65 Table 2: CFM performance requirement and their impact of the CLUPI performance  CFM requirement  Impact on CLUPI performances   Positioning  repeatability  of 20 µm Poorer repeatability  will impact the sharpness of the images  acquired due slightly off -focus lens   Positioning  resolution of 5 µm Poorer resolution may impede  the proper functioning  of the z - stacking algorithm and of the au to-focusing  algorithm.   Mobile stage parallelism at rest  position better than 0.1°  Lack of parallelism  at rest  may impact the image quality   Mobile stage concentricity at rest  position better than 50 µm Lack of concentricity at rest could impact the image quality   Mobile stage parallelism stability along  stroke better than 0.1°  Lack of parallelism  stability  may impact the image quality  and the ability to perform the z -stacking due to erroneous coregist ration  of successive images   Mobile stage lateral shifts along stroke  (concentricity stability) better than  20 µm Lack of concentricity stability  could  impact the image quality  and the capability to perform the z -stacking due to erroneous  co-registration  of successive images  

To achieve such performance the following  approaches were implemented:  

1. Completely frictionless design along with a LVDT sens or and Voice- coil actuation: with such a  configuration, the mechanism positioning capabilities are mainly driven by the control loop performance  and the electronics used to drive the actuator and sensor. With a frictionless  mechanism and  considering that CLUPI operates only when the rover is immobile , the control loop can be very simple  and achiev e excellent performance, especially after a proper sensor calibration.  

2. Fine tolerance chain between the mobile and the fixed opt ics: the parallelism and concentricity at rest  are guaranteed by a very stringent tolerance chain throughout the flexurized guiding structure.  

3. Axisymmetrical construction: the ax isymmetry of the guiding structure improves  the parallelism and  concentricit y at rest , since (undesired) lateral motions tend to counteract each other, instead resulting  in very small (benign) rotation of the optics about the optical axis . The axisymmetry of the guiding also  guaranties parallelism and concentricity stability throu ghout  the movement as the errors induced by the  movement in a perfectly axisymmetric  construction also remain axisymmetric , i.e. normal and centered   on the guiding axis.  

Such approaches  were in general well implemented with the following exceptions:  

• The CFM design is not a perfectly axisymmetric  construction. To fulfill this approach, the sensor  and actuator should have been either centered onto the optical axis or repeated around  the optical  axis. Early in the design, the trade- off selected a single set of off -centered sensor and actuator  for  mass and cost saving. This resulted in an off -centered actuation force which generates  a significant  error of concentricity  and parallelism stability. Such err or was identified early as a potential nonconformity, based first on FEM analyses and  then on breadboard testing.  

• The LVDT sensor was selected for its compactness. Because of its miniature size, the radial gap  between the mobile rod and the fixed coil body is very narrow (about 0.04 mm). With such a tiny  gap, it is not possible to guaranty the absence of contact between the two parts during the  movement especially with the errors introduced by the lack of axisymmetry . Therefore,  even with a  perfect initi al alignment of the two parts of the LVDT, during the movement, the mobile and fixed  part make contact,  generating tiny friction forces which can climb up to 50mN. This residual friction  force impacted the positioning performance, which thankfully remained within the requirements.  

In the end the compliance of the EQM and FM to performance requirement s is provided in Table 3.