passive vacuum detection switchedge.rit.edu/edge/p18351/public/final documents/p18351_paper.pdf ·...

Multidisciplinary Senior Design Conference

Kate Gleason College of Engineering

Rochester Institute of Technology

Rochester, New York 14623

Copyright © 2018 Rochester Institute of Technology

Project Number: P18351

PASSIVE VACUUM DETECTION SWITCH

David Pawlush

Mechanical Engineering Brady Hrabovsky

Mechanical Engineering

Aaron Jamison

Mechanical Engineer Kyle Bergeron

Mechanical Engineering

ABSTRACT

Team NAMASTe was tasked by Sandia National Laboratories to develop a completely mechanical switch to

sense the absolute pressure between 10-1 [mm Hg] and 10-6 [mm Hg]. The switch will be used to sense a near-space

environment, then close a circuit to the given customer requirements. After evaluating a handful of design

considerations, the chosen design uses a pliable diaphragm deflection by exploiting the pressure difference between

atmosphere and vacuum to activate a binary switch. The design was subjected to extensive vibrational testing, bell-

jar vacuum testing and computer simulated g-force testing to verify system success. The system had confirmed

activation at 1 * 10-4 [mm Hg], survived 4 G-RMS vibrational test for 20 – 2000 [Hz] on all three axes, and survived

a simulated g-force loading. Overall, the design is a success and meets the customer performance requirements.

Proceedings of the Multidisciplinary Senior Design Conference

NAMASTe - Project P18351

NOMENCLATURE

Figure 1: Exploded View of Final Design

Item

Number Nomenclature Description

5 C-clamp Small brackets to mount cylindrical assembly to the mounting bracket.

n/a Customer Sandia National Laboratories

4 Cylindrical assembly Central assembly, housing all critical systems

3 Diaphragm Circular cut nitrile.

6 Mounting Bracket Bracket to integrate device with rocket

1 Open back end End of device where switch is mounted. Open to atmosphere.

n/a Rocket The rocket/missile that the switch will be mounted to

n/a Rocket deck A flat surface in the rocket that the assembly will be mounted to.

4 Sealed end End of device that is sealed.

2 Snap action switch Small COTS switch used to close electrical channel.

n/a Team P18351/NAMASTe

BACKGROUND

The project, per the customer input, will be mounted to a missile deck and used to close a circuit that tells the

navigation systems to initiate. After preliminary talks with Sandia and initial research, it was determined that there is

currently nothing on the market that does not do what they require. Most switches in the market have a high

dependency on electronics in order to achieve the intended purpose. More mechanically driven switches either require

input derived from an electronic source, or an input from a user. For example, while the mechanism for a switch in

the key on a keyboard is mechanical, it requires the input of the human finger in order to operate. Likewise, a relay

switch may snap shut, but requires the input of a voltage to drive the system.

“Sandia National Laboratories has a need for a new type of mechanical switch for space applications. This

device is required to ensure the safety of our new systems being delivered into space. In this context, a mechanical

switch that physically actuates under vacuum of space is required.” [1]

Proceedings of the Multi-Disciplinary Senior Design Conference


Customer Requirements:

Table 1: Customer Requirements

Req.

#: Description: Type:

1 Survive through a maximum acceleration of 20-27 g’s Quantitative

2 Actuate before an absolute pressure of 10-6 [mm Hg] but never actuate before 10-1 [mm Hg] Quantitative

3 Resistance less than 0.2 [Ω] when actuated Quantitative

4 Switch can uphold current of maximum of 2 [A] Quantitative

5 Switch must be able to handle up to 30 [V-DC] Quantitative

6 Fit within wedge of 6.8 [in] radius at a 36 [degree] sweep with a height of 6 [in] Quantitative

7 Weight of system is below 4 [kg] Quantitative

8 Withstand operating temperature of -70 to 30 [°C] Qualitative

9 Must withstand 20 – 2000 [Hz] of vibration Quantitative

10 Not exceed a budget of $1000 Qualitative

11 Must be passive (driven by mechanical system) Qualitative

12 Lock after the switch activates Qualitative

13 System must be resettable for testing Qualitative

14 System must have a bolt pattern to attach to missile Qualitative

15 System must be protected against environmental exposures Qualitative

METHODOLOGY”

Assumptions:

1. Environmental Exposure is negligible.

Justification: After discussing with the customer, the temperature does not need to be

considered as a critical constraint in the design process. Because the application is a missile,

the exposure to the cold temperatures will be so rapid that it will not make a large impact on

the system. Additionally, humidity and debris were neglected in the design because their effect

was rendered negligible compared to other major factors (e.g. vibration, g-force).

Concepts:

1. Horizontal Driving, O-ring Design:

Description: This design utilized the pressure differential between Earth’s sea level and space

to drive a plunger horizontally into a switch. After rough preliminary calculations, the force

expected to drive the system would be large, and so an O-ring was implemented to resist the

motion. Additionally, a telescoping neck was designed in order to help ease into the switch to

reduce risk of damaging it. The design concept is pictured below in Figure 2:

Figure 2: Horizontal Driving, O-ring Design Concept

However, after research into the complexity of o-ring mechanics, along with discussions with

the customer, this design was abandoned. It was thought that accurate repeatability would be

difficult to achieve with the proposed base concepts.



2. Integrated Diaphragm Design:

Description: This design again utilizes the pressure differential like the previous design, but

the driving mechanism is a small diaphragm sandwiched between two enclosures. It was found

that characterizing a diaphragm deflection accurately would be more feasible than

characterizing o-ring mechanics. Ultimately, the pressure differential will cause deflection in

the diaphragm to activate a snap action switch. One of the preliminary design iterations for this

concept can be seen in Figure 3:

Figure 3: Integrated Diaphragm Preliminary Design Concept

This was the chosen design to pursue. Analysis and testing was further executed to better

characterize the deflection distance expectation and system overall size.

Analysis and Evaluation:

Diaphragm deflection theory:

Diaphragm deflection theory was investigated to determine the parameters that would drive the final

physical design of the system, such as diaphragm size and expected total deflection within the activation

range. From Di Giovanni’s Flat and Corrugated Diaphragm Design Handbook [2], the governing equation

for a diaphragm subjected to large deflections is given as:

𝑃 = 𝐸ℎ3

𝐴𝑝𝑎4 ∗ 𝑦 + 𝐵𝑝𝐸ℎ

𝑎4 ∗ 𝑦3 Eq. (1)

Where P is the differential pressure acting on the diaphragm area, E, h, and a are the elastic modulus,

thickness, and outer radius of the diaphragm, respectively, and y is the deflection of the diaphragm at its

center point. AP and BP. are constants dependent on diaphragm geometry and material properties and are

defined in Appendix 1.

A sensitivity study was performed to determine which system parameters would have the most effect

on final deflection of the diaphragm. Given a nominal diaphragm design, each parameter was varied, one at

a time, and the difference in final deflection from the nominal operating point was recorded.

Figure 4: Percent Change in Final Diaphragm Deflection for a Percent Change in Parameter Value

Examination of Figure 4 shows the outer radius and the thickness of the diaphragm contribute the

most to differences in final deflection for a given pressure range. During the final design these parameters

will need to be held to the tightest tolerances to ensure that actual deflection is as close to the theoretical

deflection as possible.



Diaphragm Deflection Feasibility Tests:

When characterizing the behavior of the diaphragm, it was found early in the deformation, the

motion can be described as fairly linear. However, after a pressure that can best be assimilated to the yielding

point on a stress curve, the motion changes to an exponential curve.

Figure 5: Feasibility and Deflection Characterization Tests

Using a vacuum chamber, the set up in Figure 5 was used to empirically characterize the deflection.

A sample of data from these tests can be found in Appendix 2. Different materials were tested in order to

find which combination of material properties and material thickness returned consistent deflection in the

elastic range.

Rough Deflection Distance Tests Using Pressure:

After testing in a large, vacuum chamber, it was determined that the testing method occurs too

slowly in comparison to what the system will see when in use. The prototype was adapted to receive pressure.

The resulting effect on the diaphragm would be theoretically identical, however the tests would have a shorter

run time. Through the high number of tests run, the team was able to roughly characterize the chosen material

behavior and verify the repeatability of the design.

Figure 6: Test set up for hand pump deflection tests

Due to elementary nature of the system seen in Figure 6 the amount of deflection observed could

not be trusted as a value of what to expect in the final design.

Bell Jar Vacuum Testing of System:

After choosing a material and completing rough characterizing tests, the switch assembly was

tested under a much more precise bell jar vacuum system. Offset shims of 0.03 and 0.09 [inches] were used

to vary the distance of the switch to the diaphragm.

Figure 7: Bell jar testing set up



Using a bisection method to find the distance of the deflection that activation occurred at resulted

in a window of 0.03 [inches]. However, due to testing design, smaller distances could not be tested in order

to more precisely find the activation deflection range.

Fine Distance Deflection Testing:

By utilizing a visual indicator and the threading of a 4-40 bolt, the test apparatus seen in Figure 8 is

able to precisely measure increments as low as 0.003 [inches].

Figure 8: Fine distance deflection testing apparatus

Using this test set up from Figure 8 the team was able to get a final deflection distance of 0.26 [in.].

Engineering Standards:

The customer stated that the design did not have to meet any known engineering standards. There was no

limit on materials, outside of hazardous materials or over-budget materials.

RESULTS AND DISCUSSION

Final Design:

The final design has a snap action switch mounted directly to the housing assembly (open back end and

vacuum end). That assembly is then set in the mounting bracket and held in place by the c-lamps. This entire

assembly is bolted to the missile deck.

Figure 9: Front and Back view of final, manufactured product

Bill of Materials: Table 2: Bill of materials for final design of product



Final Testing and Simulation Verification:

Simulation (ANSYS):

A Finite Element Analysis (FEA) study was conducted to understand how the system should

respond to the given launch conditions. The results could then be used to determine if our structural design

would be sufficient to withstand the launch conditions. The academic version of ANSYS contains a

node/element limit, so the system had to be simplified in order to simulate with the software. The simplified

model is shown below in Figure 10:

Figure 10: ANSYS Model used for simulation

Only the mounting bracket and retention clamps was physically modeled in the simulation. The

central cylindrical assembly was assumed to be a rigid assembly, and was modeled as a point mass for further

simplification. The simplified model was simulated under the 27g launch condition as well as an enveloping

random vibration spectrum (0.005 G2/Hz, from 20-2000 Hz). Results from these test are shown in Appendix

3. Because the results from all simulations gave sufficiently high factors of safety, the physical design was

not altered.

Vibration Testing (Delphi Corporation):

The system was then subjected to a 4G – RMS test for 20-2000 [Hz]. During the test, the resistance

across the switch was being monitored in order to guarantee there is no false activation in the mechanisms of

the snap action switch or from the diaphragm hitting a harmonic vibration and deflecting into the switch

plunger. The summary of these tests can be found in the Appendix 4. Ultimately, it was found that there was

no activation of any kind for each test run in all three axes.

Customer Requirements vs. Performance:

Table 3: Customer Requirements with team performance



CONCLUSIONS AND RECOMMENDATIONS

The final design is highly sensitive to the assembly process. Depending on how many turns the bolts holding the

vacuum end and the open back end together are turned, the distance the switch sits to the diaphragm could change by

a critical amount. Additionally, because of the slop in the holes that house the bolts holding the snap action switch, if

the switch gets assembled and pushed forward it could affect the activation range of the system, as well.

One of the biggest changes that would be carried out, had there been more time, would be to choose a different

switch. It was found, without enough time to get a different switch in, that the chosen snap action switch was rated to

only 28 [volts] and had a measured resistance of 0.3 [ohms]. By choosing a different switch, these customer

requirements would be met. The team also would write more comprehensive work instructions for assembling the

system by putting torque specifications on all bolts and precise assembly instructions for installing the snap action

switch. In terms of a proof of concept, the integrated diaphragm design was a successful design. Activation was able

to be achieved at 10-4 [mm Hg] for multiple trials though not all of the trials. The system survived a vibration test that

simulates the rocket launch environment. In total, the proposed design by NAMASTe served as a successful prototype

to verify that the technique proposed would satisfy the major customer requirements.

REFERENCES

[1] Leathe, Nick. “The 2017 Sandia National Laboratories Senior Design Bonanza.” Sandia National

Laboratories, 2017

[2] M. D. Giovanni, Flat and corrugated diaphragm design handbook. New York: M. Dekker, 1982.

ACKNOWLEDGMENTS

Team NAMASTe would like to make special thanks to:

Dr. Mario Gomes

Prof. John Wellin

Dr. Mark Kempski

Dr. Stephen Boedo

Dr. Michael Schrlau

RIT Machine Shop

Delphi Test Labs

For your help, expertise and facilities.

Team NAMASTe would like to make an extra special thanks to:

Chris Leibfried and Vince Burolla

For your guidance and mentorship throughout the Senior Design class.



APPENDIX 1: SAMPLE OF DATA FOR PRELIMINARY DEFLECTION TESTS

Definition of AP and BP coefficients:

𝐴𝑝 =3(1 − 𝜇2)

16∗ (1 −

𝑏4

𝑎4− 4

𝑏2

𝑎2log (

𝑎

𝑏))

𝐵𝑃 =

7 − 𝜇3

(1 +𝑏2

𝑎2 +𝑏4

𝑎4) +(3 − 𝜇)2

1 + 𝜇𝑏2

𝑎2

(1 − 𝜇) (1 −𝑏4

𝑎4) (1 −𝑏2

𝑎2)2

Where a and b are the outer and inner radii of the diameter, and μ is Poisson’s ratio of the diaphragm material.

For our analysis, b was assumed to be very small, less than 10% of the outer radius, as this has negligible

effect on the modeled stiffness of the diaphragm but allows us to use the nonlinear equation for large

deflections of the diaphragm.

APPENDIX 2: SAMPLE OF DATA FOR PRELIMINARY DEFLECTION TESTS

APPENDIX 3: DATA RESULTS FOR ANSYS SIMULATION OF G-FORCE



APPENDIX 4: GRAPHS FROM DELPHI TEST (X,Y,Z) RESULTS:

ORIENTATION:

X RESULTS:

Y RESULTS:



APPENDIX 4: GRAPHS FROM DELPHI TEST (X,Y,Z) RESULTS:

Z RESULTS:

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