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Cody... Can you help me some technical questions

F

FrankMD

Member
Hello Cody and whoever is reading this here. I noticed you are knowledgeble about the technical aspects of metal detectors. Wondered if you could answer my questions. I put this on another forum with no responses. My e-mail is: fdew@att.net in case this isn't on subject enough to talk about it here.

Hello everyone. I am trying to get a better grasp on some of the math/physics behind metal detecting. I hope you guys don
 
Your assumptions are based on the relationships between the primary and secondary winding of a transformer. You most likely know this but will post this anyhow. Primary power must equal secondary power in a transformer. If voltage is stepped up in the secondary then current in the secondary must decrease so primary power equally secondary power. If voltage is stepped down then current in the secondary must increase so secondary power is equal to primary power. The transformer core is similar to the soil and the target of interest. It is the change in the core that we detect such as a variable core in a transformer.

However, although the transmitter coil and receiver coil can be analyzed much like the primary and secondary of a transformer they are not quiet the same. The receiver coil is positioned in the search head so no voltage is induced into the secondary unless a target is detected or if soil minerals cause an induction. I am not sure of the turns ration but there is no attempt to accomplish the principles above as in a step up or step down transformer. Soil mineral signals can be rejected which leaves signals. Several detectors I can think of have adjustable voltage to the transmitter coil. A lot of testing has been done and when ground minerals are considered no voltage greater than about 20 volts Rms is of value. Also, with an increase of the voltage to the TX coil we now have a ground balance problem due to soil minerals. The electromagnetic field that is
 
Cody,

As I stated in prior posts I look at the operation in somewhat different terms. The following is a general discussion and does not necessarily relate to the design of the Explorer. For simplicity we assume sinusoidal excitation.

#1 THE MAGNETIC FIELD GENERATED BY THE SEARCH COIL IS A VOLTAGE PHENOMENON
The time rate of change of the magnetic field enclosed by a single loop of wire is directly proportional to the voltage around that loop. If the transmit coil has 10 turns and the voltage across the coil is 10V, then the volts per turn is 1V/turn. It makes no difference which is the cause and which is the effect, that is changing magnetic fields cause voltage and voltage causes changing magnetic fields.

The important thing to note here is that the above relationship is true whether or not there is any current flowing in the transmit coil. There are only two reasons why current flows in the transmit coil.
A. Magnetizing current flows based upon the inductance of the primary coil. There is no average power delivered by magnetizing current.
B. Any power delivered to a secondary winding must result in an additional current flow in the primary such that the average primary power equals the average secondary power (assuming 100% efficiency). This is the current relationship that you discussed in your post.

#2 A PORTION OF THE MAGNETIC FIELD GENERATED BY THE TRANSMIT COIL IS COUPLED INTO THE RECEIVER COIL
The ratio of the receiver coil to transmit coil is called the coupling coefficient, e.g. 75%. This depends upon the physical orientation of the transmit and receive coils. It is possible to arrange the receive coil so that in a vacuum the coupling coefficient is zero. But when a metal target is brought into the region of the two coils, then that coupling coefficient is altered to be non-zero and a target response is indicated.

#3 THE EFFECTS TARGET ON THE COUPLING COEFFICIENT
There are basically two components of all targets. They are the magnetic permeability and conductivity of the target. Most targets have both effects simultaneously.

#3.1 The magnetic property "sucks the magnetic field into the target" and distorts the magnetic field. This changes the coupling coefficient in a particular manner. This is essentially an instantaneous effect.

#3.2 The conductivity "forces the magnetic field away from the target" (because of eddy current build up) and distorts the magnetic field. This changes the coupling coefficient in a completely different manner. There is time delay involved here.

#4 THE IMPORTANCE OF THE FREQUENCY OF EXCITATION
I do not feel comfortable with my present knowledge in this area. My father used to tell me that it is better to remain silent and be thought a fool than to open you mouth and remove all doubt.

HH,
Glenn
 
If you are good with technical data you can go to the patents. There are a lot of them that explain sine wave, square wave, rectangular wave, PI, FD, TD, SW, you name it and it is there. While they don't bother with the standard circuits we find in many applications they do cover the basic information with equations to help explain the process and operation.

I have gone through a number of them and talked to the engineers at several manufactures and a couple of design engineers for detectors I have met on the Internet. There are several good books that also go into some detail starting with the old TR and BFO detectors.

The bottom line is about all I do is spit back what I have read or been told in conjunction with my own training in electronics. What I post is pretty much right from the patents so that may be the best for you if you want to take the time to go through them.

Glenn can really help to smooth over the rough spots. He is being modest and has a good grasp of frequency domain as well as time domain. Jeff Foster on the DFX Classroom Forum is one of the best I have run into as is MattR a retired engineer in the UK. Both have published very good books and are very helpful and friendly to communicate with.
 
Thanks Cody and Glenn for the information. I appreciate it. Can you tell me the names of the books Jeff Foster wrote and Glenn wrote?

Thanks

Frank
 
Glenn, I understand your post with the first one about a year ago. However, moving away from that and considering the the patent for the coil I don't see anything that deals with coefficient of coupling (k) but can see where you are in this.

I understand the coefficient of coupling in a transformer and spent many hours in the study as I am sure you did. As I recall a tight coupling, as an example, would mean that most of the flux from the primary cuts the windings of the secondary. Again as an example this would mean if we have one volt per turn in the primary and the ratio between primary and secondary is 2:1 then we have one volt per turn in the secondary so step the voltage up at this ratio.

This is true based on a high (k) coefficient. (The coefficient of coupling k is just a number indicating the efficiency of coupling. I added this for anyone that is not familiar with this term.)

I can see how you are relating this to a TX and RX coil but great care is taken to position the TX and TX so no induction takes place while the TX is ON although there is no load on the RX while the TX is ON. It is only after the TX is off that the RX is sampled and only if the flux has been distorted that there is induction in the RX coil.

If free air is used to determine the coefficient of coupling as in a transformer we don
 
Cody,

It is apparent that you and I understand the coupling mechanism in the same manner. But, I am confused about how the Explorer works. I gather from what you say is that there are two time periods.

#1 The Tx stimulates the target matrix and the Rx is in an idle state during which no information is processed about the target matrix.

#2 The excitation is removed from the Tx and the signal of the Rx is analyzed.

I have not studied the patents in any great detail, but from what I read and the waveforms shown, I did not get the idea of two serial operations as described above. Am I correctly interpreting your post?

HH,
Glenn
 
Yes you are correct.

The demodulators are switched Off during the time the TX is ON so the TX and RX are never active at the same time. Sampling starts right after the TX pulse ends by switching the demodulators ON. I think I read the patents several times before I caught this as I expected the TX and RX to be active at the same time.
 
Cody,

Perhaps, I am looking at the wrong patent. The patent I am referencing is 5,537,041 "Discriminating Time Domain Conducting Metal-Detector Utilizing Multi-Period Rectangular Transmitted Pulses" by Bruce H. Candy.

From the waveforms and the discription given in the patent, the transmitted signal is a square wave around 0V. The receive circuits essentially do a full wave rectification of the receive signal followed by some filtering and waveform analysis. I only reviewed the patent to get the basic idea of the operation. I may have missed a point, but I saw no suggestion that the transmitted signal was interrupted while the received signal was analyzed.

I might add that the patent shows a dashed line between the Tx and Rx that suggest that they are coupled in some fashion. It does not imply that the Tx excites the target matrix and then the Rx examines the target matrix while the Tx is in the off state.

Which patent are you referencing?

HH,
Glenn
 
Cody,

To me this is an important and fascinating area of discussion. Is suspect that it would be a good idea for us to carry on this dialog via email. For those who may also have an interest in this area, we could then summarize our conclusions for them.

I will try to put together an email to you relating to this last response from you. Thanks for your patience with me.

Best Regards,
Glenn
 
I appreciate your interest as always. Unfortunately the patents do not try to "unveil" the process or discuss much more than the process that is patented. It takes a lot of work to fill in the gaps. I look forward to your email.
 
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