TWO LINK COMPLIANT ROBOT MANIPULATOR FOR HUMAN

ROBOT COLLISION SAFETY

Muhammad Rehan Ahmed and Ivan Kalaykov

School of Science and Technology, AASS,

Orebro University, SE 701 82,

Orebro, Sweden

Keywords:

Motion control, Adatable compliance actuator, Smart materials, Magneto-rheological ﬂuid, Collision safety,

Human robot interaction.

Abstract:

For successful human robot interaction (HRI), collision safety as well as position accuracy are equally im-

portant. Robot is required to demonstrate safe sharing of work space with humans and to exhibit adaptable

compliant behavior that comply with interaction forces generated upon contact. We present an approach for

acquiring reconﬁgurable compliance using semi-active actuation mechanism, where compliance is achieved

by controlling the viscous properties of magneto-rheological (MR) ﬂuid. In this paper, we have discussed three

essential modes of motions required for safe physical HRI. Then, we have shown collision safety of our robot

based on static and dynamic collision testing in different motion modes. Finally, experimental results validate

the signiﬁcance of our proposed approach for human robot collision safety and high position accuracy.

1 INTRODUCTION

Next robotic generation requires to have direct and

physical contact with human in performing robotic

tasks. The successful pHRI requires to handle the

interaction forces in smart way assuring high level

of human safety by preventing injuries and damages.

This leads to development of ideal safe robot manipu-

lator offering high stiffness in non contact phases and

lowstiffness in contact phases of the task, maintaining

collision forces within the human pain tolerance limit

and to display high position accuracy. These char-

acteristics necessitate the use of compliant actuation

mechanism instead of stiff actuation.

Active compliant mechanisms (T.Lefebvre et al.,

2005), (M.Kim et al., 2004) posses severe threat

to the joints upon rigid impacts (S.Haddadin et al.,

2007) and usually suffers from delayed contact re-

sponse, higher cost and complex control strategies.

Passive compliant mechanisms (C.M.Chew et al.,

2004), (B.Vanderborght, 2007) having passive ele-

ments (springs, sliding axels) achieve the compliance

on the cost of higher system complexity. Variable

stiffness (A.Bicchi and G.Tonietti, 2004), (T.Morita

et al., 1999), (B.Vanderbrought et al., 2006) is

achievedby using elastic element in the joints with the

cost of reduced position accuracy and energy losses.

We have proposed an approach based on semi ac-

tive compliant actuation mechanisms with magneto

rheological (MR) ﬂuid based actuator. Our actuation

mechanism is an assembly of MR ﬂuid brake/clutch

and DC-servo motor. Compliance is controlled by

the application of magnetic ﬁeld to drive the vis-

cous properties of ﬂuid while the position control is

achieved by a standard DC motor control. This results

in much simpler control system compared to the com-

pliance control schemes used in active and passive

compliant devices (M.Danesh et al., 2006), (R.Carelli

et al., 2004). Here we have analyzed the human robot

collision safety for HRI tasks based on static and dy-

namic collision while keeping high position accuracy.

2 ADAPTABLE COMPLIANCE

Robots lack one major skill compared to biological

systems, namely adaptable compliance or variable

stiffness property. The successful execution of safe

interaction task necessitates the use of compliant mo-

tions, allowing a robot to comply with the interac-

tion forces generated by its contact. This property

can be mimicked by using safe actuator mechanism

with adaptable compliance instead of traditional stiff

actuation mechanism.

Human skeletal antagonistic pair of muscles is the

biological similitude of the ideally safe and adaptable

compliant robotic actuator where humans have the di-

518

Rehan Ahmed M. and Kalaykov I..

TWO LINK COMPLIANT ROBOT MANIPULATOR FOR HUMAN ROBOT COLLISION SAFETY.

DOI: 10.5220/0003178405180521

In Proceedings of the International Conference on Bio-inspired Systems and Signal Processing (BIOSIGNALS-2011), pages 518-521

ISBN: 978-989-8425-35-5

 2011 SCITEPRESS (Science and Technology Publications, Lda.)

Figure 1: Two link planar robot prototype.

rect control over these muscles to generate almost any

desired motions. Human arm exhibits the required

three principle modes of motions (stiff, compliant and

soft) by controlling the tension between their antago-

nistic set of muscles. Thus, a pair of continuously

controlled antagonistic muscles ensures the required

level of adaptable compliance and position accuracy.

2.1 Modes of motion for safe HRI

In HRI tasks, purely motion control strategy for con-

trolling the robot interaction forces is not enough,

mainly due to imprecise modeling of the robot ma-

nipulator and the highly unpredictable nature of the

HRI itself. For safe HRI, robot has to execute simul-

taneously multiple-axes motions based on feedback

signals. This usually involves a combination of sev-

eral motions varying from fully stiff to fully compli-

ant. The contact situations may vary depending upon

the speciﬁc requirement of interaction task, but in all

cases the robot has to execute three different modes

of motion namely:

Stiff motion, refers to robot motion in unconstrained

free work space. The reaching of desired position is

achieved by a standard position and velocity control.

It manifests zero compliance, therefore only this mo-

tion mode is not sufﬁcient for performing HRI tasks.

Soft motion, refers to robot motion in a dynami-

cally constrained work space. The dilemma of avoid-

able/unavoidable collision with a sudden, unexpected

intrusion of an obstacle for example, human body or

part of it, implies the necessity of switching from fully

stiff to zero-stiff joint in order to cut the transmission

of a torque to the adjacent robot link.

Compliant motion, represents all transitions between

stiff and soft motion. In some situations the human

wants to superimpose its motion over the robot’s spec-

iﬁed motion. For such conditions the robot has to ac-

complish compliant motion mode.

For our research two-link planar experimental

robot prototype was set-up (Figure 1). The desired

adaptable compliance or variable stiffness is intro-

duced by our dedicated MR ﬂuid actuator, one for

driving each joint. Operational smoothness in the

performance of the MR ﬂuid actuator mechanism is

shown in (R.Muhammad.Ahmed et al., 2008). Fully

activated MR ﬂuid actuator (max clutch current) cor-

responds to stiff motion mode. Alternatively, fully de-

activated MR ﬂuid actuator (zero clutch current) con-

forms to soft motion mode. All levels between fully

activated and deactivated actuator refers to the com-

pliant motion mode. These transitions can be tuned

depending upon the type, geometry and modalities of

the contact object accordingly.

2.2 Collision Safety Analysis

Previously, achieving high position robot accuracy

was considered as the main objective. Now, with the

emergence of new trends and applications in service

robotics, there is a strong belief that both position ac-

curacy and the collision safety are equally eminent

and indispensable for tasks involving HRI. A contact

phase is considered to be safe, only when the robot

exerted collision forces remain under the human pain

tolerance limits and never causes injury to the human.

This, consequently, formulates a criterion for the col-

lision safety analysis of the robot manipulator based

on static and dynamic collision.

Static collision, appears in situations where robot ma-

nipulator is directed to collide with the human, and

the collision is performed at very low speed, typically

less than 0.2 m/s. In order to evaluate the safety per-

formance several researchers have suggested a col-

lision force of 50N as a human pain tolerance limit

(Y.Yamada et al., 1996). Therefore, we have em-

ployed collision force threshold of 50N as a boundary

between the unsafe and safe regions of operation. We

did static collision tests and analyzed the static colli-

sion safety performance in both the stiff and compli-

ant modes of motion.

Dynamic collision, replicates the condition where

robot is forced to collide with the human at higher

speeds. Since the topic of human robot colli-

sion safety in dynamic collision is relatively new in

robotics, no speciﬁc standard has been established

yet. However, in order to evaluate the safety per-

formance at dynamic collision, head injury crite-

rion (HIC) and abbreviated injury scale (AIS) are

currently employed (A.Bicchi and G.Tonietti, 2004),

(J.Versace, 1971). Head injury criterion (HIC) deﬁnes

the index for injury severity (damages) and used by

automobile industry for car crash. HIC value greater

than 1000 refers to a very severe head injury. For

the normal operation of machines, a HIC value of

100 is suggested. Therefore, for the safety perfor-

TWO LINK COMPLIANT ROBOT MANIPULATOR FOR HUMAN ROBOT COLLISION SAFETY

519

Figure 2: Experimental setup block diagram.

mance evaluation of our proposed actuation mecha-

nism, robot link is forced to collide with a ﬁxed ob-

stacle at a certain speed in both stiff and compliant

modes of motions.

3 EXPERIMENTS

Our experimental setup is shown on Fig 2. Real time

interface between the robot and the control computer

is realized through dSPACE hardware, the Real-time

workshop of Matlab and dSPACE control desk.

3.1 Static Collision Safety Analysis

Figure 3 explains the static collision safety perfor-

mance and position accuracy without the adaptable

compliance control, where the MR ﬂuid actuator is

only working in stiff mode imitating traditional stiff

actuation mechanism. In Figure 3a a constant bias of

approximately 2N is present in a force sensor data,

where as the approximate time at which the wall con-

tact occurs is around 7 seconds. It has been noted

that within 10 seconds of wall contact, the collision

force rises to around 50N and robot goes into the re-

gion which is unsafe for HRI. Therefore, operating

in stiff mode without adaptable compliance control

is not suitable for interaction tasks involving HRI.

Robot position accuracy in stiff motion mode during

static collision is illustrated in Fig 3b.

0 2 4 6 8 10 12 14 16 18

Time (s)

Collision Force (N)

Unsafe Region

For HRI

Safe Region

for HRI

(a) Collision force.

0 2 4 6 8 10 12 14 16 18

Time (s)

Joint Angle (Deg)

3.6 Deg

(b) Joint angle.

Figure 3: Stiff mode static collision.

Figure 4 describes the static collision safety per-

formance and position accuracywith compliance con-

trol. Collision force threshold value of 33N was set to

analyze the static collision safety as shown in Fig 4a.

Initially, MR ﬂuid actuator operates in stiff mode and

as soon as collision force reaches the threshold value,

actuator mode switching occurs from stiff to compli-

ant mode, thus keeping the robot to operate within the

safe region of operation suitable for HRI and never

allows a robot to go into unsafe region and to cause

injury to human. Fig 4b demonstrate the robot posi-

tion control accuracy in compliant motion mode.

0 5 10 15 20

Time (s)

Collision Force (N)

33N

Collision Force

Threshold value

17.6N

Unsafe Region

Safe Region for HRI

(a) Collision force.

0 5 10 15 20

Time (s)

JointAngle (Deg)

3.3 Deg

(b) Joint angle.

Figure 4: Compliant mode static collision.

3.2 Dynamic Collision Safety Analysis

Figure 5 illustrates the dynamic collision safety per-

formance and position accuracy without adaptable

compliance control (MR ﬂuid actuator in stiff mode).

Fig 5a indicates a collision force of approximately

70N exerted on the ﬁxed wall at the time of the con-

tact. Then, the collision force settles down to approx-

imately 38N. Fig 5b represents stiff mode position ac-

curacy of the robot manipulator in dynamic collision.

0 2 4 6 8 10 12

Time (s)

Collision Force (N)

70 N

38 N

(a) Collision force.

0 2 6 8 10 12

100

150

200

Time (s)

Joint Angle (Deg)

184 Deg

(b) Joint angle.

Figure 5: Stiff mode dynamic collision.

Figure 6 demonstrates the robot dynamic collision

safety performance and position accuracy with com-

pliance control, where a robot manipulator operating

in compliant mode is commanded to make hard col-

lision with the ﬁxed wall. With a speed of 60 per-

cent of full scale, Fig 6a shows a collision force of

approximately 44N exerted on the wall at the time of

the contact, which is fairly small as compare to the

collision force occurred in stiff motion mode (70N).

Additionally, just after the contact, the collision force

is reduced to approximately 17N, which is in accord

with the collision force exerted upon the wall in com-

pliant mode static collision. This comparison indi-

cates the effectiveness of our proposed semi active

BIOSIGNALS 2011 - International Conference on Bio-inspired Systems and Signal Processing

520

0 2 4 6 8 10 12

Time (s)

Collision Force (N)

17N

44 N

(a) Collision force.

0 2 4 6 8 10 12

−200

−150

−100

−50

Time (s)

Joint Angle (Deg)

184 Deg

(b) Joint angle.

Figure 6: Compliant mode dynamic collision.

compliant actuator mechanism in terms of dynamic

collision safety. Fig 6 explains the position control

performance while performing dynamic collision in

compliant mode.

4 CONCLUSIONS

We have proposed an efﬁcient solution based on semi-

active compliant actuation mechanism enabling com-

pliant robot behavior needed for HRI tasks as well

as providing high inherent collision safety originated

by controlling the properties of smart materials. It has

been justiﬁed that MR ﬂuid actuator is capable of gen-

erating complete range of motions typically required

for HRI applications with high inherent safety. We

have demonstrated simultaneously, the superior capa-

bility of handling higher payload with eminent posi-

tion accuracy and guaranteed collision safety focus-

ing simultaneously on static and dynamic collision.

Finally, it has been veriﬁed that semi-active compli-

ant robot is demonstrating the required features of

safe robot manipulator (provide high stiffness in non-

contact phases and low stiffness in contact phases of

the task) and maintaining collision forces well under

the human pain tolerance limit.

Future studies will be focused on investigating the

HIC for dynamic collision safety analysis.

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