Triangulation: proven measuring principle with a future

Triangulation: proven measuring principle with a future

Triangulation has been used for land surveying in Europe and America since the 17th century. In this process, an area is divided into triangles, which are used to measure distances. If the triangle angles and the base line of the triangle are known, the missing lengths can be calculated easily with trigonometry. With the advent of electronic distance measurement devices, the trilateration method, i.e. the direct measurement of distances, was increasingly used to determine points in land surveying.

Mathematical-geodetic triangulation was probably invented by the Dutch astronomer Snellius. Today, the triangulation principle is used far beyond land surveying, e.g. in photogrammetry or 3D scanning.

Laser triangulation with the light-sectioning procedure

The triangulation-based light-sectioning procedure is used for optical shape detection and is applied in 3D metrology. The corner points of the triangulation triangle are formed by a light source, a camera and the measuring object. From the light source, a light spot or line is projected onto the measuring object at a known angle by means of a laser. The reflection of the light is recorded by the camera, so that the exact position of the measuring object can be calculated by means of triangulation. To map the contour of an object or component, either the sensor or the object must be moved in one direction. Since the movement and changes in the environment affect the measurement result, i.e. the accuracy, this method requires a stable measurement installation. As with all other optical methods, the light-sectioning procedure also has difficulties with narrow bore holes and undercuts. 

Advantages of laser scanners

Independent of ambient light conditions
Sensor head can be mounted on existing measuring systems, e.g. coordinate measuring systems

Disadvantages of laser scanners

Loss of resolution due to laser granulation (speckles) and movement
No process reliability, as temperature-dependent

Fringe projection

For fringe projection, a projector projects a fringe pattern onto the object to be measured using the optical 3D measuring method in order to reproduce the exact shape in a 3D model. The section line image is captured by the camera and the 3D coordinates of the object surface are computed. The triangulation is performed with the knowledge of the exact arrangement geometry. The distance between sensor and camera as well as the angles are known and form the basis for triangulation.

Advantages of fringe projection:

Full-field 3D information with a constant resolution
High measuring point density
Flexible measuring setup

Disadvantages of fringe projection:

Higher requirements for projection method
Translucent or reflective surfaces may require pretreatment

Functionality of the ATOS sensors

In the case of ATOS sensors, two cameras with known relative orientation are used. The distance between the two cameras forms the basis for triangulation. The projection unit is located in the middle. It projects a structured light pattern onto the object to be measured. The coded fringe pattern is subject to a phase shift during the scanning process, i.e. the pattern changes rapidly and is barely visible to the human eye. The two sensor cameras record the changing stripes and calculate the 3D coordinates for each camera pixel using optical transformation equations. A highly-detailed image consisting of millions of measuring points can be recorded within a few seconds and without physical contact. The sensor software automatically creates a high-resolution point cloud (STL mesh), which represents a precise image of the measuring object.

Technological developments from GOM

To make the measuring systems as dependable as possible and the process as reliable as possible, GOM has developed special technologies to improve the scanning process. The ATOS 3D digitizers work with Blue Light Technology: The narrow-band blue light from the projection unit enables the scanner to make precise measurements regardless of ambient lighting conditions and even of shiny surfaces.

Thanks to the Triple Scan Principle, the right and left sensor camera are used separately in combination with the projector. This results in different perspectives on the component, so that during a scan three views are captured instead of only one. Even when scanning complex parts, the measuring procedure is accelerated, due to fewer single scans. Scanning in deep pockets is a further advantage of this concept. According to the stereo camera principle, simultaneous acquisition with two cameras results in redundancy, which ensures process reliability: Sensor and measuring object movement as well as environmental changes are detected, transformation accuracy is checked and 3D sensor position is tracked in real time.

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