LQG control of a high redundancy actuator

A high redundancy actuator, comprising a relatively large number of actuation elements, is being developed for safety critical applications. Some classical control results have previously been reported and this paper will focus on evaluation of the LQG control design. Three different design approaches will be presented and compared under different types of typical faults in the sub-actuation elements. Overall a LQG design using a physically motivated reduced order model appears to be the best approach

Modelling and control of a high redundancy actuator

The high redundancy actuation concept is a completely new approach to fault tolerance, and it is important to appreciate that it provides a transformation of the characteristics of actuators so that the actuation performance (capability) degrades slowly rather than suddenly failing, even though individual elements themselves fail. This paper aims to demonstrate the viability of the concept by showing that a highly redundant actuator, comprising a relatively large number of actuation elements, can be controlled in such a way that faults in individual elements are inherently accommodated, although some degradation in overall performance will inevitably be found. The paper introduces the notion of fault tolerant systems and the highly redundant actuator concept. Then a model for a two by two configuration with electromechanical actuation elements is derived. Two classical control approaches are then considered based on frequency domain techniques. Finally simulation results under a number of faults show the viability of the approach for fault accommodation without reconfiguration

High redundancy in actuation

Actuation, the controlled movement and positioning of objects, is an essential function of many technical systems. It is crucial in many applications from central heating to aircrafts, and without actuation, the function or even the safety of the system would suffers. For example an aircraft is steered using control surfaces, and if the actuation of these surfaces fails, the aircraft may crash. Therefore, actuation is often provided by using several (typically between 2 and 4) redundant actuation elements. If one element fails, another takes over, and harm can be avoided. While this solution works, it involves increased cost, weight and energy use, reducing the efficiency of the system considerable. This project on high redundancy actuation investigates the use of a high number of actuation elements, such as 10 or even 100. This is a bionic (or bio-mimetic) idea: the use of actuation elements is similar to the composition of a muscle from many individual muscle fibres. Just like the muscle is highly resilient to damage in individual fibres (causing sore muscles, but no loss of movement), high redundancy actuation is highly reliable even if several elements have failed. The reliability analysis shows that this approach provides the same level or even superior protection against faults, without the loss of efficiency involved in the traditional solution. The basic advantage is that the law of large numbers applies, which provides a much more accurate prediction of how faults will affect the actuation elements over time. In the aircraft example, this would provide lighter actuators that provide superior reliability, leading to better fuel economy and easier maintenance. The main scientific problem of this project is how to deal with the complexity of using a high number of elements together. The results show that it is possible to determine the reliability of the system, and it is also possible to control the many elements as if they are just one big actuator. The next phase of the project is dealing with the technological challenges of combining many actuation elements. A simple experiment with four elements has been completed, and a demonstrator with 16 elements is being built. These experiments are used to demonstrate the resilience to faults, and understand the practical control issues at hand. A project leading to a more advanced version with up to 100 elements is currently being prepared. While the developed theories can be extended easily to consider such configurations, the practical difficulties of designing and manufacturing such a solution are challenging. The goal is to demonstrate that high redundancy actuation is feasible with the currently available technology, and to get an idea of the manufacturing issues involved