Mitsubishi Heavy Industries Technical Review Vol. 53 No. 4 (December 2016)75

Development of Powered Exoskeleton for Heavy Work- Coexistence of Robot's Power and People's Dexterity –TADASHI MURATA*1

HIROHISA HIRAI*2

KEN ONISHI*3

YOSHIHIRO TAMURA*2SYUHEI KOBORI*2

NAOKI KAZAOKA*4

Mitsubishi Heavy Industries, Ltd. (MHI) has been working on the development of a power
assist suit (PAS) in an attempt to reduce the manpower and working hours required for heavy
work. The key to this development is a technology to control power exceeding that of a humanbeing’s without impeding the wearer’s dexterity. So far, MHI succeeded in developing a pair ofrobotic lower limbs enabling the wearer to walk 3.9km/h while carrying over 80kg. In the
Promotion of Market Implementation by Applying Robotics (MARC)(1) launched by the New Energy
and Industrial Technology Development Organization (NEDO) in September 2016, MHI has
proceeded with the practical use of PAS by applying a pair of robotic lower limbs in combination
with a set of robotic upper limbs developed according to the types of work required and by
examining the effects.

|1. Introduction
A power assist suit is an exoskeletal device equipped with an actuator driven by a motor,
hydraulic pressure, etc. It is expected to enhance the wearer’s strength or to serve as a walking aid
for a rehabilitant. HAL(2) developed by CYBERDYNE Inc. and ReWalk(3) by ReWalk Robotics,
Inc. are famous applications for the latter use, some of which have already been released.

Figure 1

Performance and mechanical structure of the robotic lower limbs of PAS

On the other hand, the former application has only been realized as a back supporter whicheases strain on the lower back by assisting only in the moment when the wearer does heavy lifting.
However, there are almost no products anywhere in the world that can lift and hold up objects that
*1
*2
*3
*4

Chief Staff Manager, System Technology Development Department, ICT Solution Headquarters
Nuclear Plant Component Designing Department, Nuclear Energy Systems Division, Energy & Environment
Senior Manager, Nuclear Plant Component Designing Department, Nuclear Energy Systems Division, Energy & Environment
Advanced Design Group, Research & Innovation CenterMitsubishi Heavy Industries Technical Review Vol. 53 No. 4 (December 2016)76

are heavier than human beings can manage, due to the increased technical difficulty caused by thelarger devices and the significant assisting power that would end up impeding the wearer’s freemovement.
MHI has focused on the improved efficiency of heavy work achieved by an increased
assisting power and has been working on the development of PAS for heavy work applicable tovarious scenes, such as the transportation of sick and injured people at the time of a disaster,building construction, civil engineering work, assembly plants and backyard operations. In
December 2015, MHI presented prototype robotic lower limbs (Figure 1) at a robot exhibition andissued a press release(4).

|2. Functional requirements for heavy work PAS
2.1

Design of assisting power

The most significant benefit of the introduction of PAS at a heavy work site is the decreasein manpower. According to laws and regulations, the maximum weight which one person canhandle continuously is 20kg. On the contrary, the weight that one person should not handle is 55kg
and over (Guidelines on lower back pain prevention in the workplace: Notice from the
Director-General of the Labour Standards Bureau No. 547). Therefore, in order to reduce thenumber of workers from 2 to 1, the transferrable weight desired from PAS should be 40kg andover, i.e., 60kg (the weight load requiring 2 workers) minus 20kg (the weight load that 1 worker
can handle). Accordingly, it was determined that the weight transferrable by PAS should be 40kg.
Subsequently, the assisting power of PAS was determined to be a total of 83kgf including the target
weight of the lower limbs of 25kg and that of the upper limbs of 18kg.

2.2

Functional requirements

There is the risk of negative impact such as the impediment of the wearer’s movement ordecreased safety due to the high-power output of the actuator in order to achieve the high assisting
power. In an attempt to overcome such risk, the following three functional requirements were
established:(1) Light weight: It is inevitable that the frame and batteries grow in size due to the high-power
output of the actuator. As the weight of PAS itself increases, an even higher output actuator
will need to be selected. In order to break out of such a negative spiral, making the entire
device lighter is crucial. In addition to selecting lightweight materials, a concept thateliminates unnecessary functions to achieve an optimum design will also be required.
(2) Safety: Ideas for minimizing injury in case the wearer falls, and dual sensors andcommunication systems are required in order to prevent system from running amok in the caseof failure of electric devices.
(3) Compatibility with human beings: In order for PAS to be able to perform heavy lifting, thelinkage mechanism needs to be touching the ground so that the reaction force occurs. In otherwords, only the standing leg (the one touching the ground) can generate the assisting power.
Therefore, a control mechanism is required to switch the assisting power back and forth
between the standing leg and idle leg in a timely manner.
Furthermore, due to the high-power output, the speed reduction ratio increases, which leads
to a decrease in the back drivability (the drivability of power-transferring mechanisms including thedecelerator when the force is applied from the device contrary to what usually happens), whichmight possibly put the wearer in a puppet state. In order to address this problem, it will benecessary to adopt a control function that can run in advance in response to the wearer’s intentions.

|3. Design to achieve functional requirements
3.1

Light weight achieved by modularization of robotic arms

In order to break out of the negative spiral of the increase in size of PAS due to thehigh-power output of the actuator, the entire device needs to be as light as possible, which cannot
be achieved by simply slimming down the components and reviewing the materials.
In this regard, rather than making a “PAS that can do everything,” we adopted a modular
design in which the robotic upper limbs could be selected from among multiple different pairs
depending on the type of work the wearer intends to carry out, while there is just one pair of lower
limbs that serve the purpose of supporting any upper limbs selected (Figure 2). By minimizing theMitsubishi Heavy Industries Technical Review Vol. 53 No. 4 (December 2016)77

functions of the upper limbs in order to avoid adding unnecessary features to the mechanism, theweight of the entire device can be reduced. A pair of lightweight upper limbs would also help
reduce the required output power of the lower limbs, which would result in achieving a lightweight
PAS as a whole.
In order to achieve the modular design, it is necessary to control the lower limbs so that they
can generate appropriate assisting force no matter what kind of upper limbs were connected.
Accordingly, as shown in Figure 2, MHI developed a new control function in which a power sensor
is placed at the joint of the upper and lower limbs in order to detect the overturning moment
occurring around the wearer’s trunk due to an unknown pair of upper limbs and the object to belifted. Subsequently, torque is generated so that the overturning moment is cancelled out.

Figure 2

3.2

Design concept of upper limb module

Safe design

Prior to achieving a safe design, MHI conducted a risk assessment with reference toISO13482, the international safety standard for personal care robots. JIS B 8446-2, a new standard
for low-power, wearable, physical assist robots, was established in 2016. It is expected that astandard for high-power robots like PAS will also be established in the near future.
In light of the results of the risk assessment, MHI performed the following safe design:
3.2.1 Minimum restraints
In most powered suits generally known around the world (but limited to those with amechanism that covers down to the ankles), the thighs and shins are restrained by the linkage
mechanism, just like they are with ReWalk(3). However, the human joint mechanism is quite
complex. As human movement can hardly be mimicked by a simple mechanism such as a
powered suit, it ends up restricting the wearer’s movement. On the other hand, in terms of
power assisting robots for heavy work such as PAS, they only need to be able to support aheavy weight for the wearer and to follow the wearer’s actions without getting in the way.
There is no need to accurately mimic the wearer’s leg motion.
With PAS, the restrained portions are limited to the ankles, waist and chest. This design
should minimize injury to the wearer’s body due to the weight of PAS sitting on the wearer inthe case of a fall. Furthermore, the restraints around the ankles are only shoes. In the case of an
emergency, the wearer can immediately escape from PAS by simply removing the shoes.
In addition, MHI performed the inherent safe design that is intended to avoid catching thewearer’s body parts such as the fingers in the linkage mechanism with reference to ISO13482.
3.2.2 Dual CPU
In order to avoid abnormal actions such as the system running amok due to a failure of
the CPU and resolver, the CPU on the servo substrate that drives the motor is duplicated, whichenables the monitoring of the following cross-checking operations:(1) Current value: Checking the current values transmitted by the motor which are detectedMitsubishi Heavy Industries Technical Review Vol. 53 No. 4 (December 2016)78

by the individual CPUs in order to detect abnormalities(2) Calculation results: Checking the motor command values output by the individual CPUs
utilizing the Complex Programmable Logic Device (CPLD: A programmable logicalcircuit element achieved by modifying the wiring connections) in order to detect
abnormalities(3) Reciprocal signals between CPUs (Voting): Checking the periodical processing of theindividual CPUs based on periodical signals transmitted to each other(4) Resolver value: Checking the resolver values for angle detection built into the motor
and gear individually in order to detect any system failures. When a failure of
motor-side resolver, which is used mainly for control, is detected, a signal of thegear-side resolver can be used as a back-up alternative for control, keeping the system
alive as long as possible.
As the user is wearing PAS, even if a servo fails at one joint, an immediate halt of theentire device would throw the wearer off balance and result in the risk of a fall. Therefore,
currently, information regarding a state of abnormality is only transmitted to the main controller
and no further action will be taken automatically. In the future, detailed rules in the case of asystem halt will be necessary depending on the safety level of the situation, i.e., whether PAS
should shut down its entire assist control system automatically whenever it detects anabnormality even in a single servo only, or whether the assist control system should be
manually stopped by the wearer while ensuring safety.
3.2.3 Dual communication system
FlexRay was adopted as it is capable of high-speed, regular-cycle communication at ashort interval of 1ms and ensuring redundancy. PAS is equipped with 2 lines ofcommunication, A and B. In case one of them fails, the communication line will be promptly
switched to the other one in order to avoid an instant system halt.

3.3

Design of control system for increased human compatibility

3.3.1 Load-assisting function
In order to support a heavy weight to reduce the load on the wearer, the load of the heavy
object needs to be transferred to the ground via the linkage mechanism without going throughthe wearer’s legs. In an attempt to achieve this, MHI focused on the load on the soles of thewearer’s feet. If a force which cancels out this load upwards can be generated in the linkage
mechanism as the target assist power, the wearer would feel no load at all.
In order to perform load sensing at the soles of the wearer’s feet, the key to achieving thiscontrol concept, a sensor was embedded in the wearer’s shoe insoles. In an attempt to achieve
flexibility of the shoe soles and accurately detect the load value at the same time, MHI created a
prototype in which the most appropriate sensor was selected and the optimum arrangement wasdetermined (Figure 3).

Figure 3 Development of shoe-sole sensor

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3.3.2 State estimation function
As explained in Chapter 2 above, a decrease in the back drivability due to the high-power
output of the actuator is inevitable. Therefore, in order to achieve a similar level of mobility as
not wearing PAS, it is necessary to predict the wearer’s next move and control PAS in advancebased on the state estimation algorithm.
Previous examples(2) adopted a method that predicted the wearer’s intention before thenext movement by measuring the wearer’s myoelectric signals prior to activating the controls.
However, these myoelectric signals vary depending on the person. They also vary depending on
the daily physical condition of the wearer or the surroundings, even for the same wearer.
Therefore, calibration needs to be conducted every time, which we thought to be unsuitable forthe use MHI intended, such as heavy lifting and emergency response at the time of a disaster.
Accordingly, we decided to make positive use of the information provided by the shoe-solesensor or inertial sensor described above rather than adopting the myoelectric sensing method.
The two types of state estimation algorithms mounted on PAS are briefly introduced
below.
- Leg-lifting/lowering assist function: PAS estimates the next leg-lifting/lowering
movement based on the chronological change of the load on the soles of the wearer’s feetand gives a command to the actuator to assist the action. Leg-lifting/lowering areimportant state changes between the standing leg and idle leg. Therefore, this function isextremely useful for correctly predicting their movements.
- Gait assist function: PAS creates the next gait path after learning the wearer’s step pattern
while walking with the inertial sensor. This function is crucial for achieving a similar
walking speed as when not wearing PAS.
When PAS is controlled by a system that gives feedback in response to the position andspeed, the wearer’s actions are restricted to a particular posture with no flexibility. Such an
extreme ill-compatibility leads to a decrease in mobility. In order to avoid this kind of problem,
all the algorithms were configured under torque control. Figure 4 illustrates the outline of thecontrol block diagram.

Figure 4 Control block diagram|4. Results of function tests
This paper provides the results of the following tests: (1) walking test, (2) uneven load
support test in order to confirm that PAS is able to support a pair of robotic upper limbs with
uneven loads, and (3) striding test/stair walking test.

4.1

Walking test

Figure 5 shows a series of video frames of a wearer of a shielding jacket, walking. A
shielding jacket is one of PAS’s intended applications in emergency response at nuclear power
plants, which covers the torso of the wearer down to the lower hip with a 9mm-thick lead plate tominimize radiation exposure. However, the jacket weighs 87.3kg in total including four limbs,
which is too heavy for the wearer to move without assistance from PAS.

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Figure 5

Shielding jacket (right) and walking test

As shown in Figure 5, since the wearer’s head is not lowered while walking, it is confirmedthat the weight of the shielding jacket is successfully supported by PAS. Furthermore, the walking
speed achieved was 3.9km/h, which is equivalent to not wearing a shielding jacket. When the
optimum design of the mechanical parts successfully further trims the overall device weight in the
future, faster and smoother walking can be expected. This walking test was conducted as part ofjoint research with the Japan Atomic Power Company.

4.2

Uneven load support test

Postural stability was examined by attaching a 35kg weight (a dumbbell) simulating theweight of the frame. The test condition is illustrated in Figure 6. Before PAS control was initiated,as the center of the rotation axis of the motor was off from the working point of the ground reaction
occurring on the soles of the wearer’s feet, the overturning moment was so large that the wearer
was unable to maintain an upright posture. On the other hand, after PAS control was activated, due
to the overturning moment-compensated control function described in 3.1 above, the wearer had no
problem maintaining an upright posture. This indicates that the functionality can take priority indesign since the restriction on the weight balance in the upper-limb design has been eased.

Figure 6 Uneven load supporting test

Even with a 35kg weight, the controls allow the wearer’s
torso to be in a comfortable posture and the legs tostretch so that an upright position can be maintained.

4.3

Striding test and stair walking test

In order to utilize PAS for heavy work, just having a flat surface walking function would beinadequate. It should rather achieve a broader range of actions intended by the wearer, including
crouching down, bending forward, striding over objects and walking up and down stairs. Figure 7
shows a striding test and stair walking test. As described in 3.2.1 above, because the thighs andknees are free from restraints, it was confirmed that the wearer was able to perform these activities
freely. However, there are still some pending issues to be addressed, such as further reducing bulk
so that PAS would not make contact with banisters or to provide increased stability while walking
down stairs. In order to deal with the latter issue in particular, the existing state estimation function
will need to be extended to perform estimation of the transition from one state to another. For
instance, when PAS first recognizes that the wearer is going down stairs, we think an advanced
control algorithm will be required in order to give a command suitable for the situation.
Functional verification tests for these movements were conducted as part of the energy
research and development base project in Fukui Prefecture in cooperation with Associate Professor
Kawai and Associate Professor Takahashi from the University of Fukui.

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Figure 7

Striding test (left) and stair walking test (right)|5. Conclusion
MHI has so far finished the prototype of a pair of robotic lower limbs of PAS for heavy
work. PAS has successfully achieved both the dexterity of a human being in tasks such as walking
or striding and the power to lift and carry objects weighing more than 80 kg, exceeding natural
human abilities. This achievement was released to the public at the International Robot Exhibition
held in December 2015 and Maintenance & Resilience Tokyo in July 2016. Although the prototype
requires sophistication and there is plenty of room for improvement, it elicited a strong public
reaction. We strongly felt that a significant number of people have high hopes for PAS.
The name “powered suit” has been widely recognized since it first appeared in a novel in
1959. However, even after more than 60 years, the concept has had little success as a product. Partof the reason is inadequate cooperation among companies and research institutes. MHI’s PAS
adopts a concept in which a pair of modularized upper limbs can be connected with robotic lower
limbs through a very simple interface. We are currently striving for early successfulcommercialization of PAS, taking advantage of this concept with enhanced cooperation from otherinstitutions.
Figure 8 shows the future PAS. At the Tokyo Olympics in 4 years, we are hoping to provide
a dashing and confident-looking PAS that can care for and transport spectators who faint fromexcitement while watching the games.

Figure 8

Future PAS; Clean and innovation-inspiring design

References
1.
2.
3.
4.

http://www.nedo.go.jp/content/100783669.pdf
http://www.cyberdyne.jp/products/HAL
http://rewalk.com
http://www.mhi.co.jp/news/story/1512015706.html