George T.-C. Chiu

Adaptive robust motion control of single-rod hydraulic actuators: theory and experiments

High-performance robust motion control of single-rod hydraulic actuators with constant unknown inertia load is considered. The two chambers of a single-rod actuator have different areas, so the dynamic equations describing the pressure changes in them cannot be combined into a single load pressure equation. This complicates controller design since it not only increases the system dimension but also brings in the stability issue of the added internal dynamics. A discontinuous projection-based adaptive robust controller (ARC) is constructed. The controller takes into account not only the effect of parameter variations coming from the inertia load and various hydraulic parameters but also the effect of hard-to-model nonlinearities such as uncompensated friction forces and external disturbances. It guarantees a prescribed output tracking transient performance and final tracking accuracy in general while achieving asymptotic output tracking in the presence of parametric uncertainties. In addition, the zero error dynamics for tracking any nonzero constant velocity trajectory is shown to be globally uniformly stable. Experimental results are obtained for the swing motion control of a hydraulic arm and verify the high-performance nature of the proposed strategy. In comparison to a state-of-the-art industrial motion controller, the proposed algorithm achieves more than a magnitude reduction of tracking errors. Furthermore, during the constant velocity portion of the motion, it reduces the tracking errors almost down to the measurement resolution level.

Contouring control of machine tool feed drive systems: a task coordinate frame approach

We show that contouring performance can be viewed as a regulation problem in a moving task coordinate frame that is attached to the desired contour. By transforming the machine tool feed drive dynamics to this task coordinate frame, a control law is designed to assign different dynamics to the tangential and normal directions. The transformation also illustrated the effect of contour curvature and feed rate in the control action as well as the system dynamics in the task coordinate frame. The resulting control law consists of a linear time-varying proportional-plus-derivative (PD) position error feedback control law and a linear time invariant trajectory feedforward control law. Experimental results of the proposed control law on the motion axes of a machining center, Matsuura MC510V, showed significant improvement of the contouring accuracy compared to the existing servo controller as well as the successful decoupling between the tangential dynamics and the normal (contouring) dynamics for feedrate up to 6 m/min (4 in/s).

Extremum seeking control of a tunable thermoacoustic cooler

In this paper, the performance of a prototype standing wave thermoacoustic cooler is optimized using an extremum seeking control (ESC) algorithm. A tunable Helmholtz resonator was developed for a thermoacoustic cooler to change the boundary condition of the standing wave tube. The volume of the resonator is changed by changing the location of a piston on a ball-screw assembly driven by a dc motor. Multiparameter ESC was applied to optimize the cooling power via tuning both the boundary condition (piston location) and the driving frequency. Experiments were conducted for the online optimization under both fixed and varying operating conditions. The experimental results demonstrated the effectiveness of using ESC for maintaining maximum achievable performance. The effect of changing parameters in the ESC algorithm on the transient behavior was also investigated.

Coordinated Position Control of Multi-Axis Mechanical Systems

Position coordination of multi-axis mechanical systems is studied. Under the assumption that the coordination objective can be represented by smooth functions of the positions of the multiple axes control systems, a necessary coupling effect can be introduced to each axis by the proper choice of a Lyapunov-like function. The resulting control law requires the knowledge of the desired trajectories and their time derivatives as well as actual position and velocity information. Implementation of the proposed control algorithm on a CNC feed drive system shows significant improvement in dimensional accuracy during high speed contouring.

Scanner identification using sensor pattern noise

Digital images can be captured or generated by a variety of sources including digital cameras and scanners. In many cases it is important to be able to determine the source of a digital image. This paper presents methods for authenticating images that have been acquired using flatbed desktop scanners. The method is based on using the pattern noise of the imaging sensor as a fingerprint for the scanner, similar to methods that have been reported for identifying digital cameras. To identify the source scanner of an image a reference pattern is estimated for each scanner and is treated as a unique fingerprint of the scanner. An anisotropic local polynomial estimator is used for obtaining the reference patterns. To further improve the classification accuracy a feature vector based approach using an SVM classifier is used to classify the pattern noise. This feature vector based approach is shown to achieve a high classification accuracy.

Printer identification based on graylevel co-occurrence features for security and forensic applications

In todays digital world securing different forms of content is very important in terms of protecting copyright and verifying authenticity. Many techniques have been developed to protect audio, video, digital documents, images, and programs (executable code). One example is watermarking of digital audio and images. We believe that a similar type of protection for printed documents is very important. The goals of our work are to securely print and trace documents on low cost consumer printers such as inkjet and electrophotographic (laser) printers. We will accomplish this through the use of intrinsic and extrinsic features obtained from modelling the printing process. In this paper we describe the use of image texture analysis to identify the printer used to print a document. In particular we will describe a set of features that can be used to provide forensic information about a document. We will demonstrate our methods using 10 EP printers.

Forensic techniques for classifying scanner, computer generated and digital camera images

Digital images can be captured or generated by a variety of sources including digital cameras, scanners and computer graphics softwares. In many cases it is important to be able to determine the source of a digital image such as for criminal and forensic investigation. This paper presents methods for distinguishing between an image captured using a digital camera, a computer generated image and an image captured using a scanner. The method proposed here is based on the differences in the image generation processes used in these devices and is independent of the image content. The method is based on using features of the residual pattern noise that exist in images obtained from digital cameras and scanners. The residual noise present in computer generated images does not have structures similar to the pattern noise of cameras and scanners. The experiments show that a feature based approach using an SVM classifier gives high accuracy.

Motion synchronization for dual-cylinder electrohydraulic lift systems

This paper presents a nonlinear control algorithm to address the motion synchronization problem for a dual-cylinder electrohydraulic (EH) lift system. A two-step design approach is applied that utilized linear multiple input-multiple output (MIMO) robust control technique to design an outer-loop motion synchronization controller. A nonlinear single input-single output (SISO) perturbation observer-based pressure-force controller is designed for each of the lift cylinders as the inner-loop controller to handle the nonlinearities associated with the EH actuators. Experimental results on a 2-cylinder lift system verified the effectiveness of the proposed approach.

Printer and scanner forensics

Contrary to popular opinion, the use of paper in our society will not disappear during the foreseeable future. In fact, paper use continues to grow rather than decline. It is certainly true that as individuals, we may be printing less than we used to. And the role of paper has been transformed from the archival record of a document to a convenient and aesthetically appealing graphical user interface. The use of paper is now intimately linked to the electronic systems that capture, process, transmit, generate, and reproduce textual and graphic content. Paper can be thought of as an interface between humans and the digital world. If this interface is not secure, the entire system becomes vulnerable to attack and abuse. Although paper is read by humans in the same way that it has been for millennia and has had the same fundamental form and composition for almost that long, the technologies for printing and scanning documents and capturing their content have evolved tremendously, especially during the last 20 years. This has moved the capability to generate printed documents from the hands of a select few to anyone with access to low-cost scanners, printers, and personal computers. It has greatly broadened the opportunities for abuse of trust through the generation of fallacious documents and the tampering with existing documents, including the embedding of messages in these documents.

Nonlinear adaptive robust control of electro-hydraulic servo systems with discontinuous projections

Studies the high performance robust motion control of electro-hydraulic servo-systems. The dynamics of hydraulic systems are highly nonlinear and the system may be subjected to non-smooth and discontinuous nonlinearities due to directional change of valve opening, friction, and valve overlap. Aside from the nonlinear nature of hydraulic dynamics, hydraulic servo systems also have a large extent of model uncertainties. To address these challenging issues, adaptive robust control (ARC) is applied and an ARC controller based on discontinuous projection method is constructed. The resulting controller is able to take into account the effect of the parameter variations of inertia load and cylinder and the uncertain nonlinearities such as uncompensated friction forces and external disturbances. Non-differentiability of the inherent nonlinearities associated with hydraulic dynamics is carefully examined and addressing strategies are provided. Compared with a previously proposed ARC controller, the ARC controller in the paper has a more robust parameter adaptation process and is more suitable for implementation.