General “One-To-Many” (OTM) concept The “One-to-Many” (OTM) concept allows for a single actuator, e.g. electric motor to drive multiple independently actuated and controlled degrees of freedom (DoF). Each DoF has its own dedicated reservoir for controlled energy storage and release. Each reservoir can be actively coupled, either to the actuator to store energy or to the end-effector/load to release energy. Similar to the recruitment of muscle fibers in a biological muscle, OTM system can output variable stiffness actuation by controlling the recruitment of actuated DoFs coupled to a common load. The system is also capable of augmenting power by releasing energy over a much shorter period of time than was required to store that energy. Within OTM system, the state of each actuated DoF and the state of the actuator are all mechanically decoupled. This technology may prove critical for systems with many DoF that will benefit from being compact, lightweight, efficient, modular, and capable of variable stiffness and power augmentation. Existing examples of successfully realized OTM system include a cable driven Linear OTM utilizing linear springs [3-5] and a shaft driven Rotary OTM utilizing rotary springs [1-2]. Practical hydraulic OTM systems with fast multi-valves [6] currently under development within WPI Popovic Labs are discussed in this proposal. OTM Definition The general “One-to-Many” (OTM) concept allows for a single actuator, e.g. electric motor to drive multiple independently actuated and controlled degrees of freedom (DoF). Each DoF has its own dedicated reservoir for controlled energy storage and release. Each reservoir can be actively coupled, either to the actuator to store energy or to the end-effector/load to release energy. Hence, within OTM system, the state of each actuated DoF and the state of the actuator are all mechanically decoupled. Any system that encompasses this definition is considered an OTM system. OTM Power augmentation capability The emergent feature of any OTM system is power augmentation. The OTM system is capable of augmenting power by releasing energy over a much shorter period of time than was required to store that energy. This quite practical property of OTM systems may substantially lower power requirements on a single actuator for numerous applications. Standard pneumatic and hydraulic systems typically consist of one motor actuating many DoF. Ordinarily, these systems have a single energy reservoir that is utilized more as a safety measure than as a mean to efficiently store and release energy of pressurized gasses or other elastic elements. The OTM presented here is different from these and other approaches because the actuator is never directly connected to the load; instead, elastic or pressurized elements, one per each DoF, are used to store potential energy. When this energy is not being stored by the motor, it is passively stored until the energy is needed at the point of actuation. As a result, the power output at each DoF is not limited by the maximum output power of the motor itself. From Soft Robotics Exo-Musculature to OTM Concept The OTM concept was motivated by research on Soft-Robotics Exo-Musculature utilizing Bowden cable [10-13]. The goal was to design an at-home, comfortable, and portable orthotic and prosthetic (O&P) device built off of an Exo-Musculature that could assist patients in physical therapy routines and/or everyday tasks. The term Exo-Musculature, introduced first in [4], refers to a soft, thin, light-weight, and compliant self-actuated garment without rigid elements or singular joints.
Microsoft Word - GeneralOTMforproposalwithCMUGeneral “One-To-Many”
(OTM) concept
The “One-to-Many” (OTM) concept allows for a single actuator, e.g.
electric motor to drive
multiple independently actuated and controlled degrees of freedom
(DoF). Each DoF has its own
dedicated reservoir for controlled energy storage and release. Each
reservoir can be actively coupled,
either to the actuator to store energy or to the end-effector/load
to release energy. Similar to the
recruitment of muscle fibers in a biological muscle, OTM system can
output variable stiffness actuation
by controlling the recruitment of actuated DoFs coupled to a common
load. The system is also capable of
augmenting power by releasing energy over a much shorter period of
time than was required to store that
energy. Within OTM system, the state of each actuated DoF and the
state of the actuator are all
mechanically decoupled. This technology may prove critical for
systems with many DoF that will benefit
from being compact, lightweight, efficient, modular, and capable of
variable stiffness and power
augmentation.
Existing examples of successfully realized OTM system include a
cable driven Linear OTM
utilizing linear springs [3-5] and a shaft driven Rotary OTM
utilizing rotary springs [1-2]. Practical
hydraulic OTM systems with fast multi-valves [6] currently under
development within WPI Popovic Labs
are discussed in this proposal.
OTM Definition
The general “One-to-Many” (OTM) concept allows for a single
actuator, e.g. electric motor to
drive multiple independently actuated and controlled degrees of
freedom (DoF). Each DoF has its own
dedicated reservoir for controlled energy storage and release. Each
reservoir can be actively coupled,
either to the actuator to store energy or to the end-effector/load
to release energy. Hence, within OTM
system, the state of each actuated DoF and the state of the
actuator are all mechanically decoupled. Any
system that encompasses this definition is considered an OTM
system.
OTM Power augmentation capability
The emergent feature of any OTM system is power augmentation. The
OTM system is capable of
augmenting power by releasing energy over a much shorter period of
time than was required to store that
energy. This quite practical property of OTM systems may
substantially lower power requirements on a
single actuator for numerous applications.
Standard pneumatic and hydraulic systems typically consist of one
motor actuating many DoF.
Ordinarily, these systems have a single energy reservoir that is
utilized more as a safety measure than as a
mean to efficiently store and release energy of pressurized gasses
or other elastic elements.
The OTM presented here is different from these and other approaches
because the actuator is
never directly connected to the load; instead, elastic or
pressurized elements, one per each DoF, are used
to store potential energy. When this energy is not being stored by
the motor, it is passively stored until the
energy is needed at the point of actuation. As a result, the power
output at each DoF is not limited by the
maximum output power of the motor itself.
From Soft Robotics Exo-Musculature to OTM Concept
The OTM concept was motivated by research on Soft-Robotics
Exo-Musculature utilizing Bowden cable
[10-13]. The goal was to design an at-home, comfortable, and
portable orthotic and prosthetic (O&P)
device built off of an Exo-Musculature that could assist patients
in physical therapy routines and/or
everyday tasks.
The term Exo-Musculature, introduced first in [4], refers to a
soft, thin, light-weight, and
compliant self-actuated garment without rigid elements or singular
joints.
Traditionally, assistive or augmentative systems like orthotic or
exoskeleton systems consist of
rigid links and joints for the lower [21] and upper body [22]
extremities. Similarly, most previous
actuated systems for upper body rehabilitation use rigid
exoskeletons or rigid link manipulators [23-24].
However, this traditional approach limits natural DoF and reduces
comfort for the user. Further,
misalignments between biological and artificial joints are
inevitable [25]. Misalignments occur due to: (1)
substantial skin-bone relative motion, (2) changes in volume of the
limb, (3) initial imprecision when
putting on the exoskeleton etc. Clearly, misalignments make
exoskeletons uncomfortable and in regards
to the lower extremities, can even lead to skin lesions and bone
fractures due to the large forces they are
subject to.
When comparing Exo-Musculatures to Exo-Skeletons, the
Exo-Musculature utilizes natural
anatomical structures (skeletal joints and bones) to provide
support for the device and as a result, maintain
natural kinematic DoF [10]. As there are no pre-specified synthetic
rigid joints, an Exo-Musculature
avoids misalignment problems at the joint level. Misalignments that
are still present in the system are less
critical and can be addressed with more advanced sensory-motor
control system [13].
The more fundamental problem for mechanically actuated wearable
Exo-Musculature with
biologically inspired variable stiffness is need for large number
of independently actuated and controlled
DoF.
The force and torque produced by biological muscles can be
modulated by controlling the number
of fibers that are activated in parallel, a process known as
recruitment [25]. This modulation of force
enables optimized efficiency over a wide range of loads and
contraction velocities as well as accelerations
[26-28]. For the human body, there are approximately 800 skeletal
muscles, each of which is composed of
one hundred or more individual motor units [25]. For example,
biceps brachii have about 750-800 such
motor units [29-30]. Each motor unit represents a single
independently actuated and controlled DoF.
Consequently, if an artificial assistive Exo-Musculature is to
mimic even a small percentage of
human musculature, the number of actuated DoF needs to be large.
For mechanically actuated devices,
conventional approaches involving one dedicated electric motor per
actuated DoF results in systems that
are very large, heavy, and expensive. This is clearly inappropriate
for a fully mobile, wearable Exo-
Musculature.
The general OTM concept solves this problem as it allows for a
single actuator, e.g. electric
motor to drive multiple independently actuated and controlled
DoF.
The OTM biologically inspired recruitment strategy
Some of the OTM actuated DoF can be coupled to the same load,
performing as muscle fibers of
the same muscle. Engaging simultaneously combination of those DoF
provides recruitment resulting in
variable stiffness performance similar to that observed in an
ordinary muscle.
Hence, OTM systems are very similar to the biological architecture
and dynamical performance
of “muscle fibers” and muscles.
The OTM system and SEAs
On a structural level, Series Elastic Actuators (SEA) [31] and
single OTM “muscle fibers” are
similar; however, the two are very different on operational and
control levels. While both systems
introduce an elastic element between the actuator and load, SEAs do
not store energy in this element;
rather, SEAs passively transfer energy from a single actuator onto
respective load. The value of SEA is
simplicity and escape from an impractically tight position control
paradigm. This property is also shared
by OTM system. The OTM system, similar to SEA, is not about tight
position control. Unlike a SEA, the
OTM system can modulate power output and it can utilize
biologically inspired recruitment strategies in
practical manner by allowing for a single actuator to drive
multiple independently actuated and controlled
‘muscle fiber” DoF.
OTM architecture
On a structural level, all OTM systems consist of a single actuator
coupled to DoF dedicated
energy reservoirs which connect back in series with the
end-effector/load, Figure 1. On an operational
level, no single reservoir can be coupled to both the actuator (to
store energy) and load (to release energy)
at the same time. These characteristics make up the architecture of
every OTM system.
As a result of this architecture, actuators are mechanically
decoupled from end-effectors’
dynamics and may operate in safe and potentially close to optimal
regime.
Similarly, no two DoFs share the same energy reservoir. Hence, they
are all mechanically
decoupled from each other. Here, it is assumed that for realistic
application, two or more DoFs will be
active at the same time.
Figure 1: OTM System Flowchart
Conventional pneumatic and hydraulic systems often have shared
energy reservoirs. If this energy
reservoir is infinitely large, its state can be considered
independent of the end-effectors’ dynamics.
However, for finite sized reservoirs, and in particular for very
small reservoirs, the state of each reservoir
depends on end-effectors’ dynamics. Hence, the end-effectors’
dynamics are not decoupled from each
other. Furthermore, the actuator is not decoupled from load.
Although simple shared reservoir architectures require a smaller
number of structural and sensing
elements, the advantages of the OTM concept include decoupled
actuator and DoFs states.
OTM systems obtain controlled energy storage and release by means
of clutches, brakes,
transmission mechanisms etc. for mechanical drives and valves for
fluid based, i.e. pneumatic and
hydraulic, drives.
The emergent feature of any OTM system is power augmentation. The
OTM system is capable of
augmenting power by releasing energy over a much shorter period of
time than was required to store that
energy. For example, imagine that a system needs to output 500 W
for only 20 ms every second. Ignoring
energy losses, this corresponds to at least 10 J of energy stored
in an energy reservoir. During a period of
980 ms, a single actuator could store energy at rate of only 10.2 W
(49 times smaller input power than
output power). Gain is system that can be driven with only a tiny,
light weight, inexpensive actuator and
powered with a tiny electric battery.
Linear and Rotary OTM
The linear OTM system was the first physical realization of the
basic OTM architecture. The
system was capable of storing elastic potential energy in springs
through the use of a single electric motor
and a sequence of active clutch mechanisms. The prototype system
had a single motor connected to three
independent motor units (DoF). Each motor unit consists of two
clutch mechanisms and one elastic
element. This allows three possible states: Charging (mechanical
energy is transferred from motor to an
elastic element), Neutral (energy is stored in elastic element),
and finally, Release (potential energy is
transferred to load). The goal of this prototype was to realize an
OTM mechanism based on fast,
inexpensive, lightweight, and energy efficient clutches. The
solution to this was designing latching-
solenoid based clutches that could axially constrain a gear and
control the rotation of a spool mounted to
each gear. A latching solenoid is a type of solenoid that requires
an electric pulse to open or close but
does not require power to remain in either state.
Figure 2 - C1, C3, C5 are the energy storage clutches. C2, C4, C6
are the energy release clutches. Input is the motor drive wheel to
charge springs. E1, E2, E3, are storage springs. E4, E5, E6 are
experimental load springs.
Rotary OTM
The Rotary OTM system [1-4] was built using standard OTM
architecture. The primary goal
when designing this implementation was to allow for motor units
(DoFs) to be compact, lightweight,
energy efficient, and modular. This was realized by designing
independent modules that acted as motor
units.
Modules were designed to connect either to the motor or to another
motor unit in series. A
driveshaft running the length of the module allows for energy to be
transferred through itself to another
unit with minimal losses due to friction.
A planetary gear based clutch allows for the drive shaft to rotate
at a constant velocity, and
simultaneously allow for at least one other output at all times.
This allows each motor unit to engage and
disengage with the main drive shaft to transfer energy to an
internal elastic element without affecting any
other modules.
A few features of the Linear OTM system were improved with new
Rotary OTM system. Most
importantly: (1) more compact rotary spring was used instead of
linear spring and (2) Rotary OTM
incorporates a more refined means of a controlling the energy
output of the system in order to achieve
actuations that could vary in speed, force, and stiffness.
Figure 3: Exploded View of Clutch 1
Figure 4: Energy Storage/Release Components
Figure 5 - A) OTM Module (One DoF) Front, B) OTM Module (One DoF)
Back, C) Two OTM modules showing how they could be connected in
series (left) or to a motor using a coupling (right), D) Planetary
gear set used in clutch.
Hydraulic OTM
The OTM architecture can be also realized with fluid based
actuation. One example is practical
hydraulic OTM systems with fast multi-valves currently under
development [6].
Soft hydraulic artificial muscle array with simple sensing
The advantage of hydraulically and pneumatically actuated OTM
systems is that most elements
are commercially available. It is rather common for hydraulic and
pneumatic systems to have a single
actuator engages multiple DoF. For mechanically actuated OTM
systems, the majority of components
used had to be custom made.
A hydraulically actuated Exo-muscle may have several advantages
over pneumatically actuated
Exo-muscles. The system response times are typically much faster
(sound propagates faster in water than
in air), energy losses are much smaller, and for incompressible
fluid forces per unit area may be much
larger allowing for more compact design of system with same peak
force dynamical output [32-25].
Further, The Exo-Musculature directly interfacing with human body
system, consisting mostly
(>60%) of water, may provide mechanically more compatible media
which is often considered as an
advantage [33].
The currently considered system’s actuations are those similar to
conventional PAM ala
McKibben muscle, dating back to 1950s where pressurizing fluid
causes artificial muscle compression
[34-35] and those with just opposite mechanism, such that
pressurizing fluid causes artificial muscle
elongation [36-37]. Later designs are similar to commercially
available expandable garden hose [38]. The
outer non-stretchable layer is in wrinkled state when muscle is not
fully extended and it defines the
maximal elongation of the muscle. Its rationale is to confine
muscle stretching along axial direction. The
inner layer in the form of elastic tube (e.g. latex) containing
fluid returns muscle to its original state when
fluid is not pressurized, Figure 6.
Figure 6 - Expandable garden hose like artificial muscle
Independent of type, i.e. McKibben vs expandable hose like,
hydraulic muscle can be bi-
directionally controlled by controlling the pressure or total
volume of stored fluid.
Another advantage of hydraulically vs. pneumatically actuated
muscle is simplified integrated
sensing. Pneumatically actuated muscles typically require
integrated strain gauges embedded in “skin”
[17] to estimate muscle length. Hydraulically actuated muscles
solves that complexities by simple
resistance measurements of working fluid, in this case water. It
was experimentally established that
resistance of water contained in muscle is fairly linear function
of muscle length and may provide a good
estimate of this variable [25].
Multivalve system
Imagine hydraulic system with large number of fluid powered
actuated degrees freedom (DoF),
Figure 10. Each actuated DoF will necessitate at least two valves
to control pressure. System on the lines
of the OTM concept that has separate energy storage / power
augmentation unit per DoF will require at
least three valves per DoF.
muscles muscles
Figure 7 - Architecture of the fluid powered Exo-Musculature system
with conventional single energy reservoid(A) and with
However, having too many valves in the system may be problematic
due to cost, size, mass, and
energy efficiency.
Here, this problem is addressed by exchanging the large number of
conventional valves with
newly designed multivalve system based on rotary disk with embedded
fast valve, Figure 11.
There exists variety of similar design solution involving selective
rotary disk valves for both
hydraulic [39-40] and pneumatic [41] systems. For better
controllability fast standard electronic valve
embedded in rotary disk is anticipated.
one energy reservoir per DoF on the lines of OTM systems (B).
fast valve
R
R
RR
st
Figure 8 - 1-to-N multivalve: rotary, R, and stationary disks (A),
schematics (B) and cross-sectional view of embedded fa
1-to-N multivalve consists of rotary disk with single opening
embedded with fast valve (<2ms to
completely close or open) and another static disc with many
openings, each corresponding to one DoF.
Only when rotary and static disk openings are aligned and fast
valve is open fluid is allowed to flow
through. Distance between rotary and static disk is much smaller
compared to the diameter (~1-5 mm) of
disk opening, Figure 9.
Figure 9 – Renders of proposed 1-to-N multivalve
The N-to-N multivalve, Figure 13, consists of rotary disk with
single opening embedded with fast
valve. Rotary disk is sandwiched with two static discs with equal
number of openings, each
corresponding to one of DoFs. Again, only when rotary and static
disks openings are aligned and fast
valve is open fluid is allowed to flow through.
…
…
R R R R
Figure 10 - N-to-N multivalve: stationary, rotary, R, and
stationary disks (A), schematics (B) and cross-sectional view
of
Consider regime in which rotary disk is rotating with constant
angular velocity corresponding
to period . If the angle associated with single opening is then
angle associated with
space between nearest neighbor openings for densest configuration
is . This defines the
maximal number of openings as ( .( If opening diameter is D and
if
opening center is located at distance r from the center of rotary
disk then .
For example for it follows that
=
and corresponding to roughly 4 Hz frequency. Hence, each DoF can be
in
