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SuitSat pushes engineers' limits

Posted: 30 Jun 2008     Print Version  Bookmark and Share

Keywords:MCU  electronics  engineering  safety issues 

What started out as an innocuous call about their company's line of MCUs sucked a small cadre of engineers into a multi-year, space-based design project that would push the boundaries of their professional knowledge and flexibility, while reigniting the sense of wonder, exploration and possibility that led them down the path to engineering in the first place.

That call came in October 2004 from Lou McFadin, hardware manager for Amateur Radio on the International Space Station (ARISS), a volunteer program designed to inspire students worldwide to pursue careers in science, technology, engineering and math through amateur-radio communications opportunities with the space station on-orbit crew.

McFadin was calling in the wake of an AMSAT Symposium/ARISS International Partner meeting at which it was decided to convert an old Russian Orlan space suit into a satellite by outfitting it with telemetry equipment and antenna—and to toss the suit out of the space station.

The plan, code-named SuitSat, called for the novel "satellite" to transmit spoken greetings from children in multiple languages, as well as slow-scan TV images encoded in audio signals and some spoken telemetry, such as elapsed time, temperature and battery voltage. McFadin was calling Microchip Technology Inc. to see if a PIC MCU could handle the job.

It seemed simple enough, and so a small team from Microchip volunteered to help out. "We got into it thinking it's easy," said Steven Bible, technical staff engineer at the Chandler, Ariz., company. "Kind of like a high-level science fair project." They were mistaken.

What the Microchip engineers didn't fully appreciate at the time was that extreme environmental and safety issues would challenge them at every step of the design. With a vacuum to deal with, they learned about the implications of outgassing and about handling thermal hot spots with no airflow. They also learned how to avoid the Fermi region to prevent arcing, how to handle thermal extremes, and what do about ionizing radiation. They studied power-optimisation techniques for system longevity, and with no gravity for stability, they found out the hard way what a tumbling antenna does to radio reception.

The experience proved so interesting that the engineers enthusiastically signed up for SuitSat-2. Leveraging their acquired knowledge, the team has already developed a new solar-conversion technique for the follow-on satellite that will extend system life to months instead of weeks. They have also crafted a software-defined, full-duplex radio with a better antenna for two-way communication and more-refined control.

Electronics the easy part
From a schematic-diagram point of view, the original SuitSat-1 system and its electronics were fairly straightforward. According to Bible, the main design parameters were that it be easy to assemble on the space station and that it work with simple, inexpensive ground receive equipment, so that teachers and hobbyists could readily track the signals. All they should have to do would be to download free software that let them track the audio and decipher the audio-encoded images.

SuitSat-1 comprised a controller box with a control PCBsDC/DC converter and EMI filter. This connected to a separate radio box that used an off-the-shelf Kenwood TH-K2 amateur handheld two-way VHF radio with an external quarter-wave, ground-plane antenna. The controller also connected to a switch box with one power and two timer switches that were used to start the system upon deployment. The whole system was powered by the suit's own three, very expensive but high-power-density 28V silver-oxide batteries.

The controller board was where the messages were stored and where temperature and voltage monitoring took place. At its heart was the PIC18F8722, a 64/80-pin MCU with 1Mbit of enhanced flash, on-board 10bit analogue-to-digital converter and the company's nanoWatt technology. Another reason that processor got picked was its ability to handle voice signals encoded using adaptive pulse-code modulation (ADPCM). Those signals were stored on an 8Mbit SPI serial flash chip from SST (the SST25LF080A), programmed via an RS-232 interface, and a MAX3232 RS-232 level shifter from either Maxim or Texas Instruments (dual sources).

The ADPCM signal was output from the MCU to an MCP6022 op-amp-based, 4kHz low-pass filter, and from there passed on to the radio, which transmitted at 145.99MHz. Also on the board were an MCP9800 SPI temperature sensor and three MC14541 programmable timers.

Safety first
The inclusion of timers on the control board underscored the biggest problem the designers faced: safety. "We had to deal with NASA safety people," said Bible. "It was mind-boggling." Each aspect of the design had its own documentation: snag hazard, outgassing hazard, electric shock and RF hazard. To handle the latter, the timers were used to implement three interlocks to prevent the transmitter from sending out messages before a 30-minute timeout, thereby giving the cosmonauts time to reenter the space station after pushing away the SuitSat.

To keep from exposing the astronauts to toxic fumes, all the components had to be built from materials that could pass a thermal-vacuum test without outgassing. But that wasn't the only selection constraint: Tantalum and ceramic capacitors were chosen instead of electrolytic, since electrolytics are basically a can and are also potential leakers.

All mounting hardware had to be stainless steel, and no lock washers could be used, because they produce microscopic metal shavings that could short out fine-pitch SMT parts. Due to the vibration associated with takeoff, all wires were laced together and glued down. All bare-metal joints were covered with RTV to prevent any debris from shorting connections, and all bolts were glued to prevent loosening. Even the glue itself had to be specially approved by NASA.

One of the more interesting considerations when preparing for a vacuum is the Fermi effect, whereby operation at lower oxygen pressure can cause arcing or a corona effect at voltages of around 40V. To prevent this hazard, Bible said, the enclosures had extra holes to ensure rapid venting to vacuum. While ionizing radiation and its associated bit-flips and latchups were somewhat of a concern, the low-earth orbit at about 400- to 500km, and the short life expectancy of the satellite, gave this problem low priority. In the end, it proved to not be an issue.

Thermal, power management
Thermal management did become an issue, however. While convection or forced-air cooling can be used on earth, the vacuum of space meant the designers had to rely on black-body radiation to keep the system cool. To maximise heat extraction, they mounted hot spots, such as the Kenwood radio transmitter, on a large aluminium block that was itself attached to the enclosure. Also, thermal extremes had to be avoided. As it orbited, the suit would be exposed to heat and cold alternately for 45 minutes each.

While the suit was designed to keep an active cosmonaut cool, the team had no information on the environment without a human inside. The suit was not going to be actively cooled. As the electronics were placed inside, the engineers banked on it being an insulator against extreme cold and heat. They used a temperature sensor and spoken telemetry to monitor the variations. It stayed at 12°C for the entire mission.

While the electronics went inside, the switch box and antenna were bolted externally to the helmet, with the cables running inside, taking care to reduce snagging potential. To maximise life expectancy from the used batteries, which would be in an unknown state of charge, the team made sure the transmission was easy to listen to and did not require an excess amount of energy. The result was a 30-seconds-off, 30-second-broadcast mission profile.

The electronics were assembled at Microchip in June 2005, sent to Russia and transported to the International Space Station on the Progress 19 cargo ship that September. The suit was launched from the space station on Feb. 3, 2006.

Lessons learned
SuitSat-1 was a clear success, providing two weeks of transmissions before the batteries finally gave out. The suit itself finally burned up in the atmosphere after six months. However, there were a few snafus along the way.

"In the postmortem, the transmit signal was not as strong as desired," said Bible, referring to a signal 20 dB lower than expected. There were a number of theories for this, including transmitter failure, but Bible went over the schematics and could find no point of failure that would cause a reduction in output power. The coax was a possible culprit, but Bible and his team saw the antenna as the more likely problem, combined with the tumbling of the suit.

"The antenna was a hand-me-down," he said. "It wasn't designed to be used in a helmet, and it went up without a lot of testing." For SuitSat-2, he said, the antenna will be self-contained and will likely have a ground plane behind it.

That won't be the only improvement. Seeing that the original suit lasted six months in orbit, the team decided to develop a solar panel power source to extend the operating life of SuitSat-2 to possibly two months. But how do you design a converter that is efficient all the time it's out there, given that there is no control over the orientation of the suit and its panels?

That task fell to John Tray, principal applications engineer at Microchip. Tray came up with a six-panel design in which one panel will always be in full light, while two or three will be in partial light. However, with a 90-minute orbit giving 45 minutes of light and 45 minutes of darkness, Bible said, Tray incorporated a power scheme that "can run the whole system off the panels and recharge the batteries at the same time."

The converter analyses how much power is coming out of the solar panel, "and it actually maximises it for a peak detect on power," Tray said. "A solar panel looks like a constant current source, and often you'll hook it up to a battery directly and charge it at a constant current, but that's not taking advantage of the power coming out of the solar panel."

The resultant DC/DC Max Power Point Converter will take the maximum operating point of the solar panel and continually run at that point, even though the conditions change. "It'll be charging the battery at its maximum rate possible," Tray said. Power storage is via a supercapacitor.

"There's not a lot of info about this stuff out there," said Tray, "so we wrote an app on how to do it simply, for extremely low cost." It will be posted on the company's site, he said. "Anyone who's looking at having extended applications run off of solar power in a remote area, where you don't want to service it, that converter will run excellently."

Other power innovations include two-way communication to shut down portions of the circuit if it's not meeting the current budget, as well as six individual circuits to monitor each panel separately. These are based on PIC16F690 MCUs.

On the radio end, SuitSat-2 will have much more capability than its predecessor. For starters, it will be software-defined. "It uses one of our dsPICs [dsPIC33F] to do the modulation and demodulation, and to control a regular audio codec that's running at 48kHz, sampling," said Bible. The downlink is 145MHz and the uplink, 437MHz (amateur satellite service frequencies).

"We'll convert these to a 10.7MHz intermediate frequency, and that will get digitised to that 48kHz bandwidth," he said. 'And then the DSP does its processing on it." An FM signal will be stored in an SD card and sent to the dsPIC over an SPI line, then sent down.

An outstanding feature of the radio's design is the use of a quadrature sampling detector instead of a mixer. "This allows you to down-convert from 10.7MHz to audio, with better dynamic range and performance for less power," said Bible. It's not a demanding circuit, he said, and is basically a quad switch that's switching in quadrature (I and Q). Capacitors reside at the input to these switches; the whole thing is just an integrator, he said.

"So long as you're switching in a certain order [vector], then you basically get the I and Q at output: incredibly simple," he said. Bible suspects it will increase in popularity now that it can be done digitally, rather than in analogue.

Other additions to SuitSat-2 include a more-complex band plan, as more bandwidth is available, along with more temperature sensors to better understand the overall thermal environment.

Engineers being engineers
While the SuitSat project presented its share of headaches and demanded patience as the Microchip team dealt with NASA's requirements and waited for the launch (they're still waiting for a launch date for SuitSat-2), Bible said they'd do it all again—and he'd recommend it as the type of project engineering companies should encourage.

While little of what was learned from the experience is directly applicable to his job at Microchip, the intangibles are valuable, he said. For one, there's the group experience itself. "Microchip has many product groups, but they don't work together. [The SuitSat team members] have been learning a lot about each other's products and about each other," he said. Bible said he has also seen the work translate into better customer interactions.

For himself, he said, SuitSat appealed to the very core of what he's done all his life, from building bottle rockets as a kid right through his career at Microchip. It was simply an extension of who he is: an engineer.

- Patrick Mannion

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