Here are some pictures of the electronics in the RMU. We are currently assembling the whole experiment
We are working hard with the electronics in our experiment. We are working on five FFUs in parallelll and the bottom PCBs are almost finished soldered! Here is a picture from our FFU factory 🙂
The progress with the RMU electronics is going well and more pictures will be uploaded. I’m sure the other Markus will give you some really neat pictures in the near future.
On thursday we will have a droptest and we have a lot of things to work on until then.
The FFUs (free falling units) will be dropped from a balloon from 500m altitude and the data acqusition system, parachute deployment, localisation system and mechanical system will be tested.
Yesterday we got the sattelite modem working and also a zero g detector that we will use in the droptest. When the FFUs are released from a remote controlled mechanism under
the balloon they will fall down and accelerometers detect zero g. This will trigger the parachute deployment and start the localisation and data acquisition system.
Here is a video of the zero G detector that lit a blue LED:
These last days we’ve been testing the antenna in several different setups in an attempt to better understand it’s performance, and ultimately the accuracy of the temperature we are going to derive. We connected our custom made antenna to a test FFU from RAIN and sent satellite messages from the backyard of KTH Space and Plasma Physics to the Globalstar Navigation system! The sending went excellent and there seemed to be no problem in using the antenna for sending! Good news!
Then we tried connecting it to the commercial GPS of the RAIN FFU to see if it was even possible to acquire a position. This was not certain since RAIN has used another antenna, the cable to the antenna was very long (~2m) and the GPS reception through the window of the lab wasn’t likely to be splendid. But in our best engineering manner, we duct taped the antenna to a plank and stuck it out trough the window while connecting the FFU to a lab computer to read out the data. And to our happy surprise, the commercial GPS managed to lock the position and fix as many as 7 satellites! Great performance!
On to the next testing! We connected the antenna to the GPS front-end and did the same thing. Here it’s not even sure if we’re going to get any signal what so ever since the front-end is built for active antennas in RAINs FFU and our antenna (and system) is passive. We’ve read out the data but it’s yet to be analysed, so I’ll leave this one as a cliffhanger for later. 🙂
In addition, we’ve started testing the MAX2769 Evaluation-Kit to see if we can understand how the settings in the GPS front-end is connected to its performance. First step is complete. Meaning we started up the kit, got contact with the chip and it seemed to receive commands and output some kind of data. Next is to save the data the chip outputs, but we’ll wait with this until the programming of the FPGA is more finished so it understands what to do with all that data flooding its pads!
For your delight, pics:
The bottom PCB has arrived! Now we just have to solder over 200 components per needed PCB!
Heres a picture of a newly arrived PCB:
We let the PCB manufacturer solder two components because they are hard to solder without proper equipment due to lack of reachable pins.
So far we have tested all the voltage regulators except for the 3.3 V switching regulator and soldered most of the passive components (resistors and capacitors). Hopefully we will have the first PCB fully soldered by Monday! Here is a close-up on a few of the soldered components:
The Layout of the top PCB is almost finished!
Well I know there has been a lot of posts lately about the stupid layout, why is it even important? I thought I would take the time to explain the process of circuit board development to clear out some fuzzy concepts. So here it is! Your 5 step program to instant success in electrical design!
1. Determine your requirements
What is it even that you want you electronics to do? Measure the temperature, capture images in 17 Gigazillion pixels or just give you a tingling feeling once in a while? Figure out why you need an electronic system and determine some performance requirements of it. In our case, we want it to capture raw GPS signals and sensor (accelerometer, angular rate and magnetometer) data, and be able to tell us where it is with some kind of localization system. In addition, it would be nice if it could deploy the parachute when it should.
2. Pick components
Hopefully there are a set of integrated circuits and other solutions that give you the features you want. Look around and see if you can find something to meet your requirements! Google is your friend, so is experience! 🙂
We use the MAX2769 GPS front-end for raw GPS handling. The STX2 satellite modem for Globalstar® communication. The TX1 beacon transmitter for taking a bearing on the FFUs, and the ET318 commercial GPS chip for real time calculation of the position. As well as a bunch of small components (RF switches, power regulators, board connectors and other housekeeping stuff).
3. Make the schematics
In other words: Connect your shit! With our components chosen we have hundreds of pads, pins, decoupling capacitors, bounce preventing resistors and other stuff that needs to be connected in a the right way. In this stage we don’t care about the actual placement of the component on the board but are more concerned about that the signal out from the satellite modem really goes into the antenna for example. This requires a lot of “know how” in the electronic business. Breadboard testing makes it possible to test your schematics without ordering them, but we only test some parts of our system since we have a lot of heritage from RAIN and SQUID.
4. Make the layout
This is what it all comes down to! You know what you want it to do, you’ve picked your components, you know how they are supposed to be connected. Now you just want them to fit together on the stupid board. Here there never seems to be enough space to do what you thought about in the schematic stage. You fiddle back and forth with routing electrical paths and fast switching radio lines. You always have to weigh things against each other: Do I want the RF lines as short as possible or the decoupling capacitors really close to the pad?
It’s like un-entangling a huge ball of yarn that was perfect in the theoretical world of schematics. Here it comes down to the actual power line not fitting between your capacitors because they’re too big.
In this stage we get help from the computer programs that we made the schematics in. The program shows us where everything is supposed to be connected, and it doesn’t allow you to connect any to lines that are not connected in the schematics. So there is no risk running the high voltage go through the sensitive analog filter by mistake. There is however no shortage of things that can go wrong here. 🙂
When the layout is done and the PCBs have been ordered. You have to sit down and solder all of those small connections that you made in the layout. We’ll reach this stage next week with the bottom PCB ordered yesterday and the top PCB hopefully ready for order tomorrow!
It’s getting close to the real thing, soon we’ll know if it works or not!
Although this week is exam week quite a lot of work has been done on the bottom PCB layout. In fact, today we routed the last trace! This means that there are only a few minor changes left as well as adding the ground and supply planes before the bottom PCB is ready for ordering.