Magnetorquers as ADC systems

BLUEsat’s ADCS (Attitude Determination and Control Systems) team is currently designing a system to detumble a CubeSat (small satellite), which is to say that it will stop the CubeSat from spinning around when it’s in orbit. Magnetorquers are ideal for this, as they are small, light and consume minimal power. But what are they and how do they work?

What is a Magnetorquer?

Magnetorquers aren’t new. They’ve been around since the 1970s when engineers were searching for a way to control the attitude of a satellite and the design really hasn’t changed much since then. The base components are three electromagnets, a magnetometer and a microcontroller. The electromagnets are typically solenoids oriented at right angles to each other. Solenoids produce magnetic fields when a current is passed through them and so, by changing the amount of current in each coil, a magnetic field can be produced in any direction with varying strength. The magnetometer simply measures the strength of the magnetic field around it. The microcontroller is a small computer that controls the output of the solenoids and when the magnetometer takes measurements.

A photo of a commercial magnetorquer with two coils visible and a PCB.
A magnetorquer, showing two of the three solenoids

How do they work?

The aim of an ADC system is to change the orientation of a satellite in space and a magnetorquer can do this when a satellite is in orbit around Earth by interacting with the Earth’s magnetic field. As the satellite moves in the magnetic field, from its point of view, the magnetic field is varying. The magnetic fields produced by the solenoids interact with Earth’s magnetic field to move the satellite. If the magnetic field generated by the solenoids oppose Earth’s magnetic field it will slow down the satellite’s motion in that direction and if the magnetic fields align the satellite will move faster in that direction. The magnitude of the satellite’s acceleration depends on the magnitude of the generated magnetic field.

An engineer would write a program to control the direction and magnitude of the magnetic fields generated by the solenoids in response to the measurements of Earth’s magnetic field. A common example is the B-dot algorithm which is used for detumbling, but it is by no means the only algorithm. Magnetorquers can be used for both detumbling and pointing, it simply depends on the program uploaded on to the microcontroller. This is where most of the recent research regarding magnetorquers is.

BLUEsat’s Magnetorquer

BLUEsat has just completed the prototype of a magnetorquer that can detumble in one direction (pictured above). It was constructed on a prototyping board after some issues with PCBs and uses an air-cored solenoid as the electromagnet. We’ve also taken advantage of existing modules, such as motor drivers, in order to speed up production. All we need to do now is to plug in a battery and test it out!

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