- Original Article
- Open Access

# Structural Synthesis of Parallel Mechanisms with High Rotational Capability

- Xiao-Dong Jin
^{1}, - Yue-Fa Fang
^{1}Email author, - Sheng Guo
^{1}and - Hai-Bo Qu
^{1}

**31**:61

https://doi.org/10.1186/s10033-018-0260-3

© The Author(s) 2018

**Received:**24 May 2016**Accepted:**1 August 2018**Published:**14 August 2018

## Abstract

Most parallel mechanisms (PMs) encountered today have a common disadvantage, i.e., their low rotational capability. In order to develop PMs with high rotational capability, a family of novel manipulators with one or two dimensional rotations is proposed. The planar one-rotational one-translational (1R1T) and one-rotational two-translational (1R2T) PMs evolved from the crank-and-rocker mechanism (CRM) are presented by means of Lie group theory. A spatial 2R1T PM and a 2R parallel moving platform with bifurcated large-angle rotations are proposed by orthogonal combination of the RRRR limbs. According to the product principle of the displacement group theory, a hybrid 2R3T mechanism in possession of bifurcated motion is obtained by connecting the 2R parallel moving platform with a parallel part, which is constructed by four 3T1R kinematic chains. The presented manipulators possess high rotational capability. The proposed research enriches the family of spatial mechanisms and the construction method provides an instruction to design more complex mechanisms.

## Keywords

- Parallel mechanism
- Rotational capability
- Lie group theory
- Bifurcated motion
- Hybrid mechanism

## 1 Introduction

Parallel mechanisms have been extensively researched over many years, depending on their advantages of high speed, high accuracy, large stiffness and heavy-load capability in comparison with the serial counterparts. Such as the famous Delta robot [1], the surgical PMs for medical application [2] and the five-axis machine tools [3]. Several approaches for the structural synthesis of PMs have been proposed. They are the enumeration approach based on the general mobility formula [4, 5], the constraint approach based on the reciprocal screw theory, the synthesis approach based on the Lie group theory [6–8], and the approach based on the theory of linear transformations [9].

However, PMs suffer from the problems of having relatively small useful workspace and limited rotational capability, which are crucial for their performance and applications [10]. It is a more complex process to design PMs with rotational mobility with respect to the translational ones. The classical six Degrees of Freedom (DOF) PMs often suffer from limited tilting angles due to the joint range limits and the interferences between limbs or between limbs and the moving platform. For example, the famous Stewart platform can only achieve a tilting angle of 40° with respect to the horizontal axis. Hence, the research of PMs with large rotational angles has always been one of the focuses in the structural synthesis of the PMs.

To reach high tilting angles, Kim et al. [11] first proposed the Eclipse with three PPRS (P, R and S denote a prismatic joint, a revolute joint and a spherical joint, respectively) serial limbs for five-face machining in a single setup, which can continuously sweep 360° over the lateral surfaces of the workpiece. Later, the same authors presented a redundantly actuated Eclipse II [12] that can realize continuous 360° rotational motion in any direction and is suitable for the application of flight motion simulators. However, circular guideway makes the translational workspace greatly reduced, which leads to the limited application in need of large translational workspace. Kang et al. [13] developed a 3-DOF micro-positioning PM with 100° tilting angles and analyzed its positioning capability in detail. Using articulated traveling plate to amplify the rotational performance, Pierrot et al. [14–16] proposed a 4-DOF parallel manipulator known as H-4, which was applied in industrial production. Based on the design of H-4, Krut et al. [17–19] constructed a rotating amplifier using gear sets to develop an I-4 parallel mechanism which can amplify the rotating angles from ± 45° to ± 180°. A prominent characteristic for the H-4 family is that non-rigid moving platforms are used, after that many PMs with high rotational capability were synthesized utilizing the non-rigid moving platforms. Guo et al. [20, 21] developed a systematic method for the type synthesis of 4-DOF non-overconstrained parallel mechanisms with three translations and one rotation inspired by the H-4 manipulators and presented a class of 4-DOF non-overconstrained parallel mechanisms with large rotational workspace based on screw theory. Wang et al. [22] synthesized a type of 2T3R and 3T3R PMs through constructing different kinds of configurable platforms, which possess high rotational abilities. Oh et al. [23, 24] designed a high maneuverability, high accuracy and large workspace parallel positioning device of which rotational angles can achieve 100°. Liu et al. [25] proposed a family of 3-DOF fully-parallel manipulators which are capable of rotating large angles. Based on the screw theory, Li et al. [26] investigated the identification of the rotational axes of the 3-[P][S] parallel mechanism with different limbs arrangements.

In order to design PMs with large rotational angles, this paper replaces the fixed link of a CRM with additional limbs to obtain a novel PM model based on the relationship between input link and output link of the CRM. On this basis, a series of 1R2T and 1R1T planar PMs are synthesized by means of the Lie group theory. To realize 2-dimension spatial rotations, a 3-RRRR PM and a 4-RRRR parallel moving platform with rotational amplification property and bifurcated rotations are presented. Finally, a hybrid 2R3T mechanism in possession of large rotational angles and bifurcated motion is proposed and analyzed.

The organization of this paper is as follows. In Section 2, a CRM is analyzed and its degenerate model is evolved. Section 3 synthesizes a class of planar 1R1T, 1R2T PMs with large rotational angles and two kinds of spatial PMs with high rotational ability and bifurcated motion by means of the Lie group theory. Using the proposed moving platform, Section 4 constructs a hybrid mechanism, of which mobility and bifurcated property are analyzed. Finally, conclusions are drawn in Section 5.

## 2 Degenerate CRM Model

*D*as Lie group [27]. Further any motion of a rigid body can be described by a subset of

*D*, which may be either a group, called a displacement subgroup (DSG) or a displacement submanifold (DSM) [28, 29]. According to Ref. [30], the 3-dimension subgroup {

*G*(μ)} represents 2-dimension translation in a plane and 1-dimension rotation about the normal to the plane. {

*G*

_{2}(μ)} denotes 2-dimension displacement manifold, which can be obtained by removal of {

*R*(

*N*,μ)} from {

*G*(μ)}. {

*R*(

*N*,μ)} and {

*T*(μ)} are the representations of 1-dimension rotational subgroup and translational subgroup, in which (

*N*, μ) represents the axis determined by the unit vector μ and point

*N*. The displacement set of the end-effector is the product of DSGs of all pairs in a series kinematic chain and the product of groups is closed [31, 32]. Assume that the rigid bodies constructed a limb in a parallel mechanism are 1, 2, 3, …,

*j*− 1,

*j*in turn and the DSGs or DSMs (DSGs/DSMs) of the corresponding pairs are {

*D*

_{1}}, {

*D*

_{2}}, {

*D*

_{3}},…, {

*D*

_{j−1}}, the DSG/DSM of the end of the limb is the product of all the DSGs/DSMs, i.e., {

*L*

_{i}} = {

*D*

_{1}}{

*D*

_{2}}{

*D*

_{3}}∙∙∙{

*D*

_{j−1}}, where {

*L*

_{i}} is the DSG/DSM of the end of the limb. The intersection of two subgroups is always a subgroup. The DSG/DSM of the moving platform of a parallel mechanism is the intersection of the DSGs/DSMs of all limbs, i.e.,

*M*} denotes the DSG/DSM of the moving platform and {

*L*

_{i}} denotes the DSGs/DSMs of the branches.

*α*[33]. This motion principle can be used to construct a rotation amplifying mechanism and be applied to design PMs with large rotational angles.

The planar CRM is constituted by two limbs and a moving platform. Limb 1 is connected by link 1 and link 2 and limb 2 only contains the fixed link 4 and the shortest link 3 is used to be the moving platform. According to the Lie group theory, the DSG of limb 1 can be calculated as {*R*(*N*_{11}, ω)}{*R*(*N*_{12}, ω)}{*R*(*N*_{13}, ω)} = {*G*(ω)}, and the DSG of limb 2 is {*R*(*N*, ω)}.

*γ*) of link 3 will be variable due to the distance (denoted by

*d*) between joint

*A*and

*D*is changeable. For brevity and without loss of generality, we let

*l*

_{2}>

*l*

_{1}>

*l*

_{3}, the relationship between

*γ*and

*d*can be figured as

*d*is between

*l*

_{2}

*+*

*l*

_{3}–

*l*

_{1}and

*l*

_{1}

*+*

*l*

_{2}–

*l*

_{3}and the maximum rotational angle will decrease when

*d*is equal to other values. It is an important guidance for the design of the PMs with large rotational angles.

*M*

_{1}} represents the intersection between subsets of the additional limbs,

According to Eq. (4), all possible motion the end-effector 3 could output are {*R*(*N*,ω)}, {*R*(*N*,ω)}{*T*(μ)}, {*R*(*N*,ω)}{*T*(ν)} and {*G*(ω)}, which are subsets or proper subsets of {*G*(ω)}. Thus, we let the motion types of the additional limbs also be subsets or proper subsets of {*G*(ω)}, i.e., \(\{ M_{1} \} \subseteq \{ G\text{(}\varvec{\omega}\text{)}\} .\) A class of planar 1R1T, 1R2T PMs can be synthesized and further a type of spatial 2R1T and 2R PMs can be obtained by appropriate evolution.

## 3 Structural Synthesis of 1R1T, 1R2T, and 2R1T, 2R PMs

In this section, different limbs are connected with the degenerate CRM to synthesize a series of planar and spatial PMs with large rotational angles in terms of displacement group theory.

### 3.1 Planar 1R1T and 1R2T PMs

#### 3.1.1 1R1T PMs

*k*which parallel to

*x*axis in Figure 4(a), or along the dasher line

*t*which parallel to

*y*axis in Figure 4(b), or alone a circle line in Figure 4(c) and rotate with large angles about any point in the given lines.

Therefore, a class of 1R1T PMs is demanded. As known from Lie group theory, {*R*(*N*_{1},ω)}{*R*(*N*_{2},ω)} = {*R*(*N*,ω)}{*T*(μ)} = {*G*_{2}(ω)}, {*G*_{2}(ω)}∩{*G*(ω)} = {*G*_{2}(ω)}, where {*G*_{2}(ω)} is a subset of {*G*(ω)}. Because the DSG of limb 1 is {*G*(ω)}, the DSM of the additional limbs must be {*G*_{2}(ω)} if 1R1T PMs are needed.

*k*parallel to

*x*axis,

*t*parallel to

*y*axis,

*r*,

*x*,

*y*,

*z*axes in Cartesian coordinate system respectively. {

*T*(r)} is perpendicular to link 4 in Figure 4(c) and can be composited by {

*T*(μ)} and {

*T*(ν)}. CAD models of these 1R1T PMs are given as shown in Figure 5. According to Eq. (3) and Figure 3, the rotational angles are variable along with the motion of the prismatic pairs for RRR-RP

_{x}and RRR-RP

_{y}PMs and invariable for RRR-RR PM.

#### 3.1.2 1R2T PMs

Now a class of 1R2T PMs will be synthesized by connecting specific additional limbs with the degenerate CRM. The output DSG of the 1R2T PM is {*G*(ω)} and as known before, DSG of the degenerate CRM is {*G*(ω)} as well. Hence, DSG of the additional limbs must be {*G*(ω)}, without taking the subgroups which are greater than 3-dimension into account.

The two PMs analyzed above have the same motion properties in view of their DSGs, which both move with two dimensional translations in *xy* plane and one dimensional rotation about axis parallel to *z*. DOF of the two PMs are three, as a result, three actuators are needed for each one. Three actuators can be arranged on the base of the three limbs respectively for the 3-RRR PMs. However, one of the two limbs must install two actuators and another limb installs the rest one because there are only two branches for the 3-DOF 2-RRR PMs. It is worth mentioning that the two PMs have whole cycle or close to whole cycle rotational capability based on Figure 3.

*G*(ω)} can be obtained either by {

*R*(

*N*,ω)}{

*T*(μ)}{

*T*(ν)} or by {

*R*(

*N*

_{1},ω)}{

*R*(

*N*

_{2},ω)} {

*R*(

*N*

_{3},ω)}. The PMs indicated by the latter one are presented in Figure 6(a) and Figure 6(b). Now, {

*R*(

*N*,ω)}{

*T*(μ)}{

*T*(ν)} is used to present the additional limbs which are connected in the degenerate CRM in Figure 2. The DSGs of the new PMs are

The additional limbs denoted by {*T*(μ)}{*T*(ν)}{*R*(*N*,ω)} can be composed by two translational pairs P_{x}P_{y} and one revolute pair R, as a result, a 2-RRR-P_{x}P_{y}R mechanism and a 2-P_{x}P_{y}R-RRR mechanism can be derived as shown in Figure 6(c) and Figure 6(d). Although the 2-RRR-P_{x}P_{y}R and 2-P_{x}P_{y}R-RRR mechanisms output the same motion as the other two PMs, they have an advantage of restricting the workspace within a specific range through controlling the length of the two translational linkages.

### 3.2 2R1T and 2R Mechanisms

This part investigates two kinds of mechanisms with large rotational angles whose rotations are not consecutive but bifurcated.

#### 3.2.1 2R1T PM

*M*which is always parallel to the base plane represented by

*N*. When the plane of the end-effector represented by

*m*is parallel to

*N*,

*M*and

*m*are coincided to each other. The DSM/DSG of limb 1 is

According to the DSM, the end-effector has the motion type of two rotations about *x* and *y* axes and one translation along *z* axis and it is an instantaneous motion only if *m* is parallel to *N*.

*x*axis, shown as Figure 7(b),

*m*is no longer parallel to

*N*and has an intersection angle

*φ*. In other words, the axis of the last revolute joint of limb 3 doesn’t parallel to any one joint of limb 1 and 2 anymore. The DSMs of limb 1 and 2 remain unchanged, however, the DSM of limb 3 becomes

*y*axis.

Obviously, the end-effector outputs one rotation about *x* axis and one translation along *z* axis.

*y*axis, shown as Figure 7(c),

*m*is no longer parallel to

*N*and has an intersection angle

*α*, and the axes of the last revolute joints of limb 1 and 2 don’t parallel to any one joint of limb 3 anymore. The DSM of limb 3 remains unchanged, however, the DSMs of limb 1 and 2 become

At this configuration, the DSM of the end-effector is {*T*(ω)}{*R*(*N*,ν)}. Hence, the end-effector at this configuration outputs one rotation about *y* axis and one translation along *z* axis.

Combining with above analysis, note that there are three bifurcated configurations for the 3-RRRR PM including an instantaneous 2R1T motion only if the end-effector is parallel to the base and the other two 1R1T motions with rotating about *x* axis and *y* axis asynchronous as long as the plane of the end-effector intersects with the plane of base.

#### 3.2.2 Rotational Capability Analysis for the 2R1T PM

When the platform rotates about *x* axis, only one limb controls the rotational angle and the links interference is slight and can be eliminated through reasonable structural design for the platform. Hence, the rotational angle about *x* axis can reach 360°. However, the platform is controlled by two identical limbs when rotating about *y* axis, as a result, the interference between these two limbs hampers the whole cycle rotation of the platform. Therefore, it is indispensable to investigate the rotational performance about *y* axis under the interference model.

*z*axis if its posture is fixed. This means that the PM can be seen as a crank-slider mechanism (CSM) illustrated in Figure 8(b) at the fixed posture and its translational workspace is equivalent to the slider stroke.

*A*can be deduced as

*s*denotes the displacement of the equivalent slider and

*θ*denotes the input angle of the fixed joint

*A*.

The equivalent interference models are shown in Figure 8(c) and Figure 8(d). We assume that the initial posture of end-effector 3 is horizontal and define that the clockwise rotation is positive. There are two kinds of cases that need to be discussed. One is the maximum positive rotational angle is greater than 90° as shown in Figure 8(c) and another is less than 90° as shown in Figure 8(d). These two cases are determined by the displacement *s*.

*s*

_{min}and

*s*

_{max}denote the minimum and maximum displacements and

*s*

_{90°}denotes the displacement value when the maximum positive angle is equal to 90°.

*β*can be figured out in ∆

*AB*

_{1}

*P*in Figure 8(c) and ∆

*AC*

_{1}

*P*in Figure 8(d).

*l*

_{1}= 400 mm,

*l*

_{2}= 600 mm,

*l*

_{3}= 300 mm. Then the maximum rotational angle

*γ*can be calculated as

*l*

_{1}+

*l*

_{2}−

*l*

_{3}/2. In other words, when the translation of the platform makes the values

*l*

_{1},

*l*

_{2},

*l*

_{3}and

*s*conform to the geometrical relationship of a CRM, the PM has the ability of rotating about

*y*axis more than 180°. The maximum rotational angle will drastically decrease when the displacement is greater than

*l*

_{1}+

*l*

_{2}−

*l*

_{3}/2.

*x*axis and

*y*axis with large angles. If the end-effector needs to rotate about

*x*axis, it has to move its plane parallel to the base at first and then lock the drives in limb 2 and 3. The similar operation should be taken if the end-effector needs to rotate about

*y*axis. Figure 10 shows the CAD models about large rotational angles about

*x*axis and

*y*axis respectively.

#### 3.2.3 2R Parallel Moving Platform

*A*,

*B*,

*C*and

*D*are intersected at point

*O*. When the four axes are coplanar, as shown in Figure 12(a), the overconstrained chain is at the singular position where axes of joint

*A*and joint

*C*, axes of joint

*B*and joint

*D*are coincident respectively. It has instantaneous two dimensional rotations about

*x*axis and

*y*axis. When the mechanism rotates a certain angle about joints

*A*and

*C*from the singular position, as shown in Figure 12(b), it remains only one dimensional rotation about

*y*axis because joints

*B*and

*D*can’t rotate due to the structural constraints. Similarly, it remains only one dimensional rotation about

*x*axis when the mechanism rotates a certain angle about joints

*B*and

*D*from the singular position as shown in Figure 12(c).

*A*

_{i}

*B*

_{i}(

*i*= 1, 2, 3, 4) in each limb is designed that can’t rotate about its own axis but be capable of moving with the overconstrained 4R chain and it will be applied mechanical constraints when connected on the hybrid mechanism. Therefore, joint

*A*is not a compound hinge but a single revolute pair. When the PM moves to the configuration of plane

*m*parallel to plane

*N*, as shown in Figure 11(a), the DSM of the four limbs are

It can be found that the Lie group format of each limb is equal to the format of the corresponding limb in the 3-RRRR PM at the reference plane. Consequently, the DSG of the end-effector is also {*T*(ω)}{*R*(*N*,μ)}{*R*(*N*,ν)}, which illustrates it has the motion type of two rotations about *μ* and *ν* axes and one translation along *ω* axis. The two dimensional rotations are instantaneous DOF at the current configuration. At this singular configuration, axes of joints *A*_{1} and *A*_{3} are coincided and they should be seen as only one revolute pair when calculated by Lie group theory, the same to joints *A*_{2} and *A*_{4}.

*A*

_{2}and

*A*

_{4}revolute a smaller angle, the end-effector can output a relatively larger rotational angle

*α*as shown in Figure 11(b). At this configuration, the DSMs of the four limbs are

Thus, the DSM of the end-effector can be deduced as

It reveals that the PM only has 1-DOF rotation about *μ* axis at this configuration. Similarly, if joints *A*_{1} and *A*_{3} rotate a smaller angle, the end-effector also outputs a relatively larger rotational angle *φ* and the PM has 1-DOF rotation {*R*(*N*,ν)} about *v* axis as shown in Figure 11(c).

#### 3.2.4 Rotational Capability Analysis for the 2R Parallel Moving Platform

*xz*plane or

*yz*plane.

*s*in Figure 8(c). The maximum positive rotational angle is

*β*can be figured out in ∆

*AB*

_{1}

*G*,

*s*

_{c}denotes the certain displacement value.

Equation (24) represents the maximum rotational angle of positive direction. Due to the bifurcation property, this parallel moving platform can rotate about *μ* axis and *ν* axis with large angles, both of which range are ± *δ*. It is worth mentioning that if the platform would change its rotational direction from one axis to another, it must move to the singular posture at first.

In practical applications, the PMs usually input a given swinging angle *θ* and the end-effector outputs a larger angle with range of ± *δ*. Assuming that the length of all links in the equivalent 2-RRR mechanism in terms of the 4-RRRR PM in Figure 11 is known and still let *l*_{1} = 400 mm, *l*_{2} = 600 mm, *l*_{3} = 300 mm, the maximum positive angle *δ* can be calculated as *δ* = 136° according to Eqs. (24)‒(26).

## 4 2R3T Hybrid Mechanism

The 4-RRRR parallel moving platform is used as a rotating amplifier for the 2R3T hybrid mechanism in this section. As mentioned before, since link *A*_{i}*B*_{i} in each limb of the 4-RRRR mechanism can’t rotate, we let these four links connect with the remaining components of the hybrid mechanism. The DSMs of the hybrid mechanism are {*R*(*N*,v)}{*R*(*N*,μ)}{*T*} at singular configuration and {*R*(*N*,v)}{*T*} at one bifurcated configuration and {*R*(*N*,μ)}{*T*} at another one. Considering the moving platform only has 1-dimension rotation at either of the two bifurcated configurations and has an instantaneous 2-dimension rotation at the singular posture, it just need to connect the moving platform to a 3T1R parallel component. However, an additional actuator should be installed in one of the four joints of the overconstrained close-loop chain in order to avoid singularity. Since the new component doesn’t have moving platform before connecting with the 4-RRRR PM, all limbs are open-loop chains and symmetric distribution and two revolute joints are needed to provide swinging motion about *x* axis and another two to provide swinging motion about *y* axis for the parallel moving platform. Therefore, two limbs of the 3T1R parallel component associate with the DSM of {*R*(*N*,μ)}{*T*}, however, another two limbs associate with {*R*(*N*,v)}{*T*}. Since the new parallel component connects with the parallel moving platform in series, the Lie group operation for the two components obeys the product operation.

Now, we will analyze its mobility and bifurcated rotations in another way, however, the large rotational ability will not be demonstrated again which was analyzed before in detail. According to the Lie group theory, the product of two identical DSGs/DSMs is always equivalent to themselves, i.e., {*R*(*N*,v)}{*R*(*N*,v)}={*R*(*N*,v)}. As a result, we divide the hybrid mechanism into two components both of which contain the 4R overconstrained close-loop chain, as the two dashed frames shown in Figure 15.

*T*(r)} represents the DSG associated with a prismatic pair. The intersection of these four DSMs is equal to {

*R*(

*N*,v)}{

*R*(

*N*,μ)}{

*T*}.

*R*(

*N*,v)}{

*R*(

*N*,μ)} at singular posture. Thus, the DSM of the end-effector can be calculated as

In summary, when the hybrid mechanism moves to the singular configuration, it possesses instantaneous 5-DOF of 3-dimension translations and 2-dimension rotations about *x* axis and *y* axis. Nevertheless, when moves to one of the two bifurcated configurations, it possesses 4-DOF of 3-dimension translations and 1-dimension rotation about *x* axis or *y* axis. By the way, if the mechanism would move from one configuration to another, it must switch to the singular posture first. Five actuators are needed for the hybrid mechanism where the four hydraulic cylinders are taken into account and the last one is installed in one of the four joints in the overconstrained close-loop chain.

## 5 Conclusions

- (1)
Based on the Lie group theory and the rotational amplifying characteristics of the CRM, a class of planar 1R1T, 1R2T PMs is presented and a spatial 2R1T PM with bifurcated rotations is derived.

- (2)
A 4-RRRR rotating amplifier is constructed and used as the moving platform for a 2R3T hybrid mechanism. This hybrid mechanism possesses high rotational performance and bifurcation, which can be applied to manufacture the 3D printers, the flight simulators, the multi-axis machine tools, etc.

- (3)
The proposed PMs, whose reachable rotating angles are larger than 180°, are more adequate for fast and low accuracy motions.

- (4)
The future research will focus on the kinematic analysis, the structural design of hybrid mechanisms with multidimensional non-bifurcated rotations and a properties comparison between the synthesized architectures.

## Declarations

### Authors’ Contributions

Y-FF was in charge of the whole trial; X-DJ wrote the manuscript; X-DJ, SG and H-BQ assisted with sampling and laboratory analyses. All authors read and approved the final manuscript.

### Authors’ Information

Xiao-Dong Jin, born in 1991, is currently a PhD candidate at *School of Mechanical, Electronic and Control Engineering*, *Beijing Jiaotong University, China*. He received his bachelor degree from *Huaqiao University, China*, in 2014. His research interests include spatial mechanism design and hybrid robots.

Yue-Fa Fang, born in 1958, is currently a professor and a PhD candidate supervisor at *School of Mechanical, Electronic and Control Engineering, Beijing Jiaotong University, China.* His main research interests include theory of mechanisms and parallel robots.

Sheng Guo, born in 1972, is currently a professor and a PhD candidate supervisor at *School of Mechanical, Electronic and Control Engineering, Beijing Jiaotong University, China.* His main research interests include spatial mechanism design and parallel robots.

Hai-Bo Qu, born in 1983, is currently a lecturer at *Robotics Institute, Beijing Jiaotong University*. He received his PhD from *Beijing Jiaotong University* in 2013. His research interests include robotics mechanism and mechanical design.

### Competing Interests

The authors declare no competing financial interests.

### Funding

Supported by Fundamental Research Funds for the Central Universities of China (Grant No. 2018YJS143), and National Natural Science Foundation of China (Grant Nos. 51675037, 51505023, 51475035).

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## Authors’ Affiliations

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