Multi-agent Control of High Redundancy Actuation

International Journal of Automation and Computing Multi-agent Control of High Redundancy Actuation Jessica Davies 2 3 Roger Dixon 1 3 0 Department of Aeronautical and Automotive Engineering, Loughborough University , Loughborough, LE11 3TU , UK 1 School of Electrical, Electronic & Systems Engineering, Loughborough University , Loughborough, LA1 4YQ , UK 2 Lancaster Environment Centre, Lancaster University , Lancaster, LE11 3TU , UK 3 Thomas Steffen graduated from the Technical University of Ilmenau with a diploma (B. Eng.) in electrical engineer- ing/automation and control in 1999. He worked as a research assistant at the Technical University of Hamburg-Harburg (TUHH), Germany and at the Ruhr- Universit at Bochum, Germany, where he obtained his Ph. D. degree in 2004. He joined Loughborough University as a re- search associate in January 2007, and now works as a lecturer at the Department of Aeronautical and Automotive Engineering, Loughborough University. His research interests include engine, powertrain and after- treatment control, high speed / high performance control and fault tolerant control The high redundancy actuator (HRA) concept is a novel approach to fault tolerant actuation that uses a high number of small actuation elements, assembled in series and parallel in order to form a single actuator which has intrinsic fault tolerance. Whilst this structure affords resilience under passive control methods alone, active control approaches are likely to provide higher levels of performance. A multiple-model control scheme for an HRA applied through the framework of multi-agent control is presented here. The application of this approach to a 1010 HRA is discussed and consideration of reconfiguration delays and fault detection errors are made. The example shows that multi-agent control can provide tangible performance improvements and increase fault tolerance in comparison to a passive fault tolerant approach. Reconfiguration delays are shown to be tolerable, and a strategy for handling false fault detections is detailed. Fault tolerance; actuators; intelligent control; redundancy; intelligent actuators; cooperative systems - In automated processes, faults in hardware or software often produce undesired reactions. Faults can lead to system failures, where expected actions are not completed, possibly resulting in damage to the plant, its environment or people in the vicinity of the plant[1]. A fault tolerant system is able to avoid failure and achieve adequate system performance in the presence of faults. The majority of fault tolerance research to date has concentrated on sensor faults[25]. Most of these strategies are not applicable to actuator faults. This is attributable to the fundamental differences between actuators and sensors. Sensors deal with information, and the signals they produce may be processed or replicated analytically to provide fault tolerance. Actuators, however, must deal with energy conversion. As a result, actuator redundancy is essential if fault tolerance is to be achieved in the presence of actuator faults. Actuation force will always be required to keep the system in control and bring it to the desired state[6]. No approach can avoid this fundamental requirement. The common solution involves straightforward parallel replication of actuators[710] . Each redundant actuator must be capable of performing the task alone and possibly override the other faulty actuators. This over-engineering incurs penalties as cost and weight are increased and subsequently efficiency is reduced. It also can not deal with lock-up (fail-fixed) faults easily. High redundancy actuation High redundancy actuation (HRA) takes a different approach to this problem. The HRA concept is inspired by musculature, where the tissue is composed of many individual cells, each of which provides a minute contribution Manuscript received November 28, 2012; revised March 28, 2013 This work was supported by UK s Engineering and Physical Sciences Research Council (EPSRC) (No. EP/D078350/1). to the overall contraction of the muscle. These characteristics allow the muscle, as a whole, to be highly resilient to individual cell damage. This principle of co-operation in large numbers of low capability modules can be used in fault tolerant actuation to provide intrinsic fault tolerance. The HRA uses a high number of small actuator elements, assembled in parallel and series, to form one high redundancy actuator (see Fig. 1). Faults in elements will affect the maximum capability. But through control techniques, the required performance can be maintained. This allows the same level of reliability to be attained in exchange for less over-dimensioning. In addition, the combination of both serial and parallel elements allows for the intrinsic accommodation of both lock-up (loss of travel) and loose (loss of force) faults. High redundancy actuation Through careful design for specific applications, HRA can provide a solution that continues to operate within the system s performance requirements in the presence of multiple faults in the elements, and gracefully degrades after the specific redundancy design limits have been reached. Fault tolerant control of high redundancy actuation Control is often integral to providing fault tolerance. The HRA project thus far has focused on using passive fault tolerant control (FTC) to provide fault tolerance. Research to date suggests that this is a theoretically and practically viable approach[1114]. Passive FTC is where a single robust control law is designed, which should provide adequate stability and performance under both nominal and fault conditions. This concept with respect to HRA is illustrated by Fig. 2. The behaviour of the nominal HRA is represented by a point bn in the diagram. Inevitably, a bound of uncertainty for the system surrounds this point, bn, and its uncertainty bound lies within a region of acceptable behaviours BP F T within which, the system is considered fault tolerant. Passive FTC aims to design a single robust controller that keeps the behaviours of the fault perturbed HRAs (points bf ) within BP F T . Fig. 2 Representation of passive and active fault tolerant control of HRA Although the HRA has a capability level in excess of that required by the application, lock-up and loose faults reduce the overall travel or force capability respectively. And as such, there are fault limits dictated by the capability requirement. Thus, HRA under fault conditions in excess of this limit (represented by points bgd) will lie outside BP F T in BGD, a region that represents the HRA graceful degradation operation. The passive FTC approach is attractive, as its simplicity and constancy make it more easily verifiable for a high integrity application. However, if the region BP F T is restricted, then it can be difficult or impossible to retain {bf } within this region. Hence, active FTC approaches have also been investigated, which d (...truncated)