The SEA is an excellent step forward in developing compliant actuators. However, SEAs suffer from a backlash, friction, limited performance at low output impedance and high positional error. The main reason for the high error is motor saturation. This
58 amount of error is unacceptable in a practical sense as well as in terms of safety, especially in the latter case when the robot is interacting directly with humans. In addition, the selection of a spring constant that satisfies the requirements of the design is very hard to achieve, while a low spring constant is required at low output impedance to minimise the effects of nonlinear friction, a high spring constant is required with large output impedance bandwidth. Hence, the use of a linear spring can simplify the design of the controller, but at the same time reduces the efficiency of the SEA. The VSA overcomes many of the SEA drawbacks. However, there are still many limitations in the use of variable stiffness actuators, especially in unspecified and in joint work environments where safe human-robot interaction is a desirable concept. One of the limitations of VSAs is their inability to adjust their position quickly. A robot constructed by VSAs still can cause serious injury if it interacts directly with humans because it will be unable to reduce its stiffness quickly enough should an incident occur. Other limitations of VSAs are the size and weight of the actuators. The reason for this increased weight is due to the use of two motors in the actuator construction (one motor to control the position and another motor to adjust the stiffness). Furthermore, there are still notable power losses in variable stiffness actuators due to friction and the use of two electric motors in their construction.
To gain compliance and safety in robots that interact with humans directly, another type of actuator which is compliant has been proposed as an alternative to using SEAs and VSAs. PMA can provide these characteristics whilst still delivering a high power-to- weight ratio. They use only soft materials in their construction, and can be used directly without gearboxes. The most popular design of PMA is based on the McKibben Muscle structure. PMAs have been widely used in the construction of a new generation of robots called soft, or continuum, robots. The overall behaviour of the PMA is similar to a variable stiffness spring. Hence, compliance can be achieved in robots that use this type of actuator. Robots constructed from PMAs and from soft materials have the potential to allow for much safer systems, as compared to the traditional robot, when interacting directly with humans.
One of the main concepts discussed in this chapter is the use of soft continuum structures in the design of robot systems. A human-robot joint working environment
59 without any barrier between them is a potential benefit of the use of continuum soft robots. This provides for the possibility of another kind of application, that of a safe human assistive robot for elderly or disabled people.
The main, and common, drawback to the continuum soft arms discussed is the weaknesses inherent to the various control strategies that may be used to control the behaviours of these types of robot. One cause of these weaknesses lies with their highly non-linear behaviour. Another weakness is that there is no currently available precise/exact mathematical model that can describe the kinematics of continuum soft arms, which places additional limitations on the control of this kind of robot.
Another drawback arises from the limited capability to change the stiffness of the proposed continuum soft arms during operation, with the exception of the granular jamming case. The importance of variable stiffness can be simply stated as the reality that a stiff robot’s performance can be controlled easily; however, to directly interact in a safe manner with a human requires the stiffness of the robot to be reduced. Hence, developing a continuum soft robot arm with the ability to change its stiffness during operation will be a huge step forward in the development of future robots.
Different approaches to grasping techniques and end effector design have been illustrated. The whole arm grasping technique uses continuum soft robot arms constructed to have a large contact area with a grasped object. Hence, they have a good capability to grasp large-sized objects. On the other hand, it is very hard to grasp small- sized objects. Finally, controlling the performance of the gripper is particularly difficult because it deforms and bends in the working space while in contact with the grasped object.
Due to the range of size and shapes of objects, a multi-fingered soft hand was found to be a better choice to achieve grasping in robot end effectors. Although the fingers themselves are soft, they are attached to a stiff base to construct the end effector. Hence, this combination may be unsuitable for applications where human safety is a desirable requirement. In most cases, there are no sensors attached to these manipulators to give any feedback signal that can be used to control the performance of the multi-fingered soft hand during grasping operations. Furthermore, these grippers are unable to change
60 their stiffness. While highly compliant fingers may be desirable for grasping certain products, at other times stiffer fingers may be desirable.
The ability to change the stiffness in the granular jamming universal gripper plays a desirable role in the ability to effectively pick and place a wide variety of objects that are not otherwise easy to pick, such as flat and soft objects. The use of crushed coffee as a granular material limits the gripping performance of the proposed universal jamming gripper. As a result of the conclusions of this literature review a multi- fingered, variable stiffness, continuum soft robot end effector was chosen for development in the remainder of this research.
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