Patent Application
20090265149
The present invention relates generally to the field of precision structures, and more particularly to structures which must maintain very precise dynamic stability in the presence of forces induced by on-board spinning mechanisms.
Rotating mechanisms create disturbance forces and moments that can degrade the operation of various precision systems. One particular class of system encompasses optical and other telescopes, which are sensitive to vibration at the micron to nanometer level. This class includes space based observatories. A particular class of mechanism is the Reaction Wheel actuator for spacecraft pointing. This is often the largest source of disturbance forces on the observatory. The characteristic disturbance signature or a rotating mechanism consists of forces and moments at harmonics (integer and non-integer multiples of the wheel speed). Mechanisms also frequently exhibit broadband disturbances with lower forcing magnitudes. Both noise signatures are dynamically amplified by structural modes of the wheel, which are themselves functions of wheel speed through the influence of gyroscopic forces. The dynamic amplification can increase the forcing amplitude by factors of 100 or more. Reaction wheels are particularly problematic because the speed can vary arbitrarily from zero to plus or minus several thousand RPM. Similar forcing characteristics can be found in other spinning mechanisms such as pumps, filter wheels and shutters, and steering mirrors.
An accurate model of the mechanism induced forces, correctly incorporating speed-dependent dynamic amplification, is vital for predicting the resulting mechanism vibration, and thus enabling identification and mitigation of any mission-threatening vibration errors.
The state of the art approach to developing a model of rotating mechanism disturbances is to manually extract a disturbance model from measured disturbance data. Disturbance harmonics are manually identified and manually fit with a fixed speed-dependent amplitude function (most often speed-squared). The vibration data near mechanism resonances is discarded, since the dynamic amplification leads to an erroneously large disturbance prediction.
The subject invention overcomes the limitations of the state of the art in the following ways. The invention allows automatic model extraction, significantly reducing the time required to extract a model while improving the quality of the model. Manually tuned models contain fewer harmonics and ignore the broadband noise signature entirely. Furthermore, the invention simultaneously tunes the mechanism structural dynamic model along with the noise model, providing additional improvement in forcing level predictions. Certain structural characteristics, notably damping, can only be extracted from the mechanism dynamic model.
The present invention addresses the need to develop an accurate model of disturbances induced by spinning mechanisms. In a specific exemplary embodiment, mathematical models of the disturbances and the spinning mechanism structure are realized as matrix equations of motion in computer code, and a least squares optimization algorithm is used to automatically tune the model parameters to match measured disturbance data. The model can then be used to predict mechanism disturbances, and resulting degradation in stability, in the operational environment.
The invention comprises a system for extracting the parameters that define the forces and moments of a spinning mechanism, using measured force and moment data for a range of rotation speeds. The data consists of time histories of the three forces and three torques referenced to a defined center of measurement, for a range of wheel speeds. These data are acquired as part of the mechanism acceptance testing and thus require no additional resources to obtain.
The preferred embodiment is realized as computer code that creates and tunes a mathematical model of forces imparted by the rotating component, filtered by a speed dependent model of the mechanism dynamics. The code implements the following algorithm. The time data is converted to Power Spectral Densities (PSDs) using a Fourier Transform. The PSDs define the forcing amplitudes as a function of temporal frequency and rotation speed.
The PSDs are converted to order analysis data by dividing the temporal frequency by the wheel speed, so that the forcing amplitudes are functions of multiples of the wheel speed, or harmonics h. In order analysis form, disturbance harmonics can be identified as constant ridge-lines. Harmonics are automatically identified by collapsing the order analysis data to a scalar function of harmonic factor, then extracting the local maxima.
The procedure for tuning the model is diagrammed in
The mathematical models of the forcing behavior of each harmonic, the broadband noise, and the wheel structural dynamics are as follows. The general form for the harmonic forcing model (100) is a speed dependent function g(Ω),
f(t)=g(Ω)sin (hiΩt) EQ. 1
where t is time, f (t) is the force or torque time history, Ω is the rotation speed, and hi is the harmonic. A particular example of a speed dependent function is the speed squared model,
f(t)=ċΩ2 sin (hiΩt) EQ. 2
where c is a harmonic coefficient. Those skilled in the art will recognize that any arbitrary forcing function can be defined in this way. For the broadband noise (200), an arbitrary forcing function can be realized in terms of a speed dependent polynomial,
where W(f,Ω) is the disturbance as a function of temporal frequency and speed, C(Ω) is a wheel speed dependent shaping function, and N(f, ) and D(f, Ω) are the speed dependent numerator and denominator polynomials, respectively. Those skilled in the art will recognize that EQ 3 can be equivalently represented in pole/zero/gain form or state space form. The structural dynamics (300) can be written in a general form as
M{umlaut over (x)}+(V+ΩG){dot over (x)}+K x=Bff {circumflex over (f)}=Cfx+Dff EQ. 4
where M is the mass matrix, V is the damping matrix, G is the gyroscopic matrix, K is the stiffness matrix, Bf is the force influence matrix, Cf is the force observation matrix, Df is the feedthrough matrix, and {circumflex over (f)} is the filtered force that is applied to the system. Those skilled in the art will recognize that the structural dynamics matrices can be developed in numerous ways, including but not limited to first-principles modeling and Finite Element modeling. In particular, Finite Element models are always created as part of the design process for spinning mechanisms, and thus are available with no additional effort. Also. those skilled in the art will recognize that the second order Equations of Motion (EOM) in EQ 4 can be equivalently represented in various other forms such as first order EOM.
The models are tuned to the waterfall data using a nonlinear least-squares optimization procedure. The procedure involves defining an error function J that mathematically characterizes the difference between the measured and predicted forces, and that is summed over all the measurement frequencies, rotation speeds, and forcing axes,
The j index runs over the disturbance axes (3 forces and 3 moments), k runs over wheel speeds, and l runs over frequency. p is a variable that represents the variable parameters in the structural, harmonic, and broadband models, denoted Pstructure, Pharmonic, and pbroadband, respectively. The predicted force {circumflex over (F)}(p) is computed from the harmonic and broadband noise models, and the structural model,
{circumflex over (F)}
jkl
=Ĝ
jl(f, Ωk, pstructure)(Wjl(f, Ωk, pbroadband)+Hjl(Ωkptonal)) EQ. 6
The structural model EQ 4 is represented by Ĝjl which is the frequency response of the model for axis j at frequency l,
Ĝ
jl(s, Ωk, pstructure)=C(M s2+V s+GΩks+K)−1B EQ. 7
where s is the Laplace variable, s=2πf 1. Pstructure can include any element of the structural response matrices, but would typically include resonance frequencies and damping ratios. Wjl is the random noise model,
with parameters pbroadband that include the coefficients of the speed shaping function C(Ω) and the coefficients of the numerator and denominator polynomials N and D. Hjl is the harmonic disturbance for axis j at frequency l,
whose parameters pharmonic are the coefficients of the harmonic function fm(Ω).
While the above description is of the preferred embodiment of the present invention, it should be appreciated that the invention may be modified, altered, or varied without deviating from the scope and fair meaning of the following claims.
This invention was made with government support under contract NNX08CA33C awarded by NASA. The government has certain rights in this invention.
