Jung-Min Yang

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.

Brief paper: Model matching inclusion for input/state asynchronous sequential machines

The problem of model matching for asynchronous sequential machines consists of finding a feedback controller for a given open-loop machine so that the resulting closed-loop machine matches a desired model. In this paper, the control objective is extended to model matching inclusion in which the behavior of the closed-loop system should be contained in that of the model. The supremal controllable sub-model is characterized as an asynchronous machine with the largest behavior set contained in a given model that can be matched by the closed-loop machine via state feedback control. An effective computational algorithm is developed and an example is provided for illustration.

Brief paper: Supervisory control for real-time scheduling of periodic and sporadic tasks with resource constraints

In the framework of supervisory control of timed discrete event systems, this paper addresses the design problem of a real-time scheduler that meets stringent time constraints of periodic tasks and sporadic tasks which exclusively access shared resources. For this purpose, we present the timed discrete event models of execution of periodic tasks and sporadic tasks and resource access for shared resources. Based on these models, we present the notion of deadlock-free and schedulable languages that contain only deadline-meeting sequences which do not reach deadlock states. In addition, we present the method of systematically computing the largest deadlock-free and schedulable language, and it is also shown that schedulability analysis can be done using this language. We further show that the real-time scheduler achieving the largest deadlock-free and schedulable language is optimal in the sense that there are no other schedulers to achieve schedulable cases more than those achieved by the optimal scheduler.

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.

Fault Tolerance in Asynchronous Sequential Machines Using Output Feedback Control

This note presents a control scheme for fault diagnosis and tolerance in asynchronous sequential machines. The considered asynchronous machine is subject to permanent faults, so the system remains in the faulty condition indefinitely after the occurrence of fault inputs. When the machine has a certain structural redundancy, we can design a corrective controller and observer that diagnose faults and compensate the closed-loop system so that it can maintain the normal input/output behavior. The proposed framework is based on the output feedback control scheme and asynchronous techniques.

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.

Modeling and Control of Switched Asynchronous Sequential Machines

This note presents a model for switched asynchronous sequential machines (ASMs) and utilizes corrective control to solve their model matching problem. A switched ASM comprising a number of single ASMs or submachines can change its mode or the submachine in which it is operating in an asynchronous mechanism. We obtain a matrix expression for the reachability of switched ASMs, based on which we present the existence condition and design algorithm for a corrective controller that matches the stable-state behavior of the closed-loop system to that of a reference model. The corrective controller for switched ASMs provides not only control input characters but also switching signals to utilize the reachability of each submachine in generating required feedback paths. The constraint on the switching operation caused by the asynchronous mechanism is also discussed.