VISION TECHNOLOGY no longer is limited to discrete sensing applications, military reconnaissance or deep-space exploration applications. New manufacturing technologies allow chips to pack exponentially more circuitry, which translates into higher power and faster processing capabilities that allow PC systems to handle the enormous computation load of vision analysis in real time. Such systems offer unique performance features no other instrumentation can provide to control level in a process.
Vision technology based on compact charge-coupled device (CCD) cameras can offer excellent access to an industrial process, providing operators with information about level, color, foam presence and other process conditions. The technology is not dependent on the CCD unit; users can install complementary metal-oxide semiconductor (CMOS) CCD units or other combinations required by the specific application. Information technology (IT) systems even can distribute video of process conditions across the Internet to link manufacturing with engineering in real time.
The Silicon Eye
A vision system, consisting of a camera and light source, functions in much the same way as the human eye and brain collaborate. If you look at an object on your desk, your eye recognizes it because it has a different color or reflects light differently than your desktop reflects it. It is these changes in reflected-light intensity that make the object visible.
That is precisely how a vision system determines tank content level. A camera looks into a tank and determines the fluid interface with the tank wall because the wall is offset 90 degrees from the fluid surface and reflects light differently. It is also likely the fluid reflects a color or shade different from that of the wall. When the software assesses pixel color or intensity, it detects a change," up or down," in the shade or color at the interface point, so the system determines where the level is in the image. Solids within a hopper do not settle at one particular level. A vision system can view a large portion of the hopper to allow an integrated level calculation to take place.
A camera can determine the fluid interface with the tank wall because the wall is offset 90 degrees from the fluid surface and reflects light differently.
More Than Pixel Dust
The system eye is the CCD camera, which contains a semiconductor chip segmented into an array of cells called pixels. Each pixel represents a color and intensity value of an individual and small portion of the image field; together, the array changes the incoming light wave into a digital signal of the image.
The brain of the system is the image processor. Its function is to read the color/intensity values of each pixel and interpret the image according to the instructions provided by the software. The software learns about the process and, in a manner of speaking, thinks about the results.
On the initial process run, the software can be set to "learn mode" to determine what, if any, abnormal situations arise in the process during a run. This information lets the operator select the proper method to determine level and the proper image processing to apply to obtain accurate and consistent results.
For example, a highly reflective fluid such as latex or water in a polished stainless steel tank can cause the provided light to reflect at random and trick a vision system into locating the level incorrectly. When the system does its initial run, it can learn these instances, and the operator can apply the correct imaging processes to eliminate the effect of these random occurrences. If the proper filters and algorithms are selected for analysis, the software will be able to provide a thoughtful result by assessing process variables in the manner the operator desires.
To take this example further, assume the process fluid changes from red to green. The software will indicate not only proper level, but also the occurrence of a color change. This ability to assess multiple variables simultaneously gives the operator the added capability of managing several facets of the process with a single instrument, which can be critical when tank nozzle space and cost are at a premium.
The general rule for vision system applications is: "Even if you could see the process with your own eyes, a vision system will see it better."
The unique characteristics of vision technology make its application broad based. The same vision system not only handles the typical liquid-in-a-reactor scenario, but also applies nicely to hopper solids-level applications.
Solids do not settle at one particular level, so a system that can see a large portion of a hopper allows an integrated level calculation to take place. By averaging the level of product around a significant portion of the hopper, plants can avoid errors that can arise from single-point measurement.
Vision systems also can monitor product flow on a conveyor belt by integrating the detected product level on the belt over time.
Additional applications for a standard vision system include the detection of web edges, solution opacity/turbidity, pipe or column interface and phase changes and many more. Applications range as far as the eye can see.
The advantages of a vision approach are numerous. Perhaps the greatest benefit is that the system provides a visual verification of the current process status to the operator. This verification is invaluable, not only as a way to confirm process status, but also as a way to assess corrective action and determine cause/effect relationships of otherwise unknown events that might be taking place. This benefit is especially useful in pilot-plant environments.