X.P. Liu

Carleton University, Ottawa, Ontario, Canada

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Publications (24)24.84 Total impact

  • Huanran Wang, X.P. Liu
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    ABSTRACT: This paper presents a new adaptive servo-level shared control scheme for a mobile assistive robot that aims at assisting senior and disabled people to transport heavy objects in a complex environment with obstacles. Several technical problems and challenges related to the assistive robotic system and the shared controller are addressed. Specifically, a nonlinear tracking controller is developed for the robot to follow the user. An obstacle avoidance controller is developed based on the deformable virtual zone principle for the robot to avoid obstacles. The adaptive servo-level shared controller utilizes the tracking controller and the obstacle avoidance controller’s outputs to generate a new shared control output to command the robot. Experiments show that the user can guide the movement of the robot safely and smoothly in the complex environment with the developed controllers.
    IEEE/ASME Transactions on Mechatronics 12/2014; 19(6):1725-1736. DOI:10.1109/TMECH.2014.2299213 · 3.65 Impact Factor
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    ABSTRACT: The arterial blood pressure (ABP) is one of the most important physiological parameters for health monitoring. Most of the blood measurement devices in the market determine the ABP through the inflation and the deflation of a cuff controlled by a bladder. This method is very uncomfortable for most of the users and may even cause anxiety, which in turn can affect the blood pressure (BP) (white coat syndrome). This paper investigates a cuffless nonintrusive approach to estimate the BP. The main idea is to measure the pulse transit time (PTT), i.e., the delay between the R-peak of the electrocardiogram (ECG) signal and the following peak of the finger photoplethysmograph (PPG) signal. The main problem of this approach is that when the dicrotic notch of the PPG signal is unobservable, the position and the amplitude of the main peak of the PPG signal will be changed. As a result, the correlation between the BP and the PTT can be affected. To overcome this problem, three types of secondary peak detection methods are designed to reveal the secondary peak from the original PPG signal. Actual ECG, PPG, and the BP measurements extracted from the Multiparameter Intelligent Monitoring in Intensive Care II database that contains clinical signal data reflecting real measurements are used. The results verify that the proposed detection methods improve the correlation relationship between the BP and the PTT, and demonstrate that the adjusted PTT can be used as an indicator of the ABP by removing the dicrotic notch impact on the PPG signal.
    IEEE Transactions on Instrumentation and Measurement 06/2014; 63(6):1431-1439. DOI:10.1109/TIM.2014.2299524 · 1.71 Impact Factor
  • Chang-Chun Hua, X.P. Liu
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    ABSTRACT: The control design problem is investigated for network-based teleoperation systems under the condition of asymmetric and time-varying delays. The classic teleoperation model is considered for which the position of the master is transmitted to the slave site as the control command and the slave torque is directly transmitted to the master in order for the user to have a feeling of the remote interaction. The slave controller is constructed based on the master-slave position error plus a new nonlinear damping function, while the master controller is composed of the transmitted slave torque and the introduced nonlinear damping function. By employing a new Lyapunov Krasovskii functional, we prove the exponential input-to-state stability of the closed-loop system. The relationship is built among the control design parameters and the maximum allowable time delays, which is in the form of linear matrix inequality. Both the simulations and experiments are performed to show the effectiveness of the proposed method. Compared with existing approaches, transmission delays considered are both time varying and asymmetric, and the passivity conditions on the remote environment and the human operator are removed. In addition, the method is very simple.
    IEEE/ASME Transactions on Mechatronics 04/2013; 18(2):764-774. DOI:10.1109/TMECH.2012.2185506 · 3.65 Impact Factor
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    ABSTRACT: The arterial blood pressure is an essential physiological parameter for health monitoring. Most blood measurement devices determine the systolic and diastolic arterial blood pressure through the inflation and the deflation of a cuff. This method is uncomfortable to the user and may cause anxiety which in turns can affect the blood pressure (white coat syndrome). This paper investigates a cuff-less non-intrusive approach to measure arterial blood pressure that is suitable for continuous measurement. The approach is based on measuring the delay between the R-peak of the electrocardiogram (ECG) signal and the peaks of the finger photoplethysmograph (PPG) signal. The results of this paper show a high correlation between the blood pressure and the pulse transit time (PTT). Different polynomial regressions are applied for further estimation. The paper uses actual ECG, PPG and blood pressure measurements extracted from the MIMIC database that contains clinical signal data reflecting real measurements. The simulation results verify that the delay (PTT) between the R-peak of the ECG signal and the peaks of the finger PPG signal have a high correlation with arterial blood pressure and can be used as an indicator of the arterial blood pressure.
    Medical Measurements and Applications Proceedings (MeMeA), 2013 IEEE International Symposium on; 01/2013
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    ABSTRACT: Some diabetic patients experience difficulties in modulating the grip force magnitude when they manipulate objects using their hands. This difficulty is caused by the sensory loss at the fingertips that impairs the feedback loop between the brain and the aforementioned sensors. In this paper, we present a sensory substitution system called “F-Glove”, which is aimed at helping diabetic patients to manipulate objects more efficiently by using appropriate forces. This is achieved by substituting the force felt at the fingertips when grip an object with an audio feedback displayed through the earphones of a mobile phone. The patient wears a glove integrated with pressure sensors mounted on the fingertips, and the sensors' pressure signals are conditioned and wirelessly transmitted to a mobile phone interface to display an audio with a volume linearly proportional to the pressure applied by the fingers of patients.
    Haptic Audio Visual Environments and Games (HAVE), 2013 IEEE International Symposium on; 01/2013
  • Kun Wang, X.P. Liu
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    ABSTRACT: This paper presents the mechanism of visual tracking implemented on a context-aware robotic platform with layered and centralized system architecture. The visual tracking mechanism is developed using Haar-like feature detection algorithm, enhanced by modified Particle Filtering (PF) method, to realize human face tracking and following on a mobile robot platform. Experimental results demonstrate the feasibility and effectiveness of the proposed implementation of visual contexts on the context-aware robotic system.
    Haptic Audio Visual Environments and Games (HAVE), 2013 IEEE International Symposium on; 01/2013
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    IEEE Transactions on Instrumentation and Measurement 12/2012; 61(12-12):3125-3126. DOI:10.1109/TIM.2012.2219153 · 1.71 Impact Factor
  • XP Liu, A Alamri, G Liu
    IEEE Transactions on Instrumentation and Measurement 01/2012; 61(12):3125. · 1.71 Impact Factor
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    Huanran Wang, X.P. Liu
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    ABSTRACT: This paper presents a prototype of noval assistive robotic system, for which the user can interact with a mobile robot using a haptic interface, for senior and disabled people. The system consists of a nonholonomic PeopleBot mobile robot and a Phantom Omni haptic interface. Several technical problems related to the system are addressed and solved. Specifically, a nonlinear tracking algorithm is developed for the robot to follow the user. To preserve the safety operation of the haptic device, a safety control algorithm is applied. Besides, a simple haptic rendering algorithm is designed to generate the haptic feedback based on the position error. Experiments show that the user can guide the movement of the robot quite safely and smoothly using the haptic interface.
    Virtual Environments Human-Computer Interfaces and Measurement Systems (VECIMS), 2012 IEEE International Conference on; 01/2012
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    ABSTRACT: This paper addresses the position tracking problem for the teleoperation systems. With the use of neural network approximating unknown nonlinear functions, a neural network based sliding mode adaptive controller is designed to realize the synchronization of the master site and the slave site. By choosing Lyapunov Krasovskii functions, we prove that the motion tracking error converges to zero asymptotically. Finally, the simulations are performed to show the effectiveness of the proposed method.
    Intelligent Control and Automation (WCICA), 2012 10th World Congress on; 01/2012
  • Huanran Wang, X.P. Liu
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    ABSTRACT: Haptic shared control (HSC) has been widely used in human-robot interaction systems, such as car driving and assistive robots. However, the stability analysis of HSC is not well investigated in current literature, especially for the nonlinear systems. In this paper, the stability of HSC for nonlinear systems is analyzed based on the set theory and convex analysis. The authority weight calculation and haptic rendering algorithm of HSC are developed based on the stability analysis. Then, we apply these algorithms to our assistive robot system. Experimental results show the stable operation of the assistive robot and the effectiveness of the haptic rendering algorithm.
    Haptic Audio Visual Environments and Games (HAVE), 2012 IEEE International Workshop on; 01/2012
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    ABSTRACT: This technical note studies identification problems for dual-rate sampled-data linear systems with noises. A hierarchical least squares (HLS) identification algorithm is presented to estimate the parameters of the dual-rate ARMAX models. The basic idea is to decompose the identification model of a dual-rate system into several sub-identification models with smaller dimensions and fewer parameters. The proposed algorithm is more computationally efficient than the recursive least squares (RLS) algorithm since the RLS algorithm requires computing the covariance matrix of large sizes, while the HLS algorithm deals with the covariance matrix of small sizes. Compared with our previous work, a detailed study of the HLS algorithm is conducted in this technical note. The performance analysis and simulation results confirm that the estimation accuracy of the proposed algorithm are close to that of the RLS algorithm, but the proposed algorithm retains much less computational burden.
    IEEE Transactions on Automatic Control 12/2011; 56(11-56):2677 - 2683. DOI:10.1109/TAC.2011.2158137 · 3.17 Impact Factor
  • Changchun Hua, X.P. Liu
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    ABSTRACT: This paper addresses the controller design problem for teleoperation over networks such as the Internet. The forward and backward network transmission time delays are assumed to be asymmetric and time varying, which is the case for computer network communications. We propose a novel master-slave controller, which renders the entire system stable with relatively good steady- and transient-state performances. The relations among the parameters of the controller and the allowable maximum time delays are built in the form of linear matrix inequality. The designed controller is extended to the case that the velocity information is not available. A high-gain observer is designed to estimate the velocities of the master and slave joints online, and the controller is constructed on the basis of the estimated velocities. The stability of the closed-loop system is proved. Both simulations and experiments are performed to verify the correctness and effectiveness of the proposed method.
    IEEE Transactions on Instrumentation and Measurement 02/2011; 60(1-60):4 - 13. DOI:10.1109/TIM.2010.2065410 · 1.71 Impact Factor
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    ABSTRACT: The linear elastic models of soft tissue are widely used in virtual-reality-based surgery simulation due to their computational efficiency; however, it is well known that these models are only a coarse approximation of the real biological soft tissue. To achieve realistic simulation, the deformable model should incorporate many other tissue properties such as nonlinearity, anisotropy, and viscoelasticity. Among these properties, viscoelasticity is a very important one, and it directly determines the behaviors of the tissue when it is cut, deformed, or torn. In this paper, we proposed to incorporate the property of viscoelasticity into the visual model of soft tissue. One significant advantage of the developed model is its fast computation. Experiments show that the incorporation of nonlinear viscoelasticity makes the simulated tissue look much more realistic than other models, whereas the computation time is increased by only approximately 5% compared with models without the consideration of viscoelasticity.
    IEEE Transactions on Instrumentation and Measurement 02/2011; 60(1-60):14 - 20. DOI:10.1109/TIM.2010.2065450 · 1.71 Impact Factor
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    ABSTRACT: Herein, a novel, original and artful gravity compensation method, namely,the Auto-Regulation Counterweight method (ARC method) is been proposed and based on the ARC method, a haptic interface device with the well-designed ARC mechanism composed of the counterweight link, the counterweight block, the retarding mechanism and an actuator, is newly designed and implemented. A manipulator of the haptic interface device is supported with a series combined mechanism rather than parallel mechanism to obtain the large workspace. Simulations and experiments confirm the validity of the ARC method for auto- regulation gravity compensation. The kinematical equation employed to represent for interface device by the modified D-H frame is constructed and analyzed, and the scopes of workspace of the end-effector in haptic interface device is also determined.
    Haptic Audio Visual Environments and Games (HAVE), 2011 IEEE International Workshop on; 01/2011
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    ABSTRACT: Abstract-The transmission of wave variables rather than power variables in time-delay teleoperation system ensures system passivity rendering the entire system stable, however the delayed force feedback will still lead to the unanticipated motion of master, which makes the system difficult to control and unstable. The aim of this work is to propose an force feedback algorithm, which is easy to implement, to depress unanticipated and disturbing motion. This algorithm would not affect passivity of the whole teleoperation system. The simulation experiments show that the teleoperation system with designed algorithm is stable and easy to be operated by human operator.
    Haptic Audio Visual Environments and Games (HAVE), 2011 IEEE International Workshop on; 01/2011
  • Huanran Wang, X.P. Liu
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    ABSTRACT: This paper presents a novel human-robot interaction system for which haptic interaction is involved. The system consists of a nonholonomic mobile robot and a Phantom Omni haptic device. The tracking problem on the 2D plane is analyzed and the backstepping technique is used for designing the tracking algorithm for the nonholonomic mobile robot. A haptic rendering algorithm is designed to generate the haptic feedback based on the position error information. Experiments show that the user can guide the movement of the robot quite easily and smoothly.
    Haptic Audio-Visual Environments and Games (HAVE), 2010 IEEE International Symposium on; 11/2010