There is a plethora of companies that currently manufacture 3D scanners and digitizers. This growing market produces instruments able to digitize objects microscopic in size to entire constructions sites. The speeds for data acquisition vary from a few points per minute to a million points per second. The price ranges vary from a thousand dollars to a hundred thousand. This broad spectrum represents the large variety of devices now available. The market and technology base for these products may be premature and not fully developed.
Another field that also has a wide range of technology is rapid prototyping. Coincidentally, the Reverse Engineering (RE) used in this field may also be reverse-rapid prototyping. RE develops converted point cloud data, acquired through digitization or noncontact scanning in CAD models. The CAD models can be then used for fabrication materials by removing methods like milling or material incremental methods.
There are three key specifications when considering digitizers: volume, speed, and accuracy. Volume is usually not much of a limitation because scans can be stitched together to create objects that are larger than the available scanning volume. Time and accuracy are elements that need to be considered.
Accuracy is the precise measurement that correlates directly to dimension. It isn’t the same as resolution, which specifies distance or volume to the smallest measurable increment. An instrument can have a high resolution and still be inaccurate, or the opposite can occur. Problems occur when manufacturers specify one value but not the other. They do this by creating their own set of conditions and terminology. This can cause different specifications to be applied to each axis of measurement. Accuracy and resolution are vital to applications and may require additional data from manufacturers or performance testing.
Speed is the frequency determined in points/second. This area also has a tremendous amount of variation among manufacturers as they only provide anecdotal specs or no information. The best possible option is to determine which regimen the instrument falls under.
Mechanical Touch-Probe Systems
There is a large distinction in digitizing technology between contacting and non-contacting instruments. Touch-probes, known as contacting digitizers, provide consistent measuring accuracy. They are very affordable instruments. Some contact digitizers are manually positioned to provide a single measurement. Others may scan a surface to provide a series of measurements. Touch-probes can be programmed to automatically scan an object using a mechanical drive system. Many of these systems have articulated arms that provide free movement in many directions.
A disadvantage of a contacting device is that it can distort soft objects. They may also be too slow to digitize complex objects such as the human body or may require assistance in scanning complex and curved surfaces. The advantage is that they are impervious to surface colors, transparent or reflective surfaces that may affect lasers and light-based systems. Even though they are slow, they may be the most effective means of digitizing surfaces where only a few data points need to be gathered. Narrow slots, pockets and difficult to digitize surfaces may be accessed more easily by manually positioned devices.
The two classes of non-contact scanners are based on either laser technology or a non-coherent white or broadband light source. The laser scanners use geometric triangulation to obtain an object’s surface coordinates. Their simple technique and quick ability to digitize large volumes with sufficient accuracy and resolution make them popular. The system is complete with self-contained measuring heads which usually mount to touch-probe arms. They also have customizable fixtures for special applications.
If surfaces have color, are transparent or reflective, laser and light-based systems may be affected. With experience, laborers have learned to work around surface issues which have caused errors. It is imperative that safety factors be followed when using a laser. Although lasers are calibrated not to cause harm, using them on reflective curved surfaces could potentially cause harm from a focused beam.
Digitizing instruments and laser scanners often have complimentary capabilities. Laser devices are capable of scanning broad areas using lasers mounted on the arm. Areas that might cause problems for lasers can be contact-probed. Companies are now developing instruments that can simultaneously carry a contact probe and a laser head.
Some companies, such as Arius 3D, have the ability to produce color laser scanning. Arius’s scanning technology utilizes a combination of red, green and blue lasers to gather geometric data and color. Minolta and Cyberware use lasers to get measurements and then combine that data with color video.
Other Laser Systems
Other laser technologies include optical radar, laser tracking and time of flight. These systems have good accuracy and the ability to take measurements of the object from a great distance. The “stand-off” distance, which can be tens of meters, has important applications in digitizing buildings, large machines, and large structures.
The time it takes for light given off by a laser to return to its sensor is known as “time of flight”. Optical radar systems operate the same way as do radar systems, measuring the return time of radio waves. Both can capture scenes and objects with great speed and don’t usually require retroreflectors. Laser trackers search for a signal from retroreflectors on the object in its field of view. This system works with a high level of precision and is often used for aligning large machinery or verifying dimensions of a large object.