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Multidisciplinary Senior DesignKate Gleason College of Engineering

Rochester Institute of TechnologyRochester, New York 14623

Project Number: P16228

MAGNETICALLY-LEVITATED PROPELLERBernie Garcia

Mechanical EngineeringElijah Sensenig

Electrical Engineering

Joseph BernardiniMechanical Engineering

Cheng LinElectrical Engineering

Zachary LouisonMechanical Engineering

Michael PurcellElectrical Engineering

Abstract

Magnetic levitation is a phenomenon that has gained popularity and attention for its many advantages when integrated into electromechanical systems. A variety of applications across different industries make use of magnetic levitation and suspension technologies. For instance, magnetic bearings used in machinery eliminate the need for any physical contact between moving components; thus reducing mechanical wear while increasing efficiency. Perhaps a more popular example: Maglev trains integrate a magnetic suspension and propulsion system to lift and push the train down the “tracks” at very high speeds and with minimal energy loss.

Introduction

The goal of this project was to integrate magnetic technology into a propeller-based application; specifically, a submersible remote-operated vehicle (ROV). For ROV’s that operate at very low depths, a big issue is shaft seal failure. Mechanical wear from the rotating shaft coupled with the extreme water pressure at low depths causes the seals to fail and allow water ingress. This system aimed to minimize the chance of failure by eliminating the need for a rotating shaft and consequently, shaft seal. We can accomplish this by utilizing magnetic technology. For our concept, the single moving part (propeller) would be completely isolated from the system controlling it, while also being completely detached from a stationary shaft. A combination of permanent magnets and electromagnets would hold the propeller in a levitated position. Sensors and feedback control would monitor and adjust the propellers position, while a stator (mounted inside the waterproof housing) would act on the rotor (mounted on the propeller’s outer diameter) to rotate the propeller. With this design, the system would have the desired electro-mechanical isolation, as well as magnetic levitation. The intricacies of this design will be made clearer in the body of this paper.

Copyright © 2016 Rochester Institute of Technology

Multi-Disciplinary Senior Design - P16228

Design Process

The project design process began with several system sketches; each of which were evaluated based on customer and engineering requirements. Strengths from several ideas were combined into one hybrid design, and development commenced. As this was a first-generation project, there were no specific baseline requirements. The goal was simply to create a magnetic system that could, theoretically, perform at or above the level of traditional systems. This required us to obtain a set of benchmark values from these competitor propellers. Since the end-game of our system was to be part of a submersible ROV, several commercial ROV’s were chosen for comparison.

Figure 1. Exploded view of entire system (motor and enclosure)

At the start of the project, we anticipated having a full waterproof propeller prototype to test and analyze by the end of the year. Due to time constraints and a variety of unforeseen obstacles, the waterproofing and consequently, the in-water testing were trimmed from the agenda. The focus turned towards just getting the propeller system to work in air; a proof of concept.

Propeller Design

The propeller design began by using the ROV benchmarking values and a few estimations/assumptions to come up with some basic requirements. Thrust, power, and flow calculations were done to get an idea of how the final product should be able to perform.

Figure 2. Sample thrust calculations



Additionally, various propeller material types were compared to see which would be most appropriate for this particular application. Several composite-based propeller designs were drawn up in Solid Works. A total of four concepts were evaluated: traditional 5-blade, 4-blade, and 3-blade, as well as an inverted 4-blade.

Figure 3. Propeller concepts (left, center) and final model (right)

In the end, the inverted model was chosen due to our inverted motor concept. The size was determined based on the dimensions of our chosen motor. Blade pitch and other geometry was finalized and an inner ring opening was included for the anticipated magnetic bearings and shaft. Finally, the propeller was 3D printed with ABS plastic.

Enclosure Design

Before making the decision to direct focus away from the submersible requirement, a waterproof enclosure concept was designed. This enclosure would house all electrical components, including the motor’s stator. As shown in the figures below, the main enclosure is constructed out of four parts: two for the inside and two for the outside. This allows for simple installation and repair while limiting the total number of parts; thus limiting the potential failure (leakage) points. Retaining rings would also be used in order to provide a better seal along the seams and hold the center shaft and axial stability components.

Figure 4. Enclosure design concept

Motor Design

Choosing the motor design was based off of our initial system design concept. Considering the uniqueness of this design, there was concern that we would either have to build our own custom motor, or revise our system design concept. With already-tight time constraints, neither option was ideal. Fortunately, we avoided both by finding a motor model that seemed to fit the system design concept quite well. The motor, ULT-165-A-12-B-H-000, was a product of Applimotion. Besides having a unique geometry that matched our concept, it also met our torque requirements and ran on 24 volts; the voltage we were already planning on using. This allowed us to choose an off-the-shelf model, saving time and money without having to change our design. The stator was made up of 27 solenoids and the rotor was a 36-pole sintered magnetic ring.



Figure 5. Applimotion ULT-165-A-12-B-H-000 motor rotor (left) and stator (right)

Having selected a motor, a motor controller was now needed. Much like selecting the motor, we had the option of making our own controller or purchasing one. It was decided that purchasing a controller would save time and allow us to have exactly what we needed. The controller chosen was the Castle Creations Phoenix Edge 50. This controller was chosen as it worked at the same 24-volt line powering the motor, along with allowing a large input current, and control from the microcontroller.

PCB Layout

For the levitation and control system, both the solenoid controller boards and the Hall Effect sensor boards needed to be designed. The solenoid controller board required a PWM input from the microcontroller to be translated into a 24-volt analog signal to the solenoids. A pull-up circuit was used to take the high PWM input, 5 volts, to the required 24 volts. This pull-up paired with a set of optocouplers allowed the input to be isolated from the rest of the circuitry. This was a key design element as it protected the microcontroller from any high current or voltage coming back off the controller PCB. Test points for each of the outputs along with indicator LED’s were included for troubleshooting. LED’s were also added at each of the six outputs, for both the 5-volt and 24-volt rails. Additional test points before and after the pull-up, as well as at the output allowed for further verification. Flyback diodes were added near each output to prevent any current from coming back into the circuit from the solenoids.

Two main iterations of the design were ordered and tested. The first iteration boards failed; most likely due to user error on assembly. The second iteration boards were larger (to make assembly and soldering easier), and also had capacitors added at the voltage inputs to help filter out any noise in the lines. Additionally, resistors were added at each test point to prevent any accidental shorting when troubleshooting. These second iteration boards were tested and worked as expected.

Figure 6. PCB layout for solenoid board (left) and sensor board (right)



The Hall Effect sensor board was designed after the solenoid controller board and utilized the same precautions as the second iteration solenoid board: capacitor at the input voltage line and resistors at the test points. An indicator LED was added to allow quick power troubleshooting at well. All of the test points allow each of the three Hall Effect sensor pins to be monitored (supply voltage, output, and ground).

Levitation System

The levitation system was made up of two main parts: passive, permanent magnet suspension, and active electromagnet lateral control. Ideally, the passive subsystem would be able to achieve suspension and maintain radial stability. Then the active subsystem would monitor and adjust the lateral position during propeller operation. Unfortunately, the passive elements turned out to be more complex than anticipated. Due to a lack of magnetic familiarity and resources, a trial and error approach was used to find a suitable arrangement of elements.

The idea for the passive levitation was to have a magnetic bearing system. These magnetic bearings would consist of a set of small arc magnets arranged in a ring formation around a plastic cylinder. This cylinder would be mounted on the propeller shaft. A separate set of arc magnets with the same pole orientation would be mounted on the inside of the propeller hub. After several failed mock-ups, it was found that by using arc magnets of two different lengths (shorter on the propeller hub, longer on the bearing), that the arcs naturally want to center. Placing the bearing inside the hub, it snaps and lock into place in the direct center.

Figure 7. Levitation system (right), magnet bearing and its arcs magnets (left)

However, the prototypes used had several imperfections and there technically wasn’t any levitation. We anticipate with a cleaner prototype and the motor running that we can achieve full levitation.

Assuming quasi-stable levitation, the complementary active system was developed. Hall Effect sensors would be used to monitor the location of the propeller by sensing the magnetic material on the end of the hub extension. The signal from these sensors would feed into a microcontroller, which would then send out a reactive signal to a set of electromagnets. These magnets would either push or pull the propeller by acting on magnetic material at the opposite hub extension.



Active Stability Control

The original microcontroller chosen for our control system was the Beaglebone Black. Due to technical issues, a switch was made to the Arduino Mega 2650. This microcontroller provided enough PWM outputs and analog inputs, with each output pin provided 5 volts; sufficient for powering the solenoid controller board. The control begins with the Hall Effect sensors determining the propeller’s location. This is done by setting a threshold value to compare current readings against. Readings above or below this value would result in a PWM signal being sent out to the solenoid controller board. The solenoids (electromagnets) would then push or pull the propeller with a force proportional to the PWM signal. The goal was to implement a PID control for this functionality. This did not happen in time.

Figure 8. Solenoid controller input (yellow) and output (blue), and solenoid inductances

Test Plans (Electrical)

Four test plans for the four electrical subsystems were created: the solenoid controller board, the sensor board, the solenoids, and the motor. Each plan listed all equipment and procedures needed to run the individual test.

For the solenoid controller there were three different testing procedures. First: ensure the board is receiving power. The procedure includes the specific pin outs on the controller board along with the specific LEDs that should be observed. Second: verify proper signal at board output. An arbitrary waveform generator was used in place of a microcontroller in order to reduce the chance for error. Each of the input/output combinations were tested to ensure that all six worked as desired. Third: replace waveform generator with microcontroller and make sure there are no errors. Though there was no program tested, this was a necessary starting point.

For the Hall Effect sensor board, a single LED was used to ensure the board is receiving power. With the power and ground wired, the only other step was to monitor the output of the Hall Effect sensors. A magnet was placed near each sensor and the output voltage observed. The change in output voltage indicated the movement of the magnet. The signal would later be used as feedback for the axial control system.

The solenoids were tested by first measuring their inductance. Using a baseline starting frequency and voltage level, the frequency was changed until the voltage was about half the original value. Using the inductance-frequency relationship, L = 4.57 / f, the inductance was calculated. From this inductance we determined whether or not the solenoid was acceptable to use. A more ideal method to doing this would have been to use a Helmholtz coil as it measures the magnetic field directly.

To ensure proper motor operation, the input/output characteristics needed to be tested. The easiest way to do this was to supply a specific voltage/current at the input and measure the rotation via a dynamometer. If the input matched the output as per the specifications, the motor operation is verified. The next step was to integrate the motor controller and make sure the motor still ran properly.



Results and Discussion

The initial goal of this project was to create a magnetically levitated propeller system that could operate in water. Although the system did not fully reach that goal, we believe that our progress will set up a second generation group for success. With the time and resources provided, the group was able to accomplish the following things:



1. A working prototype of the motor○ the purchased motor was mounted on a mechanical ball bearing and shaft. It was run using an

Arduino software loop via the Mega 2650 and Edge 50 microcontroller○ we were unable to achieve continuous rotation due to discrepancies between the motor controller

and Arduino code 2. Hall Effect sensor operation

○ proper sensor board operation was verified by placing a magnet near each of the sensors and measuring an output signal

3. Solenoid operation○ proper solenoid board and solenoid operation was verified by sending a PWM signal the board

input and observing the now-magnetized solenoids4. Sensor-to-solenoid control

○ a preliminary Arduino program was implemented to connect the Hall Effect sensors to the solenoids. Bringing a magnet close to the sensor would immediately turn on the solenoid. Taking the magnet away would then turn it off

5. Passive magnet bearing prototype○ a magnetic bearing was printed and tested with our levitation concept. We discovered the

relationship between longer and shorter arc magnets that should enable quasi-stable levitation using a more precise and balanced bearing model

Figure 9. Motor test rig with magnetic bearing and propeller stand-in test disk



Conclusions and Recommendations

As is the case with most technical projects, there were a variety of obstacles encountered. Reflecting on these obstacles and on possible ways to overcome them is important for a team and organization to progress and grow.

1. Time Management○ The group worked at a decent, steady pace throughout the project term. Unfortunately, due to the

project complexity and absence of any prior work, as well as the group’s general lack of magnetic competency, that pacing proved inadequate. For this project to have reached all of its goals, much more time would’ve needed to be dedicated. Alternatively, the time that was dedicated should have been used more efficiently. Keeping in mind the members’ need to balance all of their academic priorities, we could have increased our efficiency with a higher level of organization. Having a set list of action items ready before each group meeting would have made the time together much more productive.

2. Obstacle Anticipation and Risk Management○ A document compiling potential project risks and their corresponding resolutions was created at

the beginning of the term and updated as the project evolved. However, this document was never given quite enough attention and thought. As a consequence, many encountered obstacles were either unanticipated or just dealt with inefficiently. Ideally, our risk analysis should have been updated with each step of the design, build, and test process. Many “what-ifs” were exchanged, but not all were documented and addressed. Doing this would have saved time and reduced frustration.

3. Resource Utilization○ The complexity of the project was mostly due to the group’s lack of knowledge or experience with

magnetic theory and technology. The research done was enough to get a basic understanding, but not for actual design implementation. Had we made better use of local resources (professors, businesses, software), many problems could have been resolved much sooner.

4. Transparency and Communication○ A successful team knows what everyone is doing at all times. Individual skills and competencies

are used efficiently, and all outstanding tasks and their resolving actions are known and agreed upon. Although there were seldom any group disagreements on how a problem should be solved, it was not always clear when and by whom they would be solved. Responsibilities were delegated but were often times too vague. Many action items were left untouched due to a lack of communication. For a successful project and team, each of these items should be fixed.

The group is proud of the work that was accomplished for this project, and hopes that it is enough to justify its continuation into another year.

References [1] Improving the Beginner’s PID. Web. <http://brettbeauregard.com/blog/2011/04/improving-the-beginners-pid-

introduction/>.[2] Richmond, Michael. "Solenoids and Magnetic Fields." Solenoids and Magnetic Fields. Rochester Institute of

Technology, n.d. Web. 10 May 2016.[3] Propeller Thrust. NASA.Web. <https://www.grc.nasa.gov/www/k-12/airplane/propth.html>

Acknowledgements

This material is based upon work supported by Boeing. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author and do not necessarily reflect the views of Boeing.


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