TriNano CMM: Nanometer uncertainty for a micro price

Oct. 4, 2011—Current micro CMMs (coordinate measuring machines) comply with the Abbe principle in 2-D or even in 3-D, which is a preferred method to reach 3-D nanometer uncertainty. Most of these CMMs are the result of academic research, where the challenge was to achieve the lowest uncertainty over a large measurement range. This resulted in technologically advanced, but expensive systems.

Within the micromanufacturing industry a large measurement range often is not the most important factor. Most objects or their molds have dimensions of a few centimeters (lenses, watch base plates, small gears, etc.), and fit in a match box.

TriNano is a new micro CMM for true 3-D measurements with nanometer uncertainty on objects with sub-millimeter features. In order to achieve this while keeping the device cost-efficient, a new working principle has been developed. This principle employs a moving workpiece table and a stationary probe. The table moves in three directions by means of three identical linear translation stages as pictured in the video and figure 1 below.

Figure 1. TriNano employs a moving workpiece table supported by VPL air bearings in order to comply with the Abbe principle in 3-D. Pictured above is an aluminium dummy table for testing purposes.

The 1-D stages are positioned orthogonally and in parallel, and support the workpiece table via vacuum preloaded (VPL) porous air bearings as shown schematically in two dimensions in figure 2 below.

Figure 2: Schematic 2-D operating principle, with the workpiece table in its neutral position (left), and after making a translation in local y’-direction (right).

On each linear stage the scale of an optical linear encoder is mounted. The probe tip is located at the point of intersection of the measurement axes of these encoders. As the orientation of the encoder scale does not vary with respect to the probe—as can be seen in figure 2—the TriNano complies with the Abbe principle over its entire measurement range. As a result, rotations of the workpiece table will have little effect on the measured dimension.

Furthermore, the orientation of the axes combined with the operating principle shown in figure 2 results in three identical translation stages that can be produced at a lower cost. Several other means have been implemented to reduce costs without compromising on quality or ease of use. The parallel configuration results in superior dynamic behavior of TriNano. On-machine measurements show that the natural frequency of the complete system is 75 Hz. This allows a high control bandwidth required for ultra-precision scanning measurements of unknown microparts with high velocity.

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