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Type Synthesis of 1T2R Parallel Mechanisms Using Structure CouplingReducing Method
Chinese Journal of Mechanical Engineering volume 32, Article number: 89 (2019)
Abstract
Direct kinematics with analytic solutions is critical to the realtime control of parallel mechanisms. Therefore, the type synthesis of a mechanism having explicit form of forward kinematics has become a topic of interest. Based on this purpose, this paper deals with the type synthesis of 1T2R parallel mechanisms by investigating the topological structure couplingreducing of the 3UPS&UP parallel mechanism. With the aid of the theory of mechanism topology, the analysis of the topological characteristics of the 3UPS&UP parallel mechanism is presented, which shows that there are highly coupled motions and constraints amongst the limbs of the mechanism. Three methods for structure couplingreducing of the 3UPS&UP parallel mechanism are proposed, resulting in eight new types of 1T2R parallel mechanisms with one or zero coupling degree. One obtained parallel mechanism is taken as an example to demonstrate that a mechanism with zero coupling degree has an explicit form for forward kinematics. The process of type synthesis is in the order of permutation and combination; therefore, there are no omissions. This method is also applicable to other configurations, and novel topological structures having simple forward kinematics can be obtained from an original mechanism via this method.
1 Introduction
Forward kinematics, which is one of the fundamental issues in the kinematic analysis of a parallel mechanism (PM) [1], refers to evaluating the pose of a platform from a set of specified values of actuated joint parameters. Generally, the forward kinematic analysis of a PM is much more complex than its reverse process, i.e., inverse kinematics. The difficulty is mainly in relation to the dependent motions of the passive joints in the mechanism.
For a better illustration, three planar open kinematic chains that can be employed to compose planar PMs are given as examples (see Figure 1). The first one is a 2degreeoffreedom (DOF) planar open kinematic chain applying a constraint on a moving platform. It is easy to see that given the position of point A, the lengths of the linkages, and input angle θ, the position of point D on the moving platform can be directly determined using the vector pointing from point A to point D. Meanwhile, for the second and third cases, because of the need to evaluate passive joint parameters α* and β*, the forward kinematics of point D cannot be solved using only the kinematic equations formulated by the open kinematic chain itself, and additional equations depicting the coupled motions among the chains of the PM have to be derived. Therefore, the higher the number of passive joints in a PM, the higher the complexity of the forward kinematic analysis. In order to calculate the forward kinematic problem of a PM efficiently, great efforts have been made to develop numerical algorithms for particular case studies [2,3,4,5,6,7,8]. Although the realtime performance benefits of these methods have been exemplified, only approximate solutions for forward kinematics have so far been attained.
Because this issue is critical to the realtime control of PMs [9], the type synthesis of a mechanism having an explicit form for forward kinematics has become a topic of interest. However, the majority of currently known type syntheses aim to realize output motion only [10,11,12]. To design PMs with analytic direct kinematics, a metric for assessing the complexity of forward kinematics at a topological level has to first be defined. For this purpose, the motion of coupling degree, which is denoted by κ, is proposed in the theory of mechanism topology [13, 14], giving the minimum number of passive joint parameters that need to be solved in the process of forward kinematic analysis. For a specific PM, the value of κ is usually an integral number larger than zero. However, when its value is equal to zero, analytical solutions for forward kinematics can be achieved. Through further examination of the coupling degree of a PM, the approach known as structure couplingreducing (SCR), which aims to reduce the coupling degree while keeping the DOF and position and orientation characteristics (POC) [14] of the mechanism unchanged, is developed in [15, 16]. In this manner, new topological structures that have simple forward kinematics can be obtained from an original mechanism by rearranging the axes and positions of the joints. With the aid of this approach, eight new 3DOF translational PMs [17], three types of 3T1R PMs [18], and sixteen novel 6DOF PMs [19,20,21,22,23] have been synthesized.
Drawing on the SCR method, this paper deals with the type synthesis of 1T2R PMs by reducing the coupling degree of the 3UPS&UP PM within the Tricept robot [24,25,26]. The rest of this paper is organized as follows. In Section 2, the topological characteristics of the 3UPS&UP PM are systemically analyzed, followed by the calculation of the coupling degree of the mechanism. In Section 3, the SCR methods are then introduced to synthesize 1T2R PMs with new topologies. Finally, in Section 4, the forward kinematic analysis of one of the obtained PMs is performed to demonstrate the validity of the method, before conclusions are drawn in Section 5.
2 Topological Characteristics Analysis
In this section, the topological characteristics [13, 14] in terms of the POC set, decomposition of singleopen chains (SOC), constraint degree Δ, and coupling degree κ of the 3UPS&UP PM within the Tricept robot are systematically analyzed.
2.1 Mobility Analysis
As shown in Figure 2, the 3UPS&UP PM consists of three active limbs and one passive limb connecting the base to the moving platform. The active limb i (\(i = 1,2,3\)) is composed of a universal joint U_{i}, a prismatic joint P_{i}, and a spherical joint S_{i}. Meanwhile, the passive limb is composed of a universal joint U_{4} and a prismatic joint P_{4}.
Based on the theory of mechanism topology [14], the POC sets indicating the motion characteristics of the U, P, and S joints (see Figure 3) can be expressed as follows:
where t and r represent translation and rotation, respectively; \({\varvec{\uprho}}_{{{\text{U}}i}}\) and \({\varvec{\uprho}}_{{{\text{S}}i}}\) are the vectors from the centers of U_{i} and S_{i} to a reference point O, respectively; the superscript is the number of translational/rotational motion; (*) describes the direction of translation or the axis of rotation; while {*} denotes the parasitic translations induced by rotations. For example, \(t^{1} ({\text{P}}_{i} )\) and r^{0} in M_{Pi} indicate that a prismatic joint P_{i} can achieve only one translational motion parallel to the direction of the joint.
The joints are serially connected in a limb, and therefore, the POC set of a limb can be expressed as the union of the POC sets of all joints. Here, the operation rules of “union” are presented in Refs. [27, 28]. The POC sets of UPS and UP limbs can then be obtained via
In Eq. (2), because the rotational axes of S_{i} could be parallel to those of U_{i}, the union of \(\{ t^{2} ( \bot {\varvec{\uprho}}_{{{\text{U}}i}} )\}\) and \(\{ t^{2} (\bot {\varvec{\uprho}}_{{{\text{S}}i}})\}\) leads to \(t^{2} ( \bot {\varvec{\uprho}}_{{{\text{U}}i}} )\), according to the rules given in Ref. [21]. Furthermore, because the directions of \(t^{2} ( \bot {\varvec{\uprho}}_{{{\text{U}}i}} )\) are perpendicular to the direction of P_{i}, the union of \(t^{2} ( \bot {\varvec{\uprho}}_{i} )\) and \(t^{1} ({\text{P}}_{i} )\) results in \(t^{3}\). In Eq. (3), the translational directions of \(\{ t^{2} ( \bot {\varvec{\uprho}}_{\text{U4}})\}\) and \(t^{1} ({\text{P}}_{4} )\) are independent; therefore, the union of these two translational motions is \(t^{1} ({\text{P}}_{4} ) \cup \{ t^{2} ( \bot {\varvec{\uprho}}_{{{\text{U}}4}} )\}\). From Eqs. (2) and (3), it can be found that the UPS limb is a 6DOF unconstrained open kinematic chains, while the UP limb has one translational and two rotational independent motions.
Because of the parallel topology of the mechanism, the POC set of the moving platform, M_{pa}, can be obtained by taking the intersection of the POC sets of the limbs. Based on the operation rules of “intersection” given in Refs. [27, 28], the POC set is defined as follows:
from which it can be seen that M_{pa} is identical with M_{b4}. For this reason, the UP limb is also known as the POC chain [29].
2.2 Coupling Degree
The coupling degree can be considered as a metric for assessing the complexity of forward kinematics. For the evaluation of coupling degree of the PM, the mechanism must be decomposed into several SOCs and independent loops [28]. As shown in Figure 4, the decomposition is given as follows:
The first independent loop, Loop1, can then be defined by fixing both end links of SOC_{1} according to Ref. [14], while the second independent loop, Loop2, is obtained by taking Loop1 as a submechanism and attaching two end links of SOC_{2} to Loop1. Analogously, the third independent loop, Loop3, is defined by attaching two end links of SOC_{3} to Loop2 (see Figure 4). With these definitions, the number of independent displacement equations of the three loops can be computed following the method presented in Ref. [28].
where dim{*} denotes the dimension, i.e., the number of independent motions, of a POC set. Subsequently, to evaluate the coupling degree, the constraint degree describing the constraint relationship of a SOC has to be calculated [14]. For this specific case, the constraint degrees of the three SOCs are
where f_{i} is the DOF of the ith joint, and I_{j} is the number of actuated joints in the jth SOC (\(j = 1,2,3\)). The constraint degree of SOC_{1} is shown to be 2, which means that two passive joint parameters have to be solved to determine the motion of SOC_{1}. Meanwhile, the constraint degree of SOC_{2} (SOC_{3}) is \( 1\), indicating that a constraint equation should be established to solve one of the passive joint parameters in the kinematic equations of SOC_{1}. Finally, the coupling degree of the 3UPS&UP PM can be evaluated via [14]
This relationship demonstrates that the constraints of the three SOCs are highly coupled. Therefore, there is no analytic solution for the forward kinematics of the 3UPS&UP PM. Approaches for the type synthesis of new topological structures with the same topological characteristics but using simple forward kinematics are investigated in the following section.
3 Type Synthesis
Three SCR methods, which aim to reduce the coupling degree of the 3UPS&UP PM and obtain new mechanisms that have simple forward kinematics, are presented in this section.
3.1 Method One: Changing the Passive POC Chain into Active Chain
In the 3UPS&UP PM, the UP limb is the POC chain because of M_{b4} being identical with M_{pa}. When the passive prismatic joint in the POC chain is changed into an active one (see Figure 5), the constraint degree \({\Delta}_{1}\) of SOC_{1} can be reduced to
because I_{1} = 2 in this equation. Furthermore, in order to achieve a PM without redundant actuation, the active joint P_{3} in SOC_{3} is taken as a passive joint, resulting in
Hence, the coupling degree of this new PM (No.1 (a) in Table 1) is
indicating that only one passive joint parameter needs to be solved in the forward kinematics. Because limb 3 becomes a 6DOF passive limb in this special case, it has no effect on M_{pa}. Therefore, it is removable, leading to the PM (No.1 (b) in Table 1) within the TriVariant robot [30].
3.2 Method Two: Making the Centers of Spherical Joints Coincident
Another approach to reduce the coupling degree is by making the centers of the spherical joints on the moving platform coincident. For example, the centers of spherical joints S_{1} and S_{3} are coincident, as shown in No. 3 in Table 1. The SOCs of the obtained mechanism (see Figure 6) and their constraint degrees can then be redefined as
resulting in the coupling degree of this new mechanism:
Therefore, a new type of 1T2R PM with lower coupling degree is synthesized. In addition, we can also make the centers of three spherical joints connected to the moving platform coincident. In this way, a new mechanism, as shown in No.4 in Table 1, can then be obtained. For this mechanism, the redefined SOCs and their constraint degrees are given as follows:
Here, the spherical joint S_{2} in SOC_{3} is degenerated into a revolute joint R^{(U1U3)} with the rotational axis parallel to the line passing through the centers of joints U_{1} and U_{3}. The coupling degree of this mechanism is then
Because \(\kappa = 0\), it can be concluded that the PM has an explicit form for forward kinematics.
3.3 Method Three: Integrating the Rotational Axes of Universal Joints
This method aims to directly reduce \(\sum {f_{i} }\) in a SOC. Note that the joint U can be decomposed into a proximal revolute joint R′ and a distal revolute joint R with their rotational axes being perpendicular. Hence, in the 3UPS&UP PM, the revolute joints R′_{2} and R′_{4} of U_{2} and U_{4}, respectively, could be made coincident and integrated into a revolute joint R_{24} (see No. 5 in Table 1). The kinematic chains U_{4}P_{4} and U_{2}P_{2}S_{2} can then be regarded as a hybrid singleopen chain, HSOC_{1} [15].
The SOCs and corresponding constraint degrees of this mechanism (see Figure 7) can then be defined as
Consequently, the coupling degree of the new PM is
Furthermore, R′_{2}, R′_{3}, and R′_{4} of U_{2}, U_{3}, and U_{4}, respectively, can also be integrated into one revolute joint R_{234}. In this manner, the kinematic chains U_{4}P_{4}, U_{3}P_{3}S_{3}, and U_{2}P_{2}S_{2} are converted into another HSOC_{1},
resulting in the R(2RPS&RP)&UPS PM (No.6 in Table 1) within the TriMule robot [31]. For this mechanism, its SOC can be expressed as
Because there is only one SOC, the coupling degree of this PM is zero. Moreover, if the revolute joints R′_{1} and R′_{3} of U_{1} and U_{3}, respectively, and the revolute joints R′_{2} and R′_{4} of U_{2} and U_{4}, respectively, are made coincident and integrated into revolute joints R_{13} and R_{24}, respectively, a new mechanism No.7 in Table 1 can be obtained. For this case, the kinematic chains U_{1}P_{1}S_{1} and U_{3}P_{3}S_{3}, and U_{2}P_{2}S_{2} and U_{4}P_{4} are converted into HSOC_{1} and HSOC_{2}, respectively.
The SOC of the obtained PM can be expressed as
This mechanism also has only one SOC, and its coupling degree is zero. Besides, if the revolute joints R′_{2} and R′_{3} of U_{2} and U_{3}, respectively, in the PM No. 1(b) in Table 1 are integrated into one revolute joint R_{23}, another new mechanism is obtained, as shown in No. 2 in Table 1. In this way, the kinematic chains U_{2}P_{2}S_{2} and U_{3}P_{3}S_{3} are converted into HSOC_{1}:
The SOC and corresponding constraint degree can be expressed as
It can be proved that the coupling degree of this PM is also zero.
4 Example
In this section, the forward kinematic analysis of the obtained R(2RPS&RP)&UPS PM is given as an example to verify that a PM with \(\kappa = 0\) has an explicit form for forward kinematics, which would illustrate the feasibility of the proposed typesynthesis method.
4.1 Coordinate System
The schematic diagram of the R(2RPS&RP)&UPS PM is shown in Figure 8, where points B, B_{1}, B_{2}, and B_{3} are the centers of joints R_{4}, U_{1}, R_{2}, and R_{3}, respectively. Point O is the intersection of the axial axis of the passive limb R_{4}P_{4} and its normal plane in which all centers of S joints, points A_{1}, A_{2}, and A_{3}, are placed. A reference frame BXYZ is attached to point B, satisfying Y⊥B_{2}B_{3} and Z⊥△B_{1}B_{2}B_{3}, while a bodyfixed frame Oxyz is established at point O, satisfying y⊥A_{2}A_{3} and z⊥△A_{1}A_{2}A_{3}. The orientation of Oxyz with respect to BXYZ can then be evaluated via
where c and s represent “cos” and “sin”, respectively.
4.2 Forward Kinematics
When point O is taken as the reference point, its position vector evaluated in BXYZ can be expressed as
where \(q_{i}\) is the length, and \(\varvec{s}_{i}\) is the unit vector along the axial axis of limb I (\(i = 1,2,3,4\)); \({\varvec{a}}_{i} = {\varvec{Ra}}_{i0}\); \({\varvec{a}}_{i0}\) and \({\varvec{b}}_{i}\) are the positioning vectors of \(A_{i}\) and \(B_{i}\) (\(i = 1,2,3\)), respectively, measured in Oxyz and BXYZ, respectively. Noting that
and taking norms on both sides of rearranged Eq. (14) leads to
Expanding Eq. (16) then gives the following kinematic equations:
where \(a_{i} = \left\ {\varvec{a}_{i} } \right\\) and \(b_{i} = \left\ {\varvec{b}_{i} } \right\\) (\(i = 1,2,3\)), satisfying \(a_{2} = a_{3}\) and \(b_{2} = b_{3}\). Combining Eq. (18) with Eq. (19) results in
Considering that s^{2}θ_{2} +c^{2}θ_{2}=1, we can get
When Eq. (21) is solved, the following equations can be obtained:
Substituting Eq. (22) into Eq. (20) then leads to
Therefore, the R(2RPS&RP)&UPS PM is proven to have an explicit form for forward kinematics.
5 Conclusions
Investigation on the type synthesis of 1T2R PMs using the method of structure couplingreducing is presented in this paper. The following conclusions are drawn.

(1)
Conducting a topological characteristics analysis of the 3UPS&UP PM shows that the motions of the three SOCs of the mechanism are highly coupled. Therefore, for this setup, explicit solutions for forward kinematics cannot be achieved.

(2)
Aiming at reducing the coupling degree of the 3UPS&UP PM and synthesizing new mechanisms that have simple forward kinematics, three different methods are proposed, by which eight new PMs having lower coupling degrees are obtained.

(3)
The forward kinematic analysis of the R(2RPS& RP)&UPS PM is presented. The analysis verifies that the obtained new mechanism, which has a zero coupling degree, uses an explicit form for forward kinematics.
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Authors’ Contributions
HL was in charge of the whole trial; KX wrote the manuscript; XS assisted with the process of analysis; TY and HS provided the assistance of theory. All authors read and approved the final manuscript.
Authors’ Information
Haitao Liu, born in 1981, is currently a professor at Tianjin University, China. He received his PhD degree from Tianjin University, China, in 2010. His research interests include hybrid robot and intelligent robotics.
Ke Xu, born in 1992, is currently a PhD candidate at Key Laboratory of Mechanism Theory and Equipment Design, Ministry of Education, Tianjin University, China. He received his master degree on mechanical design and theory from Changzhou University, China, in 2018.
Huiping Shen, born in 1965, is currently a professor at School of Mechanical Engineering, Changzhou University, China. His research interests include parallel mechanism and mechatronics.
Xianlei Shan, born in 1987, is currently a postdoctor at Key Laboratory of Mechanism Theory and Equipment Design, Ministry of Education, Tianjin University, China.
Tingli Yang, born in 1940, is currently a distinguished professor at Changzhou University, China.
Competing Interests
The authors declare that they have no competing interests.
Funding
Supported by National Key R&D program of China (Grant No. 2017YFB1301800), National Natural Science Foundation of China (Grant No. 51622508), and National Defense Basic Scientific Research program of China (Grant No. JCKY2017203B066).
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Liu, H., Xu, K., Shen, H. et al. Type Synthesis of 1T2R Parallel Mechanisms Using Structure CouplingReducing Method. Chin. J. Mech. Eng. 32, 89 (2019). https://doi.org/10.1186/s1003301904031
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DOI: https://doi.org/10.1186/s1003301904031