Chapter 5 Robust Performance Tailoring with Tuning - SSL - MIT

Table 6.1: RWA disturbance model parameters.
Name Value Units Description
f1 0 Hz minimum wheel speed
f2 50 Hz maximum wheel speed
nrwa 3 N/A number of wheels
α 30 degrees orientation angle
θ 45 degrees orientation angle
γ 0 degrees orientation angle
drwa 0.25 m distance to spacecraft origin
be used to drive the structural model, the number and configuration of the wheels
must be taken into account. A multiple wheel model was developed by H. Gutierrez
that combines the disturbance models of n wheels in a given configuration into a single
RWA disturbance model [50]. This model is a 6x6 matrix of power and cross-spectral
densities of the RWA vibrations transformed to the body-frame of the spacecraft.
The disturbance model implemented in this version of the TPF model is a stochas-
tic, multiple-wheel model created using the DOCS docs_psd_rwa_dist.m function.
The model parameters used are listed in Table 6.1. The wheel speeds are assumed
to range from 0 to 3000 RPM. The angles α, θ, andγ locate the three wheels with
respect to the body-frame spacecraft axes. The parameter drwa is the radial distance
from the wheel centers to the spacecraft origin. In the current TPF configuration,
the spin-axes of the three wheels are 45 ◦ from the TPF Z-axis, and their projections
in the X-Y plane are spaced 120 ◦ apart as shown in Figure 6-3(a). The modelled
disturbance PSDs are shown in Figure 6-3(b).
6.1.2 Vibration Isolation
Due to the precision required by the TPF instrument it is likely that the RWA will
be isolated from the truss structure that supports the optics. Simple models of a
two-stage isolation system based on the isolator design for SIM are included in the
integrated model. The first model represents an isolator located between the RWA and
the bus, and the second isolates the supporting structure from the bus. The isolator
models are included in the TPF SCI model by placing two second-order systems in
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