IEEE - Aerospace and Electronic Systems - July 2022 - 28

Development and Validation of a Canfield Joint as a Precision-Pointing System for Deep Space Instrumentation
Figure 11.
Trajectory tracking test results: Motion profiles and detector displacements for 0.5, 1.0, and 2.0-mm/s tracking rates. As in Figure 10, the top
plots represent the surface of the quadrature detector.
Inc. to study the device in the laboratory 1G environment.
This prototype, shown in Figure 6 and illustrated schematically
in Figure 7, has demonstrated the capabilities listed
in the mission requirements in open- and closed-loop tests.
This 39-kg aluminum model is designed to articulate a
payload mass of 90 kg; a spaceflight design of the same
form factor, constructed using carbon fiber, is projected to
weigh 15 kg, while maintaining the same payload mass
capability.
OPEN-LOOP VERIFICATION
Several open-loop tests were performed to verify the FOR
and resolution of the prototype system, as detailed in [23].
An optical metrology apparatus was constructed in order
to measure pointing resolution. This apparatus, shown in
Figure 8, consisted of a laser source with an electronic
beam steering system mounted on an aluminum frame. A
Thorlabs PDQ80A quadrature detector was mounted to
the distal plate of the Canfield joint, the laser directed
toward it, and the detector output used to measure the
changing position of the distal plate. The robot's FOR was
verified to fit the project requirements of 330 in azimuth
and83 in elevation, and the pointing resolution verified
to satisfy the step size requirement of 0.0025 (44 m
radians), which is commensurate with the state of the art,
as noted in Table 2. The Canfield joint accomplishes beam
precision within one order of magnitude of the ultimate
resolution requirement of 4.5 m radians obtained in the
" Orbital Analysis " section; the remainder would be
28
accomplished by FSM. The slew rate test described
in [23] verified a maximum slew rate of approximately 3/
s, comfortably above the 0.534/s required.
CLOSED-LOOP VERIFICATION
We implemented closed-loop control using D-Space via
MATLAB. Test data are available at [27]. In order to assess
the capabilities ofthe Canfield joint in a closed-loop configuration,
extra functionality was added to the test bed. The
laser was mounted to a Thorlabs LNR25ZFS linear translation
stage, controlled with a Thorlabs KST101 stepper controller.
The PDQ80A used in the open-loop tests was
replaced with a PDP90A detector, and the superstructure
shown in Figure 6 was extended so that the laser could be
mounted above the prototype at a higher angle, as shown in
Figure 9. A straightforward bang-bang feedback control
lawwas implemented in theMATLAB environment to react
to a generated error signal and drive the system to minimize
errors. The controller was tuned to yield a sufficiently fast

IEEE - Aerospace and Electronic Systems - July 2022

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