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BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to magnetic fields and levitation and more
particularly to a method and apparatus for levitating objects using a
magnet attached to the object to principally lift an object toward a core
element of an electromagnet while driving the electromagnet with a
variable duty cycle and frequency power source to fine tune the levitation
forces. The invention further relates to a method of detecting magnetic
field strength and variations therein between the core of an electromagnet
and an adjacently suspended permanent magnet.
2. Background of the Related Art
The ability to levitate objects in a magnetic field is considered to be
useful for many applications. One obvious application is in the area of
model displays and toys. Levitation is very useful for adding a sense of
realism and accuracy as part of suspending many models, such as those of
satellites, aircraft, spacecraft, and the like, in mid-air. It is also
desirable to be able to suspend some objects that comprise an artistic
formation or work in mid-air. At the same time, however, levitation has
beneficial applications for scientific work such as where isolation of a
chemical or material both electrically and physically from its
surroundings is desired. It may also be desirable to use magnetic
levitation to suspend some materials during processing or storage to
counterbalance some of the forces of gravity or to better control material
positioning in low gravity environments.
In the past, several attempts have been made to provide methods or
apparatus for levitating objects. Generally, such apparatus comprises one
or more electromagnets, although permanent magnets have been used in some
configurations, powered by an adjustable strength current source. The
electromagnets are suspended above, or below, an object to be levitated
and generate magnetic fields which are used to attract metal in the
object, or repel permanent magnets mounted on the object. The electrical
current supplied to the electromagnets is adjusted to vary the strength of
the magnetic field established by the electromagnet so as to just counter
the force exerted by gravity on a suspended object.
One major problem in previous levitation apparatus was to sufficiently or
properly balance magnetic attraction, or repulsion, against gravitational
forces on a levitated object to achieve levitation. That is, the object
must be levitated with sufficient force to prevent releasing it to fall
and, at the same time, without attracting it so strongly as to cause it to
contact the magnet or surrounding structure. This is accomplished using a
combination of sensors to detect the magnetic field strength, and feedback
control over the electromagnets based on the sensor data. However,
previous attempts at such controls have produced complicated, generally
expensive, control circuits which operate unsatisfactorily in many
applications. The sensors require very precise or critical alignment which
precludes many commercial applications. Transient lateral motion or wobble
of the object also causes severe problems for the feedback controls
typically resulting in loss of levitation.
What is needed is a method and apparatus for levitating objects that has
less stringent construction requirements or operational limitations and is
low in complexity.
SUMMARY
In view of the above problems and limitations of the art, one purpose of
the present invention is to provide a method and apparatus for supporting
objects in a magnetic field.
An advantage of the present invention is that it provides for object
support in a self-aligning mode that decreases sensitivity to transient
lateral motion.
Another purpose of the present invention is to provide support for objects
in a magnetic field using a levitation apparatus that is both very
efficient and low in complexity.
These and other purposes, objects, and advantages are realized in a method
and apparatus of levitating objects in a magnetic field comprising,
positioning at least one electromagnet above an object or objects to be
levitated and connecting the electromagnet to a switchable electrical
power source. The electromagnet employs a series of windings disposed on a
centrally positioned core comprising a material having a high magnetic
field saturation value and a rapid rise time and decay in the induced
field. The core has at least one end positioned adjacent an outer surface
of the electromagnet. At least one magnetic field sensor is disposed
adjacent this core end and is configured to provide an output signal
indicating a relative magnetic field strength. At least one magnet,
preferably a permanent magnet, is secured to the object to be levitated
having magnetic poles aligned for attraction to said energized core. A
field strength controller is connected to the sensor and in series with
the power source for adjusting electrical power delivered to the
electromagnet in response to variations in object position from a desired
position.
The magnetic field sensor preferably provides an output signal proportional
to a sensed magnetic field level. The magnetic field controller comprises
a reference voltage generator for generating a selected, but adjustable,
reference voltage level signal which is monitored by a voltage comparator
connected to both the sensor and the reference generator for comparing
respective voltages generated by each and for providing an output signal
indicative of a relative status of the two. The output from the comparator
is used to control or gate power to the electromagnet.
In further aspects of the invention, a switching transistor, generally of
the high power FET type, is disposed in series with the power source for
the electromagnet. An input gate of the FET is connected to a gating
element for applying an activation or gating voltage to the transistor in
response to the comparator output signal. An exemplary gating element
comprises a timer whose output is triggered by the comparator output.
In the method of the present invention, the electromagnet is used to
generate a pulsed magnetic field to adjust and maintain the vertical
position of the levitated object. This is accomplished by detecting field
strength of the magnet on the levitated object at a surface of the core
facing the object and adjusting the pulsed field to maintain a desired
field strength.
The field strength of the object magnet relative to the fixed end of core
position is determined by mounting a Linear Output Hall Effect transducer
on the end of the core with a fixed orientation with respect to generated
magnetic fields and detecting voltages produced by said transducer. This
transducer is able to deduce the position of the levitated object by the
magnetic field of the magnet on the object, with the signal quadrupuling
as the distance between the sensor and the magnet on the levitated object
is cut in half. The magnetic field of the electromagnet attempting to
levitate the object will complicate this relationship. To remove this
complication, this design turns on the electromagnet for a timed period,
then senses the position of the levitated object when the electromagnet is
off. When the comparator connected to the sensor determines the object to
be too far away by comparing the signal from the sensor with that of the
reference it will output a signal to the timer, which will then turn on
the field strength controller allowing current to flow in the
electromagnet for a timed interval. These events can occur at a rate of
several hundred times per second.
The levitation apparatus can further employ a second magnetic field sensor
mounted adjacent the opposite end of the electromagnet core from the
position of the first sensor and oriented to provide an output signal of
opposite sign when aligned with the same magnetic field lines. The output
of the two sensors are coupled together through a summation element to
generate a signal from the sensor nearest the levitated object that is
free of the effects of the electromagnet and represents only the effects
of the magnetic field of the magnet on the levitated object. This allows
reliable knowledge on the position of the levitated object even with
fluctuating currents passing through the electromagnet. This allows a 100%
duty cycle or nearly direct current operation, allowing very precise
control of the levitated object.
BRIEF DESCRIPTION OF THE DRAWINGS
The novel features of the present invention may be better understood from
the accompanying description when taken in conjunction with the
accompanying drawings in which like characters refer to like parts and in
which:
FIG. 1 illustrates a cross-sectional view of a levitator constructed
according to the principles of the present invention;
FIG. 2 illustrates a control system for use with the apparatus of FIG. 1 in
block diagram form;
FIG. 3 illustrates exemplary control circuitry for implementing the control
system of FIG. 2 in schematic form; and
FIG. 4 illustrates the connection of two magnetic sensors to the apparatus
of FIG. 3.
DESCRIPTION OF A PREFERRED EMBODIMENT
The present invention provides a method and apparatus for levitating or
suspending one or more objects in a magnetic field. This is accomplished
by securing one or more magnets to one or more portions of the object to
be levitated and positioning an electromagnet above the object. The
electromagnet has a fast current rise time, and decay time for the induced
field in the core, and a magnetic core with a high level of saturation. A
magnetic field sensor is mounted on a central portion of the electromagnet
core and used in a feedback control loop to monitor magnetic fields
between the core and magnets on the levitated object. A control circuit is
implemented as part of the feedback loop to adjust and control variations
in the magnetic field generated by the electromagnet.
An apparatus constructed according to the principles of the present
invention is illustrated in perspective in FIG. 1. In FIG. 1, an
electromagnet 12 having a substantially solid ferro-magnetic core 14 and
coil 16 is supported from a support frame or structure 18. The
electromagnet 12 generally comprises a cylindrical coil 16 wrapped tightly
about a cylindrical core 14, although the core can also have elliptical,
triangular, rectangular or other geometric shapes for its cross-section
and still be useful for the present invention. The core 14 is generally
positioned at the center of a central longitudinal axis extending though
the electromagnetic coil 16. However, the core 14 need not occupy an exact
centerline position within the coil 16.
The electromagnet 12 is constructed according to principles and techniques
well known in the art and a variety of such magnets are available that are
useful with the method and apparatus of the present invention. In an
exemplary embodiment, the electromagnet coil 16 was constructed from
number 26 gauge copper wire wrapped in about 570 turns about a ferrite
core. However, other material such as nickel alloys can be used to
construct the core 14. It is only necessary that the core be highly
attractive of other magnets as discussed below.
The electromagnet is held in place using one of a variety of fastening
techniques such as, but not limited to bolts 17, C or U-shaped clamps,
adhesive, or bonding agents (epoxy or casting resins). Where the magnetic
fields employed allow a sufficiently large separation distance between the
levitated object (see below) and the electromagnet 12, the electromagnet
can be supported on a non-magnetic material such as a sheet 19 of plastic
or metal which can extend between the object and the core 14. This has the
advantage of better allowing incorporation of the electromagnet into a
shell or other form of housing that is part of a display device without
leaving the electromagnet 12 visible.
The structure of FIG. 1 is for purposes of illustration and clarity, used
only in describing the invention and can have many alternative shapes or
designs. As for example, the support structure (17, 18, and 19) can employ
a part of a work of art or visual representation which matches or
corresponds to the aesthetic appearance of the object to be levitated.
Examples would be clouds or planets which present a better visual or
aesthetic impact for space or aeronautical displays.
Suspended below the electromagnet 12 is a levitated object 20. In the
illustration of FIG. 1 the object 20 is shown as a simple geometric form.
However, it will be readily understood by those skilled in the art that
the levitated or suspended object, or objects, can obtain several
variations in form, shape, and weight to be used with the teachings of the
present invention.
The object 20 has one or more magnets 22 mounted within it. In the most
basic embodiment, the object 20 is very lightweight and employs a single
magnet 22 secured to a portion of the object to be suspended directly
beneath the electromagnet 12. The magnet 22 is typically a small,
preferably high strength, permanent magnet. Alternatively, in some
applications an electromagnet can also be employed for the magnet 22. This
allows higher field strengths, especially where superconducting materials
may be available for manufacturing the conductor in the electromagnet
coil. However, use of an electromagnetic structure for the magnet 22 also
detracts from part of the advantage of the invention and adds complexity
to the stabilization of the object 20 since power leads must be accounted
for.
In the present invention, the electromagnet 12 is not used to generate
magnetic fields which levitate the object. That is, the magnetic field
from the magnet 22 is responsible for a majority of the force needed to
levitate the object 20 as opposed to the field from the electromagnet 12.
The levitated object magnet 22 is attracted to the ferrite core 14 of the
electromagnet 12. In fact, if the levitated object is raised too high, or
too close to the electromagnet core 14 even without current applied to the
coil, this attraction would overcome the force of gravity on the object 20
and cause it to strike the electromagnet 12.
Although in the embodiment illustrated the magnet 22 is orientated for
attraction to an electromagnet positioned above it, it will be understood
that a similar result may be obtained by locating electromagnet 12 below
the object with the magnet orientated for repulsion from the
electromagnet.
The magnetic field strength required for the magnet 22 is determined by the
mass of the object 20 to be supported in the generated magnetic fields.
Obviously larger objects require larger magnetic field strengths. The
field strength for attracting the magnet 22 to the core 14 at the point of
levitation is estimated to occur for a field force equivalent to at least
60-80 percent of the weight of the object. The remainder of the necessary
attraction comes from the electromagnet 12 and is typically supplied in
short pulses. An exemplary embodiment of the present invention used a
magnet having a strength of 900 gauss for supporting an object having a
mass on the order of 23 grams. However, much higher field strengths are
also envisioned for use with the invention limited only by the saturation
of the magnetic field sensors discussed below.
The allocation of forces or relative magnetic field strength away from the
electromagnet 12 differs substantially from previous approaches to
magnetic levitation. This assignment of the main object support role, or
main object levitation force (through attraction), to the magnet 22
instead of the electromagnet fields provides several advantages for the
present invention.
The magnet 22 can be secured in place on the object 20 using several
fastening means such as bonding agents, bolts, clamps, epoxy, etc. In some
embodiments, it has been found beneficial to secure the magnet 22 in place
using a resilient or spring like member between the object and the magnet.
This is illustrated in FIG. 1 by using a resilient foam or similar
material configured in the form of a thin block 23. Alternatively, a
spring assembly is used having a rod 24 attached to the magnet 22 and
terminating in a ridge, plate or other extension 25 to hold a spring 26 in
place on the rod 24. The spring 26 presses against a wall, extension, or
stop on the object 20.
A resilient member is used in order to decrease or dampen the effects of
oscillations created by some perturbations in the magnetic fields. This
resiliency can also allow greater resistance to external dislocation. In
addition, when several levitator devices are used in tandem, say to lift a
cube by each of its corners, the magnets should use resilient mounting
means to prevent oscillations between the separate support systems.
However, the method of the present invention does not require the use of a
resilient member to achieve levitation.
As seen in FIG. 2, the electromagnet 12 is connected through wires or
cables to a current driver 30 and power source 32. The electromagnet 12
generates a magnetic field according to various levels of current or power
provided by the power source 32 through the driver 30. The settings are
chosen with regards to stable positioning of the object 20.
As stated above, the structure of the present invention functions by
attraction of the magnet 22 to the material comprising the core 14 of the
electromagnet 12 to provide the main upward force for levitating the
object 20. In the alternative the invention can operate by repulsion
against a magnet. It is estimated that on the order of 75 to 90 percent of
the force required for levitating the object 20 should be provided by the
magnet, or magnets, 22 interacting with the core 14. Although, control
range is improved and the levitation device less sensitive to an exact
reference (see below), i.e., the reference voltage swing allowing
levitation, if the core attraction is providing a lesser amount of the
energy needed for levitation. The remainder of the necessary force is
provided by the electromagnet 12.
The driver 30 switches or pulses the electromagnet 12 to achieve a fine
tuning of the magnetic fields extending between the electromagnet core 14
and the magnet 22 to provide a stable, and self aligning, operation for
the levitation apparatus 10. The driver 30 is actuated by a controller 34
which uses a information or an output signal provided by the magnetic
field sensor 28 to determine the relative field strength to be provided by
the electromagnet 12.
At least one magnetic field sensor 28 is mounted on the electromagnet 12
adjacent to the core 14 where an end of the core 14 faces the object 20.
This sensor is used to detect the relative strength of the magnetic field
extending between the magnet 22 and the electromagnet 12 through the core
region.
In the preferred embodiment, the sensor 28 comprises a hall effect device
mounted on the electromagnet adjacent to the core, on the end facing the
levitated object. In alternate embodiments, additional hall effect devices
28 are mounted on an opposite end of the core 14 and allow monitoring of
relative field strengths of the electromagnet.
Using hall effect type sensors, the controller 34 is configured to compare
the voltage output of the magnetic field sensor 28 with a reference
voltage provided by the a reference source 36 to determines when the
position of the magnet 22 is changing relative to the fixed location of
the coil 12. This in turn determines when the driver 30 needs to apply
current to the coil 14 to compensate or maintain preselected object
positions.
During operation, the current driver or switcher 30 interrupts and controls
the flow of current to the electromagnet 12 from the power source 32. The
current driver 30 is connected between the power source 32 and the
electromagnet 12 and has a control input which is connected to a
comparison/timer element in the controller 34. The comparison element (not
shown in FIG. 2) is connected at a first input to the magnetic field
sensor 28 to receive an output voltage or signal from the sensor. A second
input of the comparison element is connected to the reference voltage
source 36. The reference voltage source 36 has an output which is adjusted
to match the output voltage of the sensor 26 when the object 20 is
suspended in a desired position or at a desired levitation height. It will
be readily understood by those skilled in the art that the output of the
reference voltage source 36 should be adjustable and is adjusted according
to the weight or mass of the object 20 and the type and number of magnets
22 employed. However, once adjusted for a particular levitation position,
the voltage source should not require further adjustment during use unless
the structure or weight of the object are changed.
Operation of the present invention is better understood by reference to
FIG. 3 where circuitry used to implement the driver 30, controller 34, and
reference source 36 are illustrated in further detail in schematic form.
In FIG. 3, the coil 16 is shown connected on one end to a power source 32
through a current control or limiting resistor R1. A typical value for
resister R1 is 5 ohms. The power source 32 provides the necessary voltage
and current for operating the coil 16 and represents one of a variety of
power supplies known in the art. The other end of the coil 16 is connected
to a power FET type transistor T1 which switches on and off to gate
current through the coil 16 to ground. In a preferred embodiment, the FET
T1 is used as a switchable ground, however, it can also be connected to a
lower or higher voltage level terminal of the power source 32 as desired,
taking into account the proper polarity of the transistor. The FET T1 is
shown connected through an isolation and current limiting resistor R2 to a
ground terminal of the power source 32. The resister R2 is typically on
the order of 0.25 ohms in value.
The FET transistor can comprise one of several known relatively high power
or high current FET transistors commercially available. An exemplary
transistor for T1 is an N-channel power FET supplied by the Siemens
company and designated by part number BUZ20. However, those skilled in the
art will understand how to select other FET and non-FET type transistors
to accommodate the switching function of T1 where applicable.
The control or input gate of the transistor T1 is connected to an output
terminal for the timing circuit U1, discussed below. The output terminal
is connected to the transistor T1 through a variable resister R3 to
control the voltage range applied to the gate of the transistor T1.
A diode, D1, in series with a resister R4, is connected in parallel with
the coil 16 to provide a discharge path for the coil 16 to prevent damage
to circuitry when transistor T1 cuts off.
The magnetic field sensor 28 preferably comprises a Linear Output Hall
Effect Transducer produced by the Micro Switch company division of
Honeywell corporation and generally referred to by the trademark LOHET.
This type of sensor is chosen for its highly linear, stable, and field
orientation sensitive output. In addition, this type of sensor is packaged
in a configuration that makes installation very simple and compact.
However, other types of field sensors can be integrated into the circuitry
of the present invention.
The main control circuit of FIG. 3, which corresponds to the controller 34
of FIG. 2, comprises a voltage comparator U2 connected to receive input
voltages from the transducer 28 and a variable level voltage reference on
an input side and to a timer, U1, on an output side. The comparator U2 is
connected to the transducer 28 through a resister R5 and to a variable
resister R6 through a resister R7 which is used to establish a desired
reference voltage. An exemplary circuit element found useful for the
voltage comparator, U2 is an OP-AMP circuit manufactured by the Texas
Instruments company under the part designation TOP271CP. However, those
skilled in the art will readily recognize that other circuit elements are
useful to implement the comparator U2.
The output of the comparator U2 is connected to a trigger input of a timing
device U1. Typically a resister and capacitor network is used to shape the
output from the comparator U2 to provide an appropriate trigger signal for
the timer U1. These components are shown as resisters R8 and R9 and
capacitor C1. This provides a low going trigger pulse of controlled
voltage level instead of a steady state output level as would normally be
present on the output terminal of the comparator U2.
The timing device which can comprise a 555 timer, has appropriate timing
control components such as resistor R10 and capacitor C2 connected to
terminals for setting basic timing. A variable resister R10 is found very
useful in setting the pulse duration output to the transistor T1. This
control is especially valuable if there is no resilient supports or
connections used for the magnet 22. This control fine tunes the position
setting pot and acts somewhat like a gain control in a feedback circuit.
In addition, a resister R11 may be used to set the shortest minimum pulse
length for the pulse applied to the gate of the transistor T1.
The output of the timer U1 is used as a pulse source for the FET T1 which
is applied to a gate to turn on the FET T1. The start of the gate pulse is
determined by the comparator inputs changing relative potentials, i.e., B
greater than A, to A greater than B. The output of the comparator in this
design will cause the timer to start a gate pulse when it changes from a
high state to a low state. The gate pulse duration will be controlled by
the timer's external circuitry. This gate control circuitry can be made in
many ways by those skilled in the art.
As discussed above, the transducer 28 is mounted on or adjacent to the end
of the core 14 that faces the object 20. The active portion of the
transducer 28 is positioned substantially in the center of the core and
detects the field of the magnet 22 as it interacts with the core.
The transducer's output is connected to the inverting input of the
comparator, a reference voltage is selected off the voltage divider
provided by R7 and applied to the noninverting input. The transducer's
output with no magnetic field present is approximately one half the
supplied voltage. Depending on the direction of the magnetic field, the
output will be driven either higher or lower than this midpoint.
In this design, as the magnet gets closer to the electromagnet core 14 the
output of the transducer 28 decreases toward zero volts. When the output
of the transducer 28 is lower than the reference voltage, the comparator
outputs a high level voltage which indicates that the object is too close
to the electromagnet. If the output of the transducer 28 is higher than
the reference voltage, the output of the comparator is low or zero which
indicates the object is drifting too far away from the electromagnet core
14.
The pulse shaping network transfers a low going pulse to the trigger of the
timer when the output of the voltage comparator goes low, a high going
signal has no effect on the trigger circuitry. If the timer U1 receives a
low going pulse on the trigger input it begins a timing cycle. While in a
timing cycle or mode, the timer U1 provides or generates a high level
output signal which is applied to the gate of the power transistor T1. The
transistor T1 is turned on by the presence of this signal and remains on
as long as the output from the timer U1 is high. Therefore, the pulse
width of the FET transistor T1 output is determined by the timing or duty
cycle of the timer circuit, with the start of the pulse being determined
by the comparator.
While the FET transistor T1 is turned on, the output from the transducer 28
decreases. This results from current applied to the electromagnet which
generates a magnetic field oriented in the same direction (pole to pole)
as the permanent magnet on the object. This causes the output of the
comparator to go positive which prevents constant triggering of the timer
U1 as current flows to the electromagnet.
The transducer 28 is also normally blind to the presence of the magnet 22
during the time the electromagnet is on and will not be able to sense the
object's presence until the magnetic fields in the ferrite core for the
electromagnet die down or decay to a sufficiently low value. As soon as
the output level from the transducer 28 increases above the reference
voltage input to the comparator, in other words the object magnet 22 is
too far away, the comparator output goes low turning on the timer U1 and
the electromagnet 12 and pulling the magnet closer.
To facilitate operation and account for the magnetic field generated by the
electromagnet during operation two alternative methods of operation can be
used.
In a first technique, a second transducer 28b is mounted on the core of the
electromagnet 12 on the opposite end from that of the transducer 28a. If
the transducer 28b is oriented in an opposite sense from the transducer
28a, then magnetic fields will generate an opposite polarity signal on the
transducer 28b output as opposed to the transducer 28a output. As shown in
FIG. 4, this arrangement is used to advantage by summing the output of the
two transducers to generate a signal that represents the difference in
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