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?