Technical Explanation of Semi-auto and Manual Sensitivity

[Cody]

We read some interesting post about semi-auto and manual sensitivity so let me explain what they are for. I know we all understand or at least believe the higher we have the sensitivity set the better off we are. That is not exactly true so hear me out on this one.

The transmitter and receiver coils are balanced so that in free air no part of the TX signal is induced into the RX coil. When we lower the search head to the soil the matrix couples the two coils. Soil minerals and salts are rejected; we often call this ground balanced, so we can detect metal in the soil matrix which is called the target volume. Target volume includes soil minerals, salts, and refined iron. As we sweep the search head over the soil the Q of the coil changes due to changes in the inductances of the RX and TX coils. Changes also occur in the electronics due to aging, temperatures changes, and drift as no electronics components are perfect.

We can adjust for these changes as we sweep the coil over the soil by adjusting one leg of the signals used to balance the detector to the soil in the demodulator section of the electronics. The user does that with the sensitivity adjustments. If the flow of current flow is monitored in the receiver coil then we know when to adjust the sensitivity. This can be done with a variable resistor, transformer, or a third coil to monitor the current flow.

Manual sensitivity is how the user adjusts one leg of the demodulator section. The idea is to balance the detector to the soil and therefore be able to detect the metal targets in the target volume. The user should adjust the sensitivity UP OR DOWN to the most stable point. The most stable point is where the soil component is eliminated form the target volume. The operator can hear the ground noise and current flow in the receiver as noise as we know.

Semi-auto sensitivity monitors the current flow in the receiver coil and automatically adjusts one leg of the demodulator to eliminate ground noise and balance the detector to the soil. This also maintains the Q of the coil for optimum gain.

I suspect if we understood what it means to run the sensitivity on the ragged edge we would not do so. When we do we introduce improper balance to the soil and fail to maintain the Q of the coils. That is why the Owner

 

[Captain Kirk]

Cody,

I will preface my comments with the statement that I claim no great or detailed knowledge of how the Explorer electronics work (or for that matter what the coil structure looks like). I am still trying to put all the pieces together to improve my knowledge. I would also like to state that knowing all of this does not make one a better user of the Explorer. I, like you, have a very great interest in the theory of how the Explorer works. Perhaps that quest for knowledge is ALMOST as exciting as actually detecting (because I can pursue this avenue when I am not able to detect).

You stated:
"The transmitter and receiver coils are balanced so that in free air no part of the TX signal is induced into the RX coil."

I do not dispute your statement, but I am somewhat surprised by it. For my edification can you explain to me what makes you think this way. My feeling has always been that the term "ground balance" was not applicable to the Explorer. My feeling has been that the Explorer is continuously sampling the target matrix for consistency. If it sees the same condition for multiple samples, then it judges that to be the ambient environment and gives no target response. I also have the feeling that if you turn the sensitivity up too great, then the minor variations in the ground mineralization will result in annoying faint target responses.

I would also ask some questions:

#1 How can we be sure that the Explorer even has a transmitter and a receiver coil? I do not dispute that they do, but rather how do we know one way or the other.


#2 What does the term ground balance really mean?

#2.1 From a functional standpoint, we used to describe ground balance as making an adjustment so that the detector produces no response when you lower the coil from a few feet in the air to resting on the ground.

#2.2 My last detector designed in the early 70's was wound with a outer transmitter coil excited with a sinusoidal signal. The receiver coil was wound (in the same plane but inside the transmitter coil) in a semi-figure 8 in such a manner that any homogeneous (over the entire coil structure) magnetic field would induce opposing voltages in the two sections of the receiver coil.
#2.2.1 When the coils were moved about in the air there would be no target indication.
#2.2.2 When the coils were lowered to the ground there would be no target indication if the ground under the coil was homogeneous.
#2.2.3 When the coil was moved over a ferrous or conductive target, then the balance in the magnetic field of the two sections of the receiver coils was disturbed and a target was indicated.
#2.2.4 With this design "ground balance" was automatic and worked extremely well and I had no trouble detecting a dime at 6" in the ground and up to 10" in the air.

Please bear with me in my pursuit of better understanding.

HH,
Glenn

 

[Cody]

"The transmitter and receiver coils are balanced so that in free air no part of the TX signal is induced into the RX coil."

This is almost a direct quote from the patent that goes on forever on how the detector is ground balanced. I have always understood the coils to be balanced in this manner. Coupling is accomplished by the soil matrix. Metal objects in the matrix vary the coupling as does sweeping the coil over the soil.

Consider this quote from patent 5,537,041 that goes on forever about how the TD PI detector eliminates the soil component. Minelab refers to the Explorer over an over as ground balanced so you nailed it in we need to define ground balanced. In one place they call it ground compensation so mix ground balance with other terms.

 

[Captain Kirk]

n/t

 

[Dennis 2]

Just wanted to know how you adjust your Sensitivity .... With the coil on or Off the Ground ? I've been adjusting mine with the coil about six inches ore so off the ground until the cross hairs stop bouncing all over the place . Seems to work great . But would like to know if this is the right way ( or is there a better way of doing this ) .
Thanks
Dennis 2

 

[Cody]

I don't think it makes any difference as long as the threshold is stable while searching. I can run the sensitivity at 28 most of the time in manual or semi-auto. I am using manual as I like to adjust things myself which is the major reason. If I hear ground noise then will back it down a click. I noise cancel before and after an adjustment. Sometimes noise cancel will settle the threshold down so no sensitivity adjustment is needed. If not then I back the sensitivity down a click.

 

[Dennis 2]

n/t