Proceedings of the Multi-Disciplinary Senior Design Conferencepage 1

Proceedings of the Multi-Disciplinary Senior Design ConferencePage 1

Project Number: P11566

Proceedings of the Multi-Disciplinary Senior Design ConferencePage 1

Design and Analysis of low cost solid state, eddy current vibration damper

Tiffany Heyd
Mechanical Engineering
Rochester Institute of Technology / Benjamin Hensel
Mechanical Engineering
Rochester Institute of Technology
Jacob Norris
Mechanical Engineering
Rochester Institute of Technology / Thomas Sciotto
Industrial and Systems Engineering
Rochester Institute of Technology

Abstract

Proceedings of the Multi-Disciplinary Senior Design ConferencePage 1

[Abstract]

Nomenclature

Explanation of equations used

Explanation of zeta calculations

Flexure

Stick slip

Whiskers

Stewart Platform

Project Background & Customer Needs

ITT developed a solid-state eddy current vibration damping system for space applications several years ago. They developed a prototype that exceeded their analytical model by a factor of two and were unsure of why their model did not match the calculations made by finite element analysis.

The damping system is meant to mount as one of six trusses on a Stewart platform that mounts between the bus and payload on a satellite in low earth orbit. The positioning system of the satellite is constantly moving to new orientations is order for satellite images to be taken. This induced vibration needs to be damped out before the sensitive instrumentation on the payload can take the images required.

ITT approached the RIT Senior Design program with the need for a damper whose behavior can adequately be modeled by a finite element analysis software package, like Comsol. Key deliverables to a successful project would include a functional prototype developed with space applications in mind that when tested would match the predicted performance from a model of the system. The prototype was to be delivered with appropriate test apparatuses and documentation to allow in future development.

It was hoped that after the completion of this project there would be a greater understanding of the effects that go into designing, fabricating and modeling the behavior of an eddy current damping system. From the onset it was apparent trade-offs would need to be made between the customer’s needs and actual design specs due to budgeting and scheduling constraints.

The budget was $2000 and there were only about 25 weeks to complete the design, construction and testing of the project. ITT spent five years developing theirs and, probably, more than $2000. It was assumed materials analogous to space appropriate materials could be used for fabrication due to the budget constraint. A decision was made that it was more important to develop a prototype that matches the analytical model than a prototype that strictly follows the Looking at how ITT went about their design was useful in developing design concepts for our team. ITT fixed an array of strong magnets and using a “flexure”, allowed a six finned copper vein to oscillate between the gaps in the magnet array as shown in Figure 1. The magnet array is visible in Figure 2 and it becomes apparent that a motion control system is necessary to control rotation about the center of conductive vein (shown in Figure 3).

Description E Senior Design public ITTPrototype4 JPG

Figure 1: Conductive copper vein inserted in magnet array (Photo courtesy ITT).

Description E Senior Design public ITTPrototype5 JPG

Figure 2: Magnet array (Photo courtesy ITT)

In discussions with ITT there were certain needs expressed that would need to be addressed in order to call this project successful. Chief among them was delivery of a functional prototype, the performance of which could accurately be reflected with a computer model. In order to avoid the hassle of modeling “stick slip,” there should be no rubbing of moving parts in the prototype. Also discussed was the need for the prototype to be designed to parameters appropriate for delivery and use is outer space. It would need to be less than 1 kg (2.2 lbs.) and be able to operate in temperatures from -40°C to 80°C (-40°F to 176°F). Operation needs to occur in a vacuum and because of this no tin, zinc or organic volatiles can be used due to formation of “whiskers” and potential damage from off gassing. The damping system will be one of six struts in a Stewart platform that will need to dampen the vibration of a payload of 60 kg (132 lbs.). Currently the system mounts between the bus and payload using a ball joint system, which it is not within our scope to redesign.

Description E Senior Design public ITTPrototype2 JPG

Figure 3: Motion control “flexure” on fully assembled damper (Photo courtesy ITT)

Functionally speaking, the damper would require between ½” and 1” range of motion (1.27cm to 2.54cm) at 1 Hz of vibration with spikes of 1 inch amplitude at 5 Hz. This was considered an extreme case scenario and it was to be assumed that the steady state vibration that needed to be damped would be 100 Hz at .005” of amplitude (127 µm). The resonant frequency of the total system could be assumed to be between 5-20 Hz. Another critical design constraint is seeing a damping constant (zeta) of between 10-25%. This was expressed as an optimal resistance measure to allow vibrations to be damped out quickly, but without threatening the wellbeing of any other parts of the system.

Because this system will be mounted on a satellite in space the system needs to be robust enough to function, without failure, for 10 years and because of this extreme environment everything would require a factor of safety of two. Also, due to the strength of the magnets in use, it was also expressed that magnetic shielding would be required to protect any sensitive instrumentation from the potential effects of the strong magnetic field.

Design Methodology

To meet the needs of the customer, a series of engineering specifications were devised, that is met would guarantee success in certain aspects of the project. These include:

Operating temperature: -40°C-80°C
Operating pressure envelope: 0 Pa
Weight of system: <1kg
Damping coefficient zeta:10-25%
Lifecycle: 158,000 cycles
Magnetic Shielding: 20 mG (at 6”)
Clearance of flexure movement: +/- .25-.5”
Factor of safety: 2
Load to damp: 10 kg
Frequency of maneuver: 1 Hz
Frequency of stationary: 100 Hz

Several options were investigated on how to damp vibrations induced in this situation. Understanding the nature of the vibration and the environment it is operating in are critical to making a choice between different damping options. Because the information about the nature of the vibration could be assumed to be one of a few standard cases and the environment of extraterrestrial space was defined by the customer, it was not necessary to design for multiple environments and situations.

The methods explored of mitigating the disturbance included eddy current damping, hydraulic damping, friction damping, elastic damping and fluid damping. Because of the harsh temperature fluctuations of space, most fluid and hydraulic damping systems would be rendered useless by boiling off their liquids. The friction system violated the “no stick slip rule” and the elastomer concept seemed to be difficult to make function in an environment that would damage most natural rubbers. The eddy current system seemed to be the best concept to explore.

The remaining considerations to explore would be both protecting the system from the environment it was operating in (making sure no foreign matter ended up in critical moving parts) and ensuring that the damping system didn’t have any deleterious effects on the function of the satellite payload or bus.
From this three concepts were explored in an attempt to design the optimal system that could be delivered to the customer in a short time frame.

The first concept involved a rod shaped strong magnet moving through a coiled conductor that was help fixed in place by a protective shroud. The coiling effect was explored because of the amplifying effect of the solenoid on eddy currents. Figure 4 below illustrates the concept simply, pointing out the magnet (in green) moving freely between the copper solenoid (orange spiral) and constrained at the ends by some method (preferably a flexure for all systems).

Figure 4: Concept based on magnet moving through fixed solenoid

The second concept explored again involved a rod shaped magnet moving freely within a conductor, but this time the conductor was a tube. Again the system would be incased in a protective shroud and motion would be constrained. This concept is illustrated below in Figure 5, with the conductor in orange and magnet in green.

Figure 5: Concept based on magnet moving through solid conductive tube.

The third concept explored had a fixed array of magnets that had a conductor moving between them. Again, there is a shroud surrounding the system to protect it and has a motion control system keeping it together.

We ended up choosing the third concept by weighing the strengths and weaknesses associated with modeling, fabricating, potential damping effect and weight of, concept development, amounts of magnetic leakage of and potential customer satisfaction of each system. Reasons why the third concept seemed most appropriate included its low potential level of magnetic leakage, similarity to the initial prototype the customer made and the potential to make this into the lightest system.

Feasibility analysis/ drawings
Risk analysis
Fabrication
Fabrication (insert pictures perhaps?)
Casing - (10 hours)

Step one was to machine the casing. The casing, as decided for ease of assembly, disassembly, and testing was to be broken into layers. Thus three 2” casing layers, one 1.5” casing layer, and 3 .5” casing layers were called for. Two of the three .5” casing layers are used for clamping the flexure which will be discussed in the assembly section. The other layers were used for spacing and supporting the magnets and iron fixtures.

From the design, the casing was decided to be a 4” by 4” Aluminum tube. This was ordered, cut into layers, and then milled to assure accuracy. After the layers were cut to size, thru-holes were placed in order to fix the iron fixtures as well as the angle corners to the sides.

The angle corners were fabricated similarly as the layers. These four parts were used to strengthen the casing layers as well as assemble the layers into one square tube. The aluminum corners were cut into four 9” pieces then milled to assure accuracy. After being cut, thru-holes were placed in the sides in order to line up with the respective thru-holes in the layers.

Flexure - (x hours)

Due to the intricate design, as well as the thin characteristics of the stainless steel pieces, the flexure was cut via plasma laser. The company (insert company name here) assisted the group with the skill and equipment.

Iron Magnetic Fixture - (28 hours)

The iron magnetic fixtures are cast gray iron. Although the fabrication was messy, the magnetic properties are utilized in the system. The iron first had to be milled to size and have parallel sides. After assuring this, each piece was cut to size and milled for accurate size and angles. The sockets for the magnet were milled to size assuring that the back of the socket was flat in order to take advantage of a closed magnetic circuit in the system. After the magnet sockets were milled out, a threaded hole was placed in the back of the iron magnetic fixture in order to secure it to the side of the wall of the aluminum casing. The thread chosen was a #6-32 thread size.

Copper Conductor - (7 hours)

As this is a sub assembly of the total assembly there were multiple pieces fabricated. The four conductor pieces were cut to a 7” size. The, 5/16” diameter with 24 threads per inch, threaded rod was ordered and left in an ‘as is’ state. The last part of the copper conductor sub assembly is the copper fixture. These are four pieces, to be evenly spaced on the threaded rod, were milled to attain the high accuracy and parallelism that is needed for this specific part. The center was threaded for a 5/16”-24 thread

The copper assembly makes use of the threaded rods and JB Weld. Once the four copper fixtures were threaded and spaced 2” apart on the rod, JB Weld was applied to the copper in order to fix the copper conductor to the copper fixtures. This was done over a course of four days and applied to the four conductor plates.

Assembly - (7 hours)

First, the casing was assembled. The angle corners were attached to the stacked aluminum tubing layers by #6-32 screws and nuts. Not all holes aligned between the corner angles and the tubing layers, so each layer is individually altered to assure the tubing holes match with the corner angle holes. For assembly and reassembly purposes, each layer and corner need to match the original setup.

The magnets were then inserted into the magnet sockets of the iron magnet fixtures. Due to the magnetic properties of the cast gray iron, the magnets placed in with relative ease. Magnets are also placed in alternating order. Opposite poles are facing inward, towards the iron, in each of the iron magnetic fixtures.

With the casing assembled and the magnets in their fixtures, the next step was to place the iron magnetic fixtures into the casing. First layer placed in was the middle one. After the first two were placed opposite each other, the last two of the middle layer were placed utilizing .25” thick shims. The two outside layers were assembled in a similar manner. The iron magnetic fixtures were fixed to the aluminum casing by #6-32 screws.

The copper assembly was slid into place between magnets. A lock-nut was placed on each end of the threaded rod to where the flexures would touch and stop when slid into place. The two end aluminum layers were screwed in to clamp down on the flexures. The flexures were finally secured with an additional lock-nut at each end, tightly screwed against the flexures.

Changes to design while fabricating (put in this section?)

Epoxy vs. solder

No epoxy to the magnets

Larger threaded rod

Testing plan

Modeling plan

Results and discussion

[Results of tests and discussion of similarities to model. Attempt to explain modeling difference]

Conclusions and recommendations

[How well did we meet customer needs, ways we could have fulfilled them through substitution of materials (use model to justify), and recommendations to future groups]

References

[refereces in order of appearance]

Acknowledgments

Our work could not have been completed without the assistance of the following people: Dr. Alan R for his service as our guide and mentor. His seemingly infinite wisdom allowed us to do so much more than we could have ourselves. Thanks to Mr. Phil Vallone of ITT in Rochester, NY, for representing his company and their resource contributions to the successful completion of this project. Thanks to Rob Kraynik of the RIT machine shop for his assistance, guidance and patience in the fabrication of our system. Thanks to Dr. Marca Lam and Dr. Linda Barton for their individual roles in assisting with the testing of our system. We would also like to recognize the faculty and students who attended our design reviews, including Dr. Mark Kempski and Dr. P. Venkataraman, for their crucial feedback and criticism during the formative stage in the design process. Special thanks also go to Mark Smith and the rest of the staff in the RIT senior design center; the facilities were a great place to work and the staff was accommodating.

Proceedings of the Multi-Disciplinary Senior Design ConferencePage 1

engineering specifications.