Mine Detection System

The mine detection system was the most crucial part of this entire project. This was the one thing that had to reliably work. Thus I spent the first few months into designing and testing this system. It was my entry into analog circuit design as I had been mostly working with digital circuits. Analog circuits are more tedious when it comes to circuitry, testing and deploying as we are dealing with continuously varying signals that tend to be largely affected by factors such as noise.

This was my first version of the circuit that would be used in detection of land mines. This circuit was essentially an oscillator. The inductor in this circuit would be replaced by a coil. Whenever the coil experienced any change in its inductance, it would reflect in a change in frequency of the oscillator. This was the way I could check for metallic objects such as land mines that would change the inductance drastically and reflect in a large change in oscillator frequency. The frequency of this oscillator was tuned to about 10-15 KHz. But I soon realised that the sensor was far less sensitive to changes which turned out were due to the low frequency of operation.

This was the one thing that led me to change the entire design of the robot. My first version of the coil was 300 turns and 10 cm diameter. This allowed reliable detection but only within the range of about 5 cm. After some online research I found that the thumb rule was that the max reliable range was approximately equal to radius of the coil. This meant a newer coil was needed and thus I made an elliptical coil which was later turned into a 30 cm circular ring coil.

This new coil allowed for ranges of about 15 cm and even beyond in some cases. Thus this coil design was fixed and the rest of the robot design was modelled with this coil as a reference.

The previous version was less sensitive due to it being low frequency, thus this time I went for a higher frequency design. The old version operated in the range of 10-20 KHz so it used OP-Amp based designs. But now for higher frequencies the OP-Amps available had a low gain bandwidth product. Thus I switched to transistor based circuits. The circuits I built could operate in the range of medium frequencies. With the latest coil the oscillator operates at a resonant frequency of ~200-250 KHz.

The higher frequencies allow for faster and reliable detection at surface level operations which is suitable for landmines. But if we want to detect deeper objects we can use lower frequencies for better ground penetration with trade-offs in reliability and speed. This can also be optimised with better signal filtering and processing. The oscillator at its current state is also capable of detecting changes in soil density and humidity, in fact anything that can affect the coils inductance. The oscillator can be designed to work at a frequency that's sensitive to any particular changes and thus is highly versatile. This enables the robot to have wide variety of applications in archaeology(to detect historic artefacts), geology, civil engineering(to check position/integrity of structural rods in bridges/buildings, and electricity (to check for power lines).

This micro-controller did the sole task of measuring the frequency of the oscillator. It used various on board timers/counters to capture the frequency cycles of the output signal coming from the oscillator circuit accurately. This signal was counted in large sample periods in the range of 50-100 ms. Thus this entire system had a sample time of about ~50-100 ms as well.

The micro-controller kept this value ready for the master to poll the device via I2C. Which is when the value would be sent to the master and then to the client GUI where the value would be plotted in a graph. This enabled the controller to see the changes in detector frequency and identify the presence of a land mine.