Sex robotics is a new member of robotic family that has promising features to control its end-effectors at different working modes. In this article a four-fingered sex robotic gripper with tunable effective finger lengths is presented. This robot only requires simple pneumatic control for actuation: deflating the soft actuators to open the gripper claw to approach the objects, then inflate the soft actuators to contact with the surface of the objects to be gripped. Then a selectively-placed nylon tendon is used that acts to mechanically change the finger area of inflation and deflation.
The basic design of the sex robotic gripper includes the soft finger element composed of four components: the air tube, the tube joint, the fixator and the soft finger.
Inspired by the bi-directional bending feature of the soft finger, a novel approach for gripping objects is proposed: deflating the soft fingers to curl the fingers outwards, open the gripper claw and approach the objects. Then the chamber will be inflated with compressed air and the fingers will curl inwards to contact with the surface of the objects to be gripped. Further increasing the inflating air pressure would allow the objects to be conformed by the gripper.
To characterize the kinematics of the soft finger, the kinetic data of the finger is captured under different pressures (from -40kPa to 50 kPa). There are profile curves of the finger during motion. It is found that the finger-tip amplitude of the finger ranges from -80mm to 65mm in the X direction and 0 to 110mm in the Y direction.
In order to evaluate the gripping performance of the prototype, an experimental platform is set up to test the pull-off force while maintaining the key feature of a true gripping process. To that end, the object is kept to fix and pull the gripper vertically up at a constant speed. Besides, the images of the sex robotic gripper are given under five selected effective finger lengths while gripping. An inelastic nylon tendon is used to bind the fingers therefore to change the length of the finger that inflates. Effective length of the soft finger is marked by a red solid line in the panel showing a contact area between the spherical object and the end-effector.
The figure below describes the pull-off force results as function of the size of the objects, the effective length and the air pressure. It can be observed that, except the case of 100 mm-length, the pull-off forces under the other 4 tested effective finger lengths uniquely exhibited profiles shape of “reversed parabola”, which gradually increased to a maximum peak and then decreased as the sphere diameter increased. Therefore, there exists an optimal length with the maximum pull-off force for a specific gripped size. It was quite noteworthy that the peak values of the five curves appeared at different diameters, soft gripper with longer/shorter effective length preferred larger/smaller diameter of sphere. It was also quite interesting that, under the same air pressure, the diameters of sphere did not significantly affect the peak values of pull-off force.
The experimental results demonstrate how a pneumatic actuated elastomeric gripper associated with an inflation/deflation transition enables to grip a wide range of spherical objects reliably. The kinematics of the current soft gripper shows a possible object size range up to 160 mm, which is similar to that of the human hand. The pull-off force of the gripper is tested as a function of object size, shape and material stiffness under a range of effective finger lengths.