Chuong Ngo - Academia.edu (original) (raw)

Papers by Chuong Ngo

Frontiers in Medicine

Background and objectiveSickle cell disease (SCD) is a very common autosomal recessive hemoglobin... more Background and objectiveSickle cell disease (SCD) is a very common autosomal recessive hemoglobinopathy leading to multiple pulmonary complications that are closely associated with mortality. The pathophysiology of chronic pulmonary involvement is not yet fully understood and no specific therapies are available.MethodsThe aim of this cross-sectional study was to characterize the lung function of children and young adolescents with SCD in a German single-center cohort and to extend conventional lung function testing by the use of a new imaging method. We performed spirometry and body plethysmography in 35 children and young adults with hemoglobin SS, SC, S/β-thalassemia as well as 50 controls. These data were compared with clinical characteristics and typical laboratory parameters of hemolysis and disease activity in SCD. To identify lung inhomogeneities, for example due to atelectasis, hyperinflation, air trapping or vascular occlusions, we used the promising new method of electrica...

at - Automatisierungstechnik

For many elderly, wheeled rollators play an essential role in everyday life’s mobility. In additi... more For many elderly, wheeled rollators play an essential role in everyday life’s mobility. In addition, exoskeletons are increasingly used in daily mobility as well as in rehabilitation therapy. We present a motorized rollator to combine the advantages of both support devices. Based on the developed hardware, we design two model-based control concepts. The first relies on distance measurement with ultrasonic sensors, which enables contactless navigation of the rollator, to allow balance training with the exoskeleton. The second control concept enables the balancing of the rollator on its rear wheels as a first step to overcome stairs. Through experiments on the real system, we demonstrate and discuss the functionality and reliability of both control concepts.

Current Directions in Biomedical Engineering

Lyapunov exponent is a promising parameter to ascertain the stability of the human gait. In this ... more Lyapunov exponent is a promising parameter to ascertain the stability of the human gait. In this work, we use a time-series model based on a second-order delay-system with inertial measurement units placed on the foot and wrist. Stability is analyzed in a localized sense, with the Lyapunov exponent computed in the temporal region between two heel-strike points, which are determined using a peak-detection algorithm. We have attempted to show correlations between variations in the stride time and stability of the gait under normal and abnormal conditions. In the latter case, we attach a weight on foot to emulate weakness. On comparison between both cases, we observe a statistical significance of p=0.0039 using Wilcoxon’s rank-sum test. Moreover, on observing the correlations between Lyapunov Exponent and Stride Time Variability, we notice a left-shift in the abnormal case, indicating a lower threshold for instability, with the Stride Time Variability being 0.07 as compared to 0.11 in ...

IEEE/ASME Transactions on Mechatronics

2022 44th Annual International Conference of the IEEE Engineering in Medicine & Biology Society (EMBC)

2022 44th Annual International Conference of the IEEE Engineering in Medicine & Biology Society (EMBC)

Paediatric respiratory physiology and sleep, 2020

Sensors, 2021

Creating highly functional prosthetic, orthotic, and rehabilitation devices is a socially relevan... more Creating highly functional prosthetic, orthotic, and rehabilitation devices is a socially relevant scientific and engineering task. Currently, certain constraints hamper the development of such devices. The primary constraint is the lack of an intuitive and reliable control interface working between the organism and the actuator. The critical point in developing these devices and systems is determining the type and parameters of movements based on control signals recorded on an extremity. In the study, we investigate the simultaneous acquisition of electric impedance (EI), electromyography (EMG), and force myography (FMG) signals during basic wrist movements: grasping, flexion/extension, and rotation. For investigation, a laboratory instrumentation and software test setup were made for registering signals and collecting data. The analysis of the acquired signals revealed that the EI signals in conjunction with the analysis of EMG and FMG signals could potentially be highly informati...

Sensors, 2021

The electrical impedance myography method is widely used in solving bionic control problems and c... more The electrical impedance myography method is widely used in solving bionic control problems and consists of assessing the change in the electrical impedance magnitude during muscle contraction in real time. However, the choice of electrode systems sizes is not always properly considered when using the electrical impedance myography method in the existing approaches, which is important in terms of electrical impedance signal expressiveness and reproducibility. The article is devoted to the determination of acceptable sizes for the electrode systems for electrical impedance myography using the Pareto optimality assessment method and the electrical impedance signals formation model of the forearm area, taking into account the change in the electrophysical and geometric parameters of the skin and fat layer and muscle groups when performing actions with a hand. Numerical finite element simulation using anthropometric models of the forearm obtained by volunteers’ MRI 3D reconstructions wa...

2021 Sixth International Conference on Advances in Biomedical Engineering (ICABME), 2021

Parkinson’s disease is a neurological disorder characterized by the deficiency of dopamine levels... more Parkinson’s disease is a neurological disorder characterized by the deficiency of dopamine levels in the brain. More than 75 percent of these patients suffer from tremors. Parkinsonian tremor (PT) is more characterized to be a rest tremor, but some patients suffer from action tremor as well. Usually, patients suffering from this disease are diagnosed by their physicians who perform some battery MDS-UPDRS tasks to determine the disorder. Some sensors were used to diagnose the tremor objectively, but in this study, we are using a new Body Sensor Network (BSN) designed at our institute to be used in detecting the acceleration, gyroscope, and magnetometer of the tremor patients in the clinic. Signal processing of the recorded data is performed to determine and classify the number of times throughout the day the patient suffered from tremors. This is ensured through automatic signal segmentation, extraction of several signal features, and classification with the most accurate machine learning classifier. In this study, we have proved that our BSN sensor is capable of helping clinicians in classifying tremor occurrence in Parkinson diseased patients specifically, and tremor patients generally throughout monitoring their everyday life activities.

IEEE/ASME Transactions on Mechatronics, 2021

This article investigates a closed-loop torque-controlled variable stiffness actuator (VSA) combi... more This article investigates a closed-loop torque-controlled variable stiffness actuator (VSA) combined with a disturbance observer for enhancing low output impedance. We implement the generalized extended state observer (GESO) for conveniently testing the stability of the time-varying VSA system. In our application, the GESO is also required to serve the operation of the low- and high-impedance task. Here, the most important aspect is to consider the influence of the physical stiffness on the output impedance, because the VSA has been regulated with the closed-loop torque control. Through the interaction-torque experiments, we verify that using the fast dynamics GESO with the low-stiffness actuator can achieve low output impedance and stable interaction under the reachable frequency of a human. These properties contribute to perform the low-impedance task. When performing the high-impedance task, where a large torque command is needed, the high-stiffness actuator and the slow dynamics GESO are implemented to achieve high bandwidth and proper tracking performance. The continuously variable observer responses in accordance with the stiffness values are achieved via the gain-scheduling control. Moreover, the present closed-loop linear parameter varying system can be verified to be quadratically stable. The VSA system is then implemented on a knee exoskeleton for a sit-to-stand application. The reference command of the exoskeleton is a joint torque, calculated from the inverse dynamics. This torque signal is also used as the reference command of the stiffness motor of the VSA. The effectiveness of the exoskeleton system is experimentally verified with one healthy volunteer. Subsequently, another two healthy volunteers also successfully experienced the system.

at - Automatisierungstechnik, 2020

Parallel-elastic actuators (PEA) can provide efficiency advantages. In current designs clutches c... more Parallel-elastic actuators (PEA) can provide efficiency advantages. In current designs clutches can fix or release the neutral position of the elasticity. The presented actuator combines a direct actuator with a serial-elastic actuator (SEA) in parallel. In addition to the efficiency advantages of a PEA, both actuators can also actively cooperate or actively control the neutral position of the elasticity. On the test bench, the prototype is operated as a PEA with a load similar to the hip during gait, whereby a reduction in peak power of 19 % is found. Furthermore the SEA can support 31 % of the torque during a task similar to sit-to-stand.

Actuators, 2021

The lower limb exoskeleton investigated in this work actively supports the knee and hip and is in... more The lower limb exoskeleton investigated in this work actively supports the knee and hip and is intended to provide full motion support during gait. Parallel elastic actuators are integrated into the hip joints to improve the energy efficiency in gait. The prototype was tested in sit-to-stand and gait trials, in which the actuators were cascade-controlled with position trajectories. The compliant actuation of the hip in gait experiments proved to be more efficient; the peak torque was reduced by up to 31% and the RMS power was reduced by up to 36%.

Sensors, 2020

Body sensor networks (BSNs) represent an important research tool for exploring novel diagnostic o... more Body sensor networks (BSNs) represent an important research tool for exploring novel diagnostic or therapeutic approaches. They allow for integrating different measurement techniques into body-worn sensors organized in a network structure. In 2011, the first Integrated Posture and Activity Network by MedIT Aachen (IPANEMA) was introduced. In this work, we present a recently developed platform for a wireless body sensor network with customizable applications based on a proprietary 868MHz communication interface. In particular, we present a sensor setup for gait analysis during everyday life monitoring. The arrangement consists of three identical inertial measurement sensors attached at the wrist, thigh, and chest. We additionally introduce a force-sensitive resistor integrated insole for measurement of ground reaction forces (GRFs), to enhance the assessment possibilities and generate ground truth data for inertial measurement sensors. Since the 868MHz is not strongly represented in ...

IEEE Transactions on Automation Science and Engineering, 2019

We present a novel application of the variable stiffness actuator (VSA)-based assistance/rehabili... more We present a novel application of the variable stiffness actuator (VSA)-based assistance/rehabilitation robotfeatured impedance control using a cascaded position torque control loop. The robot follows the adaptive impedance control paradigm, thereby achieving an adaptive assistance level according to human joint torque. The feedforward human joint torque command is used to cooperatively adjust the impedance controller and the stiffness trajectory of the VSA (this functional architecture is referred to as the cooperative control framework). In this way, the task performance during movement training can be improved regarding: 1) safety-for example, when the subject intends to contribute considerable effort, low-gain impedance control is activated with a low stiffness actuator to further decrease output impedance and 2) tracking performance-for example, for the subject with less effort, high-gain impedance control is used while pursuing high stiffness to enhance the torque bandwidth. Regarding the safety aspect, we demonstrate that the torque controller designed at low stiffness can be sensitive to the disturbance for low output impedance while maintaining tracking performance. A precondition for this is to treat the input disturbance separately. This is guaranteed by our previously proposed torque control of the VSA using the linear quadratic Gaussian technique. This approach is also employed here, but with additional discussion on the observer design to serve the proposed cooperative control approach. Here, the effectiveness of the proposed control system is experimentally verified using a VSA prototype and a one-degree-of-freedom lower limb exoskeleton worn by a human test person. Note to Practitioners-Control of "physical human-robot interaction" can be achieved by the mechanical parts of the variable stiffness actuator (VSA). However, the mechanical construction for stiffness variation may limit the capacity to achieve low output stiffness and fast stiffness variation in speed. These limitations may become more evident in the assistance/rehabilitation robot applications. To overcome these limitations, the impedance control scheme can be employed to achieve a programmable impedance range and impedance variation speed. This control scheme has been widely applied on the fixed-compliance joint but lacks a way to be implemented on the VSA joint because of its existing capacity to control the impedance with the mechanical construction. This article presents a novel application Manuscript

Actuators, 2019

Various actuator topologies are discussed for the purpose of powering periodic processes and part... more Various actuator topologies are discussed for the purpose of powering periodic processes and particularly walking robots. The Clutched Parallel Elastic Actuator (CPEA) is proposed to reduce the energy consumption of active exoskeletons. A nonlinear model of the CPEA is presented in addition to the mechanical design. The CPEA prototype is operated with a passive load on the walking trajectory of the hip joint. The actuator is controlled with a cascaded position control and a superimposed Iterative Learning Controller (ILC). The controller was chosen to ensure comparability between active and deactivated spring operation. The application of the CPEA has the potential to increase efficiency in the design of exoskeletons.

Computer Methods and Programs in Biomedicine, 2019

Background and Objective We hypothesized that a biophysical computational model implemented in an... more Background and Objective We hypothesized that a biophysical computational model implemented in an object-oriented modeling language (OOML) would provide physiological information and simulative data to study the development and treatment of cardiogenic pulmonary congestion. Methods This work is based on the object-oriented cardiopulmonary interaction introduced in [1]. This paper describes the novel model components required to study cardiogenic pulmonary congestion: i) interstitial fluid exchange related to the Starling equation, ii) the lymphatic pump, and iii) the interconnection of these elements with the original cardiopulmonary model. The presented model succeeds in i) describing lymphatic flow at the capillary artery and venous end, ii) activation of the lymphatic pump at elevated pulmonary pressures, and iii) the simulation of the different safety factors related to lung tissue, osmotic gradient, and the lymphatic system during the development of lung congestion. Results Simulations show a qualitative correlation between model behavior and physiological data from literature. The model also demonstrates the beneficial effect of continuous positive airway pressure therapy on fluid clearance and respiratory mechanics. Conclusion This study demonstrates the successful use of OOML to describe the development of cardiogenic congestion by introducing a model of the lymphatic system and the thoracic fluid balance system, as well as connecting them to the existing cardiopulmonary model.

Pediatric Pulmonology, 2018

Background: Electrical impedance tomography (EIT) provides information on global and regional ven... more Background: Electrical impedance tomography (EIT) provides information on global and regional ventilation during tidal breathing and mechanical ventilation. During forced expiration maneuvers, the linearity of EIT and spirometric data has been documented in healthy persons. The present study investigates the potential diagnostic use of EIT in pediatric patients with asthma. Methods: EIT and spirometry were performed in 58 children with asthma (average age ± SD: 11.86 ± 3.13 years), and 58 healthy controls (average age ± SD: 12.12 ± 2.9 years). The correlation between EIT data and simultaneously acquired spirometric data were tested for FEV 1, FEV 0.5 , MEF 75 , MEF 50 , and MEF 25. Binary classification tests were performed for the EIT-derived Tiffeneau index FEV 1 /FVC and the bronchodilator test index ΔFEV 1. Average flow-volume (FV) loops were generated for patients with pathologic spirometry to demonstrate the feasibility of EIT for graphic diagnosis of asthma. Results: Spirometry and global EIT-based FV loops showed a strong correlation (P < 0.001, r > 0.9 in FEV 1 and FEV 0.5). In all criteria, the binary classification tests yielded high specificity (>93%), a high positive predictive value (≥75%) and a high negative predictive value (>80%), while sensitivity was higher in ΔFEV 1 (86.67%) and lower in FEV 1 /FVC (25% and 35.29%). A typical concave shape of the EIT-derived average FV loops was observed for asthmatic children with improvement after bronchospasmolysis. Conclusions: Global FV loops derived from EIT correlate well with spirometry. Positive bronchospasmolysis can be observed in EIT-derived FV loops. Flow-volume loops originated from EIT have a potential to visualize pulmonary function.

Respiratory function technologists/scient, 2018

IFAC-PapersOnLine, 2018

This paper describes a novel extension of the Forced Oscillation Technique (FOT), namely the volu... more This paper describes a novel extension of the Forced Oscillation Technique (FOT), namely the volume-dependent Forced Oscillation Technique (v-FOT). This extension is based on the measurement of the respiratory impedance over the whole lung volume. A new measurement protocol was introduced: subjects breathe slowly from residual volume to total lung capacity, while oscillations are applied simultaneously. The respiratory impedance is computed by means of the windowed cosine fitting method. The respiratory impedance becomes frequency-and volume-dependent Z(jω,V). The novel impedance-volume (ZV) diagram was introduced for diagnostic purposes. Least-squares parameter estimation was performed for the extended-RIC and the constant phase models. However, both models failed in presenting the volume-dependency of Z. We proposed two new models: the volume-dependent extended RIC and the volume-dependent Mead model for a better fit of the data. The volume-dependent Mead model yields the smallest estimation error.

Frontiers in Medicine

Background and objectiveSickle cell disease (SCD) is a very common autosomal recessive hemoglobin... more Background and objectiveSickle cell disease (SCD) is a very common autosomal recessive hemoglobinopathy leading to multiple pulmonary complications that are closely associated with mortality. The pathophysiology of chronic pulmonary involvement is not yet fully understood and no specific therapies are available.MethodsThe aim of this cross-sectional study was to characterize the lung function of children and young adolescents with SCD in a German single-center cohort and to extend conventional lung function testing by the use of a new imaging method. We performed spirometry and body plethysmography in 35 children and young adults with hemoglobin SS, SC, S/β-thalassemia as well as 50 controls. These data were compared with clinical characteristics and typical laboratory parameters of hemolysis and disease activity in SCD. To identify lung inhomogeneities, for example due to atelectasis, hyperinflation, air trapping or vascular occlusions, we used the promising new method of electrica...

at - Automatisierungstechnik

For many elderly, wheeled rollators play an essential role in everyday life’s mobility. In additi... more For many elderly, wheeled rollators play an essential role in everyday life’s mobility. In addition, exoskeletons are increasingly used in daily mobility as well as in rehabilitation therapy. We present a motorized rollator to combine the advantages of both support devices. Based on the developed hardware, we design two model-based control concepts. The first relies on distance measurement with ultrasonic sensors, which enables contactless navigation of the rollator, to allow balance training with the exoskeleton. The second control concept enables the balancing of the rollator on its rear wheels as a first step to overcome stairs. Through experiments on the real system, we demonstrate and discuss the functionality and reliability of both control concepts.

Current Directions in Biomedical Engineering

Lyapunov exponent is a promising parameter to ascertain the stability of the human gait. In this ... more Lyapunov exponent is a promising parameter to ascertain the stability of the human gait. In this work, we use a time-series model based on a second-order delay-system with inertial measurement units placed on the foot and wrist. Stability is analyzed in a localized sense, with the Lyapunov exponent computed in the temporal region between two heel-strike points, which are determined using a peak-detection algorithm. We have attempted to show correlations between variations in the stride time and stability of the gait under normal and abnormal conditions. In the latter case, we attach a weight on foot to emulate weakness. On comparison between both cases, we observe a statistical significance of p=0.0039 using Wilcoxon’s rank-sum test. Moreover, on observing the correlations between Lyapunov Exponent and Stride Time Variability, we notice a left-shift in the abnormal case, indicating a lower threshold for instability, with the Stride Time Variability being 0.07 as compared to 0.11 in ...

IEEE/ASME Transactions on Mechatronics

2022 44th Annual International Conference of the IEEE Engineering in Medicine & Biology Society (EMBC)

2022 44th Annual International Conference of the IEEE Engineering in Medicine & Biology Society (EMBC)

Paediatric respiratory physiology and sleep, 2020

Sensors, 2021

Creating highly functional prosthetic, orthotic, and rehabilitation devices is a socially relevan... more Creating highly functional prosthetic, orthotic, and rehabilitation devices is a socially relevant scientific and engineering task. Currently, certain constraints hamper the development of such devices. The primary constraint is the lack of an intuitive and reliable control interface working between the organism and the actuator. The critical point in developing these devices and systems is determining the type and parameters of movements based on control signals recorded on an extremity. In the study, we investigate the simultaneous acquisition of electric impedance (EI), electromyography (EMG), and force myography (FMG) signals during basic wrist movements: grasping, flexion/extension, and rotation. For investigation, a laboratory instrumentation and software test setup were made for registering signals and collecting data. The analysis of the acquired signals revealed that the EI signals in conjunction with the analysis of EMG and FMG signals could potentially be highly informati...

Sensors, 2021

The electrical impedance myography method is widely used in solving bionic control problems and c... more The electrical impedance myography method is widely used in solving bionic control problems and consists of assessing the change in the electrical impedance magnitude during muscle contraction in real time. However, the choice of electrode systems sizes is not always properly considered when using the electrical impedance myography method in the existing approaches, which is important in terms of electrical impedance signal expressiveness and reproducibility. The article is devoted to the determination of acceptable sizes for the electrode systems for electrical impedance myography using the Pareto optimality assessment method and the electrical impedance signals formation model of the forearm area, taking into account the change in the electrophysical and geometric parameters of the skin and fat layer and muscle groups when performing actions with a hand. Numerical finite element simulation using anthropometric models of the forearm obtained by volunteers’ MRI 3D reconstructions wa...

2021 Sixth International Conference on Advances in Biomedical Engineering (ICABME), 2021

Parkinson’s disease is a neurological disorder characterized by the deficiency of dopamine levels... more Parkinson’s disease is a neurological disorder characterized by the deficiency of dopamine levels in the brain. More than 75 percent of these patients suffer from tremors. Parkinsonian tremor (PT) is more characterized to be a rest tremor, but some patients suffer from action tremor as well. Usually, patients suffering from this disease are diagnosed by their physicians who perform some battery MDS-UPDRS tasks to determine the disorder. Some sensors were used to diagnose the tremor objectively, but in this study, we are using a new Body Sensor Network (BSN) designed at our institute to be used in detecting the acceleration, gyroscope, and magnetometer of the tremor patients in the clinic. Signal processing of the recorded data is performed to determine and classify the number of times throughout the day the patient suffered from tremors. This is ensured through automatic signal segmentation, extraction of several signal features, and classification with the most accurate machine learning classifier. In this study, we have proved that our BSN sensor is capable of helping clinicians in classifying tremor occurrence in Parkinson diseased patients specifically, and tremor patients generally throughout monitoring their everyday life activities.

IEEE/ASME Transactions on Mechatronics, 2021

This article investigates a closed-loop torque-controlled variable stiffness actuator (VSA) combi... more This article investigates a closed-loop torque-controlled variable stiffness actuator (VSA) combined with a disturbance observer for enhancing low output impedance. We implement the generalized extended state observer (GESO) for conveniently testing the stability of the time-varying VSA system. In our application, the GESO is also required to serve the operation of the low- and high-impedance task. Here, the most important aspect is to consider the influence of the physical stiffness on the output impedance, because the VSA has been regulated with the closed-loop torque control. Through the interaction-torque experiments, we verify that using the fast dynamics GESO with the low-stiffness actuator can achieve low output impedance and stable interaction under the reachable frequency of a human. These properties contribute to perform the low-impedance task. When performing the high-impedance task, where a large torque command is needed, the high-stiffness actuator and the slow dynamics GESO are implemented to achieve high bandwidth and proper tracking performance. The continuously variable observer responses in accordance with the stiffness values are achieved via the gain-scheduling control. Moreover, the present closed-loop linear parameter varying system can be verified to be quadratically stable. The VSA system is then implemented on a knee exoskeleton for a sit-to-stand application. The reference command of the exoskeleton is a joint torque, calculated from the inverse dynamics. This torque signal is also used as the reference command of the stiffness motor of the VSA. The effectiveness of the exoskeleton system is experimentally verified with one healthy volunteer. Subsequently, another two healthy volunteers also successfully experienced the system.

at - Automatisierungstechnik, 2020

Parallel-elastic actuators (PEA) can provide efficiency advantages. In current designs clutches c... more Parallel-elastic actuators (PEA) can provide efficiency advantages. In current designs clutches can fix or release the neutral position of the elasticity. The presented actuator combines a direct actuator with a serial-elastic actuator (SEA) in parallel. In addition to the efficiency advantages of a PEA, both actuators can also actively cooperate or actively control the neutral position of the elasticity. On the test bench, the prototype is operated as a PEA with a load similar to the hip during gait, whereby a reduction in peak power of 19 % is found. Furthermore the SEA can support 31 % of the torque during a task similar to sit-to-stand.

Actuators, 2021

The lower limb exoskeleton investigated in this work actively supports the knee and hip and is in... more The lower limb exoskeleton investigated in this work actively supports the knee and hip and is intended to provide full motion support during gait. Parallel elastic actuators are integrated into the hip joints to improve the energy efficiency in gait. The prototype was tested in sit-to-stand and gait trials, in which the actuators were cascade-controlled with position trajectories. The compliant actuation of the hip in gait experiments proved to be more efficient; the peak torque was reduced by up to 31% and the RMS power was reduced by up to 36%.

Sensors, 2020

Body sensor networks (BSNs) represent an important research tool for exploring novel diagnostic o... more Body sensor networks (BSNs) represent an important research tool for exploring novel diagnostic or therapeutic approaches. They allow for integrating different measurement techniques into body-worn sensors organized in a network structure. In 2011, the first Integrated Posture and Activity Network by MedIT Aachen (IPANEMA) was introduced. In this work, we present a recently developed platform for a wireless body sensor network with customizable applications based on a proprietary 868MHz communication interface. In particular, we present a sensor setup for gait analysis during everyday life monitoring. The arrangement consists of three identical inertial measurement sensors attached at the wrist, thigh, and chest. We additionally introduce a force-sensitive resistor integrated insole for measurement of ground reaction forces (GRFs), to enhance the assessment possibilities and generate ground truth data for inertial measurement sensors. Since the 868MHz is not strongly represented in ...

IEEE Transactions on Automation Science and Engineering, 2019

We present a novel application of the variable stiffness actuator (VSA)-based assistance/rehabili... more We present a novel application of the variable stiffness actuator (VSA)-based assistance/rehabilitation robotfeatured impedance control using a cascaded position torque control loop. The robot follows the adaptive impedance control paradigm, thereby achieving an adaptive assistance level according to human joint torque. The feedforward human joint torque command is used to cooperatively adjust the impedance controller and the stiffness trajectory of the VSA (this functional architecture is referred to as the cooperative control framework). In this way, the task performance during movement training can be improved regarding: 1) safety-for example, when the subject intends to contribute considerable effort, low-gain impedance control is activated with a low stiffness actuator to further decrease output impedance and 2) tracking performance-for example, for the subject with less effort, high-gain impedance control is used while pursuing high stiffness to enhance the torque bandwidth. Regarding the safety aspect, we demonstrate that the torque controller designed at low stiffness can be sensitive to the disturbance for low output impedance while maintaining tracking performance. A precondition for this is to treat the input disturbance separately. This is guaranteed by our previously proposed torque control of the VSA using the linear quadratic Gaussian technique. This approach is also employed here, but with additional discussion on the observer design to serve the proposed cooperative control approach. Here, the effectiveness of the proposed control system is experimentally verified using a VSA prototype and a one-degree-of-freedom lower limb exoskeleton worn by a human test person. Note to Practitioners-Control of "physical human-robot interaction" can be achieved by the mechanical parts of the variable stiffness actuator (VSA). However, the mechanical construction for stiffness variation may limit the capacity to achieve low output stiffness and fast stiffness variation in speed. These limitations may become more evident in the assistance/rehabilitation robot applications. To overcome these limitations, the impedance control scheme can be employed to achieve a programmable impedance range and impedance variation speed. This control scheme has been widely applied on the fixed-compliance joint but lacks a way to be implemented on the VSA joint because of its existing capacity to control the impedance with the mechanical construction. This article presents a novel application Manuscript

Actuators, 2019

Various actuator topologies are discussed for the purpose of powering periodic processes and part... more Various actuator topologies are discussed for the purpose of powering periodic processes and particularly walking robots. The Clutched Parallel Elastic Actuator (CPEA) is proposed to reduce the energy consumption of active exoskeletons. A nonlinear model of the CPEA is presented in addition to the mechanical design. The CPEA prototype is operated with a passive load on the walking trajectory of the hip joint. The actuator is controlled with a cascaded position control and a superimposed Iterative Learning Controller (ILC). The controller was chosen to ensure comparability between active and deactivated spring operation. The application of the CPEA has the potential to increase efficiency in the design of exoskeletons.

Computer Methods and Programs in Biomedicine, 2019

Background and Objective We hypothesized that a biophysical computational model implemented in an... more Background and Objective We hypothesized that a biophysical computational model implemented in an object-oriented modeling language (OOML) would provide physiological information and simulative data to study the development and treatment of cardiogenic pulmonary congestion. Methods This work is based on the object-oriented cardiopulmonary interaction introduced in [1]. This paper describes the novel model components required to study cardiogenic pulmonary congestion: i) interstitial fluid exchange related to the Starling equation, ii) the lymphatic pump, and iii) the interconnection of these elements with the original cardiopulmonary model. The presented model succeeds in i) describing lymphatic flow at the capillary artery and venous end, ii) activation of the lymphatic pump at elevated pulmonary pressures, and iii) the simulation of the different safety factors related to lung tissue, osmotic gradient, and the lymphatic system during the development of lung congestion. Results Simulations show a qualitative correlation between model behavior and physiological data from literature. The model also demonstrates the beneficial effect of continuous positive airway pressure therapy on fluid clearance and respiratory mechanics. Conclusion This study demonstrates the successful use of OOML to describe the development of cardiogenic congestion by introducing a model of the lymphatic system and the thoracic fluid balance system, as well as connecting them to the existing cardiopulmonary model.

Pediatric Pulmonology, 2018

Background: Electrical impedance tomography (EIT) provides information on global and regional ven... more Background: Electrical impedance tomography (EIT) provides information on global and regional ventilation during tidal breathing and mechanical ventilation. During forced expiration maneuvers, the linearity of EIT and spirometric data has been documented in healthy persons. The present study investigates the potential diagnostic use of EIT in pediatric patients with asthma. Methods: EIT and spirometry were performed in 58 children with asthma (average age ± SD: 11.86 ± 3.13 years), and 58 healthy controls (average age ± SD: 12.12 ± 2.9 years). The correlation between EIT data and simultaneously acquired spirometric data were tested for FEV 1, FEV 0.5 , MEF 75 , MEF 50 , and MEF 25. Binary classification tests were performed for the EIT-derived Tiffeneau index FEV 1 /FVC and the bronchodilator test index ΔFEV 1. Average flow-volume (FV) loops were generated for patients with pathologic spirometry to demonstrate the feasibility of EIT for graphic diagnosis of asthma. Results: Spirometry and global EIT-based FV loops showed a strong correlation (P < 0.001, r > 0.9 in FEV 1 and FEV 0.5). In all criteria, the binary classification tests yielded high specificity (>93%), a high positive predictive value (≥75%) and a high negative predictive value (>80%), while sensitivity was higher in ΔFEV 1 (86.67%) and lower in FEV 1 /FVC (25% and 35.29%). A typical concave shape of the EIT-derived average FV loops was observed for asthmatic children with improvement after bronchospasmolysis. Conclusions: Global FV loops derived from EIT correlate well with spirometry. Positive bronchospasmolysis can be observed in EIT-derived FV loops. Flow-volume loops originated from EIT have a potential to visualize pulmonary function.

Respiratory function technologists/scient, 2018

IFAC-PapersOnLine, 2018

This paper describes a novel extension of the Forced Oscillation Technique (FOT), namely the volu... more This paper describes a novel extension of the Forced Oscillation Technique (FOT), namely the volume-dependent Forced Oscillation Technique (v-FOT). This extension is based on the measurement of the respiratory impedance over the whole lung volume. A new measurement protocol was introduced: subjects breathe slowly from residual volume to total lung capacity, while oscillations are applied simultaneously. The respiratory impedance is computed by means of the windowed cosine fitting method. The respiratory impedance becomes frequency-and volume-dependent Z(jω,V). The novel impedance-volume (ZV) diagram was introduced for diagnostic purposes. Least-squares parameter estimation was performed for the extended-RIC and the constant phase models. However, both models failed in presenting the volume-dependency of Z. We proposed two new models: the volume-dependent extended RIC and the volume-dependent Mead model for a better fit of the data. The volume-dependent Mead model yields the smallest estimation error.