proceedings - rochester institute of...

Multidisciplinary Senior Design ConferenceKate Gleason College of Engineering

Rochester Institute of TechnologyRochester, New York 14623

Project Number: P14651

THE ROCHESTER INSTITUTE OF TECHNOLOGY MICROGRAVITY DROP TOWER

Dustin BordonaroMechanical Engineer

Jake GrayMechanical Engineer

Santiago MurciaMechanical Engineer

Adam HertzlinMechanical Engineer

Yoem ClaraMechanical Engineer

ABSTRACT

The purpose of this senior design project is to design, develop and construct a fully functional drop tower within the Kate Gleason College of Engineering at RIT. A drop tower is a device that facilitates the study of microgravity by putting an object in a state of freefall. The design requires a medium vacuum, greatly reducing the effects of air resistance, in order for an object to accelerate downward at a constant rate. Developing a clear drop chamber visually shows how all objects fall at the same rate under the sole effects of gravity, and then by recording the object’s descent it is possible to accurately calculate the value of standard gravity. In addition to creating such a vacuum chamber, a precision dropping mechanism and laser recording device allowed for accurate data collection. An intuitive user interface allows the user to process or save this data to show that standard gravity here on Earth is approximately 9.81 m/s2. The aesthetic drop tower, standing at nearly 12 feet tall, is easily transported to various locations thanks to its custom support structure. Overall, the design creates an enjoyable learning experience for all students interested in the fields of science and engineering.

Copyright © 2014 Rochester Institute of Technology

Multidisciplinary Senior Design - P14651

NOMENCLATUREA = Electrical Current (ampere)Af = Projected Area of Falling Object (m2)C = Conductance (lpm) CD = Coefficient of Drag CV = Viscous flow Conductance (lpm) CM = Molecular Flow Conductance (lpm) CT = Transitional Flow Conductance (lpm)CNC = Computer Numerical Control Machining D = Pipe Diameter (m)DAQ = Data Acquisition DeviceDR = Dimension Ratio of PVC Pipe = 0.168E = Modules of Elasticity of PVC Pipe = 2.68e6 (kPa)F1 = Viscous/Transitional Flow Scale Factor F2 = Transitional Flow Scale Factor F3 = Molecular Flow Scale FactorL = Pipe Length (m)Ṗ = Average Pressure (kPa)P0 = Atmospheric Pressure = 101.3 kPa P1 = Viscous – Transitional Pressure (kPa)P2 = Transitional – Molecular Pressure (kPa)P3 = Ultimate Pressure (kPa)Patm = Atmoshperic Pressure = 101.3 (kPa)

Pcr = Critical Pressure (kPa)PWM = Pulse Width ModulationS = Safety FactorSEff = Effective Pump Speed (lpm)SP = Given Pump Speed (lpm)V = Electrical Voltage (volt)V c = Chamber VolumeW = Weight of Object (N)e = Coefficient of Restitutiong = Acceleration due to Gravity = 9.81 (m/s2)mo= Mass of the Falling Object (kg)mc = Mass of the Catching Mechanism (kg)n = Number of pipe diameters or actual lengthst = Time (seconds)ua = Velocity of Object before Impact (m/s)ua = Velocity of Object after Impact (m/s)va = Equivalent Bead Velocity after Impact (m/s)vb = Bead Velocity before Impact (m/s)vo = Velocity of the Falling Object (m/s)x = Total Drop Distance (m)ρ = Air Density = 1.225 (Kg/m3)ν = Poisson’s Ratio of PVC Pipe = 0.37

INTRODUCTION Microgravity is a condition in which objects experience the effect of “floating” from a relative perspective.

However, from the viewpoint of a person or object not experiencing these same effects, the objects would appear to be simply “falling” under the effects of gravity. This is what happens with a spacecraft in orbit when the crew and payload appear to float around, but in fact are in a constant state of freefall, traveling around the Earth at an extremely fast rate. In order to know how objects will behave in orbit or to simply better understand the general concept of freefall, a system must be created to study and test these effects here on Earth.

Microgravity can be simulated near Earth’s surface for a few seconds by putting objects in a state of freefall and eliminating all forces except for gravity. This can be accomplished in a variety of ways including, sounding rockets, parabolic airplane flight, drop chambers and drop towers. This paper will focus on the value and practical uses of an education-friendly drop tower, which can benefit both students and researchers alike when studying microgravity and microgravity-related concepts. The design was modeled after NASA’s Zero-G Facility located at NASA’s Glenn Research Center in Cleveland, Ohio [1]. While NASA’s 132 meter drop tower yields 5.18 seconds of freefall, this 2.4 meter design has been structured to accommodate classroom use while still achieving up to 0.7 seconds of freefall. The transparent vertical vacuum chamber has the ability to track one object's descent versus time, while dropping up to two objects in an environment ranging from normal atmospheric pressure to near vacuum. With this data, students from the middle school to graduate level can perform experiments to benefit and fuel their education experience in the fields of science and engineering. For instance, middle school students can utilize the drop tower to experimentally explore the effects of gravity on a ball-bearing and a feather that are dropped simultaneously in both normal atmospheric and near vacuum conditions. Also, undergraduate students can calculate and test the effects of aerodynamic drag on various objects at different air pressures. Several standardized experiment templates using a variety of engineering concepts and software techniques are created to facilitate the use of the drop tower. The design also allows for a range of modifications that can be made by instructors or researchers, as needed, to improve the educational value of the RIT Microgravity Drop Tower.

Project P14651


DROP TOWER DESIGN


Release Mechanism

Structural Frame

Data Collection & Analysis

Vacuum Pump


PROCESS

Vacuum Chamber & Energy Dissipation SystemThe central component of the drop tower is the pipe enclosure. There were several requirements for the pipe,

including visibility and size. The entire descent of the objects needed to be visible to the observer. Also, the pipe needed to be a minimum of 6 inches in diameter and as tall as economically feasible. Component selection began with the main pipe. A clear 2.74 meters tall Schedule 40 PVC piping with a 0.15 meter diameter was chosen for the pipe based on the requirements, budget, and safety. “Schedule” refers to the amount of pressure the wall can withstand based on its thickness. Schedule 40 was chosen because the pipe will not have to withstand pressures above 101.3kPa. Equation (1) ensures this by calculating the outer critical pressure of the pipe. The critical pressure was then divided by the atmospheric pressure in order to get the factor of safety, as seen in equation Eq. (2).

Pcr=2 E

(1−ν2)(DR−1)3 (1) S=Pcr

Patm(2)

The design required the two openings at the ends of the PVC pipe to be resealable for loading and unloading objects. As a result, union fittings were used with modified end caps fitted with attachments for both a vacuum pump and multiple servo motor wire feedthroughs. The threaded base of the fitting was attached to the two ends of the pipe using PVC cement. This component also includes an O-ring ideal for creating an air fit seal between the fitting and a smooth, flat surface. Consequently, clear, thick polycarbonate was used on both the lower and upper end caps. The polycarbonate pieces were water jetted to be functional and aesthetically pleasing. The lower polycarbonate end cap was machined to a standard round shape with a single hole that would accommodate a 25mm (1in) bulkhead fitting. The bulkhead connects the main pipe to the vacuum pump via a flexible hose, in order to evacuate the chamber. In contrast, the upper polycarbonate end cap was machined to a ‘raindrop’ shape in order to accommodate safe loading and unloading by securing the plate to the frame via a rope. However, at the top a flat plate was found to not function with the laser tracking system, which will be discussed later on. In short, the laser beam must travel through an angled surface to scatter light reflection, that otherwise results in loss of data. This was accomplish by water jetting a 102mm (4in) hole in the flat ‘raindrop’ plate to fit a 4in white PVC pipe section offset with a 10 degree angled top. Then a flat piece of abrasive resistant polycarbonate was secured on top of the angled surface. This three piece design was then air sealed with PVC pipe cement and silicon pipe sealant to eliminate leaks from machining imperfections. With this the laser can be adjusted to pass through the angled piece, eliminating reflection interference. Lastly, the flat polycarbonate section was machined for six, #66 through holes that allowed for stripped 20 gauge solid copper servo wire to pass through. The wire was dipped in a vacuum wax sealant, passed through the opening, and attached to servo connectors. This allows the servo motors used in the design to be controlled from outside the pipe while minimalizing leakage.

These motors are used for a release mechanism at the top of the tower. Since the objects are being dropped, they must then be caught. Therefore, a catching mechanism was implemented at the base of the tower. The catching mechanism consists of a pillow like object, consisting of a fabric shield filled with polystyrene beads. The amount of polystyrene beads needed was calculated based on the weight and size of the largest drop item. The coefficient of restitution, Eq. (3), was calculated using the initial and final velocities of the items. Based on this particular scenario, Eq. (3) can be reduced to Eq. (4), because the objects final velocity as well as the polystyrene bead initial velocity is zero.

e=vb−vaua−ub

(3) e=vbua

(4) The velocities were found using Eq. (5) & Eq. (6), listed below.

ua=g∗t (5 ) vb=

mouamo+mc

(6)

The model shows a linear relationship between the amount of beads needed and the weight of the object being dropped. However, the exact amount of beads required is extraneous. Due to a clamp around the pipe near the base, the beads must fill to at least this height in order to not obstruct the field of view of the object’s contact point. This is important because clear visibility of the entire drop was a major customer requirement. The actual height of beads required to not harm the tower is significantly less than this level.

Project P14651


Release MechanismTo achieve the important task of dropping objects inside the microgravity environment, some major issues

needed to be addressed. Firstly, the drop tower had to be able to release two objects with zero horizontal velocity, simultaneously. The dropping mechanism also was required to be removable and adaptable. Lastly, the object dropping method must be able to automatically center the objects in a consistent position.

To achieve these requirements, an object dropping device was devised. This device is dubbed the “Release Mechanism”. The release mechanism works through the use of servo actuated doors. A servo is a device that can convert an electronic signal into a mechanical rotational position. A concept sketch of this is shown below in Fig. 1. Two servos (A0 and A1) are used in combination with a 3:1 gear ratio to open and close the release mechanism doors, thus allowing for objects to be dropped. The release mechanism can support a maximum object weight of up to 0.9kg or 2lbs. The 3:1 gear ratio converts torque into velocity, giving the doors sufficient rotational velocity to drop two objects simultaneously and with zero horizontal velocity on the objects.

Figure 1: Release Mechanism Conceptual Diagram

The doors are made out of lightweight 1.6mm (~1/16”) polycarbonate to minimize rotational inertia. To auto center the objects, the doors are equipped with removable geometry. The geometry is made out of 3.2mm (~1/8”) thick black polycarbonate and is attached to the doors via Velcro. The doors are connected to the gear shown in Fig. 1 by a 4.8mm (~3/16”) shaft. The shaft is connected to door brackets which are held in placed by a tapped hole into the shaft and screws. Also, the symmetry of the doors allows for two objects to be dropped simultaneously.

The release mechanism is encased on four sides by a 6.4mm (~1/4”) polycarbonate box. The box is screwed together with #2-56 cap head bolts. The mechanism is held in position at the top of the tower by #2-56 cap head screws to a fixed ledge. This ledge is sandwiched between a union fitting and the PVC tube. This allows for the mechanism to be taken out of the tower and to be easily adaptable if changes needed to be made to it.

The servos each require power, ground, and signal input. These are supplied by an Arduino UNO microcontroller and a SainSmart Sensor Shield. The release mechanism’s circuit diagram is shown in Fig. 2 below. The SainSmart Sensor Shield is placed on top of the Arduino and fits into its pins. However, since the Arduino UNO has a current limit of 40mA per I/O port and the servos draw much more current than that, some modifications needed to be made to the sensor shield. To avoid causing severe damage to the ATmega328 Arduino chip, the 5V pin that supplies 5V to the sensor shield from the Arduino was cut. This prevents the ATmega328 chip located on the Arduino from being able to source power for the servos. The chip only provides the PWM (Pulse Width Modulation) signal, which is low current and essential to controlling the servos. To power the servos a LM7805 external 5V regulator was used to produce 5V and was connected to the 5V pin on the sensor board. The LM7805 can source up to 1A. This forces the servos to draw power through the off-board 5V regulator and not through the 5V regulator onboard the Arduino. To open and close the doors via human input, a toggle switch was used. This switch is connected to 5v, ground, and pin 4 of the Arduino. The Arduino is programmed to open the doors via servos when pin 4 is LOW and close them when pin 4 is HIGH. Lastly, the servos were connected to the Arduino via 15ft of servo cable. The cable was hidden along the inside the drop tower frame for aesthetics and safety.



Figure 2: Diagram of the Release Mechanism Control Circuit. (Black = ground, Red = 5V, and White = Signal)

Vacuum PumpOne of the main goals of the microgravity drop tower is to be able to calculate acceleration due to gravity as

accurately as possible. In order to perform such a task simply, it is necessary to isolate the forces that are acting on the falling object. In regular atmospheric conditions there are two major forces that act on the falling object: air resistance and the force of gravity. To eliminate the air resistance, it is vital to evacuate all of the particles from the control volume so that freefall is uninterrupted by the collisions with air molecules. To accomplish this task, a pump subsystem was designed that consisted of a 2-stage rotary vacuum pump, pipe fittings to create a connection from the main pipe to the pump, a vacuum gauge to read the pressure values and lever ball valves to control air flow.

During the design stage it was necessary to calculate and predict the flow characteristics of the system, in order to determine what components were necessary to meet the engineering requirements and still fit in budget. The conductance calculation method, shown in Eq. (7-11), is used to determine the time that it would take a pump with a specific pump speed to evacuate the system to a certain base pressure. This method accounts for different flow regimes, inherent pipe loss, and the dimensions of the integrated system layout.

CV=F1ṖD4

L(7)

CT=F1ṖD4

L+F2Ṗ

D3

L(8)

CM=F3ṖD3

L(9)

1SEff

= 1SP

+ 1Cn

+ 1Cn−1

+…+ 1C2

+ 1C1

(10)

t=V c

SEff−Vln( P0

P1)+ V c

SEff−Tln ( P1

P2)+ V c

SEff−Mln( P2

P3)(11)

One of the most critical components was the vacuum pump. Based on variety and cost, depending on their performance, it was necessary to find a pump that best fit the project’s needs. It was required that the pump have sufficient speed to evacuate the tower in a timely matter while still reaching a pressure low enough to show any two objects falling at the same rate. The pump chosen was a VP6D CPS vacuum pump, with a speed of 176.98 lpm (~6.25 cfm). The theoretical results indicated that the system could be brought down from atmospheric pressure to 0.002 kPa (15 microns) in about 7-8 minutes. Unfortunately, these results do not consider potential leaks or efficiency of the vacuum pump. Leaks were expected because many components, such as the lever ball valves and couplings were not vacuum rated due to cost restrictions. To help minimize this risk, PTFE tape was used between all fittings and advice and support was taken from the technicians in the Micro-Engineering lab at RIT.

Data Collection & AnalysisOnce the drop tower is able to effectively and consistently drop objects with the designed release mechanism, it

then becomes necessary to track the object’s descent and save this information in a useful manner. Based on the customer’s requirements, all of the important output data needed to make various calculations must be displayed accurately, intuitively and aesthetically. This data should then be available to determine the value of standard gravity within 1% error and also be used to demonstrate drag versus air pressure.

In order to accomplish these goals a state of the art laser distance sensor was chosen. The sensor chosen is a MICRO-EPSILON ILR 1030-8, laser class 2. This particular laser is designed for applications involving moving objects at distances up to at least 2.5 meters depending on object color, being able to track further for lighter colored objects. The sensor comes with an optional 15 ft. data/power cable perfect for running down the length of the tower, similar to the servo wires. The sensor outputs 4-20mA current which can be linearly converted to displacement once the minimum and maximum distances are set on the laser with the press of a button. It was decided that since this application is for educational and research purposes that the data could be displayed through LabVIEW. In order for LabVIEW to acquire the data the signal must be measured in voltage and transferred through a DAQ device. Figure 3 below shows the electrical setup of the laser, DAQ and power supply assembly.

Using the appropriate LabVIEW tools, the object’s position versus time data is displayed graphically. Also, the approximation of standard gravity is displayed by calculating its value using equation Eq. (12) and Eq. (13) for

Project P14651


vacuum and non-vacuum conditions, respectively. Note that in non-vacuum conditions additional variables must be known about the object as well as the air density.

g=2 xt 2 (12)

g=2mo

ρCD A f t2∗invcosh(e

xρCD A2mo )

2

(13)

All other calculations such as determining drag vs pressure can be done by saving the data from each run and copying it to a program better suited for this type of analysis, such as Excel or MATLAB.

Figure 3: Data Collection Assembly Schematic

Structural FrameThe initial scope of the design was to aim for the tallest structure possible, to achieve the greatest duration of

fall. Thoughts of a multi-story tower stretching the height of a staircase were envisioned early on. As the project progressed, changes in budget, customer requirements and location approval resulted in the need for the tower to fit within the project customer’s laboratory. Additional constraints of mobility and ease of transport also needed to be factored in. From this arose the need for a structure that could stand freely, support the tower components, and be maneuvered with ease.

Considering these factors, a unique tower support frame was developed. The frame was designed to get the maximum drop height and therefore, the most drop time possible. Factors that affected this were the ceiling height and the laser to object distance. The laser has a sensing range beginning at 0.2 meters, so the object must be at least this far away when at rest for the laser to work properly. The tower frame also had to be designed so that enough clearance was allowed at the top and bottom to remove the end caps in order to insert or remove objects. The tower structure was also required to fit through a standard doorway.

From these constraints the tower support frame was designed and built. Maximum width for tower support was achieved that still allowed entry through any doorway. A swing out leg was added to each side of the base, attached by simple brackets that could help to stabilize the tower from tipping and vibrations, if deemed necessary. To level the tower, threaded leveling mounts were added to the bottom four corners of the structure, as well as to the swing out legs. Placing levels on the pipe also contributed to leveling the tower as accurately as possible. The structure was designed with an off-center upper structure, to ease backwards tipping and rolling. Wheels were added to the back of the structure, allowing for single-person moving operations. When the leveling mounts are adjusted, these wheels are lifted from the ground. At the top, the frame extends backwards creating a mounting point for the laser, stopping point for the frame, and a resting location for when the tower is tilted downwards and placed horizontally on the floor. The tube itself had to be attached to the frame, and this was accomplished using purchased riser clamps. As these clamp’s bolt locations would cause the union fittings to interfere with the structure if placed directly on, offset brackets were created to add the appropriate space. Plates and brackets were designed to allow for the laser mounting, as well as x, y and z laser adjustment. Lastly, sheet metal coverings over the frame at the bottom of the tower allowed for placement of the pump as well as any additional components the operator may be using.



RESULTS AND DISCUSSIONS

Vacuum Chamber & Energy Dissipation SystemGiven the wall thickness and material properties of the main PVC pipe, critical external pressure was calculated

from Eq. (1). Given that the most extreme operating condition, when the outside pressure would be one atmosphere because of an internal vacuum, the critical pressure must exceed 99.28 kPa (~14.7 psi) with a reliable factor of safety. The critical pressure calculated is approximately 534.34 kPa (~77.5psi) resulting in a factor of safety of approximately five. Hence, the pipe will not implode under even the most extreme operating conditions.

The catching mechanism also needed to be tested for energy dissipation. The catching mechanism needed to be able to absorb the energy from the falling objects, without damaging the base or sidewalls of the chamber. If the coefficient of restitution is too high then the object would not stop sufficiently and hit the bottom of the chamber, but if it is too low then the object could bounce and hit the sides of the pipe. Based on theory, the appropriate coefficient of restitution would be approximately 0.71 to produce a nice, soft landing. This value signifies that 71% of the kinetic energy that the falling object has is absorbed by the catching mechanism. Utilizing Eq. (3-6), the actual coefficient of restitution matches the theory at approximately 0.71 for both atmospheric and near vacuum conditions.

Release MechanismTo test the accuracy of the release mechanism, a high speed camera was set up to capture the descent of two

objects side by side. Playback of the video was used to determine if there was any horizontal motion of either object or if one object reached the ground before the other. Multiple tests were done all resulting in zero horizontal motion and an average of 0.05 milliseconds difference in drop time. This proved that both doors were opening simultaneously and at a sufficient rate to not interfere with either object. Also, it was concluded that the geometry of the doors was adequate to properly center the objects in their preferred locations.

Vacuum PumpAfter completely assembling and sealing the pump subsystem an initial set of testing begun. Base pressure, time

of evacuation and leak rate all needed to be tested. In the first set of testing, the value for base pressure was 0.0052 kPa (39 microns), the evacuation time was 9 minutes and the leak rate found was found to be approximately 0.0187 kPa/min (140 microns/min) for the first 3 minutes, then 0.2943 kPa (1870 microns/min) thereafter. Leak rate is determined from the slope of pressure vs. time after the pump is deactivated.

These values were unsatisfactory, so further advice was sought out from the technicians in the micro-electronics laboratory at RIT. With their assistance the subsystem was re-sealed and leak tested using helium gas. The gas was sprayed over the system while monitoring any change of pressure caused by the helium entering the controlled volume. After leak proofing the subsystem it was important to figure out what would be the limiting factor that was preventing the system from reaching 0.002 kPa (15 microns) of pressure, therefore the vacuum pump and the pipe fittings were tested individually. The pump by itself was able to reach a base pressure of 0.003 kPa (23 microns). Also, testing the pipe fittings with a superior pump resulted in a base pressure of 0.0017 kPa (13 microns). Based on these results, the pump was declared the limiting factor.

A second set of testing was then performed in which the value for base pressure was 0.0043 kPa (32 microns), the evacuation time was 12 min and the leak rate found was found to be 0.0205 kPa/min (154 microns/min) for the first 10 minutes then 0.0800 kPa/min (600.4 microns/min) thereafter. Comparing the two data sets shows a major change in the leak rate for the pump subsystem, leading to the conclusion that the leak rate had been minimized.

After successfully completing the assembly and testing of the pump subsystem, the full system needed to be tested. Evacuating the entire system takes approximately 15 to 20 minutes to reach a base pressure of 0.0213 kPa (160 microns). This is slightly higher than the ideal result, but this is not an ideal system. Nevertheless, this vacuum level adequately shows, for example, how a feather and ping pong ball fall at almost exactly the same rate and hit the bottom nearly at the same time. This demonstrates that the force of gravity has been sufficiently isolated by removing the force of air resistance.

Data Collection & AnalysisIn order to use the laser distance sensor, the voltage input to the DAQ needed to be scaled to an equivalent

height or distance away from the laser. To accomplish this, the laser was first placed parallel to a table, more than 3 meters in length. The laser has an inherent specification that requires the first point of data collection to be 0.2 meters from the laser. Therefore, this corresponding voltage is fixed. The last point of data collection however, can be adjusted and a distance of 2.946 meters was chosen, corresponding to the largest voltage output. This range is

Project P14651


sufficient to capture the entire drop when the laser is integrated to the tower. Placing objects at set distances from the laser a voltage vs. distance plot is generated. This relationship is then used to linearly convert voltage to distance with minimal error.

Once the laser is set up to measure distance it can be used to measure the vertical descent of any object. The laser is attached to the top of the tower and lined up to the appropriate object’s path. Utilizing LabVIEW, the distance verses time of the drop is collected and plotted for the user to see. The user also has the ability to choose the portion of collected data that represents the objects descent and remove any data from when the object was stationary, both before and after its descent. Within Labview this chosen data range is then used to calculate acceleration due to gravity. The user can select the type of atmosphere the object was dropped in and LabVIEW will calculate appropriately using either Eq. (12) or Eq. (13). The user can also select to save this data to a text file, which can then be imported to either Excel or Matlab to perform any further analysis.

The most important requirement that the laser system must satisfy was to calculate standard gravity within 1 percent error. After many trial runs this was consistently achievable by collecting approximately 100 samples per second, or approximately 70 data points throughout the object’s descent. Also, the laser is able to track the entire object descent because of the reliability of the release mechanism. Therefore, the drop tower is able to consistently and accurately calculate standard gravity (9.81 m/s2) from a falling object.

Structural FrameAn important frame requirement for both safety and overall project quality was that the structure would not tip

or fall over from any reasonable side load. This was tested at the most critical point, which is the top of the tower. In order to load items into the tower and adjust the laser the operator must use a ladder to climb to the top and has the ability to apply a force to the tower. As a result of operating the tower numerous times it has been determined that there is no risk of tipping the tower. Additionally, all testing has been completed without the use of the swing-out brackets, which will only add more stability. It was also a concern that the vibration from the pump might affect the tower when placed on the base, but the vibration is minimal enough to not affect the tower or laser position.

CONCLUSIONS AND RECOMMENDATIONS

Design AchievementsOverall, there were many successes over the course of this project as well as with the final result. The vacuum

pump was able to evacuate the chamber to a vacuum level sufficient for the planned experiments. Considering that many of the components used were not vacuum rated, it was remarkable that the pump was able to consistently evacuate the 3 cubic foot chamber to a base pressure of fewer than 0.0267 kPa (200 microns). Even though the tower ended up not being as tall as originally planned, it is sufficiently large to get an enjoyable drop that is visible to a group of people. As long as bright colored objects are used there is no issue seeing the object’s descent within the chamber.

The functionality of the release mechanism was also a major success. Designing a system that could fit inside a 6 inch diameter space while having room for two servo motors as well as two reasonably sized objects presented many challenges. Using the 3:1 gear ratio, the doors open at a sufficient rate to drop both objects at the same time. This needed to work well enough so that the human eye could not detect a significant difference in drop time when testing vacuum environments, because the laser distance sensor only tracks one object and is not comparing data from more than one source in a single drop.

The laser distance sensor is a key component of the drop tower design. This device allows the user to numerically record the object descent for further analysis. The main desired result for the project, besides being able to drop objects in a vacuum chamber was to calculate standard gravity within 1 percent error. Since the laser is able to record approximately 100 samples per second, enough data points are recorded over the deration of the drop to consistently achieve this result.

Two non-technical aspects of the design that affected all design choices throughout the project were to create an aesthetically pleasing design with data display and to stay within budget. In order to create a nice looking design, special attention was paid to all intricate details on all components, during both the design and actual construction of the tower. The LabVIEW display was designed to display all relevant information and have intuitive functionality for the user. The design was completed for a total budget of $2,500 by utilizing many free or reduced cost components and minimizing high cost vacuum rated components whenever possible. Overall, the tower’s design can be described as simple and sleek while still retaining the ‘wow factor’.



Design ImperfectionsEven though the majority of this design process has been a success, a few undesirable results arose along the

way. Due to budget and time constraints compromises had to be made when designing certain aspects of the drop tower. The most obvious thing that is noticed when running the tower is that there is a very long cycle time. It takes up to 15 minutes to evacuate the chamber to base pressure. This is twice as much time as what was predicted, primarily due to leaks. Fortunately, when presenting the design to an audience, the operators can use this time to discuss the theory behind the concepts being demonstrating. Another aspect of the project, related to leaks, is the requirement to measuring drag vs. pressure. It is simple to hold both atmospheric and base pressure, but maintaining any other pressure is a challenge. Leaving the pump on will steadily decreases the chamber pressure and on the other hand turning the pump off causes the pressure to rise. To deal with this the operator must open the release mechanism doors and record the current pressure from the vacuum gauge at the same time. However, this can only be done between 0 and 3.333 kPa (0 to 25,000 microns) because the vacuum gauge does not display values outside this range.

All in all the laser distance sensor works very well at tracking an object if it is properly lined up with the object’s fall path and does not lose sight of the object. Even though the laser can be adjusted in all axes, it can be difficult to initially set up. Also, any time the tower is moved or adjusted in any way the laser must be repositioned. However, it has proved to be consistent once it has been set up. Another aspect that is difficult to control on the laser is the correlation between the first point of data collected and the time of release. Since the laser has a 10 millisecond response time, the first data point can inherently be 10 milliseconds after the actual zero point of the object’s position. This can slightly add to the error in the gravity calculation, but will always contribute less than 1%; satisfying the requirement.

Future RecommendationsSeveral modifications could be made to the existing drop tower in the future if the opportunity arises. From day

one it was envisioned that the tower would be of a larger scale. This could be accomplished by extending the length of the pipe by using the existing pipe union fittings. However, this would potentially require a modified support structure. Also, isolation valves could be introduced along the length of the pipe, similar to NASA’s Zero-G Facility. This would decrease cycle time significantly by holding a vacuum in a portion of the chamber, especially if the pipe length was increased. Pipe diameter could also be increased, but even for the same length it would require a complete tower redesign.

Currently the release mechanism utilizes replaceable Velcro attached door profiles. The release mechanism could be even more accurate if the doors were 3D printed or even CNC machined. Also, the method in which the servo motors are operated could be improved. Rather than running the wires through the top polycarbonate plate and compromising the vacuum chamber, an inductance method could be used as a non-contact switch through the PCV pipe wall. This would also eliminate the extra wire in-between the doors and the cap, which can be inconvenient during the object loading process.

During the design and component selection process many components were chosen for cost motives rather than guaranteed functionality. If a budget allowed for it, it would be worthwhile to replace the non-vacuum rated components, like the lever ball valves and couplings, for their vacuum rated equivalents. Also, if a higher vacuum becomes necessary for certain application, then a turbo-molecular pump could be added as a secondary pump, operating in series with the existing 2-stage rotary pump. Lastly, it may be beneficial to upgrade the existing pressure gauge with a more sophisticated model, or set of models, that can display any value between atmospheric pressure and high vacuum.

As discussed previously, the laser can be challenging to level and align with an object’s drop path. To do so the operator must manually make adjustments from the top of the tower while standing on a ladder. This could be resolved by implementing a precision laser leveling system that could be program controlled. By giving the operator 3-axis control of the laser from the computer, alignment would be a much simpler and less strenuous task.

One main concern with the tower structure is its incredible height. This makes moving the tower in and out of rooms, as well as up and down stairs, a challenge. A future retrofit could be to hinge the structure in the center, to allow for either folding the structure inwards or creating two separate halves. Either option would allow the structure to fit within an elevator, easing movement to other floors. Doing so would also allow the tower to be much more maneuverable, into and out of rooms, buildings and doorways.

The drop tower was designed to fit in the project customer’s laboratory because it was not originally considered a valuable addition to more public locations throughout RIT. This is due to the fact that the tower is not operational without the presence of the proper trained personnel. However, if there was some way to automate the tower’s basic function of dropping objects, this restriction would be reconsidered. This concept became known, amongst those

Project P14651


involved, as the infamous ‘continuous lift mechanism’. A device would need to be designed that could continuously take an object from the bottom of the tower back to the top to be dropped, creating an infinite or semi-infinite cycle of item drops without opening the chamber. It would be ideal if any person could approach the drop tower and initiate this process with the push of a button. This would also reduce cycle time significantly because the vacuum chamber would only require a single evacuation for a single set of objects.

REFERENCE

[1] Maier, S., 2008, "Zero Gravity Research Facility," NASA, http://facilities.grc.nasa.gov/zerog/.

ACKNOWLEDGEMENTS

Primary SupportDr. Satish Kandlikar – Primary Customer & SponsorCharlie Tabb – Team Guide

Vacuum System SupportBruce Tolleson – Microfab Technician at Semiconductor & Microsystems Fabrication Laboratory at RIT

Machining and Welding SupportDave Hathaway – General machining assistanceRobert Kraynik – Welding, machining assistance and design adviceJan Maneti – Welding and machining assistanceJohn Bonzo – Waterjetting

Labview and DAQ SupportProf. John Wellin – RIT College of Engineering, FacultyDr. Mark Kempski – RIT College of Engineering, Faculty

Computer and Software SupportBill Finch – RIT College of Engineering, IT

Arduino and Servo SupportDr. Wayne Walter – RIT College of Engineering, Faculty

General SupportDr. Harvey Palmer – RIT College of Engineering, DeanDr. Risa Robinson – RIT College of Engineering, FacultyDr. Michael Schrlau – RIT College of Engineering, Faculty



Project P14651

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