by Kentaro Barhydt and Fiona Lin
Difficulty: Intermediate
Duration: Active Fabrication: 2-3 hours (Total: 2 days)
Introduction
A robotic strap is a high-strength, highly-flexible semi-soft robot that can wrap itself around heavy yet fragile objects (e.g. humans) to compliantly and safely lift them, in a similar closed-loop manner to how typical straps, harnesses, and slings are used [1]. The clips to the right show a 30-joint robotic strap (1) actively bending under gravity, and (2) actively harnessing a human subject to lift their full weight, then unharnessing them.
This tutorial demonstrates how to create a simple robotic strap (with four joints and one actuator). The steps in this tutorial can be repeated to indefinitely create robotic straps with any number of joints and up to four actuators. Left: robotic strap lifting the full weight of a human. Right: robotic strap active bending against gravity.
Prerequisite & background topics
Prior to starting the tutorial, you can try familiarizing yourself with the following prerequisite topics and terminology by browsing the resources linked under each item.
3D printing (Prusa i3 tutorial, Stratasys tutorial, Markforged tutorial)
Robotic Straps background (project page, paper)
Materials and Equipment Required
Materials:
3D printing filament material:
Recommend: Onyx (carbon-fiber infused nylon)
For high strength joints, requires (any) Markforged 3D printer.
Optional: Continuous fiber reinforcement
Alternatives:
PLA (may reduce tensile strength of robotic strap backbone structure)
Onyx ESD (more rigid than Onyx for less out-of-plane deflection, but also more brittle and thus easier to crack while removing support. Requires industrial series Markforged 3D printer.)
Any other material will work as well, with varying strength and surface quality properties.
Nylon thread, OD 0.016” (alternative: Kevlar, OD 0.014”)
Thin cable ties:
Alternative: Course-adjustment 0.07” wide 0.035” thick 4” long (shorter length requires chaining two cable ties together to make one long cable tie)
Gel super glue: Loctite Super Glue Gel Control
McKibben artificial muscles:
Any McKibben artificial muscles with an idle outer diameter of ~4 mm will suffice for this version of the Robotic Strap. The resulting bending performance and pressure requirements will vary depending on the force-contraction characteristics of the McKibben muscles.
Options:
Make from scratch: Tutorial: Soft Robotics Toolkit, Tutorial: Open Soft Machines
[Purchase] S-muscle EM-40
** As of 6/15/2023, s-muscle Co., Ltd. is temporarily only selling their products to customers in Japan. This page will be updated once s-muscles opens sales back up to international customers.
Used by authors in the version of the Robotic Strap prototype shown in [1].
ALSO NEED:
[Purchase] PneuMuscle PMJ40 (NOT TESTED BY AUTHORS)
Tools:
3D printer:
Recommend: Markforged composite 3D printer (any) with Onyx filament
Alternatives:
Alternatives cannot print with Onyx filament, yielding lower strength parts.
Original Prusa i3 MK3S+ 3D printer with nozzle for <0.2 mm layer height
Stratasys F123 series (weaker material and larger layer height, but faster printing with dissolvable supports)
Any other printer (with down to 0.2 mm layer height recommended).
3D print support removal tools:
Gingher fabric shears (alternative: office scissors)
Pneumatic pressure source:
Recommend: FlowIO with large pump module
Alternative: NORSHIRE Mini Tire Inflator, Skoocom SC3402XPM, Sparkfun 12V Vacuum Pump (only up to 0.22 MPa)
Additional tools for McKibben artificial muscle fabrication:
For s-muscle EM40: Wooden skewer, Aluminum foil, Soldering helping hands
Instructions:
3D Printed Parts:
1) Download STL files from this folder.
2) Print joints and clamps (video 0:00-3:03, for Markforged printer, apply same/similar settings for other printers).
a) Import joint STL into slicer program.
b) Set orientation of joint.
Set cylinder axis aligned with z-axis of print bed so that supports are printed in direction that is easiest to remove.
c) Set joint material.
Optional: Set joint reinforcement material. Make sure reinforcement material is routed around the inside of the inner pulleys, i.e. the load bearing feature.
d) Set joint printer type.
e) Set joint layer height. (recommend: 0.125 mm, alternative: 0.2 mm)
f) Set joint infill. (recommend: triangular, 37%)
g) Save joint part.
h) Import clamp STL into slicer program.
i) Set orientation of clamp.
Set top of clamp flat on print bed.
j) Set clamp material. (same as joint)
k) Set clamp printer type. (same as joint)
l) Set clamp layer height. (same as joint)
m) Set clamp infill. (recommend: solid fill)
n) Save clamp part.
o) Create print job with both joint and clamp parts, and start print.
3) Remove supports (joints) (video 0:05-2:34).
Remove supports from the bottom of the joint, then the middle hole, then the ends of the joints. Clean up remaining support with tweezers.
4) Remove supports (clamps).
Use tweezers to pull off arbitrarily.
5) Print assembly guides (video 3:04-4:11, for Markforged printer, apply same/similar settings for other printers).
Quantity for n joints:
# of guides >= roundup(n/3)
a) Import STL file into slicer program.
b) Set orientation of part.
c) Set material. (any rigid material)
d) Set printer type.
e) Set layer height. (recommend: 0.2 mm)
f) Set infill. (recommend: solid fill)
g) Export print job and start print.
3) Remove supports.
Use tweezers to pull off arbitrarily.
Backbone Assembly:
1) Insert two joints onto assembly guide (video 2:36-2:56).
2) Cut 0.038” diameter Kevlar cords into 4 x 24” segments (video 2:56-3:31).
3) Thread needle with Kevlar cord (video 3:31-3:46).
4) Thread the inner pulleys (video 3:46
-4:33).
5) Tie ends of inner pulley thread together with half of a double fisherman’s knot (video 4:33-7:27).
Only tie half of knot to enable tightening after tying.
6) Thread the outer pulleys of the joints (video 7:27-9:08).
7) Tie ends of outer pulley thread together with half of a double fisherman’s knot (video 9:08-11:33).
8) Perform the following steps to the inner pulley thread knot in quick succession (video 11:33-12:27):
a) Apply glue to knot and tighten.
b) Tie the first half of a square knot (instructions, steps 1-4) over that knot.
c) Apply glue to the new knot.
d) Finish the square knot (instructions, steps 5-7).
9) Repeat step 8 for the outer pulley thread (video 12:27-12:41).
10) Flip the pair of joints over on the assembly guide (video 12:41-13:16).
The glue on the knots will dry while working on the threading for the other side of the joints.
11) Repeat steps 3-10 for the other set of pulleys (video 13:16-13:45).
12) Add third joint onto assembly guide (video 13:45-13:56).
13) Repeat steps 2-12 for third joint (video 13:56-15:07).
14) For fourth joint (and ALL ADDITIONAL JOINTS):
a) Shift chain of joints down one spot on the assembly guide so that a spot becomes available on the other end. Add a new joint onto that available spot (video 15:07-15:52).
b) Repeat steps 2-12 for new joint (video 15:52-16:57).
15) Fix outer pulley threads in place along the outer pulleys by filling outer pulley thread holes with gel super glue (video 16:57-17:58).
Make sure all joints are inserted into assembly guides while applying and setting glue to ensure proper joint alignment. We recommend inserting assembly guides on both sides of joints to hold them above the table. Only inserting on one side is also fine, just make sure that wet glue does not touch the table.
McKibben Artificial Muscles Fabrication:
The following instructions are for fabricating McKibben actuators using the s-muscle EM40 stock tubing and mesh. For fabricating other types of McKibben actuators, please fabricate them according to whatever instructions are most appropriate for that type.
Any McKibben actuators with an idle outer diameter of ~4 mm will suffice for this version of the Robotic Strap. The resulting bending performance and pressure requirements of the robotic strap will vary depending on the force-contraction characteristics of the McKibben muscles.
s-muscle provides their own fabrication instructional video. The instructions are in Japanese, but are still relatively understandable for non-Japanese speakers. The spoken dialog describes the actions being visually shown on screen, so the instructions can still be followed without understanding the speech. To read the text shown on screen, we recommend using the camera function in the Google Translate smartphone app (iOS, Android).
Attach McKibben Muscles to Backbone:
1) Mark the McKibben actuator with n lines 38.1 mm (1.5 in) from each other with a sharpie (where n is the number of joints) (video 17:58-18:16).
2) Thread 0.016” OD nylon thread through outer mesh of McKibben actuator at one of the marked points according to the diagram below. Repeat for all marked points (video 18:16-21:23).
3) For one of the marked points (video 21:23-22:27):
a) Slide an actuator clamp into the open loop.
b) Tighten loop around the clamp.
c) Tightly tie the thread ends together around the clamp.
4) Repeat step 3 for all marked points along the actuator (video 22:27-23:06).
5) Apply a drop of gel super glue to the knots of each thread, and then tie another knot on each (video 23:06-24:00).
6) Insert the actuator clamps into the robotic strap backbone joints (video 24:00-24:26).
7) Fasten the actuator clamps onto the joints using the no-gap cable ties (video 24:26-27:00).
8) Cut off ends of loose threads and cable ties (video 27:00-27:50).
Testing:
1) Actuate McKibben muscle to bend robotic strap (video 27:52-30:01).
Variations and Things to try
Try adding up to four actuators in parallel to the backbone to improve active bending performance.
Try printing the joints with different materials for different trade-offs (strength vs. weight vs. surface quality vs. print time vs. cost, e.g. FDM vs. SLA printers).
Try making longer backbones to fully wrap around large objects.
Try different 3D print settings to tune weight vs. strength/rigidity.
Modify the CAD file (Solidworks) to change key joint parameters such as diameter and width. As described in [1], the choice of joint diameter significantly impacts many critical performance trade-offs.
Try designing attachments for the joints (e.g. end-point loop to latch onto other devices).
Try controlling the pressure input with digital pressure values to program the robotic strap to behave autonomously.
Post-requisite topics & Going beyond
Read more about robotic straps in our recent IEEE RA-L paper [1].
[1] K. Barhydt and H. H. Asada, “A High-Strength, Highly-Flexible Robotic Strap for Harnessing, Lifting, and Transferring Humans,” in IEEE Robotics and Automation Letters, vol. 8, no. 4, pp. 2110-2117, April 2023, doi: 10.1109/LRA.2023.3246389.
* If you cannot access the full text paper on IEEE Xplore, please email kbarhydt@mit.edu for a copy of the full paper.