IEEE TRANSACTIONS ON INSTRUMENTATION AND MEASUREMENT, VOL. 58, NO. 8, AUGUST 2009
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Design and Development of a Low-Cost Digital
Magnetic Field Meter With Wide Dynamic Range
for EMC Precompliance Measurements and
Other Applications
Sandeep M. Satav and Vivek Agarwal, Senior Member, IEEE
Abstract—The design and development of a low-cost, portable,
and easy-to-operate instrument that can measure static and timevarying magnetic fields for electromagnetic-compatibility (EMC)
precompliance and other applications are presented in this paper.
The basic sensor used is a Hall-effect element. The instrument has
an accuracy of 0.5% and a wide bandwidth of 30 kHz. The resolution of the meter is 12.5 mG, enabling accurate measurement
of small fields, such as the geomagnetic field. Isotropic and linear
detection of magnetic fields is possible with true root-mean-square
(RMS) measurement. Other desirable features, such as maximum
hold, data logging, and computer interface, are also incorporated.
A graphical user interface (GUI) has been developed for computer interface and data presentation. A Helmholtz coil and a
Zero-Gauss chamber have been used for design validation. A
low-cost EMC precompliance test setup, based on the proposed
work, is also presented. All the design details and measurement
results are presented. Apart from being low cost and accurate,
the proposed meter has a lower part count and involves a simple
design-and-fabrication process.
Index Terms—Electromagnetic compatibility (EMC), electromagnetic interference (EMI), Hall-effect sensor, Helmholtz coil,
magnetic field, precompliance.
I. I NTRODUCTION
S
TATIC and time-varying low-frequency (typically less than
30 kHz) magnetic fields can severely affect the functioning of electrical and electronic equipment [1], [2]. They can
induce interference voltages in wiring loops, the amplitudes
of which depend on the area that is exposed to the fields,
forming a classical example of electromagnetic interference
(EMI). Nontoroidal transformers, switching power transistors,
and conducting wires carrying currents are some sources of
magnetic field generation, apart from natural sources, such as
the geomagnetic field and the fields from nearby magnetized
metallic objects [3]. It is desired to keep the magnitudes of such
fields well below the susceptibility levels of the systems or the
levels defined by the applied EMC standards. EMC compliance
Manuscript received December 2, 2007; revised July 21, 2008. First
published May 15, 2009; current version published July 17, 2009. The
Associate Editor coordinating the review process for this paper was
Dr. Subhas Mukhopadhyay.
S. M. Satav is with the Research Centre Imarat, Hyderabad 500 069, India.
V. Agarwal is with the Department of Electrical Engineering, Indian
Institute of Technology-Bombay, Mumbai 400 076, India (e-mail: agarwal@
ee.iitb.ac.in).
Digital Object Identifier 10.1109/TIM.2009.2016367
can be confirmed only by measurement of the fields. Hence,
an accurate and repeatable measurement of the magnetic field
is an important aspect of electromagnetic compatibility (EMC)
[4], [5].
Other than EMI/EMC, there are several other areas where
there is a need for the measurement of magnetic fields, such
as product design, material inspection, and research. Some
of the specific applications are magnetic dipole characterization, linear and rotary position sensing, current sensing and
permanent-magnet quality assurance, inward inspection, and
magnetocardiography [6], [7].
There are various techniques that are available for the measurement [8] of magnetic fields, such as induction coils, fluxgates, nuclear magnetic resonances, superconducting quantum
interference detectors (SQUIDs), and solid-state devices. A
survey of various magnetic field measurement techniques is
presented in Table I [8]. Depending on the levels of magnetic
field, cost, complexity, sensitivity, and overall performance
specification, one can choose a suitable technique for a given
application. However, solid-state magnetic field sensors (using
a Hall-effect element as the basic sensing element) have several
advantages [9] over others, such as small size, high physical
reach, and capability of high spatial resolution of the fields.
They can monolithically be integrated on a single chip, making
it possible to realize a system-on-chip, consume very low
power, and provide a direct voltage or current output, which
facilitates simpler subsequent circuit stages. They are highly
efficient in rejecting electric field during the measurements and
do not modulate the field that they are meant to measure. These
sensors can also be used to detect the polarity of a static field.
Inherent problems associated with Hall-effect [10] elements,
such as nonlinearity, temperature dependence, and susceptibility to external noise, can easily be eliminated at the sensor
integration level itself using chopper stabilization and other
techniques.
Time-varying magnetic fields are common (e.g., power lines
produce a time-varying field that is proportional to the current
in the transmission line). The requirement of time-varying
magnetic field measurement for compliance with military and
nonmilitary standards is summarized in Table II [11]–[13].
Although it does not represent the complete requirement, it does
give an idea about the nature and levels of the field required to
be measured by an EMC engineer.
0018-9456/$25.00 © 2009 IEEE
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TABLE I
SURVEY OF MAGNETIC FIELD MEASUREMENT TECHNIQUES
TABLE II
TIME-VARYING MAGNETIC FIELD LEVELS, AS DEFINED IN VARIOUS STANDARDS
The dc flow in high-voltage dc transmission, elevators,
cranes, and battery-operated power systems (e.g., inverters
and uninterrupted power supplies) and the presence of strong
magnets in medical imaging systems produce elevated levels of
static magnetic fields. Structural and reinforcing steel members
in building structures, for example, can get magnetized as a
result of being in the vicinity of static magnetic fields over a
length of time [1]. Elevated levels of the static magnetic field
are quite common on surface ships. These elevated levels of the
static field induce interference phenomenon, such as cathode
ray tube image distortion and the malfunction of navigational
instruments comprising gyros and associated electronic circuits. These examples highlight the need for the measurement
of the static magnetic field for EMI/EMC purposes. The same
situation applies to any space mission, where every system must
be compliant to the designated dc magnetic field levels [14].
Magnetic field (both static and time varying) measurements
and verification are essential in various industries, where
strong magnetic fields are generated in certain applications
(e.g., magnetic-field EMI testing and superconductivity experiments). These fields are highly localized and can be modeled
as a magnetic dipole moment. The measurement of such fields
SATAV AND AGARWAL: DESIGN AND DEVELOPMENT OF LOW-COST DIGITAL MAGNETIC FIELD METER
with high spatial resolution gives a better analytical view of the
fields. Dipole moment characterization after the measurement
of magnetic fields around appliances can be used for estimating
emissions levels [15].
EMI performance evaluation (compliance) is mandatory for
all kinds of equipment available in the market. Similarly, for
all the equipment put into an actual application, whether it is
commercial or military, assurance of the EMC is mandatory. In
most cases, the compliance of the product (EMC) is established
in accredited laboratories using the recommended test method
and apparatus for the relevant EMC standard. Noncompliance
of the product in the first attempt results in the loss of time and
money, and lengthens the life cycle of the product. Hence, an
EMC precompliance test at the designer’s (or manufacturer’s)
site with a low-cost, portable, and easy-to-operate instrument
is desirable [16]. A careful precompliance test with such an
instrument reduces the overall development cost of any electrical and/or electronic product and is an essential part of a successful product design. The proposed instrument is primarily
meant for EMC precompliance although it can also have other
applications.
In the past, several efforts have been made for the design and
development of magnetic field measurement systems through
analog and digital approaches. Arseneau and Zelle [17] proposed an isotropic detection method for field measurement.
However, with their design approach being analog, it does not
have the inherent advantages of a digital system, such as storage
of measurement data, incorporation of correction factors, removal of nonlinearity, and easy expandability. In addition, their
detection method is not suitable for static field measurements.
Sedgwick et al. [18] used a digital design approach where
the nonlinearity and temperature variations are mapped and
stored in the memory for final corrections. However, this system
requires a large number of components.
This paper presents a compact, economical, yet accurate
magnetic field meter. The salient features of the proposed
system are given here.
1) It is a low-cost, low-part-count, and easy-to-operate instrument with high performance-to-cost ratio. Small companies can easily afford it for precompliance testing.
2) Several commercial instruments are available, but they
are expensive. Furthermore, their use of applicationspecific ICs precludes customization of the design for
specific requirements. There is no such restriction with
the present design.
3) It has a wide dynamic range of 96 dB, a high resolution
of 12.5 mG, and a sensitivity of 50 mG.
4) It can measure dc magnetic fields with 0.5% accuracy (of
the reading) and also indicates their polarity.
5) It has an ac bandwidth of 30 kHz.
6) It has computer connectivity and data storage capacity for
512 readings.
7) It is lightweight (less than 300 g) and of small size (approximately 150 × 80 × 35 mm3 ). This makes it highly
suitable for field applications due to its portability.
All the details of this paper are presented in the succeeding
sections.
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II. H ARDWARE D ESCRIPTION OF THE P ROPOSED
M AGNETIC F IELD M EASUREMENT S YSTEM
The principal objective was to develop an ac/dc magnetic
field measurement system for EMC precompliance measurements and other applications that require information about the
surrounding magnetic fields.
The block diagram of the proposed system is shown in Fig. 1.
It comprises an analog multiplexer, a true root-mean-square
(RMS)-to-dc converter, a 16-bit analog-to-digital converter
(ADC), and a digital meter based on a popular low-cost microcontroller (Atmel’s 89S52, which belongs to Intel’s MCS-51
family). The man–machine interface (MMI) is provided in the
form of a keyboard and an eight-character liquid-crystal-display
(LCD) module. Computer connectivity is provided through
a standard RS232 port. Appropriate grounding, filtering, and
shielding practices have been followed to curb external noise
and reduce the internal noise coupling [19], [20].
A. Linear Hall-Effect Magnetic Field Sensor
An integrated Hall-effect sensor (Allegro MicroSystems,
Inc., Part No. 1321A) [21], [22] has been used for measuring
the magnetic field. It is a ratiometric Hall-effect sensor, which
produces a voltage that is linearly proportional to the applied
magnetic field. It has a monolithic circuit, which integrates a
basic Hall-effect element, temperature compensation circuitry,
a high-gain amplifier, and a low-impedance output stage for
an external interface. A proprietary dynamic offset cancellation
technique that employs an internal high-frequency (HF) clock
reduces the residual offset voltage, which is normally produced
by external thermal and mechanical stresses. This technique
produces an extremely stable quiescent output voltage and
precise recovery after temperature cycling. The output precision
is achieved during the manufacturing process by internal gain
and offset trim adjustments [22]. However, any other similar
Hall-effect sensor can be used, in association with the rest of
the circuit. By using a monolithic sensor, the requirement of
several external electronic building blocks, such as a precision
current source and a digital-to-analog converter, for inherent
offset compensation has been eliminated, resulting in a lowcomponent-count design.
Two probes—one transverse and the other isotropic—have
been fabricated using the Hall-effect sensor. Each probe is
fabricated out of a thin FR-4-grade printed circuit board (PCB).
In the transverse probe, the Hall-effect sensor is placed on the
tip of the PCB (Fig. 2). Copper tracks on the PCB are coated
with carbon graphite to shield the dc output signal from external
noise. A black dot is placed on the front surface of the sensor to
define the polarity of the magnetic field under measurement.
In the isotropic probe, three similar sensors are orthogonally
placed and covered in a Teflon spherical enclosure. In isotropic
detection, the output on the LCD module provides the vector
sum of the magnetic flux density from its three (the x, y, and z
probes) components as follows:
B=
Bx2 + By2 + Bz2 .
(1)
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Fig. 1. Block diagram of the proposed digital magnetic field meter.
Fig. 2. Prototype of the transverse probe employing the Hall-effect sensor.
The conducting leads are realized using a high-impedance
transmission line (nonmetallic conductive wires of FIBEROHM
make, having a resistance of approximately 8 kΩ/in, were
used) to minimize the modulation of the output dc signal by
external electrical noise.
B. Analog Multiplexer
Philips Semiconductor’s 74HC4051, which is an eightchannel analog multiplexer, is used. Of the eight channels, only
three have been used for the circuit operation, and the rest of
the channels are tied to the power supply to reduce noise pickup
and maintain parameters, such as the “ON” resistance (typically
70 Ω) and crosstalk, within limit.
In isotropic detection, the output of each X, Y, and Z Hall
sensor is routed through the three channels, which are subsequently fed to the true-RMS-to-dc converter. In transverse
detection, only one channel (channel Bx ) is selected by default.
The control signals for the multiplexer are generated from
the microcontroller. These signals have an appropriate phase
between them that is governed by the settling time of the
Hall sensor. The effect of the “ON” resistance of the device
(typically 70 Ω) is compensated in the calibration process and
therefore does not affect the field measurement. Similarly, the
sine-wave distortion through the multiplexer is typically only
0.06% at 5-V power supply, which does not affect the accuracy
of the ac field measurements [23].
C. True-RMS-to-DC Converter
For the measurement of time-varying magnetic fields, it is
necessary to measure and/or compute the RMS value of the
field levels. Before converting a time-varying signal into its
RMS value, the signal is buffered through an ultralow-offset
operational amplifier [Fig. 3(a)].
The traditional method of converting peak values into RMS
values using heat and temperature measurement suffers from
Fig. 3. (a) Circuit schematic of the buffered true-RMS-to-dc-converter section
of the magnetic field meter. (b) Interfacing of the ADC with the sensor.
many inherent limitations, such as poor dynamic range, temperature sensitivity, and large conversion errors. Maxim’s
MX536A is a true-RMS-to-dc converter that uses an implicit
method of RMS computation [24]. The transfer equation of the
device is given as follows:
(2)
Vout = avg (Vin )2 .
As it is a low-power-consumption device, it is useful for
battery-operated instruments. The conversion error of the device is compensated using the calibration process and by averaging (moving average) over eight samples. To reduce the
noise, such as the harmonics of the power line frequency (50/
60 Hz), power supplies, or relay-switching transients, an implicit antialiasing filter is provided in the true-RMS-to-dc converter. The chip employs two levels of filtering for better noise
performance. First, in the circuit, an averaging time constant of
25 ms is set through an internal resistor of 25 kΩ (connected at
SATAV AND AGARWAL: DESIGN AND DEVELOPMENT OF LOW-COST DIGITAL MAGNETIC FIELD METER
2841
pin 4 of the MX536A IC) and an external capacitor of 1 μF.
It implies that any signal above 40 Hz is averaged, thereby
significantly reducing the noise. Second, a post filter (one-pole
filter, a 2.2-μF capacitor, and a 25-kΩ internal resistor connected between pins 8 and 9 of the MX536A IC) has been
incorporated, which reduces the ripple [24].
D. ADC
This section uses Maxim’s MAX1162, which is a 16-bit
successive-approximation ADC [25]. It is a serial-output lowpower (12.5 mW at 5 V) small-sized ten-pin dual-flat-no-lead
(DFN) device, which makes it an ideal choice for batterypowered portable devices. The interfacing of the ADC with the
sensor is shown in Fig. 3(b).
It has separate analog and digital power supplies, making it
suitable for small-signal (analog-to-digital (A/D) conversion)
applications. General EMC design guidelines were followed
to keep the system noise level as low as possible. Separate
analog and digital power supplies have been used. The analog
and digital grounds are separately maintained and shorted only
at the most stable point of the two ground planes, which is
obtained after probing the planes with the help of a spectrum
analyzer.
The external reference voltage (4.096 V), which sets the
input signal range, is derived from a separate three-pin reference source regulator with heavy decoupling. The full linear
bandwidth of the MAX1162 is 10 kHz (the −3-dB bandwidth
is 4 MHz), which may pose severe problems in the form of
power line frequency, and HF and radio-frequency (RF) noise
coupling during the measurement of the dc signal. Since the
ADC is used to measure only the dc signal, it is highly desirable
to eliminate all the noise above 10 Hz. Filtering has been used
at three different levels. An EMI filter (low-pass filter) has been
incorporated just before the ADC, similar to an antialiasing
filter [Fig. 3(b)]. This filter attenuates HF/RF noise superimposed on the dc (or time-varying dc) signal being fed into the
ADC. Oversampling reduces the broadband white noise. The
low-frequency (50/60 Hz) noise is removed by the movingaverage technique (eight-sample processing). The ADC is used
to measure only the dc signal (or dc varying over time) from
the true-RMS-to-dc converter. A sample rate of 100 Sa/s was
used. The ratiometric linear sensor is powered with the A/D
reference voltage source, allowing the sensor to track changes
in the A/D least-significant-bit (LSB) value. As the reference
voltage varies, the LSB will proportionally vary. The output of
the ADC is in serial form, which further reduces the copper
track traffic on the PCB. The read clock of the ADC is properly
guarded to reduce radiated emissions and crosstalk.
E. Digital Meter and MMI
The digital meter is built around the popular low-cost 8-bit
microcontroller AT89S52 from ATMEL. The device was chosen, in particular, because of its low cost, in-system reprogrammable flash memory, and low power consumption. An MMI
is provided in the form of a low-power alphanumeric display
module and keyboard. The module is shielded with μ metal
Fig. 4. Magnetic field concentrator for volumetric measurement.
to reduce the radiated emissions from it. The backlight in the
LCD is a switchable option to extend the battery life. The keys
are interfaced in the interrupt-driven mode to reduce the RF
emissions that would occur due to continuous scanning of key
lines in the polling mode. A piezobuzzer is also interfaced to
the meter section to form a threshold alarm that can be set as a
percentage of the full-scale reading, which is 1000 G. A 1-kB
nonvolatile random-access memory (NVRAM) is incorporated
to store up to 512 measurement data points. The data can
be logged at variable rate. The maximum time for logging is
24 h, with an interval of approximately 3 min. The computer
interface is provided through a standard universal asynchronous
receiver/transmitter available in the microcontroller. Data transfer uses the RS232 protocol, and the necessary level shifting is
achieved using Maxim’s MAX232 IC.
F. Power Supply
The proposed instrument requires two +5-V (analog and
digital) supplies and a single −5-V supply (which is only used
by a true-RMS-to-dc-converter IC), and is provided by two
9-V rechargeable batteries. Linear fixed-voltage regulators are
used to regulate the power supplies. The reference voltage for
the ADC is separately regulated to avoid any missing code.
For example, a transient spike on the reference voltage of
the ADC can cause a missing code or a blank code, such as
0000F. A digital potentiometer is provided for the fine-tuning
of the reference voltage. Use of switching regulators for the
generation of negative supply from a single positive supply is
avoided to keep the system noise as low as possible.
G. Field Concentrator
The presented instrument uses a Hall probe for point magnetic field measurements. However, there are several requirements of volumetric measurement of the magnetic fields, such
as geomagnetic survey, electrical power-line-frequency interference studies, and security inspections of aircraft. The volumetric measurement can be achieved with the Hall-effect sensor
by using concentrators [6], such as that shown in Fig. 4.
The structure is symmetrical about a 3–5-mm air gap, comprises soft iron or other ferrous metals, and allows the placement of a Hall sensor in the air gap. The concentrator bends
the local magnetic flux lines, so that more lines pass through
the Hall sensor. However, an inappropriate size of the field
concentrator can distort the magnetic field under measurement;
hence, its size depends on the volume under measurement. For
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a volume of 1 m3 , a magnetic field concentrator with a diameter
of 6 cm gives results that are comparable to those obtained with
an induction coil or loop of 6-cm diameter.
III. S OFTWARE D ESCRIPTION OF THE P ROPOSED
M AGNETIC F IELD M EASUREMENT S YSTEM
This section discusses the firmware designed for the digital
meter and the graphical-user-interface (GUI) development for
the computer interface and data presentation.
A. Firmware
In this paper, the operation of the entire system is controlled
by an 8-bit microcontroller. The program of the microcontroller
(assembly code) was developed and stored on to the flash readonly memory (ROM) of the microcontroller. An operation flow
diagram was laid according to the sequential operations during
the measurement process. Each module of the program (e.g.,
the keyboard and LCD interface, analog-signal-processing control section, data acquisition from the ADC, signal processing
(as in moving averaging), and mathematical operation) is separately written and rigorously tested for several anticipated conditions. The keyboard is interfaced in the interrupt-driven mode
to minimize power consumption and reduce radiated emission
due to continuous row and column key scanning. Mathematical
operations, such as addition, multiplication, subtraction, and
division, were implemented in 16-bit format as the input data
from the ADC are of 16-bit format. Each operation functions
as an individual module. Data manipulation, such as binaryto-binary-coded-decimal (BCD) and BCD-to-ASCII conversions, was also implemented as dedicated modules/subroutines.
Similarly, memory (NVRAM/E2 ROM) write/read, maximum
hold function, and moving-average (as explained later in this
section) subroutines were also developed. These subroutines
are invoked, as required by the algorithm during execution. A
flowchart of the program flow is shown in Fig. 5.
Moving-Average Signal Processing: In a measuring instrument, such as that presented here, the data are prone to corruption by all kinds of noise (i.e., RF, power line frequency,
transient, high-speed digital switching within the system, and
overall system noise). Although the instrument is provided with
a true-RMS-to-dc readout, it is desirable to filter aberrations
and reveal the real trend in a collection of data points. “Moving
average” is a simple mathematical technique that has been
adopted in this paper to achieve this. If a digital waveform is
cluttered with noise or a slowly drifting baseline needs to be
eliminated from a higher frequency signal, a moving-average
filter may be applied to achieve the desired results. The moving
average is also a prototype of the finite-impulse response filter,
which is the most common type of filter used in computer-based
instrumentation. The moving average of a waveform can be
calculated as
A(n) =
1
S
n+(a−1)
n
Y (n)
(3)
Fig. 5.
Flowchart of the firmware of the proposed system.
where A is the new averaged value, n is the data point position,
S is the smoothening factor, and Y is the original data point
value.
Any periodic waveform can be thought of as a long string
or collection of data points. The algorithm accomplishes a
moving average by taking two or more of these data points
from the acquired waveform; adding them; dividing their sum
by the total number of data points added (smoothing factor);
replacing the first data point of the waveform with the average
just computed; and repeating the steps with the second, third,
and remaining data points until the end of the data is reached.
The new waveform consists of the averaged data, having lesser
number of data points. The number of data points in the new
form is given by
Number of new data points
= [Total number of data points−(smoothing factor−1)] . (4)
For example, if the total data points are 100, with a
smoothening factor of 8, then the new averaged data set consists
of 93 data points, which is highly undesirable for a data acquisition system. This can be resolved using a feathering technique
with waiting time in the beginning and a decreasing smoothing
factor toward the end of the data sequence.
SATAV AND AGARWAL: DESIGN AND DEVELOPMENT OF LOW-COST DIGITAL MAGNETIC FIELD METER
2843
Fig. 7. Measurement setup for the digital magnetic meter.
measurement, and product design information) can be saved in
text file format for further reference, such as while carrying out
the final compliance tests of the product.
IV. C ALIBRATION AND M EASUREMENT R ESULTS
Fig. 6. Screenshots of the GUI. (a) Offline and (b) online displays of measurement data.
B. Computer Interface and GUI Development
The main objectives of the computer interface, along with
the GUI, are ease of use, large storage space for the measurement data, and graphical presentation. Calibration (correction
factors) of the instrument, correction of nonlinearity of results,
and firmware upgradability are also achieved. A GUI has been
developed for this purpose using Matlab; however, it could
be developed using any other high-level language, such as C,
C++, or Visual Basic. Screenshots of the GUI in the offline
and online modes are shown in Fig. 6(a) and (b), respectively.
After connecting the instrument to a personal computer (PC)
via a null modem cable, the GUI can be activated. A communication is established after the appropriate port (COM1 or
COM2 in the PC) is selected. In the offline mode, the contents
of the random-access memory are transferred to the PC. The
measured field in the transverse mode can be shown against
time, whereas, for isotropic detection, all three components can
simultaneously be seen. In the online mode, the graph (display
panel) inside the GUI will show the reading of the LCD module
of the microcontroller.
For easy interpretation of the magnetic field levels in various
units, the result can be shown in different units; gauss, amperes
per meter, and tesla. The measurement data with necessary
comments (e.g., description of the measurement site, time of
To accurately measure a magnetic field [26], a system for
periodic calibration of the magnetic field meter is needed. The
current ANSI/IEEE standard specifies two possible methods for
calibration: 1) a single square loop or 2) a round Helmholtz coil
[27]–[30]. The latter option has been adopted in this paper. The
complete measurement setup is shown in Fig. 7.
For the measurement of magnetic fields in the laboratory
and the calibration of the proposed magnetic field measurement system, a circular Helmholtz coil with a coil constant of
1.6400 G/A has been used. It is a small coil with a diameter
of 0.2 m and a working volume of 4 × 4 × 4 cm3 . A field
uniformity of the coil of 100% is realized within 2 cm about the
center of the coil [Fig. 8(a)], which is sufficient for measuring
the surface area (approximately 2 × 2 mm2 ) of the Hall-effect
sensor. The normalized magnetic field uniformity of the coil is
shown in Fig. 8(b).
For generating a pseudo-Zero-Gauss ambient for zeroing of
the instrument, a specialized Gauss chamber has been fabricated. The Zero-Gauss chamber comprises a double-walled
cylinder with a thickness of about 8 mm and is made from
very high permeability μ metal. The Zero-Gauss chamber is
further enclosed in an iron (low carbon) enclosure filled with
iron wool. The arrangement is designed to provide a shielding
effectiveness (SE) of more than 120 dB for the magnetic fields.
A schematic of the arrangement under Zero-Gauss condition
is shown in Fig. 9(a). The SE of the Zero-Gauss chamber was
verified with a permanent magnet and a commercial calibrated
gauss meter.
There is a nearly Zero-Gauss field (500 nG after 120-dB
attenuation of a 500-mG ambient magnetic field) within the
chamber. After placing the Hall-sensor probe into the chamber,
the reference voltage is set at 5.000 V. This sets the full-scale
calibration of the instrument. For verification of the static field
measurement results, a known dc from a linear power supply
is fed into the Helmholtz coil. For verifying time-varying field
measurement results, a low-total-harmonic-distortion (THD <
2%) generator is used to drive the Helmholtz coil. Deviations
in the results from the calibration chart of the Helmholtz coil
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IEEE TRANSACTIONS ON INSTRUMENTATION AND MEASUREMENT, VOL. 58, NO. 8, AUGUST 2009
Fig. 8. (a) Field uniformity of the Helmholtz coil used for the calibration of the meter. (b) Normalized magnetic field uniformity.
Fig. 9. (a) Zero-Gauss setup for measurement. (b) Measurement results of the output voltage and magnetic flux density mapping.
are found to be less than 1.5% at higher field levels. These
deviations are noted and stored in the program memory of the
microcontroller, so that appropriate correction factors can be
applied while displaying.
The system’s noise floor is about 200 μV. Each bit corresponds to 62.5 μV; hence, the two lower bits cannot be read
due to a noise floor of 200 μV. Hence, the effective number
of bits of the ADC is 14. Since the ADC has a unipolar
transfer function, the full scale (Vref ) is represented by FFFF H,
whereas 0 V is 0000 H. The conversion factor of the sensor
is 5 mV/G, and 1 LSB is equal to (4.096/216 ) = 62.5 μV.
Hence, the system resolution for the magnetic field density is
62.5 μV/5 mV/G = 12.5 mG. The minimum voltage level that
the instrument can detect is 200 μV, which gives a sensitivity
of 50 mG. The reference voltage for the ADC is 4.096 V. Here,
it is worth noting that 212 = 4096, which is exactly 1/16 of 216 .
This minimizes rounding errors, because each code change is
a nice fraction of 4.096 V. Another way to say this is given
here: A group of 16 bits, each worth 4.096/(216 ), will be equal
to 1 mV with a 4.096-V reference. The complete voltage scale
calibration in terms of the magnetic flux density (in gauss) is
shown in Fig. 9(b).
During the measurement of a magnetic field using a Halleffect-sensor-based meter, there are two common sources of
errors. The first is the influence of ambient magnetic fields and
offset signals from the internal circuitry of the meter. This error
is nulled after taking a reading with the probe in the Zero-Gauss
chamber and is particularly recommended for the measurement
of fields below 1 G. The other error occurs due to the relative
angle between the Hall element face and the magnetic flux
being measured. The maximum output is generated when the
flux lines are perpendicular to the Hall sensor and is the way
each Hall-effect sensor (or probe) is calibrated and specified.
The error can be removed by rotating and tilting the probe in
different planes to obtain the highest possible output for a given
field. The use of isotropic detection can also minimize this
error [6].
V. EMC P RECOMPLIANCE T EST S ETUP
B ASED ON THE P RESENTED W ORK
One of the requirements of CE-marking standards is for
power line magnetic field immunity for residential, commercial,
industrial, and audio/visual electrical and electronic products.
The relevant standard is IEC 61000-4-8. Failures in the final
compliance tests may increase the R&D costs and the timeto-market, so there is a need for precompliance methods and
tools. A very low cost EMC precompliance test setup for
this standard, along with the proposed digital magnetic field
meter, offers a cost-effective solution. The proposed test setup
is shown in Fig. 10. It comprises an uninterrupted power
supply with a sine wave output having a THD of less than
5%, a variable transformer (autotransformer), resistive loads,
an induction coil (size = 1 m2 ) made from appropriate currentcarrying-capacity copper wires, and a digital magnetic field
meter with probe.
SATAV AND AGARWAL: DESIGN AND DEVELOPMENT OF LOW-COST DIGITAL MAGNETIC FIELD METER
2845
Fig. 10. Low-cost EMC precompliance test setup for the standard IEC 61000-4-8.
TABLE III
TYPICAL BOM COST OF THE PROPOSED DIGITAL MAGNETIC FIELD METER
The induction loop can be constructed using polyvinyl chloride pipes with tees and elbows. An area of 1 m2 is recommended for products with a volume of 0.6 × 0.6 × 0.5 m3 . For
larger test volumes, a larger induction loop or square Helmholtz
coil can be constructed according to the requirements of the
standard. One or two turns of the copper wire are sufficient for
a low-level (less than 10 A/m) magnetic field. The approximate
inductance of the coil is given by the empirical formula [31] as
follows:
2N 2 μ0 μr w w ln
− 0.774
(5)
L=
π
a
where N is the number of turns, w is the length of the copper
wire for one side, a is the radius of the copper wire, and μr is
the relative permeability of the medium.
Depending on the impedance of the coil at 50 Hz, the load
resistance can be varied over a wide range to generate the
desired magnetic fields inside the loop. The proposed digital
magnetic field meter, along with an appropriate probe, can
accurately measure the fields than the use of Biot–Savart’s law,
which can lead to erroneous results due to variations in copper
quality, mechanical tolerance, temperature, etc.
VI. C ONCLUSION
A low-cost, portable, and easy-to-operate digital magnetic
field meter having 50-mG sensitivity, 12.5-mG resolution, and
a dynamic range of 96 dB has been designed and developed
for EMI precompliance evaluation and other applications. A
computer interface and a user-friendly GUI give the added advantage of modern-day instrumentation, rendering the proposed
digital magnetic field meter a good choice for carrying out an
electromagnetic (magnetic field part) survey of the sites. As the
dynamic range of the meter is 96 dB, it can be used for various
educational, industrial, and automotive applications. A simple
and low-cost EMC precompliance test setup for the standard
IEC 61000-4-8 has also been proposed. This, along with the
designed digital magnetic field meter, provides an overall lowcost tool for achieving EMC during the product development
cycle.
The typical bill-of-material (BOM) cost of the presented
digital magnetic field meter is tabulated in Table III. The low
part cost and count lead to economical and simple fabrication
of the system; a survey carried out by the authors showed that
a similar instrument with comparable features costs more than
dollar400 in the commercial market.
The magnetic field measurement system described in this
paper has no proprietary cover; hence, it can freely be used by
any EMC engineer engaged in the use and/or design of such
systems.
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Sandeep M. Satav received the B.E. degree in
electronics engineering from Amravati University,
Amravati, India, in 1991, the Postgraduate Diploma
in business management from Indore University,
Indore, India, in 1995, and the M.Tech. degree in
electrical engineering, with specialization in electronic systems design, from Indian Institute of
Technology-Bombay, Mumbai, India, in 2005.
In 1992, he joined a private-sector company,
where he worked on test and measuring instruments
as a Research and Development Engineer. He has
designed and developed one of India’s first microcontroller-based analog oscilloscopes. Since 1999, he has been with Research Centre Imarat, Hyderabad,
India, which is a pioneer laboratory of the Defence Research and Development
Organisation, as a Scientist, working on the electromagnetic compatibility
of defense-related systems. His research interests include the design and
development of indigenous and low-cost sensors, and test and measuring
instruments for electromagnetic interference and electromagnetic-compatibility
(EMC) precompliance.
Mr. Satav is a Life Member of the Society of EMC Engineers of India.
Vivek Agarwal (S’92–M’93–SM’01) received the
B.S. degree in physics from St. Stephen’s College,
Delhi University, New Delhi, India, in 1985, the
integrated M.E. degree in electrical engineering from
Indian Institute of Science, Bangalore, India, in
1990, and the Ph.D. degree from the University of
Victoria, Victoria, BC, Canada, in 1994.
He has been with Statpower Technologies,
Burnaby, BC, as a Research Engineer. In 1995, he
joined the Department of Electrical Engineering,
Indian Institute of Technology-Bombay, Mumbai,
India, where he is currently a Professor. His current research interests include
new power converter configurations, intelligent control of power electronic
systems, power quality issues, nonconventional energy, and electromagneticinterference/electromagnetic-compatibility issues in electronic and power electronic systems.
Prof. Agarwal is a Fellow of the Institution of Electronics and Telecommunication Engineers and a Life Member of the Indian Society for Technical
Education.