Charles' Labs - ECE3SAT - Helmholtz Coils

Électronique - 11-02-2018

Context

Since the beginning of the space race in 1957, the number of objects sent into orbit is continuously growing, as does the amount of space debris orbiting the Earth. This is becoming a real threat for operational space missions around the Earth. Space debris can be the result of:

A collision between two satellites, two debris or a satellite and a debris/meteoroid
A battery which became unstable and exploded
Fuel leftovers in a satellite or a launcher stage which became unstable and exploded
A planned destruction
An out of control satellite or a launcher stage

Today, the population of space debris is estimated to be more than 500 000 trackable objects where 20 000 of them are bigger than a tennis ball. In addition, there are millions of pieces too small to be detected.

The vast majority of space debris is located in Low Earth Orbit (LEO) where most space missions are located or planned. Figure 1 illustrates the distribution of debris around the Earth in 2013.

Representation of the distribution of the space debris in LEO in 2013. Source: ESA.

The ECE3SAT project is a student project developed at the french engineer school, ECE Paris. The goal of the project is to send a CubeSat in space to verify a physical theory permitting a fast deorbiting.

A CubeSat (1U-class spacecraft) is a nanosatelite satellite for space research that is made up of multiples of 10x10x11.35 cubic units, with a weight less than 1.33 kilograms. CubeSat are most commonly put in low Earth orbit by deployers on the International Space Station (ISS).

The Attitude Determination and Control System (ADCS) is focused on the control of the rotating motion of the satellite. Through sensors it will determine the attitude and act using it’s environment to reach the required orientation.

To test and validate the ADCS of the CubeSat, the team decided to build a device that can generate a steady and controlled magnetic field: a Helmholtz coils magnetic simulation environment.

What are Helmholtz coils?

The CubeSat uses the geomagnetic field (the Earth's magnetic field) for two important tasks: detecting its orientation, using magnetometers, and rotate, with magnetorquers. Magnetorquers are coils, in the nanosatelite, that can generate a torque, according to this formula:

Equation of the magnetorquers generated torque.

The Helmholtz coils are two separate coils facing each other like described in the scheme below.

Helmholtz coils pair arrangement. By Ansgar Hellwig, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=193184

This combination can generate a magnetic field between the two coils that is uniform on one axis. Here is a spatial representation of this magnetic field on a plane orthogonal to the coils.

Magnetic field in a Helmholtz coils pair.

The norm of this field depend of the radius of the coils, the number of windings and, more importantly, the intensity of the electric current. This means that, by controlling the amperage, we can chose the magnitude of the magnetic field. In order to achieve a three dimensionnal magnetic field, we use three pairs of Helmholtz coils, orthogonal to each other.

By building a device like this, it is possible to generate a magnetic field with a control on the norm and direction using the current. Thus we can:

Test and calibrate magnetometers in a controlled environment.
Test and validate the sizing of the magnetorquers
Integrate those two in a CubeSat model to test our filtering and positioning algorithms
Build a visual representation of the CubeSat progress.

Video of the Helmhotlz Magnetic Simulation System

Sizing

These were our design specifications:

Requirement	Value
Volume of the constant filed zone	64L (40x40x40cm)
Output magnetic field range	-5 to 5 Gauss
Input voltage range (per coil)	20V to 60V
Max. input current (per coil)	3A
Coils equivalent series resistance (ESR)	22.5Ω
Time of assembly and disassembly	Less than 5 min.
Maximum variation of the field	< 10µT

To achieve this, we used the Wolfram Square Helmholtz Coils demonstration by Peter Euripides. This is a simulation with 80 windings, with 80 cm square Helmholtz coils at 1.5 A.

Wolfram simulation of the square Helmholtz coils.

With this configuration, we get an amplitude of 5 Gauss (five times the geomagnetic field in each direction). By putting a current of 3A per coil instead, this rises to 10 Gauss.

Design & Construction

Mechanical build

As discussed earlier, we chose the following dimensions for our simulator: 80cm square coils with 80 turns. This adds up to about 256m of copper per coil. The wire used for the windings is 0.5mm (AWG 24) diameter copper enameled magnet wire.

To wind these coils, the frame is made out of aluminum profile, U-shaped. The outside width of the profile is 7.5mm, which is enough to store about 140 turns of our wire. It is also fairly rigid and inexpensive.

In order to ensure that the assembly and disassembly time meet our constraints, we designed a 3D-printed clip that attaches the 6 coils of our Helmholtz simulator together and makes a rigid cage.

3D-printed clip that attaches the structure together.

Electronics

A circuit is required to control the current that goes through the 6 coils and therefore the magnitude and direction of the magnetic field.

Circuit of the Helmholtz Magnetic Simulation System.

The subsystems are the following:

Logic computational unit: this board drives the power modules according to the software that is loaded on the micro-controller
Power H-Bridge: these modules control the amount and the direction of the current that goes through each coil

Power H-bridges

This is the circuit we designed for each these modules:

Circuit of the H-bridges.

Using the open-source CAD software Kicad, we designed a compact, single-sided PCB to build 6 identical modules.

PCB layout of the H-bridges.

Logic unit

The purpose of this board is to run the software and drive the H-bridges. It is based around an Arduino Nano with an Atmel ATMEGA328P mainly because of the cost and the simplicity of the programming tool-chain compared to other more powerful micro-controllers.

We also used Kicad to design a PCB to layout the connectors to the LEDs and the power modules.

PCB layout of the logic unit.

Software

To program the Helmholtz coils, the user can simply provide a three dimensional waveform of the desired magnitude of the magnetic field. The software reads the arrays and handles all of the low level tasks: calculation of the PWM duty cycle, changing the pins’ states, etc. There are also built-in safety procedures to avoid the sudden collapse of the magnetic field, which might be dangerous for the user and the electronics.

Results

Helmholtz Magnetic Simulation System fully assembled.

To validate our simulator, it is necessary to check two aspects of the magnetic field it generates:

The constancy of it with a fixed input current on each of the three axes
The full control of the magnitude of the field on each of the three axes

Constancy of the field

To make sure that the field in the working volume is constant, we ran several series of tests. For each of the axes, a constant current is fed into the pair of coils, and a sensor is moved within the cage.

For instance, this capture was taken during a back and forth movement of the sensor along the X axis, while the X axis pair of coils was powered to reach 2.15 Gauss (215 µT).

Graph of the variation of the magnetic field in the X axis.

The field is very constant: within 0.06 Gauss of variation in the X axis. This matches our specifications.

With the same test performed on the three axes, we validated the constant nature of the magnetic field in our Helmholtz simulation environment. This is a very important result as it proves that the coils are well wound, that the electronics manages to power them with a constant current, and that any testing we will later do in the simulator is meaningful no matter where it is placed in the working volume.

Control of the field

After it is proved that the field is constant within the working zone, we ran another set of test to check the control that we have on the generated field.

The experimental protocol is the following: a description of several magnetic field waveforms is done in the software, and a sensor is placed in the Helmholtz coils. If the measured waveform corresponds to the one described in software, it proves that we have control over the magnetic field.

For instance, we described in the software triangle waveform for the magnitude of the X axis magnetic field. This waveform has an amplitude of 2.15 gauss (215 µT) and a period of 7.5s.

Graph of the control of the magnetic field.

As shown by this capture, the waveform corresponds to the one we described in software. With the testing of the magnetic field with different waveforms in each of the three axes, we validated our control of the Helmholtz simulation environment.

Ressources

By popular demand, I added an archive containing Helmholtz coil ressources (more design details, code, electronic schematics, etc.). Click on this button to download it:

Download the archive

Notice: the content of this archive is provided "as-is", under the terms of the CC0 1.0 Universal (CC0 1.0) Public Domain Dedication license.

Auteur : Charles Grassin

What is on your mind?

#1 ilya
May I know what frequency did you operate?
on November 7 2018, 13:15
#2 Alyanna
Hi! We are currently working on this project and I saw this. If I may ask, what is the advantage of driving a pair of coil separately rather than driving them in series? Do you have a current sensor for feedback at the h bridge to regulate the current? Also, at what frequency did you operate? Thank you!
on November 7 2018, 19:22
#3 Charles
Hi Alyanna and Ilya! Below are the responses to your questions:

Q1: If I may ask, what is the advantage of driving a pair of coil separately rather than driving them in series?
-> A pair of coils should have the exact same current running through them to generate a uniform field. Therefore, it is actually better to run them in series. To power the two coils in series, you need double the voltage. However, the 3 power supplies we had were limited to 35 volts but could provide up to 10A. By wiring each pair in parallel, we get the 1.5A we wanted with 30V (following ohm's law). The drawback of this solution is that we needed one H bridge per coil (6 instead of 3) and we had to make sure the current was equal in each pair.

Q2: Do you have a current sensor for feedback at the h bridge to regulate the current?
-> No we did not. Because our power supplies were always set to 30V, we could do a simple calculation in the code to get the output current for a given PWM value : current=input_voltage*PWM_duty_cycle/coil_resistance. This worked perfectly. However, adding real current feedback would be very easy with a ACS712 Arduino current sensor module, for instance!

Q3: Also, at what frequency did you operate?
-> I am not sure of what frequency you are talking about. The Helmholtz coils operate on constant current to generate a field. With constant 1.5A, we get constant 2.5 gauss. We did do some variation of the current over long periods to simulate a rotating field (see "Control of the field"). If you are talking about the PWM frequency, it is about 500Hz on the Arduino nano, but it does not really matter.
Good luck!
on November 7 2018, 20:53
#4 Alyanna
Thank you so much for this.
Our supply rating is 32V/6A. I'm planning on driving the coil in series with a maximum current of 3A. Will this work? The voltage at each coil would be 15V maximum.
on November 10 2018, 6:08
#5 Charles
This question has an easy answer: the ohms law gives you the current for a given voltage!
I=U/R
This means that the answer to your question depend on the resistance of your coils. Their resistance depend on the conductor length and diameter (http://hyperphysics.phy-astr.gsu.edu/hbase/electric/resis.html).
In our case, we got 22 ohms per coil. For two coils in series, my previous formula gives:
I=30/(22+22)=0.7A
A 30V supply would not work with our coils.
However, you can easily make it work by lowering the resistance of the coil. If you look at the formula for resistance, you have two ways to achieve this:
* Take bigger diameter wire -> not a good idea, copper is both expensive and heavy!
* Make the coils smaller -> this is the solution ! Because the Helmholtz coils magnetic field formula (https://en.m.wikipedia.org/wiki/Helmholtz_coil) also depend on the size, making them smaller will also increase the field, therefore requiring less turns to get the field you want, meaning the the resistance gets even lower! All in all, this mean making the coils a bit smaller will reduce their resistance a lot!
I will let you do the math with the ohm's law, the wire resistance formula and the Helmholtz coils formula.
on November 10 2018, 8:58
#6 Ned
I'm interested in doing a similar thing on a smaller scale. Do you know if the PWM of the H-bridges causes the current in the coils to vary at the PWM frequency? In other words, the average current is variable by choosing the PWM duty cycle, but the instantaneous current oscillates around the average value at the frequency of the PWM. Am I correct about that? p.s. - Great page describing your project.
on November 19 2018, 1:51
#7 Charles
Thank you Ned! Building this on a smaller scale is definitely a good idea !
You are absolutely right: when powering a large inductive load like these coils with a PWM signal, one would expect large voltage spikes to occur (because an inductor resists the change of current). I have not actually mesured the current output with precise test equipement, however, I have measured the magnetic field. I have not seen *any* magnetic fluctuations or spikes. At the time, my unproven hypothisis was that the energy stored in the magnetic field (1/2*L*I^2) averages the PWM wave. Because everything behaved exactly as exepected, I have not done further investigation. Unfortunatly, I don't have the equipement to try this now.
If you do more testing, I would love to hear your results!
Hope this helped,
Charles
on November 19 2018, 17:32
#8 Carlos
Hello, I have a question. Is there a specific reason to produce a 10 Gauss magnetic field? Thank you!
on March 5 2019, 14:48
#9 Charles
Hi Carlos! This system was built to test magnetorquers, which create a weak torque. Beeing able to output +/- 5 gauss (10 times the Earth's magnetosphere in every direction) gives us about 10 times more force than what we would experience in low earth orbit. This helps us overcome friction in our test environnement (that we obviously would not have in space). It gives us more freedom.
The practical minimum would be +/- 1 gauss to be able to cancel the Earth magnetic field and output an opposing one.
on March 5 2019, 18:05
#10 Melvyn
Hi Charles,
How did you get do to generate a negative magnetic field and so cancel the Earth magnetic field ?
Thank you
on March 19 2019, 9:06
#11 Charles
Hi Melvyn! Generating a magnetic field that cancel the Earth's is simple. The first step is to measure the field (with a LSM303C sensor or a smartphone for instance). Then a command to generate an equal and opposite field can be sent to the coils. I wrote my code so it takes a 3 dimensions field in input and calculate the current it needs to output in each pair of coils. This could be automated in the software so that, on startup, it does this by itself with a magnetometer.
The important part is that the coils must be able to generate at least -0.5 to +0.5 gauss in each direction.
on March 20 2019, 6:44
#12 Melvyn
Hi Charles,
Thank you for your quick answer.
The thing I don't get is how could we generate this negative magnetic field ? with negative input voltage to the coils ?
Also, I wanted to know which software are you using to automate your process ?
Have a nice day
on March 20 2019, 8:45
#13 Charles
Melvyn,
The magnetic field should not necessarily "negative" as the signs are completely arbitrary, it must just opposite to the Earth's one.
For instance, if you measure the magnetic field to be +0.5 gauss in the X axis, the X axis coils need to be generating a -0.5 gauss field to compensate for it. If you measure the field to be 0.25 in the X axis and -0.25 in the Y axis (i.e. the Helmholtz system is at a 45° angle to the magnetic north), you need to output -0.25 gauss in the X axis and 0.25 in the Y axis. The magnitude and sign of the generated field are determined by the direction and value of the *current* in the coils. Refer to the Wikipedia article of the Helmholtz coils to view the equation that relates the current to the magnetic field. Hence, the coils are driven with an H-bridge.
The coils are controlled with an Arduino board. It can read sensors and control the coils through the H-bridges. The software is Arduino code that I wrote for this application.
This project turned out to be very popular. I will release more details and an archive with all the design data (code, CAD, schematics, etc.) this week.
on March 20 2019, 10:19
#14 Melvyn
Hi Charles,
Thank you for your accurate answers.
I am really looking forward to looking into your details and archive for this project
Have a nice day
on March 21 2019, 8:53
#15 Nicolas
Hi! Will you release the codes and design data you chose for your set up please ?
on March 26 2019, 14:06
#16 Charles
Melvyn, Nicolas, I added a download button for the archive at the end of this article. I hope that you will find some useful data there.
on March 27 2019, 21:35
#17 Melvyn
Thank you Charles!!!
on March 28 2019, 9:36

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