Keywords: Control , IPC Control, drift compensation, spring rest length, adpative control

In the literature, a number of interesting control schemes has been proposed for telemanipulation robotic systems and, in particular during the last years, passivity based control schemes have been widely used in order to guarantee the stability of the whole system. This paper deals with the problem of position drift, i.e. the error between master and slave positions during the execution of certain tasks, originated in particular when an Intrinsically Passive Controller (IPC) is adopted. In particular, it is shown that by introducing a proper modification of the basic control scheme, the position drift can be completely or partially compensated.

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Fig.2 depicts the physical interpretation of a IPC controller:

Position drift is used as a parameter for evaluating performances of a telemanipulation
scheme?.
...Important factors affecting position drift are the dynamics of both the manipulators and the passive controllers, as well as the transmission delay in the communication line.

Three cases of interaction have been studied: no environment, stiff environment, viscous environment

When an IPC control is adapted, a position drift appears between master and slave. Solution: introduce springs with variable rest lengths xl: Fl=(xmaster-xslave+xl)

The proposed control system compensates for the difference between the positions of the two manipulators by varying the rest lengths of master and slave controller?s springs.
A supervisor is inserted for each variable spring in order to compute the correct rest lengths to be imposed, and in this way one obtains an adaptive controller, whose parameters change according to the different environment one has to interact with:
A.) Goal supervisor: compute (1) force applied by human operator and (2) impedance of environment
B.) Inputs supervisor: Position and force measurements, time delay
C.) Result:compute (and possibly) compensate position drift at both master and slave side

?.Main idea of this new method is that the energy storage in the spring rest length
can be allowed only if external power supplied from the human operator is injected,
otherwise one could create energy?.

Results:
As one can easily verify, position drift has been completely removed in all the cases analyzed with the first proposed controller. Transient periods depend on the parameters of the IPC and they can be further improved.
In the passive approach, the new supervisor energetic port injects or extracts a finite amount of energy in the whole telemanipulation system according to the different working situations, in order to obtain low position drift; the whole telemanipulation scheme still remains passive.