Summary and Analysis for Professor Robert Wood's Article

A Summary and Analysis of Seitz’s Article on Adhesion on an Incline for Microrobots

Seitz, Benedikt F., et al,  “Bio-inspired mechanisms for inclined locomotion in a legged insect-scale robot.” Harvard Microrobotics Laboratory. Published 14 December, 2014.

The HAMR (Harvard Ambulatory Microrobot) is an insect-scale micro robot and a vehicle for further study on adhesion methods on a microscopic scale. Three different types of adhesion methods are proposed: micro-spines, electroadhesion, and dry adhesives (comparable to gecko-foot tissue). To determine which method is most conducive to locomotion on a microscopic scale, the robot must exert shearing forces (a force that pushes one component in one direction while pushing another in a different direction) and apply normal force to detach the foot. The HAMR is tasked with measuring the degree of inclines that each method can produce adequately stable forces upon when applied to the legs of the micro robot. Tests upon the HAMR using the three different foot surface technologies analyzed the ability of the HAMR to move along varying degrees of inclined and declined surfaces, and used the results to formulate an idea for future modifications of the material. The three methods proposed for the foot materials were logical, and the results were interesting because of the ranges of locomotion were so different. The study has yet to conclude the best method for movement on a microscopic scale.

The study used three types of material to cover the foot joints of the microrobot. The first material proposed was similar to a capacitor – a surface of charge that included a dielectric material. Dielectric materials increased the ability of the plate to hold charge, and therefore its addition created a stronger electric attraction between the test surface and the electroadhesive surface. The shearing force was created through the protrusion of charged electrodes. The second method investigated a gecko skin-like surface that induced Van der Waals forces between the test surface and the foot joint. In the foot, a material that had many wedges directed outward created London dispersion forces which attracted the foot to the test material. The authors experimented with three different orientations and widths of these wedges: non-directional, directional, and directional with a larger wedge width. The last method used the concept of microscopic spines to create shearing force when interacting with the test material. The micro spines were oriented at 90 degrees from the foot joint.

Preliminary testing of these methods attracted attention to the instabilities of the HAMR. Before proceeding with the tests, the authors investigated some of these problems and attempted to reduce their effects on the locomotion of the HAMR. Two major modifications were made: a “passive tail” and a five joint foot piece. The foot piece was able to increase the range of movement and self-alignment, while the passive tail reduced the rocking motion of the HAMR.

With these three methods, the means of testing the locomotion capacity of each at varying angles from the horizontal was straightforward. The HAMR was equipped with one of the methods on its foot joint and programmed to climb up and down an incline. The incline was increased (or decreased when testing downward motion) until inadequate shearing forces were produced. The maximum angle of functionality was determined by the microbot’s ability to climb the subsequent incline without slipping or becoming unable to move forward.

The major results of the study showed that the gecko-like adhesion method with wedges of larger width (100 micrometers) had the greatest angle of incline and decline at which the HAMR functioned within parameters. The micro-spine method proved to be more efficient than the electroadhesive method at both inclined and declined angles. Limitations to the both methods were discovered, and the study mentioned the inefficiency of coordination between the electrodes and the movement of the leg (electroadhesive method) and how the restricted range of leg movement reduced the micro-spine functionality.  The study revealed that the best method to test next (surpassing any singular method mentioned and tested above) is a combination of micro-spines and gecko-like surfaces. This is a logical conclusion given the presence of both structures in nature.

This study shows the potential of micro-adhesive surfaces and how they could be useful to technology. By exploring three different micro-adhesive methods, the range of results point scientists toward better methods of adhesion and suggest that a hybrid combination of two or more technologies may be ideal. It was critical that the study first involved a modification of the HAMR to reduce negative influences on the robot’s performance. The introduction of the passive tail and new foot joint decreased the rocking effect and improved the ease of foot alignment, which made implementing the three methods easier and perhaps more accurate.

The data and conclusions are in accordance with each other. The authors found that the gecko-like material (directional) was a more effective micro-adhesive than other methods. The micro-spine method also was significant in its results. The authors suggested that the presence of both materials in nature and insect and reptilian systems is an indicator that the hybridization of the two will be promising for future micro-adhesive surfaces. The data is sufficient in that both locomotion on an incline and decline are considered, and restrictions are noted (i.e. the limited range of leg motion affecting the result of the micro-spine adhesion). Future study could be improved by further experiments of similar natures. For example, these experiments feature the HAMR moving in a line up or down an inclined plane. Study of the HAMR moving in a slight transverse direction along with forward or backward movement may be beneficial in revealing the positive attributes and negative complications of the materials.