ESA GNC Conference Papers Repository
Design of a Low Micro Vibration High Precision CubeSat Reaction Wheel
Missions involving payloads such as laser communications or astronomical objectives often have high pointing requirements. The lower budgets and small inertias of CubeSats and SmallSats increase the difficulty of achieving these pointing requirements for payloads mounted on such satellites. Reaction wheels are almost always employed as the actuators of choice for the fine pointing of satellites to achieve high precision pointing. Contradictory, they also belong to the instruments which limit the pointing accuracy that can be attained by the spacecraft, as they often represent one of the main sources of vibrations to the platform. This results in a delicate trade-off during the reaction wheel design. For example, reducing dimensions and mass of a reaction wheel while maintaining a similar momentum capacity can be obtained by increasing the operating speed of the wheel, but comes at the cost of increased exported vibrations and power consumption. Arcsec has been working on such a CubeSat reaction wheel, dubbed Zyra, in cooperation with the KU Leuven and ESA under the GSTP program. The product of this design effort is a 40x40x23mm reaction wheel of 160g, providing 27.5mNm momentum capacity. The project is now entering its qualification and testing phase. A number of strategies were integrated in the design to increase the precision of the wheel. On the control side, the wheel integrates its own drive electronics, the required sensors and a micro controller. This allows the wheel to function as a stand-alone device with custom control algorithms tuned to the reaction wheel behavior. The control loop continuously estimates the friction encountered in the bearings during operation. The estimated friction is then added to the torque demand from the ADCS, which allows the wheel to quickly respond to friction changes and restore the angular velocity in such an event. The ADCS controller itself does not need to take into account such effects, or motor dynamics, and simply passes a torque demand to the integrated reaction wheel controller. Such a control architecture ultimately results in lower torque fluctuations, lower response times to unpredictable friction changes and more stable control of the reaction wheel velocity. On the mechanical side, the reaction wheel construction allows to reduce the generation of micro vibrations and reduce the propagation to the platform. In order to achieve this, the required process to efficiently balance the rotor should already be considered during the design of the housing . The housing is therefore designed to allow for post-assembly (field-) balancing down to grade G0.4, as well as to preserve the balancing grade once it is obtained. The balancing process is performed by laser ablation, which ensures that the bearings are not overstressed or damaged during material removal. Access to the reaction wheel rotor in its fully assembled form is required for this process. However, a G0.4 balancing grade can easily be compromised by dirt or scratches on the rotor due to the small dimensions and weight of the wheel. It is therefore crucial that the rotor can be protected and closed off after the balancing process is completed. The rotor suspension is axially preloaded to avoid rattling and increase reproducibility. The preloading also increases the bearing lifetime since the rolling elements and bearing races remain in constant contact without overstressing the elements or producing excessive friction. The suspension design is a so called soft-preload suspension. It utilizes a preload screw to repeatably set the desired preload to the bearings independent of production tolerances, as well as a loaded spring to keep the actual preload within the desired limits over the entire operating temperature range. The use of a spring in combination with a sliding fit is chosen over a flexible mounting to preserve rigidity in the radial direction and avoid rotor modes at low frequencies. Rolling element bearings are known to generate higher order harmonics. Even 10th or higher engine orders can be observed, depending on the bearing type. When a rotor is used in a large speed range, these higher order harmonics might pass and excite rotor modes, increasing exported vibrations at these frequencies. For small rotors, there are some benefits to making them as one rigid unit with the shaft, web and rim all machined out of a single piece of metal. However, such monolithic metallic structures are characterized by high Q-factors, resulting in sharp amplifications of the resonance frequencies. When a higher order harmonic passes such a high quality resonance frequency, large amplifications of the exported micro vibrations at this location can be expected. A first step to minimize this Q-factor is the material choice. For example, a bronze rotor has higher internal friction than a steel one, and therefore exhibits lower Q-factors for a similar shape. To further reduce the Q-factor while keeping the advantages of a single piece rotor, a damping principle called constrained layer damping is employed in the construction of the rotor. This allows to significantly increase the damping ratio of the rotor structure, while leaving the locations of the resonance frequencies virtually untouched. During the design phase of the reaction wheel, the damping layer was tuned to the rotor shape and material and was tested on simplified monolithic steel rotors on a shaker table set-up. On these rotor dummies, a decrease in Q-factor for the first resonance frequency of 59.2% was observed with the application of a 0.3mm damping layer, while the shift in resonance frequency was negligible. This rotor construction provides a relatively low cost method to decrease the Q-factor of the rotor without lowering its resonance frequencies and maintaining the advantages of single piece rotor. The next step in the project is validating and quantifying the resulting exported micro vibrations during an extensive test- and qualification phase.