Patterned and instrumented directional adhesives for enhanced gripping with industrial manipulators

Abstract

Gripper pads are provided for use, augmentation or retrofitting in a robot manipulator. The gripper pads distinguish a dry adhesive layer with dry adhesive wedges that would interface with an object that is desired to be manipulated. The plurality of dry adhesive wedges is organized in a pattern such that the pattern has at least two different orientations for the direction of the plurality of wedges. The gripper pads further distinguish a sensor array underneath the dry adhesive layer. The data obtained from the sensor array provides an estimate of the shape and size of the contact between the gripper pad and the object when the dry adhesive layer is in contact with the object.

Claims

1. A gripper pad for use in a robot manipulator, comprising: (a) a dry adhesive layer having a plurality of dry adhesive wedges each having a first angled direction extending away from the dry adhesive layer and a second angled direction, orthogonal to the first angled direction, relative to the surface of the dry adhesive layer, wherein the plurality of dry adhesive wedges is organized in a chevron pattern, wherein the chevron pattern has at least two different orientations defined for the second angled direction of the plurality of wedges relative to the surface of the dry adhesive layer, and wherein the dry adhesive layer would be interfacing with an object; and (b) a sensor array underneath the dry adhesive layer, wherein the data obtained from the sensor array provides an estimate of the shape and size of the contact between the gripper pad and the object when the dry adhesive layer is in contact with the object.

2. The gripper pad as set forth in claim 1, wherein the sensor array comprises capacitive sensors, piezo-electric sensors, or optical sensors.

3. The gripper pad as set forth in claim 1, wherein the sensor array measures a pressure distribution between the gripper pad and the object to predict adhesion force and moment by the dry adhesive layer.

4. The gripper pad as set forth in claim 1, wherein the gripper pad further comprises a force and torque sensor.

5. A method of augmenting or retrofitting a robot manipulator with gripper pads, comprising the step of: augmenting or retrofitting the robot manipulator with at least two gripper pads, wherein each of the two gripper pads has a dry adhesive layer with a plurality of dry adhesive wedges, wherein the plurality of dry adhesive wedges each having a first angled direction extending away from the dry adhesive layer and a second angled direction, orthogonal to the first angled direction, relative to the surface of the dry adhesive layer, wherein the plurality of dry adhesive wedges is organized in a chevron pattern, wherein the chevron pattern has at least two different orientations defined for the second angled direction of the plurality of wedges, and wherein the dry adhesive layer would be interfacing with an object that is to be manipulated, and a sensor array underneath the dry adhesive layer, wherein the data obtained from the sensor array provides an estimate of the shape and size of the contact between the gripper pad and the object when the dry adhesive layer is in contact with the object, and wherein the plurality of dry adhesive wedges of each of the at least two of the gripper pads would be facing each other and the object that is to be manipulated.

6. The method as set forth in claim 5, wherein the sensor array comprises capacitive sensors, piezo-electric sensors, or optical sensors.

7. The method as set forth in claim 5, wherein the sensor array measures a pressure distribution between the gripper pad and the object to predict adhesion force and moment by the dry adhesive layer.

8. The method as set forth in claim 5, wherein the gripper pad further comprises a force and torque sensor.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 shows according to an exemplary embodiment of the invention a robot 110 picking up an object 130 with mass m away from its center of gravity. Depending how the gripper pair (120, 122) holds the object a torque is generated in the magnitude of mg times distance d of the center of gravity of the object relative to the gripper.

(2) FIG. 2 show according to an exemplary embodiment of the invention a gripper pad 200 with dry adhesive layer 220 with its dry adhesive wedges 222 with under the dry adhesive layer 220 a sensor array 210.

(3) FIGS. 3A-B show according to an exemplary embodiment of the invention force F and torque T generated by a fingertip covered with a uniformly aligned straight wedges (=0 degrees, FIG. 3A) and with a chevron pattern with wedges rotated degrees (FIG. 3B).

DETAILED DESCRIPTION

(4) In this invention, we integrate directional, gecko-inspired adhesives into the jaws of a commercial gripper, enabling it to hold very delicate objects. The adhesives are mounted to the outer surface of a tactile sensor, which provides an estimate of both the shape of contact area and the gripping force normal to the contact, and the shear force, which is parallel to the contact, and the moment taken about an axis perpendicular to the contact. All four pieces of information are important for predicting the maximum handling forces and moments that the grip can sustain without slipping. A force/torque sensor at the robot wrist could be added to measure the overall external force and moment.

(5) The solution in this invention is suitable for retrofitting existing industrial robot grippers and uses gecko-inspired adhesives that allow them to handle delicate objects with very low grasp forces. Reviews of gecko-inspired adhesives can be found in (A) Brodoceanu et al., Hierarchical bioinspired adhesive surfacesa review, Bioinspiration & Biomimetics, vol. 11, no. 5, p. 051001, 2016, and (B) Eisenhaure, A review of the state of dry adhesives-biomimetic structures and the alternative designs they inspire, Micromachines, vol. 8, no. 4, p. 051001, 2014. Among the many possible solutions, for the embodiments in this invention we desire a material that is directional and highly controllable, meaning the magnitude of adhesion can be controlled, for example, by varying the applied shear or normal force.

(6) In an exemplary embodiment, the particular adhesive material employed is an arrays of 80 m long, angled silicone rubber micro-wedges fabricated on a 25 m thick polyimide film (Day et al., Microwedge machining for the manufacture of directional dry adhesives, Journal of Micro and Nano-Manufacturing, vol. 1, no. 1, p. 011001, 2013). The same adhesive film has been used in a passive gripper for lifting objects purely through shear tractions and in grippers designed to grasp objects in space. In the present embodiments of this invention, we use the film with a small positive normal pressure to provide greatly enhanced friction. The adhesive is capable of gripping with zero normal force (as when used previously on wall climbing robots, U.S. Pat. No. 7,762,362). However, when some positive (i.e. pressing into the gripper pad) pressure is available, the ability of the adhesive to sustain shear forces parallel to the gripper pad improves further. This is particularly true for object surfaces that are not smooth. The effect is similar to having an extremely high coefficient of friction (mu much greater than one), as shown in equation (1) in the paper. The small positive force can be generated by the gripper itself, squeezing very gently upon the object that is between its fingers. The difference compared to ordinary gripping with friction is that a very small normal force (on the order of 0.01 N) is now enough to lift an object weighing 1 kg or more, assuming an adhesive contact area of approximately 2 cm.sup.2.

(7) Because the gripping performance depends on both the area of contact and the pressure, we use a gripper equipped with a tactile sensing array. Many tactile sensing technologies are potentially applicable such as capacitive sensors, piezo-electric sensors, optical sensors, or the like. Practical concerns include spatial and pressure sensing resolution, accuracy and robustness. For the exemplary embodiments reported herein we use precommercial version of a tactile array.

(8) Friction with Adhesion

(9) Friction generally has two components: one due to molecular attraction and hysteresis and one due to molecules bumping over each other. The former is an adhesion-controlled component, which depends on the real area of contact at a molecular scale. The latter is a load-controlled component, which depends on the normal force. For most hard materials, the former part is negligible and the latter part provides a maximum friction force that grows linearly with the applied load; hence f.sub.tf.sub.n, where f.sub.t is the tangential force, f.sub.n is the normal force and is the coefficient of friction. However, with gecko-inspired adhesives, even in the presence of a normal force, the area dependent part often dominates.

(10) Thus, in static conditions, we expect the tangential force for an adhesive with a positive normal force to follow:
f.sub.t.sub.A(c.sub.1p+c.sub.a)dA(1)
where p(x, y) is the pressure at a given location of the contact, c.sub.1 and c.sub.a are constants and A is the contact area. Similarly, in static conditions, the moment about an axis perpendicular to the fingertip's surface should correspond to:
m.sub.x.sub.Ar.sub.2(c.sub.1p+c.sub.a)dA(2)
where r=[x, y] is a vector from the center of pressure of A to each element in A. The embodiments in this invention involve directional adhesives, hence the constant c.sub.a becomes a function of the angle between the preferred loading direction of the adhesive and the angle of the applied tangential force: c.sub.a(). For example, FIGS. 3A-B show two different possible arrangements of directional adhesives. The invention is not limited to these two arrangement examples as a person skilled in the art would be able to optimize other arrangements as well.

(11) Moment Compensation in Manipulation Tasks

(12) If a gripper only needs to exert tangential forces in a single direction, as when lifting objects by grasping at their center of mass, it is most efficient to orient the directional adhesive parallel to the lifting direction. However, manipulation typically involves rotating grasped objects about multiple axes. In addition, the objects may be non-homogeneous, or it may not be possible to grasp them along their centerlines. Consequently, grasp attempts with a single manipulator often introduce a moment about the point of contact with the object, as shown in FIG. 1. With a tight pinch of the object this is perhaps not a problem. However, when attempting to minimize grasping forces there is the potential for slippage. A moment introduces a circular pattern of shear forces around the center of rotation, of which only a small portion is aligned with the adhesive's strongest direction. To compensate for this effect, one can arrange small areas of directional adhesive with multiple orientations. FIGS. 3A-B show the expected results for several different patterns, assuming an adhesive that has maximum strength at =0 and minimum strength in the orthogonal directions at =90. The grey scale of those plots is distributed linearly with , the angle between the load direction and the adhesive's preferred direction, and they show the case where the adhesive has a 70% efficiency when solicited against its preferred loading direction (i.e.: =180).

(13) It is also noteworthy from FIGS. 3A-B that should in general be adjusted according to the fingertip's dimensions. Furthermore, for optimal moment compensation regardless of tangential force compensation, one should orient the wedges such that:

(14) ${}_m={\tan }^{-1}\left(\frac{h_f}{w_f}\right)$
where h.sub.f and w.sub.f are respectively the height and the width of the fingertip's contact area.

(15) Fingertip Design and Construction

(16) In an exemplary embodiment, the gripper is an industrial Robotiq 2-Finger 85 gripper equipped with tactile sensors. The sensor is described in Le et al., A highly sensitive multimodal capacitive tactile sensor, in 2017 IEEE International Conference on Robotics and Automation (ICRA), May 2017. This multi-modal tactile sensor is 22 mm by 42 mm and contains a 4-by-7 matrix of taxels. The outer skin of the sensor was replaced by a film with patches of gecko-inspired adhesives in one or more directions (FIG. 1). The adhesive used in these embodiments are rows of inclined micro-wedges with a triangular cross section. A characteristic of this adhesive is that the amount of adhesion increases in proportion to the applied shear load. When the shear load is relaxed, the adhesion becomes negligible.

EXPERIMENTAL DATA

(17) The reader is referred to U.S. Provisional Patent Application 62/586,736 filed Nov. 15, 2017, which is incorporated herein by reference, for experimental data and comparisons with other solutions.