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The real time control architecture is based, at least in this first phase of activity, on standard HW/SW components. The adopted architecture consists in a PC equipped with a DSP (TMS320C32) board (FastProt) and connected with the motor drives and to an input board for the sensors. This board has been purposely designed because of the relatively high number of signals (30) to be acquired in real-time. From the software point of view, besides a real-time kernel on the DSP board, an interface between the DSP and the PC has been developed, allowing to use in an integrated fashion both real-time software and high-level environments for user interface. The position control of each finger is based on a classical PI controller, as depicted in figure. At this level, a difficulty has been the compensation of nonlinearities caused by the actuation system, in particular a relevant (and non constant) dead zone and the nonlinear characteristic of the Hall effect position sensors. The set points and the controlled variables of the servo loops are considered according to two main modalities: position control or proximity control. In the first case, the absolute position of the fingertip is controlled by planning the desired motion with a fourth-order polinomial function and assigning the desired motion time. The controlled variable is the position x of the fingertip obtained by means of the forward kinematics from the joint position measured by the Hall effect sensor. In the second case, the controlled variable is the distance of the finger with respect to the approached object. This modality is activated when the finger is sufficiently close to the object (e.g. 15 mm). The controlled variable is now the distance from the object, as measured by the proximity sensor. This information can be used both to start the grasp of the object (if all the fingers are at the same distance from it) or to maintain constant the distance between the finger and the object (e.g. if the object is moving). A proper switching logic between the above two control modalities must be adopted in order to ensure a smooth behavior of the gripper. Note that the latter control modality can also allow to `stabilize' a slowly moving object with respect to the gripper.

Besides the position/proximity, a further control loop is present in order to control the forces applied during manipulation. This scheme, that is based on the information deriving from the force/torque sensors, at the moment implements a simple compliance control.


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