How Laser Displacement Sensors Work

Muffins being scanned by  3D vision camera on conveyor belt,  3D vision camera camera inspecting gap and flush on car door panel, electronic devices being scanned on conveyor belt by 3D vision camera

Laser displacement sensors, commonly known as point lasers, measure a single point using triangular reflection. Laser profilers, on the other hand, measure an entire line. The measurement accuracy of laser displacement sensors is high, but the efficiency is low because data is collected point-by-point. Laser profilers have the opposite trade-off. They scan the surface and form 3D contours quickly, but the accuracy decreases. The choice of which type of laser measurement approach to use depends on application requirements, mainly is accuracy or speed the higher priority. 

Whether it is counting cookies in a clamshell container, verifying the alignment of car panels, or measuring the position of electronic components on a printed circuit board, the list of manufacturing applications that can benefit from a simple-to-use 3D vision system is virtually unlimited. 

Today, 3D machine vision designers have multiple options at their disposal for solving 3D applications, including laser displacement systems (also called laser scanners), stereoscopic, and time-of-flight (ToF) solutions. Of these three solution sets, laser displacement sensors are the most common and provide the fastest, most accurate, and most cost-effective 3D data acquisition for both in-line and offline operations. 

Profilers versus linear displacement versus area displacement

A laser displacement sensor can generate precise 2D and 3D surface measurements, including an object's height, width, angle, area, and position. Laser displacement sensors fall into three primary types: linear profilers, linear displacement sensors, and area-scan displacement sensors. 

Laser profilers generate either 2D slices of images (e.g., an image of the object cut in half) or 3D surface maps by stacking individual profiles into a continuous image. Typically, this is done using motion data from an encoder that tracks the object under scan.

2D height map and 3D point cloud comparison

2.5D height map (left) and 3D point cloud (right)

Laser displacement sensors take this operation one step further, generating true 3D point clouds of the entire object for better accuracy and usability (assuming the image processing software has been fully optimized to work in a true 3D environment). Laser displacement sensors are also mounted above, and occasionally below, conveyors to scan moving objects. These sensors are sometimes attached to the end of robotic arms to provide the sensor-to-object scanning motion. 

Finally, some area-scan displacement sensors use microelectromechanical (MEMs) mirrors to move the laser light around the object rather than depending on the object to move in relation to the laser. 

Laser triangulation measures height

Laser displacement sensors use laser triangulation to determine the height of a pixel from a calibrated base plane (think of the “zero” location on a line graph). 

In operation, laser displacement sensors project a laser line on an object that can be stationary or in relative motion. For example, a linear displacement sensor such as that used in the In-Sight 3D-L4000 machine vision system may be mounted above a moving conveyor or mounted on a moving robotic arm. 

Located at a known distance and angle from the laser line generator inside the laser displacement sensor, a digital sensor captures the reflected light. Laser triangulation software running within the sensor reconstructs the surface map or 3D point cloud based on the changes to the shape of the projected laser line as it moves across the object. The result is then communicated to downstream PLCs, material handling systems, and/or production tracking software for further action.

2D and 3D: The best of both worlds

Based on these simple operating principles, laser displacement sensors are widely used for profile, position, and gap measurements for many industrial manufacturing applications, including:

  • Presence/absence in automotive assembly
  • Seal and volume measurements for food and packaging
  • Mold clearance, thickness, and web inspections in extrusion applications
  • Component height, location, and flushness inspections in electronics and consumer packaged goods

In addition to their accuracy, laser displacement sensors are fast, capable of acquiring millions of 3D data points per second to construct high-resolution 3D point cloud models. The ability to work at very high speeds enables 100% measurement of products moving down a conveyor, thereby eliminating the need to stop the line to sample product. This results in lower costs, better quality, and higher throughput.

The most reliable laser displacement sensors deliver 2D grayscale images as well as 3D point clouds. By using advanced, speckle-free laser scanning systems, next-generation laser displacement sensors have the uniformity and optical power to generate high-quality 2D images. These advanced systems deliver reliable operation even if mounted upside down, under a conveyor where contamination is traditionally a problem for laser projectors.

21-032 Speckle Gif

Blue speckle-free laser line (left) and red laser with speckle (right)

As noted above, the wide measurement range of modern laser displacement sensors, coupled with the most powerful and intuitive machine vision software programs, gives laser profiler systems the ability to solve virtually any in-line 3D measurement application. This saves manufacturers and machine builders time and expense, while increasing the overall quality of their products.

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