Energy for Biomimetic Robots: Challenges and Solutions

nimals and autonomous robots need to carry theirown fuel (unlike plants, they do not generate usableenergy from their surroundings). Animals typically exceedthe normal endurance and range of all our current untetheredrobots. As an obvious example, humans have tremendousburst speed (less than 10 seconds to run 100 meters) andendurance (running a 26-mile marathon), and they can con-tinue to do everyday activities without refueling (eating) forseveraldays.Thetypicalcostoftransportforhumansisabout0.2. In comparison, most robots operate for less than 1 houron their carried fuel; the cost of transport is 15 or 20 timesmore than that for animals. An intriguing insight is thatpassive dynamic walkers can approach the human cost oftransport (the Cornell Ranger can walk nonstop for 65km),but this is a single optimized task (walking) with none of theversatilityofananimalthatcanstepoverobjectsandoperateonvariedterrain.Whatitdoesillustrateisthatstructures(andby extension, material properties) can be exploited to ‘‘getthe most’’ out of a given fuel source. Surely, this is whatanimals do on a continuous basis. What do we need to do togive our robots similar capabilities? In particular, what arethe special demands, advantages, and limitations of fuelstorage and usage in soft robots? To begin exploring some ofthese issues and to also stimulate a larger dialog in the robotcommunity, the following discussion has been compiledfrom a series of questions posed to the participants.—Barry TrimmerSoft Robotics: What are the special problems in providingfuel (energy) for biomimetic and soft robots?William Messner: Biomimetic and soft robots in particularare limited because it is very difficult to convert efficientgenerators of high-speed rotary motion (e.g., turbines orelectric motors) to low-speed/high-force motion with softcomponents. Are there alternatives such as planetary orharmonic gear trains? While biological muscles are not par-ticularlyefficientfromathermodynamicperspective,theydonotsufferfromthehugelossesofminiaturizedgearreductionsystems, and they are soft!Inaddition,heatenginesscaleverypoorly—thermallossesand friction losses reduce their efficiency enormously atsmall scales. Thus, highly efficient turbines and even mod-estly efficient reciprocating heat engines do not deliver goodperformance at centimeter scales and below. Existing batte-ries on the other hand deliver poor energy density on a per-mass and per-volume basis.BarryTrimmer:Yes,Icaninserthereafewnumbersthatwekeep coming back to. The energy density of nuclear fuel suchas nuclear uranium is 80,620,000 MJ/kg; by comparison, hy-drogen (compressed at 70MPa) provides 123 MJ/kg and hy-drocarbons such as LPG, gas, diesel, and fat range from 35 to47 MJ/kg. Our best readily available batteries (Li and Li ion)are around 0.3–1.8 MJ/kg. Even if we could get around thesafety and security issues associated with radioactivity, cap-turing that energy requires a lot of physical plant and is un-suitablefor softmobilemachines.Clearly,hydrocarbonshavethe advantage.The obvious drawbackisthatour conventionaluse of hydrocarbons involves combustion and the productionofextremeheat,pressure,andpollutants.Thelow-temperatureoxidationpathwaysusedbylivingtissueswouldseemtomakemost sense for robots used around humans.Sangbae Kim: Most of human-made electric energy storagedevices are relatively rigid or semirigid (lithium-ion poly-mer). It will impede compliant behavior if electric energy isrequired to be part of the structure. Fuel in the form of liquidwould be ideal. A possible question to answer is how to dealwiththevolumechangeoftheliquid asthefuelisconsumed.In a rigid robot, like MIT Cheetah, we need better power-density fuel cells. The power density of the current fuel cellsis not high enough to use in a robot less than 50kg.Robert Shepherd: One of the major benefits that living or-ganismshaveoverrobotsistheirabilitytoharvestenergyfromtheenvironment:eating.Whilewecandeveloprobotsthatcango to charging or fuelstations autonomously, these waypointsmust be predetermined. Thus, even though the internal com-bustion engineismoreefficientthanmuscle, a praying mantiscan catch prey when it must refuel at many locations.William Messner: Making structural components out ofenergy storage devices would be a way to increase the range