Note: Descriptions are shown in the official language in which they were submitted.
CA 02629408 2008-05-01
INDUCTION BALANCE SENSOR
BACKGROUND OF THE INVENTION
The present invention is in the field of conductivity sensing, specifically
the conductivity
evaluation of ore samples, including low grade base metal sulphide ores. More
particularly, the present
invention is in the technical field of induction balance sensing, with
applications in down the hole
sensing and conductivity based sorting.
BRIEF SUMMARY OF THE INVENTION
The present invention consists of an induction-balance type sensor system and
method for
detecting and recording electrical characteristics of conductive media,
specifically low grade nickel ore
and other conductive ores. The system comprises an arbitrary waveform
generator, signal electronics, a
sensing coil and a matched balancing coil with extension cables, data
acquisition hardware, a computer
running a software package, and power electronics. The base functionality of
the sensor system finds
use in differing applications, such as sample characterization, bore hole
mapping, and automated ore
sorting.
BRIEF DESCRIPTION OF DRAWINGS
Fig. 1 is a block diagram of the system components;
Fig. 2 is a signal flow diagram representing implemented circuitry;
Fig. 3 is an isometric view of a coil and coil housing block;
Fig. 4 is a drawing of a down-the-hole sensing application; and
Fig. 5 is a drawing of a sorting application.
1
CA 02629408 2008-05-01
DETAILED DESCRIPTION OF THE INVENTION
Referring now to the invention in more detail, in Fig. 1 there is shown a
block diagram of the
system components. An arbitrary waveform generator 10 provides an input signal
to the signal
electronics 11, where it is applied to a pair of matching inductor coils 12a
and 12b. Output from the
coils 12 is sent back through the signal electronics 11 and is captured by the
data acquisition hardware
13. The captured data is processed and analyzed by a computer software package
14. A regulated DC
power supply 15 provides power to the integrated circuits in the signal
electronics 11. The dashed lines
indicate one exemplary arrangement of the components; the waveform generator
and data acquisition
hardware are implemented on a single PCI card mounted in a computer 16, and
the power supply and
signal electronics are housed together within an enclosure 17. Other
equivalent arrangements are easily
conceivable, and the invention is not limited to the above described
arrangement.
Waveform generation control is performed by the software package 14. The
waveform
generation hardware 10 is capable of producing user selectable arbitrary
waveforms, including single
frequency signals of specifiable shape, amplitude and frequency, composite
signals of multiple
frequencies, and frequency sweep signals with specifiable range. A generated
waveform is applied at
the input of the signal electronics 11, where it is conditioned to drive the
matched coils 12a and 12b.
The input signal is filtered and amplified by the signal electronics 11 before
being applied to the coils
12a and 12b as an excitation signal.
One coil functions as a sensing coil 12a used to examine samples, and the
other as a balancing
coil 12b used as a reference to the sensing coil. The balancing coil 12b is
subject to the same static
environmental conditions as the sensing coil 12a, but is kept isolated from
the samples to be examined.
In the presence of conductive media, the impedance of the sensing coil 12a no
longer matches that of
the balancing coil 12b . This impedance change unbalances the signal
electronics 11, producing a
voltage signal of magnitude and phase related to the change in resistive and
reactive components of the
sensing coil impedance.
The unbalanced signal, along with a reference of the excitation signal 18, are
sent back through
the signal electronics 11, where they are conditioned for output to the data
acquisition hardware 13.
2
CA 02629408 2008-05-01
The data acquisition system 13 is capable of real-time data streaming into a
computer and is used to
digitize the output signals for analysis by the software package 14. Data from
the two measured signals
18 undergoes Fast Fourier Transform operations to extract and display spectral
information. The
change in impedance of the sensing coil 12a is then calculated, and magnitude
and phase values are
displayed. Depending on the application, the data can be recorded as
individual values, stored in a
database and correlated to previous entries, plotted as a positional map, or
used to make a decision and
generate a control signal.
The power supply 15 is a common component with internal operations and design
non-critical
to the invention, with the sole purpose of providing a DC voltage as required
by integrated circuits in
the signal electronics 11. The power supply is controlled by an ON/OFF switch
and indicated by a
light when operational. With the switch in the off position, no signal is
applied to the coils 12a and
12b, and the data acquisition system 13 will only receive noise.
Referring now to the individual system components in more detail, in Fig. 2
there is shown a
signal flow diagram of the signal electronics 11 previously described, and how
they interact with the
waveform generator 10 and data acquisition hardware 13. The signal electronics
11 comprise an input
filter stage 19, a signal splitter stage 20, a power amplifier with
differential output 21, a balanced
bridge network 22 incorporating the matched coils 12a and 12b, and on each of
the output channel
lines, amplification 23, and filtering stages 24.
Output from the arbitrary waveform generator 10 is applied as an input to the
signal electronics
11. The input filter 19 is a low pass filter with a cutoff frequency greater
than the highest frequency
component of the input signal. The filter is used to smooth the generated
signal and remove spectral
images produced by the waveform generator 10. The signal splitter stage 20
then produces inverted
and non-inverted versions of the original signal, which are used as inputs to
the power amplifier 21.
The differential outputs of the power amplifier 21 are used as the driving
current to excite the bridge
network 22, providing a balanced source of positive and negative signals.
Variable gain incorporated
into the power amplifier 21 circuit allows for hardware level control of the
excitation signal amplitude.
3
CA 02629408 2008-05-01
The implemented bridge network 22 is a conventional Wheatstone bridge, used to
measure
impedance differences between bridge components in the form of a voltage
signal. Variations of the
Wheatstone bridge, and other bridge networks, provide equivalent methods of
detecting impedance
differences, and as such, the invention is not limited to one specific network
arrangement.
In an ideal rest state with no conductive samples present, the bridge network
22 is perfectly
balanced, and no voltage is seen across the bridge. Error tolerances in real
components create an
inherent imbalance in the bridge network 22, producing an unbalanced voltage
signal even in the rest
state. Local environmental factors also affect the bridge balance, such that
the rest state unbalanced
voltage of a given sensor system may differ between operating locations. The
software package 14
calibrates the system by interpreting this rest state signal as a baseline
response against which
successive readings are measured.
Since the bridge network 22 is driven by an alternating current signal, the
unbalanced voltage
signal is measurable not only in magnitude, but also in phase with respect to
the driving signal.
Measurement of these two components of the unbalanced signal makes the sensor
system responsive to
conductive and reactive properties of samples. Depending on the type of sample
examined, the bridge
network 22 may produce an unbalanced signal with a change in magnitude only, a
change in phase
only, or a combination of magnitude and phase changes. From these parameters,
the sensor system can
determine the amount and type of conductive media present in the sample.
Output channels 18 are taken from a reference of the excitation signal and an
unbalanced signal
from the bridge network 22. The output channels pass through variable gain
differential amplifiers 23
for common mode rejection of any induced circuit noise. Low pass anti-aliasing
filters 24 remove high
frequency noise from the signals to prevent sampling errors. Buffer stages
condition the signals for
driving the data acquisition hardware.
Referring now to the individual system components in more detail, in Fig. 3
there is shown an
isometric view of a coil and coil housing. The coil 26 rests in a base 27
comprising a solid block of
high density polyethylene with a circular groove 28 routed around a center
spindle 29. The coil 26 is
produced by feeding one end of the wire though a cable hole 30 in the side of
the base 27 and
4
CA 02629408 2008-05-01
alternating between winding layers from the outer edge of the groove 28 inward
to the spindle 29, then
outward from the spindle 29 to the outer groove edge 28. The free ends of the
coi126 exit the base
through the cable hole 30, are twisted together to prevent crosstalk
interference, shielded with braided
cable 31 to prevent external electromagnetic interference, and finally sealed
in a protective insulating
layer 32. A flex relief grommet 33 affixed to the base 27 around the cable
prevents damage from sharp
bends in the wire. An XLR connector 34 is used to connect the coi126 to the
signal electronics 11
either directly or through an extension cable. Shield plates 35 are placed
above and below the coil 26
to limit electromagnetic interference. The shields 35 are implemented with two-
sided printed circuit
boards, each side bearing a comb of 0.3mm thick copper traces, spaced 0.3mm
apart, which are
connected together and grounded. The entire sensor block is sealed by casting
it in a polyester resin.
When excited, the coils 26 each produce a dynamic magnetic field in the
surrounding
environment, related in frequency and strength to the applied excitation
signal. Electric currents are
induced in conductive media present within the field. These currents, and
their respective magnetic
fields, alter the impedance of the coil-conductor system as seen across the
sensing coil terminals. This
change in impedance works to unbalance the bridge network 22, causing a
potential difference across
the center bridge nodes. The magnitude and phase of this signal is dependent
on the conductive
material present within the magnetic field.
To make the coils 26 less prone to internal, self-induced eddy currents, and
more responsive to
external influences, the coils 26 are wound from a conductor with minimum skin
effect characteristics.
The coils are wound using Litz wire, a multi-stranded cable with each strand
individually insulated.
This type of wire also reduces the proximity effect, an increase in resistance
of adjacent conductors
caused by field interactions.
Regardless of application, the system must undergo a startup and calibration
routine before use.
In use, power is supplied to the signal electronics 11, and the desired
operating mode and
corresponding excitation signal are selected via the software package 14.
Initiating signal generation
streaming does not necessarily initiate data acquisition streaming, depending
on the selected operating
mode. A rest state calibration reading is taken in the absence of conductive
media and used as a
baseline reading to compensate for any inherent imbalances in the bridge
network 22. Once calibrated,
CA 02629408 2008-05-01
the system is ready for use. Recalibration may occur at desired intervals to
compensate for any
changes in the local operating environment.
In operation, readings can be produced using a variety of generated signals
and data acquisition
methods. Useful signals include a frequency sweep and a multi-frequency
composite signal. Data
acquisition can function in snapshot mode, where a set of readings are
recorded on demand, or
streaming mode, where a continuous flow of data is recorded. Exemplary signals
are a repeating
frequency sweep up to 1Mhz, and a composite of 8 frequency components between
10kHz and 1MHz.
Each of these signals have their own use in application and can function with
either snapshot mode or
streaming mode data acquisition. The sweep signal produces a detailed set of
readings for each sample
but requires the sample to remain motionless for at least one sweep period.
The composite signal
produces a set of readings with lower frequency resolution, though all
readings for one sample are
captured in a single instant.
In one implementation, readings from a group of samples are recorded before
the samples are
analyzed for mineral content. The readings are then correlated to their
respective mineral content. A
sample database can then be constructed containing magnitude and phase
readings from the sensor
system along with the mineral analysis results. For this application, the more
detailed sweep function
is a preferable input signal, and snapshot mode data acquisition can be
triggered manually. With a
database in place, the sensor system can be used to judge characteristics of
unknown samples without
performing the mineral analysis process. The magnitude and phase readings
produced by an unknown
sample can be compared to similar entries in the database, thus the mineral
content of the sample can
be inferred.
Characterization of unknown samples can be performed by several conceivable
means, not
limited to desktop systems, conveyor systems, and down the hole systems. In a
desktop system, the
user manually places a sample above the sensing coil 12a, instructs the
software 14 to take a reading,
then removes the recorded sample and replaces it with a new sample, repeating
the reading process.
For the desktop application, a sweep input signal is used with manually
triggered snapshot mode data
acquisition. In a conveyor system, the sensing coil 12a is mounted beneath a
conveyor belt that moves
samples along. For this application, a multi-frequency composite signal is
used with streaming data
6
CA 02629408 2008-05-01
acquisition. The software 14 streams the data signals and determines when a
reading should be taken
based on a specified threshold value. In a down the hole system, the sensing
coil 12a is lowered down
a bore hole while a continuous streaming signal is recorded. A composite input
signal is used since the
sensing coil is constantly in motion. The recorded data can be used to map
conductive media
distribution down the bore hole. An exemplary down the hole system is shown in
Fig. 4. The sensing
coil 12a is lowered down a bore hole while the computer 16, power and signal
electronics 17, and the
balancing coil 12b remain on the surface. As the sensing coil 12a passes
through regions of non-
conductive earth 35, slightly conductive deposits 36, and highly conductive
deposits 37, a vertical
mapping of the regions is created.
Using knowledge gained from correlating recorded readings to mineral analysis
data, the system
is able to discriminate between the amount and type of conductive material
producing the readings.
Discrimination parameters can be set so the system only acknowledges samples
containing a specified
amount of a specified material. An application of this function is the ability
of the system to sort a
group of unknown samples depending on the type and grade of their mineral
content. Sorting can be
implemented in a user controlled desktop system or in an automated conveyor
system where the
software decision drives a conveyor mounted deflector.
In an automated sorting system, a composite input signal is used with
streaming data
acquisition. In Fig. 5 there is shown an exemplary automated ore sorting
system. Ore samples 38 are
transferred across the sensing coil 12a by a conveyor belt 39. The balancing
coil 12b is kept away from
the passing samples 38. Output from the signal electronics 17 is processed on
a computer 16, which in
turn controls a deflector arm 40. Samples containing desirable ore fall to one
side of a divider 41,
while undesirable samples fall to the other side.
Multiple coils can be arranged in an array to augment system performance. One
application of
a sensor array involves placing a row of sensors across the width of a
conveyor. By monitoring the
response from each sensing coil, the location of a conductive sample on the
conveyor can be
determined. It is conceivable that a sensor array may be used in conjunction
with an optical sensor to
correlate sample readings from the two sensor systems and better distinguish
multiple samples in a
single image. Such a combination would result in a more robust sorting system.
Alternatively, placing
7
CA 02629408 2008-05-01
a row of sensors along the length of a conveyor would allow for taking
multiple readings of each
sample, which could then be averaged to produce a more accurate reading.
While the foregoing written description of the invention enables one of
ordinary skill to make
and use what is considered presently to be the best mode thereof, those of
ordinary skill will understand
and appreciate the existence of variations, combinations, and equivalents of
the specific embodiment,
method, and examples herein. The invention should therefore not be limited by
the above described
embodiment, method, and examples, but by all embodiments and methods within
the scope and spirit
of the invention as claimed.
8
