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248 Andrei Mihalache et al. CMM with laser system CMM with touching probe Physical model (sample or prototype) CNC – milling machine with touching probe Digitizing software Points cloud (3D coordinates) CNC – milling machine with laser system CT Computer tomography Fig. 2 – Digitizing techniques for 3D-geometries and generated data. 3. Differences between Digitizing and Scanning Processes 3D scanning (digitizing) is the process of gathering data from undefined three-dimensional surface. During the scanning process, an analogue scanning probe is commended to move back and forth (contact or non-contact) across the unknown surface. During the process, the system records information about the surface in form of numerical data’s – and generates a points cloud matrix (3D coordinates). The terms of digitizing and scanning are often used to describe the same process. Traditionally, digitizing refers to the process of taking discrete points using a touch-sensible probe. However, with the introduction of new technologies in data capture, such as laser or camera, digitizing term is now used as general description for the process of data acquisition from undefined surfaces [2] – [4]. The digital points cloud can be captured using different digitizing techniques (Fig. 2), classified into two major groups: ‐ Mechanical techniques (by physical contact sensors); ‐ Optical techniques (by non-contact with the object). Related to the first group, usually they use a coordinate measuring machine – CMM, or a CNC-milling machine equipped with physical touching probe sensors (ex. Retroscan or Renscan – Renishaw, UK). Related to the second
Bul. Inst. Polit. Iaşi, t. LVI (LX), f. 2, 2010 249 group, we can also use CMM’s or CNC-milling machines but equipped with laser beam probes (ex. Renishaw, Metris LC Mitutoyo), or associated optical sensors (ex. CCD cameras) [2]. 3. Advantages and Weak Points of Different Scanning Equipments Scanning equipments Advantages Week points Laser Precise and fast scanning on Z-axis (0.001 mm) Equipment high price Non-contact method It is not possible to scan reflective materials It is possible to scan soft materials (or liquids) Scanning in X- and Y-axis is inaccurate (0.035 – 0.060 mm) It is not possible to scan notches areas or steep surfaces due to additional reflections. Sensibility to air dust CCD Fast High price of equipments cameras It is possible to use two or three cameras Accuracy decreases linear with simultaneously camera distance Insensible to part color Scanning angle is equal regardless the shape of part’s surface; in case of steep angles the measurement in inaccurate Non-contact method, it is possible to scan soft By camera scanning, a very sharp materials picture is required; simultaneous scanning of far and closer surfaces demonstrates linear deviation of results depending on focal distances In case of special coaxial lightning it is possible to In case of oily or wet parts the scan small diameters and high depths on Z-axis measurement in inaccurate It is possible to scan very small areas: 1 mm 2 (one scan only needed) – accuracy is above few micrometers Scanning on flat surfaces is very fast Dust causes bad scanning Contact Very precise on all axis (depends on scanning It’s inadequate for soft materials. (classic) equipment) Fast scanning of known geometrical parts Scanning unknown surfaces is either impossible inaccurate or very slow and Contact Precise scanning of coins or similar geometries Manual or automatic scanning possibility Hand-held scanning equipment is useful for scanning big products (airplanes, ships, big machines or devices) Precise on all axis (0.001 mm) Minimum diameter of stylus is 0.3 digitizers mm, scanning surface roughness is (Renishaw) not possible Relatively low price It not possible to scan soft materials It is possible to use different styluses for different Though material of stylus is wear surfaces resistant, it’s surface wears in time Oil, liquid or dust do not interfere with the Scanning speed is lower (compared scanning process It is possible to scan unknown steep surfaces to non-contact systems)
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