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.