M achine vision systems can perform faster repetitive tasks, more accurately, and with greater consistency than humans. It can reduce labour costs, increase production yields, and eliminate costly errors associated with incomplete or incorrect assembly. It can help automatically identify and correct manufacturing problems on-line by forming part of the factory control network. The net result is greater productivity and improved customer satisfaction through consistent delivery of quality products. Implementing a cost-effective machine vision system, however, is not a casual task. The selection of components and system programming must accurately reflect the application’s requirements. Factors such as the time required for system development, installation, and integration with the factory system, operator training (and retraining) costs, project management, maintenance, and software upgrade and modification, all contribute to the total cost of ownership for the system and should be evaluated before investing in a specific system design.
Sort the up-front knowledge
While selecting a machine vision system for a factory automation task it is important to closely define the requirements. There are a number of critical questions to ask up front:What task does the system need to perform?Different tasks may require different vision capabilities. Inspection requires an ability to examine objects in detail and evaluate the image to make pass/fail decisions. Assembly, on the other hand, requires the ability to scan an image to locate reference marks (called fiducials) and use those marks to determine placement and orientation of parts. A machine vision system designed for one task may not be well suited to the other.What are the key visual performance criteria?The vision system’s lighting, camera, and lens must perform adequately. Factors such as the smallest object or defect to detect, the texture of the parts, measurement accuracy needed, the image size (field of view), speed of image capture and processing, and the need for colour all affect lighting, camera, and lens choices.What are the environmental factors?Some cameras are better for stationary views while others are more suitable for viewing moving objects. Temperature, humidity, vibration, and the like can impose needs for protecting vision system components.With what equipment must the vision system interface?A vision system that only activates a solenoid to eject failed parts from a production line is considerably easier to implement than one that also reports results to a quality control network or that controls the operation of production equipment based on inspection results. Similarly, a system that interacts with a human operator has different needs than one that interfaces only to other machines.What information must the system provide?Machine vision systems in factory automation seldom operate in a stand-alone mode. Instead, they send information to other parts of the factory enterprise for a variety of purposes.What are the operator requirements?The extent to which human intervention and control of the machine vision system is required affects many system elements,particularly software. If operators are required to periodically change inspection criteria, such as acceptance tolerances, then software must support such changes. Software may also need to provide security to prevent unauthorised access and parameter changes, and include safeguard to avoid introduction of erroneous parameter values.
Building machine-vision system
All vision systems must have an image to inspect. They must operate on a continuous basis and at the fastest practical speed. All systems operate by using the following steps:Position the part or camera so the camera can view the partCapture an image with a cameraProcess the imageTake action based on the image processing resultsCommunicate results to operators and other factory systems
Because of this commonality, examination of a specific application, such as inspection of objects on an assembly line, will illustrate methods by which developers can build a suitable machine vision system for their application. The essential elements of an inspection system include a delivery system, vision system, response system, and sensors to trigger image capture and system response. The delivery system positions the part for inspection. The vision system, which includes camera, toptics, lighting, and image processor, captures and processes the object image to determine a pass/fail response.
The response system takes the required action as well as communicating results to operators or other systems. The sensors serve to trigger the vision and response systems, identifying when the object is in the proper position for the systems to perform their tasks. The reading of an identification number requires close-up imaging, front lighting, and optical character recognition software.
Ensuring factory integration
Factory and production specialists need to understand essential steps of vision system design. It is essential to resolve the sometimes-opposing needs of system design and runtime operations. To control design costs the system should have a well-specified and bounded task. This makes development and programming simpler and allows optimisation of components such as camera, lighting, and optics.
A system that targets a single task with fixed specifications is easier to develop but will require vendor involvement to make even small changes in operation. Hence, Machine vision is a key technology for improving the quality and productivity of manufacturing lines though factory automation.Courtesy: InTech, the official publication of ISA
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