Team Vision for System-Level Design Phase
This Phase requires system level development and analysis of the AABS system as a whole. As such, a fundamental high level understanding of system performance and expectations is necessary for completion of systems design and implementation. Goals of this phase include completion of the following:
- Functional Decomposition
- Systems Architecture
- Morphological Chart and Concept Generation
- Concept Analysis and Pugh Chart Development
- Systems Design Refinement
- Risk Assessment
These phases were completed and presented March 1st, 2018 to debrief both our adviser Harold Paschal and Meggitt Engineers. In addition, four team members visited the Meggitt PLC site in Akron, OH to gather data on the hydraulic rig used by Meggitt to perform brakes qualification tests. As this table-based system will be used by Meggitt throughout the life of the AABS system, sufficient knowledge of this system as well as the existing apparatus AABS will replace is necessary for system level design.
The team's visit to Meggitt's Akron Ohio site stems from the need to evaluate the customers expectations of the AABS system and how it will be integrated into their existing infrastructure. To complete this, the away team completed the following:
- Reviewed the existing test system that our project will replace.
- Met with seven subject matter experts (SMEs) and employees about the project
- Asked a list of over 30 questions we had about their system specifications and the project in general.
After returning, the team shared their knowledge with the other four members of the team. Based on this new knowledge, the complete team refined its original designs. The team immediately eliminated some concepts and added new concepts based on the new knowledge. Then, using a morphology chart, the team created nine initial designs and assessed them using a Pugh chart.
In developing AABS, each system level function is evaluated on its own attributes as well as its relationship to the AABS system as a whole. In this context, all sub-functions of the system combine to generate a pressure-displacement curve, i.e. the overall objective of the system.
The decomposition is necessary to further system development. Each sub-function is analyzed for methods of completion, by which physical characteristics are developed and paired with methods of completion.
The system is decomposed by determining why each function is conducted, and the system level procedures by which each function is completed. In this manner, descending actions represent lower sub-functions that describe how system actions are taken, while ascending the system tree illustrates why lower level functions are included.
Relation to Engineering Requirements
The functional decomposition was developed from a combination of ERs and CRs to discern the discrete functions the AABS system is expected to perform. As such, the 20 ERs as included in Problem Definition have been matched to each function. The three branches of the tree have been separated for clarity.
- Ensure flow of energy, info, material and structural forces as intended.
- Define subsystem functions, envelopes and interfaces.
This schematic serves to illustrate the basic relationships between high level system functions and how information, data, power and hydraulic fluid are parsed through the AABS apparatus. Note the broad nature of each system function. This is intended to provide a guide for concept generation. Following the selection of a solution route, this architecture is refined to detail the more specific nature of the chosen system, and allow for additional work to be added in Phase 3.
Concept Development and Morph Chart
The purpose of concept development is to generate new options or combinations that are intended to meet or exceed benchmark standards.
- Generate multiple concepts to fulfill the function requirements.
- Guarantee that the concepts are able to fill every function.
- Choose the optimal combination of concepts that will be integrate with each other to fulfill the problem definition.
Designs and Flowcharts
Create a high-level view of components necessary to system build and operation.
When reviewing feasibility, we asked three key questions, based on three criteria:
- Can we design and build a device that’s easy for the intended audience to use?
- Can we design a device so that it can be repaired with common components to meet the ten-year criteria?
- Can we design a device that the intended audience can easily configure?
We then used these questions to develop three criteria that we built into our Pugh Chart:
- Configurability/Set up
- Ease of Use
We then developed eight preliminary designs. From these designs, we came up with additional feasibility considerations regarding general mechanical engineering knowledge:
- The springs will deform during usage.
- The combination of multiple components will create compound error. For example, viscoelastic compression plates and springs include the error of the plates and springs.
- Measurements of fluid flow through the damper system will be difficult to calculate because of the expected large fluid flow rate. This will require an advanced flowmeter.
- Calculations to find the required mass to create desired resistance will result in a very high mass. Calculations shown below. *
- Torsion shaft will create resistance but it is very likely to break at the connection of the axel and block.
We refined our concept selection and narrowed down to two designs:
- Rotational Spring Selection System with Digital DAQ
- The same system with Analog DAQ.
Our primary feasibility concern for these two designs is the life cycle of the design, which we will determine using fatigue calculations. Our Team will complete detailed numerical feasibility and fatigue calculations once materials and design are developed. Life cycle is primarily driven by the expected life of moving parts within the AABS system, namely the method of resistance used to resist movement of the piston.
- Verify the chosen concepts are functional as defined by the System Architecture.
- Select the optimal values of sensitive design parameters.
- Evaluate the team's concepts with quantitative information.
Concept Selection and Pugh Chart
Originally, the team split into two breakout groups to perform Pugh Chart analysis and to work more efficiently. However when the two teams convene, we found we interpreted the criteria differently. We realized we had to further define and document the criteria. The two Pugh charts are shown above. These Pugh Charts include all designs generated by the team. The team moved forward with the top two designs from the Pugh Chart analysis, and these were used for the following Pugh charts analysis. Note that, for each Pugh Analysis,
System Analysis Design Criteria
Following the narrowing down of design options, design criteria for analyizing the merit of various AABS designs were developed to determine two systems that provide the most likely venue for system development. As shown below, the criteria developed were used to define the benefits and drawbacks of each design.
|Safety||Likelihood of user being injured during testing|
|Accuracy / Precision - Measurements||Accuracy and Precision of Measurements techniques, components, tech.|
|Accuracy / Precision - Resistance||Accuracy and precision during the application of resistance|
|Simplicity - Design||Simplicity of the prototype as a system (non-analog devices decrease simplicity in this case)|
|Cost||Cost of prototyping|
|Ease of reading measurements||How easy it is to use it - i.e. ease of reading measurements|
|Configurability/Setup||Degree and ease to which pressure-displacement settings can be changed|
|Air bleed time/ease||Time and steps that need to be taken to bleed air out of the hydraulic fluid|
|Durability||How periodically items need to be replaced/repaired|
|Repairability||Parts are commercially available and easy to replace / repair|
|Portability||Ease of transportation considering estimated sizes and weights|
Using these criteria, Pugh charts were used to compare possible designs to one another. To ensure objectivity, the datum used to analyze each design is rotated to allow or sufficient comparison between designs. This repetition provides a more holistic approach, and serves to provide added validation to the selected design's merit for system development.
First, a Pugh Chart utilizing Meggitt's existing system as the datum is used to ensure the chosen designs meet the basic requirement of functioning above the expected performance level of the system being replaced. As such, this chart serves as a method of validating the assumption that all designs selected to proceed to this stage of concept analysis are valid attempts at solving the issue at hand. As can be seen from the chart, all four designs result in an overall positive sum, thus these designs then proceed to the next stage of the Pugh Analysis:
Following comparison to the Meggitt system, the four designs are compared to one another in order to yield the design found to most effectively resolve the customer's problem. This process is achieved through the implementation of two main elements:
- Concept Screening: Identify 6-10 useful system-level selection criteria, and evaluate the system designs objectively. Compare your system concepts to a single datum concept. Run your selection analysis multiple times to uncover saturated scores.
- Concept Improvement: Look for opportunities to combine two or more promising system concepts to create an even better solution. You may need to repeat these steps several times in order to converge on an optimal solution.
The real value in this step of the process is not the comparison matrix you generate to compare your concepts, but the analysis and discussion you do to support your evaluation.
With this in mind, the system that was found to be most viable via this analysis is shown to be the Rotational Spring System with Digital Data Acquisition. This is evident from both comparisons to the datum, and when the system is treated as the datum. In both cases the design is valued above other designs, as denoted from totaled sums of +'s and -'s. A sketch of the system is shown below. This system will be further developed into a complete CAD package with included BOM in Phases 3 and 4.
As similar systems to the AABS are either proprietary to their respective developers or unavailable for public consumption, analogues to the AABS system were difficult to compare to. Thus it is more beneficial to the team to bench-mark on the sub-function level, i.e., evaluating the existing hardware that can be developed or acquired to accomplish necessary system functions as listed in the functional decomposition. In this manner, bench-marking serves to illustrate the means by which various sub-functions will likely be performed.
Detailed FeasibilityWe refined out concept selection and narrowed down to two designs:
- Rotational Spring Selection System with Digital DAQ.
- Rotational Spring Selection System with Analog DAQ.
Our primary feasibility concern for these two designs is the life cycle of the design, which we will determine using fatigue calculations. Our team will complete detailed numerical feasibility and fatigue calculations once materials and designs are further developed.
"DAQ" - Data Aquisition
Preliminary BOMBased on the selection of the system design as shown, a generalized analysis of both bench-marked and necessary system level components (as prescribed by Meggitt) is used to verify such a design will not exceed the team's budget of $10,000 nor surpass ER2, denoting a max weight of 100 lbs. Note that this budget will be used for not only system development, but overall project overhead as well. This overhead cost is estimated to be $2,500, including expenses for travel, machine shop use, and other extraneous expenses necessary to project completion. This analysis will be further developed in Phase 3, whereupon a preliminary bill of materials (BOM) will drive the detailed cost analysis.
|Item||Supplier||Part Number||Quantity||Weight (lbs)||Unit Price||Total Price|
|Spring Discs||McMaster Carr||9712K419/9712K452||5, 12 per pack||1.08/1.73||$12.52||$62.60/$68.55|
|OR Springs||McMaster Carr||96485K481/96485K147||12||101.3/19.59||$37.90/$13.38||$454.80/$160.56|
|Aluminum Stock||McMaster Carr||8975K574||2||11.06||$86.92||$173.84|
|Alumium Round Stock||McMaster Carr||8974K11||1||0.019||$4.65||$4.65|
|Aluminum Round Stock-Piston||McMaster Carr||8974K82||1||1.88||$33.70||$33.70|
|Restrictor (exact part TBD)||Lee Company||VDCA1415890D||1||0.5||quote - $50.00||quote - $50.00|
|Check Valve||Lee Company||CHFA1875501A||1||0.5||quote - $50.00||quote - $50.00|
|Hand Pump||TBD||TBD||1||N/A||$459 - $958||$459 - $959|
|O-Rings||Grainger/Carr||TBD||2 pack of 12||0.25||$6||$12|
|ENER-CAP Dyn. Seal||Green-Tweed||TBD||4||0.5||$6||$24|
|Preformed Packing||ENERPAC/TBD||TBD||2||0.15||quote - $12||$24|
Cost AnalysisAssuming an overhead of $2,500 requires the entirety of system development and build to remain under $7,500. For the sake of risk mitigation, all values as determined through the preliminary cost analysis will receive a factor of safety of 1.25, implemented encapsulate likely increases in cost due to unforeseen factors.
|Alumium Round Stock||$4.65||$4.65|
|Aluminum Round Stock-Piston||$33.70||$33.70|
|Restrictor (exact part TBD)||$50.00||$50.00|
|ENER-CAP Dyn. Seal||$24.00||$24.00|
Upon implementing this factor of safety of 1.25, values for low and high preliminary cost, to the nearest whole dollar, become $1,718 and $2,978 respectively. As such it would be expected such a system will not exceed budgetary requirements.
Weight AnalysisAs Meggitt has identified a max weight of 100 lbs. as a requirement, the system should not surpass this point. Keeping the system both mass and volumetrically efficient are important to satisfying Meggitt's request that the AABS system remain movable and easy to operate.
|Part||Low (lbs)||High (lbs)|
|Alumium Round Stock||0.019||0.019|
|Aluminum Round Stock-Piston||1.88||1.88|
|Restrictor (exact part TBD)||0.5||0.5|
|ENER-CAP Dyn. Seal||0.5||0.5|
As can be seen from the preliminary weight analysis, the driving factor of the system's rise in weight between low and high benchmarks is shown to be the difference in weight between spring discs and linear springs. From the chart the estimated 12 linear springs used in such a design would exceed 100 lbs. alone, therefore eliminating the possibility of using these piece-parts in the piston resistivity sub-assembly. Therefore, the team will be moving forward with spring discs for developing the AABS system's resistance to linear displacement of the piston/actuator.
Risk AssessmentUpdates to the risk assessment were made following the initial Problem Definition Review, the Systems Design Review, the visit to Meggitt, and our preliminary designs. New risks include: not meeting the lifetime requirements, spring wheel doesn't "lock" in place, inability to get light weight springs with the desired characteristics, simulator breaking apart due to high pressures, and the size of the simulator being too large. We also changed the risk item of "explosion of fluid due to pressure" to have a likelihood of zero. During our discussions with Meggitt, we learned that the fluid we'll use will be non-compressable and therefore won't cause more than a small spurt in the case of a leak, unlike a gas which can explode.
We also added a new risk category, Miscellaneous, to hold risks that don't properly fall under the other categories but are important to keep.
Design Review Materials
Design Review: The Systems Design Review presentation as presented 03/01/2018.
Plans for next phase
- Our Phase 3 Individual Team Plans