Computer Modeling and Synthesis

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Feb 23, 2014 (3 years and 6 months ago)

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Computer Modeling and Synthesis

of Human Centered Robots

1.
Introduction



2
.

Basic

mechanical

characteristics

defining

the

mutual

interaction

safety
.



2
.
1.
Effective inertia modulation



2
.
2.

Effective stiffness modulation
.





2.
3
. Effective damping modulation.




3.
Modeling and simulation
with
Solid Dynamics 2004+
program software.

6.Conclusions.

D
.
Chakarov
,


T
.
Tiankov,

K
.
Kostadinov

Institute of Mechanics


Bulgarian Academy of Sciences

1.
Introduction



In the future, robots will take an active role not only in industry,
but also in human life. Especially thriving is the development of
robots for domestic services and entertainment.

A big number of scientific publications and scientific
congregations and events in recent years form a new scientific
area dedicated to the mutual interaction “man
-
robot”. This is an
interdisciplinary scientific field, that includes robotics, cognition
sciences, physiology and sociology.

Investigations in this field find application as robotized systems
for search and save in urban media as robots for personal
catering, service robots, robots for domestic work (cleaning,
trimming), medicine robots (in surgery and rehabilitation), robots
for entertainment (toys, pets, guides and others).

Robots that share the space and environments with human are
named human
-
centered robots [1].




Human
-
centered robots

have to meet the safety requirements
except the traditional requirements for performance.


The manipulator safety depends on its mechanical, electrical and
software characteristics. It is known that by means of sensors and
feed backs can be cut off some potential anomalies and to be avoided
cases of not desired contact or collision. But even the most robust
systems are not guaranteed of some unpredictable electrical, sensor
or even software errors. That is why the mechanical characteristics
of the robotized systems are the key factor for increasing the whole
safety.


The aim of this paper is a evaluation of the basic mechanical
characteristics limiting the manipulator safety. The choice of these
parameters is shown with respect to impact interaction, oscillation
damping and contact configuration. The results from investigation
are shown graphically using Solid Dynamics 2004+.


2.
Basic mechanical characteristics defining the mutual
interaction safety.


The basics of the control approach of the dynamic mutual interaction
between the robot manipulation system and the surrounding
environment are founded by
Neville

Hogan in
1985г.
[2]
.
This approach
is called impedance control. The force of the manipulator end effector
interaction with the surrounding environment can be presented by the
equality:








t
v
M
]
V
V
[
B
]
X
X
[
K
F
0
0
int







The stiffness


K
, the damping


B

and the inertia
M

present the components
of the mechanical system impedance.

It is important to produce manipulators possessing naturally low impedance in
order to achieve natural safety in the mutual interaction “man
-
robot”.
Unfortunately, most of the up to date robots designed mainly for industrial
purposes possess high effective impedance.[3].

(1)

The manipulation system impedance must be specified to certain
suitable levels in order to increase the safety level. This can be
achieved by means of specifying separately the components of the
mechanical impedance effective
inertia, damping and stiffness.

2.1 Effective inertia modulation


t
v
J
M
J
]
V
V
[
J
B
J
]
X
X
[
J
K
J
F
1
q
T
0
1
q
T
0
1
q
T
int













If the end effector effective inertia is reduced successfully then the
shock impulse force is also reduced because it is dependent mainly on the
inertia and on the velocity variation.

The effective inertia can be actively modulated by means of feed backs and
then it is dependent on the characteristics of the close loop control system.
A series of restrictions exist here such as driving forces and torque’s range,
sensor delays, stability problems and others.

Another inertia modulation approach is the passive approach that requires
kinematics redundancy in the manipulation system. The redundant number of
degrees of mobility allows manipulator configuration variation at the same
positioning of the end effector [4]. The configuration variation defines
transformations not only among the co
-
ordinates, but also among the inertia
matrixes in joint and absolute co
-
ordinates. Thus, equation (1) presented by
means of matrixes of stiffness, damping and inertia in joint co
-
ordinates is
shown below:

(2)

The presented in the figure configurations of collision of
the end effector of the mobile manipulator at one and the
same point correspond to the case a) at high inertia and b)
at minimal inertia in a horizontal direction.


a)


b)

The configuration variation influences by the Jacobean
J

of the
effective inertia of the end effector.


1
q
T
J
M
J
M



(3)

2.2.

Effective stiffness modulation

.


The low inertia reduces the impulse force but after collision in the
phase of contact for the reduction of the contact force major role act
the compliance qualities of the manipulator.

Compliance is defined for the increase of the safety level at contact in the
mechanical structure. Two basic approaches are known


active and passive
for the stiffness modulation to secure levels.


The active approach is based on the use of sensors and position and force
feed backs by means of which a desired parametric proportion is balanced.
It guarantees a wide range of stiffness variation, but it does not ensure high
level of safety due to a low resolution or noise of the sensors, long
calculation time and instability in the servo system.


The passive approach is realised by means of the physical
compliance of the robot limbs and/or additional compliance
mechanisms [5
].

This approach is independent of the servo
systems, but the range of the impedance parameter variation
is limited. Passive approach is more convenient in the “human
-
robot” interaction.


The configuration variation, as with the inertia, defines
transformations among the stiffness matrixes and the compliance
matrix in joint and absolute co
-
ordinates.



1
q
T
J
K
J
K




D
4

D
3

D
2

D
1

O
1

O
2

T
q
J
B
J
B

The compliance matrix B is inverse of the stiffness matrix.
The variation of
the qualities of the compliance matrix in different directions is interpreted
graphically by the compliance ellipsoid.

In figure is shown the influence of the configuration variation
on the shape of the compliance ellipsoid [6].

(4)

It is necessary to use
redundancy

for the stiffness
modulation to safety levels by using the passive approach. The
redundancy is either in actuation or in kinematics.
The use of a
actuation redundancy is characteristic for the parallel manipulators.

Kinematics redundancy is used with the serial manipulator, as in the
joints of the open chain structure is introduced compliance. The higher
number of compliant joints allows specification of a desired matrix of
the end effector compliance.


Thus, by means of the three limbs manipulator presented in the
figure it is possible to modulate a maximal stiffness along the
tangent to the trajectory of motion.

F
or realisation of passive compliance in the robot joints controllable
compliant mechanism are used . In each joint except an actuator for
position control is added an additional actuator for stiffness variation
of a passive compliant element. Thus, joint position and stiffness are
controlled independently. A similar solution is known [9]
-
(Ogata T., T.Komiya
and Sh.Sugano, 2000) at which in the joint it is mounted a compliant
element
-
a sheet spring.
Stiffness is modelled by means of variation of the
length of the deformation part 3 of the spring.

0
1
3
4
5
2

Passive

compliance

adjuster




2.
3
. Effective damping modulation.




The introduction of compliance increases the level of safety in the
contact realisation, but increases the possibilities for oscillation in the
contact as well. Damping is modulated by means of using two basic
approaches
-
active and passive in order to overtake this problem. The active
approach uses sensors and a position and a force feed back, by means of
which an additional damping force of the drives is maintained proportional to
velocity and directed against it [4](Kang S.,2001).

q
B
F
q
q


The passive approach is realised by additional damping mechanisms. Thus, in
the shown above solution

Passive

compliance

adjuster

[Ogata T., T.Komiya
and Sh.Sugano, 2000]
except compliant devices, electro
-
magnetic pseudo
dampers are introduced. The damping effect is created by control of the
electric current in the electro
-
magnetic brakes proportional to the joint
angular velocity.

(6)

6
.
Computer experiments with Solid Dynamics 2004+
.


m
1

m
2

m
3

Fig.1. Mobile robot and fixed barrier.

The security of the “human


robot”

interaction
is been
evaluated when changing based
mechanical characteristics


inertia, damping and stiffness.
Computer simulations are made,
using Solid Dynamics 2004+
program software.

A model of a mobile robot is used, shown on fig. 1.
The experiments are made
on the
plane

where the mobile base of the robot has one degree of freedom
and the robot’s
arm

has three degrees of freedom.

The kinematic redundancy on the
plane

allows a configuration change at a
fixed position of the endefector.

The mobile base is treated like a cylindre with height 0.6[m] and diameter
0.3[m]. The joints of the manipulator are presented like cylindres with length
l1=0.4, l2=0.4, l3=0.2 [m] and diameters 0.08[m].



Two
arm

configurations are made and simulations are presented on
figures 2 a) and b). Two cases of these models are examinated with solid
(
m1=5.429 [kg], m2=5.429 [kg], m3=1.853[kg]
) and hollow (
m1=0.677[kg],
m2=0.677[kg], m3=0.387[kg]
) bodies. An evaluation of
injury

is made,
researching contact force, when the endefector realizises
collision

with
the fixed solid
barrier
, shown of figure 2. The shoc between the
endefector and the barrier is realized at a horizontal motion of the
robot with speed V=0.2[m/s]. After the contact the robot passes
additional motion equal to 0.05[m].



а)




b
)

Fif. 2. Resulting
collisions
at two
arm
’s configurations a) and b)

The experiments

are with the following s
equence
:

The manipulator is with solid bodies ,
m1=5.429 [kg], m2=5.429 [kg],
m3=1.853[kg], the stiffness and demping of joints are k1=k2=k3= 500
[Nm/rad] ,

b
1
=b
2
=b
3
=1 [Nms/rad]. On figure 3 a) and b) is shown the
variation of the contact force on collision between the endefector and
the barriere.




When the arm configuration is changed, the effective inertia,
stiffness and resulting force are changed too. When using
configuration a) the impusle force is up to 160 [N], and in
configuration b) case it reaches up to 110 [N]. The
configuration a) contact force is about 116 [N], and that of
configuration b) is about 68[N].



а)




b)

Fig. 3. The variation of the force of contact at configuration a) and b)

If the manipulator is more compliant the contact force decreases. On
graphics 4 a) and b) joint stiffnesses are k1=k2=k3=100 [Nm/rad] and
respectively the contact force decreases until 25[N] and 14[N].






а)




b
)

Fig.4. Variation of the contact force in configurations a) and b) with
low
arm stiffness: m1=5.429 [kg], m2=5.429 [kg], m3=1.853[kg] ,

k1=k2=k3= 100
[Nm/rad], b
1
=b
2
=b
3
=1 [Nms/rad].

The experiments show that the insertion of compliance doesn't
decrease the impuls force. Its dimension depends radically on
the inertia

of the manipulator. By changing the configuration the
efective inertia and efective compliance are reduced partially.



V
ariation of the
compliance ellipsoids are shown at
configurations a) and b)

for two values of the joint stiffness


k1=k2=k3= 100 [Nm/rad] and



k1=k2=k3= 500 [Nm/rad].



а)




b
)

A reduction of the inertia can be achieved by creation of robot
with light arm.

On fig. 5 a) and b) is shown the variation of the contact force on robot
with light arm m1=0.677[kg], m2=0.677[kg], m3=0.387[kg].








а
)




b)

Fig. 5 Variation of the contact force in configurations a) and b)
on light
arm robot: m1=0.677[kg], m2=0.677[kg], m3=0.387[kg] ,

k1=k2=k3= 100
[Nm/rad], b
1
=b
2
=b
3
=1 [Nms/rad].

Respectively when the hand is with low inertia the impuls force
ari
s
es until 40 [N]. The low inertia allows faster attenuation
of
impact oscillations.

The experiments are made with joint’s damping b
1
=b
2
=b
3
=1
[Nms/rad]. The low stiffness creates contact oscillations,
wherefore it's necessary a joint damping. On fig. 6 c) and d) is shown
the change of contact force: c) with damping b
1
=b
2
=b
3
=1 [Nms/rad]
and d) without damping b
1
=b
2
=b
3
=0 [Nms/rad].






с)



d)

Fig. 6. Variation of the contact force in configurations a) with
hand’s parameters
m1=0.677[kg], m2=0.677[kg], m3=0.387[kg],

k1=k2=k3= 100 [Nm/rad] and with joint damping:
с
) b
1
=b
2
=b
3
=1
[Nms/rad],
д
) b
1
=b
2
=b
3
=0 [Nms/rad].

4. Evaluations and approaches for safety realization in human
centered robots.


The mechanical characteristics are the key factor for increasing the
whole safety during the interaction “human
-
robot”.

During a collision with a robot arm the impulse force is determined from the
speed of motion and the effective arm inertia. By a kinematics redundancy and
choosing an appropriate configuration the effective inertia can be reduced till
lower level (Fig. 3). During an unexpected collision it is impossible to guarantee
the necessary arm configuration. Therefore the low arm inertia is determined
for impact security. Human
-
oriented robots have to be with light bodies links
and motors (fig. 5). In case of need of more powerful and heavier motors, they
can be placed on the base using cables and other light transmissions [8].

During a contact, the contact force depends of the manipulator stiffness (fig.
3 and fig. 4), The human
-

centered robots have to be compliant for reduction
of the contact force.

The
stiffness

of the manipulation system must be specified to certain levels in
order to increase the level of safety.

5. Conclusions.

Two basic approaches are known


active and passive

for the
impedance modulation to secure levels.

The active approach

of
impedance

modulation is more accessible. It can be applied at the
conventional robot solutions but
the safety level

in these cases
is limited
.
The passive approach

requires redundancy and a special construction with
additional mechanisms.
This approach creates a natural security

in the
interaction with human, but paying the prize of lower performance.
It is
possible to use devices with controlled passive joint compliance [9] for
achieving requirements of performance and safety.

The introduction of compliance enhances the level of safety but during an
impact or immediately change of speed, the compliant arm is trended to
oscillations. The presence of
damping in joints decreases these oscillations

(fig. 6). The human
-
centered compliant robots have to possess joint damping
that can be achieved with active or passive devices.
The safety limitations of
the active approaches and the limitations in the performance of the passive
approaches can be overtaken by using a combined one.
The hybrid approach
includes appropriate compliant and damping devices [9] or additional motors in
joints [1], as well as a presence of sensors and control loops[10].


Литература:

[1] Zinn M., O.Khatib, B.Roth, J.K.Salisbury.

A New Actuation Approach for
Human Friendly Robot Design, The Int. Journal of Robotics Research, Vol.23, No 4
-
5,
pp.379
-
398, (2004).

[2] Hogan N
. Impedance Control: An Approach to Manipulation: Part II


Implementation, Trans. of the ASME:Journ. of Dyn. Syst., Meas. and Contr. Vol.107,
pp.8
-
16, (1985).

[3]
Kostadinov K., G. Boiadjiev
,
Development

of
Impedance Control Method
for

Mechatronic Systems, in

W. Schiehlen &M. Valasek , Proceedings of NATO ASI
Virtual Nonlinear Multibody Systems, Prague, Vol.1, June 23
-

July 3, 2002, pp.101
-
106.

[4] Kang S., K.Komorya, K.Yokoi, T.Koutoku, K.Tanie
. Reduced Inertial Effect
in Damping
-
based Posture Control of Mobile Manipulator, Proc. of the 2001 IEEE/RSJ
Int. Conf. on Intell. Robots and Syst., Maui, Hawaii, Oct.29
-
Nov.03, pp.488
-
493,
(2001).

[5] Okada M., Y. Nakamura and Sh. Ban
. Design of Programmable Passive
Compliance Shoulder Mechanism, Pros. Of the 2001 IEEE Int. Conf. on Robot.&Autom.,
Seoul, Korea, May 21
-
26, pp.348
-
353. (2001).

[6] Chakarov D.

Study of the passive compliance of parallel manipulators,
Mechanism and Machine Theory, Vol.34, No.3, pp.373
-
389, (1999).

[7] Gabrielle J.M. Tuijthof, Just L. Herder
, Design, actuation and control of an
anthropomorphic robot arm. Mechanism and Machine Theory, Vol.35, pp.945
-
962,
(2000).

[8]

http://www.uni
-
saarland.de/fak8/bi13wn/projekte/roboarm_eng.html

[9]

http://www.sugano.mech.waseda.ac.jp/wendy/arm/mia1
-
e.html

[10]

http://www.ai.mit.edu/projects/cog/