TuftsUniversity
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:Samuel MacNaughton
ChandlerDowns
Dante DeMeo
Project Advisor: Sameer Sonkusale
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:
Figure 1: System Engineering Diagram Larger Version Available in Appendix.
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.
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:
- FrequencyRange: 10-106 Hz
- Current Supply: ~15 mA
- Power Consumption:<50 mW
- Weight:~6 oz.
- Dimensions6 x 4 x 1 in. or less
- Operating Temperature:~27° C
- Coil ArchitectureToroidal
- Coil Structure Inductance:>5 mH (will change slightly depending on sample)
- Coil Structure Resistance<100Ω
- Q-factor<.005 (smaller is better)
- Parasitic Capacitance<10pF
Organization:
ChandlerDowns:
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 / Start Date / Duration (Weeks) / Scheduled Finish DateSign-up Sheet / 9/7/2007 / 2 / 9/21/2007
Project Proposal / 9/28/2007 / 1 / 10/5/2007
High level circuit design / 9/21/2007 / 3 / 10/12/2007
System Engineering Diagram / 10/5/2007 / 1 / 10/12/2007
Project Plan / 10/26/2007 / 2 / 11/7/2007
Risk Assessment / 10/19/2007 / 3 / 11/9/2007
Component Selection and Acquisition / 10/12/2007 / 9* / 12/14/2007
Design Specs / 9/21/2007 / 12* / 12/14/2007
Circuit Construction / 12/14/2007 / 8* / 2/8/2008
Breadboard / LabView Tests / 1/25/2008 / 3 / 2/15/2008
Reassessment of circuit / 2/15/2008 / 3* / 3/7/2008
Design of PCB / 3/7/2008 / 2 / 3/21/2008
Computer Modeling / 3/7/2008 / 2 / 3/21/2008
Creation of PCB / 3/21/2008 / 1 / 3/28/2008
Working Prototype / 3/28/2008 / 3 / 4/18/2008
Final Testing / 4/18/2008 / 1 / 4/25/2008
Final Report / 4/18/2008 / 2 / 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:
10/5Project Proposal
11/7Project Plan
12/14Design Specs
2/15Final Testing / Beta Prototype
4/18Working Prototype Presentation
5/1Final Report
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 KyoungPark.
References:
Griffiths, David J. Introduction to Electrodynamics. 3rd ed. UpperSaddleRiver: Prentice Hall, 1999.
Park, Kyoungchil.
Sonkusale, Sameer. Project Advisor
Appendix: System Engineering Diagram (Large Version)
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