AN INTRODUCTION TO 3D SCANNING
Simply put, 3D scanning is a fast and accurate method of putting physical measurements of an object onto the computer in an organized manner, resulting in what is commonly called 3D scan data. Typically, the 3D scan data is represented with a scale digital model or a 3D graphical rendering. Once the scan data is on the computer, all of the dimensions of the physical object can be taken, such as length, width, height, volume, feature size, feature location, surface area, etc.
3D scan data is often used as a bridge between physical objects and modern manufacturing. This is achieved by converting the data into computer-aided design (CAD) models, using it to compare against the “as-designed” ideal of the part, or using it in the seemingly infinite number of computer-aided engineering (CAE) tools available.
HOW 3D SCANNING WORKS
In general, a device that captures 3D information is referred to as a 3D scanner. There are many different methods for capturing the 3D measurements of a physical part and thus, many different types of scanners:
3D LASER SCANNING
3D Laser Scanning or 3D Laser Scanners can generally be categorized into three main categories – laser triangulation, time of flight and phase shift. These laser scanning techniques are typically used independently but can also be used in combination to create a more versatile scanning system. There are also numerous other laser scanning technologies that are hybrids and/or combinations of other 3D scanning technologies such as accordion fringe interferometry or conoscopic holography.
- Laser triangulation is accomplished by projecting a laser line or point onto an object and then capturing its reflection with a sensor located at a known distance from the laser’s source. The resulting reflection angle can be interpreted to yield 3D measurements of the part.
- Time of flight laser scanners emit a pulse of laser light that is reflected off of the object to be scanned. The resulting reflection is detected with a sensor and the time that elapses between emission and detection yields the distance to the object since the speed of the laser light is precisely known.
- Phase shift laser scanners work by comparing the phase shift in the reflected laser light to a standard phase, which is also captured for comparison. This is similar to time of flight detection except that the phase of the reflected laser light further refines the distance detection, similar to the vernier scale on a caliper.
WHITE LIGHT SCANNING
White Light Scanning (structured light scanning) is used to describe a wide range of 3D scanning devices. The basic technique is to project a known pattern of light (usually white) and use sensors (typically CCD cameras) to capture images of the object with the patterns projected on it. In order to capture 3D information, multiple patterns and/or multiple sensors can be used. If multiple patterns are projected, the software uses referencing and the change in shape of the known pattern to interpret 3D measurements. If multiple sensors are used the software uses the known pattern and referencing between image angles to determine the 3D measurements.
Photogrammetry is a technology based on standard photography and projective geometry and was originally used to digitize large objects such as buildings, oil rigs and warehouses. The principle behind photogrammetry is to take multiple images of objects and manually or automatically reference common points in each photograph. Points can be added automatically or manually to create 3D measurements of the desired elements of the part. Photogrammetry is often used with other 3D scanning technology to provide full surface measurements of parts and to retain tight tolerances over large areas.
Machine Vision is generally used to detect two dimensional information, such as bar codes, to sort packages. An offshoot of machine vision is stereo vision, in which a pair of sensors is placed a known distance from a part, taking two images simultaneously. The resulting images are merged, creating correspondence points between them. The shift in position of the matching points is known as disparity and by calculating the disparity of every point of the two images, 3D measurements of entire scenes can be built.
COORDINATE MEASURING MACHINES (CMMs)
Coordinate Measuring Machines, more commonly referred to as CMMs, are measuring devices that use movable probes. The position of the probe is tracked with a series of encoders along each axis of the machine. These encoders, referred to as digital measuring tape, can track the movement of the measuring probe to within nanometers with the only uncertainty in the measurement being the rigidity of the machine itself. CMMs are also known as touch-probes because the probe must “touch” the part to obtain a measurement. CMMs can be fixed mounted or portable, with portable CMM arms being more versatile but sometimes less accurate because they have a higher tendency to flex or lose calibration. CMMs can also be combined with non-contact scanning technology to capture full surface data.
Destructive Slicing is a process in which multiple pictures of an object are taken from a fixed location. Thin slices of the object are removed between each image. The distance to the object is precisely controlled, therefore the scale of each image is known. After slicing all the way through the object, the images are stacked up in the capture software – resulting in a full 3D model of the object.
3D CT OR MRI SCANS
3D CT or MRI scans are obtained by stacking a series of CT (computed tomography) or MRI (Magnetic Resonance Imaging) scans on each other in software. This is typically done by precisely controlling the steps in between each sectional CT or MRI scan. The resultant data is a 3D model of the object and contains all of its physical measurements. This technology was originally developed for the medical field, as you may know, but is now growing into manufacturing and industrial applications as well.
A Theodolite device is a telescope mounted on two rotating axes and is a measuring technique common in surveying applications. The angle to an object is precisely tracked and by taking angular measurements from multiple locations, the distance to the object can be obtained. Modern theodolites are computer controlled to automatically target and measure objects.
Trackers are a class of 3D scanning device that work by tracking the position of a measuring device. Several techniques are used to track the measuring device including laser, magnetic position, optical position and acoustic position. The various methods all track the position of the measurement device, recording the position each time a measurement is taken either by touching the object or by using non-contact scanning technology.
This list is not meant to be a comprehensive list of available 3D scanning technology, only a brief synopsis of the more prevalent technologies in use today.