The solution for ground elimination in time domain is described in other thred:
Here is described solution in Frequency domain.
To identify frequency characteristic of target, four coils are used in the search head. Here is an example:
1. TX coil is used for excitation. It creates magnetic field with suitable frequency spectrum.
2. AIR reference coil. It is placed with large coupling with TX coil to obtain AIR signal.
3. GND reference coil. It is placed in position for induction balance with TX coil to obtain GND signal.
4. RX coil is used to receive TGT signal. At induction balance with TX coil, the RX coil receives suppressed AIR signal. The GND signal is suppressed by subtraction (both identical coils are connected as TWIN LOOP receive system).
TARGET identification
The target is represented as block 5. It has Laplace transfer function H(s) between input and output. The name of transfer function in frequency domain is FREQUENCY CHARACTERISTIC (frequency response). If we know the input signal X(s) and output signal Y(s), we can identify the target by calculating its transfer function H(s). For calculation of H(s) is used the mathematical operation DIVISION, because Y(s) is product obtained by MULTIPLICATION of X(s) with H(s). Note that in Time domain we should calculate DECONVOLUTION.
AIR signal
This is the excitating signal that induces eddy currents in conductive volume. It is produced in RX coil by mutual inductance, and by self-inductance in TX coil.
The shape of AIR signal is like di/dt (derivative of TX coil current in time domain). A turn wire placed near to TX coil can generate enough large AIR signal used for refence. The derivative of TX current appears in frequency domain as multiplication by frequency. The induced voltage has phase lead 90 deg for all frequencies.
To calculate frequency characteristic of ground, we need AIR signal.
TGT signal
Since the target is buried, the energy passes through the soil twice and induces TGT signal in RX coil. Note that TGT signal contains distorted information for frequency characteristic of target H(s). Excitating signal X(s) of target differs from AIR signal because it is output of block 4. It is obtained by multiplication of excitating AIR signal in input of block 4 with its frequency characteristic H4(s).
The output signal from the target Y(s) is distorted. Going to RX coil, the Y(s) undergoes multiplication with the frequency characteristic H6(s) of block 6. To identify target, we should calculate H5(s) by removing the distortion of TGT signal.
GND signal
We need GND signal to calculate frequency characteristic of target H(s). Therefore, we use GND reference coil.
It is assumed that the excitation signal X(s) of target has the same form of frequency spectrum as the GND signal. It is assumed also that spectral characteristics H4(s), H6(s) and H7(s) are similar in shape.
Computational algorithm
1. By division of GND signal with the AIR signal is calculated the frequency characteristic of soil H6(s).
2. By division of TGT signal with the frequency characteristic H6(s) is calculated the target output spectrum Y(s).
3. By division of output signal Y(s) with the GND signal is calculated the frequency characteristic of target H(s).
4. The problem is solved. Follows comparison of calculated spectral form H(s) with known forms of different targets in memory.
The block diagram in frequency domain shows how to design the excitating function i(t) in order to obtain a better SNR and better energy efficiency:
The form of TX current should contain frequency components in this part of frequency spectrum where frequency characteristics H5(s) of different targets differ significant.
Sometimes this is possible at only one TX frequency. That means the problem can be solved sometimes with CW (narrow band) metal detector at very high energy efficiency and very low noise.
Leave a comment: