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Cobra – a Two-Degree of Freedom Fiber Optic Positioning Mechanism

Incorporating two high-torque rotary piezo micro motors

Application overview

The Prime Focus Spectrograph (PFS) is a fiber fed multi-object spectrometer for the Subaru Telescope that will conduct a variety of targeted surveys for studies of dark energy, galaxy evolution, and galactic archaeology. The key to the instrument is a high density array of fiber positioners placed at the prime focus of the Subaru Telescope. The system, nicknamed “Cobra”, will be capable of rapidly reconfiguring the array of 2,394 optical fibers to the image positions of astronomical targets in the focal plane with high accuracy.

The system uses 2,394 individual “SCARA robot” mechanisms that are 7.7mm in diameter and use two custom rotary piezoelectric motors to individually position each of the optical fibers within its patrol region.

Updated design and test results were presented at the SPIE Conference on Astronomical Telescopes and Instrumentation in 2014 and 2012.  Initial development of the system was described at the 2009 IEEE Aerospace Conference.

The Cobra fiber positioner

Cobra is a two-degree-of-freedom mechanism that can position an optical fiber in the prime focus of the telescope to a precision of 5 µm. It is a theta-phi style positioner containing two rotary piezo SQUIGGLE motors with one offset from the other, which enables the optic fiber to be placed anywhere in a 9.5 mm diameter patrol region. This patrol region is larger than the 7.7 mm diameter of the actuator, enabling a close packed hex array pattern of positioners to have overlapping patrol areas and deliver 100% sky coverage.

The SCARA robot was given the name Cobra because it resembles a snake ready to strike.

To achieve the fiber tip positioning goal the motors must:

The overall height of the Cobra positioner is not constrained, but efforts were made to keep Cobra as short as possible to reduce the effects of gravity sag.

Custom rotary piezoelectric motors using New Scale Technologies’ patented SQUIGGLE micro motor technology were demonstrated as the best solution to meet the requirements of size, torque, resolution, and cost. A variety of other positioner concepts were explored but not prototyped, as the rotary SQUIGGLE motors performed exceptionally well.

Working with JPL’s design requirements, New Scale optimized the miniature rotary motor for high torque and precision. The custom rotary motor created for the Cobra features torque >3.0 mN m and step size <0.065°. Each Cobra fiber positioner will include one 4.4 x 4.4 mm and one 2.5 x 2.5 mm motor in a two-stage theta-phi configuration. The Prime Focus Spectrograph will have 2,394 Cobra positioners in a close-packed hex array pattern on 8-mm centers.

The Cobra positioner was tested in a lab environment in a manner that simulates its use on the Subaru telescope. The positioner was controlled in open loop and used a CCD camera to image its optical fiber to determine its location. Over 100 simulated cosmological targets were tested using the Cobra positioner and showed that the Cobra positioner can converge on over 95% of its targets within 5µm in six open loop move iterations.

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