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. 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.)

Lighting Back-lighting (Lighting methods ) 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

Sample Images - Glass Bottle Back-lighting Dark-field

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.

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.

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

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.

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.)

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.

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.

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

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