art in an embodiment of the invention a sensor target 114 is situated at or near the end of cam follower 112 opposite the end of cam follower 112 that is attached to lever 110 optionally sensor target 114 is not a separate element added to cam follower 112 but is just the far end of cam follower 112 made of the same material as the rest of cam follower 112 cam follower 112 follows the surface of a cam 116 as impression roller 102 rotates around axis 104 although cam 116 is shown some distance to the right of impression roller 102 in fig1 for clarity cam 116 is optionally directly in contact with the end of impression roller 102 cam 116 has a depression 118 on one side when cam follower 112 moves into depression 118 then gripper 106 closes by the mechanism described above when cam follower 112 moves out of depression 118 then gripper 106 opens a first position sensor 120 is located directly to the right of and somewhat above depression 118 when cam follower 112 falls into depression 118 then sensor target 114 at the end of cam follower 112 will be located radially inward from sensor 120 and sensor 120 will fall to detect target 114 however if cam follower 112 fails to fall into depression 118 but remains at the same distance from axis 104 as it normally is when gripper 106 is open then target.