Current micro CMMs comply with the Abbe principle in 2D or even in 3D, which is a preferred method to reach 3D nanometer uncertainty.

 Most of these CMMs are the result of academic research, in which the challenge is to achieve the lowest uncertainty over a large measurement range. This resulted in technologically  advanced, but expensive systems.

Within the micro manufacturing 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. To fasten market adaptation of micro coordinate metrology, a fast and cost effective micro CMM has been developed: TriNano.

Working principle
TriNano is a newly developed CMM to measure objects with sub-millimeter features in three dimensions and with nanometer uncertainty. The TriNano N100 has a 3D
uncertainty of 100 nanometer in its entire working volume [1,2]. In order to achieve this while keeping the device cost efficient, a new working principle has been developed. This principle employs a moving work piece table and a stationary probe. The table moves in three directions by means of three identical linear translation stages as shown schematically in figure 1 and pictured in figure 3.

Figure 1: Schematic 3D representation of TriNano’s moving work piece table.

The 1D stages are positioned orthogonally and in parallel and support the work piece table via vacuum preloaded (VPL) porous air bearings as shown schematically in two dimensions in figure 2. From this figure the operating principle of the TriNano becomes clear. A linear translation of a stage is transferred via a VPL air bearing to the work piece table. Translations of the work piece table with respect to the linear stage in other directions than the translation of the stage are decoupled by the VPL air bearing. In this manner, the three stages independently determine the position of the work piece table in three dimensions.

Figure 2: Schematic 2D operating principle, with the work piece 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. At the point of intersection of the measurement axes of these encoders the probe tip is located. 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 work piece table will have little effect on the measured dimension.

Cost effective design
Instead of a conventional orientation of the machine axes, i.e. two orthogonal axes plane (x and y) and a third vertically oriented axis (z), the three axes in the TriNano are oriented such that each stage experiences an equal gravitational load. This orientation of the axes combined with the operating principle shown in figure 2, results in identical translation stages which can be produced at a lower cost.

In addition, TriNano employs linear encoders to determine the position of the workpiece table. Compared to conventional ultra precision CMMs, which often employ laser interferometer systems; this principle results in a considerable cost reduction. Several other means have been implemented to reduce costs without compromising on quality or ease of use.

Figure 3. TriNano employs a moving workpiece table (testing purposes is shown) supported by VPL air bearings in order to comply with the Abbe principle in 3D.

The parallel configuration of the three identical stages supporting the superior dynamic behavior of TriNano. This mass of each stage with short and stiff structural loops. 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 micro parts with high velocity.

Kinematic design
An unconstrained rigid body has six degrees of freedom (d.o.f.), three translations and three rotations. If all d.o.f. of a rigid body are fixed once, this body is said to be statically determined or exactly constrained. If more or less than six d.o.f. of a rigid body are constrained, it is said to be over or under constrained, respectively.

An important benefit of an exactly constrained design is that it will isolate critical parts or systems from the influence of manufacturing tolerances or deformations of the support frame, due to temperature variations or loading of the frame. An over constrained design often suffers from backlash and requires tight tolerances in order to function properly. In order to obtain the highest measurement repeatability, exactly constrained designing is necessary [3]. The design of the TriNano is therefore based on well known kinematic design principles, in order to obtain an exactly constrained design.

Figure 4. Exactly constrained design: classical (left), TriNano (right)

A classical solution of exactly constraining a body is to use six slender rods, as displayed on the left of figure 4. A single (VPL) air bearing as used in the TriNano, constrains 3 d.o.f.; 2 rotations and 1 translation. Applying an elastic line hinge releases one of the rotational constraints. Combining three of these air bearing-elastic line hinge combinations as shown on the right of figure 4 results in an exactly constrained work piece table which allows repeatable positioning of the work piece table.

Thermal design
Thermally induced errors are often the largest contribution to the total error budget in precision measurement equipment despite the research efforts performed on the matter [4]. However, certain straightforward measures can be taken to reduce these thermally induced errors, such as minimizing and controlling the heat flow and decreasing the thermal sensitivity of the machine. To minimize the heat production in the actuators, TriNano employs a pneumatic weight compensation system as represented in figure 5. The weight compensation system can be adjusted to carry various object weights. Moreover, a thermal insulating box is placed over the actuators preventing heat produced by the actuators from affecting the measurement.

Figure 5. Schematic of the thermal design of the TriNano

The volume in which the parts of the metrology loop are located is enclosed by a thermal isulation shield as shown in figure 6. A temperature control system can be employed to create a stable environment inside this volume. Parts to be measured can be placed inside the machine prior to the actual measurements, in order to reach the desired temperature upfront and minimize throughput times.

The thermal sensitivity of the TriNano is further reduced by designing the parts of the metrology loop to have a large thermal time constant. For most parts this is achieved by adjusting their dimensions, instead of using expensive low thermal expansion materials. Only parts which need to be of a specific slender shape (e.g. measurement scale, elastic line hinge) are made from a low expansion material.

Figure 6: TriNano N100 CMM with housing (left, schematic) and without (right).

Gannen probing systems
In the standard configuration, a TriNano is supplied with a Gannen XM probing system, as shown in figure 7. The Gannen XM is a 3D probing system supplied by Xpress Precision Engineering (5). It is suitable for measuring micrometer sized features with nanometer uncertainty. Other ultra-precision probes will be supported as well, including the Gannen XP.

Figure 7: Gannen XM probing system.

The suspension of these Gannen probes consists of a silicon membrane with three slender rods as shown in figure 8.

Figure 8: Chip and stylus of the Gannen XM probe with example products.

The probe tip is connected to the centre platform of this chip via a stylus. When the probe tip is displaced, the three slender rods will deform. This deformation is measured using piezo resistive strain gauges on the slender rods. The strain gauges are manufactured together with their electrical connections and the slender rods in a series of etching and deposition steps.

This results in a design with an extremely low moving mass of 25 mg including the weight of stylus and tip, as shown in figure 8. Also, it allows the use of rods with a thickness down to several micrometers, which as a result are very compliant. The stylus of the Gannen XP has a length of typically 6.8 mm. In this configuration an isotropic stiffness of 480 N/m is obtained and the sensitivity of the probe is similarin each probing direction.

Since the piezo resistive strain gauges are deposited onto the silicon membrane, hysteresis is below 0.05% and the standard deviation in repeatability is 2 nm over its whole measurement range and in any probing direction [6]. This combination of a highly compliant design with low moving mass and nanometer repeatability allows the use of micrometer sized probe tips. Currently tungsten probe tips with a diameter down to 42 μm have been manufactured and used with this probe, allowing 3D measurements on micrometer sized features.

Conclusion
TriNano is a new micro CMM, developed to measure micro manufactured objects in true 3D with nanometer uncertainty. It employs a new cost effective working principle, enabling fast measurements. A TriNano is equipped with one of the Gannen micro probes, which are specifically designed for fast and accurate scanning measurements on small objects.

More information
For more information (including a video of a TriNano N100), please visit www.trinano.eu.