The apparatus has several drive motors (e.g. 40, 41, 42) for moving the movable part subgroups (e.g. 35, 36, 37) and a controller, by which the drive motors can be controlled independently of each other. One of the subgroups (35) is coupled to one of the drive motors (40), and at least one more of them to another drive motor, so that the respective parts of the subgroups can be moved independently of the subgroups themselves. The control process is also described.
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