Table of Contents
Content related to this node is located in the Detailed Design Documents directory
Team Vision for Detailed Design PhaseDuring this phase our team sought to answer any final feasibility questions necessary to the purchasing decisions for our test rig. We explored potential difficulties associated with the piston and solenoid components of the rig. We ran testing to ensure the feasibility of our dampening methodology. Comsol Analysis was used to determine the structural feasibility of the system. Lastly we established distinct and separable subsystems of the rig to ensure ergonomic portability and a simple assembly and dis-assembly of the rig. Since our last design review, we now have a acquired a new budget of $1500 courtesy of Boeing.
Prototyping, Engineering Analysis, Simulation
Questions needed to be answered
- Is the compressibility of air at the systems running region stiff enough to reach our system goals?
- Does the solenoid cycle at 100 Hz?
- Is compounding timing an issue with the solenoid, piston, DAQ and Labview for the 100 Hz?
- Will the flow rate be an issue cycling in and out of the piston?
- Do we stay under our natural frequency of our system? And by how much?
1. The speed of the air needs to be under Mach 0.3 to assume incompressible flow.
As you can see if we go with our current spec-ed out piston we would only be able to run the air at about 1.7 CFMs before we ran into issues with comprehensibility. So knowing this we might need to choose a different piston as the other inlet sizes would be more ideal.
2. We can get a solenoid that will cycle at 100 Hz After meeting with Professor Wellin he assured me not only that 100 Hz is not a big deal for solenoid valves at but he himself had some pneumatics that we could barrow to experiment with. It should be noted that he did not say our system was no problem but 100 Hz was no problem for the solenoid valve.
3. Depending on our choices 100 Hz for a DAQ is not a big deal in any way. In my meeting with Professor Wellin when discussing DAQ devices there was no question that it could handle the speed going with the MyDAQ option, which we have elected to do. These are used in the systems lab currently and could possibly be borrowed. In my meeting with Professor Wellin he also said there was no issue with labview. Then we also needed to take account that the solenoid valve and as long as we use one that does not go above our minimum response rate. Then as far as the piston goes Professor Wellin also said that this was not a concern.
4. There should be no issues with this as our Amplitudes are not large for our frequencies
From this graph we can see that the flow rate for our system will be close depending on the bore size we end up going with.
For the different bore sizes we can also look at the frequency for our ideal amplitude we get the fallowing graph:
This image is hard to see so bellow is a zoomed in version of the same graph:
We see that the ideal flow rate for 100 Hz is low and definitely reachable.
We realize though that pneumatics are not exactly the most precise controllers for such a small Amplitude. So we reassessed problem and used a larger amplitude to see what the results would be.
This shows though even if we try to hit this amplitude we are well within the range of our limits.
5. Our time constant direct to air is under 1 Hz!
For the figure above we will be assuming that the points (1) and (2) will apply to the figure below for the Resistance of the air flow for both in and out.
In this case it should be noted that the first flow rate does not equal the second because it is changing from the tubing size to the bore size. Now we can find the relating flow rate from equation 1 seen bellow
This equation can relate the flow rate and pressure to the resistance in equation 2 seen below
Plugging equation 1 into equation 2 we can then solve for the resistance and get the following solution
Those same points (1) and (2) can be assumed to act as air capacitors as shown below:
For the incoming flow and
For the outgoing flow.
The system can be assumed to have the following trend for the capacitance:
This assumes that the pressure from the piston is enough to hold the weight of the system which based off our specs it does.
Now we need to assess the time constant of our system to see if it can hit the right frequency. Using the following equation we can find out time constant tou:
From our equations from before we can calculate the time constant of our system. Subbing in our equations we get the following form:
Simplifying we get:
The thing to see in this equation we have pressure [P] and density [rho] that are dependent upon each other. This density though will be the density of our fluid (in this case air) at the first pressure. And the third pressure will be the difference in pressure from 1 to 2. Looking into the time constant to ambient conditions we get the fallowing:
- 0.2573 s for the inlet (assuming 0.86 for the C_d)
- 0.2213 s for the outlet (assuming 1 for the C_d)
Which is under 1 second for 3 times our time constant and thus under our needed Hz. Note: this would be for the full evacuation of the volume which would be the stroke for our 5 Hz cycle and is well within our time restraints. Then using the fallowing equation we can assess the effect time constant for the 100 Hz (note that the gravitational constant will be different and thus the time constant will change slightly).
Now solving to find the maximum time constant we get:
The solutions to this are:
- 0.3149 s maximum
- 0.3092 s for the inlet (assuming 0.86 for the C_d)
- 0.2474 s for the outlet (assuming 1 for the C_d)
Therefore the extremes will work.
The following specs are for the different components used:
Questions needed to be answered
1. Are the 50 durometer Sorbothane Hemispheres meet our damping goals?
2. What feasibility analysis can be done to determine that the hemispheres can get the job done?
2. Feasibility Analysis
With some help from the machine shop, we were able to manufacture a system similar to the frequency and amplitude specifications required for our vibration test rig. The sine wave generator borrowed from Mr. Wellin’s Engineering Applications Lab was capable of reaching the 10-100Hz. We were able to take the circular plate and rectangular polymer pieces from the machine shop as well. I threaded four holes into the circular base plate in order to attach and secure the cylinder to it with 5/16-24 fine thread screws. I also tapped a hole into the center of the rectangular piece in order to secure it to the top of the cylinder.
Using shop air, our solenoid, regulator and piston, we were able to run a feasibility test on our Sorbothane hemispheres. We were able to test at 5Hz, 10Hz and 20Hz frequency values and our hemispheres successfully dampened the system with about 30-40lbs of added weight. 5Hz gives our system the highest displacement (0.5 inches). 10Hz and 20Hz are the values that require the most force to vibrate the system. So, although we weren’t able to test for the entire spectrum of frequencies, we did test the frequencies that would require maximum support from our damping hemispheres.
The hemispheres also seemed to show high effectiveness through the difference in acceleration felt on the table versus on the vibrating plate itself. An overlay of the two graphs is shown below:
Plans for MSD II
I plan to continue utilizing our teams’ parts as are ordered and arrive after break to get more accuracy and assurance with our vibration damping solution.
One problem that did occur during the analysis was that something within the system gave out once we increased the frequency on the sine wave generator greater than about 25 Hz. We weren’t sure where the problem occurred but I am interested to see if there is a way we can fix it in order to get complete results.
Questions needed to be answered
How can we keep the operators safe during normal testing operations?
To better ensure safety if the device happens to tip, we will be including a gyrometer to automatically halt the program. After some sourcing work for simple gyrometers, it was found that certain accelerometers include a gyrometer on the board. Having them paired on the board instead of seperate would eliminate the need for a second control system. The accelerometer detailed below has a gyrometer on board. This gyrometer will be hooked up to the emergency stop in LabView to stop the program if any tip is detected.
Questions needed to be answered
How are we going to ensure that our rig is functioning to the specifications required to pass the vibration testing?
After a brief discussion with Professor Wellin, he confirmed that a position sensor capable to measuring to the mm would be more expensive than this project can afford. However, these sensors do exist and Professor Wellin noted that he does have one capable of meeting the requirements. Future discussion will be needed to obtain this sensor for initial testing and calibration of the system.
As mentioned above in the safety section, we have sourced a new accelerometer with a gyrometer on the board. The specifications of this new accelerometer are very similar to the first accelerometer. It allows us to vary the range it is testing over and will increase the sensitivity as the range is lessened all the way down to 1mg/LSB in ±2g mode.http://www.adafruit.com/products/1714 http://www.adafruit.com/datasheets/LSM303DLHC.PDF
Drawings, Schematics, Flow Charts, Simulations
Questions needed to be answered1.) What updates have been made to our overall system or CAD model?
2.) What are we doing to prevent the system from tipping over, or rotating?
3.) Are there any steps going forward that are planned but not on here?
4.) Why no dimensioned 2D drawings, as denoted in the previous design review?
Answers1.) No major component changes have been made, as only the type of each has changed. For example we are still using a pneumatic piston, just not the same model as initially planned.
2.) We have devised a guide rail system to prevent any rotation. This is only for last resort cases, as the accelerometer should shut off the system before contact occurs. The guide rails will be similar in concept to those on a highway, not a frequent used option to bump off of, but there in case of emergency.
Regarding the issue of the entire system tipping over we are not foreseeing this being a major issue. The test area will be clear when any test are run, and again can be shut off by the accelerometer or emergency stop switch. The options to add in a substantial factor of safety to this is to made the damping base plate even larger, thus requiring an even larger moment for it to tip, or creating an acrylic shield like structure on 4 sides to catch it before it becomes too off axis.
3.) Yes, we will be including a user manual that includes work instructions for constructing, assembling each test run, and troubleshooting. This is a MSD II deliverable.
We will also include P16012's CubeSat in the CAD files in MSD II. Right now the solid orange/brown CubeSats are just used for ease of clarification.
Small details such as accelerometers or any small componenets will be placed in the model. Currently they would be hidden due to interior placement.
4.) Contrary to our thinking at the previous design review, we do not yet need any dimensioned drawings yet as we have accounted for the required raw material in our BOM below. If any new material is not accounted for, all parts are currently sourced from McMaster-Carr ensuring next day shipping to eliminate any lead time issues for unaccounted for material. The drawings will become a necessity in MSD II once we order any new raw material, or machine select materials in the machine shop.
"Bare Bones" assembly: Shows what we need to have to test our system at the most minimal level. Once this test is run and completed, we can procede to add other pieces of our system on and run accordingly.
Subassemblies: Note that the P Pod is included in two subassembly's. This is because for ease of transportation it can be kept inside the top assembly, but if needed to be accessed or worked on further, it can be fully removed. The purpose of distinct subassemblies is to show how our system can be broken down both physically and for trouble shooting purposes. Also, the main inputs of compressed air, power, and labview are not shown since they are just external inputs, not distinct members of a subassembly. There also will be the option to add handles to the top assembly for ease of transportation. This may vary based on final design of system, but is a quick, easy, and affordable add on later on.
Minimal Guide Rail: Highlighted in blue is the minimal system we need to have in place to ensure the system does not go out of line. Ideally we would have a stand alone system that is not connected to our vibration test rig in any way, and can ensure it does not rotate or even worse, fall over. However, we are confident in our system operating properly and our safety shut off that will occur if any out of line motion great than 2 degrees will shut off the system. The advanced system could also include an acrylic shield to further ensure its proper orientation, and protection against any materials that may come loose from the vibration test rig during operation.
COMSOL Finite Element Analysis Results
Questions needed to be answered
Are any pieces of the structure in danger of vibration excitation by being near a resonating frequency during normal operations?
No, all of the resonating frequencies of the moving pieces are well above the normal operating frequency of the rig. The lowest simulated value seen was the P-Pod with an eigenfrequency at 633hz.
1. All materials are going to be made from 6063-T83 Aluminum
2. The mass of each CubeSat is at the maximum, 1.33kg
3. The P-Pod and CubeSats will act as a single rigid body and will be fixed to the baseplate
4. Simplified model will provide accurate results to analyze the system
With help from Rich, we were able to simplify the CAD model of the system to a point where COMSOL can run the analysis on the school computers. With the given assumptions above included in the model, the constraints can be applied and the results can be generated.
The above pictures show the Eigenfrequency for the top assembly and each individual piece in the assembly. The lowest Eigenfrequency experienced is seen in the P-Pod at 633hz. This is well above the normal operating frequency of the test rig and gives us a large tolerance on this analysis. While it is possible that unforeseen circumstances will arise, this should give us the peace of mind that the large moving parts of the system have resonating frequencies above the normal operations of the system.
Bill of Material (BOM)
Questions needed to be answered1.) What significant updates have been made to the BOM?
2.) What is our updated progress on inquiring free raw material to reduce overall system cost.
Answers1.) We are no longer using the piston and solenoid from the engineering basement storage. This causes a $748 increase. All smaller components were also added in detail such as any T-Slotted rail necessities, a $44 increase. We have also acquired a position sensor from Professor Wellin, for calibration purposes, eliminating that cost from our current BOM.
2.) Shown in the chart below, our inventory has been updated. This will continue to grow overtime as a predicted influx of scrap material will be put into the machine shop in the coming weeks as MSD II comes to an end.
- Updated Risk Assessment with ownership added
Plans for next phase
Deliverables by Owner
Our goals for the start of MSD II:
1. Continue to analyze any changes made to the system to ensure safety and structural integrity.
2. Thoroughly record and document build and assembly procedures. Begin work on the owner’s manual, operation instructions, and maintenance plan.
1. Get more of a hands on experience with the pneumatics so that I understand them and can design properly for our system.
2. Make sure my part of the system (the pneumatics) gets up and running in a timely fashion with the assembly of the rig.
1. Get more certainty with use of Sorbothane hemispheres from 20-100Hz as additional parts arrive
2. 70 duromenter hardness testing if time allows
1. Continue to update our BOM to ensure it is accurate and we remain in budget
2. Have all of our CAD up to date, and create 2 dimensional drawings for machining purposes as needed per GD&T standards.
1. Ensure all components purchased or otherwise sourced
2. Establish basic, minimum necessary labview code.