Topological and Kinematic Singularities for a Class of Parallel Mechanisms
We study singularities for a parallel mechanism with a planar moving platform in , with joints which are universal, spherical (spatial case), or rotational (planar case). For such mechanisms, we give a necessary condition for a topological singularity to occur, and describe the corresponding kinematic singularity. An example is provided.
Kinematic singularities of parallel mechanisms have been studied extensively (cf. [1–4]), but less attention has been paid to topological singularities in this context. The topological theory of singularities is an extensive and rapidly growing field of mathematics, with connections to complex and real analysis, algebraic geometry, and differential topology (see [5–7]). In this note, we investigate the connection between the two types of singularities (for a certain class of mechanisms), thus illustrating their relevance to robotics.
1.1. Topological Singularities
By choosing appropriate local coordinates for a given mechanism , we can think of the set of all its configurations as a topological space , called its configuration space, and try to endow it with the structure of a differentiable manifold. The points of where this cannot be done constitute the topological (or differentiable) singularities of (see Section 2).
These have been studied in detail mainly in the case of a single closed chain (see [8–11] and compare [12–14]). Here we consider parallel mechanisms with a planar moving platform, in any dimension. In the spatial case, we require the joints to be universal or spherical; in the planar case, all joints are rotational.
Evidently, topological singularities of are an inherent feature of the mechanism, independent of the actuation scheme. However, at first glance they appear to have no mechanical significance. The main results of this paper areas follows:(a)we give a necessary geometric condition for a topological singularity to arise in such a mechanism (Theorem 2.4);(b)we show that topological singularities for these mechanisms always give rise to kinematic singularities (Proposition 3.1).
The occurence of such singularities is illustrated in a specific example in Section 4.
These results are intended to exemplify the power of (higher-dimensional) topological methods for the study of singularities in robotics. In the future we hope to show how they apply to more general types of mechanisms.
1.2. Kinematic Singularities
In general, a configuration is naturally described by the vector whose coordinates correspond to the positions of the various joints and links of the mechanism. In practice, we choose a subset of input coordinates (corresponding to the actuated joints), which can serve as local coordinates for around . In addition, we often focus on a subset of output coordinates of interest (which may describe the position of the end effector). The remaining joints (if any) are passive.
The structure of the mechanism imposes relations which must hold among the coordinates ; in particular, we may assume thatidentically in a neighborhood of in . The Jacobian consists of two blocks and , where must be nonsingular if the are to serve as local coordinates near (cf.  or [16, Section 5]).
The kinematics of the mechanism are described by a time-dependent path in the configuration space. Differentiating (1.1) with respect to we getwhich can be written in the formwhere .
If is of maximal rank and is not, is called a (instantaneous) kinematic singularity of type I; this means that not every infinitesimal change in output can be obtained by changing the actuated joints.
On the other hand, if is of maximal rank and is not, is called singular of of type II; in this case the actuated joints do not determine uniquely the behavior of the outputs. Finally, if neither nor is of maximal rank, is called singular of type III (cf. ). Gosselin and Angeles give examples of all three types of singularities for a parallel 3-RRR planar mechanism.
This classification is widely used in the robotics literature (see, e.g., [18, Section 6.2]). Note that in addition to the (somewhat arbitrary) choice of and , the mechanism may have additional “passive coordinates” of interest. Thus, Zlatanov et al. provide a more detailed classification, listing six types of singular configurations (see ).
The kinematic singularities which arise in this paper are all of type I or III, corresponding to the impossible output (IO) of [2, Section 5, Definition 7], where there exists an infinitesimal output vector for which (1.2) cannot be satisfied with any combination of active and passive input vectors.
Other types of singularities for mechanisms have been considered—for example, control, constraint, and architectural (cf. [18, Section 6.2.1])—which we do not attempt to discuss here. Note, however, that there is no analogous classification of singularities for topological spaces, which can be extremely complicated in general (see ). See  for a general discussion of singularities in robotics.
1.3. Parallel Mechanisms with a Planar Platform
In this note we begin a study of the relationship between topological and kinematic singularities for a common class of mechanisms (which occur in applications).
These are polygonal mechanism in (), consisting of a moving planar -polygonal platform with chains attached to its vertices. The th chain is a sequence of concatenated links of lengths (), connected by spherical or universal joints, in the spatial case, and rotational joints, in the planar case. One end of the th chain is attached to the vertex of the moving platform, and the other is fixed at .
The restriction to these specific types of joints is intended to simplify the study of , which can then be interpreted as a space of immersions of the corresponding metric graph. In future work, we hope to extend these results to more general parallel mechanisms.
For each component, we use parenthesized superscripts to indicate the chain number, and subscripts to indicate the link number. For example, denotes the length of the th link of the th chain.
2. Topological Singularities
From now on we consider a fixed polygonal mechanism as in Section 1.3, and construct its configuration space as follows.
Definition 2.1. A chain configuration for a single -link chain
consists of vectors in of specified
lengths: for .
A chain configuration is said to be aligned if all the vectors are scalar multiples of , which is called the direction vector of . The direction line for is .
Definition 2.2. A
configuration for consists of a
set of chain
configurations for each of the chains, such
that the endpoints of the
corresponding chain configurations form a polygon congruent to the given moving
platform . Here,for , where the
points are the
vertices of the fixed platform (see Figure 1).
The set of all such configurations, topologized in the obvious way, is the configuration space of .
Definition 2.3. A
configuration for is called singular
of type (a) if for two of its chains—say, numbers —the corresponding chain configurations and are aligned,
with coinciding direction lines: (see Figure 1).
is singular of type (b) if three of its chain configurations are aligned, with direction lines in the same plane meeting in a single point (this is referred to in literature as a planar pencil) (see Figure 2).
is singular of type (c) if (at least) four of its chain configurations are aligned, with direction lines in the same plane (compare [18, Section 6.4.1, condition 3d]).
We can now formulate our main result.
Theorem 2.4. A necessary condition for a configuration of a polygonal mechanism to be a topological singularity is that it be singular of type (a), (b), or (c).
For the proof, see .
Remark 2.5. The three types of singular configurations defined above need not be topological singularities of ; in particular, we know of no topological interpretation of the three different types (a)–(c). As noted above, in general the classification of topological or differentiable singularities is very difficult (see ).
Examples 2.6. Consider the following two examples.
(1) If is singular of type (a), as in Figure 1, consider the submechanism consisting of the two aligned chains of , with the corresponding configuration . If we assume that these two chains have one and two links, respectively, then has a neighborhood in the configuration space which is a one-point union of two 2-discs (see [14, Proposition 4.1])—so is singular.
On the other hand, if is the mechanism obtained from by omitting the two aligned chains, the configuration corresponding to is nonsingular and has a Euclidean neighborhood in . Since the itself has a neighborhood in equivalent to , we see that is a topological singularity.
(2) Consider a mechanism with a triangular platform, and two chains with one link each. In this case, the workspace for the third vertex of the platform is the coupler curve for the corresponding 4-chain mechanism (see [21, Chapter 4]), while the workspace for the third chain is an annulus .
For suitable parameters, the boundary (where the third chain is aligned) will be tangent to . The configuration corresponding to the point of tangency will be singular of type (b), as in Figure 3.
Note that near each point in has two corresponding configurations, associated to “elbow up/down” positions of the third chain, which coalesce at itself; thus has a singular neighborhood in consisting of two transverse intervals.
Remark 2.7. For simplicity, in the spatial case we restricted attention to mechanisms where all joints are universal. Replacing any such joint by a spherical one simply multiplies by a circle, so it does not affect the topological singularities.
3. Kinematic Singularities
Theorem 2.4 gives a necessary condition for a configuration to be singular (topologically), namely, that some subset of its chains be aligned, with direction lines , so that the Plücker vectors of these lines span certain types of varieties, of positive codimension in (see [18, Section 5]). We now show how topological singularities give rise to kinematic singularities, for our class of polygonal mechanisms.
Recall from Section 1.2 that kinematic singularities require an actuation architecture, that is, a choice of input coordinates (corresponding to the actuated joints) and output coordinates (corresponding to the end effector of the mechanism: in our case, the position and orientation of the moving platform).
The planar case (with more general joints) was analyzed by Bonev et al. in a series of papers, summarized in . For a classification of the kinematic singularities of such mechanisms, using instantaneous centers of rotation, see .
For the spatial case (i.e., embedded in ), we assume for simplicity that all actuators are universal or spherical, while the passive joints (located at the end of the chain, say) are spherical. There are three architectures to consider.
(a)Six chains having actuators for the th chain ().(b)The first chain having actuators; three more chains with actuators for the th chain ().(c)Two chains having actuators (); the third chain with actuators.
3.2. Screw Theory
We use screw theory (see ) to describe the forces operating at each joint of our mechanism.
A screw is a Plücker vector in , describing a line—or equivalently, the position and direction of a vector—in (cf. [18, Section 5]). Thus Figure 4 depicts the equivalent kinematic chain of an arbitrary chain of a mechanism, where the movement of each spherical joint is described by three unit screws , , and attached to its center. For a universal joint (or just before a passive joint), we can make do with two screws.
Considering the th chain as an open chain, we can express the instantaneous twist of the end-effector aswhere is the input coordinate for the screw (cf. [16, Section 5.6]).
In order to eliminate the passive joints from (3.1), for the th chain, we must multiply both sides by appropriate reciprocal screws. More precisely, choose a basis for the space of common reciprocals of the screws of all passive joints for this chain.
(a)If the chain has actuators, at all joints but the last two, the reciprocal screw for these two joints corresponds to the line passing through them (i.e., is one-dimensional).(b)If the chain has actuators, at all but the last joint, is 3-dimensional, with the corresponding lines all passing through the last joint (see [16, Chapter 5]).
For the th chain we obtain a system of linear equations for :(), in which of course the unactuated inputs have zero coefficient.
The matrix will take the formwhose rows are the reciprocal screws of all actuated chains.
The matrix is block-diagonal:with the th bloc (for the th actuated chain, with passive joints) and a matrix of the form:
Proposition 3.1. For a polygonal mechanism (with no unactuated chains), there is an instantaneous kinematic singularity of type I or III at any topological singularity.
Proof. At a topological singularity at
least two chains are aligned, so the reciprocals to the passive joint(s) are
reciprocal to all screws of these chains, and thus for these
chains. Since , is singular.
Now by Theorem 2.4, a topological singularity can have the following:
(1)two coaligned chains, each with a pair of unactuated joints; they have a common reciprocal (and ), so has rank 5. The same holds in the second architecture whenever two chains are coaligned;(2)three aligned chains whose lines lie in a planar pencil, each with a pair of unactuated joints; in this case the corresponding screws are linearly dependent, so again has rank 5;(3)four aligned chains whose lines are in one plane; the lines of those with a pair of unactuated joints; each lying in a planar pencil (rank 3). In the first architecture, each of the last two lines adds at most 1 to the rank; in the second, adding the last line forms a degenerate congruence (total rank 4). Thus in any case has rank 5;
Remark 3.2. The sort of conditions in the Grassmann algebra used here to identify singularities is of course well known in literature (see, e.g., [18, Section 6.4]). Our point is that these are necessary conditions for topological singularities, and sufficient for kinematic singularities, providing an implication between two concepts of independent interest.
4. An Example
To round off the discussion we now present an example of a topological singularity of type (b) for a specific real-life mechanism , namely, the 3-URU 3-DOF mechanism (in ), introduced in . Zlatanov et al. studied the constraint singularities of extensively. Here we do not attempt a comprehensive study, since our goal is merely to illustrate topological methods.
The mechanism consists of three two-link chains, and both the base and moving platforms are equilateral triangles. It turns out that in a certain region of the configuration space , acts as a planar mechanism (see ). In particular, the three R-joint axes of the base platform meet in the base triangle but not in the center (otherwise mobility would be increased by the additional spin dexterity of the extended chain). Furthermore, in this region the three intermediate R-joints in each chain are parallel (see Figure 5).
Since the topological singularity in question is located in this region , for simplicity we may regard as if it were a 3-RRR planar mechanism . The pose of the equilateral moving platform in is determined by the two coordinates of its barycenter , and the rotation of in . Denote the common distance from the vertices to the barycenter by ().
The work space for each vertex of is an annulus centered at the fixed endpoint of the th chain, with boundary radii . Thus, if we fix the orientation of , the resulting constrained work space for (the shaded area in Figure 6) is the intersection of three annuli (with centers at ), namely, the displacements () of by a vector of length .
The configuration space is described in a neighborhood of any configuration by(a)discrete data on the elbow up/down position of each chain at ;(b)the orientation (which takes value in an open interval );(c)the location of in .
The work spaces will generically all be homeomorphic to a fixed curvilinear polygonal region (for near ). Thus topologically will be a cube, that is, a product . The discrete data yield eight identical copies of identified along their boundaries (which represent chain alignments).
For some values of , may be empty, so in fact may split up into two disjoint cubes as above (see Figure 7), where the vertical gap between them represents values of for which . This corresponds to a situation where the platform cannot rotate continuously between two orientations, each of which is feasible in itself. No singularities arise in this case.
More care is required for the analysis of the full configuration space near , because there are actually eight regions , corresponding to the eight possible choices of “elbow up/down” for the three chains. The boundaries of are arcs of the boundary circles of the annuli , where the links of the th chain are aligned. Therefore (as noted above), the boundaries of the “cubes” () are glued together in according to a combinatorial pattern represented by the colors in the two figures. For example, gluing faces for the situation depicted in Figure 7 yields two disjoint 3-dimensional tori.
Of course, this is only true in the region where the original mechanism is planar (and thus equivalent to ); all we can conclude about in the region corresponding to Figure 7 is that it has two connected components, locally isomorphic to .
However, as the parameters for (and thus ) vary, we find that in certain cases the two connected components of approach each other, and, for an appropriate , they actually touch at one point (see Figure 8).
In this case, is homeomorphic to , so that is topologically singular in . In —and therefore, in too, at least locally—we obtain a one-point union of two 3-tori.
The aligned poses can be calculated analytically using the algorithm in Gosselin and Merlet (cf. ), since each of the extreme situations can be treated as an equivalent 3-RPR robot, whose link lengths are fixed and known.
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