scholarly journals Avoiding non-linearity of optically pumped magnetometer MEG within an actively shielded two-layer mu-metal room

2021 ◽  
Vol 7 (2) ◽  
pp. 543-546
Author(s):  
Tilmann H. Sander ◽  
Urban Marhl ◽  
Vojko Jazbinšek
Keyword(s):  
Sensor Array ◽  
Open Loop ◽  
Linear Range ◽  
Periodic Signal ◽  
Optically Pumped ◽  
Active Compensation ◽  
Linearity Range ◽  
Non Linear ◽  
Magnetically Shielded Room

Abstract Some optically pumped magnetometer (OPM) sensors available for biomagnetic investigations have a linear range limited to +- 1 nT due to the specific properties of their open loop operation. In a two-layer magnetically shielded room of type Ak3b/Vacoshield Advanced with an external active compensation we studied how much sensor movement is allowed until amplitudes exceed the linearity range. Intentional movements were performed by a subject wearing an OPM-MEG sensor array. It was found that movements of 8 cm did yield non-linear amplitudes, but a reduction of the movement in half already preserves linearity. Despite movements, the heartbeat was found to generate a periodic signal, although the generating mechanism could not be identified so far.

scholarly journals Towards the non-zero field cesium magnetic sensor array for magnetoencephalography

2021 ◽  
Author(s):  
Mikhail Petrenko ◽  
Sergei Dmitriev ◽  
Anatoly Pazgalev ◽  
Alex Ossadtchi ◽  
Anton Vershovskii
Keyword(s):  
Information Transfer ◽  
Sensor Array ◽  
Response Speed ◽  
Magnetic Sensors ◽  
Zero Field ◽  
Optically Pumped ◽  
Noise Spectral Density ◽  
Information Transfer Rate ◽  
Field Sensor ◽  
Better Than

Magnetic sensors developed for application in magnetoencephalography must meet a number of requirements; the main ones are compactness, sensitivity and response speed. We present a quantum optically pumped atomic sensor with cell volume of 0.5cm<sup>3</sup> that meets these requirements and is operable in nonzero magnetic fields. The ultimate sensitivity of the sensor was estimated as (using the criteria of the ratio of the slope of the magnetic resonance signal to the shot noise spectral density) to be better than 5 fT/Hz<sup>1/2</sup>. The actual sensitivity, measured in a gradiometric scheme, reaches 13 fT/Hz<sup>1/2 </sup>per sensor. We also present a novel and fast algorithm for optimization of the geometric properties of non-zero field sensor array with respect to maximization of the information transfer rate for cortical sources.<br>

An L1 adaptive closed-loop guidance law for an orbital injection problem

2009 ◽  
Vol 223 (6) ◽  
pp. 753-761 ◽  
Author(s):  
J Roshanian ◽  
M Zareh ◽  
H H Afshari ◽  
M Rezaei
Keyword(s):  
Closed Loop ◽  
Adaptive Strategy ◽  
Analytical Form ◽  
Open Loop ◽  
Control Input ◽  
Guidance Law ◽  
Non Linear ◽  
Non Linear System ◽  
Orbital Injection

The current paper presents the determination of a closed-loop guidance law for an orbital injection problem using two different approaches and, considering the existing time-optimal open-loop trajectory as the nominal solution, compares the advantages of the two proposed strategies. In the first method, named neighbouring optimal control (NOC), the perturbation feedback method is utilized to determine the closed-loop trajectory in an analytical form for the non-linear system. This law, which produces feedback gains, is in general a function of small perturbations appearing in the states and constraints separately. The second method uses an L1 adaptive strategy in determination of the non-linear closed-loop guidance law. The main advantages of this method include characteristics such as improvement of asymptotic tracking, guaranteed time-delay margin, and smooth control input. The accuracy of the two methods is compared by introducing a high-frequency sinusoidal noise. The simulation results indicate that the L1 adaptive strategy has a better performance than the NOC method to track the nominal trajectory when the noise amplitude is increased. On the other hand, the main advantage of the NOC method is its ability to solve a non-linear, two-point, boundary-value problem in the minimum time.

scholarly journals On-scalp MEG system utilizing an actively shielded array of optically-pumped magnetometers

2018 ◽  
Author(s):  
Joonas Iivanainen ◽  
Rasmus Zetter ◽  
Mikael Grön ◽  
Karoliina Hakkarainen ◽  
Lauri Parkkonen
Keyword(s):  
Spin Exchange ◽  
Dynamic Control ◽  
Sensor Arrays ◽  
Low Frequency ◽  
Sensor Calibration ◽  
Optically Pumped ◽  
Magnetic Shields ◽  
Brain Responses ◽  
Shielding Factor ◽  
Magnetically Shielded Room

AbstractThe spatial resolution of magnetoencephalography (MEG) can be increased from that of conventional SQUID-based systems by employing on-scalp sensor arrays of e.g. optically-pumped magnetometers (OPMs). However, OPMs reach sufficient sensitivity for neuromagnetic measurements only when operated in a very low absolute magnetic field of few nanoteslas or less, usually not reached in a typical magnetically shielded room constructed for SQUID-based MEG. Moreover, field drifts affect the calibration of OPMs. Static and dynamic control of the ambient field is thus necessary for good-quality neuromagnetic measurements with OPMs. Here, we describe an on-scalp MEG system that utilizes OPMs and external compensation coils that provide static and dynamic shielding against ambient fields.In a conventional two-layer magnetically shielded room, our coil system reduced the maximum remanent DC-field component within an 8-channel OPM array from 70 to less than 1 nT, enabling the sensors to operate in the sensitive spin exchange relaxation-free regime. When compensating field drifts below 4 Hz, a low-frequency shielding factor of 22 dB was achieved, which reduced the peak-to-peak drift from 1.3 to 0.4 nT and thereby the standard deviation of the sensor calibration from 1.6% to 0.4%. Without band-limiting the field that is compensated, a low-frequency shielding factor of 43 dB was achieved.We validated the system by measuring brain responses to electric stimulation of the median nerve. With dynamic shielding and digital interference suppression methods, single-trial somatosensory evoked responses could be detected. Our results advance the deployment of OPM-based on-scalp MEG in lighter magnetic shields.

scholarly journals Magnetic Field Compensation Coil Design for Magnetoencephalography

2021 ◽  
Author(s):  
Hermann Sonntag ◽  
Christian F. Doeller ◽  
Jens Haueisen ◽  
Burkhard Maess
Keyword(s):  
Magnetic Field ◽  
Head Movements ◽  
Side Length ◽  
Coil Design ◽  
Optically Pumped ◽  
Active Suppression ◽  
Highly Sensitive ◽  
Movement Artifacts ◽  
Operational Range ◽  
Magnetically Shielded Room

Abstract While optically pumped magnetometers (OPMs) can be attached to the head of a person and allow for highly sensitive recordings of the human magnetoencephalogram (MEG), they are mostly limited to an operational range of approximately ±5 nT. Consequently, even inside a magnetically shielded room (MSR), movements in the remnant magnetic field disable the OPMs. Active suppression of the remnant field utilizing compensation coils is therefore essential. We propose 8 compensation coils on 5 sides of a cube with a side length of approximately 2 m which were optimized for operation inside an MSR. Compared to previously built bi-planar compensation coils, the coils proposed in this report are more complex in geometry and achieved 10 times smaller errors for simulated compensation fields. The proposed coils will allow for larger head movements or smaller movement artifacts in future MEG experiments compared to existing coils.

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