active ankle-foot orthotic
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Active Ankle-Foot OrthoticAir Muscle Tethered
Team P13001Nathan Couper, ME
Bob Day, MEPatrick Renahan, IEPatrick Streeter, ME
This material is based upon work supported by the National Science Foundation under Award No. BES-0527358. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author and do not necessarily reflect the views of the National Science Foundation.
Agenda• Assumptions• Customer Needs• Engineering Specifications• Test Plan• Mechanical Analysis
– Proximal Attachment• Static Analysis• Fatigue Analysis
– Distal Attachment• Static Analysis• Fatigue Analysis
• Air Muscle Testing– Transient Flow– Muscle Contractions
• Risk Assessment• Proposed Schedule• Questions and Criticism
Assumptions and Project Scope• Patient maintains zero muscle control over dorsi-flexion, plantar-flexion,
and toe extension • This product is designed to be used on a treadmill in a clinical setting; but
can be incorporated into an aquatic setting– Tethered System
• The elastomer can be adjusted on a patient basis so that when the patient’s full weight is applied on the AFO, the foot rests at angle slightly above 90 degrees with respect to the patient’s lower limb
• Designed patient has the ability to use a dorsi-flex assist AFO without receiving tone-lock spasms
• For calculations:– Anthropometric Data is from the ANSUR (military) Database
• Based on the 50th percentile man– 2D system– no resistive forces/friction associated with the joints– a normal gait cycle time of 1.2 to 1.5 steps per second is assumed– Isotropic, Elastic Materials
Customer NeedsObjective Number
Customer Objective Description Comment/Status
S1follow safety guidelinesand standards
S3 energy stored safelyAir source designed for specified pressure
S4 no sharp protrusionsAttachments designed to be flush inside AFO
S5 allergy consciousNo new materials to be in contact with user
FT1support regular gaitcycle
System designed for responsiveness necessary for normal gait
FT2hold foot up when stepping forward
Dorsi-assist AFO design has been proven successful
FT2
range of motion to allow full dorsiflexion and plantar flexion
Tamarac joint allows flexion of joint. Hard stops of AFO prevent over flexion
FT4 resist foot slapDorsi-assist AFO design has been proven
successful
FT5operate smoothly/simulate
normal muscle behavior
Regulation of air muscles will allow for adjustment on patient by patient basis
FT6allow for extended use without
straining leg from weight
CF2 non-invasiveDesigned to not interfere with normal fit
of AFO
CF3 secure foot in orthoticExisting orthotic attachment is
unchanged
CF4 non-abrasiveNo new materials to be in contact with
user
CF6 allow normal cooling of legThis is a challenge with existing orthotics:
vent holes will be drilled into orthotic CF7 allow bending of knee Orthotic will stop below the knee
CF8 allow toes to flex upToe flexion will not be hampered by air
muscle device
ST1ballow natural movement down
stairs and rampsAir muscle system will provide proper
plantar flexion during gait cycle
ST2 adapt to different terrainsTerrain sensing system will be compatible
with air muscle control
ST3fast system response between
sensing and doing
Low computative demands on system. Concern is with actuation speed. Intial testing suggests system has responsiveness required
ST4correctly interprets sensor
informationSensor integration with team 13002 is
pending
ST5 support foot drop over obstaclesDorsi-assist AFO design has been proven
successful
Engineering SpecsEngineeringSpecification
Number
EngineeringSpecificationDescription
Units NominalValue*
Ideal Value**
Method ofValidation Comments
s1 Torque on Foot N-m ≥±1.5Fmuscle =53.10 N Test Force represents requirement for 50th
percentile male
s2 Air muscle fill time Ms <150 <200 Test Based on descending stairs gait analysis
s3 predicts step up yes/no yes x - No terrain sensing
s4 predicts step down yes/no yes x - No terrain sensing
s5 predict flat yes/no yes x - No terrain sensing
s6 predicts ramp up yes/no yes x - No terrain sensing
s7 predicts ramp down yes/no yes x - No terrain sensing
s8 predicts speed of person m/s ±0.1 x - No terrain sensing
s9 measure angle of foot Degrees ±5 x - Not necessary for system operation
*Nominal value represents the initial target value for specifications.**Ideal value represents the adjusted target value for specifications based on research and adjusted objectives.
Engineering Specs
s10allowable range of
motion between foot And shin
degrees 94.5 to137.7
72 to 116with shin as
referenceTest
Equivalent to dorsi assist AFO. Measuredangle between calf of AFO
and bottom of AFO
s11 follow safety standards yes/no - -
s14 fits calf (diameter) mm 292 to433 - Use of custom orthotic
s15 fits foot (length) mm 212 to317 - Use of custom orthotic
s17 force to secureconstraints N < 80 N Test Only air muscle system considered
s18 force to removeconstraints N < 80 N Test Only air muscle system considered
s21 monitoring/display ofenergy level yes/no yes - Pressure gauge on air tank
s22 error status yes/no yes -
Engineering Specs
s23 radius of edges/cornerson AFO mm 0.5mm -
s25 Harm to user (survey) scale - survey user
s26 Noise Level (at ears ofuser) dB 60 Test
s27Moving devices and
electronics use standarddust and water shielding
yes/no yes -
s31a Minimum life untilfailure air muscle steps >18000 test
Calculated for 95% uptime. Assuming 20minute replacement, and 44 contractions/min
during use
s31bMinimum life untilfailure: Attachment
pointssteps 5.5
million >15,000 test 100 steps (50 contractions), twice a weekfor three years
s32 Allowable toeextension/flexion Degrees 0-50 test
Testing Plan – Required TestsEngr.
Spec. # Specification (description)
Unit of Measure
Marginal Value Comments/Status
ES26 Noise level (at ears of user) dB <60 Decibel testing
ES2 Flow rate – time to inflate
Sec <.20 Initial testing indicates good performance
ES1 Torque on Foot
N-m >=1.5 Force of air muscle*moment arm
ES31 Lifetime – Air Muscle
% uptime >95% Time in use versus time replacing air muscle
ES31 Lifetime – AFO Fixtures
Steps >15,000 Use of air muscles in clinic must not affect full life of AFO
ES17,18 Force to secure/remove constraint
N <80 Velcro straps pre-existing, and test force to secure muscle (4)
Clamping force: cable to air muscle
Nm .358 Verify clamping force is sufficient to hold cable
Testing Plan – Required Equipment
Engr.Spec. # Instrumentation or equipment not available (description)
ES31 Polymer to simulate AFO for lifetime analysis
Gait AnalysisStairs: Descending Percent Gait Cycle
Event mean s. d.
Foot Off 15% 3%
Foot Strike 56% 3%
Bovi, et. all
Based on 88 cycles per minute: 0.30 seconds from foot off to foot strike.
Assumed AFO Design
• Designs based around AFO of this structure
• Design is flexible so it will be able to work on many different AFO designs and shapes
• Assumed material =
Proximal Muscle Attachment
Key Components:• Weld Nut
• Exterior threading for nut
• Secures device to AFO
• Screw clamps air inlet and muscle attachment to weld nut
• Nozzle screws into block
• Relatively simple components
• Low Profile• Strong• Removable
Weld Nut
• Uses 5/16” Nut to secure against AFO
• Note external threads not shown
• 316 Stainless Steel• Allows for easy
removal of device
Stress Calculations:• Treated like a cantilever beam• 130 N force (Max force air muscle can apply)• Max Bending Stress: 57.45 Mpa• Shear Stress: 7.49 MPa
Proximal Anchor and Air Inlet• Houses weld nut
and exterior nut• Applies force on
weld nut• Also clamped on
by ¼-20 screw• 1/4-inch air inlet
channel • Threaded hole for
nozzle insertion• 316 Stainless Steel
Proximal Anchor and Air Inlet
Element Type: Solid 10node187 (tetrahedral)Max Stress: 45 MPa
All displacement is about 0 meters
Proximal Anchor and Air Inlet
Nozzle• Proposed Materials:
Delrin or Stainless Steel• Threading
• External Threading not pictured
• Screws into Proximal Anchor to allow air supply to muscle
• Air muscle clamps on to cylinder
• Max Stress: 2.85 Mpa• Yield Stress:
• 63 MPa (Delrin)• 290 Mpa (316)
Fatigue Analysis
316 Stainless Steel Properties:• Endurance Limit (Se): 270 MPa• Ultimate Strength (Sut): 579 MPa
Fatigue Results: (Using an applied force of 53 N rather than 130N)• Weld Nut
• FOS=15.53 • Proximal Anchor
• FOS=20.46• Nozzle (316 Stainless Steel)
• FOS=316.9• Nozzle (Delrin)
• FOS=37.6
Delrin Properties:• Endurance Limit (Se): 32 MPa• Ultimate Strength (Sut): 69 MPa
Distal Muscle Attachment Assembly
Tendon Cable• Use 1.5 mm diameter cable• Will use bicycle brake cable• Braided Stainless Steel cable• Tension can be easily adjusted• Preliminary calculations make
us believe this solution will be more durable than previous air muscle tendon materials
• Maximum stress = 100.2 MPa; yield stress = 290 Mpa
• Factor of Safety = 11.5• Maximum Deformation =
0.233 mm
Distal Muscle Plug
• Presses against Distal Muscle Plug Plate with slot for tendon cable to rest in
• Plugs distal end of air muscle
• No air nozzle needed at the distal end
• Proposed Material = Delrin
Maximum Stress = 8.5 MPaYield Stress = 63 MPa
Distal Muscle Plug Plate
• Presses against Distal Muscle Plug
• Creates friction on tendon cable,
• Allows for tension in tendon cable to be easily adjusted
• Proposed Material = 316 Stainless Steel
• Necessary Screw Clamping Force = 0.358 N-m
Heel Cable Attachment Point• Attaches distal end of
tendon cable to AFO heel protrusion
• Held in place by 10-24 screw at distal end, Heel Cable Attachment Pin at proximal end
• Allows for:• full range of
motion of tendon cable
• ease of cable changeover
• Proposed Material = 316 Stainless Steel
ANSYS Simulation
Fatigue AnalysisAnalyzed with stresses from 53 N force as opposed to 130 N
• This will be more realistic to values seen during normal operation
Ultimate Strength = 579 MPaEndurance Strength = 270 MPa
Factor of Safety = 36.8
Heel Cable Attachment PinProposed Material = 316 Stainless Steel
Air Muscle Construction
• Outer Sleeve• Inner Tube• Clamp• End
Muscle Testing
• Goal of .1 sec inflation time, max of .2 sec, estimated via gait analysis– Function of pressure and flow rate
• 4.45cm contraction required for full range of motion– Function of muscle construction
Transient testing
• Started by calculating the theoretical flow– Realized this is questionably accurate and very
complex• Decided it would be easier and more accurate
to directly measure inflation time• Took video of the muscle inflating and counted
the number of frames it took to move.
Transient testing
• 5 video tests
Muscle Contraction
• The muscle was loaded with 53N and inflated
Transient results
Programming Flow Chart
Input from Terrain Sensing System
Flat terrain Ascending terrain (up stairs/up ramp)
Descending terrain (down stairs/down
ramp)
Relax air muscleRelease air
Ankle angle at foot strike = -44.96 deg
Gait speed info from sensors.
Ankle angle at foot strike = -14.65 deg
Gait speed info from sensors
Test Plan
https://edge.rit.edu/edge/P13001/public/WorkingDocuments/Project%20Management
See Edge:
Updated Risk Assessment
Bill of Materials
Schedule for MSD II
Reference EDGE website for working, detailed project schedule:
Planning and Execution – Project Plans and Schedules – “Schedule of Action Items”http://edge.rit.edu/edge/P13001/public/Planning%20%26%20Execution
Questions?
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