Biomedical Engineering Reference
In-Depth Information
can be grouped into a single block representing the teleoperator, as shown by the
dashed line. For
n
-DOF manipulation, the teleoperator can be viewed as a 2
n
-port
master - controller - slave (MCS) network terminated at one side by an
n
-port operator
block and at the other by an
n
-port environment network. The force - voltage analogy is
more often used than its dual to describe such systems [98, 99]. It assigns equivalent
voltages to forces and currents to velocities. With this analogy, masses, dampers, and
stiffness correspond to inductances, resistances, and capacitances, respectively.
For passive operator and environment blocks, a sufficient condition for stability is
passivity of the teleoperator (e.g., [98]). For the operator to be able to control the slave,
a kinematic correspondence law must be defined. In position control mode, this means
that the unconstrained motion of the slave must follow that of the master module in some
predefined or programmable scaling.
Teleoperation system transparency can be quantified in terms of the match between the
mechanical impedance of the environment encountered by the slave and the mechanical
impedance transmitted to, or felt by, the operator at the master [99, 100], with the
requirement that the position/force responses of the teleoperator master and slave be
identical [101].
In spite of the significant amount of research in the area of teleoperation, there are
still very few applications in which the benefits of transparent bilateral teleoperation
have been clearly demonstrated, even though some areas have great potential, including
teleoperated endoscopic surgery, micro-surgery, or the remote control of construction,
mining, or forestry equipment. Whether this is due to fundamental physical limitations
of particular teleoperator systems or to poorly performing controllers is still not clear.
From this perspective, probably the single most important challenge ahead is a better
understanding of the limits of performance of teleoperation systems. Toward this goal,
it would be useful to have a benchmark experimental system and tasking criteria to be
completed, for which various controllers could be tested. Unfortunately, it would be very
difficult to do this entirely through simulation due to the fact that the dynamic algorithms
necessary to develop a reasonable array of tasks would be just as much under test as the
teleoperation control schemes themselves. Furthermore, the minimum number of degrees
of freedom for reasonably representative tasks would have to be at least three, for example,
planar master/slave systems.
Specific improvements could be made to the fixed teleoperation controllers designed
via conventional loop shaping or parametric optimization. In particular, a class of operator
impedances that is broader than single fixed impedance, but narrower than all pas-
sive impedances, should be developed with associated robust stability conditions. Since
the control design problem was formulated as a constrained 'semi-infinite' optimization
problem, different algorithms could be tested or new ones developed. Like many other
multi-objective optimal control problems, robust teleoperator controller design problems
are likely to be hard to solve.
There seems to be much promise in the design of adaptive bilateral teleoperation con-
trollers with relatively simple and physically motivated structures. In particular, indirect
adaptive schemes based on Hannaford's architecture [99] are likely to succeed. Whereas
fast or nonlinear environment identification techniques are necessary in order to accom-
modate contact tasks, they are, however, difficult to develop, whereas operator dynamic
identification seems to be quite feasible [102]. Some of the difficulties encountered in
