How to turn screws with robot hand

Small tips for Fanka Emika robot arm’s operating (How to turn screws)

Let the robot hand move linearly would be an easy thing, by using its OS on a browser. However, things would be more difficult when referring to rotation, threshold setting and logical judging.

Difficulties for turning screws and solutions:

1. Since when setting the path of robot arm by using its own OS, every node is confirmed by hands, that is, you have to use your hands to drag the robot hand to a proper location in a proper angle. This means there would be an inevitable error at both the angle and the location of the gripper. Therefore, we have to turn up the threshold of the torque limit and force limit on each arthrosis (lead to robot’s higher tolerance to the force against its scheduled motion).

2. The part of screw that out of the hole would be shorten along the turning process. Because the force that reacts on the gripper is provided by the spiral, which constrains to a lever. It would provide a force, the force is so big that the robot hand will still report errors even if you have already turned up the threshold. So the best solution is to let the gripper move along the screw. Manually set its complete trajectory is unpractical. In order to realize a same effect we can use the OS to set a small-value axial direction force, where the axis of gripper is along the direction of screws.

3. Another difficulty is that how can we continuously repeat the process of tightening a screw. In another word, can the robot automatically switch to the next screw after the previous screw is tightened. That where the logical part comes to use. After setting the repeating part for turning the gripper, it is suggest to add a “if” module, which can let the robot hand to the second thread when some situation is detected. And in this situation, we can set the second routine as “moving the gripper to the next screw and continue circulating of rotation”, whose trigger is satisfying the condition of striking an error. In the other words, the second routine would be activated when any error is detected and not surprisingly the error is torque threshold warning caused by turning the screw.

4. And the last difficulty is that the gripper is made of hard resin, which can hardly deform when contacting with the screws. Therefore, the gripper can hardly provide sufficient friction force and activate the trigger that have been set in “difficulty 3”. What’s more, rubbing with excessive force will shorten the rubber part of gripper’s service life. Adding a 3D printed shell is a solution, but wrap it with medical proof fabric would be cheaper and more convenient.