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A Matter of Science: Helmholtz Coils in Earth Observation

11 Jun 2021, 13:10 UTC
A Matter of Science: Helmholtz Coils in Earth Observation

Earth observation is a complex and time-consuming process. Its complexity stems not only from the payload reaching orbit by launch vehicles but first in the production and testing of cameras. Due to the difference in physical conditions in space and on Earth, it is necessary to create special test facilities. The Helmholtz coil is among the important devices needed for testing the cameras.
Basic principles of Helmholtz coil
The Helmholtz coil, named after the German physicist Hermann von Helmholtz, is composed of two identical parallel electromagnetic coils that are placed symmetrically along a common axis. Passing through both high-frequency coils in the same direction, the current creates a magnetic field between them with a high degree of uniformity in all three dimensions. This uniform field occupies cylindrical space with a radius of 25% of the coil’s radius (R) and a length 50% of the distance between the coils.
High-frequency Helmholtz coils can be one-, two- and three-dimensional. Multiaxial magnetic coils generate magnetic fields in all directions of three-dimensional space within the Helmholtz pair. High-frequency Helmholtz coils are most frequently round in shape. Square Helmholtz coils are also common in use, due to the simplicity of construction compared to round ones.
The use of Helmholtz coils
Helmholtz coils are used in the creation of magnetic fields, as well as in the cancellation of external magnetic fields, such as the Earth’s. Other areas of use of Helmholtz coils are:

Experimental testing of control algorithms for the spacecraft altitude control  system
Experimental tasks and research work (for example, studying the magnetic properties of substances)
Calibration of magnetic induction sensors, magnetometers
Magnetization / demagnetization of permanent magnets
Demagnetization of materials (workpieces, parts, tools)
Construction of installations for testing magnetic particles (detection of defects in ferromagnetic products)
Helmholtz coils in Dragonfly Aerospace laboratory in Ukraine

Helmholtz coils in Dragonfly Aerospace
Dragonfly’s laboratory in Ukraine has developed a three-coordinate system of Helmholtz coils. We use the Helmholtz coil system to calibrate magnetometers, align and measure the characteristics of electromagnets of our own production, as well as to simulate the strength of the Earth’s magnetic field during fine-tuning of the spacecraft attitude control system. The use of our own test facilities based on Helmholtz coils allows us to:

significantly reduce measurement costs associated with the production of proprietary components for spacecraft, such as electromagnets
promptly perform the necessary attitude control system tests on-site
train our own personnel to work with the test bench
conduct training for students in the space industry
receive orders for equipment testing

Dragonfly’s control unit of Helmholtz coils
Dragonfly‘s team has developed a control unit for the coil system, as well as special software. There is no parallel for such a system. Using this control unit, Dragonfly’s Helmholtz coil system automatically generates magnetic fields, simulating field parameters for given orbits.
The intensity of the magnetic field can reach +/- 200,000 nT with an accuracy of <1% at the center of uniformity and a uniformity diameter of 600 mm. The magnetic field coils can be calibrated and then adjusted statically or dynamically. The overall dimensions of Dragonfly’s Helmholtz coil system are 2350 * 2050 * 2050 mm. These dimensions enable experimental testing of spacecraft control algorithms not only at the level of attitude control system components, but also in combination with a microsatellite weighing up to ~200 kg.
The parameters of the orbit are set using a special software – control system. The control system for Helmholtz coils is based on a web interface that allows:

Remote control of Helmholtz coils from any device that supports Chrome browser or equivalent;
Mode selection (using IGRF model or manual control)
Visualization of test bench results in real time, as well as analysis of the test history, execution commands, etc.
Use of existing spacecraft models to test or creation of spacecraft with unique orbital parameters
Extended adjustment of Helmholtz coils in manual mode

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