Interaction Control
In view of the development of an underwater vehicle able to perform a completely autonomous mission the capability of the vehicle to interact with the environment by the use of a manipulator is of greatest interest. To this aim, control of the force excha
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4.1 Introduction to Interaction Control of Robots In view of the development of an underwater vehicle able to perform a completely autonomous mission the capability of the vehicle to interact with the environment by the use of a manipulator is of greatest interest. To this aim, control of the force exchanged with the environment must be properly investigated. Underwater Vehicle-Manipulator Systems are complex systems characterized by several strong constraints that must be taken into account when designing a force control scheme: • Uncertainty in the model knowledge, mainly due to the poor description of the hydrodynamic effects; • Complexity of the mathematical model; • Structural redundancy of the system; • Difficulty to control the vehicle in hovering, mainly due to the poor thrusters performance; • Dynamic coupling between vehicle and manipulator; • Low sensors' bandwidth. Limiting our attention to elastically compliant, frictionless environments several control schemes have been proposed in the literature. An overview of interaction control schemes can be found, e.g., in [36, 183]. Stiffness control is obtained by adopting a suitable position control scheme when in contact with the environment [146]. In this case, it is not possible to give a reference force value; instead, a desired stiffness attitude of the tip of the manipulator is assigned. Force and position at steady state depend on the relative compliance between the environment and the manipulator. Impedance control allows to achieve the behavior of a given mechanical impedance at the end effector rather than a simple compliance attitude [84]. In this case, while it is not possible to give a reference force value, the force measure at the end effector is required to achieve a decoupled behavior. To allow the implementation of a control scheme that fulfills contact force regulation one should rather consider direct force control [182]. This can be G. Antonelli, Underwater Robots © Springer-Verlag Berlin Heidelberg 2003
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4. Interaction Control
effectively obtained by closing an external force feedback loop around a position/velocity feedback loop [55], since the output of the force controller becomes the reference input to the standard motion controller of the manipulator. Nevertheless, many manipulation tasks require simultaneous control of both the end-effector position and the contact force. This in turn demands exact knowledge of the environment geometry: the force reference, in fact, must be consistent with the contact constraints [115]. A first strategy is the hybrid force/position control [140]: the force and position controllers are structurally decoupled according to the analysis of the geometric constraints to be satisfied during the task execution. These control schemes require a detailed knowledge of the environment geometry, and therefore are unsuitable for use in poorly structured environments and for handling the occurrence of unplanned impacts. To overcome this problem, the parallel force/position control can be adopted [40]. In th
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