Computer Modeling and Synthesis of Human Centered Robots

1
Computer Modeling and Synthesis
of Human Centered Robots
D.Chakarov, T.Tiankov, K.KostadinovInstitute
of Mechanics Bulgarian Academy of Sciences

• 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.

2
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.
3

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.
4
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 1985?.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
(1)
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.

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.
5
2.1 Effective inertia modulation
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 torques 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)
6
The configuration variation influences by the
Jacobean J of the effective inertia of the end
effector.
(3)
a) b)
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.
7
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.
8
The configuration variation, as with the inertia,
defines transformations among the stiffness
matrixes and the compliance matrix in joint and
absolute co-ordinates.
(4)
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.
9
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.
10
For 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.
Passive compliance adjuster
11
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).
(6)
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.
12
6. Computer experiments with Solid Dynamics 2004.
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.
Fig.1. Mobile robot and fixed barrier.
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 robots 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 0.6m and diameter
0.3m. The joints of the manipulator are
presented like cylindres with length l10.4,
l20.4, l30.2 m and diameters 0.08m.
13

Two arm configurations are made and simulations
are presented on figures 2 a) and b). Two cases
of these models are examinated with solid (
m15.429 kg, m25.429 kg, m31.853kg) and
hollow (m10.677kg, m20.677kg, m30.387kg)
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
V0.2m/s. After the contact the robot passes
additional motion equal to 0.05m.
?)
b) Fif. 2. Resulting collisions at two arms
configurations a) and b)
14
The experiments are with the following
sequence The manipulator is with solid bodies ,
m15.429 kg, m25.429 kg, m31.853kg, the
stiffness and demping of joints are k1k2k3 500
Nm/rad , b1b2b31 Nms/rad. On figure 3
a) and b) is shown the variation of the contact
force on collision between the endefector and
the barriere.

?) b) Fig. 3. The variation of the force of
contact at configuration a) and b)
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 68N.
15
If the manipulator is more compliant the contact
force decreases. On graphics 4 a) and b) joint
stiffnesses are k1k2k3100 Nm/rad and
respectively the contact force decreases until
25N and 14N.

?) b) Fig.4. Variation of the contact force
in configurations a) and b) with low arm
stiffness m15.429 kg, m25.429 kg,
m31.853kg , k1k2k3 100 Nm/rad, b1b2b31
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.
16
?) b)
Variation of the compliance ellipsoids are shown
at configurations a) and b) for two values of the
joint stiffness k1k2k3 100 Nm/rad
and k1k2k3 500 Nm/rad.
17
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 m10.677kg,
m20.677kg, m30.387kg.

?) b) Fig. 5 Variation of the contact force
in configurations a) and b) on light arm robot
m10.677kg, m20.677kg, m30.387kg ,
k1k2k3 100 Nm/rad, b1b2b31 Nms/rad.

Respectively when the hand is with low inertia
the impuls force arises until 40 N. The low
inertia allows faster attenuation of impact
oscillations.
18
The experiments are made with joints damping
b1b2b31 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 b1b2b31 Nms/rad and d) without
damping b1b2b30 Nms/rad.

?) d) Fig. 6. Variation of the contact force
in configurations a) with hands parameters
m10.677kg, m20.677kg, m30.387kg,
k1k2k3 100 Nm/rad and with joint damping ?)
b1b2b31 Nms/rad, ?) b1b2b30 Nms/rad.
19
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.
20
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
loops10.
21
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