Abstract: A positioning device has a pneumatic motor for longitudinally translating an arm. Four bars are parallel with the arm and form three guideways along which stopping devices can be lockable in desired positions. The stopping devices have heads that are selectively activatable into positions to form abutments for a head of the arm in order to define stop positions for the arm when the arm is moved by the motor that stalls when the arm is stopped. A shock absorber provides for a gentle stopping of the arm and its distance of operation prior to a stop position varies with the distance of displacement of the arm.