Browsing by Author "Zheng, Hao"

Now showing 1 - 3 of 3
  • Thumbnail Image
    Item
    Damage detection and sensor placement optimization in composite structures
    (University of Alabama Libraries, 2012) Zheng, Hao; Roy, Samit; University of Alabama Tuscaloosa
    Structural health monitoring, damage identification method and sensor placement optimization for carbon fiber reinforced polymer (CFRP) composite beam were studied in this thesis. In this work, different methodologies were investigated for the damage detection process to enhance the use of current structural health monitoring systems by identifying the optimal sensor placement. Carbon fiber reinforced polymer composite materials were fabricated and the fabrication process based on vacuum assisted resin transfer molding (VARTM) is briefly introduced. Numerical analysis using finite element method was subsequently performed for a composite beam based on the material properties determined by performing experimental material characterization tests. Three benchmarking tests with different types of elements were performed to verify the best method for modeling the composite panel. Moreover, shear lag analysis was also presented to model an embedded crack in the composite panel which would be used in damage detection and optimization process. Based on the finite element analysis and static strain data extracted, a comparative study on two damage detection algorithms based on artificial neural network (ANN) and support vector machine (SVM) is presented. The viability of these two methods was demonstrated by analysis of the numerical model of composite beam with a crack embedded in it and the performance for each algorithm is also presented with different number of sensors and different noise levels. Two experiments are presented for the performance evaluation of damage detection and identification. To identify the optimal locations of sensors in the optimization process, a statistical probability based method using the combination of artificial neural networks and evolutionary strategy were developed to increase the detection rate of damage in a structure. The proposed method was able to efficiently increase the detection accuracy compared with a uniform distribution of sensors for a composite beam that was damaged in different locations. The finite element model of the composite coupon was used as a representation of the real structure. Static strain data from finite element simulation was extracted with different damage scenarios and used as feature vector for the classification process. Based on the performance of the classification for a given sensor configuration, updated sensor locations would be selected by changing the coordinates of these sensor locations using strategy parameters. The viability of this method was demonstrated by conducting different examples and significant number of simulations was performed to check the repeatability of the algorithm.
  • Thumbnail Image
    Item
    Design and control of powered transtibial prostheses
    (University of Alabama Libraries, 2017) Zheng, Hao; Shen, Xiangrong; University of Alabama Tuscaloosa
    The primary purpose of lower extremity prostheses is to restore the locomotive functions of amputees’ lost sections and joints. The objective of this dissertation is to develop energetically active transtibial (TT, also known as below-knee) prostheses with powered ankle joints to generate sufficient power and torque with a compact form factor. Firstly, pneumatic sleeve muscle actuators have been designed and tested to investigate the feasibility of application of such type of actuator in TT prosthesis. Experimental results obtained on the prototypes validated and proved that sleeve muscle is a good fit for robotic systems with asymmetric force/torque requirements. Although the pneumatic sleeve muscle can provide sound force/torque capacities, it is challenging to build a transtibial prosthesis with this type of actuator that can match the size and weight of the human ankle. Hence, the subsequent efforts are directed towards utilizing a more compact actuation unit – pneumatic cylinder, which is well known for its capability in generating large force/power output with light weight and compact volumetric profile. Thus, a transtibial prosthesis has been designed using pneumatic cylinder. A finite-state impedance controller (FSIC) has been developed as a walking controller, and the parameters are tuned in walking experiments. The results from the experiments proved that the prosthesis is able to provide an improved gait compared with the traditional passive prosthesis. Additionally, a model that characterizes the stiffness and equilibrium point as functions of the chamber air masses in the pneumatic cylinder has been developed and a predictive pressure control algorithm was used to improve pressure control performance while minimizing valve actions. This enables the pneumatic actuator to be used as a variable series elastic actuator (VSEA). Experimental results showed that the proposed approach is able to provide the desired elastic characteristics of an artificial spring in stiffness control and demonstrated the advantages of this new approach for potential prosthetic applications. Lastly, the dissertation presents VSEA-powered TT prosthesis with direct implementation of the finite-state impedance control (FSIC). The human subject walking experiments were also conducted, and the results demonstrated the effectiveness of the direct FSIC prosthesis control approach.
  • Thumbnail Image
    Item
    Pneumatic Variable Series Elastic Actuator
    (ASME, 2016) Zheng, Hao; Wu, Molei; Shen, Xiangrong; University of Alabama Tuscaloosa
    Inspired by human motor control theory, stiffness control is highly effective in manipulation and human-interactive tasks. The implementation of stiffness control in robotic systems, however, has largely been limited to closed-loop control, and suffers from multiple issues such as limited frequency range, potential instability, and lack of contribution to energy efficiency. Variable-stiffness actuator represents a better solution, but the current designs are complex, heavy, and bulky. The approach in this paper seeks to address these issues by using pneumatic actuator as a variable series elastic actuator (VSEA), leveraging the compressibility of the working fluid. In this work, a pneumatic actuator is modeled as an elastic element with controllable stiffness and equilibrium point, both of which are functions of air masses in the two chambers. As such, for the implementation of stiffness control in a robotic system, the desired stiffness/equilibrium point can be converted to the desired chamber air masses, and a predictive pressure control approach is developed to control the timing of valve switching to obtain the desired air mass while minimizing control action. Experimental results showed that the new approach in this paper requires less expensive hardware (on-off valve instead of proportional valve), causes less control action in implementation, and provides good control performance by leveraging the inherent dynamics of the actuator.