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On the symbolic insimplification of the general 6R-manipulator
kinematic equations
T. Recio.”t and M. J. Gonz41ez-L6pez*t
Dpto. MatemAticas, Estadistica y Computaci6n
Universidad de Cantabria, Santander 39071, Spain
Abstract
When symbolically solving inverse kinematic
problems for robot classes, we deal with computations
on ideals representing these robot’s geometry.
Therefore, such ideals must be considered
over a base field K, where the parameters of
the class (and also the possible relations among
them) are represented. In this framework we
shall prove that the ideal corresponding to the
general 6R manipulator is rezd and prime over
K. The practical interest of our result is that it
confirms that the usual inverse kinematic equations
of this robot class do not add redundant
solutions and that this ideal cannot be “factorized”,
establishing therefore, KOVACS [7] conjecture.
We prove also that this robot class has six
degrees of freedom (i.e. the corresponding ideal
is six-dimensional), even over the extended field
K, which is the algebraic counterpart to the fact
that the 6R manipulator is completely general
Our proof uses, as intermediate step, some dimensionality
analysis of the Elbow manipulator,
which is a specialization of the 6R.
1 Introduction
Recently, a lot of attention has been devoted to
the kinematic problem for the general 6R manipulator,
see [11], [12], [15], [17], [18]. After twenty
five years of research on this subject, starting
with the work of Pieper [14] in 1.968, the complete
symbolic solution to the inverse kinematic
problem for this manipulator has been presented
in [11] and [15]. Following this pattern in a numerical
framework, [12] presented a real time algorithm
for solving the inverse kinematics of the
general 6R manipulator.
* Partially supported by CICyT PB 92/0498 /C02/01.
t partially supported by Esprit/Bra 6846 (POSSO).
On the other hand, KOVACS [7] has studied different
triangulations of the kinematic equations
system of this manipulator, searching for simplifications
on the corresponding determining equations.
In particular, he explicitly conjectured
that the kinematic equations system should generate
a prime ideal ([7], pg. 331). In fact, ([7],
pg. 334) he mentions that “extensive tests for
ideal factorization were perjormed and no jactori.
zation was jound. However the irreducibility
could not be established with certainty”. As a
consequence, he comments that, with high probability,
the solution presented in [11] and [15]
must be optimal in some sense.
In this paper we show that, in fact, the inverse
kinematic ideal of the general 6R manipulator
is a prime ideal. Moreover, we show that
this is also the best possible ideal in some other
sense that was not regarded by Kovics, namely,
that this is a real ideal, which roughly means
that it describes the solutions of the inverse kinematic
problem without redundancy. We remark
that, when working in a complex number setting,
primality alone implies this property, but
this is not true anymore when one is interested
in the real solutions (which is, usually, the case
in Robotics): (Z2 + y2) is a prime ideal in R [z, y]
but its solutions are represented in a simpler way
by the real ideal (z, y). Therefore, we consider
that proving both primality and reality, is required
to establish the idea behind KOVACS conject
ure.
Moreover, we obtain these two properties for
two different approaches to the concept of the inverse
kinematic ideal: first, as in the formulation
of Kov~cs, that follows [2] (see our Proposition
3.1); and second, with an enhanced version of inverse
kinematic ideal that involves a more symbolic
setting, in which the parameters are considered
as true indeterminates and not merely
nameholders for numbers (Theorem 4.2). Completing
the algebraic study of these ideals, we
show that the- corresponding hand ideals have
the expected dimension (six) for the general 6R
manipulator (Theorem 4.1). This is achieved
showing that the particular instance of the 6R,
namely the Elbow manipulator, has also this dimension
for the hand ideal (Lemma 4.1) and, in
order to obtain this, we use Paul solution ([13])
354
to the inverse kinematic problem of this robot. generators:
2 Symbolic approaches to the inverse kinematic
problem of the general 6R manipulator
Following Denavit-Hartenberg formalism ([4]) to
represent the geometry of the 6R-manipulator,
a coordinate system is attached to each link in
order to describe the relative positions among
the links. The 4x 4 homogeneous matrix relating
the i + 1 coordinate system to the i coordinate
system is (considering the links numbered from
the base, which is link 1):
(c; ‘Si.’!i sap% sic;
Si
A:= ~
CiA* —c*pi aiSt
A, d,
)
,i=l, ...,6
W
00 01
where s; and c; represent the sine and cosine, respectively,
of the rotation angle at joint i; A; and
~~ represent the sine and cosine, respectively, of
the twist angle between the axes of joints i and
i + 1; a; is the length of link i + 1; and di is the
offset distance at joint i (see [12] and [15]).
Let us consider now the group SE(3) of ma
trices of the form:
hll hlz h13 ‘U1
P=HV=
()(
hzl h22 h23 V2
01 h31 h32 h33 V3
0001 )
where the matrix H is a proper orthogonal matrix,
i.e. it is an element of the proper orthogonal
group SO(3). An element in SE(3) represents
the position and orientation, with respect
to some fixed universal frame, of a rigid body
in the space; in particular it gives the position
and orientation of the hand (last body) of the
6R robot. Thus, considering that the universal
frame agrees with the coordinate system of link
1 we have, for any given pose P of the hand, the
identity:
Besides these (twelve) equations, the ideal corresponding
to the general 6R manipulator must
contain the conditions expressing that each one
of the considered matrices (A:’s and P) belongs
to SE(3), which is obtained imposing that the
3 x 3 rotational part (i.e. the first three rows and
colums of the homogeneous matrices considered)
is proper orthogonal. As the rotational matrix
in A? is proper orthogonal when s? + c? = 1
and A; + ~~ = 1, we can describe the ideal
by the following (non necessarily irredundant)
@jR =
where – t denotes the transpose matrix and I is
the 3 x 3 identity matrix. In order to simplify
notation we group the variables as follows:
P= (hlI, h12. ... h33, vl, v2, tJ3),
S=(S1,. ... S6),
C=(C1,..., C6),
~=(~l,..., ~6),
i7=(/.41,. ... /J6),
A=(al,. ... a6),
~=(dl,. ... d6).
With this notation, aeR is an ideal in
R[P, S, C, L, U, A, D].
In this context, the standard symbolic approach
to solving kinematics consists in the symbolic
manipulation of the polynomials in the
ideal @~ yelding the variables S, C as a function
of the variables in P, and considering that
L, U, A, D can take any arbitrary real value. [2]
proposes the triangulation of the system of equations
using Grobner basis computations. Based
on this method there are several techniques introduced
by [9], [7] (see also the recent monograph
[8]) and [16], consisting essentially in determining
sets of parameters that make kinematic
equations simpler to solve. However, these triangulation
procedures may fail to give a complete
answer to the posed problem (see, for example,
[3] and [5]), as there can be numerical values of
the variables for which the specialization of the
triangular system is not triangular, or is not an
equivalent system to the given one. Weispfenning
construction of Comprehensive Grobner basis
([19] ) is a relevant tool to avoid such problems,
as already remarked in [5].
The more general (i.e. completely symbolic)
approach that we propose here is to consider the
variables defining the robot class of 6R robots,
namely L, U, A, D, and the hand variables P, as
true indeterminates (independent parameters)
and not merely nameholders for numbers, which
implies to work in a base field where these variables
are represented and also the possible relations
among them always hold. To show an
easy example, if we want to solve the general
2-degree equation ax2 + bz + c, with the condition
a + b + c = O on the coefficients, the
ideal to be considered must correspond to p :=
(az2 + bx + c, a + b + c) in the base field:
q.f.(R[a, b,c]/P n R.[%hc]) =
=q.j. (R[a, b,c]/(a+b+ c)).
lIn particular, the equations HH’ –I and det(H) –
1 are redundant, as they are formal consequences of
the remaining ones (see [6]).
355
where (q. f.) denotes the quotient field. For the
6R case, if we denote
/3:= (F’, L, U, A, D),
x := (s, c),
the base field is:
where a~R := afj~ n R[~].
In order to describe more explicitly the completely
symbolic ideal, we consider the following
ring isomorphisms (where –’ represents the
extension ideal to the corresponding ring):
‘[pZ]/a%= [R[~l/a’Rl[zl
where the last isomorphism determines the ideal
ox and:
oz.K[x]
is the ideal of the kinematic equations for the
general 6R manipulator in a fully symbolic approach.
3 Insimplificability and dimension of kinematic
ideals over the reals
Towards the goal of finding the best defining
equations of the ideals that appear in robotics
[7] made the following conjecture:
“Kinematic equation systems are irreducible
(i.e. they generate prime ideals). In particular,
all determining equations of all triangulations
are irreducible. ”
We will prove in the two following subparw
graphs that the kinematic ideal of the general
6R manipulator is a prime ideal and also that
it is a real ideal, which roughly means that it
describes the (real) solutions of the inverse kinematic
problem without redundancy, (this is required
to show Kovtics conjecture in the real
case, as commented in the introduction). We
also compute its dimension. The same conclusions
will be obtained for the Elbow manipulator
(see [13] for a description of this robot).
Definition 3.1 An ideal J C R[X] :=
R[q,. . . , Z8] is r-ml if it contains all the polynomials
vanishing on the real algebraic variety
V(J) = {z c R’/ p(z) = O for allp(X) E J}.
There are several characterizations and computational
issues related to this concept of real
ideal (see [1] ).
3.1 The kinematic ideal of the general 6R manipulator
Proposition 3.1 The ideal aGR (in the ring
R[P, S, C, L, U, A, D]) is real, prime, of dimension
24.
Proof:
The ring homomorphism given by:
R[P, S,C, L, U, A,D] * R[S, C, L, U,A, D]
P R A;. ..A;
and inducing the identity over the remaining
variables, is subjective and determines a ring isomorphism
R[P, S, C, L, U, A, D]/a6R =
c R[S, C, L, U, A, D]/q
where
q :=
({s~+c~– l,i=l,...,6},
{Af+p~- l,i=l,...,6}).
q is a real and prime ideal of dimension 12, as it
is the sum, with separated variables, of the real,
prime and one-dimensional ideals (s? + c? – 1),
i=l ,.. .,6and(A~+ pl),i=l, l,... ,6. This
follows from some classical results on the ideal
of the product of varieties: see [10] for the proof
of the general fact.
With respect to the dimension, we have
dim(a6R) = 12+12 = 24, where the first 12 correspond
to those independent modulo q among
S, C, L and U; and the last 12 are the twelve
independent variables in A and D. n
3.2 The kinematic ideal of the Elbow manipulator
As mentioned above, the Elbow manipulator is a
particular instance of 6R, when the parameters
of the 6R class are set to some values. Following
[13] and using the Denavit-Hartenberg representatlonl
the six homogeneous matrices A~ corresponding
to the six joints of the Elbow manipulator
are obtained gwing the following values to
the variables in (L, U, A, D) in the matrices A;:
i= 1 2 3 4 5 6
J, o 1 1 0 0 1
Pi 1 0 0 -1 1 0
ai o 1 1 1 0 0
d, o 0 0 0 0 0
Following the same notation as in the 6R case,
the ideal for the Elbow manipulator is:
a~l~o. = (PH–H$~AjA3A4AtAG,
det(~) – ~
{s~+c~– l,i=l,...,6})
Proposition 3.2
The ‘ideal aE&W C R [P, S, C] is real, prime, of
dimension 6.
356
Proofi
It suffices to remark that there is a ring isomorphism
R [F’, S, C~aElbow E
sR[S, C]/({s:+c, –1, i= 1,...,6})
and the conclusion follows as in the 6R case. n
4 Insimplificability and dimension of the kinematic
ideal of the 6R general manipulator
over K
We prove now the insimplificability (reality and
primality) over K of the kinematic ideal of the
6R manipulator, o-L. K[z], in the general symbolic
approach that we propose for this problem in
Section 2, computing also its dimension. Some
properties of the hand ideals (intersection of @~
and a~lbo~ with R [P]) of the 6R case and of the
Elbow case will be required.
Let us consider the ideal whose zeros define
the set SO(3) of all proper orthogonal matrices:
SO(3) := (llfft – 1, det(~) – 1) c
CR[hll, h12, . . ..h33]
where H is the 3 x 3-matrix with indeterminated
coefficients (hll, hlz, . . . . h33 ). In [6] we have
proved that, for arbitrary n, the ideal so(n) is
real and prime.
The following lemma will be used in Theorem
4.1, about the dimension of the hand ideal of the
6R manipulator. We remark that both results
(Lemma and Theorem) could have been proved
just making Grobner basis computations, as we
merely have to:
l compute the intersection of an ideal with
R [P], and
l test the equality of two ideals.
However, after spending many hours with the
computer, trying to get an answer using different
software packages (COCOA, Maple, Axiom), the
high number of involved variables did not allow
to finish these computations. Therefore, we have
tried to follow an alternative, more theoretical,
proof.
Lemma 4.1 a~~bo~ rl R[P] = SO(3) .R[P].
Proofi (Sketch)
To prove the inclusion SO(3).R [P] C aElbow fl
R [P] it suffices to remark that any matrix P
which is the product of the six proper orthogonal
matrices Ai is also proper orthogonal. This is
accomplished at the ideal theoretic level using
the results of [6].
To prove the other inclusion, it suffices now to
show that
v(so(3).R. [~]) C v(aElbow n ~[~]).
As v(aElb.w rl R [P]) is the Zariski closure of
the projection of v(a~ibo~) on the P-variables,
Cl (HP (V(aEIboW))), we are reduced to prove
that there exists a Zariski-dense set A in
V(SO(3).R[P]) such that A C ~p(v(aE~b~w)).
We find this set A using [13], where closed formulae
to determine real solutions to the inverse
kinematics for the Elbow manipulator are explicitly
given. A detailed look to these formulae
shows that they are valid over an euclidean non
empty open set (therefore, Zariski-dense) of the
space of the P-variables. n
Obviously the A; matrices in the Elbow manipulator
are numerical specialization of the A:
matrices in the 6R one. This is translated in
the commutative algebra framework, by means
of the identity:
aElbow =
(ae~,
~1,~2–1,~3–1,~4,~5,~6–1,
= ~1 ‘1,P2,P3,P4+1,P5 – 1,P6,
al, a2—lja3— l,a4—l, a5, a6j
dl, d2, ds, d4, d5, d6)fl R[p, s,c]
Theorem 4.1 afjn (l R[P] = SO(3) .R[P]. In
particular, the hand ideal aGn n R [P] of the general
6R manipulator is 6-dimensional.
Proof:
We have the following chain of inclusions:
SO(3) .R[P] C
a6R n R [P] C
(ae~,~l,fll –l, ~Z–l,..., dA, ds, dG)fl R[P] =
aEtbow n R[p] =
SO(3) .R[P]
n
Lemma 4.2
Proofi
We can easily prove the inclusion
(so(3) .R[@], {A~ +p~ – 1, z = 1,... ,6}) C a~R
and then, we have that
‘i~a%s < dim(so(3).R[@], {~i +Pi – 1, i = 1,. . . ,6}) =
= 24.
Let aEIbOWl C R [,6] be the ideal obtained when
giving fixed constant values ci = Cio and Si =
s$o, i= l,..., 6, (C?. +s, 2 = 1) to the 6R general
robot. As (si, ci) and t A,, Pi) have equivalent
roles in the ideal, we have that in R [~]:
dim aElbOWl = 24
Now, remark that
& c
C (aSR, {c, – C,O, S, – S,o}i=l,...,6) n R[D] =
= aElbowr
and then 24 = dim aE1bOwt < dim a~R n
357
Theorem 4.2 The ideal of the general 6R
manipulator, oT. K[z], is real, prtme and Odimensional.
Proofi
Set R = R [@]/a;~, then ot is an ideal in R[x],
R[x]/oz is a real integral domain of dimension 24
and o~ n R = {O}; hence setting S = R – {O} we
get:
K[X] = (R[X])S,
OZKIX] = Ots ,
KIX]/OZ~[XI = (R[X]/OZ)F >
where ~ = S + oz. Clearly, (R[-X]/oz)F is a real
integral domain. Thus, OZKIX] is a real prime
ideal of
dim otKIX] =
= tT(K[X]/OZKIXl, K) =
= tr(q.f. (R[X]/oz), K) =
= tr(q..f. (RIX]/oz), R) – tr(K, R.) =
24–24=0
where tr denotes the transcendence degree. n
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.O {color:white;} a:link {color:#336699 !important;} a:active {color:#0099CC !important;}
nHard real-time: systems where it is absolutely imperative that responses occur within the required deadline
nSoft real-time: systems where deadlines are important but which will still function correctly if deadlines are occasionally missed.
nReal real-time: systems which are hard real-time and which the response times are very short.
nFirm real-time: systems which are soft real-time but in which there is no benefit from late delivery of service
Introduced in 1962 by Carl Adam Petri in his PhD thesis.
Focus on modeling causal dependencies;
no global synchronization assumed (message passing only).
Key elements:
• Conditions
Either me
(Extract from PetriSim Manual)
The following example models a cooperation between two processes called Producer and Consumer. The Producer prepares data and writes them to buffers. If there is no empty buffer, the Producer must wait. The Consumer reads the data supplied by the Producer. The initial marking of the place "Empty_buffers" is the total number of buffers available (initially all the buffers are empty). The semaphore ensures that only one process can work with the data at a time. After reading the data the Consumer returns the empty buffer.
t or no met.
• Events
May take place if certain conditions are met.
• Flow relation
Relates conditions and events.
Conditions, events and the flow relation form
a bipartite graph (graph with two kinds of nodes).