INDUSTRY EXPERTS sometimes argue these days about what makes a vision system a vision system. Vision devices come in self-contained packages, a.k.a. the vision sensor, or as a modular system known typically as machine vision that consists of computer, lighting and optics, camera or sensor, image-acquiring frame grabber, and application software.
Whether you use a vision sensor or machine vision system, either one is likely to be 20% cheaper then it was 10 years ago and many times more powerful. Thanks to accumulated libraries and rapid application development (RAD) tools, analysis functions that were hand coded in the past now can be selected as a feature or an object that can applied to the vision-processing task.
Which ever is the case, manufacturers, system integrators and, more recently, machine builders have been building vision into industrial machines quite successfully for a while now.
We’ve become familiar with the usual applications in robot guidance, pick-and-place, and inspection/rejection, but there are some interesting applications that use vision to help control the process and keep it from turning out products that flunk the final vision-based inspection process. Examples of these applications are all over the map—from cutting meat, producing semiconductors, picking and analyzing leaves in crops, guiding surgical instruments, and others. We’ll consider several of them.
Sensor or System?
Vision sensors typically are self-contained devices with built-in charge-coupled device (CCD), processor, software, and communications in the same box. Smarter than photoelectric and laser sensors but not as capable as a PC-based machine vision system, vision sensors offer a lot of functionality in a price range of about $1,000-8,000 (See Figure 1).
|FIGURE 1: SENSOR SEEMS LIKE A SYSTEM|
This self-suffcient vision sensor has speeds of four, six, or 12 ms, and has an intelligent lighting and adjustable focus. Advanced algorithms provide for wide viewing area, enhanced pattern recognition, brightness, character detection, label position, target width, etc. Source: Omron
The choice of vision sensor or machine vision system often is a matter of degree and common sense. What speeds are needed and what special needs does an application require? Is vision being used for inspection or for control? “High-speed color, custom-lighting, specialty vision systems are available for press color registration,” says Jeff Schmitz, corporate business manager for Banner Engineering Vision Products. “These sensors perform far better for high-speed printing press color registration than any vision sensor today.”
Vision sensors fill a niche between photosensors and machine vision systems. While color-mark photoelectric sensors have been one solution for checking registration marks (pass-fail) for small packets such as sugar or cream used in a restaurant, vision sensors add quality optics and often are capable of speeds of 30-500 frames of video per second. Though usually not considered for printing press register applications, vision sensors, according to Schmitz, can perform decent binary large object (BLOB) and intensity or average gray-scale algorithms that can calculate the amount of darker features on a tobacco leaf.
“Typically a machine vision system has greater configurability and can handle a broad range of applications, often using generic functions,” adds Zuech. “A PC-based machine vision system can be effective for some applications with just cameras, FireWire inputs, and software. But for high-performance applications, frame-grabber boards serve as buffers and offload processing from the PC’s CPU.”
The key differentiator between a machine vision system and a vision sensor seems to be the machine vision system’s ability to combine several applications or functions, e.g., find, OCR, 2D symbol reading, presence, pattern recognition, etc. While Schmitz thinks of a vision sensor as a Swiss Army knife capable of fitting into several applications, Zuech feels that typically a vision sensor either will work or not work in an application, but a vision system can usually be engineered to work in most any application.
Vision sensors are not necessarily thought of as feedback devices for closed-loop control; rather they often play roles in pass-fail inspection systems. But technology advances slowly are pushing them beyond pattern recognition roles and into limited forms of control. While some vision sensors look for specific patterns or marks, improvements in vision sensor technology (intelligence, processing power, sensor elements) allow them to work with any unique pattern on a part, thus adapting to new registration marks.