Download Nuclear Magnetic Resonance Spectrometer

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Nuclear Magnetic Resonance Spectrometer
Aline Elad, Mengting Nan, Zachary Henderson
Abstract— Our senior design project this year is to design and
build a nuclear magnetic resonance spectrometer (NMR) that is
accessible to high school laboratories so that high school students
can gain access to important technology in the field of biology
and chemistry. A NMR spectrometer works by manipulating
unique spin properties of atomic elements such as hydrogen,
carbon and nitrogen. A NMR spectrometer is capable of
determining the chemical composition and chemical structure of
molecules by measuring the relative intensity of the atomic
elements with the substance. The NMR spectrometer outputs a
graph of relative intensity against parts per million and this data
allows scientists to work out the important details about the
substance. Details such as specific element percentage, element
isotopes, and types of connecting bonds are all available in the
output of an NMR spectrometer.
The field of biology and chemistry advances so dramatically
and relies so much on the data obtained from a NMR
spectrometer that it is critical to introduce it to young, up
incoming scientists who wish to advance our scientific knowledge.
Currently however students lack access to NMR spectrometers
due to the high maintenance and purchase cost, which, as a result
allows only research labs, and universities access to these
powerful machines. Commercial NMR spectrometers can cost
anywhere from $20,000 to beyond $100,000. This cost is
unfathomable for almost all high school districts across the
United States. Thus, there is an open market for introducing
NMR spectrometers to high school and the potential benefits
include giving students more experience that they can use for
applying to colleges and jobs later on in their life.
Our NMR spectrometer will be built with the following goals
in mind: a $500 budget, an easy to use and understandble
interface, and a reliable results comparable to commercial
devices.
I. INTRODUCTION
O
ur objective with this project is to deliver a working
Nuclear Magnetic Resonance spectrometer that is capable of
being used in high school curriculums. Our spectrometer will
follow the standard operation and procedure of current
commercial spectrometers. We foresee our spectrometer
becoming a useful tool in high school curriculums as it will
not only allow students to use an advanced research tool but
will allow them to understand the inner workings of it as well.
NMR spectroscopy is a powerful research tool used for
identifying chemical structures in fields of the physical
sciences.[1] NMR spectroscopy exploits fundamental spin
characteristics of individual atoms in a chemical compound to
determine the composition and structure. [2] Our project is to
build a cost effective, easy to use and reliable NMR
spectrometer that can easily be used in high school
laboratories that are unable to purchase high-end NMR
spectrometers.
Modern physical sciences are increasingly becoming
dependent on the understanding of complex chemical
structures that are increasingly being taught in universities and
high schools around the country. [1] In order for future
students to gain a sense of modern scientific tools and practice
it is necessary to introduce and orient them to NMR
spectroscopy as early as possible. Currently however
commercial NMR spectrometers can cost anywhere between
$25,000 and $100,000. [3] An example of a commercial NMR
spectrometer can be seen in Figure 1. This NMR
spectrometer is different than our spectrometer as it houses the
entire machine in one box and the solution is injected via a
syringe into a capillary test chamber. [4]
Figure 1[5]
Our project aims to provide comparable results of higher
end NMR spectrometers to these schools and helps introduce
this modern scientific tool to an important and expanding
market.
NMR spectroscopy relies on a being able to exploit the spin
properties of certain elements such as hydrogen, carbon 13,
nitrogen, fluorine and phosphorous. [2] These unique elements
naturally have a quantum spin number of ½ which is crucial to
being detected by an NMR spectrometer. [2]
The NMR spectrometer exploits this spin ½ because it
applies a powerful external magnetic field that forces each of
these nuclei into one of two of their spin states, +½ or -½ as
seen in Figure 2. [2]
Figure 2[2]
These spin states can be understood as miniature magnetic
moments that are responding to the larger external magnetic
field as seen in Figure 3.[2]
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field and so based on this information the necessary Larmor
frequency will be around 14MHz – 20 MHz.
When the chemical compound is placed within the magnetic
field the atoms either align in the + ½ or – ½ alignment and
remain in this equilibrium state until disturbed.[2] The number
of atoms in each spin state is determined by Boltzmann
statistics where the ratio of lower energy spin states ! ! to
upper energy spin states ! ! is given by equation (5): [2]
Figure 3[2]
!!
These + ½ and – ½ spin states exhibit different energy
levels and the difference in the energy levels is proportional to
the strength of the external magnetic field as seen in Equation
(1). [2]
∆! = ! ∗
!
!
(1)
where the magnetic moment, ! is dependent on the material
being studied.
When the nuclei are placed in the magnetic field the nuclei
is able to absorb a photon with a specific frequency of ν. The
frequency depends on the specific gyromagnetic rotation, γ of
the particle and by the external magnetic field Β given by
equation (2): [2]
! = ! ∗ !
(2)
An example of γ, in MHz/T for the following elements is
shown in table 1. [2]
Table 1[2]
The particles of the element in the magnetic field can
transition between the two energy states when the particle in
the lower energy state absorbs a photon and transitions to the
higher energy state. The energy, ! of the photon has to match
the energy difference between the two states and is directly
related to the frequency by Planck’s constant, ℎ as seen in
Equation 3. [2]
! =ℎ∗!
(3)
The technical term used in NMR spectroscopy for the specific
frequency ! is the Larmor frequency. [2]
Since we know that the energy of the photon needed is
! = ℎ ∗ ! and that ! = ! ∗ ! the energy of the photon in
terms of the magnetic field is then given in equation (4). [2]
! = ℎ ∗ ! ∗ ! (4)
In our NMR experiment our goal is to successfully measure
multiple chemical structures within a 0.3 - .5 Tesla magnetic
!!
!!
= ! !∗!
(5)
Where !, the energy difference between the spin is states, !
is the Boltzmann constant and ! is the temperature in Kelvin.
At room temperature of ! = 273 Kelvin typically the number
of lower energy spin states outnumber the number of higher
energy spin states.[2]
Individual spins and their respective magnetic moment that
are in close proximity are then summed together to form spin
packets with net magnetization vectors; this helps form a
macroscopic image overall spin orientation.[2]
When in the equilibrium state the net magnetization vector
is parallel to the direction of the external magnetic field,!!
which in this case is oriented along the z-axis; this orientation
is called the equilibrium magnetization, !! .[2] Introducing
energy with a frequency equal to the difference in energy
states as shown in equation (4) can change the equilibrium
magnetization. If enough energy is put into the system it will
eventually become saturated and the overall magnetization
vector will transition to when !! is 0, also known as !! .[2]
The energy source in NMR spectrometers is a probe coil that
produces an RF signal to change the equilibrium
magnetization of the system. After the energy source is
removed, the magnetization vector will return to the
equilibrium state and the time constant of this transition is
called the spin lattice relaxation time, !! . The transition is
governed by time, ! dependent equation shown in (6): [2]
!! = !! ∗ (1 − 2
!!
!!
) (6)
So as a result !! is the time needed to change the
magnetization vector by a factor of the natural number e.
Sometimes due to the Heisenberg uncertainty principle
which correlates the energy level and the lifetime of the spin
states the net magnetization vector will be placed not only in
the z-direction but it will rotate about the z-axis in the XY
plane as shown in Figure 6.[6] This rotation occurs at the
Larmor frequency where it is equal to the necessary photon
frequency that would cause a transition between energy levels.
[2]
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Figure 3[2]
As the sample returns to the equilibrium magnetization
vector it will emit an RF signal at the Larmor frequency. This
RF signal is called the free induction decay (FID) signal
shown in Figure 7 and it contains the specific atomic
information about the atom. [3]
where σ is a fraction that is specific to each individual atom.
The reason σ is specific to each individual atom is that the
electron density around each atomic nucleus varies according
the local chemical bonds and nuclei properties. Since the
electron density varies the effective magnetic field varies as
well. The difference in effective magnetic field due to
differing electron densities is called the chemical shift
phenomenon. [2]
Since different NMR’s are capable of producing different
field strengths the effective field produced by the individual
atoms will vary from NMR to NMR. [2] In order to keep
measurements consistent between NMRs the chemical shift
and magnetic field strength should be normalized based on the
NMR’s specifications and a reference substance’s resonance.
The chemical shift is measured in parts per million (ppm) and
is referred to as δ, given by equation (8). [2]
!=
!!!!"#
!!"#$%&'()
(8)
where ! is the measured frequency of a substance, !!"# is the
standard frequency of a reference substance and !!"#$%&'() is
the operational frequency of the NMR spectrometer.
The chemical shift is applied to the FID signal after the
probe coil receives the signal and it is converted into the
frequency domain via a Fourier transform. The frequency
domain signal is then displayed on a ppm vs. intensity graph
as shown in Figure 9. This is the final output of the NMR
spectrometer and will be further discussed in section III.
Figure 4[2]
The FID signal is then collected and examined by a separate
receiving coil. The FID signal is then converted via a Fourier
transform into the frequency domain which shows the
different frequency components and thus different atomic
nuclei of the sample.
An important factor however is the chemical shift
phenomenon. This occurs because the sample is placed within
the magnetic field and the electrons of the sample circulate
about the applied magnetic field direction.[2] The circulation
of these electrons causes a small magnetic field at the nucleus
that opposes the externally applied field as seen in Figure 8.
[2]
Figure 5[2]
The effective field at the nucleus is related to the applied
magnetic field by equation (7). [2]
! = !! ∗ (1 − !)
(7)
Figure 6 [2]
There are two types of NMR set-ups, Continuous Wave
(CW) NMR and Fourier Transform (FT) NMR. The CW
experiment set-up is the simplest NMR experiment because it
involves either having a constant RF and a changing magnetic
field or a constant magnetic field and a changing RF. Fourier
transform NMR techniques are more popular than CW
techniques because they are able to scan and identify multiple
components at once whereas CW techniques requires a time
consuming process that involves testing for each individual
components in succession. [2] For our NMR we chose to use a
CW NMR system and this will be explained in the overall
NMR system in section III.
In order to test our design against contemporary machines
and to prove it works correctly we are going to run
experiments on the NMR spectrometer that are also used in a
standard MIT junior physics lab such as the FID and spin echo
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experiments. [7] The lab involves junior students at MIT to
study the physics and properties of a NMR such as measuring
the effect of field homogeneity on NMR performance,
measuring the chemical composition of water and determining
the effect of field strength on NMR performance.[7] The
results gathered from our NMR will be tested against these
base line experiments to see if it is functioning properly. As
you can see the idea of teaching young students NMR
spectroscopy is already an established course at MIT and this
shows that understanding NMR spectroscopy gives the
students a better advantage in the future. It is this advantage
that we are going to bring to high school students with our
inexpensive and easy to use NMR spectrometer.
II. REQUIREMENTS AND SPECIFICATION
A. Review Stage
The primary and general specifications of our design for the
NMR spectrometer are that the spectrometer should be
inexpensive, safe, simple and accurate. We are planning on
staying within our allotted $500 senior design project budget.
This $500 budget has forced us to design the NMR
spectrometer around easy to obtain, simple parts. For instance
many of the parts for the RF circuitry typically have been
samples from Analog Devices or cheaper parts from Digikey.
In fact the most expensive part of the NMR spectrometer to
date has been purchasing the magnets and machining the
magnetic array.
Safety is a big concern for this project because the NMR
requires a strong magnetic field in order to properly function.
The magnetic field effortlessly attracts and accelerates nearby
ferrous materials towards it and that can easily cause harm to
an individual nearby or the magnet.
The reason for the NMR spectrometer to be simple is that it
will be hopefully used in a high school laboratory setting and
as a result the students should be able to run it easily and
quickly. If the machine is difficult to use or requires intensive
knowledge on how it works it won’t be suitable for use in a
high school setting.
To meet the primary specifications of our NMR
spectrometer we need to set technical specifications of the
system. The technical specifications for the NMR
spectrometer are that it should have around a 0.4 Tesla
uniform and homogenous magnetic field, produce RF around
14MHz – 20 MHz and display the ppm curve of simple
molecules such as water or methanol with results comparable
to commercial NMR spectrometers.
The 0.4 Tesla magnetic field strength is an important
requirement for our NMR to function properly and to meet out
general specifications of performance, safety and cost. Our
research into NMR spectrometers led us to find that a
magnetic field from 0.5 T – 1 T was a good compromise for
general purpose NMR imaging. [8] Given the available
permanent magnets and our cost restraint, the optimal
theoretical magnetic configuration we are able to achieve is
0.49 Tesla. Although this is more than was the minimum
described in [8] we have found examples of other research
groups working on permanent magnet NMRs that exhibited
field strengths between .2 T - .5 T as it is in our case. [9]
The field strength must also be uniform in direction as well
as in magnitude so that all of the nuclei of the substance are all
affected by the same magnetic field, no matter the location
within the substance. [10] High field homogeneity results in
higher resolution data gained from NMR spectroscopy that
reveals detailed information about the chemical structure. [10]
The magnet will be designed and tested as such to prove that
the field lines are homogeneous and uniform as possible
within the sample area itself.
The RF generation and receiving is also being designed with
correct components to meet the specification of around 14
MHz – 20 MHz. The RF needs to be in this range because of
our designed magnetic field strength and the properties of
hydrogen as seen in Table 1. The RF generation also needs to
be tunable to adjust for the actual measured field strength of
the magnet and not the absolute designed value.
III. DESIGN
A. System Overview
Figure 10[2]
Our built NMR spectrometer system diagram is displayed in
block diagram form in Figure 10 and for our final project
accomplishments we have broken it up into five block
components. The first component is the magnetic field itself
and it is the building block upon which all of the other blocks
rely on to function properly. [8] The magnet is essential to the
NMR process and it is entirely its own standalone part of the
device. The second part of our design is a magnetic field
sensor whose role is to measure the homogeneity and field
strength of the magnetic design and provide verification of the
magnet functioning properly. The field sensor we have
designed is inexpensive and is fine tuned toward our magnetic
field design. In addition, the field sensor we have designed is
considerably less expensive than commercial magnetic sensors
while still giving correct information. The addition of a field
sensor gives further value of our project to being used in high
school laboratories as it allows students to study the effect of
field strength and homogeneity on the outcome of NMR
spectroscopy. Our magnetic field design also lacks the proper
shimming coils necessary to correct any inhomogeneous fields
within the magnet and so it is important to measure field
homogeneity before performing a NMR spectroscopy. [10]
The third part of our design is the RF generation = circuitry
which is connected to both the transmission coil. Also the
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receiving coil and oscilloscope form the fourth component
block and the software interface that controls the pulse length
and pulse frequency is the fifth block.
The design explanation and reasoning of each of the
components will now be explained. A homogeneous and
uniform magnetic field of 0.4 T is our design specification
based on the overall project goals and specifications. After
conducting initial research into magnetic field design and
weighing it against our project goals it was determined that a
Halbach cylindrical magnetic array was the best choice for the
permanent magnet. An example of an eight segment Halbach
array is shown in Figure 11. [11]
magnetic remanence in Tesla (Br), the field orientation of each
magnet relative to overall magnetic field Bo, and finally the
inner (r1) and outer radius (r2) of the cylinder.
The magnetic field, Bo inside the cylindrical array is
determined by the equation (9). [12]
!
!! = !! ∗ !! ∗ ln ( !)
!!
The constant Cn is determined by the number of individual
magnets and is calculated by equation (10): [12]
!! =
Figure 11[12]
The array works by arranging individual smaller magnets in
orientations such that all of the individual magnetic fields add
up to form a homogeneous and uniform field within the center
of the cylinder and little to no field outside of the cylinder.
The uniform field lines are an important and integral part of
the NMR spectrometer working properly.[10] A homogeneous
field is necessary because it directly affects the alignment of
the molecules in the substance. If the field is inhomogeneous
molecules of similar elements will experience different
relaxation times and thus emit different frequencies when
returning to the equilibrium state.[10] These frequency
differences can cause the spectral lines shown in Figure 9 to
appear wider, lessening the spectral resolution of the
spectrometer. [10] A simulation of the homogeneous field
lines within an eight-segment Halbach array is shown in
Figure 12. [13]
(9)
!!
)
!
!π
!
!"# (
(10)
Using vector and matrix analysis it is possible to analyze the
magnetic contribution of each individual magnet in the
Halbach array and derive equations (9) and (10). These
equation derivations can be seen in reference [14].
The upcoming analysis concerning the Halbach array layout
concerns a densely packed magnetic structure. A densely
packed magnetic structure as seen in Figure 13 would be
preferable as it increases the overall field homogeneity
throughout the inner cylinder but a compromise must have
been made with cost and feasibility. It is important to
understand however how the ideal Halbach cylinder functions
and is formed in order to fabricate as much of an ideal
cylinder as possible.
The orientation of each magnetic field is important to
understand how orientation affects the overall magnetic field.
To understand how the orientation affects the Halbach array
we first must assign each individual magnet a position on a
ZY coordinate axis such that the center of each magnet is a
standard distance r from the origin or center of the Halbach
cylinder. [15] This coordination of the magnets can be partly
seen in Figure 13.
Figure 13[15]
The coordinates of the magnet center, denoted by vector !!!
are then easily represented with a matrix equation (11): [15]
Figure 12[13]
The important specifications and properties of the Halbach
cylindrical array that need to be considered are the number of
individual magnet segments (M), the cross sectional
dimensions of each square magnet face (a), the individual
!!
sin !
!!! = ! ! = !
!!
!"#$
(11)
where B is the angle between the magnet center and the
direction of the desired magnetic !! and
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! = ! ∗ !,
(12)
! = 0, 1, 2, … , ! − 2, ! − 1
Then we consider having M magnets leads to having the
following coordinates for each Mth magnet: [15]
!
!!! = cos( − 2 ∗ !! )
!!!
!
!
= (!!! + ) ∗
(13)
!
!
!!!
sin( − 2 ∗ !! )
!
The sign of the cos and sin are dependent on where the
magnet is ultimately oriented within the coordinate system. It
is then possible to solve for a, the cross-sectional dimensions
of the individual magnets as seen in equation (14).
! =2∗!∗!∗!
Figure 14
(14)
!
where ! = − 2 ∗ !!
!
From there given the fact that the area, A occupied by the
magnets is densely packed then it can found to be as seen in
equation 15.
! = ! ∗ ! ! = 4 ∗ ! ∗ ! ! ∗ ! ∗ ! ! (15)
and so the inner and outer radius can be found in terms of the
magnetic center radius and the orientation angle as seen in
equations (16) and (17) for inner and outer radii respectively.
!! = ! ∗ (1 − 2 ∗ ! ∗ !) (16)
!! = ! ∗ (1 + 2 ∗ ! ∗ !) (17)
The orientation of the magnet can be seen as having an
effect on both the inner and outer radius as well as the cross
sectional dimensions of the individual magnets. This analysis
also shows how each parameter is interconnected with each
other and that by changing one parameter will affect the other
parameters as well.
Based on these calculation we were able to purchase the
correct magnets and design the array that would give us
approximately 0.4 Tesla field strength within the cylinder.
The magnets ultimately chosen for the Halbach array were 16
1.27 cm x 1.27cm x 3.81cm neodymium N42 grade (NdFeB)
magnets each with an individual remanence of 1.3 Tesla.
The layout of the Halbach cylinder was chosen to be as seen
in Figure 14. The notches in the cylinder are the places for the
magnets to be secured and the magnets individual orientations
are shown in the red arrows.
Figure 15
The cross sectional view displays eight magnets and the
three dimensional view shows how the 16 magnets can be
stacked in order to create a longer cylinder. The cylinder
length was made to be 7.62 cm as it fits the length of the two
bar magnets stacked lengthwise. An aluminum casing for the
magnets was designed and machined with the end result
shown in Figure 15. Aluminum is a non-ferrous material and
does not interact destructively with the intended magnetic
field lines as a steel or iron casing would. The aluminum
casing was designed with ease of manufacturability in mind
and as much reduction in machine costs as possible. The
design was a collaborative process with machinists employed
in both the Hasbrouck physics machine shop located on
campus and the Amherst Machine Company located off
campus.
We chose to have a cylinder with an inner radius of 3.5 cm
and thus the outer diameter is around 5.29 cm because of the
diagonal of the magnet extending 1.79 cm from the inner
radius. Thus with an inner radius r1 of 3.5 cm, outer radius r2
of 5.29, individual magnetic remanence Br of 1.32 T, and 8
(M) magnet segments it is possible to calculate !! and thus
calculate the desired magnetic field !! .
Following equation (10) we see that !! is equivalent to
0.9003. Using this value in equation (9) leads us to a desired
!! of .49 T. This value is indeed higher than the desired .4
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Tesla but it is the closest we could get with the available
magnetic stock while still remaining in our tight budget.
The Halbach cylinder array also has the advantage of
producing the field strength required but also gives us the
optimal field homogeneity that keeps us within our budget.
Typically the magnet systems of NMR spectrometers include
shimming coils within the magnet field that help to offset any
stray field lines that naturally occur in the magnetic field.[2]
The shimming coils are typically electromagnetic coils that
produce small magnetic fields in order to direct the magnetic
flux in certain areas and create a more homogenous field. [16]
In our design however we will not include shimming coils as
our sample area space is limited and we foresee that we will
not have time to properly complete the shimming coils.
The magnetic field sensor will be used to measure the
magnitude and homogeneity of the field within the interior of
the magnet. The sensor design involved the programming of
the hall-effect sensor and the design of a mechanical device
used to freely move the device within the magnet as well as
determine the strength and homogeneity of the field with the
highest possible precision.
The accuracy and stability of the MLX90215 hall-effect
sensor was improved by programming the chip using the
PIC18F4550 microcontroller in MPLAB. The programming
was done by setting the 10-bit quiescent voltage (voltage at
B=0 Tesla) to half of the total voltage across the sensor in
order to maximize the output swing and the linearity. The
sensitivity of the sensor was programmed by adjusting its
rough gain (with 3-bit) and its fine gain (with 10-bits) to zero.
Setting both gains in this manner offered the highest level of
sensitivity provided by the chip, 4.1mV/mT, or 0.41V/.1T. At
the highest value of the field generated by the magnet (0.49 T)
the sensor should output a corresponding voltage of 1.96 V
which is largely within the 4.5~5 V specifications. Moreover,
we used a 1-bit inverse slope function to program the chip in
such a way that an increase or a decrease in the magnitude of
the output voltage indicates the presence of a North magnetic
pole or a South magnetic pole respectively. The signal input
signal of the sensor was set to 1 a pulse greater than 150us and
0 for a pulse less than 150us as seen in Figure 16.
The mechanical device used for the motion of the sensor
was made out of a non-ferromagnetic material in order to keep
it from interfering with the magnetic field. It had to be very
stable and balanced in order to hold the chip still during
measurement, and it had to be able to freely move the chip in
the XYZ directions. We chose aluminum and brass as our nonferromagnetic materials because they were within our budget
and we used two materials instead of one in order to avoid
connections between the same materials and reduce the
quantity of aluminum used given its large weight. Our design
is made of a wide square base (7*7 inches) 1.5 inch thick,
which gives us the support we need for foundations. We used
three ½-20 brass threaded rods, 4 inch tall, on which
aluminum knurl knobs are used to move the sensor in all three
dimensions and allow us to keep track of the exact position of
the sensor (1/20inch travelled per turn). The final product of
our design is shown in Figure 17 and temporary measurements
of the magnetic array are depicted in Figure 18.
Figure 17
Figure 18
Figure 16
The RF signal generator sends pulses to the testing sample
via a single probe coil wrapped by thick copper wires since the
coil is supposed to place in the center of the magnetic field
and copper is not magnetic material. The coil is created from
24 gauge enameled copper wire that is wrapped in a 2 cm
diameter cylinder, which allows enough space for the sample
to interact with the coil. The coil is also encased within heat
shrink so that it will remain rigid and be easier to use. The
designed transmission coil is shown in Figure 19 though the
receiving coil is created exactly the same.
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programmable waveform generator PCB implementation is
shown in Figure 22.
Figure 19
The most significant part of the RF signal generator is a RF
switch circuit to provide a 50 microsecond on/off signal
(which has a 1 Hz frequency) up sampling with around 14.9
MHz local frequency. The RF switch circuit works by using a
square wave gate signal to switch on and off the RF source
with a J310 RF MOSFET. The designed circuit diagram is
shown in FIGURE 20 with the final design shown in FIGURE
21.
Figure 22
B. Design Alternatives
IV. FINAL PROTOTYPE IMPLEMENTATION AND RESULTS
For our final prototype implementation we have created a two
coil system, one receiving coil and one transmission coil that
can demonstrate a unique but questionable free induction
decay signal for different chemical compounds such as water
and ethanol. Although our goal was to have a fully working
NMR spectrometer by our final prototype we have
encountered a great deal of problems that have prevented us
from doing so. The final system set-up is shown in Figure 23.
Figure 20
Figure 23
Figure 21
When the square wave gate signal is low, the coil provides
external energy to the sample to interrupt the atoms’
equilibrium. While when the square wave gate signal is high,
the coil collects the FID voltage generated by the sample.The
square wave gate signal is generated from a PIC18F4500
microcontroller. The LO signal is created by a programmable
waveform generator, AD 9834 evaluation board from Analog
Devices. This chip is able to provide sinusoidal signals up to
37.5 MHz. PCB design is required for this circuit due to the
package of the chip is too tiny to apply to breadboard. The
The first problem we encountered was involving the AD 9834
frequency synthesizer on the transmitting PCB board. The
synthesizer PCB board, shown in Figure 19 was fabricated
according to our design but was not producing the required
sine wave output or outputting a signal at the desired 14.9
MHz frequency. We have still yet to determine the actual
cause of the frequency synthesizer malfunction but we have
tried to troubleshoot a few areas in order to make it function
properly. We started by double-checking that the chip was
successfully soldered onto the PCB and that all the proper
signals were being inputted into the pins correctly such as
grounding the RESET and SLEEP pins. After that we went
through the programming message protocol and determined
via an oscilloscope that the correct message was indeed being
sent to the frequency synthesizer, shown in Figure 24.
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However at and even above our intended frequency of 1 Hz
for the RF input and 14.9 MHz for the LO signal we have
been unable to obtain a satisfactory output signal (yellow)
when mixing a 30 MHz LO signal (blue) with a 14.9 MHz RF
signal (purple) which can be seen in Figure 26.
Figure 24
We obtained the programming message by following the
programming equation shown in equation (18) and by
following the data sheet application notes for the correct bit
assignment. [17] In our case the required frequency, fout is 14.9
MHz and the frequency of the master clock, fMCLK was 50
MHz which we generated with a function generator.
Freq Re g =
fout * 2 28
f * 2 28
(18)
FreqR e g = out
fMCLK
fMCLK
Figure 26
We theorized that this was due to the 1 nF capacitor at the
input of the RF signal which was acting which was acting as a
low pass filter and filtering out the lower frequencies. In order
to prove this idea we performed a frequency sweep on the RF
input of the mixer and found its 3dB drop off point. This
frequency sweep vs. signal amplitude is shown in Figure 27.
The second problem that we encountered was involving the
transmitting function of the AD8342 mixer that we originally
intended to use instead of the RF switch. We tested the mixer
by inputting two sinusoidal waves into the RF and LO inputs
of the mixer and it operates properly when the waves are at
high enough frequencies as seen in the yellow signal in Figure
25 where the RF frequency was 60 MHz (purple) and the LO
frequency (blue) was 80 MHz.
RF Input Frequency vs. Mixer 10 Output Amplitude (No Capacitor) Vpp (V) And finally in order to check out programming code we used
it to program the AD9834 evaluation board. Our
programming message was indeed correct as it was
successfully programmed and outputted a sine wave at our
defined frequency. In the end we determined that it the
soldering of our PCB board that caused our transmission
circuitry to fail.
1 13 15 17 19 21 23 25 27 29 31 Frequency (MHz) Figure 27
Figure 25
After that frequency sweep we increased the capacitor’s value
by an additional 1 nF and ran another frequency sweep which
showed that the 3dB drop off point was indeed changed. The
new frequency sweep can be seen in Figure 28. However we
were still unable to get a satisfactory result from the mixer at
10
our desired frequency so these results were collected with an
LO of 35 MHz in order to show the effect of the input
capacitance. The overall amplitude of the addition capacitor
was greater over the frequency range but the actual mixing
waveforms at lower frequencies ended up looking like Figure
26.
10 Vpp (V) RF Input Frequency vs. Mixer Output Amplitude (With added capacitor) The transmission coil is oriented 90° to the receiving coil so
that the mutual inductance between the two coils is reduced as
much as possible which was partly obtainable during our CDR
presentation. The mutual inductance however is not fully
removed in our FPR progress and it introduces some
uncertainty into our results. The uncertainty arises from being
unable to determine of the receiver coil is primarily driven by
mutual inductance or by the free induction decay. Our results
suggest that it is a combination of the two and will be
discussed later on.
The receiving coil contains the sample and due to it’s close
proximity it is the strongest coupled to the free induction
decay from the sample. The two coil transmit and receive
system is visible in Figure 15.
In order to tell if we were receiving any sort of response
from the sample at any RF pulse frequency we performed a
RF frequency sweep from 12 MHz – 18 MHz and plotted the
RMS value of the amplitude. The result of the sweep for an
ethanol sample, water sample and no sample is shown in
Figure 30.
1 13 15 17 19 21 23 25 27 29 31 Frequency (MHz) Figure 28
In order to obtain some kind of free induction decay signal
we needed to try a different method other than using the PCB
transmitting and receiving boards. In order to replicate the
pulsed RF signal we obtained a J310 RF MOSFET switch that
is driven by a square wave input. Our 14.9 MHz signal from
the function generator and through our transmission coil is
connected to the drain of the switch, the source is grounded
and the square wave is connected through a voltage divider
into the gate as seen in FIGURE 20. The switch acts as a
transmission and receive switch and is used to turn the 14.9
MHz signal on and off. The resulting signal across the
transmission coil is the signal in Figure 29.
Figure 29
Figure 30
An analysis of this data shows that there is a peak in the
RMS amplitude around 16.5 MHz for all the different
11
samples. The samples do exhibit some variation in their
amplitude peaks as for water and ethanol the curve is slightly
shifted to the right with ethanol experiencing more of a shift
than just water. A proposed reason for this change in amount
of total shift is that the higher percent of hydrogen in ethanol
over water. Also ethanol experiences some electron shielding
on the hydrogen atom from the carbon and oxygen that the
water sample does not experience. The electron shielding
should result in a wider range of larmor frequencies.[2]
Another important experiment we performed was to see if
the RF pulse length had an affect on the RMS value of the
post-pulse receiving coil amplitude. The pulse was pulsing at a
period of 2.5s and the pulse length was swept from 0 to 1.5
seconds. The RF amplitude did experience an increase in
amplitude around .83 seconds as seen in figure 31.
V. PROJECT MANAGEMENT
A. Team Member Roles
An important part of this project was determining and
dividing the tasks correctly so that we can quickly and
efficiently complete it on time. Our system as has been
designed and intended for FPR has been broken into three
separate components which are the magnet design, the
magnetic field sensor and the RF generation and receiving
circuitry. Since we have three team members it was simple to
devise the tasks between ourselves. We broke down the roles
as follows; Zach will be in charge of the magnetic field
design, Aline will be in charge of creating the magnetic field
sensor and Mandy will be in charge of the RF generation and
receiving circuitry.
B. Project Outlook
The outlook on the project as of right now is that we would
need some more time in order to get a workable NMR
spectrometer. We do believe however that we are very close
to detecting the different FID signals in a variety of substances
such as water and ethanol because we can see a varied
response in the amplitude change around 16.5 MHz for the
different substances.
VI. SUMMARY AND CONCLUSIONS
Figure 31
It is necessary to study the pulse length of the RF pulse
because many NMR experiments are based around changing
the pulse length. The standard NMR experiment involves
finding the FID signal but by doubling the FID pulse length it
is possible to view the spin echo response. The spin echo
occurs when the atoms magnetization is flipped 180 degrees
rather than the 90 degrees during a regular FID pulse. This
180 degree flip causes a sample response where after receiving
sequence of 90 degree pulse the 180 degree pulse the sample
emits a sine wave that exponentially increases and then
exponentially decreases. This spin echo response is important
as it reveals more information about the molecules full
composition.
The two different samples we tested water (H20) and
ethanol (C2H60) are common samples in NMR spectroscopy.
[reference] However due to the inability to totally remove the
mutual inductance we are unable to purely isolate the pure
FID signal. If the mutual inductance signal were totally
removed then the signal should decay and not return until the
RF pulses again.
This report has summarized, clarified and outlined the
purpose of our NMR spectrometer as a senior design project.
We have described the necessary background behind the
functioning of NMR spectrometers while showing the
motivation and reason for it as our SDP project. An NMR
spectrometer will be a strong tool for high school students to
use in the understanding and comprehension of the fields of
biological and chemical sciences. Our progress so far has
involved us using techniques we have learned in the field of
electrical engineering and learning new techniques in related
fields.
The NMR spectrometer as designed but not fully
implemented will be able to decipher the chemical structure in
both water and benzene. Our progress so far up until this final
design review has taught us a great deal about designing and
implementing complex engineering systems. We have
experienced a great amount of setbacks with the machining of
the aluminum magnetic array, the magnetic field sensor, the
frequency synthesizer and the mixer necessary for the RF
circuitry. As a result we had to find another method for
obtaining a FID signal in order to give a demonstration of a
working NMR response. We do feel however that our progress
up to this point has allowed us to understand the workings of
an NMR machine
VII. REFERENCES
[1] B. Esselman and D. E. Mencer, “Inclusion of NMR
spectroscopy in high school chemistry: Two approaches,”
Modern NMR spectroscopy in education, vol. 969, p. 77,
2007.
[2] J. Hornak, The Basics of NMR, online manual of the
Magnetic Resonance Laboratory, Rochester Institute of
Technology. 2003.
12
[3] M. Edgar, “Physical methods and techniques: NMR
spectroscopy,” Annu. Rep. Prog. Chem., Sect. B: Org. Chem.,
vol. 107, pp. 308–327, Jun. 2011.
[4] M. Edgar, “Physical methods and techniques: NMR
spectroscopy,” Annu. Rep. Prog. Chem., Sect. B: Org. Chem.,
vol. 107, pp. 308–327, Jun. 2011.
[5] M. Edgar, “Physical methods and techniques: NMR
spectroscopy,” Annu. Rep. Prog. Chem., Sect. B: Org. Chem.,
vol. 107, pp. 308–327, Jun. 2011.
[6] F. P. Gasparro and N. H. Kolodny, “NMR determination
of the rotational barrier in N, N-dimethylacetamide. A
physical chemistry experiment,” Journal of Chemical
Education, vol. 54, no. 4, p. 258, 1977.
[7] S. D. Sewell, G. W. Clark, U. J. Becker, and J. Kirsch,
“8.13/8.14 Experimental Physics I & II‘ Junior Lab’, Fall
2002,” 2002.
[8] D. I. Hoult, C. Chen, and V. J. Sank, “The field
dependence of NMR imaging. II. Arguments concerning an
optimal field strength,” Magnetic Resonance in Medicine, vol.
3, no. 5, pp. 730–746, Oct. 1986.
[9] E. Danieli, J. Perlo, B. Blümich, and F. Casanova,
“Small Magnets for Portable NMR Spectrometers,”
Angewandte Chemie International Edition, vol. 49, no. 24, pp.
4133–4135, Jun. 2010.
[10] S. Vathyam, S. Lee, and W. S. Warren, “Homogeneous
NMR spectra in inhomogeneous fields,” Science, vol. 272, no.
5258, p. 92, 1996.
[11] H. Raich and P. Blümler, “Design and construction of a
dipolar Halbach array with a homogeneous field from
identical bar magnets: NMR Mandhalas,” Concepts in
Magnetic Resonance Part B: Magnetic Resonance
Engineering, vol. 23B, no. 1, pp. 16–25, Oct. 2004.
[12] B. T. Merritt, R. F. Post, G. R. Dreifuerst, and D. A.
Bender, “Halbach array motor/generators-A novel generalized
electric machine,” Halbach Festschrift Syposium February,
vol. 3, 1995.
[13] G. Küstler, “Computation of NdFeB-Halbach Cylinders
With Circular and Elliptical Cross Sections in Three
Dimensions,” Magnetics, IEEE Transactions on, vol. 46, no.
9, pp. 3601–3607, 2010.
[14] K. Halbach, “Design of permanent multipole magnets
with oriented rare earth cobalt material,” Nuclear Instruments
and Methods, vol. 169, no. 1, pp. 1–10, 1980.
[15] N. Doğan, R. Topkaya, H. Subaşi, Y. Yerli, and B.
Rameev, “Development of Halbach magnet for portable NMR
device,” in Journal of Physics: Conference Series, 2009, vol.
153, p. 012047.
[16] P. Konzbul and K. Sveda, “Shim coils for NMR and
MRI solenoid magnets,” Measurement Science and
Technology, vol. 6, p. 1116, 1995.
[17] A. Devices, “AD9834 Specifications,” AD9834
Datasheet, Feb, 2003.
[18] J. P. Heremans, “Magnetic field sensors for magnetic
position sensing in automotive applications,” in MRS
Proceedings, 1997, vol. 475.
[19] T. F. Budinger and others, “Nuclear magnetic resonance
(NMR) in vivo studies: known thresholds for health effects.,”
Journal of computer assisted tomography, vol. 5, no. 6, p. 800,
1981.
VIII. APPENDIX
A. Application of Mathematics, Science and Engineering
Our project involves multiple disciplines from the fields of
mathematics, science and engineering such as circuit design
and analysis, electromagnetic fields and waves, and computer
interfacing. Our course history here at the University of
Massachusetts has prepared us well for this project. For
instance our use of microcontrollers within the RF circuitry
draws heavily from ECE 353, Computer Systems as well as
ECE 313, Signals and Systems. The circuit design has been
strongly influenced by the design oriented ECE 323 and 324,
Electronics I & II respectively. The magnetic field aspect of
our project draws heavily from our work in ECE 333, Fields
and Waves. A specific example of us applying our
coursework in this project can easily be seen in the RF
generation and receiving circuitry. This part of our project is
heavily influenced ECE 353 because it involves the use of a
programmable microcontroller, the PIC18F4500 which
provides the IF square wave signal for the mixer. This is
reminiscent of last project we worked on in ECE 353 where
we had to program a microcontroller to do the same function
but in that case the signal was acting as a clock for the rest of
the circuit. ECE 323 and 324 were useful when designing
both the magnetic sensor circuit and the RF component
because it gave us a strong background in understanding how
to arrange the circuit together. To be specific our experience
in trouble shooting the programming of the magnetic sensor
involved measuring the output signal of the microcontroller
with an oscilloscope, a common tool used in ECE 323 and
324.
Of course our individual academic experiences have shaped
how we apply the fields of mathematics, science and
engineering. Zach, being in charge of the magnet design
process has drawn heavily on his interest in the physics side of
electrical engineering while Mengting has used her previous
REU experience designing circuits in her RF circuitry design.
Aline has also drawn upon her experience as a TA in
electronics and her knowledge of MRI technologies as a
research intern at Massachusetts General Hospital.
B. Design and Performance of Experiments, Data,
Analysis, and Interpretation
Prior to our midway design review we had performed a very
basic experiment that involved measuring the field strength vs.
distance of an individual NdFeB bar magnet at pole the north
and south poles. This was primarily a test of our working
magnetic sensor design as well as to see if the desired magnets
behaved as planned. The study involved changing the vertical
position of the bar magnet over the sensor in 0.2 cm intervals
and recording the change in magnetic field strength until the
magnetic field strength reached a value of 0. The data we
collected is shown in Figure 32 and it correlates well to other
magnetization versus distance curves in similar hall effect
sensors. [18]
13
Figure 32
The experiment showed us a few different facts about our
sensor and a few parts that need to be adjusted for perfect
performance. The experiment showed us that the sensor was
being pulled completely up to the supply voltage, Vdd and
also down to ground thus causing the inability to measure
stronger magnetic fields such as the 1.32 T produced by the
individual magnets. This affect however was accounted for in
our design because the magnetic field sensor was picked so it
could measure the Halbach cylindrical array and it’s 0.4 – 0.5
T field. This field range is easily within the sensitivity of the
sensor to measure accurately. This experiment also let us
know the effects of having not optimum voltage swing as the
quiescent voltage was not at the specified ½ of the 5V supply.
This brief collection of data will lead us to further improving
our design and will also allow us to practice creating
scientifically correct experiments for the final testing of the
NMR spectrometer in April.
C. Design of System, Components or Process Meet Desired
Needs within Realistic Constraints
The system requirements of our design are that the NMR
spectrometer should easy to use and to understand so that high
school students can use it successfully in a laboratory setting.
This is so that students can use the device with as little hassle
as possible while gaining as much information about their
sample. Our NMR spectrometer is designed to be able to give
results comparable with similar commercial devices although
not necessarily with the precision as higher end devices are
capable. The device should also be a reasonable size and be
safe to use because it will have be located primarily within a
high school laboratory setting.
Realistic constraints with our design are based around the
safety of the device and the performance of the device to
accurately measure chemical properties close to that of
commercial devices. Since typical commercial devices use
strong superconducting magnetic fields of around 18T we
needed to use a lower magnetic field that would be considered
safe in a high school setting. We have decided to use field
strength of around 0.4 – 0.5 Tesla. This magnetic field
strength is more manageable and easier to design in a safe
working environment. A magnetic field of this strength can
still be dangerous if used improperly however so we have
designed the magnetic field array to be safely housed in an
aluminum casing that is fastened to a stand so it won’t move
in the presence of magnetic materials. Since the aluminum
casing is a non-ferromagnetic material it should not
destructively interfere with the magnetic field within the
cylinder and provide strong enough housing to hold the
magnets in place. [11] The rest of the device is relatively safe
as it is railed at +/- 9 volts.
The accuracy of the device is perhaps the most realistic
constraint in our design as we will not be able to directly
compete with similar commercial devices for enhanced
resolution in identifying chemical compounds. Our design
should deliver reasonable results given our $500 budget
because we have kept the design specifications within
reasonable limits such as the .49 Tesla field that is strong
enough to properly align the atoms but low enough so we can
generate the proper 15.9 MHz Larmor frequency necessary
using conventional circuitry.
D. Multi-Disciplinary Team Functions
Our team has worked well to split up the necessary tasks for
completing this project with satisfactory results. We feel that
the work load has been distributed as evenly as possible and
that we are all contributing equally to the final design. Aline
Elad, an electrical engineering major has been responsible for
designing the magnetic field sensor and sensing apparatus.
Aline has been performing research into magnetic sensors and
has chosen the MLX90215 sensor as it is fits the requirements
of the designed magnetic field. Aline has also contributed to
part of the comprehensive report on our NMR spectrometer as
well as to all of our review presentations. Mengting Nan, also
an electrical engineering major has been responsible for
designing the RF component of the NMR spectrometer.
Mengting has been busy designing the RF component from
parts that are available to us and she also has been creating
printed circuit board design that can be manufactured in a
future date. Mengting has also contributed to the NMR design
report as well as created contributions for the PDR and MDR
presentations. Zachary Henderson has been responsible for
researching, designing and machining the magnetic field array
that is crucial to the functioning of the NMR spectrometer.
Zachary has found a suitable solution in the Halbach
cylindrical array and designed, via AutoCad a drawing that is
capable of being machined in aluminum via an automated
CNC milling machine. Zachary has also been the primary
author of the NMR design report as he has primarily written
and proofread the entire report.
E. Identification, Formulation and Solution of Engineering
Problems
An example of an engineering problem that was
encountered during this project was in the design of the
14
magnetic array. The problem primarily consisted of the
orientation of the magnets in the actual array. Since the
magnets we ordered are square faced magnets measuring 1.27
cm x 1.27 cm and the orientation had to be as described in
Figure 15 it was necessary to have the face of the square
magnets not form 90 degree angles with the tangent of the
inner circumference. The machinists preferred this orientation
as it allowed them to fabricate the aluminum casing easier.
The ease of manufacturing the part in the radial manner is that
they could clamp the part in one single vice that is capable of
rotating a full 360 degrees and mill out each notch one after
another, instead of needing to realign each time. After
explaining the situation to the machinists at the machine shop
we had to come up with other manufacturing solutions that
would allow us to have the orientation necessary for the
Halbach cylindrical array. In the end it was decided that the
notches in the aluminum casing that would hold the magnets
could be fabricated using another technique that was machined
in a computer controlled milling machine that was capable of
running the entire process unassisted. This technique will be
done at the Amherst Machine Company using their CNC
milling machine.
This problem was efficiently overcome by communicating
to the machine shop the necessity of the particular design and
by then brainstorming other ideas that could fabricate the part
as quickly and successfully. The problem was quickly
identified as we communicated with the machine shop and
they understood the situation and took more time to
brainstorm some more ideas with us. Ultimately it was their
expertise in the area of machining metal that helped us find the
solution to the issue and it served as a great learning tool for
us in dealing with engineering problems. This example serves
as a great problem for us to solve as engineers because it
involved identifying the problem, communicating together as
teammates with outside sources and working with outside
sources to solve a problem that we couldn’t solve with our
own skill set.
F. Understanding of Professional and Ethical
Responsibilities
An example of an ethical consideration that was inherent in
the design and building of our NMR spectrometer was the
safety concerns involving the use of a strong magnetic field.
Since our project goal was to introduce an NMR spectrometer
into a high school student science curriculums it was critical
that it’d be as safe as possible for students to operate.
In order to deal with the safety issue of the strong magnetic
field we had to design the NMR around a lower field strength
magnet. The issues and concerns that arose with having a
lower field strength magnet were that the field might lose field
strength homogeneity over the sample area. Researching
different magnetic field solutions that had high field
homogeneity at lower magnetic fields as well as being easy
and cheap to manufacture dealt with this issue. The chosen
magnetic array design of a Halbach array was the best possible
solution since it created a much lower magnetic field of
approximately .49 T. This lower magnetic field will still
attract nearby ferrous materials such as iron or steel so the
operators will still need to be careful of what is placed near the
magnet. Insuring that any operators receive thorough
instructions on the proper handling on the equipment can solve
the issue of attracting nearby ferrous materials.
G. Team Communication
Our team has been constantly communicating through
multiple and varied channels. We have been in contact via
email and via telephone almost on a daily basis. Our team
also meets regularly on a person-to-person basis in team
meetings usually in the senior design lab.
Our team has valued communication and sees it as a
powerful and useful asset in the completion of our senior
design project. We find that our communication has helped us
to identify and overcome obstacles that we have encountered.
H. Understanding of the Impact of Engineering Solutions in
a Global, Economic, Environmental and Societal Context
Based on our research on the importance of NMR
spectrometers in the field of biology and chemistry we saw the
positive benefits of introducing this technology to students in
their high school curriculum. Our research has revealed that
many of the topics taught in high school chemistry such as
chemical structure and composition are direct applications of
NMR spectroscopy. A societal context of introducing NMR
spectrometers to high school students is that we are preparing
them for future school and work responsibilities. For students
who are interested in the field of biology and chemistry
experience in NMR spectroscopy will potentially allow them
to attend more competitive colleges and obtain undergraduate
research roles. So when it comes to obtaining a job or
attending graduate school students with undergraduate
research roles will be favored over other students. As a result
the overall workforce in biology and chemistry would not only
be more proficient in an important area of their field but would
also be a more focused and experienced workforce. Having a
more focused and experienced workforce could further
increase a company’s profitability and allow for economic
growth in today’s slow economy.
Our NMR design has influenced primarily by the negative
societal consequences of how unsafe equipment can be
potentially harmful to people. The issue of safety was an
important aspect of our NMR design and was dealt with by
using a relatively low and safe magnetic field strength. [19] A
higher magnetic field strength could cause magnetic materials
to accelerate toward the magnet. These materials could hurt
an individual or they could break the system in unintended
ways. This is why we have tried to create as safe as magnetic
field system as possible by using a field strength between 0.4
– 0.5 T which is much less than standard NMR field strengths
of 18 T.[19] If our device was unsafe to operate or if it could
potentially cause harm to an individual while operating it
would be a harmful and negative consequence of our project.
I. Application of Material Acquired Outside of Coursework
This project involved incorporating a great amount of
material and knowledge outside of our typical coursework in
the field of electrical engineering. Examples of sources we
had to obtain are the local machine shops, the hall-effect
sensor manufacturers and sources describing the technical
operation of NMR spectrometers.
15
For designing and manufacturing both the magnetic field
array and the magnetic field sensor apparatus we had to learn a
great deal about different machining strategies and concepts.
Our work involved designing two-dimensional and threedimensional models in AutoCad and then bringing them over
to the machine shops for consultation. The use of AutoCad
was a huge hurdle for us as we had never needed to use an
advanced drafting tool in our curriculum as electrical
engineers. The best help we acquired for learning AutoCad
was online tutorials and instruction for simple tasks such as
inserting scale dimensions, combining shapes and extruding
two-dimensional objects into three-dimensional shapes. The
machinists at the machine shop were especially useful when it
came to actually designing the parts, as they would let us
know how feasible our entire design actually was. For the
fabrication of the magnet array we had discussions with the
machinists on aluminum thickness, individual magnet
orientation, and individual magnet depth. We also learned a
great deal about different machining practices such as milling,
boring and finishing that were required to finish the machining
but we never would have known about if we hadn’t attempted
this project.
Initially we needed to understand what type of field effect
sensor we needed to have to accurately measure the intended
field strength of our magnet. After doing some preliminary
research on magnetic sensor chips and systems, we decided on
a Hall effect sensor based on its capabilities of measuring
magnetic fields down to below .48 T. The problem we needed
to clarify is what actual Hall effect sensor we should purchase
that would be easy enough to manufacture and use. Therefore,
we called a manufacturer, Melexis who makes a great deal of
these Hall effect sensors and they recommended a few
different ones that they felt could be used in this project. This
helped to clarify the data sheets we were finding on sites such
as digikey and it ultimately led to our choice of choosing the
MLX90215 as its operating parameters such as supply voltage
and sensitivity fit well into our designed system.
The final piece of outside information that we obtained was
the actual functioning of an NMR spectrometer itself. When
we first decided to tackle this project barely any of us knew
how an actual NMR spectrometer works but through diligent
research we discovered the basic fundamentals of NMR
spectroscopy. In particular reference [2] was extremely
helpful in decoding the inner workings on an NMR
spectrometer as it started with explaining the fundamental spin
physics and went directly into the basic hardware overview
that was necessary to implement the NMR spectrometer. Our
background in physics and electromagnetic fields and waves
did aid us and allowed us to understand the NMR
spectrometer faster but we still had to learn a great deal about
the fundamental spin properties.
J. Knowledge of Contemporary Issues
Our project was directly aimed at helping and aiding
students in the field of biology and chemistry. As students
ourselves we know of the importance of achieving experience
in a field early and often as it grants you more opportunities
when it comes to applying to graduate school and for full-time
work positions. This and will always be a contemporary issue
because the fields of physical sciences will constantly be
advancing and it will be necessary for students to understand
fundamental tools such as NMR spectrometers which are the
building blocks of a number of cutting edge research. Our
goal of this project is to make sure that students can find a cost
effective and simple way of using an NMR spectrometer so
they can increase their tool set, thus potentially advancing the
fields of physical sciences even further.
K. Use of Modern Engineering Techniques and Tools
Our project involves the use of many different modern
engineering techniques and tools that are commonly used
today. Such techniques include printed circuit board design,
two/three dimensional computer aided drafting and knowledge
of electronic circuitry and electronic systems. For the printed
circuit board design of the RF generation circuitry we used
software called PCB artist. For the two/three dimensional
computer aided drafting we used AutoCad 2011 Student
Edition. The tool set available to us in AutoCad was powerful
and allowed us to accurately design both the magnetic circuit
array and the magnetic field sensor apparatus. Finally our
knowledge of reading datasheets and prototyping circuits on
breadboards has been helpful in putting together both the field
sensor circuitry and the RF generation and receiving circuitry.