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Abstract
To achieve precise alignment, tracking and pointing, inter-satellite laser communication generally requires a two-dimensional gimbal. To satisfy the requirements of space applications, the gimbal needs to be high-precision and as small as possible. To meet this need, we designed a small and lightweight gimbal assembly. We adopted piezoelectric ceramic motors instead of a torque motor to ensure driving precision and effectively reduce the mass of the drive unit. The yoke was made from low-volume fraction SiCp/Al with excellent specific stiffness, which ensured rigidity and reduced the mass of the structure. This allowed us to reduce the mass of the gimbal to just 5.8 kg. With a laser communication head mass of 5.55 kg, this gave a load-to-gimbal mass ratio of 95.7%. We carried out experiments on our small and lightweight gimbal, followed by a finite element analysis. The results of vibration test and shaking test showed that the structure has adequate stiffness and high precision.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Laser communication has the advantages of long transmission distances, fast transmission rates and good confidentiality, and will be one of the main modes of communication between next-generation satellites. In laser communication, two-dimensional gimbals are used to enable acquisition, tracking and pointing for spatial targets, and will be a key technology in future optical communication and satellite networks. Because of the long distances involved in laser communication and the relative motion between communication targets, gimbals used for tracking and pointing should have high tracking accuracy and good stability. The current trend in satellite technology is towards low weight, small size, low cost and high performance. The rotating components of the satellite payload (including the gimbal and scanning mirror) have a larger mass relative to the satellite. During rotation, all movements of the load will cause the satellite’s large moment of inertia to fluctuate, significantly affecting the satellite's own operational stability and attitude control. For this reason, miniaturization of the laser communication equipment has been the main trend in the development of this technology [1–5]. Miniaturization of laser communication has introduced difficulties into the design of the two-dimensional gimbal. Compared with optical systems, the traditional two-dimensional gimbal has an electromechanical structure, and thus miniaturization is much more difficult.
High-precision tracking gimbals for satellites have long been one of the focuses of satellite load research. In 1945, the world's first gimbal, driven by DC motors, was developed at the Massachusetts Institute of Technology. In 1994, the Optical Communication Demonstrator developed by JPL of the United States adopted a two-axis structure, which enabled hemisphere, or even wider, stereoscopic space scanning, pointing and tracking [6–9]. In 2010, the Space Based Space Surveillance system launched by the United States adopted a two-axis system. The two-dimensional rotating system had a positional accuracy of less than 5 µrad, a load mass of 907 kg and a structural stiffness of 100 Hz. In 2013, NASA and the MIT Lincoln Laboratory jointly developed the LTS laser communication terminal, which was adopted for NASA's Lunar Exploration Program. The mass of this system was 30 kg and the precision of the gimbal was less than 4 µrad [10,11].
The laser communication gimbal equipment developed in Europe had some similarities with that developed in the United States, but there was a clear gap in technological level with the United States. The early optical terminal system adopted by the European Space Agency (ESA) was SILEX, which was installed in the ARTEMIS satellite to enable laser communication with the SPOT-4 satellite. The mass of the optical terminal system was 157 kg, and the mass of the rough tracking module that drove the load part was 75 kg, with a load-to-gimbal weight ratio of 91.46%. An L-shaped theodolite structure was used for pointing, and stepper motors with open-loop subdivided drives, 10-bit photoelectrical encoders and brake parts were used for the drive components. Subsequently, ESA developed the new SOUT and SROIL laser communication systems, the latter has a mass of a mere 8 kg [12–17].
To study laser communication characteristics between low-Earth orbit and the ground, the Japan Space Development Agency developed its LUCE laser communication terminal, which was mounted on the OICETS communication test satellite. The mass of the terminal system was 22.4 kg, and the tracking accuracy was less than 2 µrad. A two-dimensional gimbal yoke was adopted, and the tracking system was driven by two torque motors [18–23].
The Extreme Ultra-Violet Camera (EUVC) was an important payload for the Chinese Lunar Exploration Project, Chang E Engineering, which was developed by the Changchun Institute of Optics, Fine Mechanics and Physics. It was used to observe the spatial structure and dynamics of the Earth's plasmasphere on a global scale. The two-axis gimbal used an altazimuth structure, and the elevation and azimuth axis were driven by worm gears and stepping motors. The mass of the EUVC’s imaging telescope was 8 kg, and the gimbal assembly had a mass of 8.5 kg, with an optical system-to-gimbal mass ratio of 94.12% [24–26].
These gimbals were all driven by torque motors or stepping motors, which needed special locking devices to lock the axes. However, the mass of the locking mechanism was so large that it was difficult to reduce the mass of the gimbal assembly. Reducing the dimensions and mass presents an important challenge in the application of laser communication technology, and the mass of the gimbal assembly forms an important part of this. For this reason, we designed a small and lightweight gimbal to enable miniaturization of the space laser communication device. Piezoelectric ultrasonic motors installed around a ceramic friction ring are used for the first time to drive the gimbal around the azimuth and elevation axes. Frictional force on the ceramic ring is used to lock the gimbal, thus reducing the dimensions and mass of the drive mechanism. After comparing the comprehensive performance of different aerospace structural materials, low-volume fraction SiCp/Al was finally selected for the gimbal yoke, which further reduced the mass.
This paper is organized as follows. Section 2 introduces the structural design of the two-dimensional gimbal, including the overall design, the drive design, the design of the yoke and the shaft pointing accuracy. Section 3 presents a mechanical analysis of the two-dimensional gimbal. Section 4 describes the experiments on the two-dimensional gimbal, including the vibration test and the shaking test. Finally, Section 5 summarizes the work covered by this project.
2. Design of the two-dimensional gimbal structure
2.1 Structure of the laser communication gimbal
Based on the technical specifications and the application environment of the laser communication gimbal, the traditional elevation and azimuth axis design shown in Fig. 1 was adopted. It consists of an azimuth axis unit, an elevation axis unit, a yoke, and an optical antenna. The two ends of the main scope tube for the optical antenna are connected to the elevation axis and mounted on the azimuth axis with the yoke. This gives the optical antenna an azimuth rotation of –178 to + 178 and an elevation rotation of –115 to + 115.
To meet the requirements of the optical path and cable wiring for the laser communication gimbal, the elevation axis and the azimuth axis of the gimbal have a hollow design. The distribution of the optical path and cable wiring is shown in Fig. 2. As the system tracks the laser target, the signal is reflected by the primary and secondary mirrors of the optical antenna unit, which is then emitted by the Coude mirror. The signal passes through the middle hole at one end of the elevation axis to reach the semi-silvered mirror. One part of the light beam enters the coarse tracking camera, and the other part is emitted from the bottom of the gimbal after two turns, and then enters the subsequent processing unit. The cable at both ends of the elevation axis is led to the center of the azimuth axis by the hole in the yoke, which is drawn through the outside of the light guide cylinder in the center hole of the azimuth axis [27–30].
2.2 Design of the laser communication antenna drive mechanism
Because of the limited dimensions and mass of the laser communication gimbal, a traditional torque motor cannot meet the requirements of the laser communication antenna. Therefore, piezoelectric ultrasonic motors installed around a ceramic ring are used to drive the azimuth and elevation rotations of the gimbal. The piezoelectric ultrasonic motors drive the ceramic ring, which is made to rotate against frictional force by piezoelectric traveling wave vibrations, driving the whole assembly in the elevation or azimuth directions. Compared with a torque motor, the piezoelectric ultrasonic motor has the advantages of a simple structure, large braking torque and fast response, enabling mass reduction and miniaturization of the laser communication gimbal.
The piezoelectric ultrasonic motor assembly is shown in Fig. 3. Four piezoelectric ultrasonic motors are evenly distributed around the ceramic ring. By controlling the distance between the end face of the motors and the ceramic ring, the friction between the ceramic ring and the driving foot of each piezoelectric ultrasonic motor can be set in the range 32 ± 1 N. The ceramic ring is bonded to a titanium alloy collar, which is secured to the azimuth axis by screws. The maximum output driving force of each piezoelectric ultrasonic motor is 8 N. The azimuth axis driving torque is 2.56 N·m, and the elevation axis driving torque is 2.16 N·m, which satisfies the requirements for a laser communication gimbal drive unit.
When the piezoelectric ultrasonic motors are turned off, the braking moment will lock the gimbal without the need for an additional locking mechanism, which further reduces the total mass and saves resources on the satellite.
2.3 Design of the yoke
The yoke for the whole elevation axis unit of the laser communication gimbal adopts the integral U-shaped design which has a direct effect on the gimbal’s overall stiffness and strength. A lightweight design is adopted to minimize the mass. The yoke and its corresponding finite element simulation model are shown in Fig. 4.
Fig. 4. Structural model(left), the finite element simulation model(middle) and the first-order model (right) of the yoke
The most common aerospace materials are aluminum alloy, carbon fiber, low-volume fraction SiCp/Al and magnesium-aluminum alloy. To ensure a low mass and high strength, the specific stiffness and structural eigenfrequencies of these materials have been calculated and are shown in Table 1. The table shows that low-volume fraction SiCp/Al has the highest stiffness and the highest eigenfrequency, and its first-order mode is shown in Fig. 4. As a result, low-volume fraction SiCp/Al was selected as the material for the yoke, to improve the structural stiffness, terminal response and support performance of the gimbal.
Table 1. Comparison of common aerospace materials
2.4 Designing a high shaft pointing accuracy
2.4.1 Pointing error accuracy distribution
Laser communication gimbals are required to perform long-distance tracking and aiming between two satellites, which requires very high tracking and aiming accuracy. Satellite operating conditions require gimbals with a single-axis pointing accuracy (${\delta _E}$) better than 5$^{\prime\prime}$. From the structure of the laser communication gimbal, it is clear that the pointing accuracy error (${\delta _E}$) depends on the sloshing error (${\delta _{e1}}$), encoder accuracy error (${\delta _{e2}}$) and inspection equipment error (${\delta _{e3}}$), where
The gimbal uses a 24-bit incremental encoder with a resolution of 0.078$^{\prime\prime}$ and an accuracy (${\delta _{e2}}$) of 4$^{\prime\prime}$. The inspection equipment error (${\delta _{e3}}$) can be controlled to within 0.5$^{\prime\prime}$, and thus from the above formula, the sloshing error (${\delta _{e1}}$) needs to be lower than 2.95$^{\prime\prime}$.The wobble error consists of bearing clearance and equivalent clearance errors caused by processing concentricity, fit clearance, and assembly measurement errors. The machining accuracy errors can be controlled by the selection of high-precision machine tools and by grinding in the subsequent assembly process. A way to eliminate bearing clearance needs to be incorporated at the design stage so that the wobble error can satisfy the accuracy requirements.
2.4.2 Eliminating clearance in the design of the elevation axis unit
The elevation axis unit is composed of a fixed end, a floating end and the main antenna barrel. Both the fixed end and the floating end are supported by customized face-to-face angular contact bearings. The inner and outer rings of the fixed end bearing are fixed to prevent the floating shaft from moving in the axial direction. The inner ring and the shaft are axially floated to prevent temperature deformation that will cause the elevation shaft to seize. A diagram of the elevation shaft is shown in Fig. 5. By customizing the inner and outer ring widths of the face-to-face angular contact bearings, the preload on the bearings can be kept within the required range. This enhances the stiffness of the floating shaft, eliminates the bearing clearance and improves the rotational accuracy of the floating shaft.
2.4.3 Eliminating clearance in the design of the azimuth axis unit
Due to the special requirements of the optical path in the laser communication gimbal, the axial space for the azimuth axis is strictly limited. To save axial space, the azimuth axis assembly adopts a single bearing design. There are two types of single-bearing supports: the cross-roller ring and the four-point contact ball bearing. Since the azimuth axis of the laser communication gimbal requires a small bearing torque and high rotational precision, a high-precision four-point contact bearing with a precise face and radial runout was selected. The four-point contact bearing is installed between the base and the azimuth shaft. The outer ring of the bearing is located between a rabbet in the base and the bearing’s outer pressure ring, and the inner ring is located between a recess in the azimuth shaft and the inner pressure ring. A diagram of the azimuth shaft system is shown in Fig. 6. The four-point contact bearings are designed with a negative clearance. Selecting high-precision bearing balls enables the bearing clearance to be maintained in the range –5 um to –1 um to reduce the effect of azimuth axis sway on the tracking accuracy. As the laser communication gimbal is required to function under vacuum conditions, the bearing is treated with a bonded molybdenum disulfide solid lubrication coating.
The mass of the laser communication head on the two-dimensional gimbal was 5.55 kg. Our design reduced the mass of the gimbal to just 5.8 kg, giving a load-to-gimbal mass ratio of 95.7%.
3. Mechanical analysis
Finite element modeling was used to construct the structural design model shown in Fig. 7. MSC/patran was used to model the two-dimensional gimbal, ignoring or simplifying some subtle features that have little influence on the deformation and stress distribution of the whole structure. The hood and the primary mirror cell were modeled as a combination of solid and shell units. The main mirror components, the secondary mirror components, the yoke and the satellite mounting bracket were modeled as solid units, and the base was modeled as a shell unit.
The key to simulating the two-dimensional rotation is in the rotation of the bearings and the locking of the motors. These need to be linearized during the analysis of the whole machine. The bearing simulation uses MPC approximation to simulate the bearing rotation and the characteristic radial degrees of freedom. Movement of the azimuth axis in the X, Y and Z directions and rotation in the X and Y directions is constrained; only rotation in the Z direction around the azimuth axis is unconstrained. Also, the movement and rotation of the fixed end of the elevation axis is constrained in the X and Z directions, and the movement and rotation in the Y direction (elevation axis) are unconstrained. With the ultrasonic motors in the contact frictional state, two cases are simulated: locked and freely released. The finite element model is shown in Fig. 7.
Modal analysis was performed on the finite element model for motors simulated for the two cases: locked and freely released. The eigenfrequencies in the three directions are shown in Table 2. The vibration patterns for the two-dimensional gimbal are shown in Fig. 8. The simulation results show that the two-dimensional gimbal has high stiffness.
Table 2. Eigenfrequencies of the two-dimensional gimbal structure for the two cases
4. Experiments
4.1 Vibration test
Sensors were arranged on the two-dimensional gimbal (Fig. 9) to record the response changes during the vibration test. The vibration test consisted of 0.2-g sweep tests from 5–2000 Hz and sinusoidal tests in three directions from 5–100 Hz.
A small-scale characteristic sweep test of 0.2 g was selected to verify the stiffness of the structure. The eigenfrequencies of the three-dimensional gimbal were shown as: 103.9 Hz, 88.5 Hz and 335 Hz. As the amplitude of the vibration was small, the drive was locked by the motors to maintain the stiffness of the two-dimensional gimbal. As shown in Fig. 10, the test results were similar to the results when the motors are locked. The test results and the simulation results are shown in Table 3. The maximum error rate in the three directions was 4.0%.
Table 3. Comparison of small-scale characteristic sweep test results and simulation resultswith the motor locked
A sinusoidal vibration test was used to simulate vibrations in the launch environment and to verify the gimbal strength. In the sinusoidal vibration test, the resonant frequencies of the two-dimensional gimbal in the X and Y directions were 94.0 Hz and 82.4 Hz. Because the frequency range of the sinusoidal vibration test was 5–100 Hz, the Z-direction resonant frequency became much larger than 100 Hz, and so the peak is not shown on the curve in Fig. 11. Since the sinusoidal vibration test had a high magnitude, the shaft could not be locked. The friction was equivalent to an increase in the damping, which caused a decrease in the frequency. The test results are similar to those with the motor freely released. The test results and simulation results are shown in Table 4. The maximum error rate in the X and Y directions was 2.5%.
Table 4. Comparison of sinusoidal vibration test and simulation results
The first-order eigenfrequency of the gimbal was 88.5 Hz in the small-scale characteristic sweep test, demonstrating that the stiffness of the structure was adequate and piezoelectric ultrasonic motors provide a reliable self-locking method. The first-order resonant frequency of the gimbal was 82.4 Hz in the sinusoidal vibration test, demonstrating that the gimbal can withstand the launch vibration environment. The structure remained stable and without damage before and after the sinusoidal vibration test, which indicated that the structural was viable and the performance was reliable. Even with the limitations on the dimensional space and the lightweight design, the requirements for stiffness and strength could still be met.
4.2 Shaking test
To verify the accuracy of the shaft sway in the final assembly for the gimbal, we carried out a sway test on the gimbal. During the test, the measuring shaft was suspended in the horizontal position in a jig, and an electronic level instrument was positioned on the end face of the measuring shaft, as shown in Fig. 12. The electronic level recorded every 30 degrees around the rotating axis. The level was rotated five times and ten values were recorded at each measuring point. The sloshing error distribution maps for the azimuth and elevation axes shown in Figs. 13 and 14 respectively were obtained showing a comparison between the recorded values and the normal distribution. The results show that 3σ for the azimuth axis sloshing error was 1.92$^{\prime\prime}$ and 3σ for the elevation axis sloshing error was 1.59$^{\prime\prime}$, all of which meet the requirements for a shaft system sloshing error distribution of 2.95$^{\prime\prime}$ for a laser communication gimbal.
5. Conclusion
This paper describes our design for a high-precision two-dimensional gimbal for inter-satellite laser communication. To reduce the dimensions and mass of the two-dimensional gimbal, piezoelectric ultrasonic motors with a ceramic ring drive the azimuth and elevation rotations of the gimbal; the ceramic ring can be locked by frictional force. Compared with a torque motor drive, the dimensions and mass of the gimbal were reduced for the same output torque conditions. Low-volume fraction SiCp/Al was used for the yoke, making the total mass of the two-dimensional gimbal a mere 5.8 kg, and giving a load-to-gimbal mass ratio of 95.7%. The first-order eigenfrequency of the gimbal was 88.5 Hz in the small-scale characteristic sweep test, demonstrating that the stiffness of the structure was adequate and piezoelectric ultrasonic motors provide a reliable self-locking method that can protect the gimbal. The first-order resonant frequency of the gimbal was 82.4 Hz in the sinusoidal vibration test, demonstrating that the gimbal can withstand the launch vibration environment. The azimuth axis of the gimbal adopted a four-point contact ball bearing with negative clearance. The elevation axis adopted two pairs of face-to-face angular contact ball bearings to eliminate the bearing clearance, reducing sloshing errors on the azimuth and elevation axis to 1.92$^{\prime\prime}$ and 1.59$^{\prime\prime}$ respectively. All meet the requirements for a shaft system sloshing error distribution of 2.95$^{\prime\prime}$ for a laser communication gimbal. In general, the design and tests successfully satisfy inter-satellite laser communication requirements.
Funding
National Natural Science Foundation of China (11672290); National Basic Research Program of China (973 Program) (2016YFE0205000).
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