Preliminary Detailed Design
Team Vision for Preliminary Detailed Design PhasePlanned:
• Design deployment mechanism inside 2U frame provided by SPEX club
• Create prototype with boom material and motor
• Spec out motor drive mechanism
• Updated test plan
• Updated risk assessment
• 3D Printed and assembled SPEX Cubesat Frame
• Designed and 3D printed deployment mechanism
• Created working prototype by attaching measuring tape booms and sail
• Decided on sensor to detect distance boom will travel
• Decided on sail attachment method and sail bonding method
• Updated test plan and risk assessment
Prototyping, Engineering Analysis, Simulation
After choosing the deployment mechanism with a single central spindle as the design to move forward with, a basic CAD model and 3D printed prototype were created to test the feasibility of the mechanism. Prior to this, sketches and rough CAD models were done using various mediums in order to visualize the forms that these pieces would take.
The team inquired into the possibility of utilizing TRAC (Triangular Rollable And Collapsible) boom material for the booms of the CubeSat, however the lengths required to manufacture this prototype were upwards of $25,000 in cost altogether. As a result, a one-inch wide tape measure was used for creating the booms for this prototype, which will be on a much smaller scale than the final design. After being created in Solidworks 2017, the prototype pieces were printed on a desktop fusion deposition modeling printer, and were made from PLA plastic.
Utilizing a frame design provided by the RIT SPEX team, a deployment mechanism with seven parts was created to fit within the frame. The central spindle was modeled as a single piece which will ultimately be machined from round aluminum stock. This piece has holes in its outer wall to allow for mounting the tape measure booms to the sides, and a slot for a motor shaft with a flat to be mounted to. As a motor has not yet been selected at this stage, the mounting points for the motor on both the spindle and in other areas of the mechanism may change.
The mounting plate for the motor, spindle, and boom guides was initially planned as a single, flat plate with holes to allow for mounting of various components. However, in order to accommodate for methods of reorienting the sail in the suture, this single plate was turned into a gyroscope-style sub-assembly made of two component pieces. The square outer ring mounts to the outer frame on one axis of rotation, and the inner mounting plate which the deployment components will be mounted to attaches via a perpendicular axis of rotation. For the prototype, these pieces are joined using paperclips as the axles, however the mission of allowing free movement in various directions is allowed, meaning that sail orientation could be a feasible future project. Before this is possible, though, cuts must be made in the frame for the next phase that allow for the booms to rotate without interfering with its walls.
Finally, the boom guides were designed as a means to keep the booms extending in one constant direction, perpendicular to the side of the CubeSat which they were deploying from. These were designed as if they were to be manufactured from 1/8th inch aluminum plate, and could be patterns which are able to be bent via use of a tool such as a sheet metal brake. Four of these were required for assembly, and were mounted to the inner mounting plate via M3 screws. In order to provide a boundary for the top edge of the guides to keep the booms in place, and to provide a storage space for the sails, cardboard barriers were cut and secured into the upper frame.
Solidworks provided an estimated volume for the designed parts, including the frame provided by SPEX, to be 188863.68 cubic millimeters. Ultimately, the craft will be constructed from a type of aluminum, so assuming a density of 2.699e-6 kilograms per cubic meter for aluminum, this would mean that the parts designed to date would have a combined mass of 0.510 kilograms, which is still within the acceptable range of mass for the design. This will only increase once the booms, sails, motor, and any additional components are added, but should be limited to a maximum mass of 2kg, all inclusive.
Feasibility: Prototyping, Analysis, Simulation
Blanket Bonding to form Sail
The first feasibility test done during this phase was testing ways to attach the blankets together to form a sail. This was a primary concern because if this did not work reliably then the sail material would have to be changed and acquired from another source. The bonded sails would have to be able to withstand approximately 10 Newtons of force. This is a very arbitrary number as the force from solar pressure is order of magnitudes smaller than this, but we wanted it to be able withstand enough that it wouldn't pull apart is if got temporarily snagged on something or pulled during folding.
We tested 4 main options: a heat gun, an iron, Kapton tape, and metalized Mylar tape. The first two were applications of heat. The theory behind this is that the heat would cause bonding of the blankets to each other by slightly melting the material. Upon cooling it was theorized that the heated material would then be bonded together. This was based on things we had read on the Internet describing this process for bonding aluminized Mylar bags together. We tested this with both a heat gun and an iron. The heat gun did not apply the heat evenly enough and melted the material. IN addition the airflow generated by it blew the pieces being bonded around and made using it difficult. The iron failed to bond the material together. Upon further investigation the examples we found online were using aluminized Mylar with exposed polymer on the back side that was melting and bonding the material. Our blankets do not have this, so the heat bonding will not work.
The main alternative to the heat bonding was an adhesive. After consideration, we decided not to use any type of glue material as ensuring even distributions would be difficult and the added thickness and mass to the sail from it would be hard to calculate. We then selected two adhesive tapes for comparison. The first was Kapton tape, which is commonly used for space applications. The second was metalized Mylar tape, which a member of the SPEX club recommended for our application based on his work in a research lab. After testing both, the metalized Mylar tape was chosen. It took significantly more force to remove than it should experience in use, is thinner than the Kapton tape, and is still as reflective as the sail material so we wouldn't need to redesign the sail size to compensate for reflective area that would be covered by the tape. In addition it is also less expensive. For these reasons we chose to use the metalized Mylar tape as our bonding material.
Sail FoldingTwo key constraints limited this design: the folded sail components have to fit within the CubeSat and the sail must unfold by the motion of the booms extending. In addition, there are existing fold lines from the packaging of the blankets. These lines were initially considered to be a major hindrance as in order to create new folding patterns they would likely need to be removed. During the testing of the blanket bonding where we determined the best way to bond separate blankets into a cohesive sail unit, we tested removing the existing fold lines with heat. This was done using an iron. The temperature applied by the iron was steadily increased, but the fold lines could not be effectively removed without either melting the blanket material or shriveled the material to an unusable point.
With this test completed, the natural next step was to determine if the existing fold lines could be used for the sail unfolding. This would solve the issue of dealing with the existing fold lines, and the compacted shape would fit within the CubeSat with minor shape reconfiguration through strategic applications of force to the folded shape. The thing to tested then was if the existing folded blanket pattern could be unfolded using only the extension of the booms. This was tested on the a small scale demo model of the sail and booms. Each boom was approximately 3'3" long instead of the 13' in the design, and the sails were isosceles triangles with sides of 3', 3', and 4'3". During this test, the demo model sails were properly deployed used using only boom deployment. Further testing will be done to verify this will work on full size sails made of bonded blankets.
Boom Attachment to SailWe also tested the attachment of the sail to the booms on our small scale demo model. For this there were several considerations that had to be taken into account. The main issue considered was that the booms needed to extend past the end of the sails in order to pull them taut. This makes a direct attachment method like taping or stapling the sail onto the boom impractical. Furthermore, it was determined that the best way to attach the sail to the booms to account for the need of folding is to fold the sail while it is not attached to the boom and then attach it. During a test with our demo model where we tried to fold the sail while it was attached to the booms it was very difficult and time consuming, and the full sized sailed is much larger and would be even more challenging, if not impossible, to do with this method.
Considering all of this, it was determined that a tether system would fit our needs the best. To use this, eyelets were put into the ends of the demo model booms and in the corners of the demo model sails. Once that was completed, string was used to tether the booms to the sail. The length of the tether was tested with a distance of 1" between the sail and the boom. This was determined to be far too short, and the longer tethers are needed. The tether materials considered were simple string and fishing line for the demo model. We tested with string because we were concerned about the edges of the hole in the boom cutting the fishing line. We also covered the sharp edges of the cut booms with duct tape. For the final product, either wire of high strength fishing line can be used.
Boom Distance Detection Sensor Analysis
We were able to come up with two possible options for detecting the distance the measuring tape has extended. This distance would be used to determine when the power to the motor should be stopped.
This sensor will detect the light emitted by the material in front of it. We could mark the measuring tape with electrical tape in increments. This would create a contrast on the measuring tape of light vs dark (see picture below). We could count the number of times the sensor detects a change in color and then calculate the distance the boom has extended.Encoder:
As the tape measure extends outward, it would rotate an encoder. We could code the encoder to tell us when one cycle is completed and then calculate the distance based on that information.
We decided to go with the QTI Sensor since it is a simpler idea overall. With the encoder we would have to create a part that would ensure the encoder would rotate instead of slip when the measuring tape is extended. Additionally, the coding of the encoder is complex when compared to the QTI sensor.
We completed a feasibility test of the QTI sensor. The sensor showed very different values when it was shown black vs yellow. We also played around with the light color to see if the tick marks on the measuring tape would have any effect on the readings. This was done by using the white tape as shown in the picture below. However, we got similar values using the white tape as using the yellow measuring tape. This feasibility study indicated that this idea will likely perform as designed.
Drawings, Schematics, Flow Charts, SimulationsFolder of Mechanical Drawings
Bill of Material (BOM)Link to live Bill of Materials: BOM
The combination of individual blankets into the needed sizes to make the solar sails was a key component of our design. If this was not possible, then the sail material with have to be reconsidered and a new material acquired. As such, confirming that the blankets can be bonded into a larger sail was priority for this phase.
- Clean the surface of the blankets or blanket pieces to be bonded.
- Place the edges of the materials to be bonded next to each other on a flat surface.
- Apply the metalized Mylar tape over top of the two pieces, ensuring an even amount on each side.
- Secure both bonded pieces to their respective test surfaces by placing the securing weights on them, making sure the bond is between the fastened areas.
- Ensure that the vertical hang of the bonded sail is at least a foot vertically.
- Gradually add test weights until the bond fails.
- Calculate the force that caused the bond to fail based on the mass of the test weights.
It was originally intended that this be completed by this phase. However, due to shipping and handling time associated with acquiring the metalized Mylar tape, this could not be done. Instead a very rough test was done to get a ballpark estimate if the metalized Mylar tape would work, which it was determined it would by a large margin. This test will be carried out by Eric and Mike.
Sail Folding and Unfolding
The folding of the sail material needed to be done to determine three main things: the volume of space taken up by the folded sail, the ease of sail deployment by the boom extension process, and the possibility of damaging the sail material during the process. This test would then determine if additional risk mitigation techniques would be needed for the risks associated with this process and to determine what folding method was the best. This test requires two people.
- Clean the test surface.
- Lay the unfolded sail flat on the test surface.
- Fold the sail into the compacted shape.
- Calculate the volume taken up by the packaged sail.
- Determine a value on a scale between 1 and 10 of the likelihood and severity of sail tears during the folding process (1 being the lowest and 10 being the highest).
- Attach the folded sail to two booms for testing. The attachment method used can be zip ties or string through eyelets in the boom and sail corners.
- Have one person hold down the corner of the sail that would be attached to the CubeSat frame.
- The other person slowly and evenly extends the booms to unfold the sail.
- Determine a value on a scale between 1 and 10 of the likelihood and severity of sail tears during the unfolding process (1 being the lowest and 10 being the highest).
- If the folding pattern unfolds completely without damaging the sail, the pattern will be considered. Otherwise discard the folding pattern.
This was also intended to be completed by this point, but again shipping and handling time of the metalized Mylar tape prevented this from being tested on larger sail pieces. For this phase, we were able to test the four small sails used in the demo model and full blankets, but mainly the feasibility of the patterns was tested and the likelihood of tears from folding and unfolding was not quantified according to the scale. Mike and Eric will conduct this testing, but it will likely be unnecessary based on the small-scale results. This is because of the pre-existing fold lines, which as detailed in the sail folding feasibility, make following the existing pattern used to package the blankets for shipping the most effective option.
Sail Attachment to Booms
The sails will have to be attached to the booms in order to be deployed. This attachment mechanism will have to be sturdy enough to pull the sail taut when the booms extend and also able to create enough space that the booms can extend past the end of the sail in order to pull it taut. In addition, since in the feasibility analysis the best attachment methodology involved removing the sails, folding them, and then attaching them, the sails must be easily removable and attachable to the booms.
- Create two loops of approximately 10 inches of the material being tested and tie it (5 inches when the ends are pulled tight).
- Measure the exact length of the first loop with the ends barely held taut.
- Attach the heavy test mass to the first loop and place it on the test bar, allowing it to hang for one minute (this simulates a snag or temporary heavy resistance from extending too far).
- Measure and record the new length with the ends barely held taut.
- Determine the magnitude of elongation in the material.
- Attach the lighter mass to the second weight and place it on the test bar, allowing it to hang for 12 hours (this simulates the solar pressure on the sail pulling on the tether).
- Measure and record the new length with the ends barely held taut.
- Determine the magnitude of elongation in the material.
- Calculate a predicted elongation over the mission duration.
This test will be done by Eric to determine which of the available materials can perform as expected without elongation. Any elongation will cause the sail to not be held taut, reducing its effectiveness. Once effective materials are determined from the available possibilities, selection can be done based on the properties of the individual materials such as mass, flexibility, cost, and others.
Sensor Measuring Accuracy
The sensor that will be used to measure the deployment of the booms needs to be able to accurately determine the length of their extension. The type of sensor selected is a light sensor that measures different values based on the color it detects. This will be to prevent any overextension of the booms, which would cause the deployment mechanism to begin to retract them again in the wrong direction. This could damage the booms and would result in a smaller sail area than anticipated, possibly leading to insufficient or uneven forces acting on the sale.
- Make alternating light and dark sections on the test boom at the desired increments.
- Attach the sensor to the test surface.
- Measure and record the length of the test booms.
- Place one end of the test boom just before the sensor.
- Pass the length of the boom underneath the sensor.
- Compare the measured length from the sensor to the actual length of the test boom.
For this test it is important to note that the increments chosen will limit the resolution of the sensor’s length measurement. For this reason, different increments should be tested, and the one that best reflects the needed resolution of the sensor should be determined. The sample rate of the sensor is not a major concern here, as the booms will deploy very slowly, and that speed can be changed with little consequence. This test will be done by Eric and Victor.
Design and FlowchartsFollowing the conclusion of the Systems Level Design phase, it was determined that some form of sensors would be required to determine how far out the booms have extended at any point in the extension process. Because of this, the System Architecture flowchart has been updated to accommodate the new inclusions.
Risk AssessmentThe assessed risks for the project have changed as a result of this phase. The risks of the boom mechanism failing and of the created moment being too large have been reduced through our feasibility testing and prototyping. These were our two largest risks from the last phase, and while still significant, are not as large. Two new risks have emerged from our work during this phase. One of these emerged from a discussion with the customer over the use of sensors to measure boom deployment. This enables feedback to the boom deployment to tell the motor when to turn off. The other new risk is buckling of the booms from forces of the sail from solar pressure. In addition, the risk of failing to be able to package the sail compactly enough has been reduced as we tested the folding of the sails. The full list of risks updated for this phase is shown below.
Design Review MaterialsBelow are links to documents related to and generated from the Systems Level Design Review:
Plans for next phaseFor the final detailed design phase we plan to complete the following:
- Make frame and deployment mechanism easily manufacturable
- Create ¼ of sail by combining the mylar sheets
- Interface sensor, motor and micro-controller to communicate with each other
- Design locking mechanism for the spindle
- Spec Out Drive Mechanism
- Interface Microcontroller with Motor and Sensors
- Create attachment mount for sensor
- Update project plan
- Design locking mechanism
- Manufacture locking mechanism
- Continue to update total volume and mass
- Finalize BOM with Mike
- Discuss manufacturability of deployment mechanism with Mike
- Create Quadrant of Full Sized Sail with Mike
- Solar Pressure and drag calculations
- Test unfolding functionally (no rips in sail) with Mike
- Test feasibility of sail refolding with Mike
- Research other existing deployment mechanisms in detail
- Create Quadrant of Full Sized Sail with Eric
- Buy sensors, microcontroller and motor
- Finalize BOM with Andrew
- Test unfolding functionally (no rips in sail) with Eric
- Test feasibility of sail refolding with Eric
- Discuss manufacturability of deployment mechanism with Andrew
- Update engineering drawings