This week’s weekly report is all about electronics. Currently we are at the end of the design phase of our electronics system. But let’s start at the beginning. The electrical part of our mission is based around the sbRIO-9626, an extremely powerful embedded systems board. With a 400 MHz CPU, 512 MB Memory, 256 MB RAM and a powerful Spartan 6 FPGA we have a enormous amount of power available for controlling and on board data handling (OBDH). Multiple interfaces allow excellent connectivity of our subsystems.
For example we will use the RS-485 Interface of the sbRIO to connect to the REXUS Service Module data connection, which is a RS-422 type interface. Fortunately RS-485 is a subset of RS-422 so we can “reuse” this sbRIO feature on our mission.
The main task to perform at the moment is interfacing all our sensors and actors in our experiment to the sbRIO. This is done by adding two addition PCBs (printed circuit board) in kind of a sandwich style to the bottom of the sbRIO.
The two boards are called power-sensor-interface boards (PSIB). The lower one (PSIB 1) will hold all power circuitry. Generally speaking this is the power supply for our experiment and the SMA spring activation circuitry. Our hold down release mechanism is activated by letting current flow through the SMA spring. As a result the spring will heat up and thus activate the contraction process. The circuit for this is rather simple but very effective. We are using two parallel N-Channel MOSFETs (of course galvanically isolated) to reduce unwanted power dissipation since paralleling two identical resistances (Rdson of the MOSFET) halves the total resistance. The less resistance the less power will be converted into heat and the better is the efficiency.
As mentioned above we are making excessive use of galvanic isolation since the SMA spring requires a high amount of power and we want to protect our circuitry and other experiments on the rocket in case of a failure. To achieve isolation we simply use multiple optocouplers to separate driver- and measurement circuitry from the rest of the system. Of course we also need to monitor voltage and current of the SMA spring. The results of the measurements are converted from the analogue to the digital domain using a ADC (analog digital converter) and the transferred via a isolated SPI bus to the sbRIO.
The second power-sensor-interface board (PSIB 2) holds all connectivity to temperature sensors, accelerometers + angular sensor on the solar panel and GoPro Interface. The GoPro 3 Black will capture the action of opening the panel inside our experiment. Thus the camera needs to be activated and we have a LED light source due to the dark inside the rocket.
We hope you got a brief insight into our electrical system and next time electronics Weekly Report will show up with a perfect and ready to fly PCB.