A Ground Balance Circuit (build and field tested)

[Carl-NC]

As has been mentioned in various threads, one issue found with GB circuits in PI metal detectors is that you can get a hole in the response, depending on the object being detected. The signal created by the GB circuit with the integrator can drive the signal LOW instead of HIGH (as it does normally) making the metal detector go quiet when over metal instead of going beep.

A solution often mentioned is an absolute value amplifier, so signals going negative will be reversed and made to go positive as per normal operation. In the MPP, it would be placed after the first integrator (after TP4) but before the second integrator where the comparison to the threshold value is determined. So I built one... but its not a simple answer.

An absolute value circuit won't fix the problem. The hole results in a zero response, and the absolute value of zero is still zero.

The location of the target hole is determined by combinations of timings and RC time constants. That is to say, you have the flexibility to place the target hole where you want it. So the solution is to create 2 completely separate TGT/GND channels, each with different target hole locations. Then combine them so you get continuous detection with no hole. This is where the absolute value circuit comes in handy.

[green]

Some scope screen pictures with the target swinging from a pendulum at about 8inches from coil. GB on and adjusted to cancel ground. The nickel is below the hole, the quarter above. My detector has one integrator so like Carl replied if target is at bottom of hole(ground being one of the targets)the output is zero. I put the absolute value after the capacitor coupled amplifier, no zero offset problem. Just another way to do it. Don't know if it's the best way. All signals direct coupled up to output A. Used another target near coil to get A output near zero, wouldn't have to be close to zero for B output to be zero with no target.

[green]

I have a PI detector program in Excel that simulates the integrator. Set three different target delay times 6, 12 and 18usec, then adjusted target sample time to cancel ground signal. Then changed the target TC in the hole TC column until it was in the hole bottom. Ground slope can be effected by Tx on time and shape, probably others also. Timings with the Excel program are close to what I get with my detector. Your timings might be different, but an example that could be. My Tx rate 1000pps, Tx width 160usec. Carl(reply#46)suggested two different timing sequences to eliminate the hole, don't know how far apart the two hole TC's should be.

[6666]

Digger429 are you still working on this ?