There are key elements that go into your surface inspection system that need to be understood before purchasing. This article highlights these elements.
It's important as a purchaser for you to understand what information you need to gather to gain an accurate cost of your vision system for surface inspection or print inspection. The key details are outlined in this article.
When specifying a web inspection system, there are so many different technologies and issues to consider. Let’s understand some of the critical criteria.
The three key factors are:
- Line speed – does your line run at 100mpm or 10mpm?
- Product widths – what is the maximum width of product to run on that line?
- Minimum defect size – what is the smallest defect that must be detected?
Let’s create an outline and explain how each element affects the cost and the best practice for specifying a surface inspection system. The system below illustrates a typical line scan application. Assume the web width is 2m and the line speed is 400 mpm.
The first step to solving any problem is to define the question and to make sure each element of that question is well understood. For a web inspection system, we need to define what we are inspecting.
Let’s ask the following questions:
- What are the defects that need to be detected? A clear list should be defined.
This list should be split according to a Pareto chart. There will be some key defects that represent the majority of the complaints and then a number of outliers. An outlier could be defined as a defect that would be nice to detect but not absolutely necessary.
- What are the characteristics and tolerances for each defect class? This includes width, height and contrast.
- What are the typical run time speeds for manufacturing? But also record the minimum and maximum speeds.
- What are the typical product widths? Also specify the minimum and maximum widths.
- What substrates are used? Match the defect list to the substrate. Note the percentage of each substrate used.
- Clearly define the installation envelope and the environment.
It is important to investigate the cost of a system that will address the key products, defects, and requirements and to determine what the delta in cost to address the remainder is. Many projects never get off the ground as a result of over specifying. Do not over specify your system. Look carefully at typical figures. Understand the cost delta between typical and maximum. Look at quotes on the upgrade cost from typical to maximum. Often a project dies in costing due to setting the wrong goals. Learn more about collecting defects, classification, specifying key components and meeting business objectives by downloading our collection of whitepapers.
Understanding your defect suite.
We looked at the effect of the minimum defect size, width and line speed on the complexity and cost of an inspection system in our first blog post. The next step is to analyze the defect suite and to understand which defects are key.
5 Steps to categorizing defects:
- Identify all the defects that are present where the machine vision system will be installed. Include those defects that are caused upstream such as substrate issues.
- Identify the source of each defect.
- Identify the defects that are causing customer complaints, and if possible use a Pareto chart.
- Identify any measurements that if out of specification would result in defective product downstream.
- Identify products and categorize according to color and substrate type.
The result is a table with a list of defects, the minimum acceptable size, the root cause if known, and any particular information as to the effect from the substrate. It's then important to start a collection process so that there is a set of samples that vendors can test. See sample test collection.
Which one is the Insect?
Now you have your defects list. The next step is for the defect to be categorized according to class and action. The following is an example of how to classify.
- Class 1- Critical defects that must be alarmed immediately and the operator must take action. This may require an alarm to be reset, the machine to stop until the reset is complete. It may also mean placing a physical mark put on the product such as a tag, so that it can be removed downstream prior to shipping.
- Class 2– Defects that if identified are not an issue unless there is a density of such defects within a time frame or a defined product area. Until then the defect is displayed as a warning, and when the density of that type exceeds a threshold, it is elevated to a class 1 type.
- Class 3- Small defects that need to be tracked for statistical issues but no action to be taken.
Each defect on the list should be classified. The next step is to assign priority according to returns. There may be a class 1 issue, but if it does occur on all substrates, and what percentage of returns are attributed to this problem. This is a key exercise.
You may ask why?
- Sometimes one defect class that seems easy for a human to see can define the scale of an automated optical inspection system. Simple examples are very subtle streaks. Large defects that are low contrast can be easily recognized by humans as we can integrate over large areas and can segment these problems. This is not the same for a vision system and may require special processing techniques.
- Maybe one defect out of a list of ten, may require an extra optical setup such as another bank of cameras and lights.
So the final column needs to be added to the list. This column defines if this defect is detected by system specification A, or A+B, or A+B+C, or by C alone. This will then allow you to allocate your capital where you can best get a return. So to recap, the steps to complete are:
- List of all defects
- List of all measurements
- List sources of defects
- List substrates and colors
- List minimum acceptable sizes
- Classify according to action
- Determine minimum equipment requirements for each defect
- Understand which optical setup is required for each defect
Download our whitepaper below to learn everything you need to know when purhcasing your surface inspection system. If you have any questions or would like to learn more do not hesitate to contact us today.