06 July 2001

Triangulation sensors: An overview

by Walt Pastorius

Optical triangulation is fast, long-lived, and accurate

Triangulation sensors are ideal for monitoring the distance to small, fragile parts or soft surfaces susceptible to deformation if touched by a probe. Today, thanks to the availability of good components and microcomputers, industrial use of triangulation sensors is increasing.

A light source, usually a laser, projects a beam of light onto the surface to be measured; the light is then reflected through a lens, say to point "A." Because the location of "A," the light source, and the lens' properties are all known, it's possible to infer the position of the reflection surface. Engineers can vary sensor range by selection of the lens.

Triangulation sensors operate with almost any type of light source. For best performance, the light source should be of relatively high intensity and should create a small spot on the surface. As a result, almost all triangulation sensors use a laser as the light source, and most standard sensor designs today use a solid-state laser, similar to the type used in common laser pointers. The solid-state laser diode provides a compact, efficient, long-lived light source for sensors.

Laser diodes may operate continuously, or they may be modulated or pulsed. Using a modulated laser can be useful in reducing ambient light by filtering the detector output at the modulation frequency or using lock-in amplifier technologies. Laser wavelength, or color, has no significant influence on sensor performance, provided the detector senses the wavelength.

From a practical point of view, using a visible wavelength laser is desirable. An operator can easily see the laser is on when a visible source is used-a source of comfort for the user and a quick check for diagnostic purposes. Use of a visible laser source is one of the fundamental requirements for laser sensors to be rated "Laser Class 2" devices, which limits the safety precautions necessary on the plant floor.

Surface effects

Triangulation operates by reflecting the spot of light from a surface onto a position-sensing detector. Most materials are, optically, a combination of diffuse and specular surfaces. Because triangulation operates by imaging light reflected from the surface, a change in reflectivity changes the level or intensity of light reaching the detector.

In cases where the surface changes dramatically, such as components that are different colors, the sensor must be able to respond to these changes automatically. Applications where this is a factor require a very fast feedback scheme that controls the laser intensity or some other exposure feature in real time to ensure that stable and reliable data is obtained.

This consideration leads to a second reason for preferring the small spot size that lasers offer: Differential reflectivity may shift the centroid of the image, introducing an error in the measurement.

In the case of specular surfaces, engineers must take steps to ensure that the laser light doesn't simply reflect back to the source. Where this is a possibility, the position of the light source should be obliquely to the surface, rather than normal.

Detector types

Because triangulation requires finding the location of the center of the imaged spot, the detector must be able to detect the spot location. There are two main types of detectors used in triangulation sensors. The first is a position-sensing detector (PSD), and the second is a charge-coupled device (CCD).

PSDs are available in one- and two-axis forms, with single-axis types generally used in triangulation sensors. The PSD is a single element detector that converts incident light into continuous position data. The detector chip has outputs at both ends, and the amount of current from each output is proportional to the position of the imaged spot on the detector.

Equal currents arrive at the two outputs if the spot centers on the detector. If the imaged spot moves off center, the two outputs change, and the device calculates the spot position from the relative values of the outputs. PSDs provide the highest data rates (up to 500 kilohertz) and have the fastest rates of gain control — an important consideration when dealing with surfaces of varying texture, color, and reflectivity.

CCD detectors are essentially a form of television camera and come in one- or two-dimension forms. In most simple triangulation sensors, a single-dimension CCD is used. The detector consists of a single row of discrete photodetectors, often referred to as pixels. This device is essentially a one-line TV "camera." In operation, the individual pixels electronically report their status as a train of pulses. The sensor electronics determine the center of the imaged spot for triangulation processing.

The CCD detector has several advantages. First, the "video" output of the sensor can be viewed to display light levels and cleanliness of the image, as well as show any stray light effects. Further, analysts can filter or process the image for their unique needs. This can remove unwanted multiple spots, reflections, or other light.

The chief disadvantage of the CCD detector is that it's slower than a PSD. Gain control in CCD-based sensors is not as fast as in PSD-based sensors — a drawback in applications where surface reflectivity changes rapidly.

A sophisticated version of the CCD sensor is available that collects data at multiple points in a single frame, generating contour information along a line of the part being inspected. This sensor projects a line of laser light onto the surface rather than a single point. It uses a two-dimensional CCD as the detector. The image of the laser line on the detector maps out the contour of the surface. Analyzing multiple points along the laser line using the triangulation equations generates a profile of the part. The sensor generate a three-dimensional map of the surface if the part is moving under the sensor. IT