Stuart Bennett

Modelling the dynamics of supply chains

A summary of the important issues arising in supply chain research is presented. A review of the various mathematical methods used to model and analyse supply chains is then undertaken. They are categorized as: continuous time differential equation models, discrete time difference equation models, discrete event simulation models and operational research techniques. Each method is then appraised from a system dynamics perspective. To conclude, a judgement is made on whether the merits of operational research techniques justify their predominance in industry over the other methods presented here.

Development of the PID controller

In 1939, the Taylor Instrument Companies introduced a completely redesigned version of its Fulscope pneumatic controller. In addition to proportional and reset control actions, this new instrument provided an action which the Taylor Instrument Companies called pre-act. In the same year the Foxboro Instrument Company added Hyper-reset to the proportional and reset control actions provided by their Stabilog pneumatic controller. The pre-act and Hyper-reset actions each provide a control action proportional to the derivative of the error signal. Reset provides a control action proportional to the integral of the error signal and hence both controllers offered PID control. The historical development of these controllers is discussed.<<ETX>>

A brief history of automatic control

The author looks at the history of automatic control in four main periods as follows: early control up to 1900; the pre-classical period 1900-1940; the classical period 1935-1960; and modem control post 1955. This article is concerned with the first three of the above; other articles in this issue deal with the more recent period.

A taxonomy for software voting algorithms used in safety-critical systems

Voting algorithms are used to provide an error masking capability in a wide range of highly dependable commercial & research applications. These applications include N-Modular Redundant hardware systems and diversely designed software systems based on N-Version Programming. The most sophisticated & complex algorithms can even tolerate malicious (or Byzantine) subsystem errors. The algorithms can be implemented in hardware or software depending on the characteristics of the application, and the type of voter selected. Many voting algorithms have been defined in the literature, each with particular strengths and weaknesses. Having surveyed more than 70 references from the literature, a functional classification is used in this paper to provide taxonomy of those voting algorithms used in safety-critical applications. We classify voters into three categories: generic, hybrid, and purpose-built voters. Selected algorithms of each category are described, for illustrative purposes, and application areas proposed. Approaches to the comparison of algorithm behavior are also surveyed. These approaches compare the acceptability of voter behavior based on either statistical considerations (e.g., number of successes, number of benign or catastrophic results), or probabilistic computations (e.g., probability of choosing correct value in each voting cycle or average mean square error) during q voting cycles.

The past of pid controllers

Abstract The history of pneumatic PID controllers covering the invention of the flapper-nozzle amplifier, the addition of negative feed back to the amplifier and the incorporation of rest (integral) and pre-act (derivative) actions is described. The transition of the controller from a special purpose unit to a robust, reliable, widely used unit; the change to electronic implementation and then the development of the digital controller is examined. It is concluded that a systems approach to control was important in the development of PID controllers as was a close relationship between instrument companies, plant designers and plant operators.

Nicholas Minorsky and the automatic steering of ships

Prefecture, as an Associate Professor. He received the D.Engr. degree from the Universitlof Tokyo. From 1961, he served as a Professor at the Yokihama National University. Dr. Masubuchi joined the Faculty of Engineering, Osaka University, Osaka, as a Professor in 1969. He has been working on industrial process dynamics analysis and control, and on adaptive control. Hidenori Kimura was born in Tokyo on November 3. 1941. He received the B.Engr., M.Engr., and

Smoothing voter: a novel voting algorithm for handling multiple errors in fault-tolerant control systems

Abstract Voting algorithms are used to arbitrate between variant results in a wide range of highly dependable real-time control applications. These applications include N-Modular Redundant hardware systems and diversely designed software systems based on N-Version Programming. The most sophisticated and complex voting algorithms can even tolerate malicious (or Byzantine) variant errors. Voting algorithms can be implemented in either hardware or software depending on the characteristics of the application and the type of voter selected. The behaviour of voting algorithms in multiple error scenarios is considered in this article. Complete disagreement is defined as those cases where no two variant results are the same. A novel algorithm for real-time control applications, the smoothing voter, is introduced and its behaviour compared with three previously published voters. Software implemented error-injection tests, reported here, show that the smoothing voter achieves a compromise between the result selection capabilities of the median voter and the safety features of the majority voter. The smoothing voter is an appropriate voter for applications in which maximising the number of correct outputs and minimising the number of benign errors of the system is the main concern, and a slight degradation in safety can be tolerated.

PID control for a distributed system with a smart actuator

Abstract The emergence of distributed architectures based on smart components and fieldbus networks is promoting changes in proportional-integral-derivative (PID) controller design issues. This paper explores how PID control can benefit from smart actuator and fieldbus technologies. Firstly, this paper discusses a smart actuator scheme to improve the efficiency of PID controller retuning, as well as an implementation using a low-cost stepper-motor. Then, the smart actuator is applied to on-line adaptation of PID parameters using a standard pole-placement design method. Finally, experimental validation of the proposed approach is conducted using a controller area network (CAN) bus-based distributed architecture demonstrator.

History-based weighted average voter: a novel software voting algorithm for fault-tolerant computer systems

Voting algorithms have been widely used in the realisation of fault-tolerant systems. We propose a new software voting algorithm which uses the history record of redundant modules to compute the final output. Two versions for the novel algorithm are introduced In the first version any module result is assigned to a weighting value such that module results with higher history record value, are assigned to a higher weighing value than those with lower history record value. In the second version of the novel voter, those module results which have a history record value, less than the average record value, are allocated a weight of zero and removed from the contribution toward the voter output. Furthermore, a novel method for creation of a history record of modules is proposed. Empirical results show that both versions of the novel voter give higher safety performance than the Standard Weighted Average voter with permanent and transient errors.

Experimental comparison of voting algorithms in cases of disagreement

Voting algorithms are used to mask erroneous results from redundant subsystems (variants) in fault tolerant systems. While voting algorithms are well known and widely used, the authors are not aware of any detailed investigation of voter behaviour in the presence of multiple errors. The results of an experimental evaluation of seven voting algorithms in a variety of simulated error scenarios are reported. A software error injection approach is used to simulate multiple errors in a triple modular redundant configuration. The Majority and Plurality voters produce the lowest number of catastrophic errors, in these tests. The Median voter produces the largest number of correct results, but also produces the largest number of catastrophic errors. The Three Domain voter results show a compromise between the large number of correct results identified by the Median voter and the small number of catastrophic results produced by the Majority voter.