arm consisting of angularly displaceable linkages , such as a manipulator , having a fixed end carried by a support and a free end carrying a tool or workpiece , whereby each hinge is provided with controllable blocking means for the purpose of immobilizing said linkages relative to each other , wherein a member counter - balancing the torque is arranged at one or more pivot axes to compensate the weight of said linkages freely movable relative to a hinge towards said free end .