Overview
We decided that the gripper base link would be incorporated into the outermost link in the arm mechanism. We wanted to create a gripper that could grasp and hold a diverse group of objects without crushing them. A servo was the ideal actuator for our gripper due to being lightweight yet powerful, and its ease of control. We also decided we wanted to use a simple mechanism for the gripper.
The Mechanism
With these design considerations in mind, we decided to use a geared double four-bar for the two halves of the gripper. The gearing made sure each of the two gripper fingers moved in unison and allowed us to also have the option to change the mechanical advantage between the servo and gripper.
To stop the gripper from crushing anything, we decided to add a spring between the servo input and the gripper fingers. This insured that the servo could keep moving while the gripper was held in one position, instead of instantly supplying its stall torque to the gripper. By putting a spring in each finger, we could also make each finger slightly less dependent on the other one. To include the springs, we decided to make one of the pin joints sit in a slot and become a sliding joint. We put the spring in this slot so that it would push the pin to the end of the slot. This spring transmits the force between the the crank and the output link. If the output link is held in position, the crank and pin will continue to turn, while compressing the spring in the slot. After a mechanical advantage analysis of the mechanism, we modified the link lengths to have the transmission ratio between the crank and the spring link be close to 90 degrees through the normal range of motion of the gripper. As the gripper closed on an object, the crank would continue to rotate while compressing the spring, and reduce the transmission angle and thus the mechanical advantage of the system. This means the servo will reach stall torque before fully compressing the spring, and that the max compression force of the spring will never be reached. This allowed us to pick the maximum force that the gripper would put on any object. We designed for a 4lb gripping force.
The Gripper
We decided to make as much of the gripper out of the same Delrin as the main arm linkage. While the made it easier to manufacture the entire mechanism, delrin was less than ideal for the actual gripping surface. To get grippy fingers, we used rubber bumpers with a sticky backing to give the Delrin fingers texture. Then we put heat shrink over the fingers and the rubber bumpers to increase the grippiness. Overall, this was successful for increasing the friction between the gripper and any objects.
SolidWorks model of the gripper
Final prototype of the gripper