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Magnetic levitation demonstration
One of my friends asked me about
magnetic levitation. On reading about
it, I came across the Thompson Ring,
which is made to levitate by a mainsdriven autotransformer. My design is
similar, except it uses a simple circuit
driven by a 9V DC 1A plugpack. You
can see it working in the video at:
siliconchip.au/link/abll
The incoming 9V supply is reduced
to 5V by the LP2950-5 regulator to
power a PIC12F617 microcontroller.
When switch S1 is closed, digital input
GP2 of IC1 goes high, which triggers
the software to produce two square
waves from its GP0 and GP1 outputs.
They are 180° out of phase with each
other. The voltage at analog input GP4
determines the frequency of these
square waves.
The out-of-phase waveforms are
applied to the DRV8871 H-bridge
IC, resulting in an 18V peak-to-peak
square wave across the coil. As the
frequency reduces, more current
flows into the coil due to its reactance
(inductance), resulting in the continuous aluminium ring rising above the
coil by an adjustable distance.
To use it, set the potentiometer for
the lowest frequency (maximum coil
current) with the switch open. When
the switch is closed, the ring flies into
the air and then oscillates up and down
in a damped motion until it becomes
stationary at a fixed distance up from
the coil. The frequency can then be
increased, and the ring will slowly
move down until it rests on the coil.
Reducing the frequency after that
will cause the ring to move up from
the coil. If VR1 is a ‘logarithmic’ potentiometer, the height of the ring above
the coil is roughly proportional to the
rotation angle.
The reason that the ring stabilises at
a particular position is that the downward force due to gravity is constant
but the upward force depends on the
strength of the magnetic field emanating from the coil and the distance
between the coil and the ring. Thus,
for a given field strength, there is a distance at which the upward force from
the magnetic field equals gravity, so
the ring ‘levitates’.
While aluminium is not a magnetic
material, the magnetic field from the
coil induces a varying current flow
in the conductive aluminium, creating an opposing magnetic field. This
is similar to how an induction motor
induces magnetism in the metal rotor
using coils in the stator, which opposes
the field from those coils, causing the
rotor to rotate.
The breakout board for the DRV8871
H-bridge is available from a couple of
Australian suppliers for about $15 and
also can be purchased directly from
Adafruit. The rest of the components
are off the shelf.
The PCB and programmed micro
are available from: siliconchip.
au/Shop/8/6866 & siliconchip.au/
Shop/9/6867
You can also download the firmware for the PIC from siliconchip.au/
Shop/6/282
The rod should be made of soft cast
iron to enhance the magnetic field.
Mild steel should work, but the lift
would be less.
I have also tested rings made of
copper, brass, iron and stainless steel,
plus an aluminium ring with a gap
cut between the perimeter and the
centre and a heavy aluminium ring.
The copper ring goes nearly as far
as the aluminium one, the brass one
rises slightly, while the iron, stainless
steel and split rings do not at all. The
heavier aluminium ring rises nearly
as high as the lighter one.
Les Kerr, Ashby, NSW. ($150)
POWER+
+9V
REG1 LP2950-5
GND
1000F
+5V
OUT
IN
100F
100nF
20F
100nF
IRON
ROD
100nF
ADAFRUIT
DRV8871
BREAKOUT
MODULE
10k
+5V
START
S1
FROM
9V 1A DC
POWER
SUPPLY
5
VR1
10k
SET
FREQUENCY
3
LP2950
GND
IN
2
1k
OUT
0V
4
1
100nF
Vdd
5
GP2
GP4
IC1
PIC12F617
17
–I/P
–I/P
GP0
GP1
7
6
2
3
4
GP5
VM
IC2
DRV8871
IN1
ILIM
OUT1
GND
8
30k*
POWER–
8
OUT2
IN2
Vss
10k
ALUMINIUM
RING
GP3/MCLR
1
6
COIL
PGND
7
SC
2023
* GIVES 2.13A MAXIMUM CURRENT
siliconchip.com.au
Australia's electronics magazine
November 2023 95
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