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Ablation analysis to select wearable sensors for classifying standing, walking, and running

Summary

The field of human activity recognition (HAR) often utilizes wearable sensors and machine learning techniques in order to identify the actions of the subject. This paper considers the activity recognition of walking and running while using a support vector machine (SVM) that was trained on principal components derived from wearable sensor data. An ablation analysis is performed in order to select the subset of sensors that yield the highest classification accuracy. The paper also compares principal components across trials to inform the similarity of the trials. Five subjects were instructed to perform standing, walking, running, and sprinting on a self-paced treadmill, and the data were recorded while using surface electromyography sensors (sEMGs), inertial measurement units (IMUs), and force plates. When all of the sensors were included, the SVM had over 90% classification accuracy using only the first three principal components of the data with the classes of stand, walk, and run/sprint (combined run and sprint class). It was found that sensors that were placed only on the lower leg produce higher accuracies than sensors placed on the upper leg. There was a small decrease in accuracy when the force plates are ablated, but the difference may not be operationally relevant. Using only accelerometers without sEMGs was shown to decrease the accuracy of the SVM.
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Summary

The field of human activity recognition (HAR) often utilizes wearable sensors and machine learning techniques in order to identify the actions of the subject. This paper considers the activity recognition of walking and running while using a support vector machine (SVM) that was trained on principal components derived from wearable...

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Ankle torque estimation during locomotion from surface electromyography and accelerometry

Published in:
2020 8th IEEE Intl. Conf. on Biomedical Robotics and Biomechatronics, BioRob, 29 November - 1 December 2020.

Summary

Estimations of human joint torques can provide quantitative, clinically valuable information to inform patient care, plan therapy, and assess the design of wearable robotic devices. Standard methods for estimating joint torques are limited to laboratory or clinical settings since they require expensive equipment to measure joint kinematics and ground reaction forces. Wearable sensor data combined with neural networks may offer a less expensive and obtrusive estimation method.We present a method of mapping joint torque estimates obtained from motion capture and ground reaction forces to wearable sensor data. We use several different neural networks to learn the torque mapping for the ankle joints during standing, walking, running, and sprinting. Our results show that neural networks that consider time (recurrent and long short-term memory networks) outperform feedforward network architectures, producing results in the range of 0.005-0.008 N m/kg mean squared error (MSE) when compared to the inverse dynamics model on which it was trained. As a point of reference, the typical measurement errors from inverse dynamics models are in the range of 0.0004-0.0064 N m/kg MSE. Errors tended to increase with locomotion speed, with the highest errors during sprinting and the lowest during standing or walking. Future work may investigate model generalizability across sensor placements, subjects, locomotion variants, and usage duration. The proposed method relies on learning from a motion capture dataset, but once the model is built, the method uses wearable sensors that enable torque estimation without the motion capture data. These methods also have potential uses for the design and testing of wearable robotic systems outside of a laboratory environment.
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Summary

Estimations of human joint torques can provide quantitative, clinically valuable information to inform patient care, plan therapy, and assess the design of wearable robotic devices. Standard methods for estimating joint torques are limited to laboratory or clinical settings since they require expensive equipment to measure joint kinematics and ground reaction...

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Large-format Geiger-mode avalanche photodiode arrays and readout circuits

Published in:
IEEE J. Sel. Top. Quantum Electron., Vol. 24, No. 2, March/April 2018, 3800510.

Summary

Over the past 20 years, we have developed arrays of custom-fabricated silicon and InP Geiger-mode avalanche photodiode arrays, CMOS readout circuits to digitally count or time stamp single-photon detection events, and techniques to integrate these two components to make back-illuminated solid-state image sensors for lidar, optical communications, and passive imaging. Starting with 4 × 4 arrays, we have recently demonstrated 256 × 256 arrays, and are working to scale to megapixel-class imagers. In this paper, we review this progress and discuss key technical challenges to scaling to large format.
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Summary

Over the past 20 years, we have developed arrays of custom-fabricated silicon and InP Geiger-mode avalanche photodiode arrays, CMOS readout circuits to digitally count or time stamp single-photon detection events, and techniques to integrate these two components to make back-illuminated solid-state image sensors for lidar, optical communications, and passive imaging...

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Flexible glucose sensors and fuel cells for bioelectronic implants

Published in:
IEEE 60th Int. Midwest Symp. on Circuits and Systems, MWSCAS, 6-9 August 2017.

Summary

Microfabrication techniques were developed to create flexible 24 um thick glucose sensors on polyimide substrates. Measurements of the sensor performance, recorded as voltage potential, were carried out for a range of glucose concentrations (0 – 8 mM) in physiological saline (0.1 M NaCl, pH 7.4). The sensors show rapid response times (seconds to stable potential) and good sensitivity in the 0 – 4 mM range. Additionally, we demonstrate that the sensors can operate as fuel cells, generating peak power levels up to 0.94 uW/cm2. Such flexible devices, which can be rolled up to increase surface area within a fixed volume, may enable ultra-low-power bio-electronic implants for glucose sensing or glucose energy harvesting in the future.
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Summary

Microfabrication techniques were developed to create flexible 24 um thick glucose sensors on polyimide substrates. Measurements of the sensor performance, recorded as voltage potential, were carried out for a range of glucose concentrations (0 – 8 mM) in physiological saline (0.1 M NaCl, pH 7.4). The sensors show rapid response...

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Covariance estimation in terms of Stokes parameters with application to vector sensor imaging

Published in:
2016 Asilomar Conf. on Signals, Systems and Computers, Asilomar 2016, 6-9 November 2016.

Summary

Vector sensor imaging presents a challenging problem in covariance estimation when allowing arbitrarily polarized sources. We propose a Stokes parameter representation of the source covariance matrix which is both qualitatively and computationally convenient. Using this formulation, we adapt the proximal gradient and expectation maximization (EM) algorithms and apply them in multiple variants to the maximum likelihood and least squares problems. We also show how EM can be cast as gradient descent on the Riemannian manifold of positive definite matrices, enabling a new accelerated EM algorithm. Finally, we demonstrate the benefits of the proximal gradient approach through comparison of convergence results from simulated data.
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Summary

Vector sensor imaging presents a challenging problem in covariance estimation when allowing arbitrarily polarized sources. We propose a Stokes parameter representation of the source covariance matrix which is both qualitatively and computationally convenient. Using this formulation, we adapt the proximal gradient and expectation maximization (EM) algorithms and apply them in...

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HF vector sensor for radio astronomy: ground testing results

Summary

The radio sky below ~10 MHz is largely unexplored due to the inability of ground-based telescopes to observe near or below the ionospheric plasma frequency, or cut-off frequency. A space-based interferometric array is required to probe the portion of the electromagnetic (E-M) spectrum below 10 MHz with sufficient angular resolution and sensitivity to be scientifically useful. Multi-spacecraft constellations scale quickly in cost and complexity as the number of spacecraft increases, so minimizing the number of required spacecraft for an interferometric array (while maintaining performance) is critical for feasibility. We present the HF (High Frequency, 3 to 30 MHz) Vector Sensor as a high performance spacecraft instrument in a future space-based interferometric array. The HF Vector Sensor is composed of three orthogonal dipoles and three orthogonal loop antennas with a common phase center. These six elements fully measure the E-M field of incoming radiation. We present the design of two prototype HF Vector Sensors, ground-based data collection at frequencies above the ionospheric cut-off, and imaging results using several different algorithms.
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Summary

The radio sky below ~10 MHz is largely unexplored due to the inability of ground-based telescopes to observe near or below the ionospheric plasma frequency, or cut-off frequency. A space-based interferometric array is required to probe the portion of the electromagnetic (E-M) spectrum below 10 MHz with sufficient angular resolution...

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Liquid crystal uncooled thermal imager development

Published in:
SPIE, Vol. 9974, Infrared Sensors, Devices, and Applications VI, 28 August 2016.

Summary

An uncooled thermal imager is being developed based on a liquid crystal (LC) transducer. Without any electrical connections, the LC transducer pixels change the long-wavelength infrared (LWIR) scene directly into a visible image as opposed to an electric signal in microbolometers. The objectives are to develop an imager technology scalable to large formats (tens of megapixels) while maintaining or improving the noise equivalent temperature difference (NETD) compared to microbolometers. The present work is demonstrating that the LCs have the required performance (sensitivity, dynamic range, speed, etc.) to enable a more flexible uncooled imager. Utilizing 200-mm wafers, a process has been developed and arrays have been fabricated using aligned LCs confined in 20-20-um cavities elevated on thermal legs. Detectors have been successfully fabricated on both silicon and fused silica wafers using less than 10 photolithographic mask steps. A breadboard camera system has been assembled to test the imagers. Various sensor configurations are described along with advantages and disadvantages of component arrangements.
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Summary

An uncooled thermal imager is being developed based on a liquid crystal (LC) transducer. Without any electrical connections, the LC transducer pixels change the long-wavelength infrared (LWIR) scene directly into a visible image as opposed to an electric signal in microbolometers. The objectives are to develop an imager technology scalable...

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Geiger-mode avalanche photodiode arrays integrated to all-digital CMOS circuits

Author:
Published in:
Sensors, Vol. 16, No. 495, 2016, doi:10.3390/s16040495.

Summary

This article reviews MIT Lincoln Laboratory's work over the past 20 years to develop photon-sensitive image sensors based on arrays of silicon Geiger-mode avalanche photodiodes. Integration of these detectors to all-digital CMOS readout circuits enable exquisitely sensitive solid-state imagers for lidar, wavefront sensing, and passive imaging.
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Summary

This article reviews MIT Lincoln Laboratory's work over the past 20 years to develop photon-sensitive image sensors based on arrays of silicon Geiger-mode avalanche photodiodes. Integration of these detectors to all-digital CMOS readout circuits enable exquisitely sensitive solid-state imagers for lidar, wavefront sensing, and passive imaging.

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Silicon Geiger-mode avalanche photodiode arrays for photon-starved imaging

Author:
Published in:
SPIE, Vol. 9492, Advanced Photon Counting Techniques IX, 28 May 2015.

Summary

Geiger-mode avalanche photodiodes (GMAPDs) are capable of detecting single photons. They can be operated to directly trigger all-digital circuits, so that detection events are digitally counted or time stamped in each pixel. An imager based on an array of GMAPDs therefore has zero readout noise, enabling quantum-limited sensitivity for photon-starved imaging applications. In this review, we discuss devices developed for 3D imaging, wavefront sensing, and passive imaging.
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Summary

Geiger-mode avalanche photodiodes (GMAPDs) are capable of detecting single photons. They can be operated to directly trigger all-digital circuits, so that detection events are digitally counted or time stamped in each pixel. An imager based on an array of GMAPDs therefore has zero readout noise, enabling quantum-limited sensitivity for photon-starved...

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Liquid crystal uncooled thermal imager development

Published in:
2014 Military Sensing Symposia, (MSS 2014), Detectors and Materials, 9 September 2014.

Summary

An uncooled thermal imager is being developed based on a liquid crystal transducer. The liquid crystal transducer changes a long-wavelength infrared scene into a visible image as opposed to an electric signal in microbolometers. This approach has the potential for making a more flexible thermal sensor. One objective is to develop imager technology scalable to large formats (tens of megapixels) while maintaining or improving the noise equivalent temperature difference (NETD) compared to microbolometers. Our work is demonstrating that the liquid crystals have the required performance (sensitivity, dynamic range, speed, etc.) to make state-of-the-art uncooled imagers. A process has been developed and arrays have been fabricated using the liquid crystals. A breadboard camera system has been assembled to test the imagers. Results of the measurements are discussed.
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Summary

An uncooled thermal imager is being developed based on a liquid crystal transducer. The liquid crystal transducer changes a long-wavelength infrared scene into a visible image as opposed to an electric signal in microbolometers. This approach has the potential for making a more flexible thermal sensor. One objective is to...

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