1 Basic description of the design
The minesweeper extension will enable the ASURO robot to detect metallic objects underneath the
halved Ping-pong ball-glider. This will allow you – of course within the scope of the robot's and the
kit's possibilities - to develop the scenario of a robotic mine detector respectively treasury hunter or
a simplified version of a detector and tracer for cables, reinforcing bars and I-beams.
To avoid abundant explanations for the physical theories for magnetic fields and complex AC-
currents, the following chapters strictly document the basic description of the design and the user's
manual.
An operational amplifier (Opamp) has been applied to stimulate oscillations in the resonant circuit
consisting of a capacitor (C) and an inductor (L) applying an open pot core. Application of the
magnetically open pot core allows the magnetic field to expand into the surrounding free field and
to be influenced by neighbouring metallic objects.
Figure 1: Schematics for the "Minesweeper"-Extension
Fig. 1 displays the schematic diagram. The resonant circuit consists of inductance L1 and capacitor
C1. The design allows resonant behaviour by cyclically exchanging the capacitor's electric field
energy and the inductor's magnetic field energy. The design's transfer frequency depends on the
values for the capacitor and for the inductor. Assuming negligible losses, the resonator's frequency
may be calculated by the following formula:
f
0
=
1
2
L C
Exchanging the capacitor's electric field energy and the inductor's magnetic field energy cannot be
performed without losses and the losses will cause the oscillation to decay within a few cycles. We
continually have to feed energy into the system to compensate losses. In analogy to a children's
swing, the system will have to apply the correct phase in feeding the energy into the circuit.
To achieve this goal, the design controls the capacitor's current proportionally to the capacitor's
voltage.
In this system the active element is the operational amplifier IC1A in a non-inverting amplifier
circuit with resistor R2 and the trimmer resistor TR1. This circuit will amplify the capacitor's
voltage at an adjustable rate of 1 up to 3, which will increase the current into resistor R1
proportionally to the voltage at C1. The losses in the resonator circuit may vary and to compensate a
range of tolerances, we will need an adjustable amplifier.
The operational amplifier IC1B is used as a comparator and compares the resonator's voltage with a
reference voltage of approx. 0.5V (depending on ASURO's battery voltage). The comparator's result
is applied to the extension pin INT1. To avoid signal collisions between the processor pin and the
output of the operational amplifier in a non-programmed processor, the port is being protected by
resistor R4. D4 replaces the previous line follower LED.
The left part of the circuit, containing a number of diodes and capacitors, generates a negative
voltage with respect to the ground level. The design will need a negative voltage as the resonator's
voltage swings in a positive and negative range, centred at the ground level.
Several types of designs are available for metal detectors. The ASURO design supports the
following two design types:
1. The design's amplification factor and the equivalent energy input for the resonator is to be
controlled at a level, in which electrical losses in the resonator are exactly to be
compensated as long as no metal is to be located near the coil. If metal objects are located
near the coil, the so-called
eddy currents
(for conducting materials) or
demagnetizing losses
(for non-conducting, but ferromagnetic materials) result in extra losses, which will cause the
decay of oscillations.
2. The design's amplification factor is to be controlled at a level, at which additional losses by
metals in the vicinity of the coil will be compensated and the circuit is to measure the
oscillator's frequency. In this mode any conducting materials near the coil result in eddy
currents, decreasing the field strength and the inductance and simultaneously raising the
oscillator's frequency. Ferromagnetic materials will increase the field strength and the
inductance, which will lower the oscillator's frequency. Additionally to detecting metals, this
design mode also allows a rather crude determination of the type of detected metal.
2 Constructional details
2.1 Manufacturing the coil
In case the coil has been prefabricated completely, including glueing the capacitor and applying the
cables as documented in fig. 8, you may skip this chapter. Otherwise you will enjoy the next steps!
First of all, we must apply 400 windings (yes, you are reading this correctly!) of very thin isolated
copper-wire (diameter 0.1mm) to a coil-carrier.
The kit supplies a double-sided coil-carrier for two core-halves (see fig. 2).
Figure 2: coil-carrier, complete
Figure 3: coil-carrier, halved
In order to fit for our purposes, we will have to split up the carrier with a saw. A suitable saw for
this is a fine-tooth hacksaw. We will have to remove one chamber by sawing the other chamber in
the middle. This procedure results in a singular coil-carrier. Remaining sawing edges can be
removed with fine sandpaper (grain size: 240 or 300) or by carefully using a sharp knife (protect
your fingers!). The removed parts will not be needed and can be thrown away.
In order to apply the coil to the carrier, we suggest to place the carrier to a pencil-shaft or (even
better for it's conical form) to a suitable paintbrush. In an optimal method we also carefully fix a
few centimetres of the isolated copper wire together with the carrier at the pencil's shaft as
demonstrated in fig. 4. As an extra fixation you may use some adhesive tape (cello tape) to avoid
slipping movements of the wire.
Figure 4: Winding preparations
After these preparations, you carefully start winding up the 400 turns of wire. Of course you avoid
reversing the winding direction and you fill the windings neatly, otherwise the 400 windings of wire
will fail to fit in the available place. In case the wire should break (there is no room for a repair) or
you fail to count correctly, you must restart the procedure. No problems are to be expected for
winding numbers between 380 up to 420, but do not exceed these tolerances.
Having completed the windings you are advised to fix the windings with some
nail varnish
or
instant glue.
As soon as the glue has hardened you may carefully remove the cello tape and the
pencil or brush.
You may also cut the wire, but do not forget to reserve a few centimetres at both sides. The wire-
endings have to be directed into one direction and are not allowed to pass through the hole in the
coil-carrier (see fig. 5).
Fig. 5: coil-carrier - completed
Having completed the coil-carrier, you can fix the structure into the core with some
instant glue.
The wire's endings are to leave the core at the closed core-side through a slit (see fig. 6).
Fig. 6: Coil - fixed in the core
At this stage you have to remove the isolation at the wire-endings, starting at one or two
millimetres from the core towards the outside. The optimal tool to remove isolation is a soldering
tool with some fresh solder at the soldering tip. Apply the heated top for some time until the
isolation has been removed and a thin layer of soldering tin is covering the wire. Warning: the
generated smoke may cause damage to your health and should not be inhaled!
At last you put some instant glue to the backside of the coil and fix the 10nF-capacitor (imprinted
text: 103) in a suitable position to point the wiring connections towards the slit for the coil-
windings. Fig. 7 demonstrates a location for the capacitor besides the carrier's hole – just in case we
may need the hole for some other purpose. The published design however does not really require
this exact position.
Now cut the capacitor's wiring connections to approx. 5 mm, wind the tinned copper wire-endings
around these wiring connections (maybe using a pair of tweezers) and fix the connection by
soldering.
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