Design of a hydraulic robot shoulder based on a
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1. dealt with in the next paragraph In research and development laboratories there are numerous exceptions to the suggested rule Although it would be lengthy to discuss them all it may be noticed that the greatest majority of these exceptions either fall in the category of trunks spines snakes tentacles and other segmented designs for example see 3 or in the category of platform manipulators see 17 for example These categories often overlap see 15 and 4 for example Figure 1 Following what has just been said some liberties with the conventional terminology are taken the word joint will refer to a group of joints actuated or passive instrumented or not which contribute toward a common function orientation or extension as in the shoulder joint or the wrist joint In this very brief survey a large number of examples could not be commented on Since the focus is on hydraulic actuation the reader is referred to three recent designs of hydraulic manipulators 12 19 21 for purposes of comparison All three examples have a serial linkage architecture although in the first two cases larger versions of these manipulators have one or two proximal revolute joints replaced by four bar crank piston joints Here the design of a shoulder joint of unconventional architecture is de scribed and the characteristics of a prototype discussed This paper is a sequel to 7 Please see Fig 1 for a view o2. Modelling and Control of a Modular Redun dant Robot Manipulator in Experimental Robotics 1 Hayward V Khatib O Eds Lecture Notes in Control and Information Sciences 139 Springer Verlag pp 508 527 5 Dietrich J Hirzinger G Gombert B Schott J 1990 On a unified concept for a new generation of light weight robots in Experimental Robotics 1 Hayward V Khatib O Eds Lecture Notes in Control and Information Sciences 139 Springer Verlag pp 244 286 6 Dunlop G R Johnson G R Afzulpurkar 1991 Joint design for extended range Stewart platform applications Proc Hight Worid Congress on the Theory of Machines and Mechanisms Prague Czechoslovakia Vol 2 pp 449 451 7 Hayward V 1993 Borrowing some design ideas from biological manipulators to design an artificial one In Robots and Biological Systems NATO Series P Dario P Aebisher and G Sandini Eds Springer Verlag pp 139 151 13 14 15 16 17 18 19 20 21 In Experimental Robotics 3 Yoshikaswa T Myiazaki F Eds Lecture Notes in Control and Information Sciences 200 Springer Verlag pp 297 309 309 Hayward V Kurtz R 1991 Modeling of a parallel wrist mechanism with actu ator redundancy In Advances in Robot Kinematics Stifter S Lenarcic eds Springer Verlag pp 444 456 Hirose S and Sato M 1989 Coupled drive of the multi DOF robot in Proc IEEE Int Conf on Robotics and A
3. actuator does not leave the device completely crippled in fact a notable level of functionality is preserved albeit in a greatly reduced controlled workspace by about an order of magnitude Figure 3 Combinatorial spherical mechanism analogous to the planar version shown on the left 1This paragraph applies to human arms as well In Experimental Robotics 3 Yoshikaswa T Myiazaki F Eds Lecture Notes in Control and Information Sciences 200 Springer Verlag pp 297 309 300 2 1 Kinematic Concept The operation of this architecture is best understood by viewing it as a com bination of four piston crank systems working cooperatively in a differential fashion a spatial version of a two piston one crank mechanism so to speak In this case four topological regions can readily be distinguished labelled by the relative velocities of the two pistons as seen on the Fig 4 Figure 4 The four topological regions of the simplest combinatorial mechanism For the more general case discussed in this paper in the vicinity of the central position tilting motions occur when the pistons attached to opposite edges of the square platform move with velocities whose signs are different while swiveling motions occur when pistons in opposite corners have velocities with differing signs as illustrated in Fig 5 MIU Figure 5 Velocity sign combinations Motions with all four equals velocity signs are kinematically prevente
4. In Experimental Robotics 3 Yoshikaswa T Myiazaki F Eds Lecture Notes in Control and Information Sciences 200 Springer Verlag pp 297 309 Design of a Hydraulic Robot Shoulder Based on a Combinatorial Mechanism Vincent Hayward Center for Intelligent Machines McGill University 3480 University Street Montr al Qu bec Canada H3A 2A7 Abstract In previous papers I have argued that while parallel mechanisms are well known for their favorable structural properties their utility is generally limited by an inherently small workspace I have also argued that proper use of actuator redundancy can simultaneously increase the workspace remove singularities and dramatically improve overall kinematic structural and actuator performance while keeping the complexity low This paper discusses a prototype shoulder joint more appropriately described as a combinatorial mechanism which exhibits the features Additional benefits in terms of modularity self calibration reliability self test and degraded modes of operation are briefly discussed in the conclusion 1 Introduction While a great deal pf research activity is devoted to the design of robot manipu lator wrists see 18 and articulated hands the design of other joints or groups of joints is not less challenging A robot manipulator like most technological creations automobiles airplanes excavators etc is an integrated machine Weakness of any of its constituents a
5. ature s solution to these four problems In Experimental Robotics 3 Yoshikaswa T Myiazaki F Eds Lecture Notes in Control and Information Sciences 200 Springer Verlag pp 297 309 306 sides of pistons or vanes Details on the properties and compensation control of these actuators can be found in 1 2 6 Structural Design and Collision Avoidance The structural design was carried out while attempting to take full advantage of the opportunities offered by parallel linkage mechanisms For example pistons as opposed to rotary vane actuators can be used as structural members and thus economically contribute to strength Piston actuators have in general numerous structural advantages over rotary actuator while being simpler to manufacture and maintain and they have greater efficiency The principal structural parts are listed by order of decreasing design dif ficulty a Central stem 1 unit b Double Hooke joints 2 identical units c Actuator cradles 4 identical units d Double actuator forks 2 identical units e Output flange 1 unit f Ground link 1 unit As it can be seen the number of parts is quite reasonable are they can be made to have rather simple shapes The loads types and magnitudes in each bearing was examined and correspondingly sized In the current state of prototype no systematic structural optimization was carried out using finite element methods for example but that could be done in t
6. cali bration sensors kinematics and dynamics Initial success in this direction has been achieved 16 Self testing is natural extension of self calibration Work is under way to investigate the automatic diagnostic of partial structural actua tor and sensor failures as well as backlash Finally there is built in provision for degraded modes of operation with missing sensors missing actuators and minor structural failures Moreover backlash can be actively compensated At the time of this writing an elbow joint using two actuators identical to those used in the shoulder is under construction A compact wrist mechanism with four smaller actuators is also in the design stage All these joints are based on design principles described in this paper and will result in a manipulator In Experimental Robotics 3 Yoshikaswa T Myiazaki F Eds Lecture Notes in Control and Information Sciences 200 Springer Verlag pp 297 309 308 arm with high dexterity high strength to weight ratio powered by 6 replicated piston actuators and 4 smaller ones also identical 5 Acknowledgments I am indebted to many people who contributed to this project in various ways in particular Dr Fraser Smith and David Barret from Sarcos Re search Corp for their first class engineering design and prototype realization Prof Stephen C Jacobsen for his encouragements and valuable comments Benoit Boulet and Ronald Kurtz for their excellent work Prof Laee
7. ctuators sensors structural properties materials kinematic properties dynamic properties control techniques and many more factors will affect dramatically the performance of the complete machine 5 Note that the great majority of existing manipulator arms man made or found in Nature can be viewed as the assembly of three joint groups the shoul der group which provides for the overall orientation or the pointing direction of the manipulator the elbow group which provides for radial extension or reach and the wrist group which provides for the final orientation of the effector Of course there are exceptions to this rule some crustacean or insect limbs for example are not so clearly organized This rule nevertheless appears to be well enforced for all larger animals Among man made manipulators in use in the manufacturing industry weld ing painting assembly part handling deburring polishing etc the author has seldom seen an exception to this rule In the application of robotics for in tervention in remote or hazardous locations this rule seems to be followed even more meticulously for undersea outerspace and other applications among many existing or proposed designs that the author has surveyed the only ex In Experimental Robotics 3 Yoshikaswa T Myiazaki F Eds Lecture Notes in Control and Information Sciences 200 Springer Verlag pp 297 309 298 ception found are the variable truss manipulators
8. d since the platform is constrained to a spherical motion These sign relationships are captured by the signs of each entry in the Jacobian matrix of the mechanism s kinematic map 8 Thus there is an upper bound of 2 of such topological regions but at this point there is no proof that they all exist The transition from one region to another is illustrated in the Fig 6 during a swiveling motion with no tilt 2 2 Expected Performance The study in 13 indicates that the theoretical optimal workspace free of sin gularities all parts with zero thickness is as large as 180 of tilting motion In Experimental Robotics 3 Yoshikaswa T Myiazaki F Eds Lecture Notes in Control and Information Sciences 200 Springer Verlag pp 297 309 301 Figure 6 The platform swivels two of the actuators contributions to the swiveling torque vanish while the other two s reach their maximum mechanical advantage A similar situation occurs for all the topological region transitions which explains the exceptional extent of the working region having high kine matic conditioning in both directions and 270 of swivel Of course this ideal performance must be compromised for kinematic performance structural properties and space to lodge bearings and actuators What has actually been achieved in the first prototype is about 90 of tilting motion in both directions and 180 of swivel These figures are expected to be improved substa
9. ding all the mechanism s bearings 2 5 Actuators It is suggested here that the actuator analysis problem in robotics might be split into four items e Energy Storage Under this heading one considers the form under which the energy needed to actuate the robotic device is stored In a manufacturing application this question of is small importance since it can reasonably be assumed that an unlimited supply of energy is avail able In other applications of robotics this item can play a major role for example for planetary explorations vehicles or untethered submarine rovers in which case a capacious energy storage must be part of the robot itself e Energy Transport One considers the method by which energy is trans ported from storage to a final stage where it must be available in me chanical form In most cases this function is performed in multiple steps For example in an electric industrial robot energy is transported in elec trical form converted into mechanical form in the manipulator s motors and then most often once again mechanically transported to the joints via gears shafts belts chains cables tendons hydraulic or pneumatic conduits or other mechanical energy transmission techniques A direct drive robot is characterized by the absence of mechanical transmission of energy except by the structure of the manipulator itself hence most hydraulic robots are direct drive e Energy Throttling One conside
10. f the prototype In Experimental Robotics 3 Yoshikaswa T Myiazaki F Eds Lecture Notes in Control and Information Sciences 200 Springer Verlag pp 297 309 299 2 Design The major characteristic of a manipulator shoulder joint is that it works with the worst possible mechanical advantage and yet its bulk and weight must be minimized although this last requirement is obviously less critical than it is for distal joints In the applications of robots in hazardous environments this requirement is nevertheless crucial because a manipulator is typically not a grounded structure but instead is supported by a gross positioning device a vehicle a boom or a crane In 7 a promising architecture to achieve light weight high mobility and favorable structural characteristics is discussed and represented in Fig 2 Figure 2 On the right a spatial version of the planar case shown on the right is diagrammed The kinematic structural concept is shown on Fig 3 According to the prevailing terminology such a mechanism is termed actuator redundant The author much prefers the term combinatorial because this adjective describes more accurately the underlying concept No actuator is redundant all are used and well Their combined utility can easily be appreciated by observing the dramatic loss of performance of the entire system when one actuator in removed much more than 25 anyway we look at it Interestingly the loss of an
11. h displays advantages over the piston driven McCallion Truong design 14 While this design reduces the number of pas sive joints by having a pair of pistons share two revolute joints in a double Hooke joint arrangement further saving can be obtained by noticing that a single revolute joint is needed to orient a plane the plane containing the pan tograph This fact was also noticed by Dunlop et al 6 who further pointed out that this would save torsion decoupling bearings for linear sensors mounted on screw actuators The combination of these ideas to the case of piston ac tuators leads to a simple structure illustrated Fig 7 A structurally sound implementation of this chain can be realized with 25 bearings which saves al most half the number of bearings as compared to the straightforward design Figure 7 Complete kinematic chain In Experimental Robotics 3 Yoshikaswa T Myiazaki F Eds Lecture Notes in Control and Information Sciences 200 Springer Verlag pp 297 309 304 The bearings although supporting large loads could be made very small and yet have little backlash thanks to a technique developed at the Center for Engineering Design at the University of Utah Finally yet another benefit of actuator redundancy contributes to ensure the elimination of the residual backlash In any given orientation of the mecha nism internal stresses can be actively created independently from the external load thus bias loa
12. he future It is encouraging to see that the first sizing estimates used in this first prototype already produced quite a light and compact system Certain deficiencies were noticed and will have to be corrected in future prototypes The main culprit in the reduction of the practical workspace is the possi bility of self collisions well within the singularity free work region This topic is not studied in any systematic fashion because of the great difficulty in its formalization General guidelines have been followed to alleviate this problem Ideally all parts should be as small as possible but of course this approach has its limits Except for the output flange which is made of aluminum the material se lected for the construction of most of the structure is stainless steel to optimize the strength to size ratio of its parts 3 Experimental Results The shoulder joint was tested in the laboratory using a L shaped member designed to approximate the construction of a complete arm It is made of two steel tubes welded at a right angle with lengths 0 3 m each It is bolted to the output flange shoulder such that the axis of first tube coincides with its axis of symmetry This member is quite rigid and has a mass of 1 6 Kg An additional load mass of 1 4 Kg is affixed to the end of the second tube The moments of inertia of this composite load about the joint center of rotation are approximately 0 62 Kg m 0 48 Kg m and 0 15 Kg m In an attem
13. ical design method global conditioning actuator forces minimization and global gradient index The second and third indices were introduced for the first time _ Jy Didw _ Jw1 k I dw Ds fydw fy dw o R Sy Fidw _ Jy 1 03dw 2 Swat Sy dw GD max GD max V Dil 3 Referring to Fig 3 it was found that the general case mechanism could be made isotropic with a conditioning ideally flat for the values lp lp 2lg units are irrelevant since it is a spherical mechanism its properties are scale invariant In practice for a shoulder mechanism there is no reason why it should be isotropic Actually it should have greater strength around the axis working against gravity The scale of the device is directly related to the stroke of the actuators Due to the vast number of actual design parameters a trial and error was used to precisely determine the final dimensions of the device first with the con struction of a series of physical models of increasing fidelity and then checking with the indices listed above that performance was indeed acceptable Most importantly the decision tree was rooted in the actuator choice see section 2 5 At the first level many decisions were dictated by the necessity of piston type actuators to have their shortest length exceed their maximal stroke From this observation attaching the cylinders by their ends would necessarily lead to poorly conditioned mechanisms To overcome this difficulty the cy
14. linders must protrude and be placed in cradles thus ruling out use the square plat form Fig 3 as the output link The final dimensioning decisions where made by considering avoidance of self collisions and structural strength see section 2 6 The entire decision process resembled searching for a Nash equilibrium conflicting objectives in game theoretic optimization problem 20 In Experimental Robotics 3 Yoshikaswa T Myiazaki F Eds Lecture Notes in Control and Information Sciences 200 Springer Verlag pp 297 309 303 2 4 Reduction of Passive Joints Count One fundamental disadvantage of parallel mechanisms particularly spatial mechanisms is the need for numerous passive joints In the case studied in this paper a straightforward implementation of the diagram shown on Fig 3 replacing each spherical joint by a three axis gimbal each with two forks and providing for torsion decoupling bearings for linear displacement sensors piggy backed on the pistons leads to a large number of bearings 45 Unless great care is exercised with respect to the strength and precision of all of these joints wear backlash or failure not mentioning high cost is to be expected defeat ing the supposed advantages of parallel mechanisms This problem is further compounded by the lack of room usually available to lodge these bearings Inoue et al 11 designed an iso static platform based on three panto graph 5 bar mechanisms whic
15. lly parallel robot Proc 5th Int Conf on Advanced Robotics Pisa Italy IEEE Press Rosheim M E 1989 Robot wrist actuators John Wiley amp Sons Regan B 1991 ATLAS 8F Advanced Bilateral Manipulator System In Proc of ROV 91 Conference Marine Technology Society Washington DC Vincent T L 1983 Game theory as a design tool J of Mechanisms Trans missions and Automation in Design Trans of ASME Vol 105 No 1 pp 165 170 Yoshinada Hiroshi Yamazaki T Suwa T Naruse T 1992 Design and control of a manipulator system driven by seawater hydraulic actuator Proc Second Int Symposium on Measurement and Control in Robotics ISMCR Tsukuba Science City Japan pp 359 364
16. ment of inertia of the order 0 01 Kg m Considering that the joint can produce a torque of 200 N m with a low supply pressure of 3 3 MPa 500 psi flat within a bandwidth of at least 100 Hz in isometric conditions around any of the three rotation axes its performance can be quite remarkable We are now in position to examine the tradeoffs which were confronted to come up with a practical shoulder joint In Experimental Robotics 3 Yoshikaswa T Myiazaki F Eds Lecture Notes in Control and Information Sciences 200 Springer Verlag pp 297 309 302 2 3 Dimensioning The investigations carried out to determine the range of parameters for a use ful device resulted in several important observations reported in 13 It was found that conventional kinematic indices could in fact be quite misleading in the search for a useful device if physical limitations were not taken into account Another important and related observation is the low sensitivity of kinematic performance as a function of the design parameters except at scattered trouble spots This was typified by the need to use log scales to concisely summarize results The upshot of the study is that kinematic design for manipulators is better described as the avoidance of debilitating conditions rather than targeting for sharp optima When J is the manipulator Jacobian o1 02 03 are its singular values k J 1 03 is its condition number three such indices were used in a hi erarch
17. ntially in a future prototype The large workspace increase makes it possible to take full advantage of the efficiency of parallel linkages as discussed in 9 In effect during most motions of the joint all four actuators move each contributing mechanical power combined at the output link because the actuator efforts sum and they all move The worst case occurs when two actuator velocities vanish in which case these actuators may only play a structural role The best case occurs when all four actuators move at the same velocity and exert the same effort In a competing serial arrangement during movements where the joint is kinematically well conditioned only one actuator is producing mechanical power the others have low velocity and uselessly dissipate power to support their neighbors while the strength of the joint is limited by the weakest link The worst case occurs when the joint is near a singularity when some actuators move at high speed exerting little effort thus operating at low efficiency Moreover in a serial arrangement the stiffness of the group is limited by the stiffness of the weakest joint whereas in the parallel case their stiffness add As a result of the previous observations excellent strength to weight ratio and power to weight ratio are expected over a large workspace made possible by actuator redundancy The moving mass lumped at the output link is about 1 Kg at a lever arm of about 0 1 m resulting in a mo
18. ombustion engine This is the method used in many pre robotic devices such as cranes excavators forestry or mining equipment In submarine applica tions high levels of power may be made available through umbilical cables With hydraulics the transport function is accomplished with hydraulic lines which must be run from the source of pressure to the actuators Lines have their own dynamical properties which are actually best described as distributed parameters systems For this reason their dynamics are not simple and they can account for significant losses in energy and performance Hence compact high bandwidth valves that can be co located with the actuators themselves are quite advantageous from a control view point With this technique excellent performance using local compensation can be obtained even with long and ill modelled lines The valve itself is only a part of the energy throttling mechanism since it is driven by an electromagnetic motor itself driven by an electronic amplifier In the case of the ASI actuators it was found that the electromagnetic energy conversion mechanism was in fact a limiting factor in the performance of the complete actuator The last stage of the energy throttling mechanism is based on forcing the fluid through orifices whose areas can be controlled and the last stage of energy conversion is accomplished simply by letting the fluid pressure act differentially on the 2It a worthwhile to reflect on N
19. pt to decouple the basic performance of the design from that of the various contro algorithms currently under investigation the simplest feed back controller is used It is a proportional position feedback controller about each actuator augmented by a lag term for improved steady state accuracy In Experimental Robotics 3 Yoshikaswa T Myiazaki F Eds Lecture Notes in Control and Information Sciences 200 Springer Verlag pp 297 309 307 The controller is implemented digitally at a servo rate of 1 kHz We have also implemented a force regulator of similar design We executed a small step motion about the axis of highest inertia in the middle of the joint range In this condition the shoulder is capable of producing accelerations of 130 m s at the tip where the wrist joint is to be located and a maximum tip velocity of 0 45 m s was observed The maximum static force produced at the wrist varies from a worst case of 200 N to a best case of 400 N according to direction of the exerted force at the center of the shoulder joint range The force control bandwidth is observed to be about about 100Hz Position resolution could not be measured with the available equipment The basic low velocity and force resolution performance is not a function of the design since it can be qualified of being direct drive there are no motion transmission elements other than simple linkages interposed between the actuators and the load Low velocit
20. que Daneshmend for his contribution to modeling and control of actuator systems Chafye Nemri and Duncan Baird for their engineering support Finally I would like to thank Profs I W Hunter and J M Hollerbach for their support within the project High Performance Manipulators C3 funded by IRIS the Insti tute for Robotics and Intelligent Systems part of Canada s National Centers of Excellence program NCE Additional funding was provided by a team grant from FCAR le Fond pour les Chercheurs et l Aide la Recherche Qu bec and an operating grant from NSERC the National Science and Engineering Council of Canada References 1 Boulet B Daneshmend L K Hayward V Nemri C 1993 System Identifi cation and Modelling of a High Performance Hydraulic Actuator Experimental Robotics 2 Chatila R Hirzinger G Eds Lecture Notes in Control and Information Sciences Springer Verlag 2 Boulet B Hayward V 1993 Robust control of a robot joint with hydraulic actuator redundancy Part I Theory application and controller design Part II Practical variations and experimentation TR CIM 93 6 and TR CIM 93 7 Technical Report McGill Center for Intelligent Machines McGill University Montr al Canada 3 Burdick J W and Chirikjan Hyper redundant robots kinematics and experi ments in Preprints of 6th International Symposium on Robotics Research Oct 2 5 Hidden Valley PA 4 Charentus S Renaud M 1990
21. rs here the method whereby energy is throttled to ensure control over the robot Energy throttling is hard to achieve mechanically clutches do that inefficiently Among a large number of possibilities electric energy throttling is usually the method of choice even in a multi step process as for electromagnetic servovalves Regardless of the type of energy being throttled there are three basic methods In the first method constant power is delivered by the supply and the throttling mechanism arranges for a variable amount of the effort to be diverted to the load usually by creating an effort imbalance An electric version of this principle leads to class A amplifiers a hydraulic version of it leads to the jet pipe hydraulic valve In the second method the power delivered by the supply varies with that delivered to the load class B limiting the loss in the throttling mechanism But in this case the throttling mechanism must have large bandwidth because the In Experimental Robotics 3 Yoshikaswa T Myiazaki F Eds Lecture Notes in Control and Information Sciences 200 Springer Verlag pp 297 309 305 power must be switched from throttling to quiescence and vice versa at each reversal of the direction of the energy flow In the third method in some sense a limiting case of the previous case power is switched and switching timing is used to throttle power Then it must have even higher bandwidth to approximate ideal s
22. utomation pp 1610 1616 Hollerbach J M Hunter I W Ballantyne J 1991 A comparative analysis of actuator technologies for robotics In Robotics Review 2 Khatib O Craig J Lozano Perez T eds MIT Press Inoue H Tsukasa Y Fukuizumi T 1986 Parallel manipulator In The Third International Symposium on Robotics Research Faugeras O E Giralt G Eds MIT Press pp 321 327 Jacobsen S C Smith F M Backman D K and Iversen E K 1991 High performance high dexterity force reflective teleoperator II in Proc ANS Topical Meeting on Robotics and Remote Systems Kurtz R Hayward V 1992 Multi goal optimization of a parallel mechanism with actuator redundancy IEEE Transactions on Robotics and Automation Vol 8 No 5 pp 644 651 McCallion H Truong P D 1979 The analysis of a six degree of freedom work station for mechanized assembly Proc 5th World Congress on the Theory of Machines and Mechanisms pp 611 616 Morecki A Malczyk G 1987 Mathematical Model of a flexible manipulator of the elephant s trunk type in Proc Sizth CISM IFToMM Symposium on Theory and Practice of Robot and Manipulators Morecki A Bianchi G Kedzior A Eds Hermes Paris Navhi A Hollerbach J M Hayward V Calibration of a parallel robot using multiple kinematic loops Submitted to 1994 IEEE Int Conf on Robotics and Automation Pierrot F Dauchez P Fournier A 1991 Hexa a fast six dof fu
23. witching This topic is not often studied in robotics although it is quite a thorny problem more often than generally assumed example variable reluctance electric actuators e Energy Conversion Under this last item one must examine which principle is applied to convert a transportable form of energy into me chanical energy The reader is referred to 10 for a recent survey Most actuator systems accomplish transport throttling and conversion in multiple steps Hydraulic techniques have been selected for use in this shoulder joint prototype enabling direct application in areas in which new manipulator designs are in need in particular telerobotics In this area often the levels of performance are attainable only with hydraulic actuation Also the recently available force controlled ASI integrated hydraulic actuators offered a path to ward rapid creation of a high performing prototype In a number of telerobotic applications there is need for human like performance in terms of bulk pre cision strength reach and dexterity Again only hydraulic actuators can be made compact enough to even consider approaching the level human perfor mance Thus with this in mind the four items listed above can be discussed Because of the large amount of power required by hydraulic actuators the energy storage can only be utility supplied or be stored chemically in con junction with a first conversion stage provided by a turbine or an internal c
24. y performance and force resolution depend completely on the actuators sensors and local feedback control To date experiments in this area have been limited solely by the resolution of the 12 bits analog to digital converters used for digital control 4 Conclusion A manipulator shoulder joint design based on a combinatorial mechanism which addresses several issues of concern in telerobotics was described A large workspace was achieved despite the use of parallel linkages thereby offering an opportunity to take advantage of their possibilities in terms of structural strength and precision The joint is efficient because any motion in any di rection combines the contribution of at least 3 and almost everywhere of 4 actuators where a serial joint could use only one actuator They are also ad vantages from a control view point in terms of impedance modulation and control robustness when compared to a serial arrangement as described in 2 As a result of the preceding items a dramatic improvement in power to weight ratio has been achieved while construction was kept simple and the number of parts minimized while low pressure hydraulics led to a significant torque output Moreover four identical piston type actuators which are the simplest to manufacture and to maintain were used exclusively This design provides a path toward the development of a highly reliable system since it has built in redundancy There is natural provision for self
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