Introduction and Initial Exploration of an Active/Passive ExoskeletonFramework for Portable Assistance*

Robert Peter Matthew?1, Eric John Mica?2, Waiman Meinhold?2, Joel Alfredo Loeza?2,Masayoshi Tomizuka2 and Ruzena Bajcsy1

Abstract— Assistive devices such as exoskeletons are capableof providing rehabilitative improvement and independence forindividuals suffering from musculoskeletal conditions. Typicaldevices use either active assistance methods such as DC motorsor passive methods such as springs. Active methods requirea continuous power input, while passive methods are limitedby user capability. This work introduces an Active/PassiveEXoskeleton (APEX) framework. This device can passivelyprovide continuous assistance, only requiring energy to changethe dynamic properties of the passive state. The first prototype(APEX−α) is introduced and tested on six healthy subjectswho performed hammer curls. It was found that changes inthe passive state of the APEX−α affect the number of curlsperformed by an individual. By changing the passive stateof the exoskeleton, increases in curl count of 65 − 92% wereobserved. This indicates the potential for such devices to provideassistance to an individual through the use of lightweight,energy efficient active/passive actuators.

I. INTRODUCTION

Assistive devices are capable of augmenting the abilities ofan individual. While numerous upper-limb assistive devicesexist, the majority are stationary platforms designed forrehabilitative tasks. One of the major challenges when de-signing portable assistive devices is operational life. Portableassistive devices are typically actuated using active electricmotors requiring a continuous power draw. This necessitateslarge electric cells to offset low operation times.

This work introduces an active/passive actuator frame-work. These actuators provide assistance passively, withoutany power draw. Power is only required to change thedynamic properties of the passive state.

A. Prior Work

There is substantial literature on the use of assistive de-vices to improve task performance and aid rehabilitation. Asa result, a number of clinical reviews suggest robot assistedtherapy can improve upper-limb motor control in strokepatients[1][2]. Patients undergoing robot assisted therapyhave scored higher in clinical tests than patients undergoingconventional treatment for the upper[3] and lower[4] limbs.

*This work was supported by NSF grants 1354321 and 1362172? These authors contributed equally to this project1Robert P. Matthew and Ruzena Bajcsy are in the department of

EECS at the University of California at Berkeley, USA {rpmatthew,bajcsy}@berkeley.edu

2Eric J. Mica, Waiman Meinhold, Joel A. Loeza and MasayoshiTomizuka are in department of Mechanical Engineering at the Universityof California at Berkeley, USA {ejmica, wmeinhold, jaloeza,tomizuka}@berkeley.edu

The majority of these assistive systems are driven byelectric motors[5][6], allowing various controllers to beimplemented and evaluated. From these studies, impedancecontrol is frequently chosen as a high level controller[7], re-sulting in compliant assistance. The drawback to this form ofcontrol is the high reliance on a detailed dynamic model[8].Inaccuracies in this model can result in poor assistance andinstability.

In contrast to these actively driven systems, there area number of passive systems that use material compli-ance to provide gravity compensation and assist in taskperformance[9][10]. These devices are driven solely by thewearer, reducing the risk of muscular atrophy. One suchdevice that has been used clinically is the WREX[11][12],which uses elastic bands to provide passive gravity compen-sation about a fixed point. This has led to the development ofthe (Pneu-WREX)[13][14], which uses pneumatic actuatorsto assist with upper arm rehabilitation. The Pneu-WREXcombines passive springs for gravity compensation with apneumatic system to provide active force assistance. Thistype of pneumatic system is restricted to use in rehabilitationclinics .

In addition to stationary rehabilitation devices, there area number of portable assistive devices. These devices arefrequently electrically powered and have limited operationtimes that have yet to exceed three hours[15][16].

In conjunction with developments in human kinematic anddynamic modeling[17][18], and methods for determining theoptimal passive parameters to achieve a task[19], this workaims to advance assistive device design.

B. Contributions

This work introduces a novel active/passive frameworkfor assistive devices. The APEX-α, designed for elbowassistance, is introduced and initial experimental results areshown. With variation in only initial parameters, a perfor-mance range of 62%− 192% was observed.

This work demonstrates the utility of the proposed novelactuator and controller methodology on a cohort of sixhealthy individuals performing hammer curls with their rightarm. From these experiments, the device was found toprovide significant changes to an individual’s performancewith a small initial energy input. The resulting device iscompact, has a low mass and is capable of providing portableassistance to the elbow joint.

2015 IEEE/RSJ International Conference on Intelligent Robots and Systems (IROS)Congress Center HamburgSept 28 - Oct 2, 2015. Hamburg, Germany

978-1-4799-9993-4/15/$31.00 ©2015 IEEE 5351

Fig. 1. Schematic of APEX-α actuators. The pair of cylinders are shownin parallel; their upper and lower chambers are labeled Pextension andPflexion respectively, to correspond with the direction of the resultingelbow rotation when expanded. The supply pressure (Psupply) and atmo-spheric pressure (Patm.) as well as the valve array are shown. The valvearray consists of one 3-way valve and two 2-way valves. While both thePext. and Pflex. chambers are pressurized from Psupply and vented toPatm., the valve array allows chamber pressures to be set independently.

II. FRAMEWORK

Fluidic actuators have traditionally been deemed inad-equate for wearable robotics because of their lack ofportability[20]. The APEX-α overcomes this obstacle byutilizing active/passive actuators, not requiring power oncethe dynamic parameters have been defined. This dramaticallyreduces the amount of working fluid required for operation,eliminating the need for a heavy, tethered working fluidsupply.

A. Terminology

Active / Passive StateThe APEX is in the active state when changes are madeto the valve array (see Figure 1). When these valves arenot altered, the APEX is in the passive state.

Actuator StateThe actuator state is defined by the pressures in theflexion and extension chambers (Pflex., Pext.) and theinstantaneous linear position and velocity of the cylinderrod (x, x, x).

Actuator ParametersThese parameters consist of the pressures and displace-ment of the actuator set in the active state. The actuatorparameters define the relation between the actuator states.

ModeAn exoskeleton mode is defined by a predetermined setof actuator parameters.

B. Active/Passive Actuation

Active/passive actuators are able to operate without con-tinuous power consumption. Power is only required whenchanging the actuator parameters, defining the dynamicresponse. Once the actuator parameters have been set, theactuators operate passively until an additional change of stateis required.

Fig. 2. APEX-α backplate sub-assembly. The rod ends of the actuatorsare connected with a clamping mechanism. This mechanism is secured tothe cable loop so linear translation of the actuators results in rotation of thearm sub-assembly pulley (Figure 3).

Fig. 3. APEX-α arm sub-assembly. The cable ends are fixed to the pulley.

The APEX−α aims to provide the user with assistanceusing active/passive actuators. The actuators are a pair ofidentical fluidic dual-action cylinders running in parallel.These cylinders behave as fluidic springs with variable stiff-ness when both chambers are sealed (Figure 1). The aim is tochange the dynamic response of these actuators by changingthe initial pressure, defining the amount of working fluid ineach chamber.

The aim of the active/passive framework is to tune thedynamic responses to a specific set of tasks. A passive modecould be set to change the equilibrium point of an arm givena load, reducing the required user torque to move in a givenworkspace such as box stacking. A second passive modecould increase the damping at a joint to reduce vibration dueto using a tool. The assistive device can remain in a givenmode without requiring any additional energy as the dynamicstate has been set, regardless of the number of boxes stacked,or duration of the shock absorption. An energy input isonly required when when the user wishes to actively changemode: changing from a stacking task to vibration dampingtask. In this manner energy can be conserved by choosingparameters that can assist a user effectively for a set of tasks,maximising the time in each passive mode, and minimisingactive switching.

C. Hardware

The APEX-α is comprised of two sub-assemblies. Thefirst sub-assembly is a backplate1 system which houses theactuators2 and a pulley (Figure 2). The backplate distributesthe device’s center of mass closer to the user’s and limitschanges to the effective limb mass and moment of inertia.The total mass of the APEX−α is 2.54kg with only 0.39kgon the user’s arm.

1Plate taken from FOX Titan Sport Jacket: shop.foxhead.com2Bimba Original Line Air Cylinder, Stainless Steel, 3 inch stroke, 0.75

inch bore www.bimba.com

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Fig. 4. User wearing the APEX-α. The arm sub-assembly is mountedto the user’s upper arm and forearm with Velcro R© straps so that thepulley is aligned with the user’s elbow. The backplate sub-assembly is alsosecured with Velcro R© to the user’s back and waist. The red pneumatic linescorrespond to Pext., Pflex., and the valve array; in (Figure 1): yellow,purple, and cyan respectively.

The second sub-assembly, the arm, is a cable driven pulleysystem for the elbow joint (Figure 3). The pulley is fixed toone bar and attached to the other with a bearing. A pairof Bowden cables run through a fixture on the pulley inopposite directions and up through a guide. Crimps on theends of each cable are used to secure the cables to the pulleyunder tension.

The rods of both cylinders are fixed to one cable witha clamping mechanism. The clamped cable runs down thebackplate, around the stationary pulley, and is crimped tothe end of the other cable creating a closed loop. Lineartranslation of the rods results in rotation of the arm pulleyvia the cable drive. The APEX-α has been operated usingpressurised air and carbon dioxide supplies. Both full sizedcompressors and portable cartridge systems have been usedas the working fluid supply for the device.

The APEX-α is attached to the user with adjustablealuminum cuffs3 and Velcro R© straps around the upper armand forearm; the arm pulley is aligned with the user’s elbowjoint (Figure 4). The bars in the arm sub-assembly consistof two separate sections and can therefore be adjusted foreach user. The backplate sub-assembly is also secured withVelcro R© to the user’s back and around their waist with anadjustable strap.

D. Control

To alter the active/passive actuator parameters, a valvearray (Figure 1) sets the pressure in the extension and flexionchambers and then seals them. The valve array can consistof either manually-actuated4 or solenoid-actuated5 valves. Inboth instances, power is only required when changing mode.Once the mode has been defined, the device becomes passiverequiring no additional power.

3Cuffs from Bledsoe Extender Arm: www.bledsoebrace.com4MiSUMi One-Touch Coupling Shut-Off Valves, Part No. BVHVU:

us.misumi-ec.com5Festo Fast Switching Solenoid Valve,

Part No. MHE3-MS1H-3/2G-QS-6-K: www.festo.com

III. EXPERIMENTAL VALIDATIONWhile the APEX framework provides for a wide range

of joint dynamics, hardware level impedance control andassistance capabilities, preliminary testing involved the char-acterization of the effect of a subset of these parameters on asingle joint for a simple activity. Hammer curls were chosenas it concentrates exertion to the flexors and extensors ofthe elbow, while the pronation of the wrist allows for betterergonomic fit to the exoskleton.

A. Terminology

Test An experimental test was defined by a specificexoskeleton mode. Five exoskeleton modes and acontrol were tested (see Table I, Figure 6).

Curl A curl was defined by a complete flexion and exten-sion of the elbow with the weight, one repetition.

Set A set was defined as a series of curls that a subjectwas able to perform consecutively.

B. ProcedureA cohort of six healthy individuals (five male, one female,

aged 23±3 years, mass 73.8±19.4 kg, height 1.77±0.08 m,(mean±s.d.)) were recruited under informed consent6. Eachsubject wore a motion capture suit with active motion capturemarkers7 placed on their right arm, torso and head. Subjectsperformed a set of hammer curls with a dumbell of 3.59kgusing their right arm at a frequency of 0.5Hz (Figure 5). Thenumber of curls performed was counted by the investigator,tabulated, and cross-checked against the motion capture data.Sets continued until failure. Failure was characterized by aninability to maintain the 0.5Hz pace, or a discontinuation bythe subject due to muscular fatigue or discomfort.

Fig. 5. Plane of the hammer curl. Each subject moved their lower arm ina plane parallel to the Saggital plane (shown in blue) about their elbow axisof rotation (dotted line). Directions of Flexion and Extension are shown.

Subjects were asked to perform six tests in a randomizedorder corresponding to a control test and five APEX-α modes.Experimental modes were chosen to evaluate the APEX−αthrough a wide actuator parameter range (Table I). A controltest (C) registered the number of curls an individual couldperform while not wearing the exoskeleton. Each experimentwas repeated three times with a 5 minute gap between sets.The six experiments were performed in two day intervals.

6UC Berkeley IRB 2012-12-48727Phasespace Impulse X2: www.phasespace.com

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TABLE ITABLE OUTLINING EXPERIMENTAL MODES TESTED

Testing Mode P-Flexion(gauge psi)

P-Extension(gauge psi) Line states

C- Control - - -AC - APEX Control Atm. Atm. open

2 Atm. 50 closed3 Atm. Atm. closed4 50 50 closed5 50 Atm. closed

C. Results

The results of the experiments are shown in Figure 7. Toaccount for variations between subjects, each subject’s curlcount was normalised to their corresponding APEX−α con-trol (AC) set. For a given test mode t ∈ {C,AC, 2, 3, 4, 5},subject u ∈ {1, 2, 3, 4, 5, 6} and repetition set r ∈ {1, 2, 3},the normalised value is given by Equation 1.

norm (t, u, r) =#curls (t, u, r)

#curls (′AC ′, u, r)(1)

The normalised curl count for each test and set are summa-rized in Figure 8.

D. Discussion

Figure 7 shows that for every test, there is a changebetween the first and third sets. This trend appears to beindependent across sets, although there is variability amongindividual subjects. The standard deviation of sets 2 and 3 issignificantly lower than that of set 1 for all tests. This changeindicates a fundamental difference between the initial set andsubsequent ones, and a larger degree of inherent variabilityin the initial test.

Figure 7 shows a large variation in total number ofrepetitions by subject, indicating high variability in subjectstrength. Variations in the comparison of the C and AC testsindicate that the APEX−α fit is variable across users. Thegeneral trend for each subject and each test was a decreasein repetitions in consecutive sets, however this decrease isnot constant across users. These observations respectivelysuggest normalization by subject, to the AC test, and by set.

Equation 1 was therefore used in order to obtain thenormalised values reported in Figure 8.

Figure 8 indicates a decrease in repetitions during exper-iment 2 and an increase in repetitions during experiment5. A hindering effect provided by mode 2, is also shown,and reported in Table II. The results from tests 3 and 4are highly variable, with a wide distribution across subjects,however there appears to be a moderate level of assistancein both when compared to the AC tests for each set. Test5 demonstrates a significant increase in the number ofrepetitions performed, with normalised mean improvementsof 65− 92%.

Control test values were higher than those of the AC tests.This effect is most likely due to friction and constrictionby the arm assembly and attachment. The variability in thedegree to which the control was higher than the AC tests

indicates that fit was not constant across subjects. Variationacross subjects for each test is also subject to this fitting.

Tests 3 and 4 demonstrated significantly more variationacross subjects than any of the other tests. The small numberof test subjects and tests precludes any conclusions relatingto this observation, future work may prove informative inthis testing regime.

TABLE IIMEAN AND S.D. OF AC NORMALISED VALUES

Set Mode 2 Mode 51 0.62± 0.12 1.92± 0.532 0.74± 0.12 1.85± 0.473 0.67± 0.18 1.65± 0.48

IV. CONCLUSION

This work details the design and initial testing of theAPEX−α. Experimental data demonstrates the capability ofthe APEX−α to alter an individual’s ability to perform sagit-tal hammer curls. After an initial energy expenditure to seteach mode of the exoskeleton, augmentation was providedentirely passively. By only varying the initial actuator state,an increase in normalised curl counts was observed in test5, and a decrease in test 2. This change is consistent acrosssubjects and sets. Based on these initial experiments, theAPEX−α was able to provide both assistance and hindranceto the user in a repeatable manner in the passive state.

A. Limitations

The work presented is an initial exploration of the APEXframework. While promising, the initial results only cover asmall number of experimental parameters. The test popula-tion was largely homogeneous in age, gender, height, mass,and physiological condition. Due to the small amount of datacollected, any changes between sets were not quantifiable.Changes in each subject over the course of the testingperiod due to muscle gain/recovery were not tracked. Generalsubject variations such as the psychological effect of wearingan assistive device were also not examined.

Disparities in the manner each subject performed thetask were not evaluated. Failure was characterized by aninability to maintain the 0.5Hz pace, or a discontinuationby the subject due to muscular fatigue or discomfort. Thisdiscontinuation was user specific. Variation in subject rangeof motion and axis of rotation were not analysed.

The fit of the APEX−α varied significantly between usersand between testing periods. The exoskeleton was fittedto the subject at the start of each experiment. This ledto variation in the way the device was mounted betweensubjects and for the same subject between days.

B. Future Work

This work has demonstrated the effect of the APEX−α ona population of healthy individuals. More work is needed toexamine the effect of the device on each individual’s range ofmotion and joint parameters. This will enable developmentof precise failure criteria. The motion capture measurements

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Fig. 6. Illustration showing the differences between tests. Side view of the arm is shown, with the shoulder, elbow, hand and weight illustrated. Exoskeletonis represented by the disc overlapping the elbow. Pressures on the Flexion and Extension chambers are shown on the disc. Chambers open to atmosphericpressure are shown in blue. Closed chambers initially set to atmospheric pressure and 50 psi are shown in green and red respectively.

Fig. 7. Number of repetitions measured for each test for each subject. Each subject is denoted by a coloured line. The three sets for each experiment areplotted next to each other in order. Each experiment (Control (C), APEX−α Control (AC), and modes 2-5) is then plotted.

Fig. 8. Normalised number of repetitions (Equation 1). The centre of each box is the mean, the boundaries of the box represent the standard deviation,and the tails denote the maximum and minimum values. Sets 1, 2 and 3 are shown in red, green and blue respectively.

for an experiment can be used to recover the joint angletrajectories and quantify any variation. This analysis couldbe used to more closely examine modes 3 and 4 of theexperiments conducted in this work.

There are also a number of improvements to be made tothe device hardware. Ergonomic improvements need to bemade to ensure a repeatable, comfortable fit. Modeling ofthe actuators and the device need to be performed toquantifythe changes to the joint dynamics. With this model, ac-tive/passive impedance control can be developed, laying thefoundation for a portable, energy efficient assistive device.

APPENDIX

A. Initial Experimental Setup

1) The active motion capture was calibrated using thesupplied calibration tools and software.

2) Subjects donned a form fitting motion capture suit.3) If required by the randomised test, the subject put

on the APEX−α (see Exoskelton Donning Procedure,Section C).

4) Fourteen markers were placed on the subject. The firstset were placed on the right shoulder, inner and outerupper arm, inner and outer lower arm, inside wrist and

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back of the hand. The second set were placed on theinter-clavic notch, left shoulder, nape of the neck, thenthe back, right, front and left sides of the head.

5) A video camera was then started to log the experimentsand marker placement.

6) Both the experimental protocol and the safety guide-lines were then reviewed for the subject.

B. Experimental Outline

1) Subjects prepared themselves to perform hammer curlson a Preacher Curl bench.

2) The active motion capture, and metronome (set at60bpm) were started.

3) Subjects were asked to pick-up the barbell and to startperforming curls in time with the metronome.

4) Experiments continued until failure.5) After failure subjects were given a five minute break.6) Subjects performed three sets.

C. Exoskeleton Donning Procedure

1) The upper arm brace and all mounting straps on theAPEX−α were loosened.

2) The waist strap was attached to the subject and thecuff and upper arm brace were lightly fastened aroundtheir right arm.

3) The elbow joints of the subject and the exoskeletonwere aligned by eye. The two inner straps were thensecured, taking care not to impede the muscle.

4) The lower cuff was adjusted to support the ulna sideof the forearm and tightened.

5) The upper cuff as adjusted to lie in the grove betweenthe tricep and bicep heads, then secured.

6) An air compressor was used to compress air to 120psi. The compressor’s regulator was set to fully closedto set the supply pressure to that of the atmosphere.

7) The APEX−α was connected to the supply line andall valves opened.

8) The subject was asked to fully flex their elbow, andhold it while the suit parameters were set.

9) The regulator was then opened and the pressure set to50 psi.

10) All of the valves in the APEX−α were then closed tomaintain a pressure of 50 psi in both Pflex. and Pext.

11) Based on the desired system mode (Table I), anyvalves required to be opened to the atmosphere werethen opened, with the supply valve being used tosimultaneously vent the required chambers.

12) The final line states were then set as required.

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