2-DoF Rotational mechanism is increasingly utilized in a large range of industrial applications. However, the structures of most of the existing mechanisms are very complex, which is a significant challenge for building them, due to the tight tolerance and assembly difficulties. In this paper, a class of 2-DoF tendon driven parallel kinematics mechanisms (TDPKM) are introduced, which can be structured with low manufacturing and assembly difficulties and is able to actively adjust the system stiffness. Since the unique class of mechanisms is developed, the kinematic model is established to derive the stiffness model, which considers the tendon, structural and central joint stiffness. Finally, a set of experiments of the deviation measurement under different payloads within the workspace are implemented and compared with the theoretical calculations presented in this paper. The overall deviation error between the experimental test and theoretical calculation are between 0.9% and 4.7% in the whole workspace.

Continuum robots are increasingly being used in industrial and medical applications due to their high number of degrees of freedom (DoF), large workspace and their ability to operate dexterously. However, the positional accuracy of conventional continuum robots with a backbone structure is usually low due to the low stiffness of the often-lengthy driving cables/tendons. Here, this problem has been solved by integrating additional mechanisms with adjustable stiffness within the continuum robot to improve its stiffness and mechanical performance, thus enabling it to be operated with high accuracy and large payloads. To support the prediction of the improved performance of the adjustable stiffness continuum robot, a kinetostatic model was developed by considering the generalized internal loads that are caused by the deformation of the flexure-hinge mechanism and the structural stiffening caused by the external loads on the end-effector. Finally, experiments were conducted on physical prototypes of 2-DoF and 6-DoF continuum robots to validate the model. It was found that the proposed kinetostatic model validates experimental observations within an average deviation of 9.1% and 6.2% for the 2-DoF and 6-DoF continuum robots, respectively. It was also found that the kinematic accuracy of the continuum robots can be improved by a factor of 32.8 by adding the adjustable stiffness mechanisms.

Parallel kinematic manipulators (PKMs) are increasingly used in a wide range of industrial applications due to the characteristics of high-accuracy and compact structure. However, most of the existing PKMs are structured with heavy actuators and high stiffness. In this respect, this paper proses a simple, yet effective, parallel manipulator that distinguishes itself through: (1) under-actuation: it employs only a single motor and a driving cable to actuate its three legs; (2) novel foot location: it uses a smart shape memory alloy (SMA) clutch-based driving system (SCBDS) which catches/releases the driving cable, thus, making possible the robot under-actuation; (3) adjustable compliance: its double compliant joints on each limb with a stiffness-adjustable section which renders a safe human-robotic interaction. To support and predict the performance of this underactuated compliant manipulator, a novel kinetostatic model was developed by considering the generalized internal loads (i.e. force and moment) in three compliant limbs and the external loads on the upper platform. Finally, based on the physical prototype, a set of experiments were conducted to validate the model proposed in this paper. It was found that the proposed kinetostatic model can be validated with the average deviations of 1.8% in position and 2.8% in orientation respectively. Further, the workspace of the system (e.g. discrete and continuous workspace) was studied when different actuating strategies were employed, thus emphasizing the advantages and the limitations of this novel system.

The Free-Leg Hexapod (Free-Hex) machine tool is an advancement from the conventional Stewart platform, which has the fixed base platform removed to enable the limbs to be attached to a wider range of surfaces (e.g. non-flat and curved). However, in some scenarios (e.g. in-situ repair of industrial installations), the limbs of the Free-Hex need to be attached to the surfaces with unequal stiffness, which brings the challenge of predicting the dynamics of the system for conducting machining operations under dynamically stable conditions. In this paper, after introducing the attachment stiffness (i.e. feet attached to the environment with different materials) in the conventional dynamic model of the parallel manipulator, the dynamic behavior of the Free-Hex machine tools devoted to the insitu operation environments was studied. Then, the experimental validation was conducted to prove the dynamic model developed in this paper. It was found that the errors of the proposed model are under 6% (i.e. 5.1% at symmetrical limb configuration and 5.8% at the arbitrary configuration) when the limbs are attached to the surfaces with unequal stiffness. Further, by applying the validated model, the dynamic performance of the Free-Hex with a wider range of attachment stiffness was analyzed. Overall, it was found that the attachment stiffness has a remarkable influence on the natural frequencies of the machine tool (e.g. the frequency of mode 4 at the symmetrical configuration is increased by 36.8% when the attachment stiffness of one limb changes from 0 to 1e+12 N/m). Thus, the work discussed in this paper can be utilized to avoid the dynamically unstable configurations of parallel kinematic machine tools (e.g. Free-Hex) when mounting on the surface with unequal stiffness in the in-situ operation environments.