Spatial rolling contact pair (SRCP) is a kinematic pair of which two links in contact at a line can generate the relative rolling motion along the specified spatial trajectory. By introducing the SRCP into a linkage mechanism with a single degree-of-freedom (DOF), the mechanism can completely generate the specified output motion. However, the conventional constraint method between two links of the SRCP is not strong enough. In order to enforce the connection between the two links, the novel design of the SRCP which has a hybrid elastic constraint with flexible bands and linear springs is proposed. Since flexible bands and linear springs can suppress slippage and separation between the two links, the SRCP can generate the ideal rolling motion. At first, the design methodology of the rolling contact surfaces of the SRCP, which has been proposed by the authors, is reviewed. Next, it is confirmed that constraint with flexible bands can be applied to the designed surfaces with a mathematical approach. In addition, the design methodology of the flexible bands to generate the zero torque around the contact line between the two links is described. Then, the design methodology for the constraint with linear springs is proposed. In this methodology, linear springs are optimally arranged between links so that two links in the SRCP can keep in contact at a line. Some examples of path generators with the SRCP are designed with the proposed design methodology, and it is confirmed that the SRCPs can generate the ideal rolling motion by some simulations. Finally, the designed examples are fabricated and examined to confirm the validity of the proposed design methodology.
Flexibility is a required property for a machine used in human daily life to reduce a load applying to it when it bumps with a human or adapt to an uncertain environment in a manipulation task. However, flexibility for such purposes and rigidity for force transmission to do a task are in a trade-off relationship. Therefore, to balance between them based on the desired task is an important issue.Flexible mechanisms can be achieved as an underactuated mechanism whose redundant passive DOF (degree of freedom) is constrained with elastic elements. In order to balance between flexibility and rigidity, many approaches to control compliance of kinematic pairs in the mechanisms have been proposed. One of the approaches is the active compliance approach (Salisbury, 1980) (Hogan, 1991). This approach can adjust output compliance by controlling the impedance of rotary actuators, such as motors. However, a sophisticated control system is required to respond to a sudden change of an external force, such as dynamic collision. Another approach is the passive compliance approach. In this approach, passive compliance is implemented into kinematic pairs with some elastic elements, such as linear springs. In order to balance between flexibility and rigidity, methods to adjust the passive compliance to the desired compliance with an additional actuator have been proposed. These methods can be achieved by antagonizing non-linear springs (
Linkage mechanisms with 1 DOF composed of only lower pairs cannot generate the specified motion completely because of severe kinematic limitation of lower pairs. In order to relax the kinematic limitation and to generate the specified spatial trajectory completely, the authors have proposed the spatial rolling contact pair (SRCP), where two links in contact at a line roll relatively with generating the specified relative spatial trajectory completely, as a novel kinematic pair used for a spatial-path generator with 1 DOF. However, since it is a passive kinematic pair, it must be used in a closed-loop mechanism. Thus, the spatial-path generator with the SRCP cannot be simplified enough. In this paper, the "active" spatial rolling contact pair (ASRCP), which is driven by several active elastic elements such as flexible linear actuators, is developed to achieve a simple spatial-path generator. Firstly, a design method to optimally arrange active elastic elements between two links of the SRCP based on a transmission index is proposed. Here, a method to use more than the minimum number of active elastic elements which are required to constitute force-closure state is described to design the ASRCP in order to achieve sufficient motion range. Besides, a control method of the ASRCP to generate the ideal rolling motion is proposed, where actuation forces of active elastic elements to get high stability between the links are calculated with the use of the actuation redundancy. As examples, the ASRCP driven with reeled-wires and the ASRCP driven with fluid-driven artificial muscles are designed and prototyped. Then, they are controlled with the proposed method, and their performances are investigated through motion capture experiments.
In order to synthesize a human-friendly flexible mechanism with a simple structure, a revolute pair with flexible kinematic constraint in multiple directions is proposed. It is called the multi-directionally flexibly constrained revolute pair (MFCRP). The structure of the MFCRP is composed of a link with two same spherical surfaces and a link with two same cam surfaces, and each set of the cam surface and the spherical surface is in contact at a point. The connection between the two links is kept by two linear springs arranged between the two links. The MFCRP can generate 1-axial relative rotation between the two links and relative motions in the other directions are flexibly constrained. In order for the MFCRP to have both flexibility for safety and rigidity for the force transmission, the specified non-linear stiffness can be implemented in the two relative translational directions between the two links. This flexible translational constraint is generated by the spring forces and the reaction forces between the two links. In this paper, two methods to design the cam surfaces to implement the specified non-linear stiffness are proposed. The validity of the proposed design methodology is confirmed by comparing measured stiffness characteristics between two links of some prototypes with the theoretical characteristics. As an application, a flexible closed-loop linkage with the MFCRP is fabricated and its flexibility and kinematic performance are investigated through some experiments.
A revolute pair with a flexible translational constraint on a plane is proposed as simple mechanism for safe robots. The mechanism is composed of two pairing elements, one with a circular and one with a cam profile that are connected by a linear spring. Flexible translational constraint is generated by spring forces and the reaction force between the two pairing elements. Two methods for designing the cam profile are proposed in order to implement the specified non-linear stiffness in the flexible constraint. Design examples with various stiffness characteristics are shown. Some prototypes are fabricated, and it is confirmed that they perform as designed. As an application, a flexible, underactuated link mechanism with the proposed pairs is synthesized, and its flexibility and kinematic performance are investigated.
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