Seong Woo Kwak

Realizing Fault-Tolerant Asynchronous Sequential Machines Using Corrective Control

A control scheme is presented for realizing state fault-tolerance of asynchronous sequential machines. An unobservable disturbance input can infiltrate into asynchronous machines and provoke unauthorized state transitions. The objective is to propose automatic state feedback controllers that detect penetration of the disturbance input and counteract state faults so as to return the controlled machine to its original state. A necessary and sufficient condition for the existence of such a controller is presented in a theoretical framework and an architecture of asynchronous triple modular redundancy is addressed as a case study.

Optimal Checkpoint Placement on Real-Time Tasks with Harmonic Periods

This paper presents an optimal checkpoint strategy for fault-tolerance in real-time systems where transient faults occur in Poisson distribution. In our environment, multiple real-time tasks with different deadlines and harmonic periods are scheduled in the system by rate-monotonic algorithm, and checkpoints are inserted at a constant interval in each task. When a fault is detected, the system carries out rollback to the latest checkpoint and re-executes tasks. The maximum number of re-executable checkpoints and an equation to check schedulability are derived, and the optimal number of checkpoints is selected to maximize the probability of completing all the tasks within their deadlines.

Control oriented model-based simulation and experimental studies on a compliant legged quadruped robot

Quadruped robots offer better maneuverability over wheeled mobile robots. However, a quadruped robot contains many joint actuators which have to operate in a coordinated fashion to achieve the desired locomotion. Joint actuations cause various degrees of disturbance on the robot body and may even destabilize the system. Thus, prior dynamic analysis plays an important role for development of control laws for quadruped locomotion. Here, a three dimensional dynamic model of a quadruped has been developed using the bond graph technique which can be interfaced with various controller models. This model contains a detailed sub-model for telescopic compliant legs. Results from simulations, animations and experiments are discussed. Turning motion at various leg speeds is studied for dynamic stability of the robot. The effect of leg compliance on locomotion parameters is studied which helps in selecting a suitable compliance. Performance measure is carried out using energy efficiency as deciding criteria. Study on energy efficient quadruped structure, energy efficient locomotion gait and foot trajectory have been carried out for designing an efficient quadruped. Paper presents three dimensional dynamic model of quadruped with compliant legs.Model is verified with simulation, animation and experiment results.Turning motion is demonstrated by providing differential leg tip velocity.Influence of leg compliance on quadruped locomotion is studied.Energy efficient structure, gait and foot trajectory have been carried out.

Output feedback control of asynchronous sequential machines with disturbance inputs

This paper presents a control scheme for input/output asynchronous sequential machines with disturbance inputs. An unobservable and uncontrollable disturbance input can infiltrate into asynchronous machines and provoke unauthorized state transitions. We present the existence condition for a corrective controller that automatically counteracts the effects of disturbance inputs and restores desirable behavior to the controlled machine. Unlike input/state asynchronous machines, access to the machines state is not available in the control procedure of input/output machines, so the control scheme requires state observation and fault detection modules. As a case study, the architecture of an asynchronous clock divider with the corrective controller is implemented and experimental verification on FPGA is provided for showing the applicability of the proposed controller.

Model Matching for Asynchronous Sequential Machines with Uncontrollable Inputs

The problem of model matching for finite-state asynchronous sequential machines is examined. In particular, the considered asynchronous machine may receive uncontrollable external inputs, i.e., of which values the controller cannot change or disable. For realizing model matching with a reference model, the asynchronous machine must have additional reachability to deal with transitions by uncontrollable inputs. Necessary and sufficient conditions for the existence of an appropriate controller are given in terms of a reachability relation between the machine and the model. A characterization of feasible control laws is derived and algorithms for their design are outlined.

Control of compliant legged quadruped robots in the workspace

Locomotion control of a quadruped robot requires a well-defined gait pattern (i.e., a coordinated actuation of its four legs in some particular fashion with respect to time). It is of practical importance to move the leg tips in a desired trajectory in order to achieve specific objectives such as to avoid obstacles, minimize energy consumption and locomotion time. Along with body displacement, body orientation is an equally important limit parameter during each leg step of the gait pattern. There are several possible gait patterns that maintain stable and aesthetically pleasing locomotion, and most of these are biologically inspired. This article presents quadruped locomotion control in the workspace through a novel control scheme in which the leg forward motion is controlled in the workspace while the body forward motion is controlled by providing the required effort directly to the joint actuators. In this control approach, the leg tip trajectory error drives a proportional-integral controller that is then transformed through the Jacobian to generate the corrective joint torques. For the body forward motion, leg motion is arrested and the joints are provided with opposite motion, which are controlled by a proportional-integral-derivative controller. The proposed method is simple and easy to implement in the workspace. The performance of the proposed control scheme is evaluated through simulations and animations.

Probabilistic optimisation of checkpoint intervals for real-time multi-tasks

This article considers the checkpoint placement problem for real-time systems. In our environment, multiple real-time tasks with arbitrary periods are scheduled in the system by the rate monotonic algorithm, and checkpoints are inserted at a constant interval in each task while the width of the interval is different with respect to the task. We derive an explicit formula of the probability that all the tasks are successfully completed with a given set of checkpoint intervals. Then we determine the optimal checkpoint intervals that maximise the probability of task completion. The probability computation includes the schedulability analysis with respect to the numbers of re-executed checkpoint intervals. Our method does not necessitate any algebraic condition on the periods of the scheduled tasks.

Stability Analysis of a Simple-Structured Fuzzy Logic Controller

The stability analysis for the fuzzy logic controller (FLC) has widely been reported. Furthermore some researches have been introduced to simplify the design process of FLC’s. One of them is to decrease the number of parameters representing the antecedent part of the fuzzy control rule. So we briefly explain a simple-structured fuzzy logic controller (SFLC) which uses only a single variable at the antecedent part of a fuzzy control rule. We analyze that it is absolutely stable based on the sector bounded condition. We expand a nonlinear controlled plant into a Taylor series about a nominal operating point. The fuzzy control system is transformed into a Lure system with nonlinearities. We also show the feasibility of the proposed stability analysis through a numerical example of a mass-damper-spring system.

Fuzzy logic-based tuning of the boundary layer thickness of the variable structure controller

ABSTRACT The variable structure control (VSC) is a simple and powerful nonlinear controller, but it leads to a high frequency chattering on the control input. To decrease the chattering phenomenon of the VSC, a boundary layer is commonly introduced. Then its thickness requires a compromise between the steady state error and the chattering amplitude. In this paper, we propose a new VSC that tunes the boundary layer thickness using the fuzzy logic system. The tuning methods presented are two: One uses absolute error and its derivative as fuzzy input variables, which decreases the number of tuning rules as compared with using common fuzzy variables of error and change of error. The other uses only a single fuzzy variable of a distance. This variable is derived by the property of two-dimensional rule table which is composed of absolute error and its derivative. Since the second method uses a single variable for tuning the thickness, the number of tuning rules is greatly decreased. Furthermore, we obtain the g...

Tolerating intermittent faults in input/state asynchronous sequential machines with a bounded delay

We address fault diagnosis and tolerance in dynamics of asynchronous sequential machines based on corrective control theory. The controlled asynchronous machine has input/state type where the present state is given as the output value. We consider a kind of intermittent faults that cause unauthorized state transitions to the machine, and that persist for finite time after the initial occurrence. Not only does the machines stable reachability decrease as a result of the fault occurrence, but also immediate state recovery is made impossible. We examine the existence condition and design algorithm for a corrective controller that tolerates the adverse effect of intermittent faults with degraded performance, that is, the closed-loop system regains the normal behavior within a bounded delay. Through an illustrative example, we demonstrate the proposed fault tolerant control methodology.