According to the Automated Imaging Association, machine vision encompasses all industrial and non-industrial applications in which a combination of hardware and software provide operational guidance to devices in the execution of their functions based on the capture and processing of images.
Machine vision systems rely on digital sensors protected inside industrial cameras with specialised optics to acquire images, so that computer hardware and software can process, analyse, and measure various characteristics for decision making.
What Is Needed?
With so many vision systems available today, it can be a daunting task trying to figure out which one is right for your particular application. Simply finding a system that can perform the necessary vision tasks is not enough; there are several other factors that need to be considered to ensure a successful deployment. These include:
- Variations in lighting conditions
- Networking and communications capabilities
- Accessories and product support options
- Ongoing post-deployment support
Whether you are new to machine vision or an experienced user, this guide will help you through the vision system selection process, providing answers to ten critical questions and valuable tips for evaluating specific product features.
1.Does the vision system make it easy to set up applications, create custom operator interfaces and administer vision system networks?
Vision applications do not usually require elaborate runtime interfaces, but operators typically need to interact with the vision system during part changeovers, to change tolerance parameters, and to determine the cause of part failures.
The vision system you choose should allow you to quickly and easily configure these and other facets of your application without coding in Visual Basic or a proprietary script-based language. The vision system’s software should also include network management tools to simplify remote administration of multiple systems, including tasks such as backup, image playback, firmware upgrades and context sensitive help documentation.
2. What is the importance of part location tools, and how can I assess their performance?
Part location software tools find the part within the camera’s field of view. This is typically the first step in any vision application, from the simplest robot pick-and-place operation to the most complex assembly verification task. It is also the most critical step, because it often determines whether an application succeeds or fails.
While it sounds simple enough, locating parts in an actual production environment can be extremely challenging. Vision systems are trained to recognise parts based on a “model” image, but even the most tightly controlled manufacturing processes allow some variability in the way a part appears to the vision system.
Therefore, the vision system’s part location tools must be intelligent enough to quickly compare model images to the actual objects moving down a production line, regardless of which side of the part faces the camera, its distance from the camera, shadows, reflections, line speed, and normal variations in appearance.
3. Does the vision system have a complete set of image pre-processing tools?
Image pre-processing tools alter the raw image to emphasise desired features while minimising undesirable features. This prepares the image for optimal performance by more powerful vision tools and can significantly improve the accuracy and robustness of the overall system. Pre-processing tools can:
- Increase the contrast between the part and its background.
- Mask insignificant and potentially confusing image features.
- Eliminate “hot spots” reflecting off the part surface, and
- Smooth rough surface textures. Make sure that a complete set of image pre-processing tools is included with the vision system you choose.
4. What should I look for in character reading and verification capabilities?
Whether you are reading stamped alphanumeric codes on automotive parts or verifying date and lot code information on medicine bottles or packages, there are several capabilities to look for when evaluating character reading and verification tools, including:
Statistical Font Training
This capability builds a font by learning models of characters that appear in a series of images. The images should include multiple instances of each character, and span the full range of quality likely to occur in production. The resulting font will be highly tolerant of normal variations in print quality, whether due to poor contrast, variable locations, degradations, or variations in stroke widths.
Image Pre-Processing Tools
These tools optimise a trained model by sharpening the edge contrast of characters and filtering out extraneous background in the image. Optimised models maximise the reliability and repeatability of the vision system.
Instant Image Recall
This capability enables line operators and technicians to quickly and easily view failed images on a display. Whether the failure is caused by a camera jarred out of position or a missing or damaged label, it is important to know immediately why the failure occurred so corrective action may be taken.
5. How can I determine the repeatability of a vision system’s gauging tools?
If your application involves critical dimensional measurements, the vision system’s gauging tools must be accurate and perform with a very high degree of repeatability. The vision system should have a full suite of gauging tools which will allow you to choose the right one to fit the requirements of your measurement application without having to write custom scripts or functions.
6. How do I evaluate industrial code reading tools and what are some specific features to look for?
Industrial environments demand a vision system that can read 2D data matrix codes that are degraded, poorly marked, or vary in position from part to part. The vision system should perform well regardless of the part material (such as metal) and the type of part marking method employed (such as dot peen, etching, hot stamping, and inkjet).
7. What networking and communications features should I look for?
Networking is essential to many vision applications as a means to share data, support decision-making, and enable highly-efficient integrated processes.
For example, networking enables vision systems to transmit pass/fail results to PCs for analysis, or communicate directly with PLCs, robots, and other factory automation devices in an integrated process control system.
If you need to link your vision systems to PCs at the enterprise level, choose a system that supports the complete set of standard networking protocols:
- TCP/IP client/server enables vision systems to easily share results data with other vision systems and control devices over Ethernet without any code development.
- SMTP (Simple Mail Transfer Protocol) enables you to immediately receive an e-mail on your PC or cell phone when a problem occurs on the production line.
- FTP (File Transfer Protocol) allows inspection images to be stored on the network for later analysis.
- Telnet is an Internet standard protocol that enables remote login and connection from host devices.
- DHCP (Dynamic Host Configuration Protocol) allows a vision system to automatically receive its network IP address from a server, enabling true plug-and-play performance.
- DNS (Domain Name Service) allows you to assign each vision system a meaningful name, such as “Bottling Line System 1”, instead of having to use a numeric IP address.
To integrate a vision system with the PLCs, robots and other automation devices in your plant, the system you choose must also support the following:
- Industrial Ethernet protocols such as EtherNet/IP, PROFINET, MC Protocol and Modbus TCP. These enable vision systems to be linked to the most popular PLCs and other devices over a single Ethernet cable, eliminating the need for complex wiring schemes and costly network gateways.
- Fieldbus networks, including CC-Link, DeviceNet, and PROFIBUS. A protocol gateway accessory is usually needed to add a vision system to a Fieldbus network.
- RS-232 and RS-495 serial protocols, needed to communicate with most robot controllers.
8. What should I know about vision system accessories?
Too often, too much attention is given to evaluating the vision system that accessory products are almost an afterthought. But the choice of accessories can go a long way towards ensuring trouble-free system integration and, in the case of lighting, can even make or break the application.
For quick and painless integration of your vision system, it makes sense to buy from a vendor that offers a complete family of compatible accessories. This gives you the assurance of knowing that each and every accessory has been tested and confirmed to be compatible with the vision system. Accessories to look for include:
No two production areas have the same ambient light conditions, and parts can exhibit a wide range of surface characteristics. Nearly every machine vision solution requires a unique lighting approach to meet its objectives and optimise performance.
Your vision system vendor should offer a variety of lighting options, including: ring lights, which provide soft, even illumination from all directions; back lights, which create maximum contrast between a part and its background; and dark field lights, which provide low-angle illumination for imaging of part surface irregularities.
- Communications Modules
Make sure your vendor offers communications peripherals such as I/O modules and network gateway modules that support easy, quick connectivity between the vision system and PLCs, robots, and other factory automation devices and networks.
- Operator Interface Panels
A networked operator interface panel allows easy, plug-and-go set-up and deployment, plus ongoing monitoring and control of vision systems without a PC. When selecting an operator interface panel, look for one with an intuitive, touch-screen interface
- Camera Enclosures
Some vision systems are assembled into rugged, IP and NEMA-rated metal cases to withstand dust and moisture without requiring a separate enclosure accessory.
However, if the environment in your plant is especially harsh or requires frequent wash down of equipment, ask your prospective supplier if they offer external enclosures pre-qualified for use with the system.
9. Does the vision system require a PC?
While some vision applications are complex and require more robust capabilities, many of them may be addressed with an affordable, standalone solution. For those applications, your vendor should offer a standalone vision system that does not require a PC—during configuration or in production mode.
The system should offer true plug-and-go performance that enables you to quickly configure the application, from start to finish, right out of the box.
Just as important, the vision system should not require you to roll a PC onto the factory floor every time changes to the application need to be made. Finally, a true standalone vision system should enable you to hook up a monitor for live image display without a PC.
10. Does the vision system supplier provide the support and learning services I need?
Even the highest performance vision system is only as good as the suppliers who stand behind it. Whether you buy your vision system from a distributor, a systems integrator, or direct from the manufacturer, it is important to know in advance the full range of support services available to you.
Be sure that the vision supplier you choose understands your unique support requirements and provides you with all of the resources you will need during every phase of the project, from application development and systems integration, to deployment and beyond.
Other important questions to consider include:
- Can the vision supplier provide a dedicated machine vision specialist to assist you with the initial application evaluation?
- Does the vision system manufacturer have a global network for post-sales support
- Does the vision system manufacturer offer a wide range of cost effective training and support options, including online self-help and training courses, worldwide phone support, and personalised training services?
- If you are buying from a distributor or systems integrator, are they authorised partners of the vision system manufacturer?
- Does the vision system manufacturer have a history of successful installations and the financial stability to support your needs over the long term?
- Does the vision supplier offer more sophisticated systems and tools if your application requires it, or if your needs change?
Last but not least, the best suppliers do not merely try to sell you a product—they take the time to carefully understand and evaluate all of your requirements before proposing a solution.
APMEN Machine Tools, May/June 2017