01 October 2006

Designed to run fast and cheap, with the bare minimum of features, these new sensors do not require a PC to process their images. And since they’re not designed for a specific task, they can easily be moved from one application to another.

Vision sensors are in a class of their own. They fit in between photoelectric sensors, on the low end, and
PC-based machine vision systems, on the high end. They have more capabilities, complexities, and costs
than photoelectric sensors—but no one’s claiming they are as capable as full machine vision systems, either.

Technology advances have made vision sensors easy to configure and use. They bring high-horsepower sensing benefits to a broader range of applications than photoelectrics or machine vision systems have done.

‘A vision sensor trades performance and functionality for ease-of-setup, smaller size, lower power, and lower cost,’ says Ben Dawson, director of strategic development, ipd, a Dalsa Coreco group. ‘The technology is similar, but complete vision systems have higher performance hardware and more general software.’ A machine vision system may cost $20,000, use custom software, and be hooked up to a PC to
process the images. By contrast, a standalone vision sensor may cost $1,000 to $3,000.

A major difference is the target market. Vision sensors are aimed at end-users while vision systems are
generally integrated into products by a machine builder.

One-track minds
Usually vision sensors have one purpose or task, such as reading a bar code or verifying colour. They are good for a quick ‘spot’ of vision, to monitor part dimensions or check for a limited class of defects. They are designed to be quickly and easily set up by an engineer who might not be familiar with vision technology.

While they can’t keep up with data rates of more than a few megabytes per second they have been migrating into more complex application areas where full machine vision systems once stood such as checking for presence or counting objects. They are in effect blurring the line between industrial measurement sensors and machine vision systems.

For simple, obvious vision tasks, such as ensuring a label is present, the sensors work well. Where PC-based vision systems are programmed and smart cameras are configured, vision sensors are self-learning. In fact, often a ‘learn’ button is the only input an operator has, and a single ‘pass/fail’ line is the only output. This greatly simplifies system setup, but it also obviously reduces possible applications.

Newer smart sensors, such as the Omron ZFV (photo, page 76), have an embedded 1.8 inch (4.6 cm) monitor that makes programming the unit simple through icon-driven menus, with realtime high quality CCD imaging. ZFV can detect 10,000 parts per minute, without requiring extra equipment, such as a laptop PC or complicated software. With ZFV, the user can ‘target the camera, teach the sensor, and go.’ The lighting is built in. Omron’s ZFV colour vision sensor comes with the same intuitive user interface as the grey scale version. But by using the colour information in the image, it adds more security and reliability to the application. More sensor heads and communication options make it more versatile. Eight inspection tools are included.

Some low-end vision sensors suffer because they are preprogrammed with specific programming that cannot be easily modified, but Omron says it has countered that by programming its ZFV to solve a variety of applications.

End of high end?
Thanks to advances in embedded processor technology, a new, middle class of vision products has emerged in machine vision. 'This bourgeoisie-type class of vision sensors can solve 80-90% of machine
vision applications for a fraction of the cost of typical high end systems,’ says Joshua Jelonek of Keyence. The industry appears to be moving away from highend machine vision systems, he says, for two reasons:

1 High-end systems offer a litany of complex tools and functions, but many of these tools and functions are not necessary to complete common machine vision applications. A lowend, or general purpose, vision sensor will provide many of these same tools, but in a condensed, easy-to-use format that filters out all the unnecessary extras. And they'll typically do this for a fraction of the cost. From an application solving perspective, purchasing a high-end system would be like paying for a car stereo with a six-disc CD changer when all you do is listen to the radio.

2 As technology improves and the cost of powerful integrated circuit chips decreases, low-end and generalpurpose vision sensors will become more capable. In image processing power, the line that divides low- from high-end is becoming increasingly blurred. This improvement in capability, coupled with the simple user interface provided by general-purpose systems, allows manufacturers to see higher return on investment through reduced hardware expenditures and faster integration time.

Sensing elements: CMOS or CCD?
There are two digital technologies currently used in vision sensors: CMOS and CCD.

CMOS (complementary metal oxide semiconductor) chips, at a cost of one-tenth their CCD rivals (e.g. $50 versus $500 for a 1,000 pixelsquared array), are helping make smart cameras more affordable. The advantage of using familiar CMOS technology is that chip makers can add functionality to the sensing
element, such as timing circuits, amplifiers, and analogue-to-digital converters. Because it allows more unwanted signal into the picture, CMOS image quality is somewhat less than CCD, but is improving. Most CMOS chips take in light with a ‘rolling shutter’ mechanism, which, in the worst case, creates a vertical line on a black background, with more spatial distortion, and requires more careful lighting. Chipmakers are creating CMOS with CCD performance, and are now challenging the 3 to 5 million pixel range.

CCD (charge-coupled device) sensors are better quality, but they must have timing, amplifier, and other external components added to them so cost and integration needs are greater. CCD’s big advantage is that it has a broader range of illumination and is more sensitive in low-light conditions, however, noise source sensitivity is a little higher. CCDs are engineered specifically for imaging, and so have higher image quality and lower artifacts. Instead of the rolling shutter, the sensing element takes in light row by row, for higher
image quality.

High end vision sensors
At the high end of the spectrum of vision sensors, the products can be versatile, stand-alone, armed with a number of algorithms, use different lighting and lenses, and have hundreds of software toolsets. And they can cost a lot more than the typical $2,000 vision sensor.

While Cognex says its Checker sensor isn't strictly considered a vision sensor, the device shares some of their characteristics. John Keating of Cognex calls it ‘a highend sensor that is a better solution for applications that could have required multiple photoelectric sensors.’ However, Checker can acquire and process 30,000 images per minute, detect presence, and inspect part features. Checker’s eye is a 128x100-pixel image sensor.

Unlike vision sensors, though, Checker can provide deterministic outputs, for registration or web cutting, perforating, or printing applications. ‘Checker doesn’t have vision tools,’ but is designed, for example, to detect if a part has the required features. Step-by-step setup is similar to that of photoelectric sensors.

—From an article written by Mark Hoske, Control Engineering.

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