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A portable system for DC and AC magnetic field measurements for magnetic cleanliness applications.

ABSTRACT

Mapping of planetary and interplanetary magnetic fields are important scientific objective in Space Missions. These magnetic field measurements are performed using magnetometers and are typically expected to extend to rather low levels (in the range of a few nT or less). Spacecrafts may have inherent magnetic fields which may mask the naturally occurring fields, so mission success relies on the attainment of an adequate level of magnetic cleanliness at the spacecraft sensor (magnetometers) locations. Therefore, the magnetometers are essential part of the spacecraft and very important in the measurement of magnetic fields in space. They drive the requirements for acceptable magnetic disturbances generated by the spacecraft and its elements.
In all missions utilizing magnetometers magnetic cleanliness becomes a major design goal, which needs to be addressed in the design and development phase of a spacecraft. Magnetic cleanliness is dealing mainly with DC magnetic fields but in some space missions (i.e JuICE) incorporate stringent magnetic cleanliness requirements that require to control both DC and low frequency magnetic fields. The AC frequency range can be from 1 mHz to 250 kHz. Suitable sensors for accurate DC fields are fluxgate magnetometers and for low frequency magnetic field measurement are the fluxgate and search‐coil magnetometers.
In missions susceptible to magnetic fields, magnetic cleanliness is attained throughout an adequate control of material used in the spacecraft, actual tests and prediction models. Typically, magnetic budgets are developed to predict the spurious magnetic field generated by the spacecraft, which is modeled as a set of dipole moments (MDM) representing units and components of the spacecraft. To provide data to model the magnetic field it is common practice to measure the magnetic field of individual spacecraft units and components.
Measurements are typically performed in the far field of the units and then the measurement data is then fitted with a small number of equivalent magnetic dipoles, which generate in the far field the same magnetic field as the units and, then, subsequently recombine the obtained models to get a reliable picture of the fully integrated spacecraft. This is achieved by using suitable prediction tools which simulate the far-field spurious magnetic field generated by the spacecraft in terms of remanent and induced magnetic dipole moments.
For this kind of measurements there are some facilities available. The most established one is the Magnetic Coil Facility (MCF) which employs a rotational measurement of the Unit Under Test (UUT) and are installed in a few laboratories worldwide. Another facility, introduced by EMTECH Space in the frame of a previous ESA study contract utilises multiple magnetometers, namely proto-MMF, which is installed at ESTEC (Noordwijk, The Netherlands). Both these facilities are of fixed installation.
A new, portable measurement system, the Suitcase Multi-Magnetometer Facility (S-MMF), was developed by EMTECH Space in the frame of an ESA study (contract no. 4000124641/16/NL/SC) for magnetic field measurements, useful for spacecraft magnetic cleanliness applications. The system fits in airplane carry-on suitcases, thus providing maximum flexibility and is suitable for in-situ magnetic field measurements for EUTs, payloads, cube-sats or even small-size micro-sats.
A single suitcase includes 2 measurement sensors, tripods, data acquisition units, all the necessary cables and accessories and a smartphone. Two new software applications are available to the user, one is executed in the smartphone and the other one in a desktop (laptop) PC. An overview of the system is provided below (Figure 1).
Starting from the sensors a single suitcase includes two fluxgate magnetometers for DC magnetic field measurements or two search coil magnetometers for AC magnetic field measurements. Both are commercial-of-the-self, state of the art, sensors suitable for the application with very good performance in terms of sensitivity, linearity, orthogonality and alignment errors and intrinsic noise. The sensors are installed on non-magnetic, folded, tripods using mechanical adaptors designed specifically for this system.

Figure 1 – S-MMF overview

Measurement data acquisition is performed by a new synchronous DAQ unit specifically designed for this system in the frame of the ESA study. The new DAQ unit provides power to the magnetometers thus increasing the system portability. Two different DAQ units are available, depending on the sensor powered, one for the fluxgates and one for the search coils.
One of the most critical parameters of the magnetic field measurement is to accurately know the positions of the sensors in respect to the position of the UUT. Typically, the UUT is placed in the center of the test scene and the tripods are placed around it. In order to define the positions of the sensors S-MMF employs a unique photogrammetry algorithm which was developed specifically for this system and is integrated in a smartphone application which controls the measurement procedure. Using the smartphone, the user just takes some photos of the test scene and the algorithm provides an estimation, with adequate accuracy, of the sensors positions with respect to the UUT in the 3D space, which otherwise it would be a very difficult and prone to errors task. To aid the algorithm find the positions of the sensors, special tags are used to mark the sensors and the center of the Tet Coordinate System (TCS). These were placed on sensors housings specifically designed for this system in order to know its exact position in respect to the positions of the inner sensors of the magnetometers. In addition, a reference pad with a checkerboard pattern is also placed in the test scene where the center of the TCS has been defined.
The measurement process is guided through an Android software application developed for this system hosted in an Android smartphone. It integrates the innovative photogrammetry algorithm, which calculates the position of the sensors in the test area based on photos of the test scene taken by the user. Moreover, it communicates with the DAQ units via Bluetooth interface to configure its parameters like the sampling rate, the gain, the DC offset and the measurement duration. The user follows the procedure implicated by the application and take as many measurements of the UUT as necessary. The measurements are stored in an SD card on the DAQ units and are available to the user.
A Windows software application is also available to the user to connect with the DAQ units, retrieve the measurement data together with the positions of the sensors which can be used to feed an MDM solver for modelling purposes. This application is hosted in a desktop computer and provides also measurement data reports for better preview of the results.
S-MMF employs the concept of snapshot measurement for DC magnetic field measurements, which means that measurements are collected simultaneously by all available magnetometers. Typically, DC magnetic field measurement emanating from an EUT is performed in typical environmental conditions, where the Earth’s magnetic field is present and, therefore, this should be removed from the measurement. AC magnetic field measurement requires synchronous data acquisition, higher sampling rate (for higher frequencies) and long measurement time in cases of very low frequencies and long EUT operational cycles. Both measurement processes are supported by the mobile application of S-MMF. A table with the technical specifications of S-MMF is provided below:

Table 1 – suitcase-MMF Specifications (per suitcase)

The system has been successfully validated for DC measurements by comparing model results of a current fed coil performed both in S-MMF and another, fixed location, facility available at EMTECH Space.
To conclude, S-MMF is a portable measurement system to capture the magnetic signature of a UUT and aim test engineers to perform in-situ measurements avoiding UUT transportation and to be capable to provide adequate accuracy, quality of test results and ease of use.

Photo 1 – Suitcase Multi-Magnetometer Facility

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