Peyman Yousefian, PhD

Objective: To investigate the association between a limb ballistocardiogram (BCG) and blood pressure (BP) based on data mining. Approach: During four BP-perturbing interventions, the BCG and reference BP were measured from 23 young healthy volunteers using a custom-manufactured wristband equipped with a MEMS accelerometer and a commercial continuous BP measurement device. Both timing and amplitude features in the wrist BCG waveform were extracted, and significant features predictive of… Show more

Objective: To investigate the association between a limb ballistocardiogram (BCG) and blood pressure (BP) based on data mining. Approach: During four BP-perturbing interventions, the BCG and reference BP were measured from 23 young healthy volunteers using a custom-manufactured wristband equipped with a MEMS accelerometer and a commercial continuous BP measurement device. Both timing and amplitude features in the wrist BCG waveform were extracted, and significant features predictive of diastolic (DP) and systolic (SP) BP were selected using stepwise linear regression analysis. The selected features were further compressed using principal component analysis to yield a small set of DP and SP predictors. The association between the predictors thus obtained and BP was investigated by multivariate linear regression analysis. Main Results: The predictors exhibited a meaningful association with BP. When three most significant predictors were used for DP and SP, correlation coefficient of r=0.75+/-0.03 (DP) and r=0.75+/-0.03 (SP), root-mean-squared error (RMSE) of 7.4+/-0.6 mmHg (DP) and 10.3+/-0.8 mmHg (SP), and mean absolute error (MAE) of 6.0+/-0.5 mmHg (DP) and 8.3+/-0.7 mmHg (SP) were obtained across all interventions (mean+/-SE). The association was consistent in all the individual interventions (r≥0.68, RMSE≤5.7 mmHg, and MAE≤4.5 mmHg for DP as well as r≥0.61, RMSE≤7.9 mmHg, and MAE≤6.4 mmHg for SP on the average). The minimum number of requisite predictors for robust yet practically realistic BP monitoring appeared to be 3. The association between predictors and BP was maintained even under regularized calibration (r=0.63+/-0.05, RMSE=9.3+/-0.8 mmHg, and MAE=7.6+/-0.7 mmHg for DP as well as r=0.60+/-0.05, RMSE=14.7+/-1.4 mmHg, and MAE=11.9+/-1.1 mmHg for SP (mean+/-SE)). The requisite predictors for DP and SP were distinct from each other. Significance: The results of this study may provide a viable basis for ultra-convenient BP monitoring based on a limb BCG alone. Show less

We report the design, fabrication and error analysis of a sun sensor array composed of six photodiodes. The sensor estimates the direction of the sun using a linear least squares method. The performance of the sensor is deteriorated by three major sources: fabrication errors, scattered environmental light, and inexact modeling of photodiodes. Using a calibration procedure and modeling the uniform component of the environmental light, we mitigate the first two errors and significantly reduce… Show more

We report the design, fabrication and error analysis of a sun sensor array composed of six photodiodes. The sensor estimates the direction of the sun using a linear least squares method. The performance of the sensor is deteriorated by three major sources: fabrication errors, scattered environmental light, and inexact modeling of photodiodes. Using a calibration procedure and modeling the uniform component of the environmental light, we mitigate the first two errors and significantly reduce root mean squared error from 2.63° to 0.83°. For a Field of View (FOV) of 110°, the maximum estimation error also drops from 3.8° to 1.6°. Through exact mathematical modeling of photodiodes, we demonstrate the feasibility of further reducing the root mean squared error, and discuss the effect of the maximum allowed angle of incidence on the sensor array performance. We propose a method to compute the optimum value of this angle and verify it experimentally. The effect of the number of illuminated photodiodes on the sensor accuracy is also investigated experimentally. Show less

Abstract:
Modern sensors consist of arrays of detectors specially arranged to enhance precision, capabilities, and field of view (FOV). Sensor arrays can have multiple functionalities, such as the simultaneous detection of position and motion. We use a linear sensor model and develop an optimization method to design an array of photodiodes. Our objective function minimizes bias and variance estimations. We introduce a maximum likelihood technique to approximate and determine the bias caused… Show more

Abstract:
Modern sensors consist of arrays of detectors specially arranged to enhance precision, capabilities, and field of view (FOV). Sensor arrays can have multiple functionalities, such as the simultaneous detection of position and motion. We use a linear sensor model and develop an optimization method to design an array of photodiodes. Our objective function minimizes bias and variance estimations. We introduce a maximum likelihood technique to approximate and determine the bias caused by measurement errors, and verify our theory by statistically complete simulations. We apply our theory to design an optimal sun sensor. The sensor has a predefined conical FOV, and its accuracy is controlled by a set of directionally variable weighting parameters. This enables us to compute the optimal placements of photodiodes under various operational conditions. It is found that for a uniform accuracy distribution, both the variance and bias estimations can be exactly minimized for a given array configuration. For non-uniform accuracy distributions with rotational symmetries, when a higher accuracy is required around the centerline of the FOV, both errors attain their minima when the array configuration becomes inclined toward the center. In non-uniform and non-axisymmetric distributions, when a higher accuracy is imposed along a line or a path, the bias and variance errors differ significantly, while the variance minimization implies the concentration of the cells toward the path, the bias minimization yields a uniform distribution. The method presented in this paper can be used in the optimal design of sun sensors for space systems and solar power plants. Show less

Abstract:
In this study a new nonlinear control approach is introduced to suppress libration of a tethered satellite system (TSS). It benefits from coupling between satellites and tether libration dynamics. The control concept uses the main satellite attitude maneuvers to suppress librational motion of the tether. To achieve this goal, the main satellite attitude control actuators are used as the only actuation in the system. The study considers planar motion of a two body TSS system in a… Show more

Abstract:
In this study a new nonlinear control approach is introduced to suppress libration of a tethered satellite system (TSS). It benefits from coupling between satellites and tether libration dynamics. The control concept uses the main satellite attitude maneuvers to suppress librational motion of the tether. To achieve this goal, the main satellite attitude control actuators are used as the only actuation in the system. The study considers planar motion of a two body TSS system in a circular orbit. Governing dynamic equations of motion are derived using extended Lagrange method. Controllability of the system about equilibrium state is studied and a nonlinear controller is designed using feedback linearization method to regulate libration of the system. By studying the conditions of feedback linearization method, stability of the controller is also proved. Tether tension and satellite attitude are assumed as only measurable outputs of the system. The Extended Kalman Filter (EKF) is used to estimate states of the system to be used as the feedback to the controller. By implementing the controller and observer on the system, simulations demonstrate that the controller lead to reduction of the tether libration from large amplitudes. Show less

Abstract: In this study a new method is introduced to suppress libration of a tethered satellite system (TSS). It benefits from coupling between satellites and tether libration dynamics. The control concept uses the main satellite attitude maneuvers to suppress librational motion of the tether, and the main satellite׳s actuators for attitude control are used as the only actuation in the system. The study considers planar motion of a two body TSS system in a circular orbit and it is assumed that… Show more

Abstract: In this study a new method is introduced to suppress libration of a tethered satellite system (TSS). It benefits from coupling between satellites and tether libration dynamics. The control concept uses the main satellite attitude maneuvers to suppress librational motion of the tether, and the main satellite׳s actuators for attitude control are used as the only actuation in the system. The study considers planar motion of a two body TSS system in a circular orbit and it is assumed that the tether׳s motion will not change it. Governing dynamic equations of motion are derived using the extended Lagrange method. Controllability of the system around the equilibrium state is studied and a linear LQG controller is designed to regulate libration of the system. Tether tension and satellite attitude are assumed as only measurable outputs of the system. The Extended Kalman Filter (EKF) is used to estimate states of the system to be used as feedback to the controller. The designed controller and observer are implemented to the nonlinear plant and simulations demonstrate that the controller lead to reduction of the tether libration propoerly. By the way, because the controller is linear, it is applicable only at low amplitudes in the vicinity of equilibrium point. To reach global stability, a nonlinear controller is demanded. Show less

Abstract: In the present study, finite element formulation based on higher order shear deformation plate theory is developed to analyze nonlinear natural frequencies, time and frequency responses of functionally graded plate with surface-bonded piezoelectric layers under thermal, electrical and mechanical loads. The von Karman nonlinear strain–displacement relationship is used to account for the large deflection of the plate. The material properties of functionally graded material (FGM) are… Show more

Abstract: In the present study, finite element formulation based on higher order shear deformation plate theory is developed to analyze nonlinear natural frequencies, time and frequency responses of functionally graded plate with surface-bonded piezoelectric layers under thermal, electrical and mechanical loads. The von Karman nonlinear strain–displacement relationship is used to account for the large deflection of the plate. The material properties of functionally graded material (FGM) are assumed temperature-dependent. The temperature field has uniform distribution over the plate surface and varies in the thickness direction. The considered electric field only has non-zero-valued component Ez. Numerical results are presented to study effects of FGM volume fraction exponent, applied voltage in piezoelectric layers, thermal load and vibration amplitude on nonlinear natural frequencies and time response of FGM plate with integrated piezoelectric layers. In addition, nonlinear frequency response diagrams of the plate are presented and effects of different parameters such as FGM volume fraction exponent, temperature gradient, and piezoelectric voltage are investigated. Show less