Video Probe Aligning of Object to Be Acted Upon

A computer program listing is set forth in a microfiche appendix.

The present invention relates in general to aligning and more particularly concerns novel apparatus and techniques for aligning an object to be acted upon at particular locations with a device that performs the specific operations on the specified points. A specific embodiment of the invention facilitates aligning circuit boards for receiving surface mounted components with a stencil for applying solder paste to a pattern on the circuit board while facilitating rapid reorientation for applying solder paste to a different pattern.

A typical screen printer is the commercially available ASP-24 automated screen printer available from MPM Corp., 71 West Street, Medfield, Mass. 02052 incorporated herein by reference. This screen printer includes a replaceable screen for printing solder paste on a footprint of pads on a surface mount circuit board. This system may also deposit other materials, such as epoxy, polymer, cermet and most other screen printable materials. This system includes a programmable controller for positioning each circuit board beneath the screen or stencil above that deposits the screen printable materials.

Prior art screen printers with vision require printing solder paste on the circuit board, driving a table out beneath the cameras and the cameras look at the solder paste. The solder paste in then wiped off, the board enters beneath the stencil, and the board is printed a second time, and reciprocates back out for examination by the camera.

A prior art screen printer includes a camera that looks between the board and the screen, but it does not look at an image on the screen, only at the board. The camera on that device does not reciprocate. It uses a fixed mounted camera that just looks down at the board, and it requires that the board be outside of the stencil which comes down upon it by a certain amount such that the camera does not get in the way.

It is an important object of this invention to provide improved apparatus and techniques for optically aligning an object to be acted upon at one or more points with a device for repeatedly acting upon like objects at like points.

According to the invention, at least one, and preferably two, movable video probes are located between the acting device, such as the screen or stencil, typically above, and the object to be acted upon, such as a circuit board, typically below. The one or two probes, first look at and locate a pattern, such as on the object device, such as a circuit board, store information on features in this pattern, such as x, y and .theta. information on key features in the pattern. Associated data processing apparatus processes and stores this information. The one or both video probes then rotate downward while still in the region between the object to be acted upon and the device to act, examine the other pattern such as on the acting device, such as the screen or stencil, for a matching pattern and relatively position the object and device so that the two patterns are in alignment. The one or both video probes then retract, and the device, such as the stencil or screen engages the object, such as the circuit board, to deposit solder paste or other screen printable material on selected points in registration with corresponding points on the device, such as the screen or stencil. Once a specific pattern is learned, the invention may repeatedly relatively position by looking at only the object to be acted upon.

According to a specific aspect of the invention, a positionable base supports a video probe.[.,.]. . This base is formed with at least one and preferably two slots having a generally horizontal leaning portion with a depending angled trailing portion for guiding elements that carry the video probe as it moves between the extended position at the leading end of the slots, with the video probe between object and device, and the retracted position at the trailing end of the slot, with the video probe retracted from the region between object and device.

Numerous other features, objects and advantages of the invention will become apparent from the following specification when read in connection with the accompanying drawings in which:

FIG. 1 is a perspective view of a fully automated screen printer with vision system according to the invention;

FIGS. 2 and 3 are perspective views of the video probe assembly in retracted and extended positions, respectively;

FIG. 4 is an exploded view of the video probe and portions of the supporting assembly;

FIG. 5 is an exploded view of the video probe support assembly; and

FIG. 6 is a diagram illustrating the relationship among target points and the center of rotation helpful in understanding principles of the invention.

With reference now to the drawing and more particularly FIG. 1 thereof, there is shown a perspective view of an exemplary embodiment of the invention comprising the commercially available ASP-24 fully automated screen printer available from MPM Corp. with movable video probes according to the invention. The apparatus includes a base 11 for supporting a circuit board to be printed upon, left and right video probe assemblies 12 and 13, a controller 14 and video monitor assembly 15. The system also includes a positionable screen/stencil support assembly that may be moved to align the screen or stencil with the circuit board to be printed upon. A lower platform 16 supports various packages for containing hardware used in the system.

The base components added to the ASP-24 automated screen printer include the two video probe assemblies 12 and 13, a fiberoptic light source for illuminating the screen and board during pattern recognition, the trackball teaching module 14, and a vision processor unit comprising a commercially available Cognex Type 2000 with suitable software, an example of which is set forth in Appendix A.

The invention utilizes a user-friendly operator interface. Menu-driven software, such as set forth in Appendix A, produces a display on video monitor 15 that prompts the operator through each phase of the four-step setup process. Help screens are accessible at each step, outlining in detail the instructions for performing that step. At the completion of each step, the system automatically displays the next prompt on monitor 15. Setup errors may be quickly corrected through the push-button editing feature with trackball teaching module 14.

The operator uses the independent trackball teaching module 14 to select a pattern on the PC board within each probe's field of view of left and right probe assemblies 12 and 13. Teaching module 14 capture these two select patterns and then recognizes and locates the matching patterns on the screen or stencil. Without further operator involvement, the system implements the program by calculating the pixel representation in x and y coordinates of significant features and angular orientation in azimuth, conveniently referred to a .theta. geometry. The apparatus then determines an accurate "home" position for the screen/stencil in relation to the circuit board then positioned on base 11. The offset data corresponding to the difference in x, y and .theta. positions of the observed circuit board features and corresponding screen/stencil features is automatically downloaded into the screen printer stepper motor programmable indexes. These indexes activate the screen printer stepper motor assembly to reposition the screen/stencil relative to each stationary board for subsequent print cycles with screen/stencil in horizontal alignment with each circuit board below. This automatic positioning adjustment accommodates variations in board tolerances. Between print cycles, the left and right video probes 12 and 13 automatically move in, look at the next board in place, move out again, screen/stencil is positioned if necessary, and printing occurs without operator intervention.

As an alternative feature the use may verify alignment with a post-print inspection feature. As part of the setup menu, the operator may choose how often to do post inspection. Then a choice may be made from three tolerance threshold menu selections for alignment error detection.

Printed circuit boards having patterns outside these parameters produce an error message on video monitor 15. This error signal may be used to activate a diverter to reroute that board for reworking. The error signal may also be used to pause automatic operation so that the operator can correct the problem before any more material is wasted. To begin the setup process, the operator presses a "video attention" button on teaching module 14. Actuating this button produces the setup menu on video monitor 15. Then the operator completes four steps, pressing a single button 14 on teaching module 14 to signal completion of each step. First, the operator clamps left and right video probe assemblies 12 and 13 in place so that each probe when in the extended position may observe two different patterns of significant features of the circuit board and corresponding screen/stencil. The operator then teaches these board patterns. Using trackball teaching module 14, the operator moves joy stick 14A to define the windows within the .[.probes,.]. probes' fields of view embracing significant features, typically a pattern of pads for receiving solder paste, and then pushes a single button on teaching module 14 to teach the pattern to the apparatus which then captures the patterns. The operator positions the probes of left and right probe assemblies 12 and 13 to view upward to learn the screen or stencil pattern above. The associated data processing system automatically locates the matching pattern on the screen or stencil, determines the pixel representation of it in x, y and .theta. geometry; automatically downloads offset data for x, y and .theta. for alignment purposes. The operator then rotates the video probes downward toward the circuit board, and the apparatus is now ready to print automatically. As each board is positioned on base 16, the video probes on left and right assemblies 12 and 13 extend above the positioned circuit board, determine x, y and .theta. differences between the positioned circuit board and the screen or stencil above and operate stepper motors to reposition, if necessary, the stencil or screen for alignment with the circuit board below.

Referring to FIGS. 2 and 3, there are shown perspective views of the video probe assemblies in extended and retracted positions, respectively. Each assembly includes a base 21 with vertical walls 22 and 23, each formed with front and rear slots 24 and 25, respectively, having horizontal leading portions 24A and 25A, respectively, and depending angled trailing portions 24B and 25B, respectively. A vertical bracket 26 rests upon side walls 22 and 23. The movable probe support 27 is formed with elements, such as 31 for riding in slots 24 and 25. The movable base 27 supports 90.degree. mirror tube 32 that provides an image to camera 33 in housing 34 and furnishes illuminating light to illuminate the screen or stencil and the circuit board. FIG. 2 shows the video probe in the extended position between circuit board and screen or stencil. Arrow 35 illustrates the 180.degree. rotational range of mirror tube 32 for looking above and below. FIG. 3 shows the video probe in the retracted position outside the region between circuit board and screen or stencil. Arrows 36 indicate the path traveled by the probe up and forward to the extended position and rearward and down to the retracted position.

Referring to FIG. 4, there is shown an exploded view of a number of elements forming the video probe with some portions cut away. The Volpie 90.degree. mirror tube 32 includes a mirror 32A mounted at 45.degree. angle relative to the axis of mirror tube 32 with a Volpie intrascope approximately 12 inches long mounted inside mirror tube 32. A rotate clamp mechanism is shown inside broken line 42 with the parts exploded and functions to position mirror tube 32 in one of two positions 180.degree. apart, looking up as shown for viewing the bottom of the screen or stencil, or looking down for viewing the top of the printed circuit board. Focus knob 43 allows for focusing the image of camera 33 by observing the image on video monitor 15.

The main probe clamp 44 carries two shock absorbers 45 and 46 above the left and right arms 47 and 48, respectively. Main clamp 44 also carries three ball-and-cone pieces, two of which 51 and 52 are visible in FIG. 4.

Referring to FIG. 5, there is shown an exploded view of a video probe support assembly with the housing cut away and the video probes omitted to better illustrate certain structural features. Main clamp 44 supports the vision probe at the center of the camera 30 and Volpie intrascope unit 44. By supporting this assembly at the center of gravity, when the ball-and-cone pieces 51, 52 and 61 engage reference balls, there is very low energy movement to achieve precise reference position very quickly.

A linkage allows the center of effort to be aligned along the axis of mirror tube 32 and Volpie intrascope unit 41. This linkage includes a left link 62 and right link 63. Air cylinder 64 drives the leading ends of these links with pin 65 seated in clevis 66 secured to the leading edge of air cylinder rod 64A. Main clamp 44 is formed with vertical recesses 44A and 44B that engage the left and right arms 71 and 72, respectively. These arms accommodate four rollers, a trailing pair in openings 71A and 72A, and a leading pair in vertical slots 71B and 72B. Left and right side plates 22 are cammed as shown. Left and right angle brackets 73 and 74 are attached to the top leading portions of left and right side plates 22 and 23, respectively, that rollers on the link system engage. These angled brackets provided horizontal surfaces for insuring that air cylinder 64 pulls the probe itself horizontally without a vertical component of force. Rollers 75 and 76 ride on left and right angled brackets 74 and 75, respectively. A pair of flow controls 81 and 82 mount on the trailing portion of left side plate 22 and have a needle valve which allows re-exhaust in each direction for aiding in providing smooth motion upon operating air cylinder 64 for extension and retraction free of binding or violet motion. Shock absorbers 45 and 46 also help allow smooth action when the vision probe approaches a final location against the ball and cone. Shock absorbers 45 and 46 provide damping that prevents the CCD cameras 33 from being subjected to violet movements.

Main clamp 44 clamps the vision probe firmly to the tooling bed 11 of the machine, while allowing some relative adjustment on the probe to position the mirror tube 32 between the stencil and circuit board that may vary depending on the thickness of the circuit board. Clamping screws may be loosened and two jacking set screws 94A may be adjusted to control this position.

There is a right ball mount support 83 for supporting balls 83A and 83B and a left ball mount support 84 for supporting ball 84A. Balls 84A, 83A and 83B engage ball-and-cone pieces 51, 52, and 61, respectively. Ball mounts 83C, 83D and 84B are independently adjustable. When air cylinder 64 urges the probes to the extended position, ball mounts 83C, 83D and 84B are tightened so that balls 83A, 83B and 84B are exactly seated in ball-and-cone pieces 52, 61 and 51, respectively. This position is then readily repeatable.

Left and right fiberoptic supports 85 and 86 are connected to left and right links 62 and 63, respectively, and carry fibers optics (not shown) in parallel to the Volpie intrascope. This arrangement with fiber optics along and parallel to the Volpie intrascope provides back lighting. That is to say, this structure provides a very shallow light illuminating the object being viewed. The shallow angle light reduces glare, and the image of the object being viewed appears more clearly with back lighting.

Pins 85A and 86A pivotally support left and right fiber optic supports 85 and 86 to slots 71B and 72B in left and right arms 71 and 72, respectively. This structural arrangement allows free vertical rotation of these supports so that when air cylinder 64 extends and retracts the assembled slide unit, it prevents significant free floating. Stated in other words, because the left and right links are referenced on left and right angle brackets 73 and 74, respectively, at the extended end of the stroke for precise horizontal actuation, the structural arrangement contains when not actually contacting the angle brackets. Pins 85A and 86A in slots 71B and 72B provide this containment. Pin 65 pulls left and right links 62 and 63 up and outward toward the extended end until rollers 75 and 76 engage the underside of left and right brackets 73 and 74. On retraction pin 65 moves down and toward the retracted position.

A focus hold bar 91 is secured to the top of main clamp 44 and carries a piece of felt on the bottom for engagement by focus knob 43. This holding function helps prevent focus knob 43 from rotating in the presence of repeated changes in position of the probe assembly.

A proximity switch 92 is fastened to side plate 23 for providing a signal indicating that the probe is in a safe retracted position to allow the apparatus to print on the circuit board then below the aligned screen or stencil.

There is a left clamp piece 93, left set screw adjust bar 94 and left clamp nut plate 95 secured to the extended end of left side plate 22. Similarly, there is a right clamp piece 96, right set screw adjust bar 97 and right clamp nut plate 98 secured to the extended end of right side plate 23. Set screws such as 94A and 97A, allow adjustment of set screw adjust bars 94 and 97.

Other components of the vision system include the vision processor, a commercially available type.

Cognex unit and monitor having Cognex commercially available software for image searching for matching a stored image with an observed image to permit alignment of the screen or stencil with a circuit board below together with software for controlling the menu-driven functions relative to the specific screen printer application of the vision processor.

An operator communicates with the processor for aligning a particular board through a trackball teaching module 14. This teaching module has three buttons 14A, 14B and 14C and a ball 14D. Rotating or pushing the ball with fingers allows the operator to move the window within the field of view on monitor 15, and actuating the buttons allows the operator to select a menu item or move the cursor down through the menu. Operating button 14C selects a help menu.

It may be advantageous to include a monitor mounted on an arm on the side of the screen printer for observing what each vision probe in assemblies 12 and 13 observes. Two vision probes are used to detect rotational movement .theta. in addition to rectilinear movement in x and y directions.

The light source is preferably a high intensity light coupled to the fiber optics. These fiber optics carry light where needed on each side of each vision probe, a total of four light sources. There is also a source of illumination above the stencil for backlighting the stencil to permit observation of features, such as holes, in the stencil for identifying the image for capturing.

We turn now to a description of the process from the beginning when an operator sets up the vision system to operate upon a specific run of circuit boards to operation in an automatic cycle.

First, the operator adjusts the mechanisms on the screen printer to handle the particular board size in conventional manner. That involves adjusting the tracks and board stop, and inserting the stencil needed to print on that board inside the screen printer. These mechanical steps are the same as with the ASP-24 fully automated screen printer without the vision features according to the invention.

The operator then applies power to the apparatus to provide the main menu on display. This main menu allows the operator to select setup, edit setup, autoprint or address a help menu. First the operator selects setup. After selecting setup, a menu prompt on the screen instructs the operator to locate each vision probe over the particular object on the circuit board which the operator feels is unique and will be trained upon, such as a configuration of surface mount circuit board footprint pads. The apparatus will then look for this same unique pattern on subsequent boards in an automated production run.

The operator manually positions a first vision probe, such as the one in left assembly 12, over the board centered over the particular unique object. The operator then clamps this vision probe assembly in place. The operator then pushes button 14A on the teaching track ball module to select the next prompt on the menu. That prompt directs alignment of the second vision probe, such as the one on right assembly 13. The operator aligns this second vision probe in substantially the same manner as the first and again presses button 14A to produce the next prompt. The board patterns are taught and then the apparatus .[.loos.]. looks at the stencil. This next prompt instructs the operator to rotate the vision probes .[.throu.]. through 180.degree. to look at the stencil. The operator then rotates mirror tubes 32 through the 180.degree. established by the rotate clamps 42. The operator then drives the screen into position above the board with joy stick and .theta. push button controls on the ASP-24 machine. The operator positions the overhead structure in x, y and .theta. into a position such that the vision probes observe the pattern on the screen or stencil that matches the pattern on the circuit board previously aligned centered within the field of view of the probes. In response to each actuation of button 14A the menu prompts the operator to teach the pattern that is desirable after manually locating the stencil. The operator may be required to make fine adjustments in the window by moving trackball 14D and changing the window size until the object is clearly defined and boxed in a square on monitor 15. The operator then depresses push-button 14B and thereby teaches the system the pattern it is looking for with the first probe. The operator follows the same procedure for the second probe. Then the operator pushes button 14C to indicate teaching is complete.

The screen printer then makes a number of automatic moves to learn the geometry associated with this particular setup and this particular type board.

The x stepper motor first moves the screen printer a predetermined number of steps in the x direction. This movement defines the world coordinate system for the cameras 33. Because of this feature the cameras may be placed at any angle anywhere along the front of the circuit board. The y stepper motors then move the screen printer a predetermined number of steps in the orthogonal y direction to confirm the world coordinate system. The system also recognizes the number of steps per pixel during these x-y moves. These moves enable recognition of how much the image moves for every step of the stepper drive system. Then the stepper motors move the screen printer through pure rotation a predetermined number of steps to determine how the object translates in x and y coordinates during a pure rotation. The printer repeats these translational and rotational movements.

The system has thus recognized how the object moves in x and y and how it translates in x and y during a rotational move. This information on these moves allows the system to learn trigonometric solutions of several triangles.

Referring to FIG. 6, there is shown a diagrammatic representation of moves in x, y and .theta. helpful in understanding the principles of the invention involved in learning the geometry of the circuit boards and stencils. The pattern is characterized by a center of rotation 111. The apparatus may learn from looking at the stencil or screen driven by the stepping motors with reference to two points, such as 112 and 113, that are some vector distance away from the center of rotation 111. First performing a move in the x direction facilitates learning the world coordinate system and steps per pixel confirmed by a movement in the y direction. The following rotational moves involves taking a picture of the objects, such as 112 and 113, after an incremental move in one direction, typically counterclockwise, followed by a move from the initial position in the opposite direction by the same increment from the initial position, typically clockwise. The moves in the x and y direction basically define right triangles having a hypotenuse of magnitude corresponding to the square root of the sum of the squares of the incremental displacements in the x and y directions. The angular displacements effectively create two isosceles triangles from the shifts about points 112 and 113 with the center of rotation 111 being the common vertex for both isosceles triangles. The invention facilitates learning the geometry of the board by looking at only two points and making moves in x, y and .theta. directions for each of these points.

An operator may select any target point that appears unique on the board such that it may be distinguished from other target points around it, choose a second target point similarly distinguishable from other surrounding targets, and teach the apparatus the geometry of the board such that when any circuit board enters the apparatus out of line with the stencil, the video probes looking at the board automatically download the proper x, y and .theta. moves for the stencil to bring stencil and board into alignment.

The operator then rotates both probes to look down at the board. Looking now at the board, the system learns the pattern on the board which correlates with a pattern on the screen. The operator has then completed the automatic setup.

It may be desirable to modify this procedure slightly. For example, it may be desirable to rotate the probes to look up again after the system has learned the circuit board pattern to better correlate circuit board and screen or stencil images.

With the probes then looking at the board, the operator may then select auto print on the main menu, and the apparatus is then ready for a production run. During a production run, both probe assemblies index into position after the board has been brought in, recognize the patterns that is learned on each of the probes and automatically moves the screen relative to the board to align the stencil very accurately with the board. The probes then moves into the retracted position, printing occurs and the board just properly printed exits the machine. A new board enters, set down against the vacuum stop, the probes move to the extended position, the apparatus recognizes the patterns, downloads to the stepper motors the proper movements to align the stencil with the circuit board, print, exits, and the process repeats.

As an alternative, the operator may edit a setup. If an operator notices that the screen printer is printing consistently off the pad in one direction or other, the operator may select edit setup from the menu and modify where the screen printer is printing by selecting a predetermined direction and distance of correction. Thereafter, the apparatus will automatically print consistently in the new location and continue to print in that location until modified again.

A feature of the invention is the lighting arrangement for back lighting the stencil. Stencils and screens are usually shiny or have objects on the bottom which may be confused with object features to be taught. By laying the translucent material on the stencil and providing a light behind it, back lighting occurs which prevents this problem. This arrangement disperses the light in a manner that clearly defines each hole in the stencil relative to any type of reflective background that might occur.

The invention has a number of features. The vision probes enter between circuit board and stencil and perform the alignment as distinguished from looking at the board outside the screen printer.

There has been described novel apparatus and techniques for aligning. It is evident that those skilled in the art may now make numerous other uses and modifications of and departures from the apparatus and techniques herein disclosed without departing from the inventive concepts. Consequently, the invention is to be construed as embracing each and every novel feature and novel combination of features present in or possessed by the apparatus and techniques herein disclosed and limited solely by the spirit and scope of the appended claims. ##SPC1##