P18351: Sandia Passive Vacuum Detection Team A
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Preliminary Detailed Design

Table of Contents

Team Vision for Preliminary Detailed Design Phase

Team NAMASTe's goal for this phase was to complete all assigned deliverables. This primarily included updating the design to use a deforming diaphragm sheet in order to eliminate complications associated with the previously planned o-ring sealed sliding piston design.

All assigned deliverables were completed and the new design fleshed out. With the new deforming diaphragm sheet design, team NAMASTe was able to develop a more in-depth mathematical model to predict the behavior of the system. This will significantly assist testing efforts in the coming months.

On top of this, a prototype of the new design has been built and further parts have been ordered so that multiple diaphram materials may be tested.

Prototyping, Engineering Analysis, Simulation

A model for the deflection of a diaphragm under a uniform pressure differential was created, defined by Mario Di Giovanni in his book "Flat and Corrugated Diaphragm Design Handbook". The diaphragm is modeled as having non-linear deflection as we are anticipating deflections outside of what are considered to be small deflection, which is defined as deflection less than 30% of the diaphragm thickness.

Mathematically it is modeled as a flat diaphragm with a rigid center. Currently, the rigid center has a very small diameter which attributes negligibly to the stiffness of the diaphragm. This allows us to compare the mathematical model directly with a simple diaphragm in ANSYS, which does not have a rigid center.

A number of static simulations have been run in order to investigate the approximate order of deflection that can be expected due to different parameter values. Going forward, more defined nominal parameters will be chosen, as well as further simulation to include additional environmental conditions, such as launch acceleration, temperature change, and vibration.

A simple prototype has been developed to allow us to test various materials for the diaphragm. This will allow us to validate our current mathematical and simulation models as well as to get a physical sense of how deflection in the diaphragm will occur.

Prototype for Diaphragm Material Testing

Prototype for Diaphragm Material Testing

Feasibility: Prototyping, Analysis, Simulation

The current nominal parameters of the system are given below. These parameters are preliminary and are subject to change based off testing data and further model investigation and simulation.

Nominal Parameter Values

Nominal Parameter Values

A sensitivity analysis of the model was performed in order to investigate what effects any variations in the parameters would have on the overall model. The results were obtained by varying one parameter at a time, while holding all other parameters constant at their nominal values. The model was then solved for the maximum deflection. This was then plotted against the percent change in each parameter, shown below.

Model Sensitivity Plots

Model Sensitivity Plots

It can be seen from the plot that the model for maximum displacement of the diaphragm is most sensitive to changes in the outer radius and thickness of the diaphragm. It will be important to hold tight tolerances for these parameters in order to achieve a predictable system response. Alternatively, these are the parameters that will be the easiest to change if it is determined that a different system response is desired.

A model of the diaphragm was also developed in ANSYS to do an initial validation of the mathematical model. The deflection results of this model corroborated the results that were obtained from the mathematical model.

ANSYS Simulation of Diaphragm

ANSYS Simulation of Diaphragm

With a given deflection, a switch with an appropriate pre-travel distance can be chosen. If no switch with a small enough pre-travel is available, model parameters can be adjusted to achieve a greater deflection. Additionally, different diaphragm methods can be employed, including corrugated diaphragms and bellows. Both of these will increase the linear range of the model, allowing for larger deflections for a given pressure differential, with the tradeoff being more difficult manufacturability and higher cost.

Drawings, Schematics

Conceptual Model Isometric View

Conceptual Model Isometric View

Conceptual Model Sectioned Cut View

Conceptual Model Sectioned Cut View

Conceptual Model Side View

Conceptual Model Side View

Conceptual Model Front View

Conceptual Model Front View

Conceptual Model Front View

Conceptual Model Front View

Conceptual Model Line View Front

Conceptual Model Line View Front

Conceptual Model Line View Side

Conceptual Model Line View Side

Bill of Material (BOM)

Working BOM and Budget

Working BOM and Budget

Test Plans

Preliminary test plans that will be used to validate prototype designs are provided below:

Design and Flowcharts

Design Flowchart

Design Flowchart

Risk Assessment

Due to the elimination of the unpredicatable O-ring based design, overall risk for the upcoming testing phase has been reduced.

Here is a link to the live document.

Risk Assessment View Side

Risk Assessment View Side

Design Review Materials

Relevant Materials for Preliminary Detailed Design Review on Tuesday, November 7, 2017.

Plans for next phase

For the next phase, team NAMASTe will have complete preliminary testing of the current design and begun revising it as necessary.

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