Most traditional robots are built using discrete joints with stiff connecting links, as inspired by human limbs. Every joint in these robots has one degree of freedom whose movement can be straightforwardly controlled by manipulating the movement of these joints. The use of these stiff joints will result in a heavy overall structure because of the need to use large supporting sections in the overall system construction. While these stiff/rigid robots are a desirable part of many manufacturing operations, there are many applications that require robots with different features and modes of performance. In contrast to traditional robots, continuum robot arms do not have discrete joints. Instead, their entire structure can be bent to achieve a required movement. Continuum robot arms can be considered to behave in a very similar way to an elephant’s trunk, an octopus’s legs, or a caterpillar. In addition, there are no rigid links or moving joints in the construction of these types of robots (Robinson, G., & Davies, J., 1999). The movement strategies used for continuum robots depend on continuous bending along their lengths through deformation.
As continuum arms do not have discrete joints, the way they interact with the environment is entirely distinct from that of a traditional robot. If a continuum robot is partially constrained to prevent a section of it from moving, other sections of the arm will remain free to bend. This means a continuum arm could easily operate inside a pipe, for instance, where a traditional robot would struggle. Continuum arms are also able to deform to match the shape of the object with which they are in contact. This means that if the arm is used to grasp an object, then the grasping force will be distributed over a larger area to minimise any damage to the object. Figure (2-15) shows some animals with continuum limbs.
27 Figure 2-15: Continuum arm/appendage examples found in nature: (A) bodies of snakes; (B) giraffe tongue; (C) lizard tails; (D) tail of the spider monkey; (E) elephant trunks; (F) chameleon tails; (G) squid tentacles; (H) octopus’ arms; (K) opossum tails; and (M) chameleon tongues (Godage, I., et al, 2012).
Depending on the method and location of the actuation mechanism, a number of continuum arm designs have been developed. They may be classified into three major categories: intrinsic, extrinsic and hybrid (Robinson, G., & Davies, J., 1999).
Furthermore, these three groups can be subdivided into planar or spatial systems depending on the kind of movement they produce. While planar systems can move in one plane only by bending, spatial systems have the ability to bend in all directions along their longitudinal axis. More details on all of these systems are given below.
2.5.1
Intrinsic Planar Continuum Arm
In the continuum actuator presented by Nemir, a single pressure input is used to provide bending in one plane (Nemir, D., 1989). The fluid-operated planner system shown in figure (2-16) is an example of such a system.
28 Figure 2-16: Intrinsic Planar Continuum Actuator (Nemir, D., 1989).
The resultant motion depends on the physical structure of the actuator walls. The axial stiffness of the actuator walls is not equally distributed around the actuator itself, and hence the elasticity of the actuator on one side differs from that of the others. Bending occurs by increasing the pressure inside this actuator. If the applied pressure is removed or decreased, the actuator motion will be changed due to the elasticity effect in straightening the actuator.
2.5.2
Intrinsic Spatial Continuum Arm
Robinson proposed the intrinsic spatial continuum actuator (Robinson, G., & Davies, J., 1998). Figure (2-17) below shows the basic structure of an intrinsic spatial continuum actuator as pressurised by fluid.
29 This actuator can produce motion in three dimensions. Its main characteristics lie in its simple, compact and lightweight design. Both bending direction and magnitude of movement may be straightforwardly controlled by adjusting the pressure inside each of the three parallel bellows actuators. Continuum arms with many degrees of freedom can be produced by combining several actuator sections in series.
2.5.3
Extrinsic Continuum Arm
Extrinsic continuum arms are lightweight and can provide a higher number of DOF. The actuators in this type of continuum arm are located remotely, with the motion typically transferred to the main actuator by sets of tendon cables. Various structures have been introduced with different numbers of continuum arm sections and which consist of various tendon arrangements and degrees of freedom; see, for example, (Hemami, A., 1984), (Lock, J., et al., 2010), (Mahvash, M., & Dupont, P., 2010), (Su, H., et al., 2012) and (Webster III, R., & Jones, B., 2010). Figure (2-18) shows the extrinsic actuator.
Figure 2-18: Extrinsic Actuator (Nemir, D., 1989).
It uses an extension spring with three tendons attached. Applying a force to one or more of the tendons will induce the bending of the actuator.
30
2.5.4
Hybrid Continuum Arm
The structure of this type of actuator has the same general appearance as the extrinsic actuator. However, it uses bellows instead of a passive spring. Figure (2-19) shows the construction of the hybrid actuator.
Figure 2-19: Hybrid Actuator (Nemir, D., 1989).
Actuation depends on the joint operation for both the bellows and the triads of three tendons arranged around the bellows. It is clear that, one tendon triad has to be connected to the middle of the bellows construction to control the shape of the lower half of the actuator. Similarly, another tendon triad must be connected to the far end of the actuator to control the operation of the upper half of the actuator. The internal pressure of the bellows controls the tension of the tendons, which works against the tendon operation. Hence, by changing the amount of the pressure in the bellows and by using different lengths of tendons the operation of the whole actuator can be controlled. Immega developed a commercial version of the hybrid actuator called a KSI hybrid actuator (Immega, G., & Antonelli, K., 1995).