Automated Optical Inspection (AOI) Guide: All You Need to Know About an AOI Machine.
Author: Ng Ci Xuan – R&D Intern
Editor: Bryan Ng – Marketing Manager
1. What is Optical Inspection?
Before we dive into the explanation of automated optical inspection (AOI) machines, let’s understand what’s an optical inspection in the industry.
Optical inspection plays an important role in optics manufacturing to check and maintain the optics quality, such as inspecting optics surface scratches and digs based on different standards.
The inspection can be carried out either by a person, which is known as manual inspection, or an optical inspection machine.
The manual inspection is carried out using the naked eye or using aids like single lens magnifiers, optical comparators, and optical microscopes.
However, using these aids requires shifting the optics around which increases the chances of damaging the optics and it also cannot detect the flaws quantitatively. Thus, the judgement varies between different inspectors.
This is when an AOI machine plays a part in making optical inspection easier and faster, as compared to the traditional manual inspection.
The optical inspection falls in every stage of the manufacturing process before the goods are determined as good condition and ready to be sold.
2. What is Automated Optical Inspection?
AOI is a machine-based technique typically used in the manufacturing industry of optics. It is used to inspect and certify optics such as flat, curved, coated and optics with different materials.
Flat optics include windows, mirrors, etc., whereas curved optics include lenses with different curvatures.
AOI uses optics to capture images for evaluating potential faults such as dimensional defects and surface defects (e.g. digs, scratches, edge chips, bubbles, stains, impurity flaws, etc.). It provides quick and precise inspection to ensure that the product is of high quality without any manufacturing defects.
With the increase in the complexity of products and them becoming smaller in size, manually inspecting them pose a serious challenge to workers and therefore AOI has a vital role to play in ensuring the products’ quality.
3. Working Principle of an AOI Machine
AOI machines depend on three crucial aspects – Illumination, Machine Vision Camera, and Processing Software.
The optics are placed under light sources that are highly illuminated. By selecting the appropriate light source, one can project different types of defects more easily.
Earlier AOI machines made use of various illumination styles, such as fluorescent, incandescent, ultraviolet and infrared lights but with advances in light technology, newer machines now employ LED lights in colours such as red, green, white and blue.
LED lighting provides a more stable form of lighting and even though its output of light will reduce over time, it can be offset by increasing the current. The amount of light can also be controlled using LEDs.
Thus, LEDs especially coaxial spot light, ring light and collimated light prove to be far more suitable for various vision and uniform illumination applications than fluorescent and incandescent lights.
Aside from the type of lighting, the positioning of the light source is as important. It must ensure that all areas are well lit in order to highlight imperfections and therefore adjustment maybe required for different services.
3.2 Machine Vision Camera
The image capture system takes an image of the product and sends it for analysis by the processing program within the AOI machine. One or more high definition cameras are usually installed to offer better performance and in some systems, they are able to move under command by the software.
Cameras used range from XGA units to high resolution video cameras and the images produced can be either monochrome or coloured. As compared to old cameras, the newer ones have a higher frame rate.
This leads to a faster scanning rate and thus, a huge amount of data is usually generated. Proper processing software is therefore required to be able to manage such a large amount of data.
The AOI machine’s imaging resolution defines the amount of detail it can identify and capture. Resolution is a key factor in an AOI machine as it affects inspection accuracy and inspection speed. The camera’s resolution defines the field of view (FOV), which shows the section that is included in one image. A wider FOV allows the product to be examined with lesser images.
However, it takes more time for each image to be captured as the camera has more pixels, which leads to a lower frame rate. On the other hand, cameras with a narrower FOV have lesser but bigger pixels and thus have a higher frame rate.
3.3 Processing Software
For the AOI machine to be able to inspect a product, the information of an acceptable product must already be installed in the system. There are two ways to program an AOI machine:
- Using of the ‘Golden Board’: a good product is used as a target for the AOI machine and this product is passed through the system for scanning. It will observe the product for the different aspects to be inspected and several products are usually needed to offer ample variance data for the system.
- Algorithm-based programming: the system generates its own profile for the product by feeding product data to it.
4. AOI Machine Vs. Traditional Manual Human Inspection
Humans are prone to fatigue and require breaks and rest from time to time during inspection. Each staff can also only examine one product at a time, and this brings down the efficiency of the whole process. With the market now having high production demands, and with products getting tinier and tinier, manual inspections are no longer a viable option.
Machines, on the other hand, are able to inspect multiple products at one go known as batch inspection and do not require breaks. They can be programmed and monitored remotely, and also work 24 hours a day. This can help manufacturers meet production demands as they have a higher inspection speed as compared to humans. However, machines are prone to wear and tear, which requires maintenance once in a while. Also, if it happens to break down unexpectedly, the efficiency of the production line would be affected.
Humans cannot maintain long-term consistency when fatigue kicks in and this will also affect the efficiency of the inspection process. Studies have shown that human inspectors are not able to maintain a high level of error-trapping for a long duration.
An inspector’s error-trapping capability decreases significantly within 15 minutes from the start of work. The complexity of the job and the time of the day will affect the number of errors made.
In addition, there is bound to be inconsistency between different inspectors as the requirements to pass a product could be subjective to them. These will lead to different standards in the products that are being checked.
Whereas for machines, it has extremely high stability that is able to maintain the same defined standards for all the products, and thus consistency in the inspection is maintained.
The section above talked about how a spectrometer works. In this section, the components of a spectrometer and the variety for each component will be covered.
Manual inspection incurs high labour costs as several inspectors are usually required at one time. Reports have shown that a machine and an operating staff can replace 5 human inspectors, leading to a large amount saved over time. The more production lines, the more money and manpower saved.
However, the initial cost of the machine would be very high. It will also cost quite an amount to train the staff to manage, program and fix the equipment. Not forgetting utilities, repair costs and insurance.
Manual inspection reports are mostly hand-written, so are much more prone to typographical errors. Hand-written reports databases are difficult to maintain and analyse the reports of each stage of the manufacturing process.
Machines generate printable reports in a desired file format at each stage of the manufacturing process making it easy to analyse the errors and helping in maintaining the quality of the product. Thus, preventing complaints/rejection from customers and leading to improve trust and satisfaction among customers.
5. OptiNspec Comparisons – AMF Vs. AOF Vs. MOF
Wavelength Opto-Electronic offers AOI machines known as OptiNspec in 3 variations:
- Automatic Flat Micro-Optics Inspection machine (AMF)
- Automatic Flat Optics Inspection machine (AOF)
- Manual Flat Optics Inspection machine (MOF)
As the name of the machines suggests, AMF and AOF are machines which conduct inspections automatically whereas the MOF is a manual machine which requires humans to operate. This full range of surface quality testers can perform quality assurance and quality control checks on flat, curved, coated and optics of different materials.
All the machines are calibrated to inspect the scratch and dig on the optics surface according to the standard MIL-PRF-13830B and equivalent visibility specification standard. It is designed for in-situ surface inspection of flat optics packed in common gel boxes with printed grids that are readily available in the market.
AOF and MOF machines are meant for the inspection of flat optics with diameters up to 25.4mm while AMF systems can be used for the inspection of flat micro-optics with diameters up to 12mm.
The OptiNspec-AOF101 and OptiNspec-AMF101 models are full-sized machines while MOF and the rest of the AMF models are table-top machines.
The OptiNspec-AMF103 is the first and latest model invented by WOE where more than one type of micro-lens can be tested. Apart from flat optics, curved optics can also be inspected. On top of surface inspection, this model is also the first to be able to conduct lens dimension inspection.
It can automatically detect the external dimensions of flat parts, such as length, angle, radius, roundness, etc., and also determine if the dimensions are qualified.
Additionally, OptiNspec-AMF103 can distinguish the contour and size measurement of various complicated precision parts, and automatically classify the products according to shape. This machine comes with a user-friendly design and GUI for intuitive machine operation, as well as an ergonomic design to help reduce employee fatigue.
Some benefits and features of the AMF103:
- Automatic detection of dimension and surface defects of flat, curved and coated optics of different materials
- Allows in-situ batch inspection
- Inspect attributes < 2 sec/pc and scratch & dig < 6 sec/pc
- Reduce employee fatigue thus improving productivity
- User-friendly design and occupies less physical space.
- Identification and separation of defective parts by visualizations.
- Fast and accurate auto-generation of detailed reports in desirable formats.
All in all, in this article, we covered the basic information of the AOI machine, the advantages and disadvantages of human and automated optical inspection, and the differences between the various AOI machines that we offer.
No doubt that machines are replacing human jobs due to fast technological evolution leading to more pros such as increased efficiency and lower operating cost, humans are still needed at some parts of the optical inspection process such as operating and ensuring the machines are being well-maintained.
With the invention of such technology, machine operators are able to acquire new skills to enhance the company’s productivity in other areas.