aircraft skins. This is shown in FIG. 13. For some types of 
rivets, it is undesirable to provide a mechanical chamfer on the hole 
since it reduces the amount of material available for forming the clinched 
head on the back side. On the other hand, taking away the chamfer makes 
robotic assembly extremely difficult. Endpoint sensing of the rivet-hole 
geometric relationship can be used to guide the robot if a suitable sensor 
can be found. 
This invention provides an almost ideal endpoint sensor for this 
application, using either passive or active guidance (described above) for 
reliably placing the rivet above the insertion capture region of the hole. 
For riveting aircraft skins, for example, it is better to apply pressure 
to the rivet side of the hole, or pressurize the back side of the hole, 
rather than apply suction to the back side of the hole. Pressurization of 
the rivet side of the hole is achieved using a flexible rubber cup 27 
around the rivet 28. Sensing of lateral forces takes place as described 
with respect to FIG. 1. Alternatively, the back side of the hole could be 
pressurized by pumping air into the entire aircraft assembly, such as a 
wing section. 
METHOD 
The method for pin-in-hole insertion by a robot comprises the steps of: 
(a) applying airflow through at least the selected hole (2) for insertion 
of a pin (1), creating a pattern of detectable airflow forces (7) within a 
detection region about the hole (2); 
(b) moving the pin (1) by open-loop positioning to place within the 
detection region; 
(c) sensing the airflow by airflow sensing means (6,16) closely associated 
with the pin (1); 
(d) following the airflow-forces sensed by the airflow-sensing means (6,16) 
by closed-loop positioning through at least one sensing and 
airflow-following servo move in the (x,y) direction of changing lateral 
airflow force, thus moving the pin (1) toward the hole (2); 
(e) repeating said sensing the airflow step (c) and said following the 
airflow-forces step (d) until an airflow null is sensed defining the 
insertion point 10 at the capture region of the hole (2); and 
(f) commanding an entry move (z direction) of the pin (1) into the selected 
hole (2) at the insertion point (10) which is within the mechanical or 
virtual chamfer capture region of the hole (2). 
The method also may include power drive into the hole by further comprising 
the steps of: 
(g) sensing a composite of pressure and z forces, using the pin (1) as a 
probe, during a z move, 
(h) detecting a blockage as a function of the composite of pressure and z 
forces; and 
(i) commanding backup and retry. 
The method requires simple changes for use with pressure or with vacuum. 
Airflow is out of or into the hole 2. The servo is a fight-the-flow servo 
for flow out of the hole, and a follow-the-flow servo for flow into the 
hole. The actual pin insertion must be powered, at least by gravity, to 
overcome the forces of the flow in case of flow out of the hole. In case 
of flow into the hole the airflow forms a virtual chamfer which greatly 
extends the mechanical chamfer capture region, and helps pull the pin 1 
into the hole 2. 
Atmospheric air is the choice for most operations, because of its purity 
and availability. Other fluids may be advisable in certain cases. Such 
fluids might include water, hydraulic oils, and such gases as nitrogen or 
helium. 
Whereas the invention has been shown and described with respect to 
preferred embodiments, using vacuum and using pressure, using the pin 
itself as a probe and equipping a skirt about the pin as a probe, it will 
be apparent that those skilled in the art will be aware of related 
techniques for sensing relatively small airflow forces surrounding a hole, 
and related techniques for following those small airflows using robotic 
techniques of power drive, so that a relatively fragile or flexible 
filamentary pin may be placed in a hole located by lateral force vectors 
of fluid flow through the hole, acting upon sensors associated with the 
pin and used to guide the positioner carrying the filament to the hole.