Jason and Alon's Coil Corner

After much trial and error, an order of jumper wires, and some design tweaking, the VLF metal detector prototype is complete and operational! The biggest challenge was getting the receiving circuit to process the input signal correctly. The size of the coil limited the amount of turns and therefore the sensitivity of the receiving coil, so the received signal is very, very small. I used a simple non-inverting op-amp circuit with a huge gain in order to process the signal. Figure 1  shows the output of the peak detector prior to amplification, and Figure 2  shows the output of the peak detector after amplification.  Figure 1: Transmitted Signal (yellow) and Received Signal  after Peak Detection (blue) Figure 2: Transmitted Signal (yellow) and Received Signal  after Peak Detection & Amplification (blue) There were also changes to the peak detector circuit. With the original integrating peak detector circuit, the output signal did not dro...

The prototype Oscillator is complete. The resulting frequency is around 3 kHz, and required some trial and error to output a relatively clean sine wave. Figure 1 shows the final oscillator design, and figure 2 shows the resulting waveform. There is still some distortion as the oscillator reaches the upper rail voltage of the op amp, but it shouldn't affect the rest of the detector system. Figure 1: Oscillator final design Figure 2: Oscillator waveform Figure 3: Oscillator circuit on breadboard

Here are a few updates on the work completed over break. Most importantly, the 3D-printed coil has been completed. The final dimensions are 11 cm diameter and 2 cm height. The winding also took a good amount of work to complete and resulted in some very sore hands. The completed coil can be seen in Figure 1. It consists of around 100 turns on the outside and twice that on the inside for increased signal sensitivity. Figure 1: Completed Coil The design for the amplitude detection circuit is also complete. It consists of a half-wave rectifier (just a diode) fed into a two-stage integrator to give different DC voltages for varying input amplitudes. The amplitude detection circuit can be seen in Figure 2. Figure 2: Amplitude Detection Schematic To verify the design, I performed a simulation with a 5 kHz sinusoidal signal with various amplitudes. Figure 3 shows the output waveform for input signals with amplitudes up to 5 Volts, stepped by 1 volt.  Figure ...

After sitting down with someone who has a bit more CAD experience, we were able to come up with a better model for the detector coil. It features concave surfaces to ensure that the coils don't move around too much after coiling. It also has holes for the wires to go through to enhance the stability of the coil. The new model can be seen in figure 1.   Figure 1: New Coil Model

Now that our capstone projects have finished up, we have found some more time to work on the metal detector. The prototype coil isn't going to be sufficient for testing the circuits on because it has a different inductance than the final coil, so our current priority is building the final coil.  We modeled the coil housing in SolidWorks and will be 3D printing the final design. Figure 1 shows a model of what the coil housing will look like. Figure 1: Model of Coil Housing The receiver coil will be wound around the inner cylinder and the transmitter coil will be wound around the outer surface. The inner cylinder and outer cylinder have a 4 cm and 8 cm diameter, respectively. We do have to keep in mind that this housing will be mounted on some sort of detector shaft. To speed things up, we will be working on building the oscillator while the coil housing is being printed. The oscillator works independently of the coils so we have the option of building and testing it...

The second iteration of testing the prototype was a success. We re-coiled the transmitting coil to give it a stronger hold on the container as well as add a few more turns for transmission strength. The test circuit was very basic, as shown in figure 1 . The output signal was identical to the input signal in frequency, but varied in phase and amplitude. In figure 2 , the received signal is shown in yellow on top and the transmitted is shown in green on the bottom. To test the effectiveness of this system in a metal detecting capacity, we held metal objects over the coils and saw what happened on the oscilloscope screen. The result was a small change in amplitude on the receiving coil, but no perceptible change in phase was seen. Figure 1: Test Circuit Diagram Figure 2: Transmitted and Received signals Figure 3: Circuit setup