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Tufts University
College of Engineering
Department of Electrical and Computer Engineering
EE97/98 Senior Design
Fall/Spring 2007
Handheld Magnetic and Dielectric
Susceptometer
Project Plan
Revision 2
Name:
Project Advisor:
Samuel MacNaughton
Chandler Downs
Dante DeMeo
Sameer Sonkusale
1
Introduction:
The object of our project is to create a portable, accurate magnetic susceptometer.
The susceptometer functions by finding the frequency of highest gain through an inductor
containing a paramagnetic sample. Phrased alternatively, the susceptometer finds the
frequency at which a coil structure filled with a paramagnetic sample exhibits the highest
inductance.
This frequency is unique to any substance; thus presenting a inexpensive, fast and
effective way to identify substances. At present, researchers in the medical and other
fields rely on chemical or fluorescent tags to identify pathogens, pollutants and other
microscopic objects of interest. These methods are time, cost, and labor intensive. A
method relying on magnetic susceptibility presents an easy solution to this problem with
the added incentive that it could be made readily portable.
We will also add a module to find the dielectric susceptance of the substance (the
electric analog of magnetic susceptibility). The implementation and hardware is the
exact same as the magnetic counterpart with the exception of a capacitive structure rather
than a coil structure. Our focus throughout the project will be the magnetic
susceptometer.
System Overview:
In the broadest scope, the system sweeps a wide frequency range (10-10^6 Hz) of
current through a coil structure infused with the paramagnetic sample. The signal
generated is then passed through a quadrature multiplexing circuit to separate the real
part of the signal from the complex. This yields the output of the device.
System Engineering Diagram:
System Engineering Diagram
Revision 3
Team Indigo
PCB
Power Supply
(batteries)
Voltage Supply
Toggle Switch
AC Current
Source
Current
Control
Mechanism
Modified
current
S(t)
Gain &
Frequency
A(ω)
Amplified
S(t)
Amplifiers
Analysis Network
Analog to Digital
Converter
Digital
Out
Computer
Current
Modified
current
Sm(t)
Coil Structure
Modified
current
Se(t)
Human Force
Sample
Capacitive
Structure
Figure 1: System Engineering Diagram Larger Version Available in Appendix.
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Assumptions:
The change in inductance of the coil caused by the magnetic permeability of the sample
will only negligibly affect the frequency response of the circuit.
We will devise a way to repair or negate the effect of the parasitic capacitance.
Magnetic noise will be negligible.
The phase delay due to the parasitic capacitance will be negligible.
The critical temperature for the material being tested is much lower than the temperature
at which the test is being run.
Critical Risks:
Current Source and Parasitic Capacitance:
We are using a modified Howland Current source to supply the current to our coil
structure. It is a voltage controlled current source, and supplies a constant amplitude of
current at any frequency assuming a constant impedance. However, because of the coil
structure and the inherent parasitic capacitance of any circuit, the impedance is actually
frequency dependant. Worse, until we actually implement the circuit on a PCB, there can
be absolute certainty to the magnitude of the parasitic capacitance. We have complete
control of the inductance of the coil. The frequency dependant output of our current
source would cause our output signal to either attenuate or amplify linearly with the
frequency, which could mask the small peak we are searching for.
There are several means to handle this problem. We can design the inductance of
the coil structure to counteract the effect of the parasitic capacitance. However this may
be impractical as there are already many constraints on the design of the coil.
We can also implement a negative impedance compensator circuit to counteract
the parasitic capacitance. This would effectively solve the problem. We would need to
tune the circuit once the exact value of the capacitance is known.
If all else fails, perhaps the simplest solution is to accept the attenuation due to the
parasitic capacitance and compensate for the effect in the analysis stage of the circuit.
Noise:
The peak at the resonant frequency of the sample can be easily masked by noise.
The magnitude of the magnetic susceptibility is proportional to the size of the peak, so
the ability to discern small peaks will allow us to detect more substances and increase the
value of our product. Thus, the more resilient our circuit is to noise, the more value it
will have.
Noise removal algorithms and methods have no way of differentiating the
resonant frequency peak from noise, so the only option is to prevent or dampen all noise
entering the circuit. Implementing the circuit on a PCB will reduce noise from
interconnects, which is the greatest source of error in bench tests.
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Random electric and magnetic fields can create noise in the susceptometer. We
must surround our coil, if not the whole circuit, with a conductor to act as a Faraday cage
and shield our circuit from electric field interference. Without this protection bench tests
have shown the circuit to be dysfunctional. Magnetic noise interfering with the circuit is
unlikely. The only way to block magnetic field interference is with mu-metal, which is
prohibitively expensive and fastidious. In the event of magnetic field induced noise, we
would simply move the device away from the source of noise.
Architecture:
This device will identify paramagnetic substances by their signature resonant
frequencies in an oscillating magnetic field. It will also perform an analogous process in
an oscillating electric field. The input will be a paramagnetic sample and the output will
be a plot of gain versus frequency.
The portable device will be constructed onto a printed circuit board (PCB). The
PCB will not require any input signals, for everything will be housed on-board. The only
human interaction required will be to physically put the sample on the sensor structures
(coil or capacitive). The user will also have to indicate which test to run (dielectric or
magnetic) and start the actual frequency sweep.
The major components on the PCB will include: batteries, sensors, current source,
control mechanism, amplifiers, analysis network, analog to digital converter, and an
output to a computer. The batteries will serve to power the unit and provide maximum
portability. The sensors will be where the sample is placed so as to determine the
magnetic and dielectric properties and thus identify the substance. The control
mechanism will be used to choose between the dielectric or magnetic sensor. The
amplifiers will be used to boost the signal coming from the sensors, while attempting to
minimize noise. The analysis network will consist of lock in amplifiers to find the peaks
in the current output from the sensors. The analog to digital converter will perform as its
name implies, making a signal suitable for output to a computer for further analysis and
data acquisition.
Key Specifications:
 Frequency Range:
 Current Supply:
 Power Consumption:
 Weight:
 Dimensions
 Operating Temperature:
 Coil Architecture
 Coil Structure Inductance:
 Coil Structure Resistance
 Q-factor
 Parasitic Capacitance
10-106 Hz
~15 mA
<50 mW
~6 oz.
6 x 4 x 1 in. or less
~27° C
Toroidal
>5 mH (will change slightly depending on sample)
<100Ω
<.005 (smaller is better)
<10pF
Organization:
Chandler Downs:
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Howland Current Source with negative impedance matching (if necessary).
Dante DeMeo:
Quadrature Multiplexing Module for Demodulation of Coil Signal
Sam MacNaughton:
Coil Structure and Low-Noise Amplifiers
All:
We will share responsibility for intercommunication to ensure the compatibility of
our modules. We will also collaborate on all written deliverables to ensure we stay on
the same page in terms of the overall project plan and execution. Also, the output stage
will be developed cooperatively as it will depend on every aspect of the circuit and must
be completed last.
Detailed Plan:
Task
Sign-up Sheet
Project Proposal
High level circuit design
System Engineering Diagram
Project Plan
Risk Assessment
Component Selection and Acquisition
Design Specs
Circuit Construction
Breadboard / LabView Tests
Reassessment of circuit
Design of PCB
Computer Modeling
Creation of PCB
Working Prototype
Final Testing
Final Report
Start Date
9/7/2007
9/28/2007
9/21/2007
10/5/2007
10/26/2007
10/19/2007
10/12/2007
9/21/2007
12/14/2007
1/25/2008
2/15/2008
3/7/2008
3/7/2008
3/21/2008
3/28/2008
4/18/2008
4/18/2008
Duration
(Weeks)
2
1
3
1
2
3
9*
12*
8*
3
3*
2
2
1
3
1
2
Scheduled
Finish Date
9/21/2007
10/5/2007
10/12/2007
10/12/2007
11/7/2007
11/9/2007
12/14/2007
12/14/2007
2/8/2008
2/15/2008
3/7/2008
3/21/2008
3/21/2008
3/28/2008
4/18/2008
4/25/2008
5/1/2008
This schedule is subject to change. An asterisk in the duration column indicates built in
buffer time to offset delays.
Major Milestones:
10/5 Project Proposal
11/7 Project Plan
12/14 Design Specs
2/15 Final Testing / Beta Prototype
4/18 Working Prototype Presentation
5/1
Final Report
5
Acceptance Test Plan:
Each of the main three modules (the current source, quadrature multiplexing
demodulator, and coil structure) will need to undergo basic tests. The frequency response
of the current source as a function of complex impedance will need to be empirically
found through extensive oscilloscope bench tests. The impedance (both real and
complex) of the coil structure and the parasitic capacitance will need to be accurately
measured in order to be compensated for. The quadrature multiplexing demodulator can
be tested with a modulating function generator to ensure proper demodulating over the
ranger of frequencies.
The overall system test will simply be detecting a control paramagnetic sample
from its unique susceptibility spectrum. Our measure of success will be to perform an
analysis of a substance with equal or greater precision than that of the lab test bench setup
by graduate student Kyoung Park.
References:
Griffiths, David J. Introduction to Electrodynamics. 3rd ed. Upper Saddle River: Prentice
Hall, 1999.
Park, Kyoungchil.
Sonkusale, Sameer. Project Advisor
6
Appendix: System Engineering Diagram (Large Version)
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