The 25-year history of the triangulation laser is marked by many disappointed users and unsolved measurement applications.

The introduction of the laser triangulation sensor was originally seen as a revolutionary approach to non-contact measurement based on increased standoff from the target and the ability to look at a wide variety of target materials without the effects of averaging common to field sensors. Application possibilities seemed endless.

Engineers soon realized, however, that the real-world performance of laser triangulation sensors was something less than what was promised. Performance specifications were based on a perfectly aligned white target in a static condition, but few applications exist where the target is white, static, and perfectly perpendicular. It became clear that different or changing colors and surface textures, as well as tilted and moving targets, caused performance that was less than the stated specifications of the laser. Reduced resolution and linearity, increased cosine error, and even instability or signal loss were direct results of these application issues.

The technology driving these application demons is known as position sensitive device (PSD) technology. It has been the main technology offered by most manufacturers of industrial laser displacement sensors. In order to provide repeatable and accurate measurements, PSD sensors require certain conditions including good target alignment, no stray or secondary reflections from the target, and no color or surface texture changes. In addition, the target cannot absorb too much light or be too shiny.

A new technology that avoids the pitfalls associated with PSD sensors is based on a charged coupled device (CCD) laser. Featured in today's smart displacement sensors, it has revolutionized the scope of applications for triangulation sensors. CCD sensors overcome almost all the application problems that PSD lasers have not been able to solve. For example, engineers now have the option of using Class II CCD technology lasers to replace Class III PSD technology lasers in order to get better performance and eliminate the safety issues associated with Class III lasers.

General purpose high-speed CCD lasers were introduced in Europe in 1993. They have evolved over time so that now CCD receiving elements, in combination with powerful DSPs, can measure what PSD elements where supposed to be able to measure from the beginning. For instance, they can measure accurately on almost any target surface or color. Target alignment is also not an issue. The effects of stray and secondary reflections are eliminated because the CCD element, unlike PSD elements, works as a function of light intensity and not light quantity. For sensor stability, as little as 1 percent diffuse reflectivity is necessary, which means black and shiny targets do not cause problems.

A CCD element is a digital pixelized array detector. It has 1,024 discrete elements, representing the amount of light on each pixel of the detector. A CCD element detector can carry 1024 x 1024 pieces of light intensity information. With the help of a powerful DSP, the intensity distribution of the imaged spot can be "viewed" completely. Image processing is then incorporated for linear triangulation measurement. The post data processing of the intensity distribution makes it possible to overcome almost all the problems of non-ideal targets.

The DSP reads only light intensity and, therefore, finds the one single pixel with the highest light intensity. It uses a sophisticated algorithm to perform sub-pixel resolution by interpreting the light intensity of the adjacent pixels. The technology of thresholding is used to discard unwanted information about stray and secondary reflections. This technique also helps to eliminate any optical or electrical noise.

Smart CCD sensors also use closed loop control to adjust the power of the transmitting laser according to the amount of reflected light from the target. A constant light quantity for the sensing element can be achieved regardless of the target color. The best CCD laser triangulation sensors on the market can accomplish all this in real time at 10 kHz.

Martin Dumberger is technical director at Micro-Epsilon. More information on CCD laser triangulation sensors is available by contacting the company at 3200 Glen Royal Rd., Raleigh, NC 27617 or calling (919) 787-9707.