Lilong Cai

A fuzzy basis function vector-based multivariable adaptive controller for nonlinear systems

In this paper, a new fuzzy basis function vector (FBFV) approach for the adaptive control of multivariable nonlinear systems is presented. With this method, the nonlinear plant is first linearized. The linearized bias and uncertainties as well as disturbances are assumed to be included in the model structure and their upper bound will be adaptively learned by the FBFV method. The output of the FBFV is used as the parameters of the robust controller in the sense that both the robustness and the asymptotic error convergence can be obtained for the multivariable nonlinear system. The effectiveness of the proposed analysis and design method is illustrated with a simulated example.

Iterative algorithm with a constraint condition for numerical reconstruction of a three-dimensional object from its hologram.

A novel method to obtain the three-dimensional mathematical model of a transparent object from its hologram is presented. The proposed method can numerically extract the object information from the fringe pattern of the hologram. Then an iterative algorithm is used to imitate an imaging system by focusing on different layers of the object; and by operating in both the spatial domain and the frequency domain, the algorithm produces a series of two-dimensional layer images. The object is finally reconstructed layer by layer. A constraint condition should be satisfied, and the noise distribution can be rearranged in different reconstruction cycles so as to get better reconstruction quality. Numerical simulations have proved the effectiveness of the proposed method.

Decentralized nonlinear adaptive control of an HVAC system

This paper presents a new decentralized nonlinear adaptive controller (DNAC) for a heating, ventilating, and air conditioning (HVAC) system capable of maintaining comfortable conditions under varying thermal loads. In this scheme, an HVAC system is considered to be two subsystems and controlled independently. The interactions between the two subsystems are treated as deterministic types of uncertain disturbances and their magnitudes are supposed to be bounded by absolute value. The decentralized nonlinear adaptive controller (DNAC) consists of an inner loop and an outer loop. The inner loop is a single-input fuzzy logic controller (FLC), which is used as the feedback controller to overcome random instant disturbances. The outer loop is a Fourier integral-based control, which is used as the frequency-domain adaptive compensator to overcome steady, lasting uncertain disturbances. The global DNAC controller ensures that the system output vector tracks a desired trajectory vector within the system bandwidth and that the tracking error vector converges uniformly to a zero vector. The simulated experimental results on the HVAC system show that the performance is dramatically improved.

A smooth robust nonlinear controller for robot manipulators with joint stick-slip friction

The design of a smooth robust nonlinear controller for the regulation of an n-degree-of-freedom rigid robot with internal joint stick-slip friction is presented. It is shown that not only positioning accuracy, but also smoothness of motion is improved through nonlinear feedback. The Lyapunov direct method is employed to prove the global stability of the closed-loop system. The desired accuracy can be achieved by adjusting the parameters of the controller. The nonlinear compensator is robust with respect to the character of the slipping torque. Only the maximum static friction torque of the internal joint is required for controller design. The numerical implementation of this controller on a two-link robot is presented.<<ETX>>

An approach to force and position control of robot manipulators

An approach is proposed to simultaneously control the contact forces exerted on a constraint surface and the position of the end-effector, both in the presence of frictional forces on the constraint surface. The approach, using a novel formulation of the dynamics model, is based on a linear state feedback. A feedforward and PI (proportional-integral) feedback control strategy is proposed along with a design procedure. Using the proposed control method the trajectory following problem is solved and the closed-loop system is shown to be asymptotically stable. Numerical examples of a two-link robot moving along a constraint surface are presented for illustration.<<ETX>>

Numerical reconstruction of digital holograms with variable viewing angles

Here we describe a new method for numerically reconstructing an object with variable viewing angles from its hologram(s) within the Fresnel domain. The proposed algorithm can render the real image of the original object not only with different focal lengths but also with changed viewing angles. Some representative simulation results and demonstrations are presented to verify the effectiveness of the algorithm.

Interferometer for small-angle measurement based on total internal reflection

We describe a new method for angle measurement based on the internal-reflection effect and heterodyne interferometry. A novel prism assembly is designed that can always parallel retroreflect the incoming light beams so the optical configuration is compact. As a differential common-path optical configuration is integrated into the design, the linearity of the method is greatly improved. Details of theoretical analysis of the method and experimental verification of the principle are presented. The resolution can be better than 0.3 arc sec. The experimental results and further improvements of the proposed method are also addressed.

A learning controller for robot manipulators using Fourier series

We proposed a new learning controller for decentralized tracking control of nonlinear robot manipulators. When the desired trajectory of each subsystem of the robot lasts for a finite duration, it can be approximated by a Fourier series with constant harmonic magnitudes. For each subsystem of the robot, a learning controller is designed to individually control each harmonic component of the actual output, although it is cross-related to other components in nonlinear systems. The learning algorithm is designed such that each harmonic magnitude of the actual output converges to that of the desired trajectory within the system bandwidth. Since this decentralized learning controller is designed in Fourier space instead of time domain, the systems time-delay could be easily compensated. This learning controller is only based on the local input and output information; no a priori structure or parameters of the system model are required. The experimental results on a 3-DOF direct-drive robot are presented.

Multidimensional data encryption with digital holography

A new method based on the concept of virtual optics for both encryption and decryption is proposed. It analyzes the potential reasons of the limitations of our earlier work and presents some new approach to improve the original algorithm. Different methods of numerical reconstruction for data encryption are compared, the limitations caused by off-axis setups are discussed, and the resolution of the decrypted image could be flexibly adjusted, which makes it possible to decrypt the image with virtually no loss of data. Numerical experiments are performed to validate the proposed method, and sensitivities of some parameters are quantitatively analyzed and illustrated.

An autofocusing measurement system with a piezoelectric translator

An autofocusing system that consists of a compact disc (CD) optical head and a piezoelectric translator (PZT) is developed for measuring the displacement or profile of a tested surface. The displacement or the profile of the tested surface can be obtained with varying accuracy by measuring the curve of the driving voltage to the expansion distance of the PZT or by using a secondary sensing device of displacement. The resolution of the system can reach 10 nm and the accuracy is 0.05 /spl mu/m. Some design considerations, such as the control algorithm of the focus servo and error analysis of the system, are addressed in this paper.