CMOS technology for sub-micron accuracy in micro-seconds

Paul Boughton

Achieving micro-meter accuracy sampled in micro-seconds and almost independent of the shapetexture and colour of the object is indeed a major undertaking. The new smart sensor ZS-L combines CMOS image capture technology and laser light beam to meet this challenge.

The pressure from end-users to sub-suppliers for zero-defect performance calls for fast and accurate in-line inspection and measurement. The concept of ‘quality integrated’ not ‘quality controlled’ is now a reality in leading-edge industries such as automotivesemi-conductors and electronic goods. In-line measurement is also helping manufacturers in other industries to reduce waste by optimising material size and thickness.

There are a number of laser displacement sensors in the market using various technologies. However PSDCCD and CMOS are the most popular methods. Let us give you a brief overview on how they work.

  • PSD (Position Sensing Detector): Commonly used in low costlow accuracy applications. It achieves sampling speeds in the order of micro seconds but a resolution of 1-5µm at best. The PSD technology is not recommended for use on colouredshiny or structured surfacesglassor other glossy materials.

  • CCD (charge coupled device): This is well-proven technology used in professional digital cameras and vision systems. It provides high picture uniformity but it is costly. It consists of tiny pixels that convert light into an analogue voltage signal. The signals are bufferedthen amplified.

  • CMOS (complementary metal oxide semiconductor): This consists of tiny cells that operate as light-to-volt transducers. In this case howeverthe amplifier for each cell is integrated. The signals are digitalised before being sent off-chip and often additional functionality is already on board.

When a light beam hits the surface of an objecta certain amount of the light is reflectedsome is transmitted through the object and the rest is absorbed.

In the case of transparent materials such as glasswe can obtain reflected light from the top surfacefrom the middle and from the bottom section. The PSD detector receives light from two or more different directions of reflected light. This means that we may get a measurement error from stray or secondary reflections.

CCD and CMOS technology can distinguish between the true object reflection and background reflection.

CMOS image sensors are very useful and cost-effective in some mid- and top-performance imaging applications. CMOS offers more integration (more functions on the chip)lower power dissipation (at the chip level)and smaller system size. It is well suited for high-volumespace-constrained applications such as automotive and electronics applications.

CMOS technology is now standard in booming consumer products like digital cameras.

Omron EuropeHoofddorpThe Netherlands.


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