High Redundancy Actuation
Encyclopedia
High-redundancy actuation (HRA) is a new approach to fault-tolerant control in the area of mechanical actuation.
Fault-tolerant operation in the presence of actuator faults requires some form of redundancy. Actuators are essential, because they are used to keep the system stable and to bring it into the desired state. Both requires a certain amount of power or force to be
applied to the system. No control approach can work unless the actuators produce this necessary force.
So the common solution is to err on the side of safety by over-actuation: much more control action than strictly necessary is built into the system. For critical systems, the normal approach involves straightforward replication of the actuators. Often three or four actuators are used in parallel for aircraft flight control systems, even if one would be
sufficient from a control point of view. So if one actuator failts, the remaining actuator can always keep the system operation. While this approach certainly successful, it also makes the system expensive, heavy and ineffective.
).
Faults within the actuator will affect the maximum capability, but through robust control, full performance can be maintained without either adaptation or reconfiguration. Some form of condition monitoring
is necessary to provide warnings to the operator calling for
maintenance
. But this monitoring has no influence on the system itself, unlike in adaptive methods or control reconfiguration
, which simplifies the design of the system significantly.
The HRA is an important new approach within the overall area of fault-tolerant control,
using concepts of reliability engineering
on a mechanical level. When applicable, it can provide actuators that have graceful degradation, and that continue to operate at close to nominal performance even in the presence of multiple faults in the actuator elements.
However, there is one fault that is difficult to deal with in a parallel arrangement: the locking up of one actuator element. Because parallel actuator elements always have the same extension, one locked-up element can render the whole assembly useless. It is possible to mitigate this by guarding the elements against locking or by limiting the force exerted by a single element. But these measures reduce both the effectiveness of the system and introduce new points of failure.
The analysis of the serial configuration shows that it remains operational when one element is locked-up. This fact is important for the High Redundancy Actuator, as fault tolerance is required for different fault types. The goal of the HRA project is to use parallel and serial actuator elements to accommodate both the blocking and the inactivity (loss of force) of an element.
s and rotational actuators.
However, initial experiments are performed with electric actuator
s, especially with electromechanical and electromagnetic technology. Compared to pneumatic actuator
s, the electrical drive allow a much finer control of position and force.
Overview
The basic idea is to use a lot of small actuation elements, so that a fault of one element has only a minor effect on the overall system. This way, a High Redundancy Actuator can remain functional even after several elements are at fault. This property is also called graceful degradation.Fault-tolerant operation in the presence of actuator faults requires some form of redundancy. Actuators are essential, because they are used to keep the system stable and to bring it into the desired state. Both requires a certain amount of power or force to be
applied to the system. No control approach can work unless the actuators produce this necessary force.
So the common solution is to err on the side of safety by over-actuation: much more control action than strictly necessary is built into the system. For critical systems, the normal approach involves straightforward replication of the actuators. Often three or four actuators are used in parallel for aircraft flight control systems, even if one would be
sufficient from a control point of view. So if one actuator failts, the remaining actuator can always keep the system operation. While this approach certainly successful, it also makes the system expensive, heavy and ineffective.
Inspiration of high-redundancy actuation
The idea of the high-redundancy actuation (HRA) is inspired by the human musculature. A muscle is composed of many individual muscle cells, each of which provides only a minute contribution to the force and the travel of the muscle. These properties allow the muscle as a whole to be highly resilient to damage of individual cells.Technical realisation
The aim of high redundancy actuation is not to produce man-made muscles, but to use the same principle of cooperation in technical actuator's to provide intrinsic fault tolerance. To achieve this, a high number of small actuator elements are assembled in parallel and in series to form one actuator (see Series and parallel circuitsSeries and parallel circuits
Components of an electrical circuit or electronic circuit can be connected in many different ways. The two simplest of these are called series and parallel and occur very frequently. Components connected in series are connected along a single path, so the same current flows through all of the...
).
Faults within the actuator will affect the maximum capability, but through robust control, full performance can be maintained without either adaptation or reconfiguration. Some form of condition monitoring
Condition monitoring
Condition monitoring is the process of monitoring a parameter of condition in machinery, such that a significant change is indicative of a developing failure. It is a major component of predictive maintenance. The use of conditional monitoring allows maintenance to be scheduled, or other actions...
is necessary to provide warnings to the operator calling for
maintenance
Maintenance, Repair and Operations
Maintenance, repair, and operations or maintenance, repair, and overhaul involves fixing any sort of mechanical or electrical device should it become out of order or broken...
. But this monitoring has no influence on the system itself, unlike in adaptive methods or control reconfiguration
Control reconfiguration
Control reconfiguration is an active approach in control theory to achieve fault-tolerant control for dynamic systems . It is used when severe faults, such as actuator or sensor outages, cause a break-up of the control loop, which must be restructured to prevent failure at the system level...
, which simplifies the design of the system significantly.
The HRA is an important new approach within the overall area of fault-tolerant control,
using concepts of reliability engineering
Reliability engineering
Reliability engineering is an engineering field, that deals with the study, evaluation, and life-cycle management of reliability: the ability of a system or component to perform its required functions under stated conditions for a specified period of time. It is often measured as a probability of...
on a mechanical level. When applicable, it can provide actuators that have graceful degradation, and that continue to operate at close to nominal performance even in the presence of multiple faults in the actuator elements.
Using actuation elements in series
An important feature of the high-redundancy auctuation is that the actuator elements are connected both in parallel and in series. While the parallel arrangement is commonly used, the configuration in series is rarely employed, because it is perceived to be less efficient.However, there is one fault that is difficult to deal with in a parallel arrangement: the locking up of one actuator element. Because parallel actuator elements always have the same extension, one locked-up element can render the whole assembly useless. It is possible to mitigate this by guarding the elements against locking or by limiting the force exerted by a single element. But these measures reduce both the effectiveness of the system and introduce new points of failure.
The analysis of the serial configuration shows that it remains operational when one element is locked-up. This fact is important for the High Redundancy Actuator, as fault tolerance is required for different fault types. The goal of the HRA project is to use parallel and serial actuator elements to accommodate both the blocking and the inactivity (loss of force) of an element.
Available technology
The basic idea of high-redundancy actuation is technology agnostic: it should be applicable to a wide range of actuator technology, including different kinds of linear actuatorLinear actuator
A linear actuator is an actuator that creates linear motion .Mechanical and hydraulic actuation are the most common methods of achieving the linear motion...
s and rotational actuators.
However, initial experiments are performed with electric actuator
Electric actuator
Actuators are used for the automation of industrial valves and can be found in all kinds of technical process plants: they are used in waste water treatment plants, power plants and even refineries. This is where they play a major part in automating process control. The valves to be automated vary...
s, especially with electromechanical and electromagnetic technology. Compared to pneumatic actuator
Pneumatic actuator
A pneumatic actuator converts energy into motion. The motion can be rotary or linear, depending on the type of actuator.Some types of pneumatic actuators include:*Tie rod cylinders*Rotary actuators*Grippers...
s, the electrical drive allow a much finer control of position and force.
Further reading
- M. Blanke, M. Kinnaert, J. Lunze, M. Staroswiecki, J. Schröder: "Diagnosis and Fault-Tolerant Control", ISBN 978-3540356523. Springer, New York, 2006.
- S. Chen, G. Tao, and S. M. Joshi: "On matching conditions for adaptive state tracking control of systems with actuator failures", in IEEE Transactions on Automatic Control, vol. 47, no. 3, pp. 473—478, 2002.
- X. Du, R. Dixon, R.M. Goodall, and A.C. Zolotas: "LQG ControlLinear-quadratic-Gaussian controlIn control theory, the linear-quadratic-Gaussian control problem is one of the most fundamental optimal control problems. It concerns uncertain linear systems disturbed by additive white Gaussian noise, having incomplete state information and undergoing control subject to quadratic costs...
for a Highly Redundant Actuator", in Preprint of the IFAC Conference for Advanced Intelligent MechatronicsMechatronicsMechatronics is the combination of mechanical engineering, electronic engineering, computer engineering, software engineering, control engineering, and systems design engineering in order to design, and manufacture useful products. Mechatronics is a multidisciplinary field of engineering, that is...
(AIM), Zurich, 2007. - X. Du, R. Dixon, R.M. Goodall, and A.C. Zolotas: "Assessment Of Strategies For Control Of High Redundancy Actuators", ACTUATOR 2006, Germany.
- X. Du, R. Dixon, R.M. Goodall, and A.C. Zolotas: "Modelling And Control Of A Highly Redundant Actuator", CONTROL 2006, Scotland, 2006.
- T. Steffen, J. Davies, R. Dixon, R.M. Goodall and A.C. Zolotas: "Using a Series of Moving Coils as a High Redundancy Actuator", in Preprint of the IFAC Conference for Advanced Intelligent Mechatronics (AIM), Zurich, 2007.