Contact Digitizers

A contact digitizer is a mechanical device designed to move a measuring probe to determine coordinates of points on a workpiece surface with respect to a fixed coordinate system. It shares the same basic principle of operation as a Coordinate Measuring Machine (CMM). A contact digitizer is comprised of four components: machine body, measuring probe, computer hardware, and software. Contact digitizing machines are available in different sizes and designs.

The probe in a contact digitizer is moved manually or by a software program. When the probe's tip touches the part surface with a distinct force, it gets deflected. This deflection is sensed through a transducer in the probe head. The current probe's position is registered and the probe is lifted by a fixed increment and moved along the same axis. The shape and size of the probe tip affects the accuracy of measurement.

The software of the digitizer saves the (x, y, z) coordinates of the measured points, which can be imported into commercial CAD/CAM systems. This set of points is called a point cloud. Some contact digitizers have built-in software that automatically regenerates surfaces and even numerical control programs directly from the measured data.

MicroScribe® digitizer uses a linkage having rotation measurement sensors (Ref: MicroScribe website). The physical model to be digitized is mounted firmly by using a fixture. The probe head is then operated manually to go over the surface to be digitized. While the probe tip is in contact with the surface, the digitizer records the sensory data. Another contact digitizer is MAXNCtm (Ref: MAXNC website). It is basically a desktop 3-axis milling machine, whose spindle head can be fitted with a probe in replacement of a cutter. The probe detects change in height along Z-axis as the probe moves in XY plane. The software provided by MAXNCTM facilitates setting values of process parameters. The output data is a DXF file, which is a translated file that contains the information of x, y and z coordinate values and the lines connecting them. From these files it is easy to extract the point cloud data. The digitized data can be imported by AutoCAD and other commercial software. Pictures of Microscibe® and MaxNC™ are shown in Fig. 8.10. The digitization of a stone model with 5 implants using a MaxNC is shown in Fig. 8.11(a). The point cloud data extracted from the measurement is show in Fig. 8.11(b). The advantages of contact digitizers relative to non-contact digitizers are:

• The contact digitizer is relatively cheap.

• The computer hardware requirements are less.

• Good accuracy (±0.01 mm) can be achieved at a more affordable price.

• No special setup or work environment is required.

The drawbacks of contact digitizers are:

• The digitizing process is relatively slow.

• The digitization accuracy is limited by the diameter of the contact probe and the probe deflection.

Fig. 8.10 Contact digitizers: (a) MicroScribe® (Ref: MicroScribe website, reproduced by permission of Immersion Corp.); (b) MAXNCTM

• It is difficult to obtain oral data directly from a patient's mouth because of the requirement of physical contact.

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