Method and apparatus for visual alignment of parts

An apparatus (10) for visually aligning parts (13) for placement using one camera (18) comprises a device (12) for picking and placing a part on an object (15), a device (24, 26 & 28) for moving and removing optics (30, 36, 38) into and out of a work envelope having the object, a device (18, 40, 42 & 44) for locating landmarks on the object within the work envelope, and a device for aligning the part with the landmark in the work envelope.

TECHNICAL FIELD 
This invention relates generally to robotic pick and place stations and in 
particular, to a robotic gripper for closed loop image guided placement of 
parts. 
BACKGROUND 
Traditional vision systems for robotic stations use a dual cameral system 
including associated lighting and lenses. Typically, one fixed camera in 
the robotic station faces up and views the bottom of a placement part for 
inspection. A second camera is then typically mounted on the robot arm 
facing down toward the area where the part is to be placed. More 
particularly, in electronics, the second camera would be looking down at a 
printed circuit board having fiducials or landmarks for a standard of 
reference. The first camera would view the general shape and pitch of an 
electronic component such as a quad flat pack (QFP), a flip chip, a solder 
bumped chip carrier, or any other number of fine pitched components. The 
robotic system would then need to calibrate and adjust to align the 
coordinates mapped into a pixel coordinate grid from the fixed first 
camera to the robot's coordinate system. Likewise, the second mobile 
camera would need to be calibrated and adjusted to the robot's coordinate 
system. Generally, a robotic system has a 4 axis coordinate system: X 
(east/west), Y (north/south), Z (up/down), and A (rotation about a central 
axis). 
In actual placement of parts, the dual camera system utilizes both cameras 
to pick and place the part on a printed circuit board. Generally, as a 
printed circuit board rides on a conveyor towards a mechanical stop in a 
work envelope (typically, the area within reach of the robot arm), the 
robot arm picks a part from a feeder. Then the mobile camera is moved over 
to the printed circuit board (PCB). With the mobile camera, two fiducials 
are found on the PCB and the system calculates the robot coordinates of 
each fiducial. Inside the vision system, a database of the part placement 
location with respect to the fiducials is stored. The robot moves the part 
over the fixed camera, and the center and orientation of the part are 
measured. The placement of the component relies solely on the robot's 
accuracy to move to the placement location and place the part. 
The drawbacks of a dual camera system are evident. The traditional dual 
camera system provides an "open-loop" placement that relies strictly on 
the robot's accuracy. The open loop system fails to confirm the actual 
placement of the part in the desired location. Furthermore, the 
calibration errors of each of the cameras stack up. In other words, the 
mobile camera calibration is dependent on the fixed camera's calibration. 
Another disadvantage of the dual camera system is that the fixed camera is 
in a remote area away from the placement location, thus requiring an 
additional robot motion. Finally, the added cost are evident in using two 
cameras and two calibration fixtures. Thus, a need exists for a cost 
effective vision system using a closed loop verification scheme. 
SUMMARY OF THE INVENTION 
An apparatus for visually aligning parts for placement using one camera 
comprises a device for picking and placing a part on an object, a device 
for moving and removing optics into and out of a work envelope having the 
object, a device for locating landmarks on the object within the work 
envelope, and a device for aligning the part with the landmark in the work 
envelope.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring to FIG. 1, there is shown a side view of an apparatus 10 for 
visually aligning a part 13 for placement onto an object such as a printed 
circuit board 15 using one camera (18) in accordance with the present 
invention. The apparatus preferably comprises a means for picking and 
placing (12) a part on the object such as a robotic gripper or vacuum tip. 
The gripper 12 is preferably mounted on a robotic arm A axis plate 14 and 
coupled to a camera 18 via vertical plates 16 and 22. The apparatus 
further includes a means for moving and removing (24, 26, and 28) optics 
(30, 36, and 38) in and out of a imaging envelope (the region between the 
part 13 and the printed circuit board 15) which allows the apparatus 10 to 
align the part 13 with the object 15 and then place the part on the object 
once the optics are removed. The means for moving and removing preferably 
includes a rotary actuator 24 coupled to an optics support bracket 28 via 
a beam 26. The rotary actuator rotates the optics in and out of the 
imaging envelope. The optics preferably include a 45 degree mirror 30, a 
beam splitter 36 and a first surface mirror 38. The mirror 30 is 
preferably aligned on the camera's imaging axis below the camera lens 20, 
the beam splitter is preferably centrally aligned to reflect the camera's 
imaging axis and below the gripper, and the first surface mirror 38 is 
likewise preferably centrally aligned to reflect the camera's imaging axis 
and adjacent to the beam splitter when the optics are placed in the 
imaging envelope. Preferably, a backlight 32 above a vacuum tip 11 
provides sufficient light to provide a shadow image of the component 13. 
Likewise, an LED ringlamp 34 provides light to reflect off of the PCB 15 
so that the images of the PCB 15 and the component 13 can be aligned. The 
function of the lights become more apparent in the description of FIGS. 2 
and 3. 
The apparatus 10 operates under the control of a known robot controller 42. 
The robot controller 42 controls the movements of the robotic gripper 12 
through a well known robot interface 44. The gripper 12 can hold a 
component 13 by any appropriate mechanism, such as holding fingers or a 
vacuum tip 11 which allows the gripper to hold the component by suction. 
The component 13 is preferably picked by the gripper 12 from a tray (not 
shown) for placement onto the printed circuit board 15. Once all the 
necessary components are placed on the PCB 15, a conveyor (not shown) 
preferably moves the PCB 15 and provides the next PCB to be populated. 
It is well known that the PCB 15 when positioned on the conveyor may be 
oriented in any arbitrary position. In order to accurately placed the 
component 13 on a predetermined area of the PCB 15, the coordinates of the 
PCB 15 and the component 13 with respect to a reference must be 
determined. The coordinates of the PCB 15 and the component 13 are 
provided by converting video pixel data from the camera 18 to Cartesian 
coordinate data which is applied to the robot interface 44 for movement of 
the gripper 12. The camera 18 captures the image of the circuit board and 
the component via a well known camera interface 40, and provides the pixel 
data to the robot controller 42 for processing. The PCB 15 includes on its 
surface prepositioned landmarks or fiducials (not shown.) The position of 
the landmarks are recorded by the camera 18. The robotic controller 42 
accurately determines the orientation of the PCB 15 based on the position 
of the landmarks. Once the orientation of the PCB 15 is determined, the 
robot controller 42 may also calculate the coordinate of any area on which 
an electrical component is to be placed. 
When a component is picked by the gripper 12, the component 13 may also be 
in an arbitrary orientation. For accurate placement of the component 13 on 
the PCB 15, the orientation of the component 13 as held by the gripper 12 
and with respect to a reference coordinate, must also be determined. 
According to the invention, the image of the component 13 and the PCB 15 
is deflected by a first surface mirror 38 via the beam splitter 36 towards 
the 45 degree mirror 30. The deflected image is captured by the camera 18 
which located the component 13 and transmits the corresponding pixel data 
to the robot controller 42. The vision system of the apparatus 10 can be 
calibrated to establish the relationship between the video pixel data 
provided by the camera 18 and the Cartesian coordinate system used by the 
robot gripper 12. A well known software algorithm may be programmed to be 
executed by the robot controller 42 to achieve the calibration of the 
vision system. The calibration program would be written to accommodate for 
the variation of particular parameters and distortions with optics as is 
known in the art. 
Operationally, the gripper 12 picks the part 13, rotates (24, 26, 28) the 
optics (30, 36, 38), into an imaging envelope area above the PCB 15, and 
the camera 18 locates the landmarks or fiducials (not shown) on the PCB. 
The component or part 13 is aligned with the landmarks of the PCB 15. Once 
the parts are aligned, the optics are rotated away to allow the gripper 12 
to place the component 13 onto the desired location on the PCB. 
Referring to FIGS. 2 and 3, emphasis is given to the concurrent imaging 
aspect of the present invention. An apparatus 100, in its simplest form, 
comprises a robot gripper 102 having a vacuum tip 101 for retaining a 
component 104. A light holding assembly 130 has an upper portion for 
retaining the upper lights 112 and a lower portion for retaining lower 
lights 114. A beam splitter 106, preferably having a 50% mirror 116 
reflects a portion of the image of the component 104 (when the upper 
lights 112 are on) towards a first surface mirror 105. Then, the 50% 
mirror 116 allows the reflection off of the first surface mirror 105 to 
pass through and provide an image to the lens 120 and camera 118. 
Likewise, referring to FIG. 3 when the lower lights 114 are on, the beam 
splitter 106 reflects the image of the printed circuit board 110 directly 
towards the lens 120 and camera 118. Thus, the image of the printed 
circuit board 110 (and its fiducial or landmarks) and the image of the 
component 104 are superimposed to provide the proper alignment before 
placement. The camera 118 and associated electronics can confirm that the 
placement of the part was correct. Thus, the present invention provides a 
closed loop image guided device for the pick and placement of parts. Of 
course, the pick and placement of all surface mounted electronics and 
mechanical devices onto substrates and other objects are within the 
contemplation of the present invention.