An alternative approach is to actuate both gripper jaws independently.Ī parallel jaw mechanism with a larger range of motion can be accomplished using two 4-bar linkages, Figure 10.2.2, right. This can be accomplished by either measuring forces in the wrist and moving the gripper to minimize lateral forces or a compliant mounting mechanism or structure, such as a robot equipped with series-elastic or pneumatic actuators. Here, compliance can help, allowing the gripper to adjust its pose to the object. Note that force-closure with a static object, such as a screw mounted to a structure, requires both jaws to make contact with the object at the same time, thereby imposing high accuracy requirements of both object detection and robot motion. The control strategy for parallel jaw grippers requires an accurate pose estimate of the object of interest and positioning the gripper so that the object is right in the center of the two jaws. Electric mechanisms instead trade accuracy and torque with speed. Pneumatic grippers, where air pressure coming in on either end of the piston can drive the gripper into an “open” or “close” position very quickly (2-3 times per second), but cannot be controlled accurately. The mechanical design also affects the speed at which a gripper can operate. Constraints like this often make the gripper body twice as wide as the maximum aperture, making it difficult for the robot to enter tight areas. In order for this design to fully close, the two rack gears must be mounted at an offset in order to slide against each other. While using gears on racks is unusual in an industrial design - the gripper jaws typically travel on threads actuated by worm gears or are attached to a pneumatic piston - this drawing illustrates the relationship between the range of motion of the gripper jaws, the length of the mechanism it is sliding on, here a rack gear, and the resulting body size. \): Left: Parallel jaw gripper driven by a single actuator via a system of coupled gears.Right: 4-bar linkage parallel jaw gripper.įigure 10.2.2, left, shows a minimalist implementation of a parallel jaw gripper that can be actuated by a single servo motor, driving two rack gears to which the gripper jaws are mounted.
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