Page 208 - Mechatronics for Safety, Security and Dependability in a New Era

Ch40-I044963.fm Page 192 Tuesday, August 1, 2006 8:21 PM
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The Motor controller, which is shown in Figure 4, is able to control the coefficient of viscosity of
ISA-4's output axis dynamically by controlling the induced electromotive force of the Motor unit. Fig-
ure 5 shows the property of the controlled coefficient of viscosity. And as shown in the figure, the values
of the coefficient of viscosity are monotone increasing for the normalized input which is sent to the
Motor controller. By applying this function, ISA-4 achieved to control SDR-4XII's joints' stiffness in a
soft or solid way.
Furthermore, some kind of shock absorbing mechanism against the loads by impact shock is needed
for the robot safe design. For this purpose, it is popular to apply torque limit mechanisms between
mechanical links and actuators. However those kinds of mechanisms sometimes cause a deterioration of
the robot's control performance such as a deviation increase of angular position. Therefore SDR-4XII
adopted a new shock absorbing system in ISA-4 instead of a mechanical method to settle the problem.
As the magnitude of the impact shock which causes the plastic deformation is able to be detected by
monitoring the kinetic energy, the Control module monitors the 2nd order differential term of the kinetic
energy to detect quickly by calculating the approximate expression Eqn.l. And if the detected value is
large enough to break the robot's body, the Control module immediately cuts off ISA-4's output torque
and makes the stiffness of the output axis soft by the viscosity friction control function described above.
Figure 6 shows the advantageous effect of this shock absorbing system. We added the impact shock
which is equal to 24-joules kinetic energy to the mechanical link attached to ISA-4M for one hundred
times by dropping an 8kg weight from 30cm height, and compared the values of Gear unit's backlashes.
The changes of backlashes with the shock absorbing system are about 15% at the maximum, though the
changes without the system are about 75% at the maximum as shown in Figure 6.

f(I,6,t) = K—lit)—9(t) (1)
dt dt
K : Torque constant [N-m/A], / (t) : Electrical current [A], 8 (t): Angular position of output axis [rad]
:
|
. 0.3 ; *- *
f
i \ 1
I ? I 1
;§ 5. o.i .!• •'1 with Shock Absorbing System
•*j
* • ; •
o
0 20 40 60 80 100 0 20 40 60 80 100
Normalized input (%) Tims of the impact shock (about 24 J)
Figure 5: The controlled coefficient of viscosity Figure 6: Effect of the Shock Absorbing System

SAFETY FUNCTIONS ON SDR-4XII

As SDR-4XII is supposed to be used in a home environment, the robot has several capabilities for
self-protection and user-protection (Iribe 2004-2). For the safety operation of home robots, they must be
able to fall over softly by controlling their posture to protect themselves and the environment from the
damage of their falling over, and of course, home robots must stand up by themselves (Kuroki 2003-1)
(Fujiwara 2003). Therefore SDR-4XII makes its joints soft and loose to soften the damage when it falls
over. And when the robot stands up, it makes its joint solid and stable to get high accuracy position
control. The behavior of these features, the Falling over motion control and the Standing up motion
Control are shown in Figure 7 and 8, and were achieved world first on the previous prototype SDR-4X.
And SDR-4XII has capabilities of detecting added loads and its internal temperature rise to protect
the robot itself and users from its overloads and overheats. When constant loads or impact loads are
added to the robot, it measures the strength of the loads by ISA-4 which is used in each joint. If the loads