P16103 SPEX Vibration Test Rig Page 1

P16103 SPEX Vibration Test Rig Page 1

Project Number: P16103

RIT SPEX VIBRATION TEST RIG

Abstract

This 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.

Nomenclature

SPEX - 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. 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 Development

Vibration 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 Subsystem

In 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 Subsystem

In 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 Structures

When 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.