Evaluation of the Capabilities of a Hybrid Driven Exoskeleton in

Passive Mode of Interaction

Dimitar Chakarov

, Ivanka Veneva

, Pavel Venev

and Mihail Tsveov

Institute of Mechanics, Bulgarian Academy of Sciences, “Acad. G. Bonchev” str., Block 4, Sofia 1113, Bulgaria

Keywords: Exoskeleton Arm, Electric Motors, Pneumatic Cylinders, Interaction Force, Patient, Harmonic Motion,

Mechanical Impedance, Experiments, Simulations.

Abstract: A new construction of upper limbs exoskeleton with hybrid drive was studied in this work. The paper presents

mechanical structure and actuation of exoskeleton with hybrid drive including pneumatic cylinders and

electric motors. In order to evaluate the transparency and safety, the capabilities of the exoskeleton in passive

mode of interaction was estimated. A model of the interaction force between the patient and the exoskeleton

arm in passive mode was built. The model was based on harmonious movements imposed in one joint of the

exoskeleton arm. Experiments and simulations were performed to assess the interaction force between the

patient and the exoskeleton because of the mechanical impedance of the device. The force of interaction was

obtained from passive forces, such as inertial, frictional and gravitational forces, as well as from the elasticity

of the pneumatics. The patient-initiated harmonic motion was studied in two cases- without pressure in the

chambers and with pressure for gravity compensation. The results where demonstrated graphically.

Conclusions where made about the behavior of the exoskeleton in the passive mode of interaction.

1 INTRODUCTION

Robotic rehabilitation using exoskeletons is an

alternative to conventional manual therapy to

improve motor function in post-stroke patients

(Manna, 2018). The rehabilitation exoskeleton

(Jarrasse, 2014) should be able to create a great force

for assisting and directing the patient's hand in the

early stages of recovery, as well as following the

human hand without opposition or being able to react

to any movement made by the patient in the full stage

of recovery. In the control design of the rehabilitation

exoskeleton in general, two "extreme" ideal regimens

can be defined, covering the entire spectrum of

therapeutic interventions: "robot in charge" and

"patient in charge" (Veneman, 2006).

In the "robot in charge" regime, it is important that

the robot has enough strength and power to realize the

desired movement with a relatively high impedance.

In a "patient in charge" it is important that the forces

of interaction between the exoskeleton and the patient

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https://orcid.org/0000-0001-5051-4411

are low; in other words, the perceived impedance of

the robot should be low. The main feature here is

transparency.

There are two main approaches to reducing the

mechanical impedance of the rehabilitation

exoskeletons and to ensure security and transparency

in interaction: active and passive. Electric motors,

pneumatic, hydraulic and other active drives are used

to reducing the impedance and to control the contact

force through an active approach. The active

approach is based on sensors and algorithms for

motor control. There are two methods to control the

contact force: direct and indirect. In the direct

approach, the controller regulates the force with a

control loop and using force feedback of the

measurement and desired value of the force

(Ansarieshlaghi, 2019). Indirect force control

methods are known as impedance or admittance

(Hogan, 1985). Impedance controlled systems detect

the deviation in position and control the force applied

by the device, while admittance controlled systems

442

Chakarov, D., Veneva, I., Venev, P. and Tsveov, M.

Evaluation of the Capabilities of a Hybrid Driven Exoskeleton in Passive Mode of Interaction.

DOI: 10.5220/0010569004420449

In Proceedings of the 18th International Conference on Informatics in Control, Automation and Robotics (ICINCO 2021), pages 442-449

ISBN: 978-989-758-522-7

detect the contact force and control the position and

velocity or the device.

The passive approach involves passive or

inherently safe drives. The pneumatic actuation

allows force control according to the active

approach. Also, pneumatic actuation has a natural

flexibility and allows to achieve inherent safety and

transparency in the process of rehabilitation in a

passive way (Morales, 2011). Due to the inherent

natural compliance of air and its storage in closed

chambers, pneumatic actuation can create passive

support force.

There are three types of pneumatic drives:

pneumatic cylinders, pneumatic artificial muscles

(PAM) and rotating pneumatic motors. High natural

compliance and non-linearity of air compression of

pneumatic drives result in a low dynamic force

response, which limits the efficiency of the actuator.

To take advantage of pneumatic actuators while

reducing their shortcomings, they are combined with

electric drives. (Rouzbeh, 2019) integrates a low

friction pneumatic cylinder and a rotating electric

motor into a compact device. A similar design of

hybrid propulsion including a linear electric motor

and pneumatic cylinder is presented in (Nakata,

2015). The hybrid pneumatic and electric actuator,

compared to high-gear electric drive, produces a

lower impedance while maintaining a high driving

force value.

A pneumatically driven exoskeleton for training

and rehabilitation, aided by interactions in virtual

scenes, was developed by the authors. PAM bundles

included in an antagonistic circuit are used for

propulsion. In order to overcome the shortcomings

of pneumatic muscles, the possibilities of using a

hybrid drive involving a pair of PAM bundles and a

parallel working DC motor are explored (Chakarov,

2019). The structure of the exoskeleton is further

developed with hybrid drive, integrating pneumatic

cylinders and electric motors.

The subject of the present work is the study of

the mechanical structure and the actuation of the

hybrid exoskeleton with pneumatic cylinders and

electric motors. The aim of this study is to evaluate

the transparency and safety by assessing the

interaction force between the patient and the

exoskeleton in passive mode. Experiments and

simulations for estimating the interaction force

which is a result of the mechanical impedance of the

device are object of the present work.

2 MECHANICAL DESIGN AND

ACTUATION OF THE UPPER

LIMBS EXOSKELETON

To design a rehabilitation exoskeleton of the upper

limbs, which provides transparency and natural

safety, it is necessary to build a structure with low

values of mechanical impedance. This means that the

exoskeleton has extremely light segments attached to

the limbs and all heavy components are placed on the

back or ground. An exoskeleton of the upper limbs is

designed in a similar manner as shown in the diagram

of Figure 1 and the photograph of Figure 2. The

mechanical structure of the exoskeleton system

includes two arms R and L, arranged on a fixed base

0, as each arm includes two identical rotating joints

respectively for clavicular motions J1, J2, shoulder

motions J3, J4 and elbow motions J5, J6 (Figure 1).

The arm has a total of 6 degrees of mobility (h = 6),

resembling the natural movement of the human arm

from back to elbow. Structure in Figure 1 is chosen in

order to design an exoskeleton arm, in which modules

with two equal type universal joints are created and

so, unlike other solutions, circular guide and three

axes joints are avoided. Each arm of the exoskeleton

is composed of six movable segments (1, 2, 3, 4, 5

and 6) made mainly of aluminium alloy. The plastic

shells with straps are placed on the segments for

attachment to a human limb (Figure 2). The

generalized masses of the six main segments of the

arm are M

= 0.463 kg, M

= 0.321 kg, M

= 0.497

kg, M

= 0.782 kg, M

= 0.510 kg and M

= 0.793 kg.

The arm and forearm lengths of the exoskeleton

are set with initial lengths L

= 0.286 m and L

0.370 m. Their construction includes lightweight ball

bearings in aluminium housings and built-in position

sensors in their cavity. A high-precision rotary sensor

(BOURNS AMS22B) is used to measure the effective

displacement. The ranges of movements in the joints

J2 J3

Figure 1: Constructive diagram of a two-handed

exoskeleton.

Evaluation of the Capabilities of a Hybrid Driven Exoskeleton in Passive Mode of Interaction

443

are as follows: J

(15°),J

(120°), J

(120 °), J

(150 °), J

(135 °). They are consistent with the

workspace of the human hand, as shown in (Abane,

2016).

In order to build an actuation that produces a low

impedance while maintaining a high value of the

driving force, the hybrid approach was chosen,

including pneumatic and electric drives operating in

parallel.

Figure 2: Prototype of a two-handed exoskeleton.

The hybrid actuation of each joint of the

exoskeleton is built as a separate unit located in the

fixed base. A picture of the drive unit of the joint J

the shoulder is shown in Figure 3. In the base 0 there

is a wheel 1 and mounted thereto reel R with a radius

equal to 31.5mm for winding a cable. Bowden cables

T1, T2 were used to connect the reel R and a similar

reel located in the joint of the exoskeleton.

The electric drive includes a DC electric motor,

on the shaft of which a wheel 3 is mounted (Figure2).

The motor is brushless (MAXON EC 90 flat), with

Ø90 mm, 260 W, 18V, nominal torque Q

=1010

mNm, operated with ESCON Module 50/8 HE driver.

To transmit movement from wheel 3 of DC motor

to wheel 1, a backdrivable transmission is used,

including two timing belts B2 and B1 and two wheels

4 and 2 with common shaft. The first gear B2

connects wheels - 3 and 4 and the second gear B1

connects wheels 2 and 1. The first stage of the

transmission have gear ratio n

= 3:1 and the second

stage - gear ratio n

= 2:1. The overall gear ratio is

n = 6:1.

Figure 3: Hybrid actuation unit for the joint J4 of the

exoskeleton: (a) front view; b) side view.

The pneumatic drive consists of a pair of

pneumatic cylinders, C

and C

mounted in the base

unit 0, as shown in Figure 3. Pneumatic cylinders

(Aignep) with diameter D = 0.02 m are used. The

cylinders simultaneously drive the opposite sides of

the timing belt B1. Two connected chambers of the

cylinders are supplied in parallel with compressed air

, and the other two chambers with pressure, p

. The

pipes of each pair are connected to two parallel valves

(SMC S070-SDG-32), one of which supplies

compressed air to the chambers and the other releases

the pressure into the atmosphere. Pressure sensors

(Honeywell 40PC100G) are mounted on each

pipeline.

The torque created by both pneumatic cylinders is

r))s(sp-)s(sp(Q

21b21ap

++=

(1)

where p

and p

are the supply pressures in both

chambers, r is the radius of puley 1 and s

and s

are

the areas on both sides of the piston.

3 MODEL FOR INTERACTION

FORCE ASSESSMENT

Experiments have been conducted to assess the force

of the interaction in the recovery phase of patients

when they are able to initiate complex independent

movement in a relatively safe and transparent

manner. This is the so-called "patient in charge"

stage, where it is important that the forces of

interaction between the exoskeleton and the patient

are low. If the forces are close to zero it can be said

that the exoskeleton is completely "transparent".

To assess the forces of interaction, dynamic

experiments were performed with movement in one

joint of the exoskeleton, similar to the approach used

in (Bembli, 2019). All joints are locked, and joint J4

is controllable, where flexion - extension in the

shoulder is performed. The operator moves the

exoskeleton arm as shown in Figure 4. The subject of

ICINCO 2021 - 18th International Conference on Informatics in Control, Automation and Robotics

444

the present study are the forces in passive mode,

when they are defined only by the mechanical

impedance of the exoskeleton.

The force of the end effector (EE) applied to the

operator's hand, which overcomes the mechanical

impedance of the exoskeleton and the actuation, is

determined by the inertial, frictional and gravitational

forces, as well as the elastic forces according to the

equation:

N/)QQQQ(F

gfr

+++=

(2)

Above N is the value of the radius vector of the EE

where force F

is applied to the patient's hand. In

equation (2), the joint torque Q

is result of the motor

and the transmissions inertia as well as the

exoskeleton inertia according to equality

)qJq

J(Q



++=

ηη

(3)

Here,

represents the inertia of the motor and

wheel 3, J

represents the inertia of the transmission

wheels 2 and 4,

represents the inertia of the

exoskeleton and wheel 1. Above n is the overall

transmission gear ratio, n

is the gear ratio of second

stage

, η is transmission efficiency, for the case, when

the motor is driven by human (Giberti, 2010).

In equation (2)

represents the friction torque

which is mainly the result of the friction forces

generated in pneumatic actuators and Bowden cables.

The friction force for each side of the parallel drive

can be represented by the following equation

xB)x(signFF



(4)

where:

represents the Columbus friction force, B is

viscous damping coefficient and

ẋ is cable velocity.

These coefficients represent a general estimate of the

various energy effects in pneumatic cylinders,

Bowden cables, DC motors, etc., for which empirical

rather than analytical estimates are known

(Andrighetto, 2006), (Schiele, 2006). The EE friction

torque as a result of the friction forces of the parallel

drive is

rF2Q

(5)

where

r is radius of pulley 1.

Figure 4: Harmonic motion imposed by the operator on the

exoskeleton joint J4.

In equation (2), Q

is the torque resulting from

the exoskeleton gravity according to the equation

)qcossinqg(MQ

e2e1e

ρρ

(6)

where:

represents the mass of exoskeleton moving

parts 4, 5, 6; ρ

=[ρ

; ρ

]

represents the radius vector

of the mass center C in а local frame and g is the

gravity acceleration coefficient.

In equation (2) with

is denoted the torque of

the pneumatic drive as a result of the elastic forces

due to air compressibility of the chambers of the both

pneumatic cylinders (C1 and C2), (Figure 5).

Chamber -

Chamber - a

Chamber - b

Chamber - a

Cilinder – C

Cilinder - C

Figure 5: Scheme of the pneumatic drive of the exoskeleton

joints.

Assuming that air is an ideal gas undergoing an

isothermal process, the change in pressure inside each

chamber (a,b) of the two connected cylinders can be

expressed according to (Shen, 2007) as the ratio of the

mass flow rate to the volume of the chamber as

follows

)ss)(xX(

RTm

210

(a)

)(a,

+−

(7)

)ss)(xXL(

RTm

210

(b)

++−

(8)

Evaluation of the Capabilities of a Hybrid Driven Exoskeleton in Passive Mode of Interaction

445

where: m

(a)

, m

(b)

are the mass flow rate entering the

chamber (a, b);

R is the universal gas constant, T is

the gas temperature at the orifice, x is the piston

displacement measured from starting position X

for

actuator C

and C

according to the scheme of Figure

4. In equations (7), (8) the volumes of the connected

chambers are taken into account where

and s

are

the areas on both sides of the piston.

In the conducted experiment it was accepted, that

one chamber of the pneumatic cylinders is supplied

with a pressure

and the other p

, after which the

supply and discharge valves of the pneumatic

cylinders are closed. It is assumed that this is done in

the starting position of the cylinders

, where x = 0.

In this position the pressure inside each chamber (a,b)

of the cylinders can be expressed as follows

)ss)(X(

RTm

210

(a)

(9)

)ss)(XL(

RTm

210

(b)

+−

(10)

After excluding the general parameters from (9),

(10) to (7), (8), the equations for changing the

pressures in the chambers of the cylinders when

changing their strokes are obtained as follows

)xX(

(a)

−

(11)

)xXL(

)XL(p

(b)

+−

−

(12)

The elastic torque created by both pneumatic

cylinders is determined by (1), where the pressures in

the connected cylinder chambers are represented by

(11), (12). After substituting in (1) it is obtained the

equality of elastic torque as a result of the pistons

deviation x from the starting position

)rs](s

xX-L

)X-(Lp

+−

(13)

When the starting position

of the cylinders

corresponds to the initial position of the arm q

(Figure: 3) the displacements x of the pistons from the

starting position depend on the deviations

q of the

joint angle from the initial position q

as follows

qrX =

(14)

4 EXPERIMENTS AND

SIMULATIONS FOR

EVALUATION OF

INTERACTION FORCE IN

PASSIVE MODE

In this mode, the authors suggest that the patient has

the motor capacity to move his hand independently

and intensively. In passive mode, the electric and

pneumatic drives do not generate active forces. The

operator performs harmonic movements from the

original position with equal amplitude and constant

oscillation frequency. The angle

q in the joint J4

determines the position of the arm, assuming that q =

when the arm coincides with the Y axis (Figure 4).

To measure the interaction force, a load cell is located

on the end effector, where the force is applied to the

operator's hand.

4.1 Passive Mode of Interaction

without Pressure in the Chambers

A. Experiments. In this experiment, a passive mode

is realized, as the valves of the pneumatic actuator are

open to the atmosphere (

= 0 and p

= 0) and no

voltage is applied to the electric actuator. The human

operator has performed harmonic movements from

the initial position

= 260

at an amplitude of about

= 20

with an oscillation frequency of about ω =

2.1 rad/s. The forces applied by the hand of operator

to overcome the mechanical impedance of the

exoskeleton changes as shown in Figure 6 a). As can

be seen from the experiment, the interaction force has

an average value of -8.2 N determined mainly by the

weight of the exoskeleton. Dynamic deviations

around it are about ±2 N.

B. Simulations. To assess the parameters of the

actuators and to evaluate the correctness of the

constructed mathematical model, dynamic

simulations of the movements in joint J4 are

conducted. To model the harmonic motions from the

initial position

with amplitude q

and oscillation

frequency ω, the following law of motion is used

q)tsin(qq +=

(15)

as well as the laws of velocity and acceleration

obtained after differentiation of (15) as presented in

(Chakarov, 2020).

ICINCO 2021 - 18th International Conference on Informatics in Control, Automation and Robotics

446

Figure 6: Interaction force in passive mode without pressure

in the chambers: a) experiment; b) simulation.

The simulations were conducted with the same

values of the kinematic parameters used in the

previous experiment: starting angle

= 260°,

amplitude q

= 20° and frequency of ω = 2.1 rad/s.

The forces applied by the hand of human operator

to overcome the mechanical impedance of the

exoskeleton are calculated according to equation (2).

The graph in Figure 6 b) shows the change in the

interaction force (2) as result of the inertia torque (3),

the friction torque (5) as well as the exoskeleton

gravity (6). The values of the mechanical parameters

using for calculations with equations (3) – (6) are

shown in Table 1.

4.2 Passive Mode of Interaction with

Gravity Compensation

A. Experiments. In this experiment, the exoskeleton

arm is gravitationally compensated in the initial

position

. For this purpose, one chamber of the

pneumatic cylinders is supplied with a pressure equal

= 250 kPa and the other - p

= 0, after which the

supply and discharge valves of the pneumatic

cylinders are closed. In this case, the passive forces

applied by the operator hand are result not only of the

sources mentioned in the previous experiment but

also of the forces

resulting from the

compressibility of the air in the cylinder chambers.

The operator has performed harmonic movements,

from the initial position

= 260

, with amplitude of

about

= 23

and frequency of about ω = 2.7 rad/s.

The change in interaction force is shown in Figure 7

a). The experiment shows a force change in both

directions about ±4.5 N.

Figure 7: Interaction force in passive mode with gravity

compensation: a) experiment; b) simulation.

Table 1: Electric and pneumatic drives parameters.

Parameter name Value

Motor efficiency η

90 %

Motor inertia J

508.10

-6

kgm

First stage reduction n

3 : 1

Second stage reduction n

2 : 1

Transmission efficiency η

97 %

Transmission inertia J

173 10

-6

kgm

Piston area side 1 s

314 10

-6

Piston area side 2 s

264 10

-6

Pneumatic cylinder diameter D 0.020 m

Pneumatic cylinder stroke L 0.125 m

Piston starting position X

0.050 m

Actuation viscous friction B 665 Ns/m

Actuation Coulomb friction F

8.61 N

Exoskeleton mass M

2.085 kg

Exoskeleton inertia J

0.198 kgm

Radius of EE N 0.660 m

Radius of pulley 1 r 0.0315 m

Coordinate 1 of mass center ρ

0.256 m

Coordinate 2 of mass center ρ

0.031 m

B. Simulations. In the conducted simulation, the

interaction force includes all components according

to equation (2). The torque of the pneumatic drive

(13) is calculated in the initial position

with

pressures of the two chambers, respectively p

= 250

Evaluation of the Capabilities of a Hybrid Driven Exoskeleton in Passive Mode of Interaction

447

kPa and p

= 0. The same harmonious movement as

in the previous experience is simulated with the

kinematic parameters: starting position

= 260°,

amplitude q

= 23° and frequency of ω = 2,7 rad/s.

Figure 7 b) shows the force of interaction which is the

result of the sum of dynamic forces, gravity forces

and elastic forces in the chambers of the pneumatic

cylinders. The elastic force acts as an elastic balancer

of the gravity load. The interaction force is 0 at the

initial position and fluctuates ± 4.5 N around this

value, as in the previous experiment.

5 CONCLUSIONS

The paper reveals the mechanical design of an

exoskeleton of the upper limbs, which uses the

scheme of hybrid drive with electric and pneumatic

actuation, thus producing a lower impedance due to

pneumatics, while maintaining high driving force and

rapid force response due to electric actuation. Further

reduction of the mechanical impedance is achieved by

placing the components of the hybrid drive in the

fixed base.

Several experiments with the available

mechanical prototype of the exoskeleton were

performed in the work to test the hypothesis that the

proposed hybrid drive is able to provide the two

modes of interaction: a) “robot in charge” mode,

when the exoskeleton applies forces with high

impedance and b) “patient in charge” mode when the

forces of interaction with the operator are low due to

the reduced impedance of the robot.

In the experiments, the force between the operator

and the exoskeleton was evaluated. The force of

interaction was obtained from passive forces, which

are the result of inertia, friction and gravity, as well

as the elasticity of pneumatics. In “patient in charge”

way the patient-initiated harmonic motion was

studied in two cases - without pressure in the

chambers and with pressure for gravity

compensation.

In the first case, the force of interaction

determined mainly by the exoskeleton gravity

indicates that the exoskeleton arm is relatively heavy

(

=-8.2 N). As a result of added inertia and friction

forces from harmonic movements, this force

oscillates from -5.8 to -10.2 N. When gravity is

compensated passively by pressure in the chambers

of pneumatic cylinders, the force of interaction is

determined by the elastic forces of compressed air, as

well as by the inertia forces and friction. For the

selected harmonic deviations, the force of interaction

reaches relatively high values (

Fh = ± 4.5 N).

The work also includes several computer

experiments to assess the parameters of the actuators

and the correctness of the constructed mathematical

model. Computer experiments show a similar change

in the interaction force as in the real experiment. It

can be noted that the parameters introduced in the

model such as exoskeleton mass and coefficients of

Coulomb and viscous friction in the actuators (Table

1) have quite high values.

The conducted experiments show that the

resulting forces of interaction in passive mode are

essential in terms of transparency, but are not a threat

of security. In active mode of operation, the forces of

interaction can be reduced by active compensations,

but the created low values of the forces in passive

mode are a guarantee of general security.

The design and control of this exoskeleton are

under development. Future work will be done by

incorporating the controller and assessing the

transparency and safety of the interaction between the

patient and the exoskeleton in active mode.

ACKNOWLEDGEMENTS

This research was supported by the Operational

Program "Science and education for smart growth"

through the project “MIRACle”, № BG05M2OP001-

1.002-0011, to which the authors would like to

express their deepest gratitude.

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