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BLUEsat’s  NUMBAT rover is due for completion early next year, and it needs a new and updated control system to go along with it. That’s where the prototype for the new joystick comes in.

First, let me take you through the previous controllers. The BLUEtongue rover was controlled via two different Xbox controllers; one to control the movement of its wheels, and the other to control the arm and claw.

The one I was to make was to incorporate all the above features onto one single joystick. An Xbox controller does not have sufficient number of buttons so creating one using an Arduino seemed to be the best option.

Speed Control

The first task was to implement speed control. Initially, I had an idea that this could be achieved by using a potentiometer as the speed control, but I wasn’t sure how. With a bit of experimentation and fumbling about, I realised how it could be done.

Firstly, to control the old space rover, two different joysticks are used, one to control the rover’s movement in the ‘Y-axis’ and the other in the ‘X-axis’.  This is done to ensure that the rover moves precisely forwards or backwards without any deviation. This was a good method to control the movement and thus I will not change this.

A joystick itself is simply two different potentiometers combined, one for each axis. They both work simultaneously to provide a reading. The values of both axis range from 0-1023, as does with any potentiometer. Therefore, the resting position of a joystick is somewhere between 500 – 530 (this is where the velocity is zero). With this idea in mind, as an example, if the joystick reads a value greater than 530 in the X and Y-axis, it means that the joystick has been move to the top-right position and the rover should also move towards the North-East direction. Using this, I can get the rover to move in any direction I want, so what about the speed control?

Arduino joystick attachment overlaid with 2D graph axis
This illustrates the direction of both potentiometer ranges for the axes

This is where the second potentiometer comes in. As mentioned above, it has a range from 0 – 1023. When the reading is 0, the velocity of the rover should be zero and when the reading is 1023, the rover should be moving at max speed. Thus, I needed a way to map the readings from the joysticks to the respective maximum boundaries set by the potentiometer. Lucky for me, there is a ‘map()’ function in the Arduino library that does just that. It, in simplicity remaps  a number from from one range to another. Testing this  on the BLUEtongue rover proved that this method worked and slowed the rover down to a specific speed depending on the reading from the potentiometer.

A small snippet of this implementation is illustrated below.


...

int yaxis = analogRead(A0);
int xaxis = analogRead(A1);
//Sensor reading from the potentiometer
double sensorValue = analogRead(A2);
//remapping the yaxis vector between 0 and the reading form the potentiometer reading
double remapy;
//remapping the xaxis vector between 0 and the reading form the potentiometer reading
double remapx;

if((yaxis >= 493 && yaxis <= 553)&&(xaxis >= 493 && xaxis <= 553)){ //No movement
      command.drive_vector.y = 0.00; 
      command.drive_vector.x = 0.00;
 }else if((yaxis >= 553)&&(xaxis >= 493 && xaxis <= 553)){ //FORWARDS
      remapy = map(yaxis, 553, 1023,0, sensorValue);
      command.drive_vector.y = remapy/1023; 
      command.drive_vector.x = 0.00; 
 }else if((yaxis <= 493)&&(xaxis >= 493 && xaxis <= 553)){ //BACKWARDS
      remapy = map(yaxis, 493, 0,0, sensorValue);
      command.drive_vector.y = -(remapy/1023);
      command.drive_vector.x = 0.00;
 }
...
else if(yaxis >= 553 && xaxis >= 553){ //TOP-LEFT
      remapy = map(yaxis, 553,1023,0,sensorValue);
      remapx = map(xaxis, 553, 1023,0, sensorValue); 
      command.drive_vector.y = (remapy)/1023;
      command.drive_vector.x = (remapx)/1023; 
 }else if(yaxis >= 553 && xaxis <= 493){ //TOP-RIGHT
      remapy = map(yaxis, 553,1023,0,sensorValue);
      remapx = map(xaxis, 493, 0,0, sensorValue);
      command.drive_vector.y = (remapy)/1023;
      command.drive_vector.x = -((remapx)/1023);
 }

It basically does the same thing as aforementioned, detecting the direction each joystick faces and maps them to their respective direction and velocity for the rover.

Buttons

Now with the speed control done, next was to implement buttons to control the extending and retraction of the arm, claws and their rotation. This was just using buttons and depending on what the PWM (pulse width modulation) value was for the specific action. This has been mostly completed, however some revision is still required.

Webcam switching is also something that is to be completed via buttons on the joystick itself, but it has yet to begin implementation.

A code snippet of the button implementation is shown below.


if(digitalRead(ARM_LOWER_EXTEND) == HIGH){
    command.arm_lower_pwm = 2000; //Extend the lower arm
}else if(digitalRead(ARM_LOWER_RETRACT) == HIGH){
    command.arm_lower_pwm = 1000;//Retract the lower arm
}else{
    command.arm_lower_pwm = 1500;
}

//UPPER ARM
if(digitalRead(ARM_UPPER_EXTEND) == HIGH){
    command.arm_upper_pwm = 1000; //Extend the upper arm
}else if(digitalRead(ARM_UPPER_RETRACT) == HIGH){
    command.arm_upper_pwm = 2000;//Retract the upper arm
}else{
    command.arm_upper_pwm = 1500;
}

In the Near Future

Prototype Joystick for BLUEsat UNSW's NUMBAT Rover - A breadboard with buttons on it, two joysticks and an arduino uno.
This is what the joystick currently looks like

As shown above, that is what the prototype currently looks like. There are a lot of buttons, and they will continue to increase. After the prototype is confirmed to be working properly to a satisfactory level, a casing for it will being to be designed. The design would have to incorporate space for all of the button, the two joysticks as well as the potentiometer. Not only that, they must all be in comfortable positions that are easy to access without causing any hand strain due to long time usage. This design will then be 3D printed when decided upon. Hopefully the prototype joystick will have a home soon.


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Fifteen years passed since Australians had last launched a satellite into space. In early 2017, this interlude was interrupted by a team at UNSW who launched EC0, a CubeSat into low Earth orbit. The CubeSat industry, although in its infancy, has allowed for an affordable method of building and launching satellites into space. The CubeSat was built by the Australian Centre for Space Enginer Research (ACSER) while BLUEsat provided the groundstation.

GPredict tracking EC0’s range of communication on our groundstation - view of the earth showing radio ranges for GOMX1, UNSW-ECO, ISS, and RS-40
GPredict tracking EC0’s range of communication on our groundstation

 

Moving at approximately 28,000 kilometres per hour and equipped with a mass spectrometer, an instrument used to measure the masses within a sample by detecting ion charged particles, EC0 sought to extend our limited knowledge regarding the thermosphere. In doing so, EC0 is capable of aiding scientists in undercovering unexpected phenomena pertaining to this largely unknown region of the atmosphere. Further developing our understanding of weather models, improving GPS and radio transmitted communication methods while deepening our understanding of the interaction that the sun’s radiation has with earth are all within the scope of EC0’s potential. First launched to the ISS and subsequently sent into orbit, EC0 formed part of the QB50 project, an international network consisting of Cubesats built by universities from around the world whose collective input help advance scientific research in this particular region of the atmosphere.

Artist impression of the EC0 cubesat orbiting the stratosphere
Artist’s impression of EC0 orbiting the Earth

 

In the true fashion of space launches, EC0’s launch wasn’t without its problems. Complications quickly arose as groundstation was unable to send or receive any signals to EC0 preventing any communication from occurring. It was believed that the satellites antenna had failed to deploy upon departure into space. After several months of unsuccessful experiments, the use of a high powered European satellite dish was employed to send extra powerful signals to EC0, and for the first time a communication was finally successful. This gave ACSER the clue that they were looking for and after further deliberation, finally the issue was detected. The groundstation was mistakenly transmitting signals to another satellite, ‘Challenger’, as both the Cubesat’s Two Line Elements (TLE), a sort of barcode, had been mixed up before the launch. Upon detection, the complication was promptly corrected, allowing EC0 to truly begin its mission. BLUEsat Satellite CTO, and groundstation lead Timothy Guo reflected upon this moment, claiming the “image of success is strange and there is no euphoria for success, just relief for a problem being solved. Successful communication meant we entered into a new phase of the mission and the fact that you’ve successfully sent a Cubesat into orbit only really hits about a week later”.

A replica of EC0’s internal structure.
A replica of EC0’s internal structure

 

For the initial six months of the mission, communication with EC0 was only achievable at the time it would pass over Australia within its orbit path. This was highly problematic as it often occurred inconveniently between midnight and the earlier hours of the morning. To combat this limitation, ACSER recently installed sophisticated software, allowing for automation of the communication process. At this point in time, a large portion of EC0’s instruments are fully operational, already collecting data about such a fascinating region of the atmosphere.

Due to the high friction environment that EC0 occupies, it’s lifespan is limited to two years. Upon completion of its mission in this period, earth’s gravity will drag EC0 towards its solid surface. The moment of impact will mimic a conventional meteorite crash, burning up when entering the atmosphere, slowing down due to gravity and having very low chances of impact repercussions.


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The IAC had been an amazing experience (read all about it here) but all good things must come to an end. But let it never be said that this IAC didn’t go out with a bang. For the entire week, anticipation had been lingering around the convention center and today it defuses as the man himself arrived on stage. A jubilant tumult erupted from hall as Elon Musk, the man of the hour, took the stage.

Elon Musk at IAC2017
Who is SpaceX?

Despite his nervous disposition, he did not disappoint and his plans and premise to great effect. We all listened intently to his vision and plans to go forth and take a great step into making humanity an multiplanetary species with his latest daring plan. There were many exciting parts to this highlight presentation. He had gone through the rough history that showed the mettle at the heart of SpaceX and detailed the problem that underlies the way we’ve done space travel and how it hinders interplanetary travel. He had given his solution in the form of SpaceX’s latest project, a new rocket with revolutionary reusability features but once again, Elon Musk uses his greatest signature move and makes another stunning announcement. He announced that SpaceX is planning to launch multiple missions to Mars starting from 2022 in a bid to create and expand a Mars colony. The first mission would carry the building blocks necessary for the establishment of a Mars base with the next mission carrying the first people to step foot on mars. Additionally, he concluded the presentation with a potential to use his new rockets as a means of long distance travel on Earth. Naturally, these two revolutionary ideas were met with overwhelming applause and praise from the audience. As far as we know, this may be the start of our journey to the stars.

BFR Concept
New York to Shanghai in 39 mins, still 4 hours in border control (image courtesy of SpaceX)

The day, however was not over and I took my seat to observe the final event of the day, the closing ceremony. There we celebrated achievements of members of the space community which have excelled and many words encouraging the next generation to take up the torch of our great journey were sent with vigour. The contract for the next IAC in Bremen was signed on stage by the esteemed president of the IAF, Dr. Jean Yves Le Gall and the ceremony concluded with the passing of the IAC flag from the Chair of the 68th IAC to the Chair of the 69th IAC.

As I left the convention centre. I had time to reflect on the journey that the past few days had been. It had been a trip of excitement and anticipation, of anxiousness in the face of finally presenting our work and of joy in meeting new people and seeing new sights. This event had revolutionised the way I see the space industry. I had learned to see it not as some dream that only a few will ever experience but a future that I hope will one day be realised through the strength and unity of humanity as we take a step towards space, the final frontier

IAC 2017 Logo
Truly a life changing experience. Anyone keen for IAC2018?

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In July of 2004, a much younger me watched from a distance as Cassini entered Saturn’s orbit. At about that time, I had begun to develop a fascination with space and as Cassini charted a course around Saturn, I began working towards becoming a spacecraft engineer. About two weeks ago, Cassini’s journey came to a spectacular close it plunged into the atmosphere of Saturn. Now I am here, standing proud among giants of the space industry and the first leg of my journey has reached its end

Cassini Grand Finale
♪♪Goodbyyyyyyyyyyyyye Moon Men♪♪ (image courtesy of NASA)

With all of the excitement we have had over the last 3 days, (read all about yesterday’s adventures here), today started slowly for me. I decided to take it easy, getting ready for the big event and didn’t arrive until two hours before our session. Still, I managed to get my long awaited selfie with the curiosity rover and launched into many conversations with the people we had met over the course of the week. It was great to see that we had become part of the group, integrating ourselves within the assembly of students, startups and established names.

Curiosity Selfie at IAC2017
He was very rude about it. He seemed nicer on TV

I arrived early to guarantee I was there on time. Slowly the crowd shuffled in behind me, including members of my team and the friends we had made. Show time!

First up was the NASA Glenn Research Center with their talk about NASA’s progress in producing In-Situ Resource Utilization (ISRU). From there the presentations became more and more incredible. We saw swarms of jumping robots for asteroid study, Martian zeppelins and computer chips built for Venus. There where four legged robots to explore the moon, a lunar rover becoming a superstar in Japan in order to raise money for launch and integrated navigation systems using star trackers and proximity sensors. We were the last presentation of the day, so I had plenty of time to talk myself into a nervous sweat. Of course I did

GreenSat at IAC2017
Dont screw it up dont screw it up dont screw it up

By the time I got up on stage, many people had seen what they had came for and left. However, even more had shuffled in to watch us. Our presentation was a smashing success: we explained the importance in agriculture throughout history and its role in making human kind a space faring race. We showed off our first prototype and our plan to breed bacteria better suited to the space environment. Question time gave us the exact questions we where looking for: talking about things we couldn’t fit into the presentation such as our plan to simulate gravity and the steps we plan to take to achieve that. We were followed out into the corridor by a mob of people curious about our work and where asked many more questions about the specifics of our project

This trip has been an amazing experience, made possible by the tremendous work done by our incredible team. Thank you to everyone who made this possible, the Greensat team itself, those within BLUEsat who helped organise this trip and our amazing faculty who helped guide us to where we are now. With one more day left, this trip is not over yet and I look forward to another day of amazing presentations and incredible new ideas


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Whilst one would think an ecology and molecular biology student would feel daunted and lost at an Astronautical Conference it has actually been both an enjoyable and educational experience so far (see our previous blogs). Admittedly, at the start it was a little bewildering trying to navigate around the labyrinth of exhibits plastered with unrecognisable words associated with space technology. In one incident we, Jess, Yasmin A and I (the only non-engineers in the GreenSat party attending the IAC), managed to find ourselves in a position where we could only slowly nod our heads in agreement as we had an exhibitor persistently tell us about his new advancements in thermal control systems for satellites. I shan’t even attempt to expand on what he explained to us.

NextAero Aerospike Engine
What on Earth is this?

However, it has been a very pleasantly surprising experience to have professionals within these industries and other students from other institutions being enthusiastically curious and willing to hear about the biological aspects of the GreenSat project. Whilst most talk sessions and presentations so far have been somewhat inaccessible to me due to their technical terminology and content, on Wednesday morning (at the begrudgingly early start time of 7am) I dragged myself to attend the ‘First Woman on the Moon-Diversity Breakfast’ talk session along with two other GreenSat members (who also happened to be other female members on the GreenSat team). Despite the excessive complaining I know I carried out about my lack of sleep and need for caffeine, the talk was undoubtedly an incredibly worthwhile endeavour. There is no sugar coating the reality of the situation either, this talk had the best gender balance ratio I have seen so far on the trip; with nearly-nearly, almost half the room being women!

First Women on the Moon Breakfast
Jaxa bringing biology to the stars

Before launching into the session, the welcome started with the President of the IAF, Dr Jean-Yves Le Gall, talking about the potential historical significance of the first woman on the moon, but the questioning of what the first woman would represent really took off with Steve Durst, Founding Director of the International Lunar Observatory Association. He brought to everyone’s attention that there has been a total of 60 woman in space, but of those none have walked on the moon compared to the 12 men who have. He began to question what a female on the moon would represent, whether she would represent a nationality, a professional background or an age too, would it be another giant leap for mankind? Whilst he did not emphasise on a definitive conclusion, it was easily interpretable the unfortunate reality is the ‘great leap’ may be more of a ‘little pounce’ for the first woman on the moon. Professor Jan Woerner, Director General of the European Space Agency, dazzling everyone with vibrant and humorous slides even referenced that moon itself is considered to have a feminine identity (including ‘la lune’ in French being a feminine noun and he even referred to the Maya moon goddess).

The truly impacting ideas from the breakfast however came from the two final speakers; Danielle Richey from Lockheed Martin and Dr. Sandy Magnus, Executive Director of AIAA and astronaut. Together, these two prominent, inspirational and determined women simultaneously balanced the same conceptual thinking and complimented one other in a powerful way, conveying the very significant message of the necessity of diversity within not just the space industry but in all industries. They both, carefully selecting their words, created their own concept of equality and diversity to ensure as much inclusivity and cementing the idea that diversity does not just come from the fact that there is representation but from having a collectively rich source of experiences and knowledge. Their talks were greatly crafted as they both brought up how diversity within a team also includes more energy requirement, more stepping out of the comfort zone and more open-mindedness.  A memorable part of the talk by Dr. Sandy Magnus was she said that a diverse team means: “a richer source of solutions…. because each person’s tackling of a problem is different…. creating a more creative and stronger community”. The key challenges that building a diverse team, community or industry faces is the “breaking down of stereotypes and most importantly being able to listen”.

First Women on the Moon Breakfast
Man on the moon is so 1969

It is safe to say that, even though I very much look forward to the days yet to come (especially since there will be many talks coming soon on astrobiology and biology), this talk was a highlight because whether it is the field of engineering, science, business or arts, these principles of equality and diversity are translatable to any area of life. It may not have been the most popular or technical talk at the IAC but I personally found it an incredible session as society and teams are what hold projects and progression together.


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The first day of the IAC was action-packed to say the least, with thousands of people ranging from bright-eyed students such as us, to earnest young professionals and finally the fatigued yet robust juggernauts of the space industry.

Thanks to the catering provided by the Adelaide Convention Centre, we were able to network with the very same people mentioned above during the IAC’s welcome reception, filled with delicious finger foods and rich wine from South Australia vineyards. Albeit shy at the start, Anuraj and I managed to start a conversation with Ralph Mcnort, who runs an aerospace laboratory at the renowned John Hopkins University in the states. He discussed everything from his profile of work (which was impressive to say the least) as well as his deep passion for space, which runs in family (his youngest grandchild is studying to be an aerospace pilot). Overall it was a once in a lifetime chance to converse with someone so well-versed in the topic of space.

Boeing Starliner at the IAC2017
Making new friends past Jupiter 2

While IAC day one had the liberty of packing us all into the same room to force us to talk to each other, the second day was much more relaxed, with a multitude of talks (about space obviously) which we could attend. Ben and Nathan woke up bright and early to attend their talk session concerning the exploration of the Moon and space system architectures while I attended a midday session on human physiology in space. This session discussed the effects of short and long term spaceflight on the human body, as well as the countermeasures being developed to mitigate these effects (astronauts not being able to walk after they return to earth is a fairly big problem). There was a lot to cover during this lecture but the finer points included a state of the art MDS system developed by the Russians (essentially a compact, full body gym in space), insights into fascial tissue studies and its relation to reactive jumping and finally a return of ballistocardiography. Don’t worry if you didn’t understand most of the things I listed above, I had a hard time deciphering the PowerPoint presentations myself!

Nathan Kristain in Space Suit
Captain Nathan Kristian Ready for Duty

All in all, the IAC so far has been a riveting experience, due to the conversations with reputable people on day one as well as some intense yet deeply interesting talk sessions about space on day two. I have high hopes for the rest of the week as I attend more events, in particular Elon Musk this coming Friday!

Bill Nye Light Sail
Bill Nye giving us the Birds and the Bees

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The first day of the International Astronautical Congress has passed, and what initially shocked me the most about the conference has now turned into a different feeling. There are about 3500 delegates at this conference, which was most clear during the opening ceremony. The auditorium for the opening ceremony was absolutely enormous – so large that a person presenting at the front appeared no larger than my thumb held at arm’s length from where I sat near the back. Expansive monitors were required to get a good look at the performances and the speakers.

ISS Model
Someday I’m gonna buy my mama a place like this

I entered the auditorium somewhat early, when about one third of the seats were taken. While I knew the number of delegates attending the conference beforehand, seeing them filter in ahead of the opening ceremony and rapidly fill up all available seats was what truly allowed me to understand the size of the number. Indeed, there were so many that many delegates sat in the aisles – not something OH&S would be too thrilled about!

The opening ceremony was marked by spectacular performances and inspiring speeches. However, each of these paled in comparison to the announcement of a national space agency for Australia by the Honorable Simon Birmingham. A cheer rose at this, its volume and length surpassing those one might hear at the cricket. The sheer joy in the delegates’ surrounded us and, for a moment, bound us together as one.

IAC pannel
The judges are set

Afterwards, a break session began, which I spent well collecting colourful brochures from various organisations, such as ArianeSpace and Surrey Satellite Technologies Limited. This was followed by the Heads of Agencies plenary talk, the topic of which was “Business before Science or Science before Business”. Unfortunately, I was unable to attend the undoubtedly interesting plenary talk as I had to make time to practice my own presentation, which I gave at the Space Exploration Overview session. My presentation’s topic was on how it was becoming easier for small organisations to send up small spacecraft, such as CubeSats, to explore the solar system. Despite some nerves, I’m happy to say that I absolutely nailed it.

Taofiq Huq at the IAC2017
Taofiq presenting his work on CubeSat Exploration of the Solar System

As the day progressed in this manner, my awe at the number of attendees progressed into, as I mentioned earlier, a different feeling. This new feeling was awe at myself and my fellow members of GreenSat. The 3500 delegates of the International Astronautical Congress can be considered 3500 of the most prominent members of the global space industry, and here we were, a small subset of a student projects society among them!

By the end of the day, rather than thinking of 3500 as a large number, I began to think on how tightly this number constrained the size of the global space industry. There are obviously tens, if not hundreds of thousands who are directly involved with the space industry globally, but only 3500 of these came to the IAC. And to think our group makes up 10 of this distinguished 3500! It’s truly incredible that we’re able to be here today.

Christopher Miller with Curiosity Rover
Have you seen my Cat?

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Batteries have become one of the most important devices in modern society with the rise in demand for portable devices,  smartphones becoming ubiquitous, and even cars shifting towards battery technology. Batteries are also very important in satellites, storing energy to power the system when solar energy is not available.

Re-chargeable batteries can be charged by forcing an electric current through it. Different types of batteries have different properties when it comes to recharging so this blog will be focusing on lithium ion batteries (4.2V) as BLUEsat will be using these, and because it is one of the most common batteries.

Li-ion batteries have an intercalated lithium compound as electrodes, discharging when Li-ions move from the negative electrode to the positive electrode. Charging forces the ions back onto the negative electrode. Unfortunately, every time ions are shifted some ions react with the electrodes, decreasing the number of free ions in the battery. Overcharging and overheating will also severely reduced battery life and potential damage to the battery. A carefully designed charging process must be used to maintain maximum battery life and battery condition.

Charging a Li-ion battery occurs in two steps:

  • Constant current: the charger applies constant current to the battery. The voltage applied increases steadily towards the voltage limit of the battery. Current should be kept between 0.5 and 1 C to prevent high temperatures.
  • Constant voltage: the charger applies a voltage equal to the voltage limit of the battery, as current declines.

Graph showing current and voltage during lithium-ion battery charging.
The constant current stage quickly charges the battery until battery voltage is about 4V. The charger switches to the constant voltage stage to prevent overcharging. The constant voltage is maintained until current falls to around  0.1C and then should be terminated when the battery reaches full charge. Most modern chargers can detect full charge and stop at this point.

If more than one cell is being charged, then the batteries need to be balanced to the same level of charge before the constant voltage phase. This can be achieved through using a balancing circuit. We are charging our batteries as a single unit at 8.4V.

Thien Nguyen's original lithium-ion battery charging board.

Thien’s battery charging board

Thien, a former member (and ex-president) of BLUEsat, developed this battery charging board a few years ago, but it had not been tested until now. Although the board will be modified slightly – to fix any problems with the circuit and to make it compatible with the new battery unit – the way it’s supposed to work remains the same.

The circuit is built around the LTC4012-1, a battery charger controller chip. In the circuit diagram below, you can see:

  • Connection points for the power source (Vin, top left), battery unit (VBAT, bottom right) and load (Vsup, top right)
  • The LTC4012-1 chip (that whole yellow rectangle)
  • Various resistors and capacitors
  • MOSFETs (Q1, Q2 and Q3) and an inductor (L1)
  • A diode for the BOOST supply

Circuit diagram for a charging board based around the LTC4012-1

 

Let’s take a closer look at each of those categories…

Connection Points

These are the light green screw terminals on the actual circuit board. There are actually two connections on each block because one of them is ground.

Vin would be connected to the solar panels, either directly or, in the case of our CubeSat, through a MPPT converter (see the previous BLUEsat blog post on Maximum Power Point Tracking). VBAT is the connection to our battery unit, which is made up of two lithium-ion batteries in series. Vsup is the output that supplies power, from either the solar panels or the batteries, to the other parts of the satellite.

The LTC4012-1

This chip has many pins, for voltage/current sensing, indicator outputs, forcing shutdown, supplying power to other pins, and more. We will elaborate on its function in the next section of this blog post.

Resistors and Capacitors

The LTC4012-1 can be programmed to control battery charging at a range of voltages and currents, not just the ones suitable for our particular battery unit. To select the required charging voltage and current, we have to connect appropriate resistors to certain pins. The datasheet for the LTC4012-1 provides a helpful circuit diagram for a typical application of the chip, along with suggestions for capacitor values and tables to look up the resistances we need.

Extract from the LTC4012-1 spec sheet showing a typical usage. Extract from the LTC4012-1 charging chip data sheet showing resistances

MOSFETs

The MOSFET (metal oxide semiconductor field effect transistor) is a device that basically acts like a switch. It can either block or allow current flowing from the drain terminal to the source terminal, depending on the voltage at the gate (compared to the source). Note that the symbol used in Thien’s circuit diagram includes a diode between the source and drain, indicating that the MOSFET still allows current to flow from the source to drain.Circuit diagram extract showing MOSFET's

There are different types of MOSFET that can be made. For example, Q1 and Q3 are both enhancement mode MOSFETs which means that the switch is normally off. The directions of the arrows show that Q1 is n-channel, requiring a positive gate voltage to turn it on, while Q3 is p-channel, requiring a negative gate voltage compared to the source.

In Thien’s circuit, Q3 is used to maintain a certain voltage needed between two pins of the chip (DCIN and CLP), and to prevent current flowing backwards from the batteries to the solar panels. Q1 and Q2 are controlled by logic inside the LTC4012-1 to create a PWM waveform that is smoothed out by the inductor (L1). This means that the charging voltage and current won’t be affected by Vin being higher than required.

Diode

Instead of SW, the diode was meant to be connected from INTVDD to BOOST, as shown below:Together with the inside of the chip and charge on the capacitors, this part of the circuit generates a voltage even higher than the chip’s internal 5V supply. This voltage would be needed at the gate of Q1 to turn on the MOSFET when its source terminal is already at a high voltage.

LTC 4012-1

This chip is the one we are using for our battery charging circuit and it provides a good introduction into how lithium-ion battery charging is actually achieved. It is a battery charging chip which is able to provide the constant current and constant voltage features necessary for charging lithium-ion batteries. This chip is also designed specifically for lithium ion batteries as it features precision internal resistors to supply 4.1V per cell. (The LTC 4012-2 provides 4.2V per cell).

The chip achieves constant current by quickly switching on and off the connection to the power supply and averaging the current over time at the desired limit. In constant voltage mode, the switching is intended to maintain a steady voltage rather than a steady current. The current going in to the batteries is determined by the batteries’ own capacity to take in current at this point, and the chip does not directly reduce the current flow anymore.

Current Charging

The MOSFETS in Figure 1 are both n-channel MOSFETs, corresponding to Q1 and Q2 in our circuit. The two MOSFETs are controlled by the two pins, TGATE, and BGATE, which are switched on and off quickly. The FETs are alternately switched on, so that when the top FET is ON, the bottom FET is OFF, and vice versa to ensure that the system power supply Vsup is never shorted to ground. Circuit diagram of a current and voltage switching mechanism from the LTC4012 datasheet

When the top FET is ON and bottom FET is OFF, the system supplies power to the inductor (L1) and the batteries, which increases the current flowing to the batteries (ICHRG). When the FETs are switched (the bottom FET is ON, and top FET is OFF), power is no longer supplied to the inductor (L1). As inductors resist the change in current, the inductor starts discharging and will attempt to keep the current ICHRG relatively constant.

The switching of the FETs is controlled by pulse width modulation (PWM), shown in Figure 2. While the top FET is turned ON, ICHRG is constantly increasing. The value of ICHRG is measured through the voltage drop across the sensing resistor VSENSE. When this voltage is higher than a programmed value, the FETs are switched for a fixed period of time to discharge the inductor, before switching to charge the inductor again via the system power supply.

While in constant current mode, the chip sends an active signal on the Input Current Limiter pin (ICL).

Osiliscope output of PWM switching to control currentVoltage Charging

In voltage charging, the switching of the top FET and the bottom FET are no longer coupled with ICHRG. They are instead controlled by the voltage as measured from VBAT, which is then compared with the value programmed by the pins FVS0 and FVS1. While this voltage is kept constant, the current ICHRG is determined by the amount of current the batteries can absorb, which in turn is dependent on their state of charge. As the batteries approach full capacity, the amount of current they absorb drops towards zero. The chip has no inherent ability to shut off the current going into the batteries. However, it is able to send a signal on its CHRG pin to indicate when current has fallen to a tenth of programmed value. This will let us know that the lithium ion batteries have finished charging.

 


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A view of the curvature of the earth from our most recent high-altitude balloon mission.

 

Things have changed since the glory days of Yuri Gagarin when we first reached for the stars. Now, it is easier than ever to get involved in a space mission.

The High-Altitude Ballooning project at BLUEsat UNSW brings together passionate students from a wide range of degrees to build, test and launch equipment to altitudes over 30km.

Our most recent mission was on the 5th of August, where we sent our custom-built scientific research payload up 33km to gather data on the atmosphere and capture video of the ascent.

So, you’re probably thinking to yourself, how can I do this from home?

Objectives

Do you want to take stunning pictures of the curvature of the earth? Perhaps take a photo of your teddy bear against the backdrop of space?

Our main objective was to study the atmosphere to gain knowledge for future missions. This involved sensors, an onboard computer, and code to log data. We used a Raspberry Pi for computing, with an accelerometer, gyroscope, magnetometer, barometer, temperature probes and humidity sensor recording data to an SD card.

We also captured video and photos with two cameras mounted on our payload.

Tracking

Without knowing where your payload is during flight, you have just launched a glorified party balloon. We used 3 discrete self-contained systems to track our flight, pictured below.

The SPOT GPS Tracker simply transmits and uploads coordinates to a website, where we can view location updates on a map. An APRS radio device, the yellow box, transmits GPS coordinates over amateur radio bands which are uploaded to an online network. The third device is a short-range radio beacon which plays a continuous tune over radio frequencies, allowing us to pinpoint the landing area.

Balloon Tacking Equipment. Left to Right: SPOT Tracker, ARPS, Radio Beacon
Tracking equipment

Separation

Typically, a mission is ended when the balloon bursts, but if you are feeling fancy, you can build a device that can remotely cut down your payload. We have developed a mechanism that runs a current through a piece of nichrome wire, which is wrapped around the rope connecting the balloon to the payload. An on-board radio receiver decodes a termination signal which we can send from the ground, causing the nichrome to heat up and sever the rope.

Approval

Before you can send anything into the air, you must seek approval from CASA, the Civil Aviation Safety Authority. You will also need to perform predictions of your flight using online prediction software, such as that found at HabHub, which will allow you to visualise the estimated flight path and burst altitude based on weather forecasts. The figure below shows a predicted trajectory for our launch.

Prediction of the stratospheric balloon's flight trajectory from Muswellbrook, NSW using HabHub.
Prediction of flight trajectory from Muswellbrook, NSW

Logistics

You now need to hire a tank of helium and drive all your equipment out to your launch site. Make sure you take a toolbox and spare parts with you for any last minute adjustments.

BLUEsat's high-altitude balloon team preparing for the launch.
Final preparations

The Launch

Once you have inflated the balloon with enough helium to attain the desired lift and attached your payload, you are ready for launch. Perform a final flight prediction, check your on-board systems, turn on your cameras and cross your fingers!

The stratospheric balloon being launched by the BLUEsat team from an oval in Muswellbrook, NSW
It’s time to let go…

 

Watch the promotional video of our launch below:

Watch our student-led group BLUEsat launch a high-altitude balloon! Spectacular footage as it reaches an altitude of 33,000 m…. 3 times higher than a commercial aircraft.

Posted by UNSW Engineering on Wednesday, 23 August 2017

 

 

 

If you would like details on anything you have seen in this article, please contact us at info@bluesat.com.au


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MPPT or Maximum Power Point Tracking is a crucial element in any solar powered satellite. We all remember that power = voltage x current, so it makes sense that if you want more power, you increase voltage or current, or both. However this is not the case with solar panels: in fact the maximum output power will DECREASE if you draw more current or voltage, past a certain point. There is a bit of a ‘sweet spot’ where voltage and current peak, this is called the maximum power point (MPP).

Power output and maximum power point of a solar panel.
As can be seen from the graph, if this particular system starts drawing over about 33V at 7A, the net power output starts decreasing.

 

On the ground, the MPP of a solar panel can be calculated and the system can be optimised to draw maximum power. The problem is that the MPP is highly dependent on the temperature of the panel, and in the vacuum of space where temperature differentials from shade to sun can be as high as 300°, this becomes a huge problem.

Enter MPPT, or the ability to track exactly where the maximum power point is, and adjust the system accordingly so we are squeezing every watt of power we can get out of the panels. The way this is achieved is through Pulse Width Modulation. By adjusting the duty cycle of the output, we can effectively control how much current and voltage we draw, and ensure that we are drawing the correct amount in order to be at or sufficiently near the Maximum Power Point.

Block Diagram of the Maximum Power Point Tracking (MPPT) system.

This is a block diagram of the MPPT Implementation we’re currently testing. At the moment, our panels will supply us with about 4.5V. The grey SM72445 box is the brains of the MPPT system, and provided with current and voltage output information, it adjusts the duty cycle of the square wave in our DC/DC converter. After being boosted to 9V, the system can charge all the batteries onboard the satellite and ensure that it never goes without power!