MHI.human Power Amp.1996

8/13/2019 MHI.human Power Amp.1996

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THE HUMAN POWER AMPLIFIER TECHNOLOGYAPPLIED TO MATERIAL HANDLINGH. KazerooniMechanicalEngineering DepartmentHuman Engineering Laboratory (HEL)University of California, Berkeley, CA 94720-1740 USAE-Mail: [email protected]

ABSTRACTWe define extenders as a class of robot manipulators worn by humans o augmenthuman mechanical strength, while the wearer's ntellect remains the central control systemfor manipulating the extender. Our researchobjective is to determine the ground rules forthe design and control of robotic systems worn by humans through the design,construction, and control of several prototype experimental direct-drive/non-direct-drivemulti-degree-of-freedom hydraulic/electric extenders. The design of extenders s differentfrom the design of conventional robots because he extender interfaces with the human ona physical level. Two sets of force sensorsmeasure he forces imposed on the extender bythe human and by the environment (i.e., the load). The extender'scompliances n responseto such contact orces were designed by selecting appropriate orce controllers. This papergives a summary of some of the selected researchefforts related to Extender Technology,during the last decadeat the Human Engineering Laboratory (HEL).1. INTRODUCTIONThis article presents an overview of human-power amplification technology beingdeveloped .at the Human Engineering Laboratory (HEL), University of California,Berkeley. Extenders are robotic systems worn by humans to increase human mechanicalability, while the human's intellect serves as the central intelligent control system formanipulating the load. The human becomesa part of the extender, and feels a force thatis related to the load carried by the extender. Figure I shows an example of a hydraulicextender (Kazerooni and Guo 93). Some major applications for extenders nclude loadingand unloading of missiles on aircraft; maneuvering of cargo in shipyards, foundries, andmines; or any application which requires precise and complex movement of heavy objects.The goal of our research s to determine the ground rules for a control system which lets usarbitrarily specify a relationship between he human force and the load force. In a simplecase, the force the human feels is equal to a scaled-down version of the load force: forexample, for every 100 pounds of load, the human feels 5 pounds while the extendersupports 95 pounds. In another example, if the object being manipulated is a pneumaticjackhammer, we may want to both filter and decrease he jackhammer forces: then, thehuman feels only the low-frequency, scaled-down components of the forces that theextender experiences. In the most general case, one may wish an arbitrary relationshipbetween he human orce and load force as a control specification.Three elements contribute to the dynamics and control of this system: the humanoperator, an extender to lift the load, and the load being maneuvered. The extender s inphysical contact with both the human and the load, but the load and the human have nophysical contact with each other. Forces between he human and the extender and forces

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between the load and the extender are measured and processed o maneuver the extenderproperly. These measuredsignals create wo paths of information transfer to the extender:one from the human and one from the load. No other external information signals fromother sources (such as oysticks, pushbuttons or keyboards) are used to drive the extender.The information signals sent o the extender computer must be compatible with the powertransfer to the extender hardware. This is fuI.lY described in (Kazerooni 90b, Kazerooniand Guo 93).

Figure 1: Experimental Six-Degree-of-FreedomHydraulic Extender designed at UC,Berkeley.

2. WORK AT UC BERKELEYIt is important to note that previous systems (Mosher 60, Makinson?1, Clark 62,Groshaw 69) operated based on the master-slave concept (Kazerooni and Tsay 93), ratherthan on the direct physical contactbetween human and manipulator inherent n the extenderconcept. Unlike the Hardiman and other man-amplifiers, the extenders designed at HELare not master-slave systems (i.e. they do not consist of two overlapping exoskeletons.)There is no joystick or other device for information transfer. Instead, the human operator'scommands o the extenderare taken directly from the interaction force between the humanand the extender. This interaction force also helps the extender manipulate objectsphysically. In other words, information signals and power transfer simultaneouslybetween the human and the extender. The load forces naturally oppose the extendermotion. The controller developed for the extender translates his interaction force signalinto a motion command for the extender. This allows the human to initiate trackingcommands to the extender in a very direct way. The concept of transfer of power andinformation signals is also valid for the load and extender. The load forces are measureddirectly from the interface between the load and the extender and processed by thecontroller to develop electronic compliancy in response o load forces (Kazerooni, Waibel91, Kazerooni 90a). In other words, information signals and power transfersimultaneously between the load and the extender. Several prototype experimentalextenders were designed and built to help clarify the design issues and verify the controltheories for various payloads and maneuvering speeds.

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2.1 One-Degree-of-Freedom ExtenderTo study the feasibility of human force amplification via hydraulic actuators,a one-degree-of-freedom extender was built (Figure 2). This experimental extender consists ofan inner tube and an outer tube. The human arm, wrapped in a cylinder of rubber for asnug fit, is located in the inner tube. A rotary hydraulic actuator, mounted on a solidplatform, powers the outer tube of the extender. A piezoelectric load cell, placed betweenthe two tubes, measures he interaction force between the extender and the human arm.Another piezoelectric load cell, placed between he outer tube and the load, measures heinteraction force between the extender and the load. Other sensing devices include atachometer and encoder (with corresponding counter) to measure he angular speed andorientation. A microcomputer is used for data acquisition and control. We developed astabilizing control algorithm which creates any arbitrary force amplification and filtering.This study led to understanding the nature of extender instability resulting from human-

envirnment (load)

hydraulicrotary actuator~ ~during an unconstrained maneuver.2.2 Two-Degree-of-Freedom Direct-Drive Electric Extender

To rapidly maneuver light loads (weighing less than 50 lb), the bandwidth of theextender's actuators must be wider than the human largest maneuvering bandwidh. Thedynamic behavior of the extender system at high speeds s non-linear. To develop and testnonlinear control algorithms, a direct-drive, electrically-powered extenderwas built (Figure3). The direct connection of the motors to the links (without any transmission systems)produces highly nonlinear behavior in the extender. This extender has two degrees offreedom corresponding to a shoulder and an elbow. Two motors are located at the sameheight as the average human shoulder. Force sensorsare located at the human-extenderand extender-load nterfaces. A third degree of freedom may be added: either rotation abouta vertical axis or roll about a horizontal fore-aft axis. Figure 3 shows the seven-bar-linkagemechanism used for our prototype laboratory system. Force sensors are mounted at thehuman-machine and machine-environment interfaces. Motor 2 rotates link 4 causing themain arm (link 6) to move up and down via a four-bar linkage (links 4, 5, 6, and 3 as theground link). In another four-bar linkage (links 1, 2, 3, and 7), motor 1 rotates link 1causing the follower link (link 3) and the main arm (link 6) to move in and out. Bothmotors 1 and 2 are connected to bracket 7 which is mounted on a platform at the sameheight as the human shoulder. A gripper is mounted on link 6 where the operator force, ismeasuredalong two directions. When the human holds onto the gripper, his/her upper arm

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lengthy foot similar to a snow ski. We have developed an actuation mechanism and acontrol technique which stabilize the mass on top of the second ink without the use of anygyro. The control of this under actuated dynamic system is the very first and veryfundamental ssue in the design and control of walking machines for the extender (Pannu,Kazerooni 95).

Figure 4:wears an extender,a computer-drivenXY table was designedand built for measuring thehuman amI dynamics n horizontal maneuvers

load

~ Transmission

iMotor 1Figure 5: -Only one oint ispowered. The goal ofour research zas been o stabilize this under actuated system.

2.5 Hydraulic .Industrial ExtenderA six-degree-of-freedom hydraulic extender (Figure 1) was designed and built formanipulating heavy objects. The extender's hand linkage performs the grasping functionwhile the arm mechanismexecutes he load manipulations. The arm mechanismconsists ofa forearm and an upper aml. The rotational axes of the extender arm are designed to

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coincide with those of the human arm joints. Both the upper arm and the forearm areplanar four-bar linkages. The extender hand mechanism s described in (Kazerooni andGuo 93). Several orce sensors,not shown in the figure, are also mounted at the gripper tomeasure he load force in six directions. The materials and the dimensions of the extendercomponentsare chosen o preserve he structural-dynamic integrity of the extender. Eachlink is machined as one solid piece rather than as an assemblyof smaller parts. The linksare made of high strength7075 aluminum alloy to reduce he weight of the extender.This extender s capable of lifting of objects up to 500 lb when the supply pressureis set at 3000 psi. Since the high frequency maneuvers of 500 lb load is rather unsafe, theexperimental analysis on the extender dynamic behavior was carried out at low level offorce amplification. In order to observe the system dynamics within the extenderbandwidth, in particular the extender nstability, the supply pressurewas decreased o 800psi and low force amplification ratios were chosen for analysis. This allows us tomaneuver the extender within 2 Hz. Force amplifications of 7 times in the verticaldirection and 5 times in the horizontal direction was prescribedon the controller.

30U~.a 20

((';;;;~i\10

0..-10gco. -200N -30'0-1 100 101 102 10'

Frequency (rad/s)Figure 6: Theoreticaland experimentalorce amplification ratio along the horizontaldirection.

Figure 6 shows the FFT of the ratio of the load force to the human force along thehorizontal direction where the load force is more than human orce by a factor of 5. It canbe seen hat this ratio is preserved only within the extenderbandwidth. Figure 7 shows theload force versus the human force along the horizontal direction where the slope of -5represents he force amplification.

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2.6 Electric Industrial ExtenderThe system s composed of two arms and two legs for maneuvering boxes. Toverify perfonnance of each subsystemprior to coordination of the entire machine, the armsand legs have been built separately, as shown in Figures 8 and 9. Each arm has five serialjoints. An additional false degree of freedom exists between he operator's hand and themachine, allowing the operator arbitrary control of his/her hand position and orientationwithin the allowable work envelope. The first three arm oints are powered by actuatorswhich will be described below. The wrist of the machine is comprised of the remainingtwo joints, and the false degree of freedom previously mentioned. A molded rubberhand piece servesas the interface between he operator and the machine. The wrist jointsare unpoweredand are arranged n a spherical configuration with the handpiece at the centerof their axes. The consequenceof this feature is that positioning of a load is accomplishedby actuation of the first three powered joints, while orientation of the end effector isachieved through wrist motions powered by the operator. The wrist has been designed tohave a degenerate configuration for all possible orientations of the wrist in the targetedwork envelope. Such an architecture insures that any oads applied to the end effector willnot manifest hemselves n moments which must be supportedby the operator.

8. The Electric Anns Maneuvering a Box. TheLegs of the Electric Extenders.In between the elbow and the wrist are two mechanisms for adjustment of theposition and orientation of the wrist relative to the rest of the arm. These adjustmentmechanisms are included to explore issues of comfort, and are essentially oints that are

locked in place during machine operation. A three axis piezoelectric force sensor ismounted in between the rubber hand piece and the end effector. Each leg has four serialdegreesof freedom, and in this case, hree additional false degreesof freedom are used inthe interface between the operator and the machine. Similar to the those of the arm, theupper three leg joints are powered. To avoid the use of a long foot to support torque at

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MHI.human Power Amp.1996