6 A Highly Integrated Single Unit CubeSat Solar Panel
6.3 Solar Panel Assembly and Test
6.2.4.2 Panel-Mount Pico Fluid-Dynamic Actuator
As precise and agile attitude control actuator, the pFDA shown in figure 5.10 on page 76 is integrated with the solar panel. Solar panel design work with respect to the actuator comes down to the mechanical and electrical interface, including power monitoring and latching.
Monitoring and latching circuitry is already discussed in section 6.2.2. As the pFDA implements an external I2C data interface, the physical electrical interface is realized using board-to-board connectors between the solar panel and pFDA PCBs. The interface provides logic and pump power to the actuator, and uses one of the two I2C buses that are available on the solar panel for communication.
6.2.5 Command and Data Handling
A 32-bit, low-power microcontroller is used as central processing unit on the solar panel. The controller connects to the redundant CAN bus using two transceiver chips. Algorithms for MPPT and attitude estimation are executed there, and the determined attitude is provided to the data bus. Commands for the attitude actuators are received and either translated and passed on to the pFDA, or directly executed for the coil driver. The implemented PCDU software monitors the subsystem power demands, and latches them in case of excess.
6.3.1 Assembly
Images of the front and the back of the pre-assembled solar panel are shown in figure 6.6. The back shows the soldermask islands that are required for gluing the PCB substrate directly to the pFDA or the primary satellite structure, respectively (cf. figure 6.6(a)). This image also shows the pFDA outline, where no electronic components can be placed, and the opening required for the antenna connector. At this step, the front of the solar panel does not hold any components, which allows to see the gridded structure of the solar cell and antenna pads in figure 6.6(b).
(a) Back (b) Front
Figure 6.6:Pre-assembled multi-functional solar panel
Figure 6.7 shows images of the front and the back of the fully assembled multi-functional solar panel. The solar antenna was integrated at Fraunhofer IZM, who also equipped the solar panel with electronic components. The solar cells were glued to the panel and the pads of the smaller cells wire-bonded to the pads on the PCB. The actuator was fully assembled by the author, and then integrated with the pre-assembled solar panel.
(a) Back with actuator (b) Front with solar antenna Figure 6.7:Fully assembled multi-functional solar panel
After fully assembling the solar panel, a fit check using a 3D-printed mock-up of the adapted BEESAT satellite structure was performed (cf. figure 6.8). No difficulties were encountered during fit-check, and the solar panel was easily assembled and disassembled.
Finally, the solar panel mass was determined for the three different versions.
The basic version weighs approximately 40 g, which is very competitive, if compared against the solar panels listed in tables 6.1 and 6.2, as there are only two other solar panels, that weigh less. This seems quite interesting, as the presented multi-functional solar panel includes more electronic components than any of the commercially available panels.
The mass of the enhanced attitude control version, which includes the pFDA mass (cf. chapter 5), was determined to approximately 85.5 g. This version weighs about twice of the average solar panel available on the market, but offers the unique feature of having two different attitude control actuators integrated on the same panel.
(a) Adapted structure (b) Solar panel on adapted structure
Figure 6.8:Fit-check of the fully assembled multi-functional solar panel With the additional solar antenna integrated (cf. page 35), the mass of the communication and enhanced control version is approximately 104 g. The fully equipped solar panel has a mass that is roughly equivalent to 7.78 % of a standard single unit CubeSat. While the advantage of using highly integrated multi-functional solar panels is not obvious on first sight, it is explained in detail in section 6.4.
6.3.2 Functional Verification
Functional verification carried out for some of the components found on the multi-functional solar panel has been addressed in earlier sections (cf.
sections 3.4 and 5.6). The sun sensor [126] or the magnetic field and angular rate sensors have been functionally verified in other projects at TU Berlin, and have gained flight heritage on multiple spacecraft in orbit. MPPT hardware and software was functionally verified at Fraunhofer IZM using a demonstrator with one large and five small solar cells and a development board of the microcontroller which is used on the solar panel [125]. The only component
that requires functional verification on system level is the optimized magnetic actuator.
For functional verification of the embedded air coil, power consumption and magnetic dipole were measured. At the targeted supply voltage of 3.30 V, the coil consumes 205 mW of power. This is only a 2.30 % difference from the targeted value (cf. section 6.2.4.1). In order determine the magnetic dipole 𝜇, the magnetic flux density𝐵 was measured at a defined distance from the solar panel along a line that passes perpendicularly through the center of the coil. Distance𝑥was chosen to be large enough, so that the relation between magnetic dipole 𝜇and magnetic field strength𝐵 is given by
𝐵 = 𝜇0 4𝜋
2𝜇
𝑥3. (6.1)
Here, 𝜇0 is vacuum permeability. From this equation and the sensor data, magnetic dipole was calculated to be 47.5 mA m2. This value is differing from the targeted value by 5.56 %.
6.3.3 Environmental Tests
Environmental tests were carried out by a colleague of the author, and only preliminary results were published in [64]. Vacuum tests still need to be conducted. Tailored thermal tests were carried out at TU Berlin according to ECSS with a reduced number of cycles. Thermal test results showed no failure of components.
For mechanical tests, two fully integrated solar panels were mounted in a carrier (cf. figure 6.9), and tested on a shaker at Fraunhofer IZM. During vibration tests, the solar antennas teared off from their mounting points, and the solar cells on the antenna and around the antenna got partially destroyed.
Analysis of the pads on the solar panel and the bottom of the antenna shows, that not enough adhesive was applied during integration of the antenna with the panel at IZM (cf. figure 6.10).
Except for the torn-off solar antenna, no other subsystem showed signs of damage after mechanical testing. Fluid conduits didn’t show any signs of leakage, and all subsystems operated nominal. Due to the fact, that
Figure 6.9: Solar panels mounted for mechanical tests
(a) Back (b) Front
Figure 6.10:Damaged solar panel and torn-off solar antenna
the solar panels are connected to the frame using screws and adhesive to simulate the assembly situation found on the satellite, the panels can’t be removed from the frame. Therefore, no repetitive functional verification of the actuators’ dynamical properties was possible. Measurements of actuator power consumption, however, showed results comparable to the ones presented in section 5.6.1. It is therefore assumed that neither pump driver electronics nor pump components have suffered any damage from mechanical testing.