# State of the Art of Tactile Micro Coordinate Metrology
## 2 Probe Developments  
When measuring small features using a tactile probe, the influence of the probe-surface interaction gets more and more critical with decreasing size of the probing element. For smaller probe sphere radii, the contact area gets smaller and therefore effects of elastic or even plastic deformation of the surface due to static or dynamic measurement forces become very critical. The effects of surface deformation by over-travel forces due to the probe stiffness and of impact forces due to the probe mass, dynamic excitations due to vibrations, surface forces as well as tip rotations due to stylus bending or rotation has been extensively studied by [PERSON] [2]. Conventional probe systems of CMMs did not fulfil the stringent requirements regarding low stiffness and low dynamic mass nor the accuracy requirements set by ultra precision CMMs, therefore new systems had to be developed. A review over a wide range of probing systems has been given by [PERSON] _et al._[1]. One the first micro probes has been developed jointly by PTB and Werth in 1998 [5] and soon after implemented on commercial multi-sensor CMMs. It is based on a glass fiber with a quasi spherical melted end whose position is measured with an opto-electronic system. Tip diameters down to 20 \(\upmu\)m can be realized. Whereas the first probes had only 2D capability, Werth has developed in the mean time a fiber probe with full 3D capability [6]. The probe is available on Werth multi-sensor CMMs and has proven its usefulness in many applications, in particular for the measurement of small holes. Probing errors of a few hundred nanometers are specified, limited mainly by the relatively large form deviations of the melted glass sphere tips. PTB developed a probe based on a micro-fabricated silicon membrane with piezo-resistive sensors [7], first published in 1999. The system measures two angular and one translational movement of the probing element. The stiffness in horizontal direction depends on the shaft length and is usually not the same as in the vertical direction. The probe design has been commercialized in the meantime and is available on the F25 micro CMM from Zeiss. The manufacturer specifies a probing error of 250 nm [8]. Approximately at the same time, NPL has also developed a micro-fabricated probe based on three angled flexible elements with capacitive sensors [9]. This original design has been commercialized as the Triskelion probe by IBS [10]. The manufacturer indicates the 3D measurement uncertainty of tip deflection to be below 20 nm. More recently NPL developed a novel vibrating micro-probe, based on the Triskelion design with six piezoelectric sensors and actuators, two on each flexure [11]. The design is optimized to feature high isotropy and very low contact forces in the \(\upmu\)N range, thus reducing effects of surface interaction forces [2]. Experimental results to qualify the probe are not yet published. A similar probe design using three angled flexible elements, but based on a monolithic silicon chip with piezo-resistive sensors and suitable for mass production, has been developed by TUE [12] and first published in 2001. Meanwhile, this is commercialized as the Gannen probe by XPRESS [2,13]. It is specified with a combined 3D probe uncertainty of 45 nm. Another micro probe which is commercially available was developed by the CMM manufacturer Mitutoyo. The UMAP probe, implemented on the UMAP vision system since 2002 [14], is based on an ultrasonic micro-vibration sensor and available with stylus tip diameters between 15 \(\upmu\)m and 300 \(\upmu\)m. The accuracy of the probe itself is not specified. In order to overcome the trade-off between the stiffness requirements of the probing system at contact and during approach, the University of Nottingham is actually developing a probing system for micro CMMs with variable stiffness [15]. While still under development, first promising experiments resulted in an expanded uncertainty of the probing system of about 60 nm.