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Machine Vision Applications

Machine Vision Applications. Case Study No. 6 Inspecting Clear Glass or Plastic Bottles. Manufacturing Process. Plastic bottles are made by blow-moulding. Glass bottles by a blow-blow or press-blow moulding process.

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Machine Vision Applications

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  1. Machine Vision Applications Case Study No. 6 Inspecting Clear Glass or Plastic Bottles

  2. Manufacturing Process • Plastic bottles are made by blow-moulding. • Glass bottles by a blow-blow or press-blow moulding process. • Bottles are transported away from the forming machines and through the filling and inspection stations on a continuously moving linear conveyor belt. (This is more economical & more robust than an indexing conveyor.)

  3. Lighting • Back-lighting (Lighting methods 17 - 19) • Dark-field illumination (Lighting methods 20 & 21) • "Freeze" motion of bottles, with strobe lighting triggered by proximity detector. (Lighting method 143) • Continuous lighting • Sensitive progressive-scan camera with fast electronic shutter • Line-scan camera is used. • Strobe lighting is potentially dangerous (epilepsy & migraine, moving objects appear stationary) Good screening is needed

  4. Sample Images - Glass Bottle Back-lighting Dark-field

  5. Machine Vision -generic system The diagram opposite does not specificy items of equipment needed for bottle inspection and does not define the layout of the lighting-viewing sub-system.

  6. Inspecting widgets on a conveyor -generic system • To inspect plastic or glass bottles, back-lighting or dark-field illumination would probably be used. • An array with N processors improves throughput by factor N but does not reduce latency at all. • Notice the spacing between the camera and the accept/reject device.

  7. Points to note • The lights and camera are located on opposite sides of the conveyor belt (assuming back-lighting is used) • Camera is placed up-stream from the accept/reject mechanism. • When using an N-processor concurrent array, there are at least N bottles in transit on the conveyor belt, from the camera to the accept/reject device

  8. Conveyor Belt • The speed of the conveyor belt should be controlled within tight limits. • The clock speed of the camera must be adjusted in real time to compensate for any small remaining variations belt speed.

  9. Parts-present (Proxinity) detector • This might typically consist of an IR LED placed on one side of the conveyor and a photo-detector (photo-diode or photo-transistor) on the other. • The IR LED should be switched off during image capture to avoid degrading the image. (Most solid-state cameras are sensitive to IR.)

  10. Camera • When working with a continuously moving object, a line-scan camera is ideal. • If an array camera is used, a fast electronic shutter should be fitted, in preference to using strobe lighting.

  11. Concurrent Array: Task Assignment Array of N identical processors. Inputs from the camera via a rotary switch: • Processor 1 receives image from bottle 1. • Processor 2 receives image from bottle 2. • ..... • Processor N receives image from bottle N. • Processor 1 receives image from bottle N+1. • ..... • Processor 8 receives image from bottle 2N. • Processor 1 receives image from bottle 2N+1 • etc.

  12. Concurrent Computing Array • Increased throughput rate (latency unchanged) • Cost effective • Simple software organisation • Self checking • Only one type of circuit board: standard processor unit • Quantity discounts • Easy to repair: hot wiring possible • Graceful degradation of performance • Easy to upgrade: add new processor cards

  13. Human factors. • Avoid exposing workers to flashing lights: add shrouding if necessary • Show performance statistics • Step-by-step checking of processor cycle possible (with any processor) • Show status of each processor

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