Abstract
This paper studies a structural–parametric synthesis of the four-bar and Stephenson II, Stephenson III A, and Stephenson III B six-bar function generators. A four-bar function generator is formed by connecting two coordinate systems with given angles of rotation using a negative closing kinematic chain (CKC) of the RR type. Six-bar function generators are formed by connecting two coordinate systems using two CKCs: a passive CKC of the RRR type and a negative CKC of the RR type. The negative CKC of the RR type imposes one geometrical constraint to the relative motion of the links, and its geometric parameters are defined by least-squares approximation. Passive CKC of the RRR type does not impose a geometrical constraint, and the geometric parameters of its links are varied to satisfy the geometrical constraint of the negative CKC. Numerical results of the four-bar and six-bar function generators parametric synthesis are presented.
1 Introduction
The first studies on the design of function-generating linkages are due to Svoboda [1,2], who designed a Watt II function generator for the generation of the logarithmic function. Kinematic synthesis (dimensional or parametric synthesis) of mechanisms, including function-generating linkages, is carried out on the basis of the kinematic geometry of finite positions of a rigid body, approximation methods (polynomials), and computers [3]. The kinematic geometry of finite positions of a rigid body, which in the case of plane motion is known as the Burmester theory [4], is used for the synthesis of function generators in the works of Hunt [5], Bottema and Roth [6], Angeles and Bai [7,8], Pira and Cunaku [9], McCarthy and Soh [10], and others. The synthesis of mechanisms by kinematic geometry is clear and simple. However, these methods are applicable for a limited number of positions. For the kinematic synthesis of mechanisms with unlimited positions of the output links, the approximation methods are used, the foundations of which were laid by Chebyshev [11]. Approximation (algebraic, optimization) methods for the kinematic synthesis of four-bar and six-bar function generators Watt II, Stephenson II, and Stephenson III were used in the works of Freudenstein [12], Hartenberg and Denavit [13], McLarnan [14], Subbian and Flugrad [15,16], Kiper et al. [17], Hwang and Chen [18], Bulatovic et al. [19], Plecnik and McCarthy [20–22], and others. In Refs. [18,19,20], the polynomial homotopic software bertini [23] was used. At the intersection of kinematic geometry and approximation synthesis, a new direction in the kinematic synthesis of mechanisms—approximation kinematic geometry—has been created by Sarkissyan et al. [24–26]. Approximation kinematic geometry combines the advantages of methods of kinematic geometry and approximate synthesis of mechanisms, such as simplicity and unlimited positions of the output links. Based on approximation kinematic geometry by Baigunchekov et al. [27–31], the parallel mechanism and manipulator are synthesized.
In this work, a structural–parametric synthesis of four-bar and six-bar function generators is carried out, where the structures and geometric parameters of the links of the synthesized mechanisms are determined in arbitrary given discrete values of the input and output link angles. In structural–parametric synthesis, according to the given laws of motion of the input and output links, structural schemes and geometric parameters of the links of the synthesized mechanisms are simultaneously determined. At the same time, the structural–parametric synthesis of the designed mechanisms begins with the smallest number of links and becomes more complicated depending on the implementation of the specified laws of motion of the output links within the required accuracy. Depending on the complexity of the given laws of motion of the output links, it is possible to form mechanisms with complex structural schemes. Structural and kinematic analysis of complex mechanisms containing Assur groups of higher classes [32] is the subject of work [33–41]. In this paper, to analyze the positions of complex mechanisms, which is necessary to evaluate the results of parametric synthesis, a simple method of conditional generalized coordinates is proposed.
2 Structural Synthesis of the Planar Four-Bar and Six-Bar Function Generators With Revolute Joints
According to the developed principle of mechanism formation, they are formed by connecting the output object to the base using active, passive, and negative CKCs [27,28]. Active CKCs have active joints, passive CKCs have zero DOF, and negative CKCs have negative DOF. The active and passive CKCs impose geometrical constraints on the motion of the output object. The passive CKCs do not impose geometrical constraints.
The output object of the function generators is a link that performs a given rotary or translational motion relative to the base at a given motion of the input link. Let the input link and the output object perform rotational movements. We choose as the input and output links the coordinate systems Ax1y1 and Bx2y2 rotating relative to the base with rotation angles φi and ψi (Fig. 1(a)).
If we connect the planes of two moving coordinate systems Ax1y1 and Bx2y2 by a negative CKC CD of the RR type, then we obtain a structural scheme of a four-bar function generator ACDB. Connection of the planes of two moving coordinate systems Ax1y1 and Bx2y2 by binary link CD of the type RR is possible when the plane of the moving coordinate system Bx2y2 has a circular point (a point moving along a circle) D in relative motion to the coordinate system Ax1y1, or vice versa, i.e., when there is a circular point C in the plane of the coordinate system Ax1y1 in relative motion with respect to the coordinate system Bx2y2.
If none of the planes of the moving coordinate systems Ax1y1 and Bx2y2 do not have circular points in relative motion, then the planes of the two coordinate systems are connected by passive CKC CED of the RRR type. As a result, we obtain a structural scheme of the five-bar linkage ACEDB with two DOF (Fig. 1(b)). To form six-bar function generators from this five-bar linkage, we connect its non-adjacent links by binary link FG of the type RR, having one negative DOF (or a constraint that reduces the DOF of the system by one), defined by the Chebyshev formula
Consequently, the negative CKC FG, imposing one geometrical constraint on the five-bar linkage with two DOF, forms six-bar function generators with one DOF. Figures 1(c)–1(f) show the structural schemes of the formed six-bar function generators. If links 1 and 2 of the five-bar linkage are connected by binary link FG, we obtain a Stephenson I function generator (Fig. 1(c)). If links 3 and 2 of the five-bar linkage are connected by binary link FG, we obtain a Stephenson II function generator (Fig. 1(d)). When link 3 of the five-bar linkage is connected to the base by binary link FG, we obtain a Stephenson III A function generator (Fig. 1(e)). When link 4 of the five-bar linkage is connected to the base by binary link FG, we obtain a Stephenson III B function generator (Fig. 1(f)).
3 Parametric Synthesis of Four-Bar Function Generator
It is easy to show that the Hessian of matrices D(1) and D(2) are positive definite together with the principal minors [23], and then the solutions of the systems of linear Eqs. (16) and (17) correspond to the minimum of function (13). Therefore, for given values of the vector parameters p(2) = [p4, p5, p3], the vector parameters p(1) = [p1, p2, p3] are determined by solving the system of linear Eq. (16). Based on the obtained values of the vector parameters p(1), the vector parameters p(2) are determined by solving the system of linear Eq. (17). In this case, the sequence of function S(k) values will be decreasing and have a limit as a sequence bounded from below. This allows using the linear iteration method based on kinematic inversion to solve the least-square approximation.
4 Parametric Synthesis of Six-Bar Function Generators
Synthesis parameters for the negative CKC FG of the Stephenson I mechanism (Fig. 1(c)) are , which are determined similar to the parametric synthesis of the four-bar function generator (Fig. 2). Therefore, the functionality of the Stephenson I mechanism is the same as the functionality of the four-bar function generator.
Synthesis parameters for the negative CKC FG of the Stephenson II function generator (Fig. 3) are , where and are the coordinates of the joints F and G in the coordinate systems Cx3y3 and Bx2y2 of the links 3 and 2, respectively.
Further, the synthesis parameters of the link FG are determined similarly to the determination of synthesis parameters of the link CD of the four-bar function generator.
Synthesis parameters of the binary link FG of the Stephenson III A function generator (Fig. 4) and the Stephenson III B function generator (Fig. 5) are and , respectively, and are for both function generators, where and are the coordinates of the joint F in the coordinate systems Cx3y3 and Dx4y4, respectively, and are the coordinates of the joint G in the absolute coordinate system OXY.
Further, synthesis parameters of the binary link FG are determined similar to the parametric synthesis of the four-bar function generator.
5 Numerical Results
Let us consider the parametric synthesis of the four-bar, Stephenson II, Stephenson III A, and Stephenson III B function generators. The following coordinates XA = 10.0, YA = 20.0, XB = 70.0, and YB = 20.0 of the four-bar Stephenson II, Stephenson III A, and Stephenson III B function generators pivots A and B are given in the absolute coordinate system OXY.
5.1 Four-Bar Function Generator.
Table 1 gives the values of the angles φi and ψi of the input and output links for N = 12 of the four-bar function generator (Fig. 6).
N | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | 11 | 12 |
---|---|---|---|---|---|---|---|---|---|---|---|---|
φi | 0 deg | 30 deg | 60 deg | 90 deg | 120 deg | 150 deg | 180 deg | 210 deg | 240 deg | 270 deg | 300 deg | 330 deg |
ψi | 36 deg | 38 deg | 51 deg | 69 deg | 88 deg | 102 deg | 108 deg | 108 deg | 104 deg | 98 deg | 84 deg | 57 deg |
N | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | 11 | 12 |
---|---|---|---|---|---|---|---|---|---|---|---|---|
φi | 0 deg | 30 deg | 60 deg | 90 deg | 120 deg | 150 deg | 180 deg | 210 deg | 240 deg | 270 deg | 300 deg | 330 deg |
ψi | 36 deg | 38 deg | 51 deg | 69 deg | 88 deg | 102 deg | 108 deg | 108 deg | 104 deg | 98 deg | 84 deg | 57 deg |
Table 2 presents the obtained values of the synthesis parameters of the four-bar function generator.
N | 1 | 2 | 3 | 4 | 5 | 6 |
---|---|---|---|---|---|---|
φi | 0 deg | 30 deg | 60 deg | 90 deg | 120 deg | 150 deg |
ψi | 36.17 deg | 37.60 deg | 51.30 deg | 69.09 deg | 88.10 deg | 102.32 deg |
N | 7 | 8 | 9 | 10 | 11 | 12 |
φi | 180 deg | 2100 | 2400 | 2700 | 3000 | 3300 |
ψi | 107.91 deg | 107.71 deg | 104.44 deg | 97.67 deg | 83.77 deg | 57.27 deg |
N | 1 | 2 | 3 | 4 | 5 | 6 |
---|---|---|---|---|---|---|
φi | 0 deg | 30 deg | 60 deg | 90 deg | 120 deg | 150 deg |
ψi | 36.17 deg | 37.60 deg | 51.30 deg | 69.09 deg | 88.10 deg | 102.32 deg |
N | 7 | 8 | 9 | 10 | 11 | 12 |
φi | 180 deg | 2100 | 2400 | 2700 | 3000 | 3300 |
ψi | 107.91 deg | 107.71 deg | 104.44 deg | 97.67 deg | 83.77 deg | 57.27 deg |
5.2 Stephenson II Function Generator.
Table 4 gives the values of the angles φi and ψi of the input and output links for N = 12 of the Stephenson II function generator (Fig. 8).
N | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | 11 | 12 |
---|---|---|---|---|---|---|---|---|---|---|---|---|
φi | 0 deg | 30 deg | 60 deg | 90 deg | 120 deg | 150 deg | 180 deg | 210 deg | 240 deg | 270 deg | 300 deg | 330 deg |
ψi | 104 deg | 98 deg | 93 deg | 90 deg | 90 deg | 92 deg | 98 deg | 105 deg | 112 deg | 116 deg | 116 deg | 111 deg |
N | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | 11 | 12 |
---|---|---|---|---|---|---|---|---|---|---|---|---|
φi | 0 deg | 30 deg | 60 deg | 90 deg | 120 deg | 150 deg | 180 deg | 210 deg | 240 deg | 270 deg | 300 deg | 330 deg |
ψi | 104 deg | 98 deg | 93 deg | 90 deg | 90 deg | 92 deg | 98 deg | 105 deg | 112 deg | 116 deg | 116 deg | 111 deg |
Table 5 presents the obtained values of the synthesis parameters of the Stephenson II function generator.
8.658 | 5.003 | 45.983 | 50.017 | 12.496 | 21.645 | 23.775 | −7.725 | 20.715 | 13.975 | 40.211 |
8.658 | 5.003 | 45.983 | 50.017 | 12.496 | 21.645 | 23.775 | −7.725 | 20.715 | 13.975 | 40.211 |
To check the parametric synthesis results of the Stephenson II function generator, we determine the values of the angle ψi corresponding to the values of the angle φi. To do this, the positions of the Stephenson II function generator are analyzed.
Consequently, the residual (39) is a function of the conditional generalized coordinate ψi. Minimizing this function by the bisection method, we obtain the values of the angle ψi for a given value of the angle φi. By changing the values of the angle φi, we similarly find the corresponding values of the angle ψi.
N | 1 | 2 | 3 | 4 | 5 | 6 |
---|---|---|---|---|---|---|
φi | 0 deg | 30 deg | 60 deg | 90 deg | 120 deg | 150 deg |
ψi | 104.12 deg | 98.35 deg | 93.18 deg | 90.24 deg | 89.54 deg | 92.07 deg |
N | 7 | 8 | 9 | 10 | 11 | 12 |
φi | 180 deg | 210 deg | 240 deg | 270 deg | 300 deg | 330 deg |
ψi | 97.57 deg | 105.21 deg | 111.87 deg | 116.16 deg | 115.75 deg | 110.83 deg |
N | 1 | 2 | 3 | 4 | 5 | 6 |
---|---|---|---|---|---|---|
φi | 0 deg | 30 deg | 60 deg | 90 deg | 120 deg | 150 deg |
ψi | 104.12 deg | 98.35 deg | 93.18 deg | 90.24 deg | 89.54 deg | 92.07 deg |
N | 7 | 8 | 9 | 10 | 11 | 12 |
φi | 180 deg | 210 deg | 240 deg | 270 deg | 300 deg | 330 deg |
ψi | 97.57 deg | 105.21 deg | 111.87 deg | 116.16 deg | 115.75 deg | 110.83 deg |
5.3 Stephenson III A Function Generator.
Table 7 gives values of the angles φi and ψi of the input and output links of the Stephenson III A function generator (Fig. 10).
N | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | 11 | 12 |
---|---|---|---|---|---|---|---|---|---|---|---|---|
φi | 0 deg | 30 deg | 60 deg | 90 deg | 120 deg | 150 deg | 180 deg | 210 deg | 240 deg | 270 deg | 300 deg | 330 deg |
ψi | 11 deg | 03 deg | 07 deg | 15 deg | 24 deg | 33 deg | 44 deg | 57 deg | 71 deg | 83 deg | 77 deg | 43 deg |
N | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | 11 | 12 |
---|---|---|---|---|---|---|---|---|---|---|---|---|
φi | 0 deg | 30 deg | 60 deg | 90 deg | 120 deg | 150 deg | 180 deg | 210 deg | 240 deg | 270 deg | 300 deg | 330 deg |
ψi | 11 deg | 03 deg | 07 deg | 15 deg | 24 deg | 33 deg | 44 deg | 57 deg | 71 deg | 83 deg | 77 deg | 43 deg |
Table 8 presents the obtained values of the synthesis parameters of the Stephenson III A function generator.
12.979 | 7.503 | 45.947 | 40.014 | 16.971 | 29.438 | 23.775 | −7.725 | 39.983 | 21.087 | 24.897 |
12.979 | 7.503 | 45.947 | 40.014 | 16.971 | 29.438 | 23.775 | −7.725 | 39.983 | 21.087 | 24.897 |
N | 1 | 2 | 3 | 4 | 5 | 6 |
---|---|---|---|---|---|---|
φi | 0 deg | 30 deg | 60 deg | 90 deg | 120 deg | 150 deg |
ψi | 11.21 deg | 02.98 deg | 06.87 deg | 14.91 deg | 23.93 deg | 33.16 deg |
N | 7 | 8 | 9 | 10 | 11 | 12 |
φi | 180 deg | 210 deg | 240 deg | 270 deg | 300 deg | 330 deg |
ψi | 43.70 deg | 57.01 deg | 70.96 deg | 82.91 deg | 77.04 deg | 42.83 deg |
N | 1 | 2 | 3 | 4 | 5 | 6 |
---|---|---|---|---|---|---|
φi | 0 deg | 30 deg | 60 deg | 90 deg | 120 deg | 150 deg |
ψi | 11.21 deg | 02.98 deg | 06.87 deg | 14.91 deg | 23.93 deg | 33.16 deg |
N | 7 | 8 | 9 | 10 | 11 | 12 |
φi | 180 deg | 210 deg | 240 deg | 270 deg | 300 deg | 330 deg |
ψi | 43.70 deg | 57.01 deg | 70.96 deg | 82.91 deg | 77.04 deg | 42.83 deg |
5.4 Stephenson III B Function Generator.
Table 10 gives the values of the angles φi and ψi of the input and output links of the Stephenson III B function generator (Fig. 12).
N | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | 11 | 12 |
---|---|---|---|---|---|---|---|---|---|---|---|---|
φi | 0 deg | 30 deg | 60 deg | 90 deg | 120 deg | 150 deg | 180 deg | 210 deg | 240 deg | 270 deg | 300 deg | 330 deg |
ψi | 223 deg | 244 deg | 266 deg | 264 deg | 245 deg | 240 deg | 245 deg | 256 deg | 268 deg | 267 deg | 245 deg | 221 deg |
N | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | 11 | 12 |
---|---|---|---|---|---|---|---|---|---|---|---|---|
φi | 0 deg | 30 deg | 60 deg | 90 deg | 120 deg | 150 deg | 180 deg | 210 deg | 240 deg | 270 deg | 300 deg | 330 deg |
ψi | 223 deg | 244 deg | 266 deg | 264 deg | 245 deg | 240 deg | 245 deg | 256 deg | 268 deg | 267 deg | 245 deg | 221 deg |
Table 11 presents the obtained values of the synthesis parameters of the Stephenson III B function generator.
12.978 | 7.493 | 55.017 | 29.962 | 7.507 | 13.004 | 15.029 | 10.134 | 39.995 | 19.917 | 25.545 |
12.978 | 7.493 | 55.017 | 29.962 | 7.507 | 13.004 | 15.029 | 10.134 | 39.995 | 19.917 | 25.545 |
Consequently, the residual (39) is a function of the conditional generalized coordinate ψi. Minimizing this function by the bisection method, we obtain the values of the angle ψi. Table 12 presents the obtained values of the angle ψi, and Fig. 13 shows a graph of its change.
N | 1 | 2 | 3 | 4 | 5 | 6 |
---|---|---|---|---|---|---|
φi | 0 deg | 30 deg | 60 deg | 90 deg | 120 deg | 150 deg |
ψi | 223.29 deg | 244.38 deg | 265.81 deg | 264.01 deg | 245.44 deg | 240.18 deg |
N | 7 | 8 | 9 | 10 | 11 | 12 |
φi | 180 deg | 210 deg | 240 deg | 270 deg | 300 deg | 330 deg |
ψi | 244.96 deg | 255.87 deg | 267.45 deg | 267.04 deg | 244.83 deg | 220.71 deg |
N | 1 | 2 | 3 | 4 | 5 | 6 |
---|---|---|---|---|---|---|
φi | 0 deg | 30 deg | 60 deg | 90 deg | 120 deg | 150 deg |
ψi | 223.29 deg | 244.38 deg | 265.81 deg | 264.01 deg | 245.44 deg | 240.18 deg |
N | 7 | 8 | 9 | 10 | 11 | 12 |
φi | 180 deg | 210 deg | 240 deg | 270 deg | 300 deg | 330 deg |
ψi | 244.96 deg | 255.87 deg | 267.45 deg | 267.04 deg | 244.83 deg | 220.71 deg |
6 Conclusion
Structural synthesis of four-bar and six-bar function generators with revolute joints has been carried out. A four-bar function generator is formed by connecting two rotating coordinate systems with given rotation angles using a binary link of the type RR, which is a negative CKC that imposes one geometrical constraint. Six-bar function generators are formed by connecting these two rotationally moving coordinate systems using a passive CKC of the type RRR and by connecting non-adjacent links of the resulting five-bar linkage by binary link of the type RR. As a result, Stephenson I, Stephenson II, Stephenson III A, and Stephenson III B function generators have been formed. Passive CKC of the type RRR does not impose a geometrical constraint on the movement of two moving coordinate systems, and its geometric parameters are varied to satisfy the constraint of the negative CKC. Geometric parameters of the negative CKC of the type RR are determined by least-square approximation. In this case, the least-square approximation problem is reduced to a simple kinematic inversion problem based on linear iteration. Structural and parametric synthesis of the four-bar and six-bar function generators are carried out simultaneously, starting with the smallest number of links of CKCs. Numerical results of parametric synthesis of four-bar and six-bar function generators are presented.
Acknowledgment
This work was funded by the Science Committee of the Ministry of Science and High Education of Kazakhstan (Grant No. AP14872115 “Development and Research of the Novel Tripod Type Parallel Manipulators With Six Degrees of Freedom”).
Conflict of Interest
There are no conflicts of interest.
Data Availability Statement
The datasets generated and supporting the findings of this article are obtainable from the corresponding author upon reasonable request.