When you see time domain that describes a metal detector consider the following. This tells us that the primary method of detection is best analyzed based on the temporal nature of the signals. A TD detector is a subgroup of a pulse induction detector. Also, consider that a square or rectangular wave drive to the transmit coil implies pulse induction and therefore time domain. However, a square wave or rectangular pulse contains multiple frequencies and for the Explorer it is 28.
Inductance is the ability of a conductor to have current induced by a magnetic field. A reference signal can be used and the distortion to the received signal compared to that signal and the electromagnetic properties of the metal determined to some % of a predicted value.
If you study patent 5,506,506 we find that the primary consideration is a time constant or rate of change of eddy current induction that is used to determine if a metal is ferrous or nonferrous. In addition nonferrous metal can be further classified such as gold, silver, copper and so forth. Also, the primary consideration for ferrous is the permeability of iron. Much of discussion assumes frequency and time domain.
Patent 5,537,041 is primarily concerned with the rectangular transmitted pulse and the receiver circuits to eliminate the soil minerals, demodulators to extract data at different frequencies, filter those signals and that is about as far as it goes. The discussion mostly time domain.
When we combine both patents and look at the entire explanation we find that the Explorer is properly classified as a time domain subgroup of a PI detector. The invention deploys both time domain and frequency domain so the the analysis depends on convenience and the extent depends on which patent we study.
The electrical properties of metals can be determined with no electrical connection to the metal other than the coupling or inductance that takes place between the electromagnetic field of the TX coil and RX coil taking into consideration the target. The primary method for the Explorer is the rate of change of eddy currents which is expressed as a time constant. That of itself implies time domain excepth for the extration of data at different frequencies for the same purpose then we have frequency domain. The eddy current rise and fall so their is a rate of change that has a time constant associated with iron and precious metals.
Because the time constant depends on the electricaland magnetic properties of the metal the time constant is not 100% accurate. Factors such as decay, oxidation, soil matrix, flux that induces eddy currents, wet or dry metal or soil, salts, size and shape of the target and other variables can give erroneous indications of the metal type. In short they produce a TC that is the same as one for another type of metal. That is why is is so hard to tell aluminum pieces of metal from silver as an example.
Time constants are stored in the circuits and a detected target
