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Research Papers

Integrated Manufacture of Exoskeletons and Sensing Structures for Folded Millirobots

[+] Author and Article Information
Duncan W. Haldane

Department of Mechanical Engineering,
University of California,
Berkeley, CA 94720
e-mail: dhaldane@berkeley.edu

Carlos S. Casarez, Jessica Lee, Andrew O. Pullin

Department of Mechanical Engineering,
University of California,
Berkeley, CA 94720

Jaakko T. Karras

Robotic Actuation and Sensing Group,
NASA Jet Propulsion Laboratory,
Pasadena, CA 91101

Chen Li

Department of Electrical Engineering and
Department of Integrative Biology,
University of California,
Berkeley, CA 94720

Ethan W. Schaler, Dongwon Yun, Hiroki Ota

Department of Electrical Engineering and
Computer Sciences,
University of California,
Berkeley, CA 94720

Ali Javey

Professor
Department of Electrical Engineering and
Computer Sciences,
University of California,
Berkeley, CA 94720

Ronald S. Fearing

Professor
Department of Electrical Engineering and
Computer Sciences,
University of California,
Berkeley, CA 94720
e-mail: ronf@eecs.berkeley.edu

Software is currently being developed to automate some of the design process (e.g. http://www.popupcad.org). We contend that even when the software is in a fully functional state, users will benefit from knowledge of good design practices.

This is not necessarily the case when using higher temperatures. The reflow temperature should be chosen such that the molten adhesive has the appropriate viscosity.

Except cardboard, which did not fail in the bending test, because the specimen was too compliant for the bending gauge length. Tensile strength is given instead.

Ripstop Nylon is used in the commercialized version of DASH [3] (made by DASH Robotics).

The robot in the shell preferentially falls top first due to aerodynamic effects.

Some of the information presented for the binary hair array is a refinement of work that was previously presented at a conference [43].

O2 plasma etchers cost approx. $10,000 USD (e.g. SPI Plasma Prep III), or machine time can be rented in many microfabrication facilities.

Manuscript received August 16, 2014; final manuscript received December 23, 2014; published online February 27, 2015. Assoc. Editor: Aaron M. Dollar.

J. Mechanisms Robotics 7(2), 021011 (May 01, 2015) (19 pages) Paper No: JMR-14-1219; doi: 10.1115/1.4029495 History: Received August 16, 2014; Revised December 23, 2014; Online February 27, 2015

Inspired by the exoskeletons of insects, we have developed a number of manufacturing methods for the fabrication of structures for attachment, protection, and sensing. This manufacturing paradigm is based on infrared laser machining of lamina and the bonding of layered structures. The structures have been integrated with an inexpensive palm-sized legged robot, the VelociRoACH [Haldane et al., 2013, “Animal-Inspired Design and Aerodynamic Stabilization of a Hexapedal Millirobot,” IEEE/RSJ International Conference on Robotics and Automation, Karlsruhe, Germany, May 6–10, pp. 3279–3286]. We also present a methodology to design and fabricate folded robotic mechanisms, and have released an open-source robot, the OpenRoACH, as an example implementation of these techniques. We present new composite materials which enable the fabrication of stronger, larger scale smart composite microstructures (SCM) robots. We demonstrate how thermoforming can be used to manufacture protective structures resistant to water and capable of withstanding terminal velocity falls. A simple way to manufacture traction enhancing claws is demonstrated. An electronics layer can be incorporated into the robot structure, enabling the integration of distributed sensing. We present fabrication methods for binary and analog force sensing arrays, as well as a carbon nanotube (CNT) based strain sensor which can be fabricated in place. The presented manufacturing methods take advantage of low-cost, high accuracy two-dimensional fabrication processes which will enable low-cost mass production of robots integrated with mechanical linkages, an exoskeleton, and body and limb sensing.

Copyright © 2015 by ASME
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Figures

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Fig. 1

Example SCM [7]. (a) DASH [9], (b) DynaRoACH [10], (c) OctoRoACH [8], and (d) VelociRoACH [11].

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Fig. 2

Overview of the SCM process. (a) Holes for flexures are laser cut into a rigid material/thermal adhesive sandwich. (b) The layers of rigid material are aligned, and bonded to a flexible layer. (c) SCM parts are released with a final laser cutting step. (d) An SCM component, and its jointed rigid body approximation.

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Fig. 3

A summary of the SCM design process, and CAD workflow. The cartoon on the right shows how CAD would be generated for a simple SCM four-bar linkage. The links are color coded for clarity, and the red rectangles represent the flexures as they would be cut in Fig. 2(a).

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Fig. 4

The open source SCM robot, OpenRoACH. (a) CAD layout of OpenRoACH SCM parts. (b) The OpenRoACH carrying a computational payload.

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Fig. 5

The self-fixturing / reflow process. (a) Cardboard sandwich with cutouts for flexure joints. (b) Thermal adhesive is applied over a Teflon template. (c) The template is removed leaving thermal adhesive on the cardboard. (d) Part outlines and tab holes are laser cut into the cardboard. (e) Parts are assembled. (f) Entire assembled robot is placed in oven to reflow thermal adhesive.

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Fig. 6

Images showing hot melt adhesive applied with a Teflon template. (a) The Teflon template with thermal adhesive applied (as in Fig. 5(b)). (b) Carboard base after template is removed (as in Fig. 5(c)).

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Fig. 7

Images of joints before and after reflow. Arrows show orientation of gravity as parts were reflowed (a) adhesive facing up; (b) adhesive facing down; and (c) adhesive strip aligned with gravity. These images demonstrate that the reflow process is not sensitive to the orientation of the parts.

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Fig. 8

Images of experimental setups for material tests. (a) Bending test; (b) delamination test; and (c) crush test for the B-P-CA OpenRoACH, shown with a 1.9 kg load.

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Fig. 9

Structure of composites: (a) balsa-PET (B-PET) composite; (b) balsa-paper (B-P) composite; and (c) balsa-paper-CA (B-P-CA) composite

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Fig. 10

A meter long hexapedal SCM robot made in the BIRDS lab at the University of Michigan. Developed by Devin Miller, Ian Fitzner, and Shai Revzen, with thanks to Stacie Desousa.

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Fig. 11

Schematic of the thermoforming process. (a) Heating of a thermoplastic sheet softens it above a positive mold. (b) Pressure difference from a vacuum source forms the softened thermoplastic onto a positive mold into the desired shape.

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Fig. 12

Top: molds constructed using (a) clay modeling, (b) slice forms, (c) 3D printing, and (d) laser cutting. Bottom: exoskeleton shells made from these molds by thermoforming using ((a) and (b)) 750 μm polystyrene, (c) 250 μm polycarbonate, and (d) 50 μm polyethylene.

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Fig. 13

Simple top shell for traversing cluttered terrain. (a) Design of shell shape resembling a thin slice of an ellipsoid. (b) Side view of VelociRoACH with the simple top shell.

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Fig. 14

Sealed bag layer manufacturing process. (a) Side view: forming of the bottom of bag layer with protrusions for freedom of leg motion; (b) side view: forming of the top of bag layer; (c) underside view: bag layer assembly with details (i) bottom piece of the bag, (ii) top piece of the bag, (iii) joining “ziploc” zipper on the bag, (iv) heat welds at the edge of the two layers, (v) formed out-of-plane pockets, shown topologically in this view, and (vi) extra vertical webs that form in the contoured bag layer between peaks, due to the high aspect ratio of the mold.

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Fig. 15

Leg mounts that clip through the sealed bag layer: (a) diagram with details (i) half of the leg mount is attached to the robot hip inside the bag layer, (ii) the other half of the leg mount is outside the bag layer and snaps into the inner component, (iii) bag layer wedges between the two leg mount halves, (iv) VelociRoACH legs slide into dovetail connections on the outer component; and (b) close-up of molded leg clips capturing the sealing bag on the robot

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Fig. 16

(a) Side view and (b) underside view of solid model renderings of VelociRoACH assembled in the structural shell. The structural shell clears the leg motion of the robot.

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Fig. 17

(a) Top view and (b) side section view of the thermoformed structural top shell over a 3D printed mold. (i) 3D printed PLA plastic buck with a 3 mm hull and 30% fill. (ii) Outwardly protruding lip at the base.

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Fig. 18

Bottom structural shell manufacturing process: (a) overhead view of multilayer laser cut mold. The pattern in the bottom layer will form the structural ribs (i), and thin cuts provide vacuum feed (ii). The top plate of the mold forms the undercut lip feature (iii). Alignment pins (iv) are added to stack up the mold layers. (b) Cross section view of thermoformed plastic shell in mold, where the undercut lip (iii) formed by separate layers of the mold is shown. (c) To release the formed shell, the shell is roughly cut around the top layer of the mold, the base plate with dowel pins is pulled downward and the top two plates defining the undercut lip are pulled outward. (d) The roughly cut shell is aligned to a fixture plate (v) for the laser cutting which duplicates the alignment pin pattern as in the stack-up above, and then holes for legs and final outline release cuts are made by the laser (vi). (e) Images of the bottom shell mold, thermoformed bottom shell, and finished bottom shell after release cuts.

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Fig. 19

Assembly process for adding shell to the robot: (a) cardboard robot body; (b) with PE bag and mounted legs; (c) with bottom structural shells; and (d) complete with top structural shell

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Fig. 20

Cockroach (B. discoidalis) leg spines. Arrows indicate the direction of the force applied to the spines.

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Fig. 21

Process to create the claws. Figures (a) and (b) are pictures of the fabricated components. To make the legs, the outline of the claws was first laser-cut into a rectangle of fiberglass. (a) Then the fiberglass was glued onto a curved leg. (b) Claws stick out because the fiberglass remains flat in the released sections.

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Fig. 22

Plot of the forces exerted by the claw onto a test substrate as it is pulled in the Fx and Fy directions and preloaded with Fz. Fy indicates the fore-aft forces while Fx shows lateral forces.

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Fig. 23

The forces required to disengage the spines from a piece of corkboard. The arrows indicate the disengagement force's direction.

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Fig. 24

(Left) 5 × 4 array of binary hair sensors. (Right) Hair sensor array mounted to bottom of hexapedal SCM robot [43].

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Fig. 25

Hair-activated switch in open (top) and closed (bottom) configurations. Sensor consists of curled polymer hair (A), rigid lever arm (B), polymer flexure (C), and copper contacts (D) [43].

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Fig. 26

Layers that comprise the hair sensor array: (A) base layer, (B) bottom contacts, (C) spacer, (D) insulating polymer film, (E) spacer (F), top contacts, (G) paper backing, (H) flexure polymer film, (I) switch levers, (J) precut sheet adhesive, (K) prestressed hair film (L), precut sheet adhesive, and (M) hair mounting layer [43]

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Fig. 27

Total loading required for sensor activation as a function of the number of hairs loaded [43]

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Fig. 28

(Left) Force-sensing taxel array. (Center) Bumper attached to front of hexapedal SCM robot. (Right) Three subcomponents of bumper: cardboard mounting structure (A), array of proximity sensors on flex-circuit (B), and outer layer of foam sensory structures (C).

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Fig. 29

Cross section illustration of force-sensing bumper taxel. Taxel structure consists of 50 μm thick reflective Mylar-film (A), outer and inner urethane foam walls (B), foam-supporting cardboard layer (C), sharp GP2S60 proximity sensor on flex circuit (D), and bumper mounting structure (E).

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Fig. 30

Laminar fabrication of sensory foam structures: Patterned cardboard base for foam structures (a), patterned sheet adhesive laminated over base (b), regions of adhesive exposed for anchoring inner foam walls (c), foam strips laminated down onto exposed adhesive regions (d), inner foam walls laser-cut from foam strips (e), taller, patterned foam laminated onto reflective Mylar-film (f), taller foam laminated onto base (g), and foam structures are isolated by cutting through Mylar-film and underlying foam, but not cardboard base, and peeling away excess material ((h) and (i))

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Fig. 31

Estimated force versus applied displacement for single taxel

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Fig. 32

Fiberglass robot appendage with integrated CNT-AgNP piezoresistive sensor (black traces)

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Fig. 33

Manufacturing process for the strain sensor: (a) fiberglass leg substrate (i) is sequentially covered with a sheet of 3M 468MP tape (ii) and a sheet of PDMS (iii); (b) oxygen plasma etch; (c) PET mask (vi) is aligned and a uniform layer of the CNT-AgNP ink (iv) is applied with a spatula (v); (d) ink dries at ambient conditions and the mask is removed; and (e) ink baked/annealed in an oven

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Fig. 34

Amplified bridge voltage (V) for the strain gauge as a function of time (s). Data is presented for 20 loading/unloading cycles to an applied uniaxial compressive force of approximately 25 mN (uniaxial compression of 6.0 mm).

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Fig. 35

Amplified bridge voltage (V) versus applied uniaxial compressive force (N) for the strain sensor. Data is presented for 5 consecutive loading (red)/unloading (blue) cycles.

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Fig. 36

Amplified bridge voltage (V) versus applied uniaxial compressive force (N) for the strain sensor. Data average (black) with ± σ (gray regions) is presented for 50 consecutive loading/unloading cycles. The sensor provides reliable sensitivity to compressive loads of 0 to ∼24 mN and saturates for loads above this. A quadratic regression function (red dash) was fit to the data for sensor calibration.

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