Chips in Space - The Building of an Amateur Satellite
Chips in Space: Let’s look inside ARISSat-1 (part 2)
Steve Bible
8/21/2011 3:47 PM EDT
Welcome to Part 2 of our dive into each of ARISSat-1’s subsystems, where I will focus on the solar and battery power systems that are managed by the Power Supply Unit board discussed last week. It’s going to take a total of three parts to cover everything, and next week’s blog post will wrap up the remaining subsystems.
ARISSat-1 has been in operation for two weeks now. Last week the battery failed, causing the satellite to go silent during eclipses. However, once its solar panels are back in the sun, the systems power up and it begins operation. The Mission Elapsed Time (MET) sent in the telemetry is reset to zero each time the satellite falls into eclipse. I explain more about the battery below. All other subsystems are working nominally.
X, Y and Z Axes Labeled
In previous blog posts, I showed you the cross-sectional drawing of ARISSat-1. Above, I am showing you the axes labeled. In the telemetry transmitted down, several of the subsystems are labeled -X, +X, -Y, +Y, -Z and +Z. The axes are often associated with the solar panels, MPPTs and cameras. This way you will know which way is up (and for those that have read Ender’s Game, remember, the enemies’ gate is down).
Solar panel on the - X axis
Solar panels
The six solar panels were donated to the project by NASA. These are space-rated solar panels that were left over after the Small Explorer (SMEX) satellite program ended. Each panel measures 19-inches by 10.5-inches and consists of 50 cells. In full sunlight, each panel can produce about 50 volts open-circuit and more than 19 watts of electrical power. A panel is mounted on each of the six surfaces of the space frame.
Maximum Peak Power Tracker (MPPT)
Maximum Peak Power Trackers (MPPTs)
The six MPPTs are intelligent, SEPIC, switching power supplies that will either boost up the solar-panel voltage (when the sun is low) or buck down the solar-panel voltage (when the sun is high) to the battery voltage of approximately 28 volts. The minimum operating voltage is 15 volts, and the maximum is 100 volts. The peak operating point of the panels is specified as 45 volts. So far, telemetry is showing that the panels are running between 15 and 46 volts. Each MPPT uses a PIC16F690 8-bit MCU as its SMPS controller. The algorithm running in the PIC16F690s quickly shifts the operating point of the SEPIC up and down, to hunt for the peak power point. As the satellite was expected to tumble, the MPPTs must hunt very quickly and then track the fast-moving peak power point. Each panel is expected to have six seconds between sunrise and sunset.
Silver-Zinc (AgZn) type 825M3 battery
Battery
The battery was donated by RSC Energia. This is a type 825M3, and is the same exact type used to power the Russian Orlan space suits. It internally consists of eighteen rechargeable, Silver-Zinc (AgZn) cells and is specified for 14 ampere hours at 28 volts.
The battery has failed much earlier than we expected. This particular battery is not constructed to be a long-life battery. It is only rated for five charge cycles. We knew this going in. Since this was the battery we were given, we did our best to prolong its life through shallow charging. Now the battery is not holding a charge, and thus goes silent and resets all of its circuitry during eclipse. When ARISSat-1 returns to sunlight, the satellite begins operating after the 16-minute safety timers expire. The Mission Elapsed Time (MET) telemetry is reset to zero each time. This can be seen by monitoring the telemetry.
In next week’s blog post (Part 3), I’ll finish summarizing the subsystems. In the meantime, please check out the below links for more background info and the latest news on this project. And, please post comments about what you’d like me to cover in future posts, as well as any questions I can answer for you.
- Steve Bible, 73 DE N7HPR SK
ARISSat-1 Official Web Site
The Radio Amateur Satellite Corporation
Read the earlier Chips in Space blogs, here.
ARISSat-1 has been in operation for two weeks now. Last week the battery failed, causing the satellite to go silent during eclipses. However, once its solar panels are back in the sun, the systems power up and it begins operation. The Mission Elapsed Time (MET) sent in the telemetry is reset to zero each time the satellite falls into eclipse. I explain more about the battery below. All other subsystems are working nominally.
X, Y and Z Axes Labeled
In previous blog posts, I showed you the cross-sectional drawing of ARISSat-1. Above, I am showing you the axes labeled. In the telemetry transmitted down, several of the subsystems are labeled -X, +X, -Y, +Y, -Z and +Z. The axes are often associated with the solar panels, MPPTs and cameras. This way you will know which way is up (and for those that have read Ender’s Game, remember, the enemies’ gate is down).
Solar panel on the - X axis
Solar panels
The six solar panels were donated to the project by NASA. These are space-rated solar panels that were left over after the Small Explorer (SMEX) satellite program ended. Each panel measures 19-inches by 10.5-inches and consists of 50 cells. In full sunlight, each panel can produce about 50 volts open-circuit and more than 19 watts of electrical power. A panel is mounted on each of the six surfaces of the space frame.
Maximum Peak Power Tracker (MPPT)
Maximum Peak Power Trackers (MPPTs)
The six MPPTs are intelligent, SEPIC, switching power supplies that will either boost up the solar-panel voltage (when the sun is low) or buck down the solar-panel voltage (when the sun is high) to the battery voltage of approximately 28 volts. The minimum operating voltage is 15 volts, and the maximum is 100 volts. The peak operating point of the panels is specified as 45 volts. So far, telemetry is showing that the panels are running between 15 and 46 volts. Each MPPT uses a PIC16F690 8-bit MCU as its SMPS controller. The algorithm running in the PIC16F690s quickly shifts the operating point of the SEPIC up and down, to hunt for the peak power point. As the satellite was expected to tumble, the MPPTs must hunt very quickly and then track the fast-moving peak power point. Each panel is expected to have six seconds between sunrise and sunset.
Silver-Zinc (AgZn) type 825M3 battery
Battery
The battery was donated by RSC Energia. This is a type 825M3, and is the same exact type used to power the Russian Orlan space suits. It internally consists of eighteen rechargeable, Silver-Zinc (AgZn) cells and is specified for 14 ampere hours at 28 volts.
The battery has failed much earlier than we expected. This particular battery is not constructed to be a long-life battery. It is only rated for five charge cycles. We knew this going in. Since this was the battery we were given, we did our best to prolong its life through shallow charging. Now the battery is not holding a charge, and thus goes silent and resets all of its circuitry during eclipse. When ARISSat-1 returns to sunlight, the satellite begins operating after the 16-minute safety timers expire. The Mission Elapsed Time (MET) telemetry is reset to zero each time. This can be seen by monitoring the telemetry.
In next week’s blog post (Part 3), I’ll finish summarizing the subsystems. In the meantime, please check out the below links for more background info and the latest news on this project. And, please post comments about what you’d like me to cover in future posts, as well as any questions I can answer for you.
- Steve Bible, 73 DE N7HPR SK
ARISSat-1 Official Web Site
The Radio Amateur Satellite Corporation
Read the earlier Chips in Space blogs, here.
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David Ashton
8/22/2011 4:07 AM EDT
Steve, could you not have used a battery with a better projected lifespan? Or were you stuck with this one? How would you do it better next time?
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Steve Bible
8/22/2011 2:23 PM EDT
Hi David, the battery is what we were given for the project. We did expect it to live a little bit longer than it did. But we did learn! So we are discussing options for the next ARISSat such as NiCd with supercaps. It will be experimental so we can learn from it. It will also have to meet all safety requirements.
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David Ashton
8/23/2011 8:47 PM EDT
Thanks Steve. Interesting you mention NiCds, I would have thought you'd go for something like Li-ion? I'm quite a fan of NiCds,but it's mainly as I have access to lots of them. If you look after them they are good batteries...but they do have an annoying habit of going permanently short circuit if they are left discharged for any time....which, as with your current battery's problems, would be disastrous. Any reason for choosing NiCd?
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Steve Bible
8/25/2011 1:30 PM EDT
Hi David, I only mention NiCad because past amateur satellites have flown them with good results. In other words, it’s something we know. But we would like to explore other chemistries and charging management methods to both learn and expand our level of knowledge – within the safety boundaries we are given.
If you look at past amateur satellites and their battery track records, the life of each most often is dictated by the battery. As you mentioned, they like to short which renders the satellite power system inoperable. If we could have a smart system that could remove the short, at least we can continue to operate in the Sun; my like ARISSat-1 is doing now. We were lucky; it appears that the battery failed open.
For an extraordinary story, take a look at the life of OSCAR 7. It was rendered dead for several years because of a shorted battery. Then one day it was discovered operating in sunlight. http://en.wikipedia.org/wiki/Oscar_7
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David Ashton
8/27/2011 5:33 AM EDT
Hmmm..that is exraordinary.. I've had batteries go open and short, but not both...then again I've never kept a shorted one (that I could not recover) for 20 years either.
With solar panels, supercapacitors and a bit of intelligent circuitry, it should be possible to give dodgy NiCds a "zap" of high current charge if it looks bad when the sat comes out of eclipse.
Would it be possible to work with supercaps alone - I would emagine the eclipse periods are not very long? I'd think supercaps have a better lifespan than the best batteries?
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mac_droz
8/22/2011 4:21 AM EDT
Great article! I'm following it as well as the project website. There's just one think I don't understand: Why on Earth did you pick the battery with a maximum lifespan of 5 cycles and unknown characteristics in the temperature range that you are going to use?
I understand all about the safety (there are good articles in the technical section on the project website), but this seems almost like a political decision. Any off the shelf battery from supermarket would do a better job it seems.
Anyway I enjoy reading!
73 de sq2ahr
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Steve Bible
8/22/2011 2:26 PM EDT
Hi SQ2AHR, thank you! It really wasn't a political decision. In the beginning ARISSat-1 was to be SuitSat-2. We would get the suit and its battery. Thus we always planned from the beginning to use the Orlan suit battery.
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Sanjib.Acharya
8/22/2011 11:58 AM EDT
Steve, thanks a lot for sharing the information. I'm curious to know what measures are taken to protect the electronics from harsh conditions in space, which are not usually needed on earth. Again, I am also wondering why you did not do something better than using that battery.
Great article! will look forward for the next parts.
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Steve Bible
8/22/2011 2:29 PM EDT
Hello Sanjib, no special protection was given to the electronics. However, we did put in redundancy and monitoring so that the subsystems will continue to operate if they detected a failure. We shall see during the life of ARISSat-1 how well it performs in Low-Earth Orbit radiation environment.
On the battery choice, please see my replies above.
Glad you like the blog!
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Sanjib.Acharya
8/23/2011 11:18 PM EDT
Hi Steve, thanks a lot for the information. While browsing through the parts e.g FPGAs, controller, I've seen some manufacturer sells "radiation hardened" chips. Do you really need them? They are usually sold at a premium. Also, what is the environmental spec. (temperature etc.) for which you design your electronics?
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Steve Bible
8/25/2011 1:35 PM EDT
Hi Sanjib, it is true that FPGA manufactures make radiation hardened components for the aerospace industry. They are very expensive for an amateur project. Thus we chose to go the cheap and easy route. I do believe that we shall see the proof of the pudding (as it were) with ARISSat-1. Let’s see how long it operates with industrial temperature grade components.
On the question of temperature specification, simulations were telling us the range of -20 to +40 degrees C. However, we are seeing in the telemetry higher temperatures. We are still investigating why that is.
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Robotics Developer
8/23/2011 10:58 AM EDT
Bummer about the battery!! It seems that (in hindsight) a backup/redundant storage system would have been in order. Is there any consideration for a low power down standby/hibernate mode for the next satellite? This coupled with a small redundant power backup could be used to enable the electronics to not get reset during short periods of eclipse. Thanks for the details! A classic case of "this is what we have to work with" and making due. Keep up the good work.
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Steve Bible
8/25/2011 1:38 PM EDT
Hi R.D., yes it is a tale of making do with what you have. Yes, we would like to make the battery system robust. We learned a lot from ARISSat-1 and we will be exploring new ideas.
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mspitze
8/24/2011 1:58 PM EDT
Would hate to be the poor Cosmonaut that gets failed battery for his his life support systems.
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Steve Bible
8/25/2011 1:40 PM EDT
This particular battery was designed with high reliability in mind, not longevity. The number of charges and discharges are carefully monitored. After the specified number are met, the battery is disposed of.
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jdesbonnet
8/25/2011 9:40 PM EDT
I would imagine that Li-ion was out because the satellite had to be stored inside the ISS alongside the astronauts for several months. So any battery technology capable of leaking, outgassing or bursting into flames is certainly a no no. Although I do see lots of Thinkpads in photos of the ISS... Do they have Li-ion battery packs?
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