A durable gripper tool was designed for use by RoboSimian robots intended for use in disaster scenarios that demand high-force, robust manipulation. The resulting Cam Hand fills a previously unaddressed niche that emphasizes grip strength and robustness over dexterity. The design uses a number of unique features to ensure high operational flexibility. While this gripper was created for use on a robot, its basic design could be refined for other applications; in particular, as a new class of prosthetic that would exist between the traditional hook and pinch models and the dexterous models currently under development.
The Cam Hand, shown in Figure 1, is characterized by a number of design features. The most significant is the continuous rotation of the fingers about the “palm” surface of the gripper. This unique range of motion offsets the lack of finger dexterity by enabling an unprecedented number of grasping angles and operational modes through repositioning the fingers. Of great importance to its use on RoboSimians are the “foot” modes that allow the robot to use all of its limbs for mobility or manipulation. The gripper is also capable of a new type of grasp: a “cam” mode for internal grasps of objects or features as shown in the lower left of Figure 1. This mode is similar to the cam protective equipment used by human climbers. Even though these cam grasps rely on friction, they are secure due to the self-locking action of the cam mechanism. Also unique to this gripper is the ability to independently grasp two items. This capability can be realized by the double-hook or by using one set of fingers to create a grasp against one side of the gripper body while the other set uses the other side. Additional functionality can be realized from the continuous finger motion that, when coupled with specifically designed fingers, enables the functions of scissor-mechanism tools such as wire cutters. Another unique attribute enabled by the finger rotation is that when all of the fingers are rotated simultaneously in the same direction, a “wrist” degree of freedom is created. In addition to giving the gripper more utility for the same design complexity, the wrist motion is very close to the grasp location, an advantage for the kinematics of a robotic manipulator and an enormous design hurdle for more traditional gripper and arm combinations.
While the basic ideas of the Cam Hand are achievable at different scales and various detailed design instantiations, the version created for the RoboSimian robots emphasizes strength, as shown in Figure 2. In particular, the gripper was designed to allow the full weight of the 125-kg RoboSimian to hang from one finger without damage. This design point was chosen both to allow a RoboSimian to climb, but also as an estimate of the maximum loads that the robot could impart on the gripper during mobility or manipulation operations. This desire for strength drove the rest of the design. While there are four fingers, to save volume, the outer two fingers are slaved together as the output of one drivetrain resulting in only three independent degrees of freedom (DoF) with four fingers. However, these three DoF are sufficient for relatively complex operations such as grasping and triggering human tools like electric drills. It has a maximum grip strength of 304.5N at the tip of each outer finger (assuming each is simultaneously engaged and evenly loaded) and 609N at the tip of each inner finger (or an overall hand grip strength of 609N at the tips, increasing the deeper into the fingers the grasped object is located). Well over the grip strength of the human hand, this actuated strength is sufficient to grip a concrete block on two faces and lift it, or pierce through half-inch drywall. However, the hand is actually more resistant to forces put into it than what it can actively exert on an object. Passively, each reacted tip force for each outer finger is 1.5kN finger (assuming each is simultaneously engaged and evenly loaded) and the inner fingers handle 3.0kN.
In order to minimize impact on the RoboSimian system design, the Cam Hand was designed to only require power and communication inputs. Integrated electronics handle all motor control and sensor interfaces, creating a convenient package that can be added not only to RoboSimian, but any other robot manipulator of the same scale.
As mentioned, while this version of the Cam Hand is intended as a robotic gripper, the principles could be adapted to use as a practical prosthetic. In particular, it can emulate many more of the simple grasps humans use on a daily basis than is possible with the traditional hook prostheses. The advent of highly articulated prostheses promises dexterous functionality, but these designs will remain either expensive, weak, or both. A Cam Hand prosthesis can be designed that is relatively inexpensive, strong, and physically robust, giving prosthesis users great capability at a lower price point than currently possible.