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Design and Analysis of a

Human Exoskeleton to Enhance Maximum

ARCHNE

Dynamic Performance

MASSACHUSETTS INSTITUTE

OF TECHNOLOGY_

byMAR

0

2 2016

Michael S. Farid

LIBRARIES

B.S., Massachusetts Institute of Technology (2014)

Submitted to the Department of Mechanical Engineering

in partial fulfillment of the requirements for the degree of

Master of Science in Mechanical Engineering

at the

MASSACHUSETTS INSTITUTE OF TECHNOLOGY

February 2016

@

Massachusetts Institute of Technology 2016. All rights reserved.

Author...Signature

redacted

Department of Mechanical Engineering

-7

Oec

jJ ,-215

Certified by...Signature

redacted

angbae Kim

ssociate Professor

Thesis Supervisor

Accepted by...Signature

redacted

Rohan Abeyaratne

Chairman, Department Committee on Graduate Theses

Dynamic Exoskeleton:

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Dynamic Exoskeleton: Design and Analysis of a Human

Exoskeleton to Enhance Maximum Dynamic Performance

by

Michael S. Farid

Submitted to the Department of Mechanical Engineering

on Dec 11, 2015, in partial fulfillment of the

requirements for the degree of

Master of Science in Mechanical Engineering

Abstract

Most existing research in powered human exoskeletons aims to increase load bearing capability or reduce the metabolic cost of walking. Current exoskeletons are typi-cally bulky and heavy and thus impede the motion of the user. Therefore, they are not suitable for highly dynamic motions. This thesis describes the first attempt to develop a powered exoskeleton suit that improves the maximum dynamic capability of a human. This Dynamic Exoskeleton is intended to enable to the user to run faster, jump higher, or traverse challenging terrain. This thesis presents a study on improving human vertical jump height using a powered exoskeleton. A simple human jump model is created, and dynamic simulation is utilized to determine the effective-ness of actuating the human hip joint for improving vertical jump height. A control system is developed and a series of human experiments with three test subjects are conducted. The test subjects improved their vertical jump heights by 13%, 6% and

5% respectively. The general challenges of actuating human joints and interfacing

with the human body are presented. Thesis Supervisor: Sangbae Kim Title: Associate Professor

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Acknowledgments

I would like to thank Jacob Tims for his incredible help in the design and build of the exoskeleton, and Albert Wang and Joao L Ramos for their assistance in the motor build and for all the parts they were happy to lend throughout the project. I would also like to thank David Otten for his help in configuring the motor controllers, and Professor Neville Hogan for his insignt and ideas. I would like to thank Professor Sangbae Kim for his invaluable guidance and advice throughout the project, and his amazing mentorship and support throughout my masters. Finally, I would like to thank Lockheed Martin for supporting this research.

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Contents

1 Introduction

9

1.1 Designing Powered Exoskeletons . . . . 10

1.2 Selected Design Approach . . . . 11

2 Modeling and Simulation 12 2.1 Hum an M odel . . . . 12

2.2 Human Model with Exoskeleton . . . . 16

2.2.1 Jump Timing . . . . 16

2.2.2 Initial Joint Angles . . . . 18

2.2.3 Conclusion ... ... 19

3 Exoskeleton Prototype

21

3.1 Mechanical Design . . . . 21 3.2 Electrical Design . . . . 23 3.3 C ontrol . . . . 25 3.3.1 EMG Control . . . . 25 3.3.2 Control Algorithm . . . . 27

4 HUMAN TESTING

28

4.1 Testing Procedure . . . . 28

4.2 Jump Height Measurement . . . . 29

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5 Future Work

5.1 Design Challenges . . . . 5.2 New Exoskeleton Prototype . . . .

5.3 Physical Challenges of Actuating the Human Body

5.3.1 Lack of Rigid Attachment Methods . . . . . 5.3.2 Rigid Components Impede Natural Motion .

5.3.3 Large Variation in Human Body Dimensions 5.4 Conclusions . . . .

A Tables

A.0.1 Test Subjects' Age, Height and Weight . . . . B Figures

B.O.2 Current Control on Electric Motors . . . . 32 . . . . 32 . . . . 34 . . . . 35 . . . . 35 . . . . 36 . . . . 36 . . . . 36 38 38 39 39

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List of Figures

1-1 Directly actuating the end effector (foot). . . . . 10

2-1 Four segment human model connected by frictionless revolute joints. 13 2-2 Dynamic simulation of a maximum height vertical jump. . . . . 15

2-3 Joint activation during a maximum height vertical jump. . . . . 15

2-4 Joint torque during a maximum height vertical jump. . . . . 15

2-5 Dynamic simulation of a maximum height vertical jump using a hip exoskeleton. . . . . 16

2-6 Dynamic simulation of a maximum height vertical jump using a hip exoskeleton with optimized activation timing. . . . . 17

2-7 Joint activation during a maximum height vertical jump using a hip exoskeleton with optimized activation timing. . . . . 17

2-8 Joint torque during a maximum height vertical jump using a hip ex-oskeleton with optimized activation timing. . . . . 18

2-9 Dynamic simulation of a maximum height vertical jump using a hip exoskeleton with optimized initial joint angles. . . . . 18

2-10 Joint torque during a maximum height vertical jump using a hip ex-oskeleton with optimized initial joint angles. . . . . 19

3-1 Exoskeleton prototype. . . . . 22

3-2 Overall electrical schematic. . . . . 23

3-3 Exoskeleton custom PCB. . . . . 24

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3-5 EMG data for three test subjects during a maximum height vertical

ju

m p . . . . 26 3-6 Algorithm for commanded torque . . . . 27 4-1 Test subject reaching for the highest tab on the Vertec device. .... 30 4-2 Exoskeleton testing data. . . . . 31 5-1 Problems with the current exoskeleton design. . . . . 32 5-2 Chaffing caused by friction between the thigh and the thigh attachment. 33 5-3 New exoskeleton prototype. . . . . 34 B-i Current tracking plot . . . . 39

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List of Tables

2.1 Optimized Starting Configurations . . . . 19

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Chapter 1

Introduction

There are several ongoing research and commercial projects that aim to improve human performance through the use of powered exoskeletons. The HULC exoskeleton developed by Lockheed Martin is designed to allow soldiers to carry heavy loads and traverse long distances with reduced stress and fatigue on their bodies, therefore decreasing the likelihood of injury. HULC works on the principle of transferring load to the ground through powered titanium legs [1]. The Soft Exosuit is being developed at Harvard to augment the capability of both healthy and impaired individuals by decreasing the metabolic cost of walking [2]. Cyberdine's HAL exoskeleton is designed to allow individuals that have disorders in their lower limbs to improve their walking gait, or to assist healthy individuals in carrying heavy loads for work in factories or elsewhere [3]. Other notable existing exoskeletons include the ReWalk Personal Exoskeleton, by ReWalk, the EKSO by Eksobionics, and the Stride Management

Assist by Honda [4] [5] [6].

Exoskeletons have many medical, industrial and millitary applications to assist human movement. In many exoskeleton research projects, the success metric is iden-tified as the reduced metabolic cost or increased load bearing capability. To our best knowledge, there are no existing actively powered exoskeletons that enhance maxi-mum dynamic performance such as maximaxi-mum running speed or maximaxi-mum vertical jump height. Dynamic performance is selected as the success metric for this project because natural human motion relies heavily on dynamic movements. In order for

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a human exoskeleton to be useful, it should augment and enhance dynamic motion, rather than limit it.

The goal of this project is to create a powered exoskeleton that allows a human to demostrate improved maximum dynamic performance -specifically maximum vertical jump height. A maximum height vertical jump is quite simple to model and simulate and is also a relatively safe way to test the exoskeleton with human subjects. This thesis discusses the design approach of the exoskeleton, modelling and simulation of a human vertical jump, followed by a description of the exoskeleton prototype and results from human testing.

1.1

Designing Powered Exoskeletons

In order to improve dynamic performance, an exoskeleton must increase the force produced at the end effector (the foot) by the leg without restricting the leg's natural motion. This can be accomplished by actuating one or more of the leg's joints (hip, knee, ankle) in order to increase the effective torque produced by that joint, or by actuating the end effector (the foot) relative to the torso without directly actuating any of the joints. Lockheed Martin's HULC exoskeleton utilizes the latter approach, while Honda's Stride Management Asisst exoskeleton employs the former approach.

F.

F' F.

Figure 1-1: Directly actuating the end effector (foot).

The main disadvantage of actuating the end effector relative to the torso is that the exoskeleton must extend from the torso to the foot, thus increasing the inertia

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of the leg and reducing the maximum acceleration and deceleration of the leg when unpowered. Furthermore, this design approach can be challenging to implement using rotary actuators. Conversely, actuating a single joint or multiple joints is potentially unsafe for users, since the leg comprises a complex network of bones, muscles and tendons that are interlinked, so forcibly moving a single joint using an exoskeleton could have adverse effects on other joints or tendons. This risk is inherently minimized by actuating the end effector relative to the torso since the joints are free to orient themselves in any configuration that satisfies the location of the end effector. The risks of directly actuating single or multiple joints on the human body are not fully understood.

1.2

Selected Design Approach

The selected design approach is to actuate the human hip joints. This is due to several reasons. Firstly, the human hip joint does the most mechanical work during fast running and jumping compared to the knee and ankle [7]. Therefore, increasing the torque at the hip joint could allow a human to significantly improve their maxi-mum dynamic performance. Further analysis of actuating the hip joint is presented in Chapter 2. In addition, actuating the hip joint allows the critical mass of the exoskeleton to be concentrated near the torso, thus minimizing the leg inertia. Al-though there may be a risk of injury that is not fully understood due to the actuation of a single joint, it is hoped that this risk is minimized since the location of the hip joint is upstream of the knee and ankle joints.

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Chapter 2

Modeling and Simulation

In order to verify the effectiveness of the selected design approach, a human model is identified and dynamic simulation is used to predict how a powered exoskeleton actuating the human hip joints could improve maximum vertical jump height.

2.1

Human Model

There are several models that have been created to simulate a human vertical jump. M.F. Bobbert and AJ. Van Soest produced a model that accounts for the six major agonist and antagonist muscles involved in a vertical jump motion, and represents those using Hill-type muscle models [8]. Although Hill's muscle model is widely ac-cepted in literature, accounting for the six major muscles in a vertical jump geometry is quite complex. K.B. Cheng produced a simplified model that couples the effects of the major muscles into three torque sources at the ankle, knee and hip. This model has produced results that are very similar to measurements of human test subjects doing vertical jumps [9]. Due to its relative simplicity and validity in modeling verti-cal jumps, K.B. Cheng's model is selected to simulate the effect of a hip exoskeleton on maximum vertical jump height. As shown in Figure 2, the model consists of four rigid links that represent the foot, shin, thigh, and torso. The mass, center of mass, length and moment of inertia parameters for each of the limbs are given in Cheng's study.

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Figure 2-1: Four segment human model connected by frictionless revolute joints.

In Cheng's model, the torque produced at each joint is a product of the maximum torque and three variable factors:

T = Tmaxf(6)h(w)A(t)

(2.1)

The functions

f(6),

h(w) and A(t) are factors between 0 and 1, and Tmax is the

maximum torque that can be produced by each joint. The joint angle factor is the maximum static torque each joint can produce given a certain angle, and is given by Pandy et al. (1990) in extension and Hoy et al. (1990) in flexion, respectively [10],[11]. The angular velocity factor is taken from Selbie & Caldwell, (1996) [121. This equation preserves the characteristics of muscle force production, which depends on maximum isometric force, muscle length, and shortening velocity. The activation factor corresponds to joint activation level, which models the effective stimulation of the muscles across the joint.

The initial angles are given by the initial starting configuration of the test subject in Cheng's study. The activation is given by:

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A

=

Ai,

t

=<

to

A(t) A = Ae((to)/Tda't ), to < t <= to + t1 (2.2)

A

= - e((to+t1-t)/rct)

+

Ae(-(tto)/Tdeact),

t

>

to

+ ti

There is a period of constant activation followed by a period of relaxation (where the simulated human lowers into the jump position), followed by a period of extension activation. Here, Ai represents the initial activation; Tact and Tdeact are muscle activa-tion rise and decay time constants, to is the period for which the initial activaactiva-tion is maintained and t, is the time elapsed from to to the instant when extension activation begins. In Cheng's model, to and t, are optimized to maximize jump height. Jump height is derived from the position and velocity of the center of mass at the time of take-off. The time of take-off is defined as the point in time where the ground reaction force reaches zero, and the initial joint angles in Cheng's study were found by analysing a human test subject.

The dynamic model for a vertical jump was recreated using identical parameters to those used by Cheng. The to parameters were taken from Cheng's study. A similar optimization procedure was used to find t, for each joint with the objective of maximizing jump height. This was done as an excercise to ensure that the simulation had been recreated accurately. As expected, the result of the dynamic simulation is identical to that of Cheng's dynamic simulation and is presented in Figures 2-2, 2-3 and 2-4.

As shown in Figure 2-3, the optimized t, values result in the activation of the knee first, followed by the hip, followed by the anlke. These t, values were identical to those produced by Cheng. Figure 2-4 shows the torque produced at each joint with respect to time. The dynamic simulation produced a jump height of 0.32m.

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1.5 1 r 0.51-0 0 0.1 0.2 0.3 0.4 0.5 Time (s)

Figure 2-2: Dynamic simulation of a maximum height vertical jump.

1 C 0 4-C.) 0.8 0.6 0.4 0.2 0 0 0.1 0.2 0.3 0.4 0.5 Time (s)

Figure 2-3: Joint activation during a maximum height vertical jump.

0 0.1 0.2 0.3 0.4 400 E 300 z 8 200 0 I- 100 0 Time (s)

Figure 2-4: Joint torque during a maximum height vertical jump.

-- ankle knee hip

-/

/

ankle knee hip

5555155

E

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2.2

Human Model with Exoskeleton

In order to evaluate the effect of a hip exoskeleton on a human's vertical jump, the model was modified. 6kg of mass was added to the torso to account for the mass of the actuators, electronics, and frame. 10ONm of torque was added to each hip joint (200Nm total) as a step function at time t1, to account for the torque produced by

the actuators. (A description of the actuators and the overall mechanical design is included in Chapter 3). All other parameters were kept unchanged.

As shown in Figure 2-5, this simulation does not result in a successful vertical jump. The derived jump height is 0.13m, which is significantly lower than the jump height derived from the original simulation which does not model a hip exoskeleton.

1.5 1 0.5-0 0 0.1 0.2 0.3 0.4 0.5 Time (s)

Figure 2-5: Dynamic simulation of a maximum height vertical jump using a hip exoskeleton.

It is hypothesized that human subjects will adapt their vertical jump technique in order to account for the extra hip torque produced by the exoskeleton. Human test subjects could change their joint activation timing (ti for each joint), or their initial joint angles. Both possibilities are explored and tested using dynamic simulation.

2.2.1

Jump Timing

Cheng's simulation was recreated with all parameters (including the initial joint an-gles) unchanged, except the timing parameter t, for each joint. A simple optimization procedure was used to find the new t, for each joint that would yield the maximum

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height vertical jump. The results are presented in Figures 2-6, 2-7 and 2-8. 1.5r 0.5 0 0 0.1 0.2 0.3 0.4 Time (s) 0.5

Figure 2-6: Dynamic simulation of a maximum exoskeleton with optimized activation timing.

1 C 0 CU 4-0.8 0.6 0.4 0.2 n 0 0.1 0.2 0.3 Time (s)

Figure 2-7: Joint activation during a maximum exoskeleton with optimized activation timing.

height vertical jump using a hip

0.4 0.5

height vertical jump using a hip

As shown in Figures 2-7 and 2-8, the optimized ti values result in the activation of the knee first, followed by the anlke, closely followed by the hip. The dynamic sim-ulation produced a jump height of 0.46m, which represents a 44% increase compared to the dynamic simulation without the exoskeleton. While this is an exciting result, the difference in the activation timing parameters between the simulated jump with-out the exoskeleton and the simulated jump with the exoskeleton is significant. The

7.

/

/

ankle -knee hip-I

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400 -- ankle E 300 - knee

z

hip 200 0 100

0

0 0.1 0.2 0.3 0.4

Time (s)

Figure 2-8: Joint torque during a maximum height vertical jump using a hip exoskele-ton with optimized activation timing.

optimized simulation requires that the order of activation of the joints be changed (with the hip leading the ankle). It is difficult to predict whether human test subjects would be able to adjust their jump timing to such an extent.

2.2.2

Initial Joint Angles

A more likely hypothesis is that human subjects will adapt their initial joint angles to

maximize vertical jump height. Cheng's simulation was recreated with all parameters (including the activation timing parameters) unchanged, except for the initial joint angles. A simple optimization procedure was used to find the initial joint angles that maximize vertical jump height. The simulation results are presented in Figures 2-9

and 2-10. 1.5 -1 E 0 0.5 -r 0 0.1 0.2 0.3 0.4 0.5 Time (s)

Figure 2-9: Dynamic simulation of a maximum height vertical jump using a hip exoskeleton with optimized initial joint angles.

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400 400 __ ankle E 30 knee hip 9 200 I- 100

0

0 0.1 0.2 0.3 0.4 Time (s)

Figure 2-10: Joint torque during a maximum height vertical jump using a hip ex-oskeleton with optimized initial joint angles.

The jump height derived from this simulation is 0.49m, which represents a 52% increase in vertical jump height. As can be seen in Table 2.1, the difference in the optimized initial joint angles between the original simulation without an exoskeleton and the simulation that models a hip exoskeleton (with optimized initial joint angles) is small (less than 2.50). This implies that human subjects will not have to drastically adjust their jump initial angles while using a hip exoskeleton, which supports the hypothesis that human test subjects will adjust their initial joint angles to account for the extra torque produced by the hip.

Table 2.1: Optimized Starting Configurations

Without Exoskeleton

With Exoskeleton

Ankle Angle 87.30 88.40

Knee Angle 92.20 92.20

Hip Angle 88.20 85.90

2.2.3

Conclusion

Table 2.2 summarizes the dynamic simulation results. The highest jump was achieved in the dynamic simulation with which modeled the exoskeleton and optimized initial joint angles. As shown above, the new optimized initial joint angles are close to those of the original simulation without the exoskeleton. This result is encouraging, and

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suggests that a hip exoskeleton could improve the maximum vertical jump height for human test subjects.

Table 2.2: Simulation Results

Test

Height

Jump w/o hip exo 0.32m Jump w/ hip exo 0.13m Jump w/ hip exo (optimized activation timing) 0.46m Jump w/ hip exo (optimized initial joint angles) 0.49m

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Chapter 3

Exoskeleton Prototype

3.1

Mechanical Design

Attaching a mechanical exoskeleton to a human body is non-trivial, due to the natural compliance of the body and skin, and the variability in human body sizes. Further-more, transmitting torque to a joint requires parts of the body to withstand some force, which can be uncomfortable.

An exoskeleton was designed and constructed with the primary purpose of actuat-ing the human hip joint duractuat-ing a vertical jump. The exoskeleton is comprised of two carbon fiber thigh attachments that push on front of the users thighs, and a carbon fiber torso part that is attached to the users torso using two commercially available back braces. The first is a lumbar lower back brace with a semi-rigid ABS back plate that supports the lower back. The second is a fabric upper back brace that goes over and around the shoulders to constrain the exoskeleton to the upper torso.

The carbon fiber torso attachment extends from the lower back to the shoulders to spread the load on the torso such that it is distributed between the stomach, shoulders and lower chest as shown in the picture in Figure 1. The upper back brace does not have any rigid components and therefore does not significantly impede the users natural torso motion. A traditional climbing harness goes around the human subject's upper thighs to prevent the torso attachment from moving up the torso (due to the reaction force on the motors) during jumps.

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Figure 3-1: Exoskeleton prototype.

The major mechanical components are made of carbon fiber to minimize the weight of the exoskeleton but maximize stiffness. The mass of the assembled pro-totype is about 6kg, which is consistent with the parameter used for the mass of the exoskeleton in the dynamic simulation. The brushless, 3-phase motors are very similar to those designed for use on the MIT Cheetah Robot [13]. The mass of each motor is about 1.5kg, and the peak torque is 180Nm. Each motor has a single stage planetary gearbox with approximately a 6:1 gear ratio, which allows the motor output to be highly back drivable.

The exoskeleton has three degrees of adjustability to account for the variety of human body sizes. Through the use of slots, the horizontal and vertical location of the motor relative to the torso can be adjusted. Additionally, the motor output allows for an abduction degree of freedom on the thigh. Although this design does not strictly allow for the hip to rotate, the carbon fiber thigh attachments are designed to be quite loose fitting to allow the thigh to move within the thigh attachment.

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3.2

Electrical Design

All of the necessary electrical components are mounted on the exoskeleton, except for

the batteries, which are off-board in order to minimize risk and reduce weight. An overall schematic of the electrical system is shown in Figure 3-2.

Analog Voltage (-3V to 3V) Microcontroller Bluetooth UART U RT TT

oRS422

TTL to RS422 Conurter Converter Encoe RS422 Encoder

mattlerstr

Power ower

Figure 3-2: Overall electrical schematic.

T e he microcontroller used is the PSoC P by Cypress Semiconductor. The motor controllers are designed specifically for use on the MIT Cheetah Robot [14]. The ficrocontroller communicates with the motor controllers using a serial protocol. The encoders communicate directly with the motor controllers. An internal current con-trol loop is implemented on the motor concon-trollers that allows them to be used in torque mode; the mnicrocont roller commands a desired current at a frequency of I kHz, and the motor controllers report back the actual current and encoder position (further details on the current controller are provided in Appendix B). Data logging is achieved wirelessly using a low-power Bluetooth module. Two EMG sensors are used to track activation in the gluteus maximus muscles (further details in Section

3.2). The signal is amplified using a dedicated EMG amplifier manufactured by

Del-sys, then read through a differential ADC on the Microcontroller. The EMG signal filtering algorithm is implemented in software on the microcontroller.

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Figure 3-3: Exoskeleton custom PCB.

In order to mount all the electronics on the exoskeleton, a custom PCB was de-signed to interface the required components with the microcontroller (shown in Figure 3-3). All the PCBs are mounted on the back of the carbon fiber torso attachment as shown below.

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3.3

Control

Increasing the maximum vertical jump height of a human wearing the exoskeleton requires an effective control strategy for actuating the motors. In the dynamic simu-lation, the torque was increased by 10ONm per hip at the activation time ti for the hip joint. This represents a simple step in the motor torque from ONm to 10ONm at time t1. While this proved to be successful in simulation, in practice, a sudden increase in torque could surprise or injure the human subject. Therefore, an initial ramp function is more appropriate. Furthermore, although activation time ti for the hip joint is defined in simulation, it is not clearly defined in practice. Thus, an ap-propriate algorithm is required to estimate t, in real-time for tests invloving human subjects.

t, is defined as the time that the activation of the muscles across the hip joint begins to increase towards maximum activation. Therefore, one possible method for estimating t, in real-time is by measuring EMG on the human test subject. The gluteus maximus muscles are the primary muscles that cause extension in the hip joints, thus, could be an accurate representation of the hip's torque source in Cheng's simulation.

3.3.1

EMG Control

A simple experiment was conducted to determine the feasibility of using EMG to

estimate time t, for human subjects. Three test subjects were asked to perform a series of maximum height vertical jumps (while wearing the unpowered exoskeleton), and their gluteus maximus EMG was measured. The results are shown in Figure

3-5. It was found that the EMG signal varied significantly for different human test

subjects, and that the signal measured was severely affected by sweat produced by the test subject. This is consistent with studies involving real-time control using EMG

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-- -Hip Angle Angular Velocity EMG -- Hip Angle -- Angular Velocity -- EMG -Hip Angle Angular Velocity EMG 0 0.2 0.4 0.6 0.8 1 1.2 Time (s)

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3.3.2

Control Algorithm

It was determined that EMG is too unreliable to consistently determine ti for hu-man vertical jumps. An incorrect EMG measurement could result in the motors prematurely firing, which poises a risk of injury for the test subjects.

0 0.2 0.4 0.6

Time (s)

0.8 1 1.2

Figure 3-6: Algorithm for commanded torque

Instead of using EMG, ti is approximated to be the time when the angular velocity of the hip joint changes from negative (the 'wind-up' phase of the jump') to positive (the actual jumping motion). Although this is not strictly equal to ti as defined above, it is a reliable and repeatable method that test subjects find intuitive. At this point in time, the motor torque ramps up from ONm to the commanded torque in 50ms. The commanded torque is then maintained until the hip joint angle reaches 1800 , and the torque returns to ONm. Figure 3-6 shows the commanded torque with respect to time.

-- Hip Angle

Angular Velocity Desired Torque

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Chapter 4

HUMAN TESTING

4.1

Testing Procedure

Six healthy human test subjects were recruited to participate in a preliminary ex-oskeleton test session. The purpose of this session was to determine the three test subjects with the most potential to improve their vertical jump height using the exoskeleton. All six subjects regularly participated in land-based sports. The test subjects were asked to perform two maximum height vertical jumps without wearing the exoskeleton in order to determine their baseline jump height, followed by five vertical jumps while wearing the exoskeleton with the motor torque set relatively low (20Nm). All jump heights were measured and recorded. The methods used for mea-suring jump height are explained in Section 4.2. None of the six test subjects matched or exceeded their baseline jump height while using the exoskeleton at 20Nm per hip. The three test subjects that came closest to achieving their baseline jump heights were asked to participate in a series of subsequent test sessions. These test subjects are hereby referred to as Subject A, Subject B, and Subject C. All three subjects are male. The height, weight and age of each subject is included in the Appendix.

In Session 1, the test subjects were asked to perform a series of maximum height vertical jumps using the exoskeleton. The motor torque was increased incrementally after each jump as the test subjects became more comfortable. The human test subjects were permitted to request higher or lower torque. The purpose of this session

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was to allow the test subjects to become accustomed to using the exoskeleton. The jump heights were not recorded.

The purpose of Session 2 was was to obtain data to determine the effectiveness of the exoskeleton at improving a human maximum height vertical jump. The test subjects were asked to perform two maximum height vertical jumps without the exoskeleton in order to dermine their baseline jump height, followed by two maximum height vertical jumps while wearing the exoskeleton set to a certain torque. The torque was increased after each pair of jumps. The higher of the two jumps was selected as the data point for its corressponding torque level. The test subjects were permitted to select their torque increment after each pair of jumps. The torque was not increased beyond 90Nm.

The exoskeleton experienced a mechanical failure at approximately 90Nm and was repaired and slightly modified. Session 3 was identical to the Session 2, except with a maximum torque of approximately 165Nm per motor. Subject A was the only test subject that participated in Session 3. The modified exoskeleton experienced another mechanical failure at approximately 165Nm. Subjects B and C did not participate in Session 3 due to the high risk of injury involved with testing at high torque levels. The data presented below is from Session 3 for Subject A, and from Session 2 for Subjects B and C.

4.2

Jump Height Measurement

In order to motivate the human test subjects to jump as high as possible a 'Vertec' product was used. The Vertec is a vertical jump height measuring device, consisting of horizontal tabs that protrude from a tall, vertical rod. The human test subjects were instructed to jump up and touch the highest possible tab with their hand. The tabs rotate when touched, so the device is capable of recording the height of the jump. Although the Vertec is effective at motivating the human test subjects to maxi-mize effort, it is'less effective at measuring vertical jump height. The tabs measure 2.54cm increments, therefore the resolution is quite low. In addition, the highest tab

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reached is affected by the test subject's ability to stretch their arm and shoulder. The exoskeleton can sometimes reduce test subjects' reach, due to the way it attaches to the human torso.

Figure 4-1: Test subject reaching for the highest tab on the Vertec device.

For Subjects B and C, jump height is measured by analyzing high speed video footage and deriving jump height based on time spent in the air. For Subject A., the video footage does not clearly show the subject's feet on the ground, so the Vertec tab measurements were used instead, and compared to the highest tab reached while standing on flat feet and reaching as high as possible.

4.3

Results

The testing results for Subjects A, B and C are presented in Figure 8. All three sub-jects exceeded their baseline jump heights (represented in the figure with a dashed line). The data shows that the exoskeleton is clearly effective at increasing the max-inum vertical jump height for all the test subjects and all three test subjects' jump heights increased as torque was increased.

The maximum improvement recorded is 13% for Subject A , 5% for Subject B and 6% for Subject C. The data suggests that Subjects B and C could have recordeded

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-.- Subject A -+-Subject B -+-Subject C 0.45 0.4 0.35 0.3 20 50 80 110 140 170 Motor Torque (Nm) Figure 4-2: Exoskeleton testing data.

higher improvements had they participated in Session 3. A discussion of the results and and challenges faced is included in Chapter 5.

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Chapter 5

Future Work

5.1

Design Challenges

Torque

transmission

Figure 5-1: Problems with the current exoskeleton design.

As previously mentioned, actuating the human joint is non-trivial, and there are several problems associated with the current exoskeleton design. These can be

sepa-Motion

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rated into three distict categories: Torque transmission challenges, motion challenges and comfort challenges. The torque transmission challenges are related to the dif-ficulty in effectively transmitting the motor torque to the human hip joint. The current design is ineffective for providing torque during unsymmetrc leg motions such as running or walking. This is because actuating a single leg at a time causes a re-action torque in yaw and roll on the torso attachment. Since the torso attachment is not rigidly attached to the human torso, the torso attachment tends to twist in yaw and roll relative to the human torso when the torque produced by the motors is not symmetric. In addition, the torso attachment experiences backlash when torque is applied. The skin on the human torso tends to stretch, and some fat is squeezed resulting in about 5' to 10' of backlash. Finally, the reaction force on the torso attach-ment causes it to ride up the human torso. This is prevented in part by the harness that goes around the thighs, but the problem still exists because the fat and skin on the buttocks tends to get squeezed, thus the torso attachment is not completely constrained.

Figure 5-2: Chaffing caused by friction between the thigh and the thigh attachment.

The second set of challenges are related to constraining the natural human motion. On the current exoskeleton design, the human spine has a limited range of motion, since it is constrained to the carbon fiber torso attachment. Additionally, although the torso attachment has two degrees of adjustability for aligning the motors with the

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axis of rotation of the hip joint, aligning the two axis of rotation in practice has proven to be challenging. The current design allows the human some abduction movement, and the leg can twist within the thigh attachment, but the range of motion is limited. Finally, as can be seen in Figure 5-2, the sliding of the thigh attachment on the thighs causes chaffing and rashes.

The third set of challenges limit comfort for the exoskeleton user. In the current design, the position of the motors prevents the exoskeleton users from being able to sit down while wearing the exoskeleton. Finally, the torso attachment must be tightly attached to the torso which limits the user's ability to breathe comfortably while wearing the exoskeleton. When torque is applied during jumps, human test subjects report a moment of discomfort when the exoskeleton squeezes the chest and their ability to breath is severely compromised.

5.2

New Exoskeleton Prototype

Figure 5-3: New exoskeleton prototype.

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prototype design is proposed. The challenge identified is to effectively apply unsym-metric torque to the human hip joints. In the current prototype design, the reaction torque would cause the torso attachment to to twist in yaw and roll relative to the torso. In order to counteract this, the thigh attachment is extended to the shin via a commercial knee brace. In addition, the thigh attachments make contact with the front and back of the thigh, thus holding on to the thigh more rigidly. In theory, these features would prevent the torso attachment from twisting relative to the torso due to the reaction torque from the motors.

5.3

Physical Challenges of Actuating the Human

Body

The key challenge in this project and in other exoskeletons is effectively actuating the human body. The actuators in this project are capable of providing plenty of torque to assist the hip joints, and the exoskeleton prototype was relativley lightweight. However, transmitting the torque to the hip joints while also attaching the exoskeleton to the body such that it is comfortable for the user proved to be difficult. In this study, motion was limited to vertical jumping, and therefore the problem was simplified. During the course of the project, the physical challenges of actuating the human body were identified and generalized, and are described below. These do not include challenges related to control, which are independent.

5.3.1

Lack of Rigid Attachment Methods

Almost all surfaces on the human body are soft and somewhat flexible. In order to actuate a joint on the human body, an exoskeleton would ideally attach rigidly to the two segments that rotate about that joint. In practice, this is challenging since there is no obvious way to attach an exoskeleton rigidly to segments of the human body. Without a rigid attachment, it is difficult to effectively transmit torque. Existing solutions include wrapping around segments of the human body or strapping

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to certain parts of the body. However, these attachment methods are not rigid, and therefore tend to slide or stretch the skin due to reaction forces resulting from actuation. Essentially, existing attachment solutions cannot effectively resist forces that are not normal to surfaces on the human body. In addition, since actuators cannot be placed directly at the center of rotation of the joint being actuated, there are reaction torques resulting from actuation that are orthogonal to the desired torque, and also tend to cause sliding and stretching of the skin at the attachment point.

5.3.2

Rigid Components Impede Natural Motion

The human body is well accustomed to wearing fabric and highly malleable clothing. Even shoes are designed to be soft and flexible. The human body has many degrees of freedom that tend to become impeded when rigid components are worn on the body. The torso in particular is incredibly flexible and useful during natural motion. Existing exoskeletons consist of rigid components that tend to impede natural motion.

5.3.3

Large Variation in Human Body Dimensions

The human body varies significantly in sizes and proportions between different people. In some designs, human dimensions are critical in order to effectively align the axis of joints on the body with actuators to ensure that the exoskeleton works with the kinematics of the user. In addition, some attachment methods work more effectively for certain body types compared to others.

5.4

Conclusions

The human testing data shows that the exoskeleton was effective at improving vertical jump height. According to the dynamic simulation, increasing torque at the human hip joint can result in increased vertical jump height. This was validated by testing with human subjects using the exoskeleton, although the jump height improvements achieved by the test subjects using the exoskeleton are significantly lower than those

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predicted in the simulation study. There are several factors that could contribute to this. Firstly, Cheng's human model simulates a relatively light human (59kg), while the test subjects are significantly heavier. Furthermore, the exoskeleton is not rigidly attached to the human torso. The natural compliance of the human body makes it challenging to effectively transmit torque to the hip joint. In addition, the test subjects may require more time to adapt and become accustomed to using the exoskeleton in order to achieve maximum performance. Finally, the human model does not account for muscle co-contraction or other subconcious effects that may be caused by externally actuating a human joint.

It is believed that this is the first actively powered exoskeleton that enables a human to jump higher. By taking a simple and direct approach to actuating a human joint, it has been demonstrated that electric actuators are capable of increasing the dynamic capability of a human. Future similarly designed exoskeletons have the potential to make humans able to run faster, traverse challenging terrain, perform fast agile movements, and generally become more athletic.

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Appendix A

Tables

A.0.1

Test Subjects' Age, Height and Weight

Subject Subject Subject A B

C

Age 23 21 23

Height

1.84m 1.80m 1.80m

Weight

86kg 82kg 74kg

"

i i

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Appendix B

Figures

B.O.2

Current Control on Electric Motors

The motor controllers are used in torque mode; an internal current control loop is implemented on the motor controllers in order to achieve the desired current. In order to ensure that the actual current is closely following the desired current, the current data from one jump is logged and presented below.

30 25 20 -15 S10 5 0 -5 0 0.1 0.2 0.3 0.4 0.5 Time (s)

Figure B-1: Current tracking plot

0.6 0.7 Desired Current

- Actual Current

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-Bibliography

[1] Lockheed Martin HULC Exoskeleton (Accessed 2015 September 14: http://www.lockheedmartin.com/us/products/hulc.html

[2] Asbeck AT, Schmidt K, Galiana I, Walsh CJ. Multi-joint Soft Exosuit for Gait Assistance, in International Conference on Robotics and Automation (ICRA) (2015) 368711

[3] Cyberdine HAL Exoskeleton (Accessed 2015 September 14): http://www.cyberdyne.jp/english/products/HAL/

[4] ReWalk Personal Exoskeleton (Accessed 2015 September 14): http://www.rewalk.com

[5] Eksobionics EKSO Exoskeleton (Accessed 2015 September 14): http://www.eksobionics.com

[6] Honda Stride Management Assist Exoskeleton (Accessed 2015 September 14): http://asimo.honda.com/innovations/feature/stride-management-assist/

[7] K.B. Cheng The mechanisms that enable arm motion to enhance vertical jump performance Journal of Biomechanics 41 (2008) 18471854.

[8] M.F Bobbert and A.J. Van Soest Effects of muscle strengthening on vertical jump height: a simulation study, Official Journal of the American College of Sports Medicine (1994) 1012-1020.

[9] K.B. Cheng The Relationship Between Joint Strength and Standing Vertical Jump Performance Journal of Applied Biomechanics 24 (2008) 224-233.

[10] Pandy, M.G., Zajac, F.E., Sim, E., & Levine, W.S. "An optimal control model for maximum-height human jumping." Journal of Biomechanics 23 (1990) 1185-1198.

[11] Hoy, M.G., Zajac, F.E., & Gordon, M.E. A musculoskeletal model of the hu-man lower extremity: the effect of muscle, tendon, and moment arm on the moment-angle relationship of musculotendon actuators at the hip, knee, and ankle. Journal of Biomechanics, 23 (1990) 157169.

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[12] Selbie, W.S., & Caldwell, G.E. A simulation study of vertical jumping from different starting postures. Journal of Biomechanics, 29 (1996) 11371146.

[13] Seok, S., A. Wang, D. Otten, S. Kim, "Actuator design for high force pro-prioceptive control in fast legged locomotion", Intelligent Robots and Systems (IROS), 2012 IEEE/RSJ International Conference on, Vilamoura, Portugal, IEEE, 10/2012.

[14] S. Seok, A. Wang, Chuah , M. Y. (Michael), D. Otten, J. Lang and S. Kim "Design principles for highly efficient quadrupeds and implementation on the MIT cheetah robot", Proc. IEEE Int. Conf. Robot. Autom., pp.3 3 0 7 -3312 [15] Castellini C, Artemiadis P, Wininger M, Ajoudani A, Alimusaj M, Bicchi A,

Caputo B, Craelius W, Dosen S, Englehart K, Farina D, Gijsberts A, Godfrey SB, Hargrove L, Ison M, Kuiken T, Markovi M, Pilarski PM, Rupp R and Scheme E (2014) Proceedings of the first workshop on Peripheral Machine Interfaces: going beyond traditional surface electromyography. Front. Neurorobot. 8:22. doi: 10.3389/fnbot.2014.00022

Figure

Figure  1-1:  Directly  actuating  the  end  effector  (foot).
Figure  2-1:  Four  segment  human  model  connected  by  frictionless  revolute joints.
Figure  2-2:  Dynamic  simulation  of  a  maximum  height  vertical  jump.
Figure  2-5:  Dynamic  simulation  of  a  maximum  height  vertical  jump  using  a  hip exoskeleton.
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