In-Flight Performance of Cassini Reaction Wheel Bearing Drag in 1997–2013

Abstract

As the first spacecraft to achieve orbit at Saturn in 2004, Cassini has collected science data throughout its four year prime mission (2004)(2005)(2006)(2007)(2008) and has since been approved for a first and second extended mission through September 2017. Cassini uses reaction wheels to achieve the spacecraft pointing stability that is needed during imaging operations of several science instruments. The Cassini flight software makes in-flight estimates of reaction wheel bearing drag torque and made them available to the mission operations team. In trending these telemetry data (for the purpose of monitoring the long-term health of the reaction wheel bearings), anomalous drag torque signatures have been observed over the past 15 years. One of these anomalous drag conditions is bearing cage instability that appeared (and disappeared) spontaneously and unpredictably. Cage instability is an uncontrolled vibratory motion of the bearing cage that can produce high-impact forces internal to the bearing, which will cause intermittent and erratic torque transients. Characteristics of the observed cage instabilities and other drag torque "spikes" are described in this paper. In day-to-day operations, the reaction wheels' rates must be neither too high nor too low. To protect against operating the wheels in any undesirable conditions (such as prolonged low spin rate operations), a ground software tool named a reaction wheel bias optimization tool was developed for the management of the wheels. Flight experience on the use of this ground software tool, as well as other lessons learned on the management of Cassini reaction wheels, is given in this paper.Nomenclatures c = viscous coefficient of wheel bearing lubrication system, N · m · s∕rad Gω = function of wheel spin rate ω defined in Eq. (5) I RWA = moment of inertia of reaction wheel rotor, kg · m 2 K I = integrator gain of reaction wheel assembly drag torque estimator, kg · m 2 ∕s 2 K P = proportional gain of reaction wheel assembly drag torque estimator, kg · m 2 ∕s L −1 = inverse Laplace operator s = Laplace variable, rad∕s T D = Dahl drag torque of reaction wheel bearing system, N · m T Drag = total drag torque of reaction wheel bearing system, N · m ξ D = damping ratio of reaction wheel assembly drag torque estimator τ = time constant of wheel speed decay [see Eq. (4)], s Ω 0 = initial wheel spin rate of coast-down test, rad∕s ω = angular rate of the reaction wheel, rad∕s ω D = bandwidth of the reaction wheel assembly drag torque estimator, rad∕s

Composite infrared spectrometer (CIRS) on Cassini

“…The cause was tracked to interference from the spacecraft reaction wheel assemblies (RWA's). The reaction wheel rates vary considerably during rolling, over the range 0 to ±1600 rpm [37,38]. RWA1, 2 and 3 were used at beginning of mission, but because of excess drag RWA3 was swapped with RWA4 in July 2003 and has been used only occasionally since then during checkouts.…”