Game Development Reference
In-Depth Information
Control of a force feedback interface
Claude Andriot and Florian Gosselin
Contributor: Philippe Fuchs
9.1 INTRODUCTION
This chapter deals with the control of force feedback interfaces. It is restricted to linear
systems of one degree of freedom. Virtual manipulation of an object by a user leads
to a transfer of energy from the user to the simulation. With a haptic coupling, this
energy transfer becomes bidirectional and the product of force-position represents the
mechanical work (Burdea, 1996). This energy transfer may have an adverse effect on
the stability (on the automatic control) and may harm the quality of the simulation.
This, in return, affects the feeling of the user via the haptic interface. This is especially
true since there is an input data sampling of the simulation, but also an output data sam-
pling to send information to the haptic controller. If the bandwidth of the system is too
low or if the simulation latencies are too large, the quality of the force feedback will be
very poor. Generally, an input and output sampling of the order of 1 to 2 kHz is recom-
mended to obtain a quality interaction (Fuchs, 1996). There are twomain concepts that
need to be taken into account in the design and control of the force feedback system:
First, the transparency of the manipulator that helps to evaluate the aspects related
to the performance of the haptic system (Lawrence, 1993). The dynamic behaviour
of the haptic interface, as it is felt by the user, must be controlled by the virtual
environment. For this, the user must not be disturbed by the mechanics of the
manipulator during his interaction with the environment. This involves a com-
pensation of its mechanical imperfections (balancing, friction and inertia) in the
control and a quality in the calculation of the interaction forces;
Second, as the stability of the interface is affected by the delays due to data sam-
pling, it is necessary to establish methods using which no energy is created by
the bidirectional interaction via the interface between the simulation and the user.
In fact, stability is guaranteed if we can prove that no energy has been created.
Actually, as we shall later see, it is the passivity that needs to be guaranteed and
not simple stability. This condition is applicable to systems using sampled data,
like the haptic systems (Colgate & Shenkel, 1994).
A global terminology can be defined to model the haptic interaction with vir-
tual environments (Adams & Hannaford, 1999). Haptic devices as electromechanical
interfaces help to provide proprioceptive sensations (force sensations) to a human
operator.
 
 
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