Michael Short

Fault-Tolerant Time-Triggered Communication Using CAN

The controller area network (CAN) protocol was originally introduced for automotive applications but is now also widely used in process control and many other industrial areas. In this paper, we present a low-cost redundancy-management scheme for replicated CAN channels that helps to ensure that clocks (and, hence, tasks) on the distributed nodes remain synchronized in the event of failures in the underlying communication channels, without the need for expensive or proprietary interface electronics. We argue that, when using this framework with duplicated channels, the probability of inconsistent message delivery drops to acceptable levels for a wide range of systems. Through an analysis of the protocol and a case study, we conclude that the creation of reliable, low-cost, distributed embedded systems using CAN is a practical possibility.

Adaptive and Nonlinear Fuzzy Force Control Techniques Applied to Robots Operating in Uncertain Environments

For robots to perform many complex tasks there is a need for robust and stable force control. Linear, fixed-gain controllers can only provide adequate performance when they are tuned to specific task requirements, but if the environmental stiffness at the robot/task interface is unknown and varies significantly, performance is degraded. This paper describes the design of two nonlinear, fuzzy force controllers, developed primarily using analytical methods, which overcome the problems of conventional control. Using simulation and an experimental robot, they are shown to perform well over a wide range of stiffness and both a quantitative and qualitative assessment of their performance compared with conventional force control is presented.

Calorimeters and Techniques Used for Power Loss Measurements in Electrical Machines

This article provides a quick reference guide to calorimeters for accurate power measurement.

Near-optimal scheduling of residential smart home appliances using heuristic approach

This paper presents an efficient heuristic approach for scheduling residential smart home appliances. Using available hourly prices for electricity, the starting times of a supplied set of appliances are optimized so that the economic cost of the energy consumed is reduced, while satisfying the operational and peak power constraints. The algorithm schedules appliances one after the other based on a greedy strategy. The heuristic (c.f. exact) approach is taken to reduce the computational burden to a level allowing re-optimization to take place at regular intervals by a modest computing device without specialized software, which could be embedded in a smart meter. The proposed algorithm is evaluated through a preliminary experimental study comparing the obtained costs and computation times with an exact algorithm. Results indicate that the obtained cost was within 5% of the optimal cost, while the computation time reduced by exponential factors.

Improved Task Management Techniques for Enforcing EDF Scheduling on Recurring Tasks

The management of tasks is an essential requirement in most real-time and embedded systems, but invariably leads to unwanted CPU overheads. This paper is concerned with task management in real-time and embedded systems employing the Earliest Deadline First (EDF) scheduling algorithm. Currently, the best known techniques to manage EDF scheduling lead to overheads with complexity O(log n), where n is the number of recurring (periodic/sporadic) tasks. In this paper it will be shown that if both the ready and waiting queues are represented by either i) timing and indexed deadline wheels or ii) digital search trees, then all scheduling decisions may be made in time proportional to the logarithm of the largest time representation required by the system, pm. In cases where pm is relatively small, for example in some embedded systems, extremely efficient task management may then be achieved. Experimental results are then presented, and it is shown that on an ARM7 microcontroller, when the number of tasks is comparatively large for such a platform (≫ 250), the worst-case scheduling overheads remain effectively constant and below 20 µs. The results indicate that the techniques provide some improved performance over previous methods, and also seem to indicate that there is little discernable difference between the overheads incurred between employing a fixed- or dynamic-priority scheduler in a given system.

Simple bounds on deadline failure probabilities in fault-tolerant real-time networks

Real-time communication networks are often required to operate reliably in harsh environments which expose the system to random errors. Although probabilistic schedulability analysis can employ rich stochastic error models to capture these random behaviors, this is most often at the expense of increased analysis complexity. In this paper, some recent results on probabilistic real-time schedulability analysis are extended to propose an efficient method of time complexity O(n log n) to tightly bound the deadline failure probability for a fault-tolerant real-time network transmitting n periodic/sporadic messages. The paper assumes Earliest Deadline First (EDF) message scheduling is employed and considers both random errors and bursts of errors. A simple example is first used to illustrate the technique, and a more realistic example related to EDF scheduling of a Controller Area Network (CAN) then helps to show the technique has practical value. Although EDF message scheduling is assumed, the technique is readily adaptable to other forms of scheduling.

RTE-SIM: A Simple, Low-Cost and Flexible Environment to Support the Teaching of Real-Time and Embedded Control

Contemporary teaching of real-time control, basic elements of which are part of a modern control engineering syllabus, can be restricted by the availability of laboratory space and the high costs of equipment, development tools and test beds. This paper describes RTE-SIM – a novel, flexible and low-cost environment developed by the authors to help alleviate these problems. The environment is in the form of an ARM7 microcontroller-based hardware-in-the-loop (HIL) environment. The paper describes each component in the HIL configuration, followed by an example of a typical student exercise. Most institutions can implement the proposed environment with few (if any) additional cost, space or time penalties incurred, and based on initial experiences of its use, some possible areas of future development are suggested.

Improving information throughput and transmission predictability in Controller Area Networks

The Controller Area Network (CAN) protocol was originally developed for distributed automotive applications in the 1980s, and has previously proved to be extremely popular for the implementation of low-cost distributed systems. Whilst CAN has many features that make it suitable for such applications, it also has a number of well-discussed drawbacks; data transmission is limited to a fixed 8-byte payload at 1 Mbps, and the hardware bit-stuffing mechanism and automatic retransmission scheme can act to severely decrease the predictability of a CAN network. This paper will describe a modified CAN-like protocol controller that can be implemented on a programmable-logic device such as an FPGA. The modified protocol controller can help to ameliorate one of the drawbacks of CAN, whilst operating as closely as is possible to the original protocol specification. Specifically, this paper will discuss an implementation of a simple extension to the original protocol that allows for larger payloads, coupled with modifications to the bit-stuffing mechanism to increase predictability. The effectiveness of this modified CAN controller is illustrated in a series of experiments employing a novel test facility. The paper also discusses the potential drawbacks of the new controller, and is then concluded by suggesting possible areas of further research.

Improved schedulability analysis of implicit deadline tasks under limited preemption EDF scheduling

Limited-preemption forms of uniprocessor scheduling provide a practical trade-off between flexibility and system overheads in embedded kernels. This paper considers the limited-preemption scheduling of implicit deadline tasks using the EDF algorithm. Upper bounds for the occurrence of a deadline miss in these task sets are derived, and employed to create several sufficient schedulability tests of increasing complexity and tightness. Exact analysis is then considered, and an algorithm with polynomial-time complexity is developed for situations in which the CPU utilization is bounded to be less than unity. Experimentally, the sufficient tests are shown to have acceptance ratios increasing from ≈60% up to ≈ 99%, and the number of evaluated deadlines required for the exact test is reduced by several orders of magnitude over previously known techniques.

Bandwidth-efficient burst error tolerance in TDMA-based CAN networks

Many distributed control systems employ TDMA-based communication over CAN in order to meet realtime constraints. Whilst this form of media access control brings several timeliness benefits, studies have also illustrated negative effects on transmission reliability. This paper extends the ‘window transmission’ technique which was recently proposed by the authors to help overcome this problem in TDMA-based networks to include the effects of correlated (burst) errors. This paper employs a simple Markov model to describe burst error behaviors in a CAN network, and the model is used to develop an algorithm for calculating TDMA slot sizes which aim to provide prespecified statistical guarantees of message delivery. Computational results are presented which indicate that the technique can reduce the amount of bandwidth needed for specified reliability levels by a significant factor when compared to the use of message duplicates. The paper is concluded with an empirical study which provides further supportive evidence for the described technique.