Ranging Scaling Scope and Induction Balance

Assume that we have a Transmit coil and a Receive coil.  Let's arrange the Receive coil so that it maximises the signal received from the Transmit coil.

Let's assume that we are reading this with an Analog to Digital (A/D) converter that has a range of -1 to +1 Volts.  To maximise our reading we amplify the signal from the Receive Coil to the A/D and adjust this amplification so that the waveform received by the A/D without a target being present is a -0.9 to + 0.9 Volts Peak to Peak waveform.

Now let us bring a typical metal target object near this pair of coils.  Because the magnetic field produced as a secondary field by this metal target is very small compared to the primary field produced by the Transmit coil, the change in voltage seen by the A/D is extremely small.  Of the order of a millionth of a volt.  It's quite a few orders of magnitude smaller than that produced by the Transmit coil.

Arrangements like this are very inefficient.  They waste most of the precision available from the A/D and don't allow the computer to get much information about the target.  For this reason, VLF metal detectors normally use an Induction Balanced coil arrangement.

Induction Balance requires that the Transmit and Receive coils are arranged in such a manner that at any time somewhere near half of the Transmit field is acting to create a current in one direction in the Receive coil and the remaining half of the Transmit field is acting to create a current in the opposite direction.  There are many coil arrangements that can be used.  One method is the use of overlapping Double D coils.  You can see a picture of overlapped Double D coils on our hardware page at http://humanise.org/demining/hardware

Having created a coil arrangement that provides Induction Balance, the designer can now provide amplification many orders of magnitude greater than that possible without Induction Balance and yet not exceed the limits of the A/D inputs.  The amplification is now just amplifying the secondary signal from the target, not the complete field from both target and Transmit coil.

The principles of Induction Balance rely on the fact that the hardware can easily amplify the received signal.  Hardware amplification can include increasing the number of windings in the coil, creating a high Q receiver circuit optimised for the frequency of interest or using an active amplifier.  Both the high Q and active amplifier approaches are shown in our hardware page at http://humanise.org/demining/hardware  While large amplification using electronic amplifiers (such as Op Amps) is cheap and easy, doing so without introducing excessive noise, distortion or instability requires careful design. 

Designers are divided as to whether the coils should be designed so that without a target there is as close as possible to zero signal, or whether it is better to have some base signal all the time.  Generally, it's not critical for the coil arrangement to be exactly zeroed.  The computer reading the values from the A/D can make appropriate adjustments for this.  What is critical is that the relative arrangement between the coils is very fixed as the coil is moving across the ground.  Small changes to the arrangement (from flexing etc) can cause false readings that the computer cannot differentiate from a target or alternatively can mask an actual target.