The goal of the Extra Large Telescope Actuator (ELTA) development project was to demonstrate operation of a relatively high stiffness, single stage optical positioning actuator capable of achieving diffraction-limited performance (<10 nm) in the visible optical band for weeks at a time while consuming no electrical power and dissipating no heat.
The design challenge was to develop a linear positioning mechanism exhibiting high stiffness, low power, zero backlash, and thermal stability over extended time periods. The key to achieving high resolution, and stability with low power is to eliminate the closed-loop control system that is normally employed to overcome the nonlinearities and hysteresis inherent in some technologies, such as piezoelectric and magnetostrictive transducers. This was accomplished by using the patented elastic transducer developed by Alson E. Hatheway (AEH Inc.) This device consists of two elastic elements; a soft spring and a stiff flexural member. Deflection of the soft spring applies a force input to the stiff flexure, which responds with a proportionally reduced output deflection. To maintain linearity, the displacements, and hence the stresses, developed in both elastic members are kept below the micro-yield strength of the material. The AEH transducer is inherently linear and hysteresis free. The unique design features of this actuator which contribute to its extremely precise motion capability include an electric motor driving a leadscrew through a zero backlash harmonic drive gear reduction. The already fine incremental motion of the leadscrew nut is further attenuated by the elastic action of the AEH transducer, to provide output motion with resolution <10 nm.