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Introduction to cable parallel mechanisms

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A cable parallel mechanism is a conventional parallel mechanism whose rigid legs have been replaced by cables. The latter are rolled up at one end (usually the ceiling) and by varying the length of the cables, one can position and orient the effector in space (Figure 1).

Fig. 1: A cable parallel mechanism.

The advantages and disadvantages of these mechanisms differ from those of conventional parallel mechanisms (which are usually rigid). The two main advantages are undoubtedly the very large workspace and the possibility to easily obtain a redundance with respect to the actuators. One is also able to produce very large accelerations.

Cable mechanisms do, however, have two intrinsic disadvantages which must be considered. First, cables produce only tension forces (a cable cannot push an object). Moreover, cables have a certain flexibility which will necessarily influence the precision of the mechanism.

These mechanisms are used in several applications, mainly in cases where a large workspace is required. The best known application is undoubtedly the Skycam, a camera used, among other things, for the televization of professional American football games.

Fig. 2: Camera controled by a cable mechanism (courtesy of Skycam).

   

Fig. 3: Example of positioning in a stadium (courtesy of Skycam).