Machine Vision Applications

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 N1.
• .....
• Processor 8 receives image from bottle 2N.
• Processor 1 receives image from bottle 2N1
• 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