Getting the Electrons from This Box to That Box (Power System Design)

Before BRRISON was a panicked scramble to write a year’s worth of code in two months, it was a panicked scramble to design and build a year’s worth of electronics in two months. Clearly, the theme of panic pervaded the project from day one. Starting from scratch, the DayStar team built up a computer system that would control two science cameras, two optical mechanisms, a $150k fast steering mirror, and a bench full of heaters. Most components in the system are off-the-shelf — a desktop motherboard, specialized PCIe cards, motor and heater controllers, etc — but one thing that could not be bought was a power supply. The UVVIS power stack is the one purely custom piece of electronics on the bench.

What components require power? All of the above. And how much power do they need? Oh, just a paltry 300W. Between the powerhouse computer (100W or more), the ProEM camera (100W), the Zyla camera (50W), and all the motor and heater controllers, the UV-VIS power system is throwing around quite a few amps. And those amps are critical for every single mission critical component.  At least a whole team was in charge of building it, to mitigate the risk that any of a thousand mission-ending failures would occur. NASA payloads require oversight like this, since they’re worth millions of dollars. So, who made up this team of experts? … Nick Truesdale. Alone. And scared.

All kidding aside, Nick built the power system for the DayStar star tracker, which used a similar computer and science camera. And of the DayStar team, he is the expert in PCB design and soldering tiny things. So, in keeping with BRRISON’s emotional theme (panic masked by confidence), as well as the unofficial DayStar slogan (“You… you want us to do what?”), Nick replied “Challenge accepted!” Less than two months later, the power system was integrated with the rest of the electronics, barely tested but apparently working. This showcased another unofficial DayStar slogan: “Test while you fly; flight is your test.”

The power system has three major tasks: power the instruments, power the computer and control the heaters. To this end, there are three custom PCBs, assembled in a stack. These are the Main Power board, the Computer Power board, and the Heater Control board. Atop the stack sits an Arduino Mega 2560 microcontroller board, provided by Sparkfun, which acts the brains of the stack and interfaces with the flight computer. The power stack is mounted to an aluminum plate for heat dissipation, which in turn mounts within the electronics pressure vessel. A separate power conversion card, the VME-550 from Aegis, provides all of the computer voltages.

The Main Power PCB is by far the most complicated of the three custom boards. It converts raw DC voltage from the gondola batteries to 24V, 12V, +15V and -15V. The 24V rail powers the ProEM scientific camera, the Fold Mirror and Filter Wheel motor controllers, and the Heater Controller. 12V is used to power the Zyla scientific camera, as well as a brake on the Fold Mirror mechanism. Finally, the +15V and -15V rails both power the FSM controller. These voltages are all supplied by Mini and Micro DC-DC converter bricks from Vicor. Between the bricks on the bottom and the Arduino footprint on the top of the PCB, there is very little space for additional components. What’s left houses MOSFET switches, ADCs and current sensors, common mode inductor filters, and more power conversion circuitry.

Main Power PCB

The Main PCB, before 12 hours of hand soldering (right) and after (left). The backside is filled by six large power bricks, seen mounted via the giant thru-hole sockets that criss-cross the board. The top holds common mode input filters, buck converters, MOSFET switches, current sensors and ADCs. There is so little real estate that some capacitors are stacked, and one of the inductors straddles two power brick pins.

The Computer Power board comes second in the stack. It gets clean 12V, 5V, 3.3V and -12V power from the VME-550. With these voltages as input, the Computer Power board does all of the control required to imitate a standard ATX power supply one would find in their desktop computer.  The board routs and switches power using MOSFETs on the front and back. These create the five voltage rails found in the ATX 24-pin connector (12V, 5V, 5VSB, 3.3V and -12V), the 12V line to the CPU, and 5V/12V rails for SATA hard drives.  The PCB uses the PS_ON signal generated by the motherboard to switch on these power lines with the correct timing. It also uses ADCs and current sensors to confirm the system is healthy, and outputs a PWR_GOOD signal. For more information on ATX power supplies, Don Woligroski has a great article on the Tom’s Hardware website. Along with the ATX specification documents, Don’s article was integral in designing the Computer Power board.

comp_complete

The Computer Power PCB is less cluttered than the Main Power board, because the conversion is done by a separate power card. This board focuses on switching and sensing the various 12V, 5V, 3.3V and -12V rails used by the computer, and is close to compatible with the ATX standard. It also has a standalone AVR (bottom right) with customizable user input switches.

The third PCB in the power stack is the Heater Control board. This board was designed last, taking advantage of the stack design to simply slip it in at the last minute. It interfaces with the similarly named Heater Controller, which uses thermocouples all over the UV-VIS optical bench to generate on/off signals for the heaters. These signals go to the power stack, where they turn on/off MOSFETs for each of eight heater zones. The MOSFETs rout raw battery power through current sensors and out to the various heaters.

ebox_back2

The Heater Board, seen here fully integrated with the electronics box, is the top of the stack. Its sole function is to switch on and off heaters based on commands from the Arduino or the Heater Controller (far right). This requires few components, and the spare real estate is used to mount up to two LCD screens. In this way, the Heater PCB doubles as a power system display.

Because the Heater Control board needs so few components, there was a lot of extra space in the design process. What’s more fun than adding more electronic toys to your design?  And what is cooler than having your very own tiny LCD screen sitting atop 300W of power? Nothing, of course. So, using the open bottom half of the Heater board, two LCD screen footprints were added. The screen itself is a 1.8″ TFT screen from Adafruit, intended to interface with an Arduino. One of these was soldered to the Heater board directly, and the other was placed on the exterior of the electronics pressure vessel. This allows the DayStar team to monitor the power stack visually, whether the box is open on the table or sealed up for flight… Or we can just display images of the DayStar logo and roflcopters.

DayStar LCD

An LCD screen, mounted to the exterior of the electronics box and controlled by the power stack, shows off the DayStar logo as well as the versatility of kapton tape.

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