Space is a hostile environment. Sending hardware to space means putting it under irradiation tests: exposing the object to radiation and see what happens :-). For this, under the lead of the ETHZ (Mathematical and Physical Geodesy), we had the opportunity to put the CubETH payload board under a proton beam. The test facility is at the Paul Scherrer Institute (PSI) in Villigen, Switzerland:

The tests were conducted by our project partners from the ETH Zurich. The proton beam is produced by an accelerator and allows an initial beam energy between 74 MeV and 230 MeV (see http://pif.web.psi.ch/pif.htm). Using the proton beam, the space environment and its impact on semiconductors can be tested in the Proton Irradiation Facility at PIF. It allows to test the radiation hardness, handling of SEUs (Single Event Upsets) and SELs (Latch-Ups). So we wanted to see how our microcontroller board (ARM Cortex-M3 running FreeRTOS with MRAM and 4 GNSS receivers) behaves under a proton beam.

The satellite/payload board with the GNSS receivers is on a moveable XY table. An external test unit is connected to it with the flat band cable:

The board then get mounted into the beam area:

Below a picture of the board from the backside and beam rear view:

Below on the right there are the different energy reduction plates, followed by the ionization chamber. Behind it the board has been mounted. The cables are for monitoring and testing the board while it is running inside the proton beam. The payload board is just one board of many in the satellite, and it communicates with two SPI busses to the other systems:

To focus the beam, there is a really strong bending magnet (blue box below):

One need to be careful near the bending magnet wall, as you can see below: 😉

The area and chamber is heavily shielded with lead pates and massive blocks of concrete:

Below are Michael Meindl (ETHZ, left) and Flavio Kreiliger (ETHZ, former HSLU student, right) working on the test setup:

Results

The setup was to test the board with a dose of 20 kRad in one hour. That would be the dose we would expect during three years.

The details of the test logs still need to be evaluated, but the general findings are the following:

With 20 kRad/h, the SPI communication to the board did not work right after start of the beam. One thing we observed was that the payload board was not able to properly come out of this situation, because the emulation/test environment was powering the board barely enough with the MISO/MOSI lines. Which is bad for such a situation, so we need the adjust the hardware design as such that the SPI lines can be completely cut to allow a proper power-up in such a situation, so we have learned that lesson.

Reducing the dose to 15 kRad per hour worked better, and the board survived for about 45 seconds, plus it has been observed that the on-board I²C communication was affected too.

With a dose of 0.95 kRad the board was able to work for about 100 seconds, then stopped with the SPI problem mentioned above. Because time was running and another team needed the test facility, the decision has been made to continue the irradiation for the rest of the time, and to verify/inspect the hardware when we get it back. It is not possible to remove the device from the radiation zone until it is released by the PSI Health Physicists.

So the results are not very encouraging. Personally I did not expect that the hardware will shut down so fast, and I admit I’m not an expert in proton radiation on silicon. The tests showed a needed improvement with the SPI communication, so we have to be able to cut the SPI lines to do a proper restart of the board in all cases. On the plus side, it showed that the error handling (latchup detection and hard fault reporting) were working. But still a long way to go :-). And if you have experience with proton irradiation, let us know!

Happy Protoning 🙂

PS: Many thanks and credits to Michael Meindl, Flavio Kreiliger, Hügi Flurin und Senn Rahel for taking/providing pictures.

PS2: Dear atmosphere, thank you for shielding and  protecting all of us from these nasty protons!

Links

• CubETH Mission: http://www.video.ethz.ch
• ETH Zürich, Mathematical and Physical Geodesy: http://www.mpg.igp.ethz.ch/
• Paul Scherrer Institute: http://pif.web.psi.ch/pif.htm
• Sending a Satellit to Space: What could possibly go wrong?