M .
Tanguay : Etudiant
à la m aîtrise, U niversité deSherbrooke,
Facultéde
génie,Dépar
tement- de génie électrique et de génie inform atique.
D. Létourneau : Professionnel de recherche, laboratoire de robotique intelligente, interac tive,
intégrée et interdisciplinaire
(In tR o L a b ), U niversité de Sherbrooke, Faculté de génie. Départem ent de génie électrique et de génie inform a tiqu eF. M ichaud : Professeur et d irecteur du la b ora to ire de robotique intelligente’ , in teractive , intégrée' e't interdiseûplinaire (In tR o L a b ), U niversité de Sherbroeeke, Faeulté de' génie, Dé- parréme’n t ele' génie’ éle>e:triejue et de génie informaticien’
E ta t de l’accep ta tio n
: Soumis le 14 a v ril 2013.T itre anglais :
Using a Diffe.rc.ntial Drive Mechnnism to Double the Torque o f D ifferentialFAastic Ac.tuat.ors of a Compilant, Legged Robot
T itre français
: U tilis a tio n d ’un diffé re n tiel mécanique p our doubler le couple d ’action- nemrs différentiel élastique d ’une jam be robotique com plianteR evu e
: R obotics Society o f Japan Advanced RoboticsC ontrib ution du d ocu m en t
: L ’a rtic le présente le système développé dans le cadre dutra v a il ele m aîtrise, composé de deux actionneurs
différentiel élastique et
d ’un d iffé re n tiel mécanique, dans le b u t de les e xploite r dans la réalisation d ’une a rtic u la tio n ro b otiq u ecompilante.
La caractérisation du mécanisme en terme de couple et de vitesse y estétudiée.
R é s u m é fra n ç a is : Les actionneurs élastiques peuvent amener d ’im p o rta n ts bénéfices aux robots marcheurs ejui doivent évoluer en te rra in irrégulier. A fin de m inim iser Us dim en sions cet. le poids de' la jam be to u t en m axim isant le couple et la vite'sse, e-et article’ présente l ’u tilis a tio n de deux actionneurs différentiels élastiques (A D E ) couplés à 1111 elifférentiel rné-
C H A P IT R E 3. U T IL IS A T IO N D ’U N D IF F É R E N T IE L M É C A N IQ U E P O U R
20 D O U B L E R L E C O U P L E D ’A C T IO N N E U R S D IF F É R E N T IE L É L A S T IQ U E
oanic|ue perm ettant l'a b d u c tio n /l'a d d u c tio n et la flexion/T extension d ’une jam be de robot. L ’e xplicatio n des actionneurs et, du mécanisme de couplage est présentée, accompagnée de la caractérisation du système résultant.
3.1
Abstract
Elastie aetuators can b rin g im p o rta n t benefits for legged robots lia vin g to operate on irreg u la r terrain. To m in im ize leg size and w eight w hile m a xim izin g torque and velocity, tin s pap('r présents tlie use o f two d iffe re n tial elastie aetuators (D E A ) coupled throu g h a d iffe re n tia l d rive ineehanism to provide a b d u c tio n /a d d u c tio n and flexion/extension o f a robot leg. E xplanations o f the D E A and tlie mechanism are provided, along w ith its charaeterizaticm.
3.2
Introduction
D e s ig n a n d c o n t r o l o f lc'gge'd r o b o t s c a p a b l e o f o ] ) e r a t i n g o n i r r e g u l a r t e r r a i n b r i n g s i n
terest ing chall<‘ng('s [34] [8], one o f which being tlia t the legs m ust liave tlie c a p a b ilitv o f dealing wit.li large forces over short periods o f tim e, as tlie y makc contact w ith the ground [2]. T h e conventional design approach consists o f using s tiff m otors pow erful enough to deal wit.h tliese forces, and measure contact or forces between the leg and the environm ent.
h b r instane e-, using geared D C motors, legs use contact switches [49] [12], spring loadeel shanks w ith a poteuitiom eter [48] or three degrees o f freedoin (D O F ) stra in gatigc's [ i l ] [28] to di'te'e:t eontact or measure forces applied o r com ing from the grounel. Tem jue/fbrce semsors plae exl w ith in jo in ts [27] [3] can also be useel. One exam ple is the D L R -C ra w le r [1G], w hich use's geared D C m otors, torque measurement at, eacli jo in t, anel 6 -D O F stra in gauge1 at the tip e»f the leg for impédance co ntro l at 1 kHz.
However, the use o f s tiff m otors lim its the a b ility o f these leggeel robots to handle u nanti- cipatoei shocks o r interactions w ith the w orld. A n o th e r approach involves using an elastie com ponent to provide passive (i.e., a spring) or active m echanical com pliance to the leg. For instance, R hex’s legs [47] are made o f a curve plastic band w hich acts as a spring tlia t dc'forms when the leg touches the ground. T h e leg is actuated bv a geared m o to r a tta cher! to one end o f the plastic band. T his b u ilt-in passive com pliance enables the ro b o t to move 011 a great va rie tv o f uneven terrains. For active com pliance, Serial Elastie A etuators
(S E A ) are- compilant, aetuators capable o f e q u ilib riu m controlled stiffness [51], offering in- trin s ic niechanie'al security to shocks. B y measuring the deform ation o f the spring using
.12. IN T R O D U C T IO N 21
stra in jauges and H ooke’s law, impédance control [18. 19, 39], i.e., the a b ility to dynam i- cally regulate stiffness throu g h torque control of the actuator, is possible. T he m axim um stiffness is sert by a spring tlia t acts as the elastie component. S EA have been tised on S pring Turkey [40], S pring Flam ingo [41] and M2 [44] two-legged robots, dem onstrating t liat tlie y can liandle external forces (sucli as being pushed around). S im ulation o f using variabh* impédance co ntro l m ethod for quadruped robot tro t lias been dem onstrated in [38]), showing b o tte r perform ance and s ta b ility tlia n tixed impédance co ntro l in tro ttin g on irreg u la r terrain.
One issue w ith SEA is tlia t having to place the elastie component in sériés w ith tlie ac tu a to r makes it more d iffic u lt to place in sm all volumes. Instead o f using serial eoupling as in SEA, D iffe re n tia l Elastie A etuators (D E A ) [29. 31] use a d iffe re n tia l eoupling bet- wceii a high impédance mechanical speed source and a low impédance meehanical spring. D E A show s im ila r com ]iliance capabilities to SEA, but w ith higher torque/encum brance ra tio , lower ro ta tiv e in e rtia , lower ro ta tio n ve lo city o f tlie flexible element, and im proved e ffort transm ission through tlie m o to r’s châssis. D E A have been used to design co m p lia nt locom otion or m a n i])u la tio n [10] mechanisms but not for legged robots, w liile SEA lias récent lv been used in quadruped robots [20, 22]. Legged robot design involves m in im iz in g leg size and w e ig lit w liile m a xim izin g torque' and velocity. In tliis p ro ject, our interest lies in stu d vin g how D E A ’s advantages can be used in the design o f com pliant quadrupède robot. For instance, D E A ’s sm aller form factor comparer! to SEA allows the aetuators to be placée! on the p la tfo rm ra th e r than on tlie leg. However, using one D E A for eaeh D O F lia n ts the torque capabilities o f eaeh leg to the ones provided by the actuator. Consi- (k’rin g tlia t D E A can be placer! close to eaeh o ther to actuate a 2-D O F h ip jo in t (one for a b d u c tio n /a d d u c tio n movement, and one for flexion/extension m oveinent), increasing torque capabilities can be accomplished by eoupling the D E A throu g h a differential d rive (D D ) mechanism, sucli as in [1G, 43]. A sim ila r mechanism using S EA is presented in [22], w ith o u t characterizing its performances. T liis paper therefore présents the designer! me- chanism using D E A and the resulting performances in terms o f torque m agnitude, torque b an d w id th, current versus torque and velocity versus torque, and is organized as follows. Section 3.3 explains the w orking principles o f a D E A along w ith its design for our legged p la tfo rm . Section 3.4 describes the 2-D O F DD -eoupled D E A design and its controller, followed bv Section 3.5 w ith results obtained using a real im plém entation o f the designer! mechanism.
C H A P IT R E 3. U T IL IS A T IO N D 'U N D IF F E R E N T IE L M E C A N IQ U E P O U R 22 D O U B L E R L E C O U P L E D ‘A C T IO N N E U R S D IF F É R E N T IE L É L A S T IQ U E