Vol.2 No.4 2010

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Research paper : Portable national length standards designed and constructed using commercially available parts (J. Ishikawa)−251−Synthesiology - English edition Vol.2 No.4 (2010) change in the recommended uncertainty (10 kHz converted to frequency), even when all the operating parameters of the iodine stabilized He-Ne laser are set within the recommendations of CIPM. It was learned that extremely high linearity was required for the movement mechanism of the laser mirror to ensure the stability of laser wavelength of the iodine stabilized He-Ne laser. The piezoactuator used in the control of the resonator length has excellent rigidity and micro-displacement capability, but was a bottleneck for the precision of conventional iodine stabilized He-Ne laser due to the linearity issue. I learned that it was essential to develop a movement mechanism with excellent linearity for further universalization, and employed the method of combining the piezoactuator with excellent rigidity and micro-displacement and a guide mechanism with excellent linearity in the new iodine stabilized He-Ne laser.In general, the mechanical linear guide can be categorized into three: parallel spring, sliding, and rolling guides. The parallel spring guide has been used most commonly as the linear guide combined with the piezoactuator. Although the range of motion is limited, the parallel spring guide is considered particularly suitable for fine position control since there are no allowance, friction, or backlash. However, to realize a high degree of linearity, the parallel spring structure must be somewhat enlarged, and the weight increases when the rigidity of the system increases. Moreover, it is expensive. The sliding guide inherently has allowance and cannot be used for this purpose. In this development, I use the rolling guide because it is inexpensive and light, even though it had almost no history of being used as a guide for laser cavity length control.5.1.1 Linear motion guide by ball splineRolling linear motion guide includes ones with finite stroke and infinite stroke. The finite stroke rolling guide has structure where the rolling body such as balls or rollers is placed between the opposing linear guides, as shown in Fig. 8a. When the lower guide is fixed and the upper guide is moved, the rolling body in between moves halfway along the upper guide. To maintain the movement of the guide, the rolling body is arranged at the range of 50~70 % of the total length of the guide. The problem of the finite stroke rolling guide is that the support point (position of the rolling body) moves along with the movement. While it is dependent on the rigidity and load, the movement of the support point is the cause of unavoidable position shift.The infinite stroke rolling guide has a mechanism to return the rolling body, which is expelled from the rear as the upper guide moves, to the position between the upper and lower guide, as shown in Fig. 8b. In addition to allowing long distance movement, the movement of the support point is small (less than the interval of the rolling bodies), and it is an excellent mechanism with little position shift.The first mechanism I used as the rolling linear motion guide was a ball spline with a circular hollow shaft structure (THK Co., Ltd.: LF13)[4]. The ball spline is a kind of infinite stroke rolling guide, and I decided to use this because high-level linearity can be obtained. Also it was shaped similarly to the ring-form piezo stack actuator and it could be incorporated easily to the laser resonator. The spline shaft diameter of the ball spline is 13 mm, and there is a hole of 5 mm diameter in the center through which the output laser passes. Figure 9 shows the laser mirror linear movement mechanism with an incorporated ball spline. The laser mirror is set at the end of the spline, and the spline shaft is pulled out front with a preload spring in the control arm. When the adjustment mechanism for the laser mirror is installed on the laser body, the tip of the linear piezoactuator of the main body contacts the spline shaft control arm, and the spline shaft is pushed in about halfway. Control voltage is applied to the linear piezoactuator, and the mirror position is controlled by the expansion and contraction.Unlike the sliding guide, the allowance can be removed in the rolling mechanism by applying preload. For the ball spline tested, the gap between the main body and the ball spline shaft was adjusted to less than the ball diameter by 2 ~ 6 m, and there was no looseness. However, when the spline shaft was pushed in the side direction during laser emission, a change in laser power that was thought to arise from the mirror tilt was observed. The rigidity against the side moment of the ball spline guide is not enough. In this ball spline guide, the ball is circulated. According to the specification, since the gap between the main body and the spline shaft is smaller than the diameter of the ball (-6 ~ -2 m), large resistance is produced when the ball enters the interval and hampers smooth movement. However, no change in resistance due to entry and exit of the ball was observed. In both ends, the interval is slightly larger to prevent resistance when the ball enters, and it is estimated that the actual Fig. 8 Finite stroke rolling linear motion guide (a) and infinite stroke rolling linear motion guide (b).a. Finite stroke rolling linear motion guideb. Infinite stroke rolling linear motion guideFig. 9 Linear motion guide for laser mirror with the ball spline.Control armSpline shaftPush with piezoelement

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