Game Development Reference
In-Depth Information
important that this performance be obtained with limited voltage and currents
(the robot actuators are located close to the user);
Quality of force: A robot actuator must generate a torque that is as 'proper' as
possible. It is important that the friction be as low as possible, particularly dry
friction, which is difficult to offset;
Force dynamics: The robot actuators must have a large acceleration capacity to
be able to give a quick response to the movements of the user. Their inertia must
be as low as possible.
The robot actuators used most commonly on the present haptic interfaces are
direct current electric motors that help to obtain favourable performance under good
safety conditions and at a reasonable cost.
There are a lot of other actuation technologies (ultrasound motors, AMF, electric
and hydraulic actuators, artificial muscles, etc.). Some of these technologies are used
on laboratory prototypes. However, they are used in very few commercial interfaces.
8.3.3.3 Reduction stages
The electric motors used on most of the haptic interfaces have been designed for high-
speed operation with a low torque. The user, on the other hand, can apply force that
is relatively high, but with low speed.
To adapt the actuation to these capacities, we introduce the reduction stages,
between the motors and the articulations of the robot, which improve the force capacity
and reduce the rotation speed.
It is important to note that if a reducer can help to increase the capacities of
the robot (in the absence of losses, its articular torques are increased with respect to
reduction r and its apparent stiffness by the square of the reduction ratio), it also
magnifies its defects (the inertia of the rotors of the motors is multiplied by the square
of the reduction ratio while the internal friction of the motors is multiplied by r ).
It is thus necessary to find a compromise between the improvement of the force and
stiffness capacities of the robot and the reduction of its transparency.
In practice, force feedback interfaces do not often use the gear reducers, Harmonic
Drive reducers or belt reducers that are used frequently on machine tools or industrial
robots, because these solutions have a limited reduction ratio and/or transparency.
Thus the robots of the German Aerospace Center which use Harmonic Drive reducers
are equipped with articular force sensors to measure and offset the friction, which
increases the complexity of the system.
What is generally preferred is the use of cable capstan reducers consisting of two
pulleys connected by a flexible cable. The cable is connected by adherence to the leading
pulley (this requires winding it around several times (Massie, 1993)) and is fixed at each
of its ends on the pulley being led. This type of reducer is very transparent. It provides
very low play as well as very low friction. However, it requires a minimum winding
size of the cables below which it does not operate properly and tires quickly. Thus,
the reduction ratio is limited for a small overall dimension. In practice, it is generally
less than 20, which does not allow application of a very high force. Beyond this, it
is necessary to use other solutions such as reversible ball screw reducers (Friconneau
et al., 2002; Garrec & Riwan, 2002).
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