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How Laser Displacement Sensors Work

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.

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