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SNAP RING DIMENSION VERIFICATION. Diameter. Gap. PURPOSE. Inspect Diameter and Gap on 3 snap ring variations; Allowable inspection time = 500 ms; Diameter Inspection: 41.0 +/- 0.4 mm; 48.0 +/- 0.4 mm; 49.0 +/- 0.4 mm. Gap Inspection: 35.6 mm +/- 0.4 mm; 42.6 mm +/- 0.4 mm;
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SNAP RING DIMENSION VERIFICATION
Diameter Gap PURPOSE • Inspect Diameter and Gap on 3 snap ring variations; • Allowable inspection time = 500 ms; • Diameter Inspection: • 41.0 +/- 0.4 mm; • 48.0 +/- 0.4 mm; • 49.0 +/- 0.4 mm. • Gap Inspection: • 35.6 mm +/- 0.4 mm; • 42.6 mm +/- 0.4 mm; • 45.0 mm +/- 0.4 mm.
CHALLENGES • Technical: • Achieving the required accuracy of the measurements; • Designing an appropriate equipment layout to fit into existing machine; • Variation in snap ring distance to camera; • Measuring the gap which is defined as the shortest distance. • Commercial: • Deviating plant vision spec from Cognex.
SYSTEM OVERVIEW • Working Distance – 227 mm; • Field of View – 89 mm x 67 mm; • Camera resolution – 1600 x 1200 pixel; • Pixel Resolution – 0.056 mm / pixel.
EQUIPMENT • VA41 Vision Appliance; • IPD-A-IOB Breakout Board; • AVT-FM201B 1600 x 1200 Firewire Camera; • CCS LDL-TP 83 mm x 75 mm Red LED Backlight; • CCS PB-2430 Light Controller; • Opto-Engineering TC1280 telecentric lens.
SOFTWARE • Sherlock 7; • Visual Basic .NET GUI.
PRESENTATION • Snap rings are fed down a chute into a rotating index table. • A cut-out in the index table allows the backlit snap ring to be imaged by the camera. • The system passes/fails based on diameter, gap, correct type, error. • The part is dropped in one of four bins: • Good; • Over Gap; • Under Gap; • Other Failure.
SHERLOCK • Program Flow Control; • Scripts; • Advanced Tools; • Sub Routines.
PROGRAM FLOW • Variables are initialized; • Wait for inspection trigger; • Read part select inputs and call inspection sub routine; • Send outputs; • Log Results; • Update Display elements.
DIAMETER • 24 Edge Points were located; • Rakes are at the most circular section; • A circle is fit to the points; • Diameter of the fit circle is used for pass/fail criteria; • Produced accurate, repeatable results.
GAP • Line ROI – Max Edge for positioning; • Right and Left ROI with Edge Crawler. Edge Crawler – This algorithm is the key to the accuracy of this inspection. Using the extracted points and a script the minimum distance can be extracted.
GAP • Gap Script; • A rough check and precise check are used for speed; • Rough Check: • Every third distance. • Precise Check: • +/- 5 points from rough position checked. 80 x 80 = 6400 Every 3rd = 606 +/- 10 = 100
GUI FEEDBACK • Overall and individual results; • Fail counters; • Inspection Time; • Reporter Window: • Updates as inspection progresses; • Allows peak into flow of Sherlock program; • Useful for debugging and operator feedback.
GUI FEEDBACK • Current Measurements; • Current Limits; • Last five results displayed: • Especially useful for quick cycle time.
GUI LIMITS • VB Interface allows for users to simply read and adjust pass / fail criteria. • This section can be password limited or access controlled through username and passwords.
CHALLENGE ACCURACY +/- 0.4 mm Tolerance Ideal 0.04 mm pixel • Camera resolution – 1600 x 1200 pixel for 0.056 mm / pixel; • Balance system capability and cost; • Calibration – A modified grid distortion target was used for four point calibration; • Telecentric Lens – High quality optics / low distortion.
CHALLENGE LAYOUT • An inspection window cut-out; • Camera and lens below machine; • Guard required to reduce debris from gathering on lens; • Maintenance procedures for cleaning cut-out.
CHALLENGE RING POSITION • The snap ring is held on the dial table that rotates on an angle. • The part tends to shift closer and further from the camera. • The telecentric lens allows for accurate measurement despite the movement. • Pre-build testing was performed to verify the expected movement would not have a negative effect on the measurements.
CHALLENGE GAP • The gap measurement presented a difficulty for two reasons: • The part tended to move and rotate. • The area where the gap is formed changes shape because the rounding operation is not consistent. Using a template match was inconsistent due to the change in shape. Edge Crawler and a custom script was used to accurately find the correct gap points.
IMPLICATIONS • The inspection system successfully sorted out improperly formed parts and therefore eliminated labour intensive mechanical sorting. • Feedback from vision system is used to tweak the forming process. • The success of this application has been followed up by the replacement of an existing Cognex system that was inspecting large snap rings for diameter, gap, tang length, bend radius and cut angle.
BENEFITS OF VA41 • Large hard drive for saving image log used for debug and set-up purposes. • Easy to implement VB GUI: • Graphical display is a significant selling feature; • Visual feedback reduces required technical support; • Remote assistance capability: • Reduces travel and gives customer a level of comfort; • Choice of firewire, analog, GigE cameras: • Built in I/O.
BENEFITS OF SHERLOCK • Large array of inspection algorithms; • Large pre-processor selection; • Scripting capability; • Visual Basic Interfacing; • Easy to control flow of program; • Sub routines allow for re-use of code; • In general, the flexibility of Sherlock allows for creative solutions to be developed.
SUGGESTIONS FOR EASIER DEPLOYMENT • Software – Ability to create a folder and check if it exists. • Software – Ability to delete files and folders. • Software – Ability to save the inspection so that variable state is not lost if power is cut. • Hardware – The i/o, and the requirement for a pull-up resistor on the VA41 complicates the integration process. In comparison the VA31 is much more straight forward for a technician to integrate into a panel. Suggestions that have been implemented: • Sub Routine re-ordering; • Ability to run script without running inspection.
