3D Machine Vision - measuring methods
With what could popularly be called '3D vision' the purpose is to measure an area or objects in one or more dimensions for subsequent calculation of dimensions and positions or even recognition of objects. Typically the measurements are made in a two dimensional plane and the third dimension is added by moving the object or the measuring device along a known track. Using this technique a three dimensional model of the object is created along with other features for instance the ability to reflect light.
- Photograph combined with computer generated wireframe model of scanning
General
In Puls Design A/S we distinguish between two groups of measuring devices: Laser Scanners and Laser Cameras. The technology in these devices differs, but in short they both rely on illuminating the particular object or area with laser light. Depending on the principle used the shape of the object or area can be deduced from the way the light is reflected. Both technologies are, compared to traditional vision, mostly unaffected by surrounding light sources and can therefore be used without the need for light shielding and can even operate in sunlight outdoors.
There are a number of advantages worth noticing to using either technology:
- No requirements for extensive lighting.
- No physical contact with object.
- Practically unaffected by surrounding light conditions.
- High measuring speed and efficient result calculation.
- High measuring accuracy can be achieved.
The choice of technology and equipment used in a particular solution is based on the requirements for the final result. The technology offers great scalability from small to big dimensions. Below is a general description of laser scanners and laser cameras.
Partnership with SICK
Puls Design A/S has been working with these technologies since 1997, when we designed our first volume measuring system using laser scanners from SICK. Our close relations to SICK has provided us with an intimate knowledge of the products' workings and possibilities and in more recent times we have developed several laser scanner products for SICK's development department in Germany.
Experience and platform
The devices are complicated which makes our long time experience necessary in order to achieve the expected results. The key to the solutions is the software. One example is our general software platform VoluMatrix which has been developed for the needs laser scanners and laser cameras have. This software platform is in constant development since it is used in most new solutions.
Also see the Case Studies for examples of solutions with these devices. We make extensive use of our options to adapt our existing solutions for new needs. The economical advantage of this is considerate, as most seemingly unrelated solutions can often be solved using techniques similar to earlier solutions. In most cases we can adapt previous solutions to new requirements and applications.
Application areas
We are experienced within a long range of application areas such as cargo transport, pharmaceutical industry, wood industry and food industry. We also have solutions with specific demands for encapsulation of the devices in stainless steel with a polycarbonate window as required by the food industry.
Laser scanners
Laser scanners are built with technologies based on knowing the speed of light allowing distances to be determined from the time it takes the reflected light to return to the source. This technology is used in point measurers who only measure the distance to a point. In laser scanners the emitting and receiving angles are rotated gradually to form an array of points centered on the laser scanner. The points measured are points on the object illuminated by the laser light. The laser scanner's function is similar to that of radar. Depending on the characteristics of the material, structure and the angle between the laser light and the object surface, more or less light will be reflected. In some types of laser scanners the amount of light reflected is also measurable providing a distance and a reflection value for each 'point'. The reflection value can disclose surface patterns, labels, changes similarly to vision.
Laser cameras
In the case of laser cameras the object is usually illuminated with a continuous laser source spread out in a so called light curtain. Observing an object illuminated from above with such a light source and looking at the object from an angle as shown in the picture, the laser light will form a contour of the object which the camera can see. If the camera has a color filter before the lens (adapted to the laser light's frequency) it is possible to minimize unwanted light and end up with sharp contour of the surface of the object. To compensate for the geometry between the laser source and the camera as well as the internal geometry of the camera, it is possible to calibrate each of the camera's pixels to actual coordinates in the 'space' the camera sees. Laser cameras are available as complete, calibrated solutions or they can be built from a laser and camera in the wanted geometries. In this case the calibration must be done manually. Our laser cameras are optimized for processing contours which makes them very fast compared to normal cameras.
Laser safety
Both laser scanners and laser cameras can use visible (mostly red) and invisible light (infrared). The more powerful a laser, the better the device can measure on dark or shiny surfaces. The laser's effect defines the laser classification the device can be categorized as. Mostly we use devices in laser category 2 which is of maximum 1mW average. These are called eye safe because they do not harm a person's eyes under normal conditions. Devices in laser class 3B on the other hand are shielded to prevent possible risks of eye injuries to personnel. In this category effects of up to 500mW are allowed even though we rarely use lasers of more than 10mW.
Three dimensions
Both types of devices measure the distance to a number of points in a two dimensional plane - also called a contour. If this process is repeated while device or object is moved relative to the other along the thirds axis, we have a number of two dimensional contours which in turn is a three dimensional description. The faster and more precise this process is, the finer the description of the object becomes. Having this description, one can extract all sorts of parameters with the right software. This could be dimensions, positions, verifying an object shape against a model, counting objects, determining how fast the object is moving etc. There are a large number of possibilities when a 3D description is captured. A few examples are shown on the right side where the objects are visualized in 3D.
Measuring accuracy
Generally laser scanners are good for measuring in larger spaces than laser cameras, whereas laser cameras in most cases offer higher accuracy with less tolerance than laser scanner. The precision of a final solution will be determined by needs such as measuring speed, minimizing vibrations of the object and measuring device, the size of the objects, material characteristics of the objects and surroundings and in some cases the lighting conditions of the operating environment. Typically the accuracy is between ±0.1mm to ±20mm depending on the solution.
Certification
Puls Design A/S has experience with certification of dimensioning systems according to International Organization of Legal Metrology's (OIML), MID (Measuring Instruments Directive). This approval is in some cases necessary in the cargo transportation industry.
