The design of soft actuators is often focused on achieving target trajectories or delivering specific forces and torques, rather than controlling the impedance of the actuator. This article outlines a new soft, tunable pneumatic impedance module based on an antagonistic actuator setup of textile-based pneumatic actuators intended to deliver bidirectional torques about a joint. Through mechanical programming of the actuators (select tuning of geometric parameters), the baseline torque to angle relationship of the module can be tuned. A high bandwidth fluidic controller that can rapidly modulate the pressure at up to 8 Hz in each antagonistic actuator was also developed to enable tunable impedance modulation. This high bandwidth was achieved through the characterization and modeling of the proportional valves used, derivation of a fluidic model, and derivation of control equations. The resulting impedance module was capable of modulating its stiffness from 0 to 100 Nm/rad, at velocities up to 120°/s and emulating asymmetric and nonlinear stiffness profiles, typical in wearable robotic applications.
OnalCD, ChenX, WhitesidesGM, et al.Soft mobile robots with on-board chemical pressure generation. Springer Tracts Adv Robot, 2017; 100:525–540.
3.
MartinezRV, BranchJL, FishCR, et al.Robotic tentacles with three-dimensional mobility based on flexible elastomers. Adv Mater, 2013; 25(2):205–212.
4.
VealeAJ, XieSQ, AndersonIA. Modeling the peano fluidic muscle and the effects of its material properties on its static and dynamic behavior. Smart Mater Struct, 2016; 25(6):065014.
5.
CalistiM, PicardiG, LaschiC. Fundamentals of soft robot locomotion. J R Soc Interface, 2017; 14(130):20170101.
6.
HawkesEW, BlumenscheinLH, GreerJD, et al.A soft robot that navigates its environment through growth. Sci Robot, 2017; 2(8):3028.
7.
StokesAA, ShepherdRF, MorinSA, et al.A hybrid combining hard and soft robots. Soft Robot, 2014; 1(1):70—74.
8.
RoseCG, O'MalleyMK. Hybrid rigid-soft hand exoskeleton to assist functional dexterity. IEEE Robot Autom Lett, 2019; 4(1):73–80.
9.
ChenY, WanF, WuT, et al.Soft-rigid interaction mechanism towards a lobster-inspired hybrid actuator. J Micromech Microeng, 2018; 28(1):014007.
10.
BartlettNW, TolleyMT, OverveldeJTB, et al.A 3D-printed, functionally graded soft robot powered by combustion. Science (80-.), 2015; 349(6244):161–165.
11.
KumarK, LiuJ, ChristiansonC, et al.A biologically inspired, functionally graded end effector for soft robotics applications. Soft Robot, 2017; 4(4):317–323.
12.
Keiko Ogura, WakimotoS, SuzumoriK, et al.Micro pneumatic curling actuator—Nematode actuator. In: 2008 IEEE International Conference on Robotics and Biomimetics. IEEE; 2009; pp. 462–467.
13.
GallowayKC, BeckerKP, PhillipsB, et al.Soft robotic grippers for biological sampling on deep reefs. Soft Robot, 2016; 3(1):23–33.
14.
GorissenB, ReynaertsD, KonishiS, et al.Elastic inflatable actuators for soft robotic applications. Adv Mater, 2017; 29(43):1604977.
15.
VillegasD, Van DammeM, VanderborghtB, et al.Third–generation pleated pneumatic artificial muscles for robotic applications: Development and comparison with McKibben Muscle. Adv Robot, 2012; 26(11–12):1205–1227.
16.
GaffordJ, AiharaH, ThompsonC, et al.Distal proprioceptive sensor for motion feedback in endoscope-based modular robotic systems. IEEE Robot Autom Lett, 2018; 3(1):171–178.
17.
WangZ, HangG, LiJ, et al.A micro-robot fish with embedded SMA wire actuated flexible biomimetic fin. Sens Actuat A Phys, 2008; 144(2):354–360.
18.
GuptaU, QinL, WangY, et al.Soft robots based on dielectric elastomer actuators: A review. Smart Mater Struct, 2019; 28(10):103002.
19.
LiWB, ZhangWM, ZouHX, et al.A fast rolling soft robot driven by dielectric elastomer. IEEE ASME Trans Mechatron, 2018; 23(4):1630–1640.
20.
DudutaM, BerlingerF, NagpalR, et al.Tunable multi-modal locomotion in soft dielectric elastomer robots. IEEE Robot Autom Lett, 2020; 5(3):3868–3875.
21.
SheaH, BingZ, ShintakeJ, et al.Jellyfish-inspired soft robot driven by fluid electrode dielectric organic robotic actuators. Front Robot AI, 2019; 6:126.
22.
ZhangJ, ShengJ, O'NeillCT, et al.Robotic artificial muscles: Current progress and future perspectives. IEEE Trans Robot, 2019; 35(3):761–781.
23.
YangD, VermaMS, SoJ-H, et al.Buckling pneumatic linear actuators inspired by muscle. Adv Mater Technol, 2016; 1(3):1600055.
24.
ConnollyF, WalshCJ, BertoldiK. Automatic design of fiber-reinforced soft actuators for trajectory matching. Proc Natl Acad Sci U S A, 2017; 114(1):51–56.
25.
PolygerinosP, WangZ, GallowayKC, et al.Soft robotic glove for combined assistance and at-home rehabilitation. Rob Auton Syst, 2015; 73:135–143.
26.
O'NeillC, ProiettiT, NuckolsK, et al.Inflatable soft wearable robot for reducing therapist fatigue during upper extremity rehabilitation in severe stroke. IEEE Robot Autom Lett, 2020; 5(3):3899–3906.
27.
KotikianA, MoralesJM, LuA, et al.Innervated, self-sensing liquid crystal elastomer actuators with closed loop control. Adv Mater, 2021; 33(27):2101814.
28.
BaidenD, IvlevO. Independent torque and stiffness adjustment of a pneumatic direct rotary soft-actuator for adaptable human-robot-interaction. In: 2014 23rd International Conference on Robotics in Alpe-Adria-Danube Region (RAAD). IEEE; 2015; pp. 0–5.
29.
NiiyamaR, RusD, KimS. Pouch motors: Printable/inflatable soft actuators for robotics. In: 2014 IEEE International Conference on Robotics and Automation (ICRA). IEEE; 2014; pp. 6332–6337.
30.
KangBB, ChoiH, LeeH, et al.Exo-Glove Poly II: A polymer-based soft wearable robot for the hand with a tendon-driven actuation system. Soft Robot, 2019; 6(2):214–227.
31.
CappelloL, GallowayKC, SananS, et al.Exploiting textile mechanical anisotropy for fabric-based pneumatic actuators. Soft Robot, 2018; 5(5):662–674.
32.
ConnollyF, PolygerinosP, WalshCJ, et al.Mechanical programming of soft actuators by varying fiber angle. Soft Robot, 2015; 2(1):26–32.
33.
Bishop-MoserJ, KotaS. Design and modeling of generalized fiber-reinforced pneumatic soft actuators. IEEE Trans Robot, 2015; 31(3):536–545.
34.
WehnerM, QuinlivanB, AubinPM, et al.A lightweight soft exosuit for gait assistance. In: Proceedings—IEEE International Conference on Robotics and Automation. IEEE; 2013; pp. 3362–3369.
35.
MooneyLM, HerrHM. Biomechanical walking mechanisms underlying the metabolic reduction caused by an autonomous exoskeleton. J Neuroeng Rehabil, 2016; 13(1):1–12.
36.
O'NeillCT, McCannCM, HohimerCJ, et al.Unfolding textile-based pneumatic actuators for wearable applications. Soft Robot, 2022; 9(1):163–172.
37.
SimpsonC, HuertaB, SketchS, et al.Upper extremity exomuscle for shoulder abduction support. IEEE Trans Med Robot Bionics, 2020; 2(3):474–484.
38.
NeslerCR, SwiftTA, RouseEJ. Initial design and experimental evaluation of a pneumatic interference actuator. Soft Robot, 2018; 5(2):138–148.
39.
FangJ, YuanJ, WangM, et al.Novel accordion-inspired foldable pneumatic actuators for knee assistive devices. Soft Robot, 2020; 7(1):95–108.
40.
NguyenPH, ZhangW. Design and computational modeling of fabric soft pneumatic actuators for wearable assistive devices. Sci Rep, 2020; 10(1):1–13.
41.
RusD, TolleyMT. Design, fabrication and control of soft robots. Nature, 2015; 521(7553):467–475.
PaolettiP, JonesGW, MahadevanL. Grasping with a soft glove: Intrinsic impedance control in pneumatic actuators. J R Soc Interface, 2017; 14(128):20160867.
45.
PrattGA, WilliamsonMM. Series elastic actuators. In: Proceedings 1995 IEEE/RSJ International Conference on Intelligent Robots and Systems. Human Robot Interaction and Cooperative Robots. IEEE; 1995; pp. 1399–1406.
46.
HaldaneDW, PlecnikMM, YimJK, et al.Robotic vertical jumping agility via series-elastic power modulation. Sci Robot, 2016; 1(1):eaag2048.
47.
VanderborghtB, Albu-SchaefferA, BicchiA, et al.Variable impedance actuators: A review. Rob Auton Syst, 2013; 61(12):1601–1614.
48.
VerrelstB, Van HamR, VanderborghtB, et al.The pneumatic biped “Lucy” actuated with pleated pneumatic artificial muscles. Auton Robots, 2005; 18(2):201–213.
49.
PolygerinosP, CorrellN, MorinSA, et al.Soft robotics: Review of fluid-driven intrinsically soft devices; Manufacturing, sensing, control, and applications in human-robot interaction. Adv Eng Mater, 2017; 19:1–22.
50.
XiloyannisM, AliceaR, GeorgarakisA-M, et al.Soft robotic suits: State of the art, core technologies, and open challenges. IEEE Trans Robot, 2022; 38(3):1343–1362.
51.
TamburranoP, PlummerAR, DistasoE, et al.A review of direct drive proportional electrohydraulic spool valves: Industrial state-of-the-art and research advancements. J Dyn Syst Meas Control Trans ASME, 2019; 141(2):020801.
52.
RennJC, TsaiC. Development of an unconventional electro-hydraulic proportional valve with fuzzy-logic controller for hydraulic presses. Int J Adv Manuf Technol, 2005; 26(1–2):10–16.
53.
SaravanakumarD, MohanB, MuthuramalingamT. A review on recent research trends in servo pneumatic positioning systems. Precis Eng, 2017; 49:481–492.
54.
HuangH, WuL, LinJ, et al.A novel mode controllable hybrid valve pressure control method for soft robotic gripper. Int J Adv Robot Syst, 2018; 15(5):1–12.
55.
FicanhaEM, RibeiroGA, DallaliH, et al.Design and preliminary evaluation of a two DOFs cable-driven ankle-foot prosthesis with active dorsiflexion-plantarflexion and inversion-eversion. Front Bioeng Biotechnol, 2016; 4:36.
56.
MajorMJ, TwisteM, KenneyLPJ, et al.The effects of prosthetic ankle stiffness on stability of gait in people with transtibial amputation. J Rehabil Res Dev, 2016; 53(6):839–852.
57.
Koehler-McNicholasSR, Savvas SlaterBC, KoesterK, et al.Bimodal ankle-foot prosthesis for enhanced standing stability. PLoS One, 2018; 13(9):e0204512.
58.
AuS, BernikerM, HerrH. Powered ankle-foot prosthesis to assist level-ground and stair-descent gaits. Neural Networks, 2008; 21(4):654–666.
59.
RibeiroGA, KnopLN, RastgaarM. Multi-directional ankle impedance during standing postures. IEEE Trans Neural Syst Rehabil Eng, 2020; 28(10):2224–2235.
60.
BrockettCL, ChapmanGJ. Biomechanics of the Ankle. Orthop Trauma, 2016; 30(3):232–238.
61.
NesterCJ, JarvisHL, JonesRK, et al.Movement of the human foot in 100 pain free individuals aged 18–45: Implications for understanding normal foot function. J Foot Ankle Res, 2014; 7(1):51.
Supplementary Material
Please find the following supplemental material available below.
For Open Access articles published under a Creative Commons License, all supplemental material carries the same license as the article it is associated with.
For non-Open Access articles published, all supplemental material carries a non-exclusive license, and permission requests for re-use of supplemental material or any part of supplemental material shall be sent directly to the copyright owner as specified in the copyright notice associated with the article.
