Design of a Hybrid Positioner-Fixture for Six-axis Nanopositioning ...

Design of a Hybrid Positioner-Fixture for Six-axis Nanopositioning and Precision FixturingSubmitted to Precision Engineering1500Y displacementCommand [nm ]100050000 500 1000 1500Command [ nm ]Figure 22: Displacement test results for coupling displacement while balls are engaged in groovesAlthough smooth motion of the HPF may be assumed via use of the guiding and groove flexure bearings,it is helpful to validate this behavior and understand the smallest motion increment which may beachieved through the HPF. Figure 23 shows a magnified view of the rectangular detail in Fig. 22. Thefigure indicates that the engaged resolution of the HPF is approximately 4nm. This limit is due tolimitations of the electronics that drive the piezo actuators.Measured [ nm ]250225200175Y displacement (detail)Figure 23: Close up of rectangular detail in Fig. 22150150 175 200 225 250Command [ nm ]6.5. HPF repeatability in fixture modeOne thousand cycles of engagement and disengagement were performed over the course of the 14 hourrepeatability test. The HPF was placed in fixture mode, meaning that the actuators were not used toenhance positioning performance. Positioning occurred solely due to the ball-grove interface interactionsand groove flexure bearing motions. Ideally, the actuators would be commanded to hold their positionthroughout the test. Although the piezos are equipped with internal strain gage sensors which could beused to maintain the position to better than 5nm, the limitations of our dSpace system prevent us fromsimultaneously reading six capacitance probes, reading six strain gages and supplying independentvoltages to the six piezos. A compromise approach was implemented in which the piezo actuators wereenergized with a constant voltage during the duration of the test. As such, the repeatability data willinclude errors from the piezo actuator creep. The results of the test are shown in Fig. 24.22