Active structure
Encyclopedia
An active structure is a mechanical structure with the ability to alter its configuration, form or properties in response to changes in the environment.
The term active structure also refers to structures that, unlike traditional engineering structures (e.g. bridges, buildings), require constant motion and hence power input to remain stable. The advantage of active structures is that they can be far more massive than a traditional static structure: an example would be a space fountain
, a building that reaches into orbit.
could reduce the maximum stress or strain level, while a shape change could render a structure less susceptible to dynamic vibrations. A good example of an adaptive structure is the human body where the skeleton
carries a wide range of loads and the muscle
s change its configuration to do so. Consider carrying a backpack. If the upper body did not adjust the centre of mass of the whole system slightly by leaning forward, the person would fall on his or her back.
An active structure consists of three integral components besides the load carrying part. They are the sensors, the processor and the actuators. In the case of a human body, the sensory nerves are the sensors which gathers information of the environment. The brain acts as the processor to evaluate the information and decide to act accordingly and therefore instructs the muscles, which act as actuators to respond. In heavy engineering
, there is already an emerging trend to incorporate activation into bridge
s and dome
s to minimize vibrations under wind and earthquake loads.
Aviation engineering and aerospace engineering
have been the main driving force in developing modern active structures. Aircraft (and spacecraft) require adaptation because they are exposed to many different environments, and therefore loadings, during their lifetime. Prior to launching they are subjected to gravity or dead loads, during
takeoff they are subjected to extreme dynamic and inertial loads and in-flight they need to be in a configuration which minimizes drag but promotes lift. A lot of effort has been committed into adaptive aircraft wing
s to produce one that can control the separation of boundary layers and turbulence. Many space structures utilize adaptivity to survive extreme environmental challenges in space or to achieve precise accuracies. For example, space antennas and mirror
s can be activated to precise orientation. As space technology advances, some sensitive equipment (namely interferometric optical and infrared astronomical instruments) are required to be accurate in position as delicate as a few nanometre
s, while the supporting active structure is tens of metres in dimensions.
actuators.
A good active structure has a number of requirements. First, it needs to be easily actuated. The actuation should be energy-saving. A structure which is very stiff and strongly resists morphing is therefore not desirable. Second, the resulting structure must have structural integrity to carry the design loads. Therefore the process of actuation should not jeopardize the structure's strength. More precisely, we can say: We seek an active structure where actuation of some members will lead to a geometry change without substantially altering its stress state. In other words, a structure that has both statical determinacy
and kinematic determinacy
is optimal for actuation.
The term active structure also refers to structures that, unlike traditional engineering structures (e.g. bridges, buildings), require constant motion and hence power input to remain stable. The advantage of active structures is that they can be far more massive than a traditional static structure: an example would be a space fountain
Space fountain
A space fountain is a proposed form of space elevator that does not require the structure to be in geostationary orbit, and does not rely on tensile strength for support. In contrast to the original space elevator design , a space fountain is a tremendously tall tower extending up from the ground...
, a building that reaches into orbit.
Function
The result of the activity is a structure more suited for the type and magnitude of the load it is carrying. For example, an orientation change of a beamBeam (structure)
A beam is a horizontal structural element that is capable of withstanding load primarily by resisting bending. The bending force induced into the material of the beam as a result of the external loads, own weight, span and external reactions to these loads is called a bending moment.- Overview...
could reduce the maximum stress or strain level, while a shape change could render a structure less susceptible to dynamic vibrations. A good example of an adaptive structure is the human body where the skeleton
Skeleton
The skeleton is the body part that forms the supporting structure of an organism. There are two different skeletal types: the exoskeleton, which is the stable outer shell of an organism, and the endoskeleton, which forms the support structure inside the body.In a figurative sense, skeleton can...
carries a wide range of loads and the muscle
Muscle
Muscle is a contractile tissue of animals and is derived from the mesodermal layer of embryonic germ cells. Muscle cells contain contractile filaments that move past each other and change the size of the cell. They are classified as skeletal, cardiac, or smooth muscles. Their function is to...
s change its configuration to do so. Consider carrying a backpack. If the upper body did not adjust the centre of mass of the whole system slightly by leaning forward, the person would fall on his or her back.
An active structure consists of three integral components besides the load carrying part. They are the sensors, the processor and the actuators. In the case of a human body, the sensory nerves are the sensors which gathers information of the environment. The brain acts as the processor to evaluate the information and decide to act accordingly and therefore instructs the muscles, which act as actuators to respond. In heavy engineering
Engineering
Engineering is the discipline, art, skill and profession of acquiring and applying scientific, mathematical, economic, social, and practical knowledge, in order to design and build structures, machines, devices, systems, materials and processes that safely realize improvements to the lives of...
, there is already an emerging trend to incorporate activation into bridge
Bridge
A bridge is a structure built to span physical obstacles such as a body of water, valley, or road, for the purpose of providing passage over the obstacle...
s and dome
Dome
A dome is a structural element of architecture that resembles the hollow upper half of a sphere. Dome structures made of various materials have a long architectural lineage extending into prehistory....
s to minimize vibrations under wind and earthquake loads.
Aviation engineering and aerospace engineering
Aerospace engineering
Aerospace engineering is the primary branch of engineering concerned with the design, construction and science of aircraft and spacecraft. It is divided into two major and overlapping branches: aeronautical engineering and astronautical engineering...
have been the main driving force in developing modern active structures. Aircraft (and spacecraft) require adaptation because they are exposed to many different environments, and therefore loadings, during their lifetime. Prior to launching they are subjected to gravity or dead loads, during
takeoff they are subjected to extreme dynamic and inertial loads and in-flight they need to be in a configuration which minimizes drag but promotes lift. A lot of effort has been committed into adaptive aircraft wing
Wing
A wing is an appendage with a surface that produces lift for flight or propulsion through the atmosphere, or through another gaseous or liquid fluid...
s to produce one that can control the separation of boundary layers and turbulence. Many space structures utilize adaptivity to survive extreme environmental challenges in space or to achieve precise accuracies. For example, space antennas and mirror
Mirror
A mirror is an object that reflects light or sound in a way that preserves much of its original quality prior to its contact with the mirror. Some mirrors also filter out some wavelengths, while preserving other wavelengths in the reflection...
s can be activated to precise orientation. As space technology advances, some sensitive equipment (namely interferometric optical and infrared astronomical instruments) are required to be accurate in position as delicate as a few nanometre
Nanometre
A nanometre is a unit of length in the metric system, equal to one billionth of a metre. The name combines the SI prefix nano- with the parent unit name metre .The nanometre is often used to express dimensions on the atomic scale: the diameter...
s, while the supporting active structure is tens of metres in dimensions.
Design
Man-made actuators existing in the market, even the most sophisticated ones, are nearly all one dimensional. This means they are only capable of extending and contracting along, or rotating about 1 axis. Actuators capable of movement in both forward and reverse directions are known as two-way actuators, as opposed to one-way actuators which can only move in one direction. The limiting capability of actuators has restricted active structures to two main types: active truss structures, based on linear actuators, and manipulator arms, based on rotaryactuators.
A good active structure has a number of requirements. First, it needs to be easily actuated. The actuation should be energy-saving. A structure which is very stiff and strongly resists morphing is therefore not desirable. Second, the resulting structure must have structural integrity to carry the design loads. Therefore the process of actuation should not jeopardize the structure's strength. More precisely, we can say: We seek an active structure where actuation of some members will lead to a geometry change without substantially altering its stress state. In other words, a structure that has both statical determinacy
Statical determinacy
Statical determinacy is a term used in structural mechanics to describe a structure where force and moment equilibrium conditions alone can be utilized to calculate internal member actions....
and kinematic determinacy
Kinematic determinacy
Kinematic determinacy is a term used in structural mechanics to describe a structure where material compatibility conditions alone can be used to calculate deflections....
is optimal for actuation.