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Research The International Journal of Robotics
DOI: 10.1177/027836498700600303 1987; 6; 49 The International
Journal of Robotics Research
Jean-Jacques E. Slotine and Weiping Li On the Adaptive Control
of Robot Manipulators
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49
On the AdaptiveControl of Robot
Manipulators
Jean-Jacques E. SlotineWeiping LiNonlinear Systems
LaboratoryMassachusetts Institute of TechnologyCambridge,
Massachusetts 02139
Abstract
A new adaptive robot control algorithm is derived, whichconsists
of a PD feedback part and a full dynamics feedfor-ward compensation
part, with the unknown manipulator andpayload parameters being
estimated online. The algorithm iscomputationally simple, because
of an effective exploitationof the structure of manipulator
dynamics. In particular, itrequires neither feedback of joint
accelerations nor inversionof the estimated inertia matrix. The
algorithm can also beapplied directly in Cartesian space.
1. Introduction
Adaptive control, as a branch of systems theory, is notyet quite
mature (see, for instance, Astr6m 1983;1984). Yet, the practically
motivated drive to makerobot manipulators capable of handling large
loads inthe presence of uncertainty on the mass properties ofthe
load or its exact position in the end-effector, aswell as the old
&dquo;cybernetic&dquo; ideal of developing learn-ing
capabilities in machines, has spurred much re-search on adaptive
control of robot manipulators (see,e.g., Hsia 1986, for a recent
review). The nonlinearityof robot dynamics, however, makes them
even morecomplex to analyze than the linear dynamic systemson which
most of the existing adaptive control theoryhas been traditionally
focused.
Several approaches have been considered. Somechoose to ignore
the dynamic complexity and fit themeasured data to a second-order,
linear, time-varyingmodel, using for instance a recursive
least-squaresapproach (see, e.g., Koivo 1986). Others do exploit
the
known structure of the system dynamics (e.g., Khoslaand Kanade
1985; Atkeson et al. 1985; Craig et al.1986), although they
generally require estimation ofjoint accelerations. Another class
of algorithms con-siders the &dquo;learning&dquo; of
specific tasks through the useof feedforward signals (Arimoto et
al. 1985; Atkeson etal. 1986), without explicitly updating the
manipulatormodel itself.
In this paper a new adaptive robot control algorithmis derived,
which consists of a PD feedback part and afull dynamics feedforward
compensation part, withthe unknown manipulator and payload
parametersbeing estimated online. The algorithm is computation-ally
simple, because of an effective exploitation of theparticular
structure of manipulator dynamics. As inKhosla and Kanade (1985)
and Atkeson et al. (1985),we use the remark that the dependence of
the systemdynamics on the unknown parameters can be madelinear in
terms of a suitably selected set of robot andload parameters.
However, contrary to most algo-rithms in the literature, there is
no need to measurethe joint accelerations or to invert the
estimated inertiamatrix.The layout of the paper is as follows:
Section 2
presents our basic adaptive structure in joint space,and in
Section 3 we discuss its extension to Cartesian
space control. Simulation results are presented in Sec-tion 4.
Section 5 offers brief concluding remarks.
Extensive experimental results are presented in Slo-tine and Li
( 1987).
2. Adaptive Robot Controller in Joint Space
~.1. Dynamic Model of Robot Manipulators
In the absence of friction or other disturbances the
dynamics of an n-link rigid manipulator can be writtenas
This research was supported in part by a grant from the Sloan
Fund.
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50
where q is the n X 1 vector of joint displacements, i isthe n X
1 vector of applied joint torques (or forces),H(q) is the n X n
symmetric positive definite manipu-lator inertia matrix, C(q, q)q
is the n X 1 vector ofcentripetal and Coriolis torques, and G(q) is
the n X 1vector of gravitational torques.Two simplifying properties
should be noted about
this dynamic structure. First, as remarked by sev-eral authors
(e.g., Arimoto and Miyazaki 1984;Kodistcheck 1984), the matrices H
and C are not in-dependent. Specifically, given a proper definition
of C,the matrix H - 2C is skew-symmetric, as shown inAppendix II.
Physically, this property can be easilyunderstood: The derivative
of the manipulator’s ki-netic energy qTHq must equal the power
input pro-vided by the actuators and the gravitational torques:
which implies that at all times
Another important property is that the dynamic struc-ture is
linear in terms of a suitably selected set ofrobot and load
parameters (Khosla and Kanade 1985;Atkeson et al. 1985), as
illustrated in Appendix I for atwo-link manipulator.
2.2. Controller Design
The controller design problem is as follows: Given thedesired
trajectory qd(t), and with some or all the ma-nipulator parameters
being unknown, derive a controllaw for the actuator torques and an
estimation law forthe unknown parameters such that the
manipulatoroutput q(t) tracks the desired trajectories after an
ini-tial adaptation process.
-
We derive our adaptive controller in two steps. First,in Section
2.2.1 a simple globally stable adaptive con-troller is obtained
from a Lyapunov stability analysis.The controller strongly exploits
the structure of themanipulator dynamics pointed out in the
previous
section. After the initial transients, however, althoughthe
adaptive controller does yield zero velocity errors,it may present
nonzero position errors. We solve thisproblem in Section 2.2.2 by
restricting the residualtracking errors to lie on a sliding surface
(see Slotine1985), thus guaranteeing asymptotic convergence ofthe
tracking.
2.2. J. A Globally Stable Adaptive Controller
To derive the control algorithm and adaptation law, ,we consider
the Lyapunov function candidate
where a is an m-dimensional vector containing theunknown
manipulator and load parameters, and i isits estimate; Kp and r are
symmetric positive definitematrices, usually diagonal; q(t) = q(t)
- qd(t) is thetracking error; and à = i(t) - a denotes the
parameterestimation error vector. Differentiating i~ yields
where we have used the property of skew-symmetry toeliminate the
term 2 qT(H - 2C)q. Let us define thecontrol law as
where the positive definite matrix K~ may be chosento be time
varying. Then
where
Choice (3) cancels the terms associated with the known
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51
manipulator parameters, so only the unknown manip-ulator
parameters have to be retained and estimated ini. Further, since
the matrices H, C, and G are linearin terms of the manipulator
parameters, we can write
where Y = Y(q, q, qa, qd) is an n X m matrix, andtherefore
This suggests choosing the adaptation law such that
that is
Note that a = a, since the unknown parameters a areconstants.
The resulting expression of V is
Therefore the control law (3) and the adaptation law(5) yield a
globally stable adaptive controller.
Expression (6) implies that the steady-state jointvelocity error
is zero. However, it does not necessarilyguarantee that the
steady-state position error is alsozero. We now modify the previous
adaptive scheme inorder to solve this potential problem.
2.2.2. Elimination of Steady-State Position Errors
Undesirable steady-state position errors can be elimi-nated if
we restrict them to lie on a sliding surface
where A is a constant matrix whose eigenvalues arestrictly in
the right-half complex plane. Formally, weachieve this by replacing
the desired trajectory qd(t) inthe above derivation by the virtual
&dquo;reference trajec-tory&dquo;
Accordingly, 4d and qd are replaced by
If we define
the control law and adaptation law become
Note that the matrix Y is now a function of q, and Q,rather than
4d and 4d, We can again demonstrateglobal convergence of the
tracking by now using theLyapunov function
instead of (2), which yields
instead of (6). Note that control law (8) does not con-tain a
term in Kp, since the position error q is alreadyincluded in qr.
Expression (11) shows that the outputerrors converge to the sliding
surface
This in turn implies that q ~ 0 as t ~ 00. Thus, theadaptive
controller defined by (8) and (9) is globallyasymptotically stable
and guarantees zero steady-stateerror for joint positions.The
previous proof of tracking convergence may
seem somewhat unorthodox to readers not familiarwith sliding
control theory. Let us detail the basicfeatures. First, the vector
s conveys information aboutboundedness and convergence of q and q,
since thedefinition of s can also be viewed as a stable,
f’-crst-orderdifferential equation in 4, with s as an input. Thus,
forbounded initial conditions, boundedness of s impliesboundedness
of 4 and q and, therefore, of q and q;
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similarly, one can easily show that if s tends to 0 ast ~ 00, so
do q and q. Second, the function is actu-ally a quasi-Lyapunov
function, in our case simply apositive continuous function of time.
Let us now detailthe proof itself. Since V is negative or zero and
V islower bounded (by zero), V tends to a constant ast ~ 00 and
therefore remains bounded for t E [0, 00].Given the definition (10)
of V, this in turn implies,since H is uniformly positive definite
(i.e., H ~ hI forsome strictly positive h), that s is bounded and,
there-fore, that q and q are bounded; it also implies that a
isbounded and, therefore, that i is bounded. From thesystem
dynamics this then makes s bounded, and thuss is uniformly
continuous on t E [0, 00]. Assuming thatthe (perhaps time-varying)
matrix Ko is chosen to beuniformly continuous (as is typically the
case, for in-stance, with KD constant, or with KD = ..18), Y is
thenuniformly continuous on t E [0, 00]; therefore, since Vis
bounded on that time interval and Fis of constantsign (T~ -- 0), V
tends to zero as t ~ assuming thatK~ is uniformly positive definite
(as is again the case ifKD is chosen to be constant, or if KD =
AH), this im-plies from (11) that s ~ 0 as t ~ ~, and therefore
thatq ―~ 0 as f ―~- oo.The structure of the
adaptive controller given by (8)
and (9) is sketched in Fig. 1. The controller consists oftwo
parts. The first part consists of three feedforwardterms
corresponding to inertial, centripetal and Corio-lis, and
gravitational torques. The second part con-tains two terms
representing PD feedback. The re-quired inputs to the controller
are the desired jointposition qd, velocity 4d, and acceleration qd
from thetrajectory planner, and the required measurements arethe
joint position q and velocity q. Contrary to severalalgorithms in
the literature (e.g., Craig et al. 1986),there is no need for
measuring the joint accelerationsq or for inverting the estimated
inertia matrix. Notethat if measurements of joint accelerations
were indeedexplicitly available online, one could easily
show(Slotine 1986) that the effect of parametric uncertaintyon
performance could in principle be made arbitrarilysmall by simply
increasing the value of the accelera-tion gain, without using
adaptation; however, thisprocedure would be extremely sensitive to
imprecisionon the joint acceleration measurement, which
thenessentially would enter as a pure disturbance added to q.Note
from Fig. 1 that the integral term f o 4 dt of (7)
Fig. 1. Structure of the jointspace adaptive controller.
need not be actually computed, since only qr and q~(not qr) are
explicitly used in the control law. There-fore, the formal
definition of qr is, in effect, equivalentto adding a feedback
loop.
2.3. Discussion
In this section we discuss implementation aspects,computational
efficiency, and strategies that combineadaptation on certain
parameters with robustness touncertainty on others and to
disturbances.
2.3.1. Implementation Aspects
Since the load is usually fixed with respect to the lastlink, it
can be regarded as part of that link. In practice,the parameters of
the robot itself can be measured orestimated beforehand (Khosla and
Kanade 1985; At-keson et al. 1985), so only the parameters of the
loadare unknown. Models of Coulomb and viscous friction
may also be included in (1), and the correspondingcoefficients
can be identified similarly.Although convergence of the trajectory
tracking is
guaranteed in the previous derivation, the parameterestimates
themselves do not necessarily converge totheir exact values.
Intuitively, to guarantee parameterconvergence, the desired
trajectory must be &dquo;su~-ciently rich&dquo; so that
only the true set of parameterscan yield exact tracking. A
formalization of this con-cept in the context of robot control and
the generationof trajectories that speed up parameter
convergenceconstitute interesting research topics in
themselves(Morgan and Narendra 1977; Craig et at. 1986).We stop
updating a given parameter when it reaches
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53
its known bounds, and we resume updating as soon asthe
corresponding derivative changes signs. This intu-itively motivated
procedure can easily be shown topreserve convergence of the
tracking.
2.3.2. Computational DeficiencyIn the practical implementation
of the previous adap-tive controller, the matrices H, C, and G may
be up-dated at a low rate, whereas a high update rate is usedfor
q&dquo; Q,, and s, since typically the error terms varymuch
faster than the dynamic coefhcient matrices(see, e.g., Khatib
1986). Further, the matrix Y, whosecalculation is naturally coupled
to the dynamics com-putation, can also be updated at the slow rate,
sincethe choice of the adaptation gain matrix r is generallysuch
that the adaptation process is slower than thecontrol
bandwidth.
Because of the presence of q, in the second term ofcontrol law
(8), however, the controller cannot beimplemented directly with
fast recursive formulations,such as the Newton - Euler method, and,
therefore,requires explicit computations of H, C, and G. Thesame is
true of adaptation law (9). We now introducea recursive
Newton-Euler method as an alternative
way of implementing the control and adaptation laws.This Newton
- Euler formulation can be seen as anapproximation of the previous
development, for whichnew stability conditions are derived.Assume
that the second term t4, in (8) is approxi-
mated by Cq. Then we can compute the first threeterms in (8) by
a recursive Newton - Euler method,based on the parameters obtained
from the adaptationlaw. The resulting control torque is
which is computed through a number of operationsproportional to
the number of links. Accordingly, thesame approximation is made in
the calculation of thematrix Y, namely,
Let us examine the effects of these approximations.We have
with now
From (10),
Thus from (13), (15), and (16), we obtain
using the skew-symmetry of the matrix (H - 2C).Therefore, the
stability of this recursive formulation ofthe adaptive controller
is guaranteed as long as KD ischosen large enough (perhaps time
varying) to satisfyKD > -tH.
2.3.3. Combining Adaptation with Robustness
In practice, we may simplify the algorithm by notexplicitly
estimating all unknown parameters. Someparameters may have
relatively minor importance inthe dynamics, in which case we may
choose to makethe controller robust to the uncertainty on these
pa-rameters rather than explicitly estimating them on-line.
Similarly, some geometric parameters may al-ready be known with
reasonable precision or may havebeen estimated through sorting
devices or visual infor-mation. Further, the controller must be
robust to re-sidual time-varying disturbances, such as stiction
ortorque ripple.We categorize the unknown parameters a into two
groups: group a~ contains the parameters estimatedonline; group
aR contains the parameters not estimatedonline. A sliding control
term is then incorporatedinto the torque input (8) to account for
the effects ofuncertainties on the parameters in aR and of
distur-bances.
Assume, without loss of generality, that only thefirst a unknown
parameters are to be actually estimated:
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54
and let, correspondingly, Y = [YE YR]. Assume thatthe
uncertainties on aR, as well as the disturbancetorques dt reflected
to the manipulator joints, arebounded:
Add a sliding control term to torque input (8):
where the notation k sgn (s) stands for the n X 1 vectorof
components ki sgn (si), with the k; yet to be speci-fied. With aE
and r~ in place of a and z’ in the Lya-punov function (10), we
obtain
Since
we let
where the tJi are positive constants. This yields
The system trajectories are thus guaranteed to reachsliding
surface s = 0, and therefore convergence of thetracking is
achieved.
Further, to avoid undesirable control chattering, wecan use
saturation functions sat (siloi) in place of theswitching function
sgn (si), with the 0, representingthe thicknesses of the
corresponding &dquo;boundarylayers.&dquo; Similarly to
Slotine (1984), s is then guaran-teed to converge to the boundary
layers, with corre-
sponding small tracking errors; further, the Oi can bemodulated
based on bandwidth considerations. Simi-
larly to Slotine and Coetsee (1986), parameter adapta-tion must
then be stopped when the system trajec-tories are inside the
boundary layers; indeed, bydefinition, disturbances and errors on
aR can drive thetrajectories anywhere in the boundary layers
withoutthis providing any information about the estimationerror on
a~. This procedure also has the advantage ofavoiding long-term
drift of the estimated parameters.Note from (18) that K~s can be
eliminated from
control input (17), since the sliding control actionmakes it
unnecessary; however, this term must be keptin a Newton - Euler
implementation of the algorithmto compensate for the approximation
of Cq, by Cq, asdiscussed earlier. It may also be retained in order
toaccelerate convergence. Note that fixed-parametersliding control
is obtained if none of the unknown pa-rameters is explicitly
estimated (a = m).
3. Extension to Cartesian Space Control
In this section we extend the previous joint spaceadaptive
controllers to task space. To this effect, for anonredundant
manipulator, we simply replace thereference trajectories in (7b)
and (7c) by .
and, accordingly,
so that
The same control and adaptation laws (8) and (9) arethen used,
again with (10) as the Lyapunov function.Following the same
derivation as before, we obtain
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Fig. 2. Two-link manipulatorcarrying a large unknownload.
which implies convergence to
Using the kinematic relation x = Jq, we recognizeexpression (20)
as the equation of the sliding surfaceac + Air = 0, which in turn
guarantees that x - 0 ast ~ 00. Therefore, the previous adaptive
controller isglobally stable and guarantees zero steady-state,
Carte-sian space, position error.Note from (19a) and (19b) that
only the desired
trajectories in Cartesian space Xd, Xd, and xd have tobe given
(i.e., explicit inverse kinematics is not neces-sary). The
quantities to be measured are joint posi-tions q and joint
velocities q. End-effector position xand velocity x can be obtained
from the direct kine-matics, and therefore do not need to be
explicitly mea-sured. Also, note that the inverse Jacobian J-’
appearsin (19a) and (19b), and therefore singularity pointsshould
be avoided (see Khatib 1986 for a relaxation ofthis condition).
4. Simulation Results
We present computer simulations using the two-linkplanar
manipulator considered in Appendix I, carryinga large load of
unknown mass properties (Fig. 2). The
Fig. 3. Desired joint trajec-tories for Examples 1 and 2.
two links are identical uniform beams, with actuatorsmounted at
the joints. In the simulations the un-known load actually has the
same geometry as thelinks but is twice as heavy. For simplicity,
the parame-ters of the robot itself are assumed to be exactlyknown.
The parameters to be adapted are a, ~3, E, and?7, whose true values
are a = 6.7, /3 = 3.4, E = 3.0, and~ = 0. The initial estimates of
the load mass propertiesassume that the load is identical to the
second link.The corresponding initial parameter estimates area =
4.1; /3 = 1.9, E = 1.7, and ( = 0. In the simulationplots the
estimates of the first three parameters arenormalized by the true
values, and ( is normalized by3 (the true value of E), since p is
itself zero.
Example 1: Comparison with conventional con-trollersThe task is
to move the load from position A to
position C, as indicated in Fig. 2. Three controllers areused:
(1) PD controller, (2) PD + full dynamics feed-forward
compensation, and (3) adaptive controllergiven by (3) and (5). The
desired joint trajectories arechosen to be fifth-order polynomials
and are shown inFig. 3. The matrices Kp and KD are chosen to be
iden-tical for all three controllers, with Kp = 8001 and KD =160/.
The results are plotted in Fig. 4 for controller a,Fig. 5 for
controller b, and Fig. 6 for controller c. Themaximum joint
position errors are about 7.5 ° forcontroller a, 3 for controller
b, and only about 0.5 °
for the adaptive controller. The maximum actuatortorques are
smaller for the adaptive controller than for .controllers a and b.
The parameter estimates do notconverge to their exact values, since
the desired trajec-tory is not persistently exciting. Also, as
anticipated inSection 2.2.1, the joint position errors do not
exactlyconverge to zero, a problem that we now remedy usingthe
development of Section 2.2.2.
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56
Fig. 4. PD controller inExample 1.
Example 2: Elimination of steady-state position errorThe
adaptive controller given by (7) and (8) is simu-
lated with the same parameters as in Example 1, andA = 30/. The
joint position errors now converge tozero (Fig. 7). We also note
that the maximum jointposition errors have been reduced to only
0.08 with-out significant increase in actuator torques.A smaller
value of A is also simulated. With A = 51,
the product of KD and A is the same as Kp of con-troller c in
Example 1; however, the resulting maxi-mum position errors are only
0.12°, and convergenceto zero is observed.
Example 3: Parameter convergenceIn this example the desired
trajectory is chosen to be
The coefficients a; and bi are chosen to make the de-sired
trajectory satisfy the initial and final conditionson position,
velocity, and acceleration. The sameadaptive controller as in
Example 2 is used. Althoughit may not be necessary to have six
frequency compo-nents for the desired trajectory to be persistently
excit-ing, this example demonstrates that sufficiently richdesired
trajectories do yield convergence of the param-eter estimation
(Fig. 8).
Fig. 5. PD + full dynamicsfeedforward controller inExample
1.
-
’
Example 4: Cartesian space adaptive controllerThe same task as
that in previous examples is per-
formed by the adaptive Cartesian space controller ofSection 3.
The desired path is now a straight line fromA to B in Fig. 2. A
fifth-order polynomial is con-structed for the desired displacement
along the path,which has zero velocities and accelerations at the
startand the end of the path. The feedback gains and allother
parameters are the same as before. The perform-ance of this
controller (Fig. 9) is similar to that of thejoint space adaptive
controller. The steady-state Carte-sian position errors are zero,
and the maximum Carte-sian path errors in the x- and y-directions
are about8 X 10-4 m.
_
Extensive experimental results (Slotine and Li 1987)confirm
these simulations.
5. Concluding Remarks
It is of interest to further investigate specific choices ofthe
adaptation gain matrix r that yield optimal con-vergence rates
while still avoiding the excitation ofhigh-frequency unmodeled
dynamics (such as struc-tural resonant modes, actuator dynamics, or
samplingeffects). This may involve employing a time-varying
To,based, e.g., on a Gauss-Newton algorithm. Although
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57
Fig. 6. Adaptive controller(3), (5).
in principle an approach similar to that of Slotine andCoetsee
(1986) could be used to this effect, we believethat in this
instance it may be more effective to tryagain to take full
advantage of the specific structure ofthe manipulator dynamics.
This will be the object of aseparate study.
Further, in the more general context of control sys-tem design
for physical nonlinear systems, we believethat the approach that
consists of modifying, throughfeedback, the system’s natural energy
function ratherthan its explicit expanded dynamics is worthy of
fur-ther investigation in its own right.
Appendix I: Two-Link Manipulator withLarge Unknown Load
A two-link planar manipulator carrying an unknownpayload is
shown in Fig. 2. The second link, with thepayload attached, can be
regarded as an augmentedlink with four unknown parameters, namely,
mass ma,moment of inertia le, the distance lee of its mass centerto
the second joint, and the angle 5, relative to theoriginal second
link. The dynamics of the manipulatorwith payload can then be
written as
Fig. 7. Adaptive controllerwith steady-state positionerror
eliminated.
where
where g is the acceleration of gravity, and the fourunknown
parameters a, /3, E, and 17 are functions ofthe unknown physical
parameters:
Conversely, the four unknown physical parameters areuniquely
determined by a, (3, E, and ’1.
Appendix II: The Matrix H - 2C
We show here that, with a proper definition of thematrix C, the
matrix H - 2C is skew-symmetric, thusmaking more precise the result
obtained earlier fromconservation of energy.
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Fig. 8. Showing the conver-gence of the estimates
forpersistently exciting trajec-tories: (a) normalized a andÎ3; {b)
normalized E and ~.
The ith element of the vector C4 is (see, e.g., Asadaand Slotine
1986)
where the Christoffel coefficients hijk verify
Thus, (A 1 ) can be written
where we used reindexing to obtain the second termon the right
side. Now take
and let W = H - 2C. Then
Fig. 9. Adaptive controller inCartesian space.
Thus for all i, j
which shows the skew-symmetry of H - 2C. Althoughother choices
of Cij could satisfy (A I ), they usually donot possess this
skew-symmetry property.
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