Advances in Robot Kinematics - Jadran Lenarcic and Bernard Roth (Eds) Part 9 potx

236 A. Degani and A. Wolf 3.3 The Singularity/Self Stress Connection Once the MLG of a PPM and a reciprocal figure are constructed, one can use them for the singularity analysis of PPMs. Maxwell’s theory (section 2.2) presents a connection between the existence of a reciprocal figure and self stress in a framework. We will now analyze a self-stressed MLG (Fig. 7) in order to demonstrate the connection between self-stress and singularity. Based on the definition of self stress framework, when a bar-joint framework is in self stress, the sum of the forces of the bars connected to a joint is equal to zero. Three equations corresponding to the sum of forces in the three vertices 1,2,3 can be written as: Sum of forces in vertex 1: a+d+f =0 , (2) Sum of forces in vertex 2: b-d+e=0, (3) Sum of forces in vertex 3: c-e-f =0. (4) These three equations are vector summations. We arbitrarily assign the direction of the forces and consistently add the forces. Therefore, some of the forces in Eq. 2-4 are negated. Summing Eq. 2, 3, and 4: a+b+c=0 (5) a b c d f e 2 1 3 Figure 7. “Singular” (self stress) configuration of MLG Equation 5 confirms a linear dependency of the three forces a , b, and c . These three forces are the forces corresponding to the lines of action of the three limbs. The meaning of this dependency is that these three limbs cannot generate instantaneous work (virtual work (Hunt, 1978)) on the end-effector while it is moving in an instantaneous twist deformation resulting from an external wrench applied on it. Therefore, self stress in a framework is equivalent to a type-II singularity of a PPM. It is now evident that the existence of a reciprocal figure indicates a self stress framework, and in a similar way indicates a singular configuration in a mechanism. 3.4 Locating the Singular Configurations To find the configurations where there exists a reciprocal figure to a particular PPM, and therefore it is in a singular configuration, one should move the manipulator by changing its joint parameters while tracking for configurations in which the reciprocal figure is visually . 237 Graphical Singularity Analysis of 3-DOF PPMs connected, e.g. in Fig. 6e, vertices p and q merge with p’ and q’ respectively. Figure 8 shows examples of PPM configurations in which the reciprocal figure is connected and the manipulator is in a singular configuration. Figure 8. Two examples of singular configurations and the connected reciprocal figures (3-R PR left, 3-RRR right) So far the search for a singular configuration was carried out by changing the joint parameters of the manipulator and checking for the existence of a connected reciprocal figure. If the analysis is constructed the other way around, so that a connected reciprocal figure is first constructed and only then an MLG is constructed to be reciprocal to it, we can trace the loci of the singular configurations of the manipulator by changing the reciprocal figure while keeping it connected (Fig. 9 left). Note that the construction of the reciprocal figure in this case is based on mechanical constraints of the PPM, e.g. the fixed shape of the end- effector. Moreover, the singular configuration’s loci are traced relative to a constant orientation of the PPM in order to enable us to plot the loci as a 2-D graph. We refer the readers to (Sefrioui and Gosselin, 1995) to examine the consistency of the results. More examples, including JAVA applets of this method, can be found at: www.cs.cmu.edu/~adegani/graphical/ . Figure 9. (Left) Singularity loci of 3-RPR in two different constant orientations of the end-effector. (Right) A loci plot from six different orientations of the end- effector (0,5, 10, 15, 20, and 25 degrees) . . 238 A. Degani and A. Wolf 4. Conclusion and Future Work References Bonev, I.A. (2002), Geometric analysis of parallel mechanisms. Ph.D. Thesis, Laval University, Quebec, QC, Canada. Crapo, H., and Whiteley, W. (1993), Plane self stresses and projected polyhedra I: Gosselin, C., and Angeles, J. (1990), Singularity analysis of closed-loop kinematic chains. IEEE Transactions on Robotics and Automation no. 3, vol. 6, pp. 281- 290. C. Galletti, Advances in Robot Kinematics, Kluwer Acad. Publ., pp. 89-96. Maxwell, J.C. (1864), On reciprocal figures and diagrams of forces. Phil. Mag no. 27, vol. 4, pp. 250-261. Sefrioui, J., and Gosselin, C.M. (1995), On the quadratic nature of the singularity curves of planar three-degree-of-freedom parallel manipulators. Mechanism and Machine Theory no. 4, vol. 30, pp. 533-551. Tsai, L.W. (1998), The Jacobian analysis of a parallel manipulator using reciprocal screws, Proceedings of the 6th International Symposium on Recent Advances in Robot Kinematics, Salzburg, Austria. The graphical method which is presented and implemented in this paper results in comparable outcomes to those obtained by other approaches (e.g. Sefrioui and Gosselin, 1995), yet avoids some of the complexities in- volved in analytic derivations. It is worth mentioning that the method we present can be potentially applied to non-identical limb manipulators and to other types of mechanisms as well. The method makes use of reciprocal screws to represent the lines of action of PPMs’ limbs in a Mechanism’s Line of action Graph (MLG), an insightful graphical representation of the mechanism. Maxwell s theory of reciprocal figure and self stress is then ’ applied to create a dual figure of the MLG. Analyzing this dual (reciprocal) the loci o figure provides us with the singular configurations of the PPM and with We are currently facing the challenge of expanding this graphical able to use our relatively simple method to find the singular configura- tions of complex three-dimensional manipulators, such as a 6-DOF Gough-Stewart platform. One possible way to achieve this goal is to method to the analysis of three-dimensional manipulators. We hope to be project the spatial lines of action of the limbs on one or more planes (Karger, 2004). We believe a self-stress analysis of these projected graphs, similar to the one done on PPM, will offer insight into the singular confi- guration of these non-planar manipulators. The basic pattern. Structural Topology no. 1, vol. 20, pp. 55-78. Hunt, K.H. (1978), Kinematic Geometry of Mechanisms, Oxford, Clarendon Press. singular configurations of the manipulator. f J. Lenarcic and Karger, A. (2004), Projective properties of parallel manipulators, ^^ DIRECT SINGULARITY CLOSENESS INDEXES FOR THE HEXA PARALLEL ROBOT Carlos Bier*, Alexandre Campos, J¨urgen Hesselbach Institute of Machine Tools and Production Technology - TU Braunschweig Langer Kamp 19b, 38106 Braunschweig, Germany * c.bier@tu-bs.de Abstract Direct kinematic singularities constrain the internal robot workspace and the proximity to them must be detected online as fast as possible for non deterministic trajectories. Direct singularity proximity for the Hexa parallel robot is measured by means of three measure indexes with two different physical bases. In this paper a new index based on Grass- mann geometry to measure the singularity closeness is introduced. This method and methods based on constraint minimization are applied and validated in the Hexa robot. From the results we observe, for instance, that the new index requires less time than the constraint minimization methods but requires a better knowledge of the robot structure. Keywords: Parallel Manipulator Singularities, Grassmann Geometry, Constrained Minimization 1. Introduction A measure of the direct singularity closeness for parallel manipulators is required aiming at a safe operation space. For parallel robots as the Hexa robot (Fig. 3d), workspace is limited by direct kinematic singu- larities as well as by inverse kinematic singularities. Direct kinematic singularities allow the end effector to gain unconstrained movements. Its identification has been studied from different perspectives. The van- ishing of the Jacobian determinant has been used for particular parallel robots. However it is a product of factors and thus it suffers from the fact that close to a singularity, where a factor shrinks to zero, other factors may be big enough and the determinant does not indicate the singularity closeness. Additionally, the physical meaning of the determinant is not clear. Qualitative conditions, based on Grassmann geometry, are proposed to detect singularities of triangular simplified symmetric manipulators [Merlet, 2000]. Quantitative approaches use a numerical measure to determine how close a robot position is to a singularity. Different mea- © 2006 Springer. Printed in the Netherlands. J. Lenarþiþ and B. Roth (eds.), Advances in Robot Kinematics, 239–246. 239 - sureshavebeenusedforthistask,e.g. the natural frequency measure [Voglewede and Ebert-Uphoff, 2004], the power and the stiffness inspired measure [Pottmann et al., 1998] based on a constraint minimization In this paper a new method for quantitative measures of the direct singularity closeness based on Grassmann geometry is presented. This new method as well as the minimization based methods are applied to the Hexa robot and the results are analyzed. The six DOF Hexa robot is composed by six limbs connecting the basis to the end effector, see Fig. 3d. Each limb contains an active rotative joint A i (for i =1, ··· , 6). Its axes are fixed to the basis plane, a passive universal joint B i and a passive spherical joint C i connected to the end effector, so that all C i joints define the end effector plane. The cranks and the passive links are connected at B i . The six limbs of the Hexa robot are arranged in three pairs of two active joints with collinear rotational axes. The pairs of active joints are axisymmetrical, i.e. 120 o betweeneachpair. 2. In spatial parallel manipulators the relationship between actuator co- ordinates q and end effector Cartesian coordinates x can be stated as a function f(q, x) = 0, where 0 is the 6-dimensional null vector. Therefore the differential kinematic relation may be determined as J q ˙q − J x $ t =0;J q ˙q = J x $ t (1) where $ t is the end effector velocity twist in ray order and, J x , J q and J = J −1 q J x are the direct, inverse and standard Jacobian matrices, re- spectively. The rows of the direct kinematic matrix J x may correspond to the normalized screw of wrenches, in axis order acting upon the end effec- tor through the passive link, i.e. thedistallinkofeachlimb[David- son and Hunt, 2004]. Therefore, a static relation may be stated as J T x τ =  ˆ $ r1 , ··· , ˆ $ r6  τ =$ r ,where$ r is the result wrench acting upon the end effector in axis order, τ =[τ 1 , ··· ,τ 6 ] are the input wrench mag- nitudes and the columns of J T x are the normalized screws (axial order) of wrenches acting on the end effector. Singular configurations appear if either J x or J q drops rank. If J x is singular, a direct singularity is encountered and the end effector gains one or more uncontrollable degrees of freedom (DOF), on other hand if 240 matrix algebra. method as well as the condition number [Xu et al., 1994] based on Parallel anipulator Singularities M C. Bier, A. Campos and J. Hesselbach J q drops rank it looses at least one DOF. The direct singularity occurs in the workspace and is the main aim of this paper. The new method as well as the minimization based methods are in- troduced and applied to the Hexa robot. 3. The constraint minimization method determines closeness to singu- larity through an optimization problem that results in a corresponding generalized eigenvalue problem. Using this methodology it is possible to describe the instantaneous behavior of the end effector near singular- ities for parallel manipulators in general [Voglewede and Ebert-Uphoff, 2004, Hesselbach et al., 2005]. In this approach, an objective function F ($ t ) to be optimized is subject to move on a constraint h.Thisis formulated mathematically as: M(X)=  min/max F ($ t )=$ T t J T SJ$ t subject to h =$ T t T $ t − c =0 (2) where S (positive semidefinite) and T (positive definite) are n ×n sym- metric matrix and c is some positive constant, e.g. c =1. Thesolution of Eq. (2) gives the closeness measure to a singularity M(X)atapar- ticular position and orientation X of the manipulator. The proposed constrained optimization problem is found with the application of a La- grange multiplier λ. The local extrema of the Lagrangian ζ($,λ)= F ($ t ) − λh($ t ) are determined by its derivation. For a nontrivial solu- tion to exist, the minimization (or maximization) of Lagrangian yields det(J T SJ − λT ) = 0, which is called the corresponding general eigen- value problem. The smallest eigenvalue λ min will be the minimum value oftheobjectivefunctionF ($ t ), and so it can be utilized as a measure value. In general, this minimization problem was formulated based on an arbitrary quantity for S and T [Voglewede and Ebert-Uphoff, 2004]. Taking J = J x , S = I 6x6 and T = diag[000111], then √ λ min is associ- ated to the minimum power [∼ W ] of the system, which indicates the manipulator singularity closeness. Another possible way is to choose S as the stiffness matrix of the actuators K Act and T as the mass matrix of the manipulator M EE (or for simplicity the end effector mass matrix, i.e. neglecting the limb masses). Therefore, √ λ min is associated to the ω natural frequency [∼ Hz]ofthe system (M EE ¨ X −ω 2 K EE = 0), indicating the singularity closeness. Both methods are applied in the Hexa robot (Fig. 3d) for its singu- larity approximation measure. The measure behaviors of the minimum power of the system through a singularity (Fig. 1a) is showed in Fig. 1b, Direct Singularity Closeness Indexes for the Hexa Parallel Robot 241 Constraint inimization M here the end effector twists θ o around the $ min (the end effector twist which requires minimum power in this singularity). The singularity oc- curs when √ λ min = 0, but a singular range exists due to clearance and compliance of the system, where the end effector is still unconstrained. The singular range bound is experimentally identified as 0.029 ∼ W and upon it the manipulator stiffness is warranted into the whole workspace. q $ tmin 20 30 40 50 60 70 80 0 20 40 60 80 q angle [°] singular range distance [mm] Grassmann algorithm V5b limit value (55 mm) singularity V5b c) d) B i A i C i 3 4 1 2 5 6 V5b L angle [°] q 0 10 20 30 40 50 60 0.01 0.02 0.03 0.04 singular range power method limit value (0.029 W) ~ singularity V5a power ( )[ W ] ~ l min $ tmin q $ r1 $ r2 $ r3 $ r5 $ r4 a) b) 4 5 1 2 6 3 V5a Figure 1. a) Grassmann variety V5a on the Hexa; b) Power based index; c) Grass- man variety V5b; d) Grassman V5b based index The same behavior is obtained through the frequency method. It is important to notice that both methods detect all the singularities with a unique index. 4. Grassmann Geometry A new index to determine closeness to singularities is obtained based on Grassmann geometry. eties of lines, i.e. the sets of linear dependent lines to n given indepen- dent lines, and characterized them geometrically according to their rank (1, ··· , 6) [Hesselbach et al., 2005]. A singular configuration of the ma- nipulator may be associated with a linear dependent set of lines, also 242 Grassmann (1809–1877) studied the vari- C. Bier, A. Campos and J. Hesselbach . called line based singularities. In general, the reciprocal wrenches $ r (Fig. 1a) to the passive twists of each manipulator leg are associated to lines in the direction of the forces acting upon the end effector, also called Pl¨ucker vectors. Linear dependence among these lines represents a direct singularity. These wrenches compose the J x matrix (Sec. 2). Using the Grassmann geometry we recognize that the Hexa robot may be associated to several varieties. Some singularities of the Hexa robot as well as correspondent varieties are shown in Figs. 1a, 1c and 2. In the Hexa configurations of Fig. 2a, two wrenches are collinear and so $ r1 and $ r2 represent a Grassmann variety 1 (for short V1). Figure 2b shows that four wrenches ($ r1 ,$ r2 ,$ r3 and $ r6 ) are on a flat pencil V2b. Given that the Hexa robot has six wrenches acting upon the end effector, the configuration in Fig. 2a may be associated to V5a and the configuration in Fig. 2b to V4d. In Fig. 2c all wrenches are parallel to each other and they form a bundle of lines V3b. In Fig. 2d all wrenches lie in a plane with different intersection points and represent a V3d. In Fig. 1a all wrenches belong to a linear complex V5a, and Fig. 1c shows an example where all wrenches are meeting one given line V5b. Each unconstrained DOF of the end effector is represented by one $ min in the Figs. 1a, 1c and 2. V2b 1 2 3 c) d) $ tmin $ tmin $ tmin $ tmin $ tmin $ tmin $ r1 $ r2 $ r1 $ r2 $ r3 $ r6 $ r4 $ r5 a) b) $ tmin $ tmin $ tmin a b 2 V1 1 V3b 1 2 3 4 1 2 3 4 V3d Figure 2. Grassmann variety on the Hexa robot: a) V1; b) V2b; c) V3b; d) V3d Considering the workspace of the Hexa robot which is limited by the actuated joint angles, the possible Grassmann varieties may be reduced to two: V5a and V5b. Thus only singularities of Fig. 1a and 1c may Direct Singularity Closeness Indexes for the Hexa Parallel Robot 243 . occur. With the help of the Grassmann geometry all possible singular configurations on the Hexa robot are known. Aiming at quantify the closeness to the singularities of Fig. 1a and Fig. 1c, a measure algorithm is presented in the sequence. A complex is generated by five skew symmetric lines (e.g. wrench axes). Let π be a plane tangent to the complex that contains a point of the line B 6 C 6 (correspondent to the sixth wrench). The distance between a certain point of this line and π gives a closeness measure to that singularity. It is possible to build a 4×4 skew symmetric matrix G so that B T i GC i =0whereB i and C i (Fig. 1c) are in projective coordinates. This linear system has six unknowns and five equations. For simplicity and without loss of validity one unknown is set to 1. A pencil of lines of the complex that contains B 6 defines π. Mathematically, if a projective point X p =[xyz1] T = B 6 is an element of π,thenB T 6 GX p =0. Considering the vector U =[u 1 , ··· ,u 4 ] T = B T 6 G, the affine component of plane is u 1 x + u 2 y + u 3 z + u 4 = 0. The distance between C 6 and π may be interpreted as a measure for the line B 6 C 6 of the complex: d(C 6 ,π)=v 1 C 6,x + v 2 C 6,y + v 3 C 6,z + v 4 =0;v i = ui  u 2 1 + u 2 2 + u 2 3 (3) If all the lengths B i C i are the same and no other variety occurs, d(C 6 ,π) is a distance measure of the manipulator to a singularity V5a. Singularity of V5b occurs if all six wrench axes B i C i intersect one line L. This line crosses two wrenches in the points C.Thesepointsmust belong to legs whose drive axes are collinear and L must be parallel to these drive axes. The maximal distance between L and all the six wrenches is a measure to a singularity V5b of the Hexa robot. This algorithm is applied in order to measure the closeness of the Hexa robot to a singularity V5b as shown in Fig. 1c. The resulting distance measure to the singularity is presented in Fig. 1d, where it linearly falls down to zero in the singularity. Similarly to the minimization method, a singular range is observed under the limit of 55 mm. 5. Conclusion The Grassmann approach as well as the power and frequency meth- ods are experimentally validated in the Hexa robot and investigated for online singularity detection. All three methods allow a safe monitoring of such positions and present some properties are described next. movement (manually drived) from a rigid position, through a singularity V5b and twice singularity V5a, to a rigid position. Figure 3a compares 244 The experiment presented in Figs. 3a and 3b shows the end effector C. Bier, A. Campos and J. Hesselbach both Grassmann algorithms with the power method and shows that a Grassmann algorithm V5a does not detect a singularity of V5b and vice versa. A combined Grassmann index, the lower of the both algorithms, may be used due to that both present the same limit range of 55mm, which is a general property. Comparing the combined Grassmann index with the power and the frequency method in Fig. 3b, it can be observed that all three methods have an equivalent behavior. It is important to notice that a scale factor is required due to the different physical base of each method. Additionally, it allows the use of a unique singularity limit value. 2 4 6 8 10 12 0 time [s] a) 20 40 60 80 100 120 b) 2 4 6 8 10 12 0 20 40 60 80 100 120 time [s] singularity index 1900 x power [ W]~ combined Grassmann [mm] 38000 x frequency [ Hz] ~ 1900 x power [ W] ~ Grassmann V5a [mm] Grassmann V5b [mm] singularity index 0 2 4681012 20 40 60 80 100 120 time [s] computation time [µs] power frequency (simplified )M EE frequency (full )M EE combined Grassmann c) d) singularity V5b singularity V5a Figure 3. a) and b) Comparison of the singularity closeness indexes; c) Computa- tional time; d) Hexa robot of Collaborative Research Center 562 For the online application of an approach, the computing time is a decisive factor. For the same trajectory of example Fig. 3a, a comput- ing time comparison is presented in Fig. 3c. The combined Grassmann algorithm is notably faster than the minimization methods. Addition- ally, in the frequency approach has been observed that using only the end effector instead of the whole manipulator mass matrix M EE ,thecompu- tation time decreases without any loss of measure accuracy. Therefore, it seems plausible to only use a simplified model of M EE . Direct Singularity Closeness Indexes for the Hexa Parallel Robot 245 . [...]... forward kinematics (Husain and Waldron, 199 4, Wohlhart, 199 4, Bruyninckx, 199 7 and 199 8) In addition to the forward kinematics analysis, Wohlhart ( 199 4) obtained also the singularity condition of the general decoupled robot (also called 3-1 - 1-1 ) Following some algebraic manipulations of the Jacobian 265 J Lenar i and B Roth (eds.), Advances in Robot Kinematics, 265–274 © 2006 Springer Printed in the... Bier, C., Campos, A., and L¨we, H (2005) Direct kinematic sino gularity detection of a hexa parallel robot Proceedings - ICRA, pp 324 9- 3 254 Barcelona Merlet, J (2000) Parallel Robots Kluwer Academic Publisher Pottmann, H., Peternell, M., and Ravani, B ( 199 8) Approximation in line space– applications in robot kinematics and surface reconstruction In Andvances in Robot Kinematics, pp 40 3-4 12, Salzburg Kluwer... curvature be described in the vicinity of the singularity by a linear approximation with a still-to-be-determined positive constant By substituting for and using de l’Hospital’s rule, the right- and left-hand side limits of become, for , 261 A Robust Model for 3D Tracking From the condition one obtains and left-hand limits become and the right-hand (12) This creates a finite value, but induces a switch of... Sciavicco, L and Siciliano, B ( 199 6) Modeling and Control of Robot Manipulators McGraw-Hill Tsai, L ( 199 9) Robot Analysis: the Mechanics of Serial and Parallel Manipulators John Wiley & Sons, New York Voglewede, P and Ebert-Uphoff, I (2004) Measuring closeness to singularities for parallel manipulators In Proceedings - ICRA, New Orleans Xu, Y X., Kohli, D., and Weng, T C ( 199 4) Direct differential kinematics. .. avoided using the blending procedure 0.3 A 0.25 C B C 0.3 A A 0.25 0.15 B C A 0.2 0.15 in in 0.2 C 0.1 0.1 0.05 0.05 0 0 sing.-free standard 4 5 6 7 8 9 10 in Comparison of 11 12 sing.-free standard 4 5 6 7 8 9 10 11 12 in and for standard and singularity-free Frenet frame formulas The presented approach for generating guided spatial motion using Frenet frames prove to be useful for parameter-independent,... Wirfs-Brock, R and Wilkerson, B ( 198 9), Object-oriented design: A responsibility-driven approach In OOPSLA ’ 89 Proceedings, pp 71–75 SINGULARITY OF A CLASS OF GOUGH-STEWART PLATFORMS WITH THREE CONCURRENT JOINTS Patricia Ben-Horin Department of Mechanical Engineering Technion – Israel Institute of Technology patbh@tx.technion.ac.il Moshe Shoham Department of Mechanical Engineering Technion – Israel Institute... Conference on Robotics & Automation, San Francisco, CA, April 2000, pp 7-1 2 Karger, A.: Singularities and self-motions of equiform platforms MMT 36, 80 1-8 15, 2001 Karger, A.: Singularities ans Self-Motions of a special type of platforms In J Lernarˇiˇ cc and F Thomas, Advances in Robot Kinematics, Kluwer Acad Publ 2002, ISBN 0-7 92 3-6 42 6-0 , pp 35 5-3 64 Ma , O., Angeles J.: Architecture Singularities of... quadratical in translations, for the non-planar case the problem remains open References Bottema, O., Roth, B.: Theoretical Kinematics, Dover Publishing, 199 0 Husty, M.L.: An algorithm for solving the direct kinematics of general Stewart-Gough platforms Mech Mach Theory 31, 36 5-3 80, 199 1 Husty, M.L., Karger A.: Self-motions of Griffis-Duffy type parallel manipulators Proceedings of the 2000 IEEE international... Parallel Manipulators Int Journal of Robotics and Automation 7, 2 3-2 9, 199 2 Merlet, J.P.: Singular Configurations of Parallel Manipulators and Grasssmann Geometry Int Journal of Robotics Research, 8, 4 5-5 6, 199 2 Abstract The article presents an object-oriented representation of Frenet frame motion along spatial curves in multibody systems In this setting, the spatial track is regarded as a kinetostatic transmission... frame, singularity treatment, kinematics Guided motions along spatial curves have many applications in engineering such as, for example, in the simulation of railways and roller coaster tracks, in ˇı CNC machining (S´r and J¨ ttler, 2005), for guiding robot end effectors or for u describing the motion of bodies measured with tracking systems (Kecskem´ thy e et al., 2003) While in unconstrained motion . H., Peternell, M., and Ravani, B. ( 199 8). Approximation in line space– applications in robot kinematics and surface reconstruction. In Andvances in Robot Kinematics, pp. 40 3-4 12, Salzburg. Kluwer. position is to a singularity. Different mea- © 2006 Springer. Printed in the Netherlands. J. Lenarþiþ and B. Roth (eds.), Advances in Robot Kinematics, 2 39 246. 2 39 - sureshavebeenusedforthistask,e.g case of S. and base the equation of the singular set becomes only quadratical and © 2006 Springer. Printed in the Netherlands. 247 J. Lenarþiþ and B. Roth (eds.), Advances in Robot Kinematics,