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Comparison of Industrial Vision Sensors to Standard Photoelectric Sensors
by AIA Member - Pepperl + Fuchs, Inc. - VMT Posted 07/21/2010
Requirements for product test vary widely within the market. Clearly, there are a variety of means to accomplish this task from simple photoelectric sensors capable of evaluating a single feature to expensive custom vision systems with nearly endless capabilities, limited only by the size of one’s checkbook. In the end, however, the questions are “How much testing is necessary?” and “How can one minimize the costs of such testing?” Let’s look at some decision points of a new series of product called “vision sensors,” and how they might relate to this continuum of testing needs from simple to complicated.
The vision sensor emerged on the market several years ago. Recently, with the participation of many vendors and an array of products in their offering, the number quickly escalated so that today there are myriad products to choose from.
From a hardware standpoint, vision sensors share common capabilities with their bigger brother’s vision systems and smart cameras. Generally, they are not so different and consist of imaging optics, imager, processor, I/O, firmware or software, etc. Where they do differ is in the flexibility of the firmware or software to accomplish certain tasks.
A vision sensor has a much-abbreviated set of methods that are specific to its testing capabilities. If the function is not included from the manufacturer it is generally not possible to add the capability. On the other hand, the smart camera has a large array of functions from which to build an application with the possibility of writing external routines and hooking them into the program environment to semi-customize tasks. Finally, a vision system may be completely freeform, allowing it to be configured to specialized test and inspection needs.
Flexibility of Position
Unlike a traditional sensor, an important feature of the vision sensor is its ability to evaluate pixels; these pixels are in a region of interest rather than at a specific spatial point. Therefore, an evaluation can be made at any location within the field of view of the imager as we see in Figure 1.
In this example, the component is free to move vertically or horizontally so long as the attribute we are inspecting (the c-ring and its opening) remains in the region of interest (the yellow box). Making a presence measurement of the c-ring with traditional sensors would likely be quite easy, but the ability to make that evaluation over a dimensional variation that exceeds the thickness of the c-ring would be impossible; it simply would not remain in the field of view of the sensor (typically called the “spot”).
Traditional sensors do a single job very well; however, they do not typically have the latitude to make several tests, for instance, with the gum pack in Figure 2. Inspecting the presence of any one of the products in the wrapper is easy, but inspecting all six using traditional sensors either requires six sensors connected in some logical AND configuration, or an elaborate indexing and control mechanism to carry out six separate tests and some storage mechanism to tabulate the index vs. result for each position and logically AND them together at the end of the steps.
For the vision sensor, there is also no requirement that the method used in the first evaluation be repeated. We could just have easily asked for an inspection of printing and sealing components in the same field of view as the count. All can be accomplished in a single test so long as the vision sensor can see all the attributes that it needs to inspect.
Comparison of Industrial Vision Sensors
A great example of multiple checks is the milk carton (Figure 3). During production, several attributes need to be checked, these include:
- Seal integrity
- Printed date and lot coding
- Cap integrity
It is easy for the vision sensor to locate the part feature of a specific check, such as the cap, and test for its presence then move to the date code, the sealed edge, etc.
Flexibility of Inspection Technique
As noted earlier, in most cases, a photoelectric sensor comes out of the box ready to use. There may be some setup, certainly alignment, and also perhaps some sensitivity adjustment, which may be mechanical or programmed via pushbutton. The point being, that outside of minor adjustments, when it comes to detecting a single aspect of an object, the capability of a photoelectric sensor cannot be matched by vision sensors.
In the case of the vision sensor, flexibility lies in its ease-of-use. The ability to vary the test method is generally accomplished via a program, a graphical user interface that facilitates customizing the application to achieve the test result. Once the user has setup the program, it is downloaded to the vision sensor and the vision sensor carries out the set of instructions.
There are a variety of ways to accomplish this. Figure 4 is an example of the programming environment for one type of sensor. It is a single screen divided into sections for operation, configuration, results, setup, and display. Some manufacturers of vision sensors compartmentalize the user interface into a step-bystep wizard. In either case, under the umbrella of “ease-of-use,” the functionality of the program environment is huge, albeit fixed to certain methods.
As with any vision system, the need to appropriately illuminate the subject matter in order to achieve unambiguous results is paramount in the setup. Considerations for interchangeable optics and external lighting to maximize image contrast and minimizing ambient lighting variations are real considerations that need to be factored into one’s decision and budget.
The vision sensor is a great addition to the tool box of industrial process engineers and technicians; it brings new capability to error-proofing in a variety of production environments from automotive to consumer packaged goods.
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