Dual Hybrid Teleoperation

This concept of "dual hybrid teleoperation" has been introduced, studied and demonstrated experimentally in [38]. It has been shown that when the geometric constraints for a teleoperation task are known, the master and slave workspaces can be split into dual position-controlled and force-controlled sub- spaces, and information can be transmitted unilaterally in these orthogonal subspaces, while still providing useful kinesthetic feedback to the operator.

Control for Teleoperation and Haptic Interfaces 61 2.11 V e l o c i t y C o n t r o l w i t h F o r c e F e e d b a c k

For some teleoperation systems, such as remotely-controlled excavators [36], position control is not a realistic option due to issues of safety and vastly different master and slave manipulator workspaces that would imply very poor motion resolution if scaling were to be used [50]. Instead, velocity control mode is used, in which the slave velocity follows the master position, so ideally Gp = npsI in Eq. 2.1. Transparency based on transmitted impedance can be defined in a similar manner, and requires that the derivative of the environment force be returned to the master, so ideally G s = n f s I in Eq. 2.1 [50]. To avoid returning the derivative of environment force that could be very noisy, velocity mode control can be modified to include a low-pass filter making Gp and G / proper.

Experiments with velocity-mode teleoperation systems have indeed shown that direct force feedback leads to poor transparency and poor stability mar- gins, especially when stiff environments are encountered. As an alternative, a new approach called "stiffness feedback" has been proposed. Instead of returning direct force information, the master stiffness is modulated by the environment force, from a minimum positive stiffness corresponding to the minimum expected force to a maximum positive stiffness corresponding to the maximum expected force. In order to avoid blocking the slave against a stiff environment, the stiffness law applies only when the environment force opposes slave motion. It can be shown that this control scheme is locally transparent when the environment force opposes slave motion and experi- mental results have been very positive [30, 36].

3. T e l e o p e r a t i o n C o n t r o l D e s i g n C h a l l e n g e s

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, in spite of areas of great po- tential, such as teleoperated endoscopic surgery, microsurgery, or the remote control of construction, mining or forestry equipment. W h e t h e r this is due to fundamental physical limitations of particular teleoperator systems or due to poorly performing controllers is still not clear. From this perspective, proba- bly the single most important challenge ahead is a better understanding of the limits of performance of teleoperation systems. Towards this goal, it would be useful to have a benchmark experimental system and task to be completed for which various controllers could be tested. Unfortunately, it would be very difficult to do this entirely through simulation, as the dynamic algorithms necessary to develop a reasonable array of tasks would be just as much un- der 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, e.g. planar master/slave systems.

62 S.E. Salcudean

Specific improvements could be made to the fixed teleoperation controllers designed via conventional loop shaping or parametric optimization. In par- ticular, a class of operator impedances that is broader than a single fixed impedance but narrower than all passive impedances should be developed with associated robust stability conditions. Since the control design problem was formulated as a constrained "semi-infinite" optimization problem, dif- ferent algorithms could be tested or new ones developed. Like many other multi-objective optimal control problems, robust teleoperator controller de- sign problems are likely to be hard to solve.

T h e r e seems to be much promise in the design of adaptive bilateral teleop- eration controllers with relatively simple and physically motivated structures.

In particular, indirect adaptive schemes based on Hannaford's architecture [17] are likely to succeed. Whereas fast or nonlinear environment identifica- tion techniques are necessary to accommodate contact tasks and these seem quite difficult to develop, operator dynamics identification seems to be quite feasible [16]. Some of the difficulties encountered in developing identification algorithms may be circumvented by the use of dual hybrid teleoperation or newly developed variants that are not based on orthogonal decomposition of the task space into position and force controlled spaces. Another interesting research area is the automatic selection of the position and force controlled subspaces.

4. T e l e o p e r a t i o n in V i r t u a l E n v i r o n m e n t s

Manipulation in virtual environments has potential applications in training systems, computer-aided mechanical design and ergonomic design. For virtual environments, the master (more often called haptic interface in this context) control algorithms differ from bilateral teleoperation control algorithms in that the slave manipulator and its environment become a dynamic simulation.

T h e simulation of systems dynamics for graphical or haptic rendering is a topic of substantial research. See, for example, [14] and other articles in the same proceedings.

Two approaches have been proposed for interfacing haptic devices to dy- nanfic simulations. The impedance display, used by most researchers, taking sensed motions as inputs, passing them through a "virtual coupler" [9] to the dynamic simulator, and returning forces to the device, and the admit- tance display, taking sensed forces as input and returning positions to the haptic device. T h e relative advantages of these display modes have barely been touched upon, with the ability to build modular systems ("summing forces and distributing motion") [49] with non penetration constraints [47]

presented in favor of the admittance approach.

Looking back at the debate on teleoperation "architectures", it seems that a four-channel coupling of haptic interface and dynamic simulation via a virtual coupler should be used. This would allow the haptic interface to

Control for Teleoperation and Haptic Interfaces 63 behave as a force sensor or position sensor depending on the impedance of the task. The implication on dynamic simulators remains to be determined, but there is no reason why forces from the virtual coupler could not be added to sensed forces.

From a control point of view, the existence of a full dynamic model of the slave has both advantages and disadvantages. On the one hand, the design be- comes easier because no environment identification is necessary. On the other, the design becomes more difficult because dynamic simulations require signif- icant computing power which is often distributed, so one can expect to deal with multiple rate asynchronous systems. The argument for building complex systems using passive building blocks [1] is quite compelling, especially since techniques for passive implementations of multi body simulations are being developed [9].

B e t t e r understanding of hybrid systems is needed for the control of haptic interfaces, as manipulation of objects in the presence of non penetration constraints often require switching of controller/simulation states [40, 49].

5. C o n c l u s i o n

A survey of teleoperation control for scaled manipulation and manipulation in virtual environments has been presented in this chapter. It seems that contributions from the areas of systems identification, adaptive control, multi objective optimal control and hybrid systems could be integrated in novel ways to provide solutions to problems of transparent bilateral control. T h e scope of the survey was quite limited, Interesting work in the design of haptic interfaces, novel ways of achieving passivity using nonholonomic systems, and issues of dynamic systems simulation for virtual reality have not been addressed.

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