Compliant mechanisms and decoupled parallel mechanisms are topics of high interest. The combination has been manifested for lower-mobility mechanisms, yet not attempted for higher degrees of freedom. The scarcity is countered by designing a decoupled compliant spherical parallel mechanism (DCSPM), having three independent rotational degrees of freedom. Reasoning, that the spherical parallel mechanism has been studied extensively and is used for various applications, however not benefiting from the advantages that compliant- and decoupled mechanisms offer.
Extensive background knowledge is provided, which explains the common principles, applications, state- of-the-art and benefits of the decoupled compliant parallel mechanisms and forms the foundation for the design towards a decoupled compliant spherical parallel mechanism. Literature has revealed the type synthesis and optimization methods used for generating these intricate mechanisms and different methods are summarized for synthesizing compliant mechanisms. Besides, various grading criteria are introduced, such as parasitic error, range of motion and manufacturability. All possible kinematic architectures, which are able to produce a decoupled motion with three independent rotational degrees of freedom, are synthesized by using Screw Theory. This entails the number, sort, orientation and placement of joints present in each leg of the parallel mechanism. Moreover, strict decoupling criteria must be satisfied, resulting in the enumeration of several kinematic structures. Upon comparison and grading, a final structure (3-3R) is chosen, which via the use of rigid-body replacement techniques, is extended to a compliant variant of the mechanism.
An inevitable drawback of compliant mechanisms is their inability to produce continuous motion. As a result, parasitic errors are present during actuation. Because the final mechanism consists of revolute joints only, a remote center of motion mechanism based on curved flexures is developed, which is optimised to have minimal parasitic error. The latter is achieved by implementing a sequential quadratic optimisation algorithm in conjunction with surrogate models of the proposed compliant RCM mechanism. The latter results in design variables which have been optimised for minimal parasitic error, while adhering to the yield stress, minimum rotation and manufacturability constraints. The final RCM mechanism, containing flexures which are laser cut from 0.2 mm spring steel, is experimentally validated to have an axis drift of ±13.3 μm in x- direction and ±47.1 μm in y-direction for a rotation up to 18.3 deg. Upon comparison with existing compliant RCM mechanisms and several compliant revolute joints, the proposed compliant RCM mechanism shows satisfactory behaviour and complies with the conducted nonlinear FEA in Ansys.
During the experimental validation of the parasitic error present in the proposed RCM mechanism, existing measuring techniques showed several imperfections. As a result, a novel quantitative assessment method is proposed for identifying the parasitic error for planar rotary mechanisms, which aims to find the remote center-of-motion with the least amount of drift for various angles of rotation. By means of an arbitrary example, the latter is verified and the proposed method is compared with existing methods. Moreover, via a nonlinear finite element analysis on an existing compliant revolute joint, the applications, benefits, ease of use and results of the proposed method are exhibited.
The final DCSPM, 3-3R mechanism, is built up gradually; as one revolute joint, which is the proposed RCM mechanism based on curved flexures, is extended to a 2R, 2-2R, 3R and 3-3R mechanism. All individual sub parts have been experimentally investigated and exhaustive nonlinear FEA is carried out. Two versions of the final 3-3R mechanism are developed, one of which is assembled with spring steel flexures and a monotonically printed PLA variant.