edge.rit.eduedge.rit.edu/edge/P16103/public/Final Documents... · Web viewMany vibration test rigs we were able to research, including the table used by RIT’s packaging science

Multidisciplinary Senior Design ConferenceKate Gleason College of Engineering

Rochester Institute of TechnologyRochester, New York 14623

Project Number: P16103

RIT SPEX VIBRATION TEST RIG

Brian HerzogMechanical Engineering

Project Manager

Richard MaroneyMechanical Engineering

Manufacturing Lead

Peter ArtusoMechanical Engineering

Vibration Subsystem Lead

Melissa St. PreuxMechanical Engineering

Damping Subsystem Lead

Timothy WilhelmMechanical EngineeringControl Subsystem Lead

AbstractThis project is part of a partnership between the RIT Space Exploration Team and the Multidisciplinary Senior

Design program which aims to use senior design teams to develop and construct systems necessary to the long term goal of building, testing, and successfully launching a CubeSat to perform research in low earth orbit. The focus of this project was to develop a test rig capable of ensuring CubeSats designed by the RIT SPEX team meet all requirements for sinusoidal vibration testing. This project aims to provide the RIT SPEX team with a low cost alternative to test their designs and ensure structural integrity when subjected to a launch environment.

NomenclatureSPEX - The Rochester Institute of Technology Space Exploration team. SPEX is the primary customer for this

project.CubeSat – A miniature satellite used primarily for small scale research and experiments in space.P-Pod - Poly-Picosatellite Orbital Deployer. A specialized container used to eject CubeSats from the launch

vehicle and into low earth orbit. Durometer - A measure of the stiffness of a sample of rubber.

Background Before a CubeSat can be launched it must meet all requirements as outlined by the CubeSat launch initiative

including a vibration test which tests at a range of frequency from five hertz up to one hundred hertz. Test facilities exist in the Rochester area with the equipment necessary for the RIT SPEX team to test their designs according to the guidelines of the CubeSat Launch Initiative. These facilities provided the RIT SPEX team with a price quote for access to testing equipment but the cost was too high for the team to be able to test and use iterative design practices.

Copyright © 2016 Rochester Institute of Technology

P16103 SPEX Vibration Test Rig Page 2

In an effort to secure a cost effective alternative for testing student designs, the RIT SPEX team partnered with the Multidisciplinary Senior Design program with the intention of teams developing equipment tailored to the team's test requirements. This partnership will provide the RIT SPEX team with equipment which can be easily operated by students on the team, be easily transported and stored, and save the team time and money during testing.

In order to successfully complete the project, the vibration test rig must be able to step through a range of frequencies at two octaves per minute. The rig is required to begin testing at a frequency of five hertz and double the vibration frequency every thirty seconds until the rig reaches a frequency of one-hundred hertz and begins to step back down at the same rate. At each frequency the rig must output a corresponding acceleration between 0.6g and 1.2g. The design was required to be reproducible, safe and simple for the RIT SPEX team to use while also maintaining a low operating cost.

Many vibration test rigs we were able to research, including the table used by RIT’s packaging science lab, use a hydraulic system to control a piston which provides the necessary vibrations. Due to spacing and cost, as well as a lower anticipated test load, our test rig used a similar solenoid valve and piston assembly that utilized pneumatics in place of a more complex and expensive hydraulic system.

Methodology As an initial step to the project our team met with Dr. Dorin

Patru and visited the test rig used by the RIT packaging science department. Using the information gathered, we were able to develop a set of customer needs which were leveraged to establish a list of engineering requirements. Our critical customer need was that the test rig provided a low cost alternative replicating the expected vibration profiles of common launch vehicles. Other requirements included that the test rig would be durable, safe and able to be operated by student members of the RIT SPEX team. The test rig was requested to be designed in a fashion so that it could be portable and could be stored in the RIT SPEX laboratory.

A key step in defining the engineering requirements for the test rig was establishing a vibration testing profile. This profile outlines the frequencies and amplitudes the test rig would need to achieve. This was accomplished by researching the sinusoidal vibration requirements provided in the payload user guides for common launch vehicles. Our final vibration profile was developed as a combination of the requirements listed in the payload user guides for the Atlas V, Delta II, Delta IV, and Falcon 9 rockets. The vibration profile encompassed all of the testing frequencies detailed in each of the rockets. The acceleration values were determined by comparing the separate vehicles and applying the maximum acceleration value for each testing frequency. By testing at the highest required accelerations the test rig would ensure a CubeSat would meet or exceed the expectations for all common launch vehicles.



Design and DevelopmentVibration Subsystem

During the initial stages of our design a wide variety of subsystem assemblies were considered for the purpose of causing and controlling the vibrations. These subsystem designs included using a spinning mass, a rotary motor, a magnet driven system, or a hydraulic system. The rotary motor and spinning mass systems proved to be ineffective in controlling the frequency and amplitude with respect to the linear acceleration of the system. Magnetic controls were eliminated due to their high cost and the team’s lack of expertise in manipulating magnetic systems. A pneumatic system and a hydraulic system both presented a similar system setup of using a piston driven by the working fluid and control valve. A pneumatic system was able to meet the necessary specifications at a significantly lower cost than a hydraulic system. Additionally, a pneumatic system was more portable and allowed for the system to be assembled by student members of the RIT SPEX team. Due to these factors we decided to pursue a system which would function by running compressed air through a solenoid valve and into a dual acting piston. The frequency would be controlled by feeding a square wave signal into the solenoid driver. The amplitude would be controlled by varying the peak voltage provided to the solenoid driver. Adjusting the voltage changes the percentage that the solenoid valve is opened or closed. This in turn changes the amount of air which enters the top and bottom port of the piston.

In order to select a piston which would meet the requirements of our system we had to consider the type of piston, maximum static load, and the compressibility analysis of the air moving through the piston. The largest design factor in piston selection was the necessity for a dual acting piston. Another option was a piston which used a spring to return the piston to the original position. The dual acting piston allowed for better control of the frequency.

Additionally we had to ensure that the piston we chose would be able to hold the weight of the CubeSat mounting subsystem and withstand the dynamic forces of the vibration testing. The most significant factor in piston selection was the compressibility analysis. This analysis showed the importance of pressure, flow rate, and bore size. Since it is critical that our system runs within parameters that allow for air to be considered incompressible, we selected a piston with a small enough bore size that the necessary flow rate and pressure would not force the air to enter the compressible region.

To achieve the high end frequency of one hundred hertz, the rig required a solenoid valve with a response time of five milliseconds or lower. Working with Ruessel, we were able to source a solenoid valve capable of oscillating at a maximum frequency of one-hundred and nine hertz. The solenoid driver was selected because it is designed to work with the solenoid we selected. While the driver represented a larger investment than attempting to source a driver elsewhere, this cost was justified by the knowledge that the connections between the driver and solenoid would function as expected.

Damping SubsystemIn order to prevent damage to the test rig or table, ensure the data is not affected by reflected vibrations,

and reduce noise, a damping subsystem was a necessary component of the design. In order to determine the best option to serve as the damping subsystem for the rig several calculations for force transmissibility and damping ratio were made based on assumed damping coefficients. The smaller the value of the damping ratio, the smaller the resulting transmissibility ratio. In cases where the transmissibility ratio is the lowest, the vibrations are the most isolated from the surrounding environment.



Two main damping concepts were considered to damp our system, rubber and a shock and spring assembly. Although the shock absorbers are very promising for isolation, the cost proved them to not be a realistic option for our budget. We determined that it would be possible to reduce the transmissibility through the base plate by using a rubber mat to separate the rig from the table or bench it rests on. By researching different durometer rubbers and similar damping products we found an appropriate design in the Sorbothane damping hemispheres.

The Sorbothane 50 durometer hemisphere mounts are visco-elastic polymers that provide a quick and cost-effective method of isolating the rig. It combines shock absorption, long fatigue life and vibration damping characteristics. The Sorbothane hemispheres have a higher damping capabilities compared to other similar polymers and rubbers.

During feasibility analysis, we manufactured a system similar to our final design that could output the frequency and amplitude specifications required for our test rig. The damping hemispheres were attached to a base plate and piston assembly vibrating from five to one-hundred hertz frequencies. We tested the rig with a twenty pound load at five hertz, ten hertz, and twenty hertz and our hemispheres successfully damped the system as expected. To validate the effectiveness of the Sorbothane hemispheres, data was collected using vibration sensor app. By placing the sensor on the baseplate of the system, on the table while using the damping hemispheres, and the table while not using the hemispheres we were able to compare the undamped system with the equivalent damped system. The data was imported into Matlab and plotted in order to compare the data for vertical acceleration. During testing without the Sorbothane damping hemispheres, the resulting data from the vibration sensor recorded average accelerations between 0-25m/s2. During testing with the damping hemispheres at various frequencies, the data showed a steady average acceleration at around 9m/s2, which was almost entirely from the force of gravity on the system. Due to compressibility issues with the piston we used for testing, we were unable to achieve vibrations passed thirty hertz during initial testing. However, the results provided proved the Sorbothane hemispheres acted as expected.

CubeSat Mounting SubsystemIn order to provide the most realistic launch experience possible

a replica P-Pod was constructed to rest on top of the shaking plate and hold the CubeSats during testing. This allowed us to safely secure the CubeSat during testing without directly contacting the CubeSat. The CubeSat would only contact the P-Pod at the rails as it would during launch. This would also allow for a broader range of testing since the P-Pod was designed as a standard P-Pod and could handle a 3U CubeSat. This enables the SPEX team to test a 1U, 2U, or 3U CubeSat by placing the current design in the P-Pod and using spacer cubes to fill the remaining space in the P-Pod. The spacer cubes were made by machining aluminum cubes down to the outer dimensions of a CubeSat and then removing material from the center until the mass was that of a standard CubeSat. At the bottom of the CubeSat a solid aluminum table was inserted to represent the displacement of the fully compressed P-Pod spring.

The original design for the P-Pod sought to cut the sides, bottom, and top of the P-Pod from sheet metal and then weld the pieces together to form the final shape. Due to the tight constraints associated with P-Pod and the clearances between the P-Pod and CubeSat rails, welding was determined to not be a feasible option. As an alternative to welding the sides together, the sides were designed to be cut using a water jet with twelve clearance holes on each side. The holes on the sides would be aligned with threaded holes on the rails and held together by tightening screws. This option provided more opportunity for complications due to tolerance stack up during assembly, but was able to provide a more realistic opportunity to meet the tight tolerances required than welding or any other options. During the assembly process the original design had too large of a gap between the rails: allowing for the CubeSat to move freely within the P-Pod in an uncontrolled fashion. To remedy this problem



one row of holes on each of the P-Pod sides were expanded to form slots which would fit the desired dimensions for the P-Pod.

In order to secure the P-Pod to the shaking plate the team considered incorporating a latch system into the P-Pod and shaking plate, create a rod attached to the P-Pod with a collet in the shake plate, and creating a frame around the P-Pod. In order to ensure the P-Pod would be secure while maintaining a design which was not overly complex it was decided to use 80/20 rail to build a frame around the P-Pod. This option would allow us to properly secure the P-Pod, while also making the P-Pod easy to remove from the rig to load and unload CubeSats. When assembling the first iteration of the P-Pod we tested the possibility of using the holes in the rails for attaching the bottom side of the P-Pod to run a longer screw through holes in the baseplate. We found this option would provide sufficient holding power to safely secure the P-Pod and reduced the overall complexity of assembling the structure and loading the P-Pod into the test rig. After thread strength analysis and testing of the system we concluded that screwing the P-Pod directly into the shaking plate would provide sufficient anchoring while simultaneously reducing the complexity of the top assembly and reducing the assembly time. Our final design uses holes in the baseplate which match the holes in rails of the P-Pod to secure the corners of the P-Pod to the shake plate.

Controls StructuresWhen deciding how to load the vibration profile and control the test rig a

number of options were considered including LabVIEW and a DAQ, Python and a Raspberry Pi, Arduino, and physical dials to control input. The usage of physical

dials or switches would have been inaccurate and non-responsive to a feedback system. While the option of physical switches would have reduced the overall complexity of the system it was determined that the loss of accuracy was too much to justify the usage. The RIT mechanical engineering coursework provided our team with familiarity with LabVIEW coding as well as access to a DAQ device and the ability to consult staff with expertise in the coding language. The potential use of resources available within the department, as well as an unfamiliarity with Python and Arduino code contributed to the decision to design our test rig to be controlled using LabVIEW and a DAQ device to provide the necessary voltage signals into the solenoid valve.

The main component of the LabVIEW code is a while loop which uses a loop counter and the known loop time to create an artificial test timer. Using the value from the loop counter and a series of if loops the code determines the current test time and associated frequency and amplitude. The frequency and amplitude values are fed into a waveform generator built to output to a National Instruments MyDAQ. The waveform generator section also accepts a DC offset value which can be varied during testing. By adjusting the DC offset during testing the user can prevent the piston from topping or bottoming out. The code includes an emergency stop button which once activated will immediately cease sending a signal to the MyDAQ. Additionally, the code has a time delay built in with a preset value of thirty seconds and a minimum value of five seconds. This allows for the user to set a time delay and clear the immediate vicinity of the test rig before testing begins.

Validation and Feedback StructureIn order to validate the outputs of our system two main options were considered: A laser position sensor

and an accelerometer. Both systems could validate the system by sampling a section of data and using the reported out values to calculate frequency and amplitude. The frequency would be calculated by locating the peaks and finding the number of samples between the peaks. Using the known sampling rate the time between peaks can be found and then inverted to find the instantaneous frequency. Once the frequency is known, the amplitude can be found by using the peak displacement and frequency to find the acceleration. The accelerometer outputs an acceleration value which directly correlates to the vibration testing profile. The accelerometer is more useful to the requirements of our system since one of the measurable outputs is acceleration. Additionally, a laser position sensor



with the response and sampling rate the test rig required was not a financially viable option for this project. The accelerometer provided a cost effective and more efficient option to arrive at the necessary validation.



Testing and Results In order to ensure the durability of the test rig and the

safety of its users, we performed COMSOL analysis on the test rig assembly and on the individual members of the assembly. The COMSOL analysis showed that the P-Pod sides would reach resonance if we used the material we were originally able to source. In response to this analysis, we sourced new material and increased the part thickness from 0.060 in to 0.109 in. This put the resonance frequency well out of the expected range of testing.

When working to calibrate the accelerometer and begin validation of our system our team ran into difficulties working with the communication between the I2C signal from the accelerometer and LabVIEW programs. After working with professors in the engineering departments, we were able to overcome many of the difficulties. However, the accelerometer was still not switching the data collection mode from a 2g range to anything larger. When testing with a 2g data range the data is clipped during the negative acceleration of the rig. After discussion with our customer regarding the timeline and state of the project, the accelerometer was determined to be a non-essential component. In order to provide validation of the frequency and acceleration values the team scheduled time with the packaging science lab to test using validation equipment owned by the packaging science department.

During the design process our team attempted to take a fast works approach to designing which focused on testing early and often. This methodology allowed us to provide consistent validation of components and make adjustments as necessary. The earliest tests were to hook the piston up to a known solenoid valve and test for expected behavior. Additionally, we wired our solenoid valve and driver into a function generator and oscilloscope to verify the output. After our critical components were validated they were assembled with an initial baseplate and connected to shop air for testing. The damping hemispheres were attached to the bottom of the baseplate and a 25lb cube was fixed to the top of the piston shaft to simulate the mass of a P-Pod. During this phase of testing we saw the system experiencing significant motion from the force of the vibrations and in some cases even jumping off the table. To counteract the motion of the system the baseplate was replaced with a larger baseplate made of steel. The new baseplate increased the overall mass of the system and reduced the portability, but with modular design of the subsystems was still within the allowable weight values.

While making final adjustments to the P-Pod, our team sought alternatives to allow for an earlier test date for P16102. Using excess materials we were able to construct a miniature 1.5U P-Pod. This smaller assembly would use the same rails as the P-Pod but allow to test a smaller 1U load while the full P-Pod was still being adjusted. This design decision allowed for the testing of the P16102 CubeSat design at an earlier stage to provide an initial level of validation for their design.

After all final adjustments were made to the P-Pod,the team performed quantitative testing on the system comprised of assembling the system by following our user guide instructions, loading the P-Pod with spacer cubes, and running the program with no user interference. The test was successful and the rig provided the expected frequencies and accelerations. After successfully validating the results of the system a time was scheduled with our customer and P16102 to test the P16102 design in our final test rig. The test was run three times at full length with the test rig performing as expected and the P16102 design passing all of the tests.



Conclusions and RecommendationsThe vibration test rig is currently able to run safely at all of the required frequencies and accelerations

required. The control structures currently lacks an efficient method to adjust the amplitude of the vibrations and requires manual adjustments to the DC offset of the signal. The DC offset and amplitude controls could be possibly be automated in the future if acceleration data were continuously fed back into the LabVIEW code. The accelerometer was removed from our final design due to complications with calibration and controlling the data sampling mode. The decision to remove the accelerometer was made after discussing the issues faced with our customer as well as the state of the project at the time of that meeting and the remaining timeline of the project. The P-Pod has been adjusted to meet all specifications. The adjustments made changed the assembly process of the top rig. In the initial design the P-Pod would be assembled and loaded before being attached to the baseplate. After the adjustments were made to the P-Pod design the top assembly process begins by securing the rails to the shaking plate and then attaching the sides to the rails and building the P-Pod upwards from the shaking plate. Once three sides are fixed the spacer table, spacer cubes, and testing CubeSat are loaded. The final side and top plate are attached and all screws are tightened and checked before testing begins.

While the controls structure is functional, we recommend that a future team with experience in continuous controls improve upon it. While LabVIEW was the right choice for our current application, it may not be the best decision as time passes and a team with experience in controls may be able to provide a more efficient structure. A future team may be able to design a controls structure which utilizes a continuous sweep rather than a step function. Other improvements to the controls could also be made if a future team were able to successfully integrate an accelerometer. Using accelerometer data, a team could design the controls structure to make real time adjustments to the amplitude and frequency of the system if the values were out of the range of expected values.

It is recommended that the connection between the piston and the shake plate be examined for improvements. Our team observed out of plane movement in the piston shaft. Overtime we expect the out of plane motion to increase over time due to usage and wear down in the piston. A structure could be designed to provide additional support to the shaft or guide the top assembly during vibrations. Currently our customer plans to monitor out of plane motion over time and if it reaches an unacceptable value the piston will be replaced. With the relatively low cost of the piston, our customer is willing to treat the piston as a consumable component which can be replaced when required. Lastly, the CubeSat mounting system could be reexamined for potential improvements. The current design may provide opportunities for improvements which could reduce complexity of assembly and loading the CubeSats. Our customer also expressed interest in the possibility of a P-Pod design which allows the test rig user to see the CubeSat during testing.

While there are opportunities for improvement in the design of our test rig the project can be considered a success. The test rig performs to the required specifications and replicates a launch profile which the RIT SPEX CubeSats will be experience before being launched. The system is safe and simple for a RIT SPEX team member to operate. The rig can be disassembled to keep the components within a reasonable size and weight for portability and storage.

References GSFC-STD-7000A: Goddard Space Flight Center standard on verification standards for flight projects.LSP-REQ-317.01: NASA Launch Services Program CubeSat requirements document.Atlas V Launch Services User’s Guide 2010Delta II Payload Planner’s Guide 2006Falcon 9 Launch Vehicle Payload User’s Guide ASM Metals Reference Book, Third Edition: Materials property reference used for analysis of structure

designCubeSat Design Specification Revision 12: California Polytechnic State University standard on CubeSat

requirements .



Acknowledgements We would like to thank the following parties for their assistance during the design and development of this

project: Dr. Dorin Patru and the RIT SPEX team for providing this opportunity and for providing information and

guidance throughout the design and development process.The Boeing Company for their support, without which this project would not have been possible.Mr. Edward Hanzlik, Mr. Thomas Bitters, Mr. Russell Phelps, Professor John Wellin, Dr. Mark Kempski,

and Dr. Marca Lam for providing continued expertise, knowledge, and advice during our design process.The staff of the Mechanical Engineering machine shop and Brinkman Lab for assisting us in the machining

process of components for our test rig and providing expertise on machining options and techniques.
