Weekly Update #22: Payload Prototyping

In our last entry we presented the first prototype. We built a device to characterise solar cells and that is exactly what the payload will be doing later in space. The prototype had only limited features, nevertheless it was sufficient to verify the concept. The major task afterwards was to check if it was working as expected.  The first tests were done under a standard lamp and under the sun. The shape of the gathered data made us cheer, we saw a beautiful characteristic I-V curve as described in the literature. However, a similar shape is not enough to be sure that our device measures the characteristic curve properly, therefore we went to AIRBUS to compare the prototype with the standard method used by professionals to characterise solar cells.  At AIRBUS we had the opportunity to use a AM0 sun simulator for the characterisation of a triple junction solar cell. Furthermore we tested the prototype also with a small cell that consists only of one junction of the triple junction cell. The test was essential since such types of cells will be characterized on the CubeSat. Overall, the results of the prototype were very satisfying, we saw deviations of maximum one percent in comparison to the standard method. The outcome of this test is displayed in the picture below.


As we verified our concept we were able to step forward. The components used in the prototype were very simple to use and were therefore chosen. However, because of the high radiation level in space, we had to choose hardware that survives at least six months in this hostile environment and because we are not able to afford radiation hard components which are normally used in space missions, we searched publicly available databases from NASA that contain information about test results under high radiation of commercial off-the-shelf components. We found very good replacements for our ADC and DAC. The new ADC is an AD7714 produced by Analog Devices and able to generate data with a resolution of 24bit. Due to this excellent resolution we can gather data at highest accuracy. It has a programmable gain amplifier but has no internal voltage reference. The ADC for the new concept is the component DAC7512 produced by Texas Instruments and provides a 12bit resolution which is the same resolution of the component in the first prototype. Unfortunately these parts are rarely used and therefore there are no software libraries released. Additionally there are also no breakout boards with implemented necessary circuits available. For the next prototype we had to develop more complex circuitry and software. Finally we got the prototype running with all the new components. Below is a picture of the second prototype:



We tested the second prototype the same way as we did with the first one and the quality of the gathered data was similar to his predecessor. As we had verified the new components we were able to implement further improvements. This prototype has one connection for the possibility to characterise only one solar cell. Actually we plan to have up to six cells on our Payload. Therefore we had to adjust our concept for more measuring ports. Additionally the prototype is not enough fast in taking measurements. To remediate the issue we distributed voltage and current measurement to different ADCs. To implement all new features we decided to build another prototype. As the complexity advanced, we had to design a PCB which has exactly the dimensions to fit on the structure prototype of the CubeSat. Below is the third and new prototype:


With this prototype we can characterise up to three solar cells. One of them can be attached onto the PCB. Currently we are working to improve the measurements. The payload should produce reliable data and has to be enough fast. In addition to the hardware improvements, we are developing an algorithm that will speed up the measurements significantly. Furthermore we are investigating different techniques of sensing the temperature of the solar cells. As the characteristics of the cell depend on the temperature this is a key task to make sure that we understand the solar cells in detail.

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