In the face of very high opposing forces — forces exceeding the normal preload times the friction coefficient — the runner would slide between the piezo actuators clamps (analogous to a slip clutch effect) and be driven to the mechanical hard stop. Those forces would need to be greater than 800 N for the N-216 model.
The development of the piezoceramic linear motors described here demonstrates that it is possible to combine long travel ranges, ultra -high resolution, and high force in a unified drive design. This drive additionally due to its high stiffness can move heavy loads dynamically. The drive is intrinsically self-locking and does not expend energy to hold a position. This development, as noted earlier, was undertaken for applications in the semiconductor industry, and multiple units in our lifetime test facility have demonstrated lifetimes of >108 slew and >109 dither mode cycles to date in continuing durability tests. The lifetime design goal of these actuators is 10 years in a vacuum environment. More testing is necessary for cryogenic applications and for mountain top observatory environments. From our experiences in extreme industrial applications, a simple and low cost dry air purge of a few cc’s per hour will ensure long actuator lifetime for ground based observatory environments.
The applications for ground and space astronomical telescopes with their associated instrumentation packages range from primary mirror segment positioning, active truss structures, and embedded coarse/fine position control in space constrained instrument packages. From a cost standpoint, NEXLINE actuators are higher in cost than conventional micropositioning drives. Yet when the requirement demands coarse/fine position control with nanometer level resolution, they are cost competitive, simpler to integrate, and occupy much less volume and mass than equivalent force hybrid mechanical actuation designs.
1) K. Lorell et al; “Design and preliminary test of precision segment positioning actuator for the California Extremely Large Telescope”, Proceedings of the SPIE, Volume 4840, pp. 471-484 (2003).
2) United States Patent 6,800,984, “Piezo linear drive with a group of piezo actuator stacks as well as method for operating such a drive”, October 5, 2004.
3) Pitz, E. et al; “Five –axis secondary system for UKIRT”, Proc SPIE Conf. on Advanced Technology Optical Telescopes V”, ed. L.D. Barr, 2199, 516–522, 1994.