[tt] mobile robots: motor challenges and materials solutions

[Eugen Leitl]

http://www.sciencemag.org/cgi/content/full/sci;318/5853/1094

Science 16 November 2007:

Vol. 318. no. 5853, pp. 1094 - 1097

DOI: 10.1126/science.1146351

REVIEW

Mobile Robots: Motor Challenges and Materials Solutions

John D. Madden

Bolted-down robots labor in our factories, performing the same task over and
over again. Where are the robots that run and jump? Equaling human
performance is very difficult for many reasons, including the basic challenge
of demonstrating motors and transmissions that efficiently match the power
per unit mass of muscle. In order to exceed animal agility, new actuators are
needed. Materials that change dimension in response to applied voltage,
so-called artificial muscle technologies, outperform muscle in most respects
and so provide a promising means of improving robots. In the longer term,
robots powered by atomically perfect fibers will outrun us all.

Department of Electrical and Computer Engineering, University of British
Columbia, Vancouver, BC V6T 1Z4, Canada. E-mail: jmadden at ece.ubc.ca

In this article, the application of actuator technologies is considered
specifically for robots that are humanlike in form. Marc Raibert and his
group at Massachusetts Institute of Technology (MIT) showed in the 1980s that
robots can walk, run, and do flips (1). These robots are not free, however,
but rather are attached to their power supplies. The incredible achievements
and the limitations of successive lifelike robots provide insight into the
challenges of using conventional actuators to drive machines that mimic human
form and motion. The focus of this article is on robots and humanoids in
particular, but much of the discussion of actuators is relevant to any active
mechanical system and particularly those that involve intermittent rather
than continuous motion, such as prosthetics, medical devices, valves, locks,
and toys.


Combustion Engines: Powerful But Hard to Carry

The power per unit mass achieved in internal combustion engines is 1000 W/kg,
about 10 times greater than the continuous power output of our own muscle
(2). High power makes combustion engines excellent for the propulsion of
vehicles, and particularly for highway driving, where abrupt changes in speed
or direction are unusual. This power is combined with the long range afforded
by the use of gasoline, which has an energy per unit mass that is about 20
times higher than that of a good battery, even after accounting for the 30%
efficiency typical in an internal combustion process. There are two
particularly notable challenges to using the combustion engine on a robot,
however. The first is that the engine operates best over a narrow range of
rotation speeds, providing no torque at all at zero speed. Cars have
transmission systems, including clutches and gears, that enable acceleration
from a complete stop up to high speed. This transmission is not suited to the
abrupt motions required of a robot, such as reaching for an object, then
holding it for some time at a fixed position, and then throwing it away. The
second challenge is simply carrying the hot, loud, and fuming engine on a
robot while operating it efficiently and effectively, with space left for
fuel.

Steve Jacobsen and his colleagues have demonstrated particularly impressive
use of hydraulics to drive robots (3). Hydraulic actuation is a sophisticated
version of the system used to drive the shovel on a front-end loader.
Jacobsen's hydraulic robotics perform extremely lifelike movements and have
been demonstrated in Disney theme park humanoid robots and Jurassic Park
dinosaurs. However, these rely on an external power source. The Berkeley
Robotics Laboratory has shown that a hydraulic motor can be taken on board
(4, 5). Its 75-kg device is not a free-standing robot but rather an
exoskeleton with powered ankles, knees, and hips. The robot is attached at
the feet and the hips, and it works in parallel with the wearer, allowing an
additional 75 kg to be carried. This capability is intended to relieve a foot
soldier's burden. The combined hydraulic system, empty fuel tank, valves,
actuating pistons, and internal combustion engine exhibit a power-to-mass
ratio that is about the same or perhaps a bit lower than that of muscle
itself (6). Hydraulics are not terribly efficient for walking, which requires
high power output only for brief periods of time. For the remainder of the
time the system is needlessly shunting fluid. Primarily as a result of this
inefficiency, BLEEX expends three times more energy in walking than a human
does (4). A further drawback is the noise and heat of the combustion engine.
The device certainly augments human strength, but so far soldiers are better
off building up their own muscle if they can.

One key to reducing weight and increasing efficiency, and thereby making
hydraulics more practical, may be to redesign the internal combustion engine
to allow for the bursts of power needed during walking, running, or jumping
(7, 8). A potential weight-saving measure is to use lightweight pneumatic
actuators in place of heavier hydraulic pistons, although this increases the
mass of the pump (9). Either way, it is very hard to beat muscle.


Electric Motors: Jogging But Not Sprinting

Electric motors are attractive because they feature high continuous power per
unit mass [up to 300 W/kg when using rare earth magnets (10) and twice that
when actively cooled (11)] and high efficiency (can be >90%) (2). They are
also relatively quiet and generate high torques at low speeds, making the
transmission easier than it is in the combustion engine. Honda's impressive
ASIMO is a battery-powered, untethered humanoid robot driven by electric
servomotors (12–14). There is a motor for each of the 34 joints, including
arms, legs, hips, hands, feet, head, and fingers. The fast rotary motion of
the electric motors (which deliver maximum power at high speed) is converted
to slower joint rotation by using a compact reduction system known as a
harmonic drive. The drive has the same effect as going into very low gear on
a bicycle. This transmission system, however, is heavy, bringing the overall
power per unit mass down to or below that of muscle. Honda's latest robot,
shown in Fig. 1, is able to do a slow run (6 km/hour, equivalent to a
16-min-mile pace), with both feet leaving the ground simultaneously between
steps, clearing the ground by about 3 cm (13). It can also do light work,
picking up 1 kg (about four coffees) when using both hands. Similar
complexity and performance are demonstrated in other battery-powered
servomotor-driven robots, including Sony's QRIO robot (15, 16), which is much
smaller than ASIMO and was the first to run, and Kawada's HRP-2 (16, 17),
which is about the same size as ASIMO but does not run.



	Fig. 1. Honda's humanoid robot ASIMO on the run. Reproduced from (13)
with the permission of the Honda Motor Company. [View Larger Version of this
Image (74K GIF file)]
 
Why can't ASIMO and the others go faster, jump higher, or carry a larger
load? Speed is limited by the peak power output. Peak power requirements
triple in the progression from walking to sprinting (18), so ASIMO's motors
need to be three times heavier to achieve a fast run than they do for a
moderately paced walk. In a human the size of ASIMO, the peak power at the
ankle is about 200 W (4). At a sprint pace, the power rises to 700 W (18).
Factoring in the inefficiency of the transmission, the power needed from an
electric motor is more than 1000 W in each ankle. With transmission included,
the power density of the motor is roughly halved, so when using a
high-performance uncooled electric motor and gear-head the output is 150 W/kg
(10), resulting in the need for a 6.5-kg motor on each ankle. Imagine the
effect on the quadriceps of carrying an extra gallon of milk on each calf
during a sprint: The actuator is simply too heavy.

Mammalian skeletal muscle, the form of muscle we use to move our limbs, has a
peak power to mass of about 300 W/kg for fast twitch muscle and lower in
aerobic forms (19). On the basis of the 700 W required at the ankle during
sprinting and optimistically assuming fast twitch performance, 2.3 kg or
about 2 liters of calf muscle are required. That is a very large calf muscle,
particularly for a person the size of ASIMO (54 kg). Nature gets away with
significantly smaller muscles. This is achieved by shunting more than 50% of
the energy in a stride in to tendon extension, muscle stretching, and flexion
of the foot (18). The running motion has been likened to the travel of a pogo
stick, and the legs each modeled by a spring in series with muscle. This
approach is being mimicked in robotics by inserting springs in series with
actuators (20) and has been used in several bipedal robots (9). In time these
may be able to match our own mechanical performance, particularly if metal
springs are avoided (the small strains of metals make them low in energy
density compared to tendons and rubbers).

Can the electric motors used in robots be improved? The Lorentz force used to
drive these motors produces a force that is proportional to current. Current
is limited by the heat generated due to resistive losses. Power output can be
doubled by adding cooling. One means of improving ASIMO's performance is to
add a water circulation system that enables perspiration. In expending 1 kW
of energy continuously (a strenuous activity level in a human), little more
than 1.5 liters of water per hour would be evaporated. The addition of water
cooling is not trivial because it adds complexity, weight, and cost, but
making robots that drink to keep cool should dramatically improve agility.

Batteries, hybrids, or fuel cells? ASIMO has a 51.8-V lithium ion battery
pack, which can sustain it for 1 hour and takes 4 hours to recharge. Humans
can continue for days on their reserves. Our fat, when combined with oxygen,
generates enough adenosine triphosphate (ATP) (21) (the molecule used to
power muscle and other processes) to provide 15 MJ/kg, 30 times more usable
energy than the same mass of lithium ion battery. At present ASIMO, with its
image and voice recognition abilities, can act as a receptionist, sitting
plugged in between making small deliveries or after guiding visitors to their
meetings. How can endurance be improved?

Some reduction in energy expenditure may be possible. HRP-2 runs its 11-kg
batteries down in about 1 hour, corresponding to an average power expenditure
of about 300 W. A person walking at a moderate pace burns about 3.3 W/kg of
body mass, a 220-W expenditure for someone weighing the same as the robot (58
kg). The comparison suggests that there are opportunities to reduce power
consumption in robots, but what is really needed is a high energy density
storage method.

One option is to create the robotic version of a hybrid car. A portable
combustion engine driving a 1-kW generator weighs about 15 kg including fuel
for up to 8 hours. The effective energy density of the fuel plus the
generator over 8 hours is about five times better than that of a battery, but
still about five times worse than storing energy as fat. The key to matching
fat is to make the motor smaller and lighter. In the long run, the
development of turbine generators on a chip could solve the energy challenge.
These are millimeter-scale turbine blades, combustion chambers, and electric
generators microfabricated in silicon. Fuel-driven microfabricated turbines
exhibit power densities that are more than 100 times larger than those in
traditional combustion engines, making their size negligible compared to the
stored volume of fuel and thus enabling a 20-fold longer running time than is
possible with batteries (22). Some fabrication challenges remain, however,
before these devices are fully demonstrated.

Fuel cells are a promising option but are not sufficiently developed. A
commercial portable hydrogen fuel cell (23) can provide the same power output
per unit mass as the portable gas generator, but the space required is larger
because of the fuel volume needed, making it more cumbersome.

Muscle: hard to surpass. The skeletal muscle used to actuate our limbs
(24–26) is a beautifully refined linear actuator, typically capable of
contracting by 20% of its length.rves that enable rate, force, and speed
control. Additionally, the digestive and circulatory systems provide amino
acids that enable muscle to build up, repair itself, and regenerate, allowing
it to adapt to demand and to last a lifetime. Our technology is not yet ready
to interface with such a complex system.


Artificial Muscle

Many materials have been investigated as candidates for artificial muscle
(26–28), including gels that swell and contract by more than 100% in response
to changes in pH and temperature; shape memory alloys, whose change in
crystal structure with changes in temperature or applied magnetic field
produce relative changes in length of up to 10% at high loads; intrinsically
conducting polymers that charge and discharge like batteries and swell or
contract by about 8% in the process; ionically conductive polymers in which
ions and solvent are shuttled from one side of the material to another,
producing a bending motion; and liquid crystals, whose change in alignment
with temperature or electric field leads to displacements. The two most
immediately promising technologies are dielectric elastomers and relaxor
ferroelectric polymers. Both are electric field–driven, and they feature high
work per unit volume [reaching 1 J/cm3, compared to 0.04 J/cm3 in muscle
(26)]. The high work density compared with muscle means that less volume and
mass are needed (because densities are similar to that of muscle), enabling
lighter and thus more agile devices. The relatively good coupling between the
electrical input energy and the mechanical work performed (20% to 90%)
enables them to operate with efficiencies that are comparable to or better
than that of muscle. Dielectric elastomers in particular are ripe for
application, having been demonstrated in multilegged robots (29) (Fig. 2B)
and an arm-wrestling device (30), as well as being commercially available
from the start-up Artificial Muscle Incorporated of Menlo Park, California.



	Fig. 2. (A) Mechanism of actuation of dielectric elastomers (21) and
(B) SRI's FLEX 2 six-legged robot operating with sheets of dielectric
elastomers rolled around a sprinterial needed to perform the same work as all
of our muscles put together. Our biceps could be replaced by an 8-mm-diameter
wire. Such compact muscle would be enormously enabling for robots, making
them far lighter and more agile.

In order to extract the energy from nanowires or nanotubes, there needs to be
a mechanism of stretching them in the first place. In a cross-bow our muscle
provides the stretching, but what can we use to stretch these tiny filaments?
Also, if they are to be used in robots, the size needs to be scaled up
substantially without producing defects. The stretching can be done
electrostatically, as has been shown in carbon nanotubes (38), platinum
nanoparticles (39, 40), and most recently nanowires (41). Charging is
achieved by submerging films of these materials into an electrolyte and
applying an electrical potential through the solution, as depicted in Fig.
3B. The resulting charging of the surfaces of the nanotubes, wires, and
nanoparticles is sufficient to expand these stiff materials because of their
high surface area–to–volume ratios.



	Fig. 3. Actuation of metal nanofibers. (A) Scanning electron
micrograph of niobium nanofilaments formed by drawing a copper-niobium
composite. Reprinted from (39). [Copyright 1978 American Institute of
Physics] (B) A depiction of how such fibers might be actuated, showing two
individual fibers to which voltage has been applied through an electrolyte.
The charging of the surfaces of the fibers is expected to lead to both
expanding relative to their neutral states when charging levels are
sufficiently high. [View Larger Version of this Image (92K GIF file)]

At present, however, the problem is that the coupling between input
electrical energy and output mechanical work is low. Spun nanotube yarns show
charge-induced strains of 0.5%, and stresses can exceed 100 MPa. However, the
electromechanical coupling is less than 1%. The problem is that a lot of
energy is expended stretching the nanotubes, but very little is extracted
because the stiffness of the yarn is far lower than that of the individual
nanotubes.

How can the coupling and the strain be improved? The conceptually simple but
practically challenging answer is by making the macroscopic structures as
stiff as the microscopic ones. It has been known for some time that bundles
of superstrong nanowires (42–44), as shown in Fig. 3A, can be as strong as
the individual wires from which they are composed. If the bundles can be made
porous, then it may be possible to ionically charge them in order to induce
deformation.

No actuator technology yet matches the muscular system's combination of high
energy density fuel, relatively efficient operation, scaleable force, elastic
energy storage, and power output. Developments in transmissions, series
elastic elements, and energy storage and generation mechanisms should make it
possible to equal muscle's performance using traditional motors. Electric
field–driven polymers outperform muscle in most respects but need creative
solutions for delivering the electrical power in a safe and compact manner.
If the incredible properties of nanofibers can be extended to macroscopic
scales in actuators, as has been achieved for passive mechanical structures,
then artificial muscle will enable robots to outrun and outjump us all.


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