Machine Vision

Vision Sensors See More Details

September 1, 2006 By: Steve Maves Sensors

As prices fall and functionality rises, vision sensors are an increasingly popular choice for complex inspections. Is it time for you to make the switch?

For decades, photoelectric sensors have been ubiquitous on packaging lines—triggering processes, counting, checking for label presence, sorting objects of different sizes, and performing dozens of other tasks. Even so, their usefulness has been limited by their single beam and lack of brain power. That's where low-cost vision sensors can help. Vision sensors are especially well suited to applications that are too complicated for photoelectrics or that require complex fixturing.

Often the Obvious Answer

Because vision sensors capture and analyze a multipixel image, they are an obvious choice for inspections that require several photoelectric sensors but not complex enough for human inspection: for example, ensuring that package labels are present and properly aligned, that bottles are capped, or that cartons contain the proper number of units (Figure 1).

Figure 1. Package inspection by a PresencePLUS P4 GEO 1.3 vision sensor
Figure 1. Package inspection by a PresencePLUS P4 GEO 1.3 vision sensor

In these relatively simple applications, vision sensors provide the usual advantages of automation—speed and accuracy—at a lower cost per inspection than human inspections. But creative packaging engineers are also discovering ways to use vision sensors for more complex applications:

  • 1. A major food packaging company uses one vision sensor to detect the label on top of a package and another to read the bar code on the bottom. A PLC then determines whether or not the top and the bottom match.
  • 2. Nice-Pak Products, which makes and packages most wet wipes on the market, uses a vision sensor to peek into the tiny opening of packets of individual towelettes to make sure the towelette is present at the moment a tube squirts in the liquid.
  • 3. A bottling plant in Hong Kong inspects bottled soft drinks for foreign matter—such as a piece of plastic or paper—that might fall in during the bottling process. The sensor captures an image of the cap and bottle neck and then checks to see if the distance between the top of the bottle cap and the top of the liquid is within specifications. Foreign matter displaces the liquid, changing that distance.
Figure 2. The PresencePLUS P4 OMNI and P4 OMNI 1.3 vision sensors
Figure 2. The PresencePLUS P4 OMNI and P4 OMNI 1.3 vision sensors

The Critical Factors

Is a vision sensor the right solution for your application? Keep these three parameters in mind:

  • 1. Lighting
  • 2. Response time
  • 3. Resolution

Lighting is the most important technical factor of any vision application because it determines the contrast between the feature of interest and the background. Changing the angle and position of lighting makes a drastic difference (Sidebar, "How You Light Is What You See,").

How You Light Is What You See
How You Light Is What You See

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