project summary - university of pittsburgh

MEMS 1043 – Senior Design Project Department of Mechanical Engineering and Materials Science Fall 2019 Swanson School of Engineering, University of Pittsburgh

Project Summary

Title

Development of a System to Test Anterior Cruciate Ligament Failure

Sponsor

Dr. Patrick Smolinski 631 Benedum Hall [email protected]

412-624-9788

Team Members and Roles

Coordinator Sydney Leonard [email protected]

Planner Austin Bussard [email protected]

Resource Manager Alexander Houriet [email protected]

Presenter Griffin Monaghan [email protected]

1. Project Background Industry Served

The end goal of this project is to design a mechanical device complete with a fixation system for the

lower leg and an actuator system to impart the forces required to simulate ACL failure. This project has the potential to positively impact many fields of industry including sports medicine, orthopaedic practices, and physical therapy. Specifically, it could allow researchers to find out how factors like age, gender, and history of physical activity affect the predisposition to ACL injury. Additionally, it could allow surgeons to compare ACL reconstruction techniques. With this information, much more could be done to prevent this type of injury.

According to the American Orthopedic Society of Sports Medicine, 150,000 ACL injuries occur annually in the US alone [1]. With recovery times exceeding nine months, these types of injuries can be career altering for athletes. This provides the context for the long-term goal of this project, which is to create a mechanical device capable of creating clinically significant ACL injuries in cadaver specimens. This is a continued project sponsored by Dr. Smolinski, an associate professor for the Swanson School of Engineering. In addition to Dr. Smolinski, we have been working with an orthopedic surgeon based in Cranberry, PA, named Dr. Joshua Szabo. Dr. Szabo is a physician for the US Ski team and treats athletes at the UPMC Lemieux Sports Complex.

Figure 1: A figure showing the anatomy of the knee

Motivation

Once this project is completed, we will have a machine developed that can consistently cause failure

in ACLs similar to how they fail during in vivo jump landings. This is a simulation that has been completed before, but it can be modified to create a unique study. Our sponsor, Dr. Smolinski, is working with Dr. Joshua Szabo to create this testing apparatus as a baseline. Once it is completed, the machine can be further modified to test ACL failure for skiers. Beginning with an already-realized apparatus lends legitimacy to a new study.

Significance and Impact to Industry

Dr. Smolinski specializes his research in orthopedic engineering. This makes him a well-suited

sponsor to work between a surgeon and a team of future engineers to complete a project like this. Better understanding how ACLs fail is the first step to creating better training regimen for athletes, more effective treatment methods for injuries, and better preventive braces.

Completion of our project will positively impact Dr. Szabo, USA Ski Team, and the skiing community as a whole because it has the potential to drastically reduce the number of ski related ACL injuries through the development of a new ski boot.

The long-term impact of our project will impact the USA Ski Team and potentially the ski community as a whole. Our project will initially help Dr. Joshua Szabo to further understand the mechanisms associated with “slip-catch” injuries. Understanding the “slip-catch” will allow Dr. Szabo to study the effects that current ski boots have on ACL related injuries. This information can be used to develop a new generation ski boot to reduce the number of ski related injuries linked to boot design. The new generation boot could then be offered to the ski community.

A device that tears ACLs in a fashion that match in vivo tears will be very useful for development of new surgical repair techniques. It is far safer to test experimental procedures on cadaver legs because if they go poorly, additional damage is not being caused to a patient. Once a new repair technique has been practiced and perfected on cadavers, it can be applied to human patients at a much lower risk.

Project Continuation

This project has been in progress for multiple terms. Full history of the project is not known, but

completed portions at this point include:

● Apparatus framework ● Femur mounting bracket ● Loading platform ● Tibial force mount ● Drop sled

The project is approaching its final stages, our group will be responsible for the tendon loading, but

the project is not anticipated to be finished this term. As the project stands now, it could be used to test ACL failure, but without actuated muscle forces and rotational moments, results would not correlate to real world injuries and other injuries would be more likely to be caused, such as bone shearing.

2. Project Objectives Project Goal

The goal of this project is to simulate muscle loads in quadriceps and hamstring muscles of a cadaver

specimen using pneumatic actuators and a cable pulley system.

Objectives

Objective #1 - Team Development: Learn and share personal skills and backgrounds. Communicate effectively and manage deadlines. Find efficiency in what works best for our group. Share and listen to feedback to ensure best solution. Compromise and resolve problems. Strive for success. Objective #2 - Structural Comprehension: Develop a thorough understanding of the existing base/framework associated with this project. Recognize current limitations and area changes may need to be made pending design/layout finalization. Determine what existing components need fixed and/or modified to accept adjustability during testing. Objective #3 - Project Analysis: Approve appropriate material, size, layout of components. Support all added features with technical knowledge. Develop models of components to aid in prototyping and fabrication. Design components for accuracy and dependability considering the end goal of the project. We will also need to make use of hand calculations and computational analysis (ANSYS) to ensure correct forces are exerted on the system. Objective #4 - Development and Material Selection: A prototype of our system will be constructed and tested to ensure that the requirements and specifications of our project have been met. We will need to select appropriate materials to ensure that the end goals are achieved. We will be making use of the SSOE Swanson Center during this process. Objective #5 - Validation: Since we do not have a cadaver specimen to test with, we will simulate tendon attachment sites by drilling screws into our fake leg and attaching cables to them. We will then provide results to our sponsor in addition to any supporting improvements to the current or future projects.

Specifications and Requirements

1. Design a Pneumatic System to Create Tensile Muscle Forces - There are three separate tendon

attachment points for this scenario, so three separate pneumatic actuators are required. Two smaller actuators mount on the hamstring (rear) side of the leg and one larger actuator mounts on the quadriceps (front) side of the leg. The smaller actuators were already in the device, but they were relocated and then a third actuator was purchased.

2. Design a Pulley System to Direct Tensile Muscle Forces - The muscles forces created by the

actuators are transferred to the leg through the cable-pulley system. We designed a system to match the lines of action of the hamstring and quadriceps muscles as accurately as possible.

3. Adjustability in Pulley Position and Orientation - Our device must be adaptable to a wide range of cadaver specimens, because people come in a wide range of sizes. to accomplish this we placed the pulleys at the base of the femoral mount in grooves to adjust their position. All pulleys are able to rotate about their mounting screw to allow them to self align during loading, creating the best possible lines of action.

Deliverables 1. Mounted Three Actuators (Physical)

-Attached to the existing fame. These actuators simulate the force exerted by the quadriceps and hamstring muscles.

2. Improve Actuator to Cable Attachment (Physical) -Eliminates existing undesired torque (moment) during actuator stroke.

3. Relocated Pulley Lines of Action (Physical) -Position cable and pulley such that the line of action accurately represents hamstring and quadricep muscle loading criteria.

4. Attachment Cable to Bone (Physical) -Place attachment points on test leg

5. Validated Line of Action (Physical) -Light loading to verify degree of freedoms

6. Collected Validation Data (Digital) -ANSYS simulation and hand calculations

7. Created Presentation Materials (Digital) -Summarization of our project (CAD Model, drawings, bill of materials, specifications of equipment (manual), suggestions, etc)

The physical components created for deliverables 1-4 will remain with the device in Dr. Smolinski’s laboratory. Physical copies for deliverables 5-7 are included in our final binder, along with digital versions included on the flash drive in the binder. Our project sponsor, Dr. Smolinki has been closely working with us and has approved our designs to this date.

3. Project Planning Resources

The anticipated list of materials will include:

● Pulleys ● Steel cable ● Pneumatic force actuators ● Assorted fasteners

The anticipated list of software will include:

● Solidworks/AutoCAD ● ANSYS

Design Iterations

We made three preliminary designs for our cable-pulley system. The first was our jump-off point from what the previous group devised; it did not create accurate lines of action. Our second design was made to be easily integrated into the device and created accurate lines of action. The third design was similar to the second, but was more adaptable at the cost of significantly more complex. Design 3 was deemed unnecessary and out of the scope of our load case so we moved forward with design 2.

Support from Project Sponsor Our project sponsor, Dr. Smolinksi, has provided our team with a lockable office/workspace for the duration of this project. The room contains the current assembly as well as other resources our team may utilize. Dr. Smolinski will be a vital asset to our team to aid in answering subject matter questions or concerns.

Support from Swanson Center for Product Innovation

The anticipated support from the Swanson Center for Product Innovation will be minimal. We are in the process of becoming certified so that we can utilize the facility. We anticipate using simple machines/tools to perform tasks such as drilling holes in existing aluminum plates, cutting various metal cables, and potentially cutting sheet metal. All other actions are anticipated to be performed using hand tools.

Support from Subject Matter Experts

Dr. Joshua Szabo (surgeon located in Cranberry, PA) is our top-level subject matter expert. Our team communicated with Dr. Szabo by email. Any top-level anatomical question we had about lines of action and attachments, he was able to answer.

Potential Risks and Anticipated Failures

The rupture is a non-repeatable action per cadaver. Therefore the actuators, pulleys, and cables should be designed to never fail. Such a failure could result in lost experimental data and potential waste of a cadaver ACL if rupture were to occur. A potential workaround to prevent such phenomena may include the design of a secondary system. This system is outside the scope of our project but could be a way to always capture the axial force per drop. The axial force could then in turn be used for analysis.

Milestones

There were 4 main milestones associated with this project. The first milestone was to finalize a design/layout to the existing system. The frame/base provided is the mounting surface we use so understanding what can and cannot be used will be important. Therefore, deciding our system layout was a fundamental milestone to build off of for this project. Next, our main focus was to complete our three main design objectives. Milestones 2 and 3 included the completion of the cable and pulley and control system. At the completion of these milestones, our system will be ready for assembly. Once all components are assembled, our fourth and final milestone was to validate our system using a fake test leg. The validation of the system will ensure the system properly translated loads to the specimen.

Chronological Order of Milestones 1. Design/Layout Evaluation 2. Cable and Pulleys 3. Control System 4. Calibration and Testing

Budget and Bill of Materials

Item Name Description Quantity Vendor Part

Number Price Per

Unit Total Price

Round Body Air Cylinder

Double-Acting, Univsersal Mount, 2" Bore, 12" Stroke Length 1 McMaster-Carr 6498K568 $125.93 $125.93

Steel U-Bolt 3/8"-16 Threads, 2-1/2" ID 4 McMaster-Carr 8880T91 $2.65 $10.60

Mounted Pulley Mounted, for 1/4" wire, 3' OD 1 McMaster-Carr 3099T38 $13.16 $13.16

Pressure Regulator

5-100 PSI outlet range, 1/4" NPT ports 1 McMaster-Carr 9891K43 $63.55 $63.55

Small Coupling Nut 1 3/4" long, 7/16-20 thread 2 McMaster-Carr 90977A190 $5.09 $10.18

Big Coupling Nut 1 3/4" long, 1/2-20 thread 1 McMaster-Carr 90977A034 $5.59 $5.59

Small Eye Bolt 7/16-20 thread 1 3/8" shank 2 McMaster-Carr 3013T961 $9.03 $18.06

Big Eye Bolt 1/2-20 thread 1 3/8" shank 1 McMaster-Carr 3013T344 $4.42 $4.42

Pressure Relief Valve

100 psi set pressure fast release valve, 1/4" NPT Port 4 McMaster-Carr 48435K72 $5.41 $21.64


100 psi set pressure fast release valve 1/8" NPT Port 1 McMaster-Carr 48435K82 $16.61 $16.61

Hose Adapter for Large cylinder

3/8" NPT threading to 1/4" hose 1 McMaster-Carr 5350K33 $2.14 $2.14



Hose Clamps 7/32" to 5/8" worm hose clamp 1 McMaster-Carr 5388K14 $6.26 $6.26

Hex Screws 1/4"-20 threaded, 1.5" length 3/8 hex drive screws 1 McMaster-Carr 90044A123 $13.25 $13.25

Compressor fitting

1/4" NPTF to 1/4" barb adaptor 1 McMaster Carr 5346K42 $15.94 $15.94

Compressor to Regulator connector 1/4" barb T Connector 1 McMaster Carr 91355K47 $6.55 $6.55

The total estimated cost is $335.89. Utilizing the existing components purchased by past groups our budget was nearly half of the anticipated amount. Excess hardware exists in our workspace which also can be utilized in our project. Our intent is to use all resources currently available and any missing components will be purchased.

4. Project Execution Analysis & Design

Our project has been divided into three objectives of design which include the actuators, cable and pulley layout, and an overall control system. The most significant portion of our project will be the contribution of the control system. The control system will deliver the air to the each actuator with a desired force to the attached tendons. The input and output of the system will be transferred from pneumatic actuators to the tendons via some cables and pulleys. The forces imposed on the tendons will be controlled via the two independent air regulators. The analysis of this system will rely on techniques learned in Mechanical Measurements (MEMS 1041). In addition to the control system, analyzing the pulleys and cables will contribute to a dependable design. Utilizing knowledge gained in Mechanical Design II (MEMS 1029) and prior supporting classes, we will decide on the following: Material suitability, imposed torque at pulley locations, reaction and moments generated due to applied loads, and fatigue considerations. We utilized ANSYS to simulation static components to support the project objectives accordingly.

Verification

The project endpoints and milestones are organized on a gantt chart. The gantt chart was utilized as a way to track progress and most importantly check off items in a sequential order. The order of events are in a linear manner such that progress needs to be made before the next task. Ultimately this progression will lead up towards a final assembly to test the control system. The final verification will be testing the system for accuracy by conducting numerous tests. This final verification will be achieved on time by using the gantt chart process as described above.

5. Project Organization Team Meetings

Meeting every Tuesday at 6:30pm in the Smolinski Reasearch Lab on the 6th floor of Benedum, with Fridays and Sundays available for spill-over. On Tuesdays we would go over what we have accomplished in the past week and discuss any questions and concerns anyone has. We then outlined our goals for the week and divide up tasks. Finally prepared for the next day’s meeting with Dr. Smolinski, such as preparing a project update and a list of questions for him.

Work Space

We worked primarily in the Smolinski Research Lab, the basement computing lab in Benedum, and occasionally G26.

Sponsor Meetings

We met with our sponsor on Wednesdays at 1pm in person, in the lab.

Sharing, Distribution and Archival of Project Data

We used Google Drive in between group members to collaborate. We also used Teamwork to plan out milestones and share our progress with Dr. Schmidt. All material was also uploaded to Teamwork for review by Dr. Schmidt. The Google Drive folder has been integrated to our Teamwork page.

Return of Equipment

All equipment, documents, or materials associated with our project will be returned to Dr. Smolinksi

(including room key). Other components such as gauges and instruments borrowed, will be returned to the according lab department. The completed project will be delivered to Dr. Smolinski (anticipated location will be the workspace given to our team for the duration of this project). Austin Bussard and Sydney Leonard will ensure return/delivery of these components.

6. Project Outcomes

Community

With the development of a device to simulate ACL failure we hope to gain a more comprehensive view of the knee joint. Knowing exactly what it takes to tear an ACL could aid the physical training community by aiding in the development of a muscular training regime designed to prevent these injuries from happening in the first place. Our project could also lead to improved rehabilitation techniques thereby supporting the sports medicine community.

Educational

We improved our understanding of the planning and organization required to complete a complex

project like this one. Our group also refined our understanding of biomechanics, especially that of the knee joint. Our main objective in this project is the utilization of pneumatic actuators to simulate muscle loading. The completion of this objective reinforced our understanding of pneumatic systems and the control systems needed to use them.


Preliminary Designs

Title


Sponsor


412-624-9788






Design Requirements We are designing a cable and pulley system to integrate into the existing aluminum frame for our device. The cables will be attached to the hamstring and quadriceps tendons to account for muscle loading during an ACL injury. One of the requirements being satisfied with this system is the alignment of tensile forces with the lines of action of the tendons. The pulley system will also allow us to keep the proper tension in the cable during testing.

Design 1 This design is simple and utilizes the design from a previous group. The cable will run directly from the tendons to the outer pulleys. This will cause inaccurate lines of action, but has less failure points. It is also the easiest and cheapest to implement.

Design 2 This design would do a far better job at aligning the cables with in-vivo tendon lines of action. Eye bolts would be placed in the femur and at the base of the femoral mount to direct the cables. This design requires minimal adaptation of the current apparatus but does not maximize adjustability.

Design 3 This design builds on design 2, where the lines of action correctly reflect the direction of muscle forces in a real knee. A circular rail system will be fabricated to attach to the base of the mount. This rail will have pulleys mounted that can rotate to the optimal position. Once position is determined, the pulleys would be locked into place.

Design Comparison

Design 1 Design 2 Design 3

Strengths ● Simplest Design ● Less failure points

● Creates more accurate lines of action

● Easy fabrication and testing

● Creates the most accurate lines of action

● Full variability

Weaknesses ● Lines of action do not accurately represent muscle forces

● Could potentially cause additional injuries in testing

● Limited variability

● Requires mounting eye bolts to femur

● Most difficult to fabricate

● Position slip potential

Validation These designs will be validated with static analyses to determine accurate forces. After doing further research into the anatomy of the knee and consulting the orthopedic surgeon also working on this project, Dr. Josh Szabo, we will have a better idea about the lines of action of the quadricep and hamstring tendons. We then plan to run semi-full scale testing (we will be loading an artificial leg instead of a cadaver specimen) to see which design will do an adequate job. Once validated, the chosen design will be tested in the actual apparatus with a cadaver specimen.


Peer Design Review

Title


Sponsor


412-624-9788






Review Team Group 7: Michael Ewan, Derek Sitt, and Stephen Selvidge Design 1 This is our least practical design, and our peer review group agreed with us. While it is the simplest and would get us to testing the fastest, the lack of accuracy in tendon line of action would be difficult to compensate for. While we could compensate for angled forces by increasing actuator force, this would increase the stress in our pulleys and cables which could lead to failure.

Design 2 Our initial plan for this design was to place eye bolts at the base of the femoral mount. It was brought to our attention that using an eyebolt for such a drastic change in angle is not ideal. This would cause a large friction force that would wear on the cable as it is directed to the rest of the system. In the long term, this friction force could cause a failure in the cable as it wears away over repeated testing. A better choice in this design would be to place pulleys at the base of the mount. A pulley would have far less friction and would be better suited for the large angle change anticipated. This design could also be improved by machining channels into the base plate. These channels would be normal to the mount and could provide additional variability to our pulley placement.

Design 3 Group 7 agreed with our team that design 3 is the best theoretical design, but also agrees that it is not entirely necessary. This design will increase our cost and take much more time to prototype and test than the other designs. Therefore, this will be used as a backup to design 2, should design 2 yield inaccurate results. Another option with this design would be to place multiple rings on the mount. A ring at the top of the mount would take the place of eye bolts in the femur, saving prep time, and providing and easy alignment method.


Adopted Design

Title


Sponsor


412-624-9788






Pugh Chart Criteria Weight Baseline Design 1 Design 2 Design 3

Correct Lines of action 5 0 - + +

Position Adaptability 3 0 - 0 +

Cost 1 0 0 0 -

Ease of Implementation 2 0 + - -

Complexity 1 0 ++ + -

Sustainability (e.g. maintenance needs) 1 0 + + -

Total 0 -3 5 3

Table 1: Design Option Pugh Chart

After creating a Pugh chart we could clearly see that the best design to select was Design 2, which involved mounting the first set of pulleys at the base of the femoral mount. Although implementing this design would not be the easiest, it provides the best tradeoff between simplicity and cost, while giving us adequate anatomical accuracy. Detailed Description

Figure 1: Initial cross-section sketch of system

Figure 2: Final CAD Model of system

Our entire system setup is modeled and shown in Figure 3, with a zoomed in shot of the cable system

in figure 2. Pulleys will be mounted at the base of the cylinder. A single pulley in a fixed location will be mounted on the anterior side of the leg to attach to the quadriceps tendon. Two pulleys will be mounted in grooves on the posterior side of the leg to attach to seperate groups of the hamstring tendons. These grooves allow placement of the pulley to be adjusted based on the size and placement of the tendons, which vary from knee to knee. From these pulleys the cable will run outward to a pulley mounted at the same height on the outer frame. From here the cable will be directed upwards where it will attach to the pneumatic actuator. Change Notice:

Design 2 was modified from its state in the peer review. Initially using 3 pulleys and 2 eye bolts per cable, it has been simplified to just two pulleys, as seen in figure 2. The eye bolts and additional pulley were deemed unnecessary after further analysis; the parts added more complexity to the system with no increase in performance.

Bill of Materials

Item Name Description Quantity Vendor Part Number Price Per Unit Total Price

Round Body Air Cylinder

Double-Acting, Univsersal Mount, 2" Bore, 12" Stroke Length 1 McMaster-Carr 6498K568 $125.93 $125.93

Steel U-Bolt 3/8"-16 Threads, 2-1/2" ID 4 McMaster-Carr 8880T91 $2.65 $10.60

Mounted Pulley

Mounted, for 1/4" wire, 3' OD 1 McMaster-Carr 3099T38 $13.16 $13.16

Pressure Regulator

5-100 PSI outlet range, 1/4" NPT ports 1 McMaster-Carr 9891K43 $63.55 $63.55

Small Coupling Nut 1 3/4" long, 7/16-20 thread 2 McMaster-Carr 90977A190 $5.09 $10.18

Big Coupling Nut 1 3/4" long, 1/2-20 thread 1 McMaster-Carr 90977A034 $5.59 $5.59

Small Eye Bolt

7/16-20 thread 1 3/8" shank 2 McMaster-Carr 3013T961 $9.03 $18.06

Big Eye Bolt 1/2-20 thread 1 3/8" shank 1 McMaster-Carr 3013T344 $4.42 $4.42


100 psi set pressure fast release valve, 1/4" NPT Port 4 McMaster-Carr 48435K72 $5.41 $21.64


100 psi set pressure fast release valve 1/8" NPT Port 1 McMaster-Carr 48435K82 $16.61 $16.61





Hose Clamps 7/32" to 5/8" worm hose clamp 1 McMaster-Carr 5388K14 $6.26 $6.26

Hex Screws

1/4"-20 threaded, 1.5" length 3/8 hex drive screws 1 McMaster-Carr 90044A123 $13.25 $13.25

Compressor fitting

1/4" NPTF to 1/4" barb adaptor 1 McMaster Carr 5346K42 $15.94 $15.94

Compressor to Regulator connector 1/4" barb T Connector 1 McMaster Carr 91355K47 $6.55 $6.55

Total Cost: $335.89

Anticipated Analysis Procedure Our chosen design (# 2) more accurately simulates the true lines of action of the quadriceps and

hamstring muscles. Considering our steel cable is rated for 340 lbf (from vendor) and given that the maximum tensile load is 101.2 lbf (450 N) in the quadriceps tendon. We do not expect failure in the cable due to tensile failure.

OS .36F = F max

F expected= 340 lbf

101.2 lbf = 3

Another potential failure point lies in the pulleys that were purchased by a previous senior design

group. The worst loading scenario will be at the first pulley attached to the quadricep tendon. The pulley will be providing a 90 degree direction change.

Figure 1: Free body diagram of the first pulley.

Based on the free body diagram, we can determine the force on the pulley (Fpulley) required to

maintain the tensions (T1 and T2):

50 NT 1 = T 2 = T quadricep = 4

36.4 N 143.1 lbf F pulley = √T 21 + T 2

2 = √2 * T 2quadricep = 6 =

Our pulleys are rated for 200 lbf which gives us a factor of safety of 1.4. A larger load capacity

would yield a more conservative design. After initial testing we will reassess the pulley choice if needed.

The last potential failure mode in our cable pulley system is failure in the eye bolts. The eye bolts are used to attach the cable to the pneumatic cylinder. The weakest eyebolt used is rated to a vertical load of 1,800 lbs, making it more than strong enough for this application.

Sponsor Approval Our sponsor, Dr. Smolinksi, has approved of this design concept.

Figure 3: Full CAD model

Figure 2

Figure 3

Figure 4


Supporting Analysis

Title


Sponsor


412-624-9788






Analysis #1 One potential failure mode would be a failure in the aluminum bar supporting the pneumatic cylinders. We are dealing with large forces from the pneumatic cylinders (225 N and 450 N). There are two beams that support the cylinders. One beam, responsible for imparting two 225 N loads to the hamstring tendons (beam #1), and another beam responsible for imparting one 450 N load to the quadriceps tendon (beam #2). In order to determine which beam is more likely to fail, a static analysis will be done on both beams. Both beams will be modeled as simply supported with point loads along them. Beam #1: This beam has two 225 N point loads along the length of the beam, where L = 24 inches. Figure 1 shows the schematic of beam #1.

Figure 1: Diagram of beam #1.

Calculating the force balance in the Y-direction and by symmetry we can determine the magnitudes of the two reaction forces, R1 and R2.

25 NR1 = R2 = 2 (1)

Now that all forces values have been found, we can create the shear and moment diagrams (shown in figures 2 and 3).

Figure 2: Shear diagram for beam #1.

Figure 3: Moment diagram for beam #1.

From figure 3 we see that the maximum bending moment in the beam is 225*L/3 [N*m]. Beam #2: Beam #2 is the same as beam #1 aside from the loading condition. Instead of two point forces along its length, there is one point force in the middle. This can be seen in figure 4.

Figure 4: Schematic of beam #2.

Calculating the force balance in the Y-direction gives us the same values for R1 and R2. The shear and moment diagrams can be seen in figures 5 and 6.

Figure 5: Shear diagram for beam #2.

Figure 6: Moment diagram for beam #2.

Based on the diagrams, the maximum bending moment in beam #2 is 225*L/2 [N*m]. From this, we determined that there is a greater maximum bending moment in beam #2. As a result we will carry out the remainder of our analysis on beam #2. The L-beam being used has a geometry we are not used to dealing with in hand calculations. Because of the cross section and the way it is being loaded we will see an additional imparted moment on the flange that is being loaded. In addition, holes that have been drilled will give us stress concentrations. Based on the calculations done above we can run a static structural simulation using ANSYS to determine the expected deflection and maximum stress. A CAD model of the beam was created and loaded into workbench and can be seen in figure 7. The four drill holes on the edges are modeled as fixed supports. The hole in the center of the beam is where the pneumatic cylinder will be placed. A 450 N load is applied to the area where the cylinder comes in contact with the beam. This simulation is valid because the stress and strain can be represented by the linear relationship with Young’s modulus acting as the constant of proportionality. It also does not violate the fundamental principle of finite element analyses because deformation values do not vary inside of a single mesh.

Figure 7: CAD file representing beam #2.

We chose to solve for total deflection and Von-Mises stress. Von-Mises stress was chosen because there is both bending and torsion and the distortional energy theory accounts for this. Figures 8 and 9 show the deformation and equivalent stress respectively.

Figure 8: Plot of total deformation.

Figure 9: Plot of equivalent stress.

The average deformation was found to be 0.78 mm which will not lead to any negative effects. The maximum equivalent stress (not including the “fake” stress at the fixed supports and force application area) was found to be on the order of 12 MPa. The yield strength of aluminum alloy is 28 MPa leading to a factor of safety of 2.33 at the absolute worst. While beam #2 had a larger bending moment, beam #1 has a hole that was drilled in the center by a previous group. A static structural simulation using ANSYS was performed to quantify the effect of the stress concentration due to the existing hole to determine the expected deflection and maximum stress. A CAD model of beam #1 was generated to be used for the simulation. The four drill holes on the edges are modeled as fixed supports. The hole located in the center of the beam is the existing hole drilled by a previous group (no load applied at that location). In addition to the center hole, the two holes located on each side of the existing hole is where the pneumatic cylinder will be placed. A 225 N load is applied to the area where each cylinder comes in contact with the beam. This simulation is valid because the stress and strain can be represented by the linear relationship with Young’s modulus action as the constant of proportionality. Additionally, it does not violate the fundamental principle of finite element analysis because deformation values vary only linearly throughout each element.

Figure 10: Plot of total deformation

Figure 9: Plot of equivalent stress.

The average deformation was found to be 0.897 mm which will not lead to significant failure. The maximum equivalent stress (not including the “fake” stress at the fixed supports and force application area) was found to be nearly 12 MPa. The yield strength of the aluminum alloy is 28 MPa leading to a factor of safety of 2.33 under worst loading conditions.

Analysis #2 Our design involves a cable pulley system that directs forces from pneumatic cylinders to attached tendon locations to accurately simulate the true lines of action of muscle. Considering our steel cable is rated for 340 lbf (from vendor) and given that the maximum tensile load is 101.2 lbf (450 N) in the quadriceps tendon. We do not expect failure in the cable due to tensile failure.

OS .36F = F maxF expected

= 340 lbf101.2 lbf = 3

Another potential failure point lies in the pulleys that were purchased by a previous senior design group. The worst loading scenario will be at the first pulley attached to the quadricep tendon. The pulley will be providing a 90 degree direction change.

Figure 1: Free body diagram of the first pulley.

Based on the free body diagram, we can determine the force on the pulley (Fpulley) required to maintain the tensions (T1 and T2):

50 NT 1 = T 2 = T quadricep = 4

36.4 N 143.1 lbf F pulley = √T 21 + T 2

2 = √2 * T 2quadricep = 6 =

Our pulleys are rated for 200 lbf which gives us a factor of safety of 1.4. If a larger design force is required with future testing, a pulley re-design may be necessary to achieve a more conservative approach.

Analysis #3 Our design does not include the initially anticipated eye bolts however we have included a simple analysis verifying the eyebolts could be used. The eye bolts are being used to direct the line of action from the tendon to the first pulley. After researching quadricep and hamstring tendon line of action we anticipate a maximum angle change of 10 degrees. With this angle change, the anticipated horizontal load is 18 lbf. We do not predict failure in the eye bolts due to the small magnitude of the load.

Figure 2: Diagram showing the eye bolt loading case.

Analysis #4 The air pressure required in each actuator depends on the hamstring and quadricep loading criteria. For our design, a 450N (quadricep) force and two 225 N (hamstring) forces are desired to simulate the muscle loading specification. Our design requires three actuators: two 1-1/4” bore and one 2” bore. The supplied actuators from BIMBA have corresponding power factors to calculate the required input air pressure for a specific output force. Each actuator has an extend and retract power factor, but our calculations are based on retraction. The 1-1/4 ” bore actuator has a power factor of 1.08 while the 2” bore actuator has a power factor of 2.83. The expression to calculate the input air pressure is:

nput P ressure [psi] Output F orce [lbs] / P ower F actorI =

Converting our initial specifications of 225 N and 450 N force, each force required is 50.58 lbs and 101.2 lbs respectively. Applying the above equation,

= 46.83 psiamstring Input P ressure [psi] 50.58 lbs / 1.08 H =

= 35.76 psiuadricep Input P ressure [psi] 101.2 lbs / 2.83 Q =

The calculated air pressure for each actuator is subject to 15% - 20% reduction in output force due to friction and/or mechanical inefficiencies.


Archive and Cleanup Plan

Title


Sponsor


412-624-9788






Archive Plan All relevant assignments, mini presentations, research materials, and any other miscellaneous documents have all been uploaded to our group’s google drive folder. We plan to include our group member’s contact information so that future groups can reach out to us this way, we can grant them access to the folder. All assignments, presentations, research, drawings, CAD models, simulations, etc. will also be printed out and given to our sponsor to be distributed to the next group. In this physical package, we will include a flash drive with digital copies of everything as well. Providing multiple forms of the same information guarantees that at least one will survive to reach the next group. One issue we ran into was not being sure of the part dimensions of parts ordered by previous groups and having difficulty determining things like thread size, pipe size, etc. All products we have purchased will be documented in a bill of materials that will also be included in the google drive. We will also include all invoices so that future groups can see exactly what we purchased and where we purchased it from complete with part numbers and all other relevant information. If more of a certain part needs to be ordered, it can be guaranteed to be correct and there will not be a trial and error with guessing part sizes. We will provide this information as a paper copy and will scan them in order to provide this information digitally as well. Lastly, because we constructed the device in the Smolinski research laboratory, the device will remain there as well as any additional remaining parts until the next group comes to work on it. We will provide an inventory list of the left over parts so that the future group can make use of the excess parts. Clean Up All of our work has been conducted in the Smolinski research laboratory. The device and all parts used will remain in this room for use by future groups. Any messes we have made will be cleaned so that the condition of the room is exactly how we found it. We will organize and sort the leftover parts we have remaining and provide a detailed inventory for the future group. Our usage of SCPI was very minimal, dropping in for a few holes drilled as-needed. We did not require any long-term space there so we do not need to work out any clean up plan.


Design Validation

Title


Sponsor


412-624-9788






Requirements: 1. Design a pneumatic system to create tensile muscle forces

Our group has successfully designed a system to simulate tensile muscle forces byselecting three actuators, appropriately sized to handle the forces that need to be applied to the muscles. We purchased a larger cylinder for the quadricep muscle and kept the two smaller cylinders that a previous group had purchased for each of the hamstrings since the loading on the quad is larger than that of each of the hamstrings. We securely mounted these to the existing frame. We also purchased another regulator and a pressure relief valve so that we could easily control the air flow from the air compressor to the cylinder and ensure proper tensioning so that they are indicative of human muscle loading.

2. Design a pulley system to direct tensile muscle forces Our team looked at three options to find the best way to properly align the muscle forces with the tendon lines of action. We narrowed down the design of a pulley system to a system that moved the location of the pulleys closer to the base of the femoral mount to help guide the cables to the correct lines of action. This was the best design due to factors such as cost, simplicity, sustainability, and ensuring that there were proper lines of action.

3. Adjustability in pulley position and orientation The design of our system made sure that the system was adjustable for all specimens, meaning that cadaver legs of various sizes could fit. We accomplished this by allowing the angle of the femoral mount to be adjustable to account for different specimen sizes. We also cut out a notch in the plate below the femoral mount so that the distance from femoral fixture to the first pulley in the system could be adjustable to account for different lines of action for various sized specimens. Additionally, our first pulleys have rotational freedom so that if we decide to alter the position of the pulley, the cable will still be able to run to the second pulley properly.

We were successful in achieving all of these requirements and specifications and our project sponsor, Dr. Smolinski has also approved of these and our validation methods.

project summary - university of pittsburgh

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