On the 18th of June we finished thermal vacuum cycling of the BiSon Sunsensors that was agreed upon in frame of the running ESA GSTP program.
At the VaB thermal cycling facility has 9 slots for mounting Sunsensors, and all Lens R&D Sunsensors have the same mounting interface, we decided to test a MAUS along with the BiSon Sunsensors.
As the test temperatures were well above the specified -40° to +80°C operational range specified for the sensors, this experiment should be treated as an accelerated life test for the sensors. As far as we know, accelerated life testing is not something which is commonly performed for cubesat components, but does give some insights into the lifetime to be expected.
As the MAUS uses soldered connections on the nano-D connectors, these are expected to be driving for the reliability of the sensor which makes it acceptable to use a modified Coffin-Mason equation which is tuned to predicting the reliability of soldered connections. This equation leads to considerable acceleration factors especially for the wider temperature range.
The test program consisted of 1000 thermal cycles between -45°C and +85°C followed by another 1000 cycles between -45°C and +105°C. Calculating the acceleration factor of this test program (based on the above formula) leads to an equivalent number of 27100 cycles. This in turn is equal to some 5 years in LEO orbit (presuming 15 thermal cycles as day)
As the sensor will probably never see temperature cycles this wide, the 5 years lifetime calculated for the full temperature range will easily calculate into lifetimes that will exceed the lifetime of the satellite itself.
Knowing that the performed tests are equivalent to surviving five years in LEO while going through the maximum temperature range the question is what has been the influence of this testing on the performance of the device.
The answer is “very limited” as can be seen in below graph.
Both before, during and after the testing a number of calibrations have been performed all of which showed very good stability of the sensor under test.
The above graph shows the difference in measured angle over the full field of view between calibrations performed before and after TVAC cycling. In addition, it shows the average displacement of the pattern (associated with the re-mounting accuracy) and the 1σ and peak errors (3σ).
comparing specified values with measured values we find:
- re-mounting accuracy <0.05° specified and 0.04° measured as a maximum in the beta direction
- accuracy after calibration correction <0.5° specified measured <0.15° 3σ
It should be noted that the accuracy is only measured at 20°C and temperature and radiation effects should be included on top of the measured 0.15° to determine the actual end of life accuracy.
It can be calculated that the temperature effects are significantly smaller than 0.1°, and it has been shown through qualification testing that the diodes are extremely tolerant to radiation loading. In addition the algorithm leads to insensitivity to common mode effects, thus leading to the conclusion that with the proven performance the stated 0.5° accuracy can be guaranteed.
The sensors are guaranteed to be single event upset (SEU) and single event latch-up (SEL) free and very radiation tolerant. In combination with the performed thermal cycles it can therefore be concluded that the MAUS Sunsensors are very suited for use on board of reliable cubesats.
It would not be surprising to find that the newby MAUS turns out to have the best on-ground qualification status of any Cubesat Sunsensor to be found anywhere in the world.
15 MAUS sensors are expected to launch into space by the end of June 2021, leading to a true TRL9 status without skipping TRL8 level.