Moodz' Awesome Gold Pulse Induction Version 3 - MAGPI V3 Project

[pito]

and how you do here ?

[moodz]

short circuit the capacitor

If you short the cap the coil is still terminated by low impedances at both ends by the bridge so there is still no target detection.
However I can see you have advanced skills in cutting and pasting random off topic images from the internet.

[pito]

short circuit the capacitor

[moodz]

and no patent

Interesting ... though this is a different circuit and I cant see how you receive anything.

[pito]

and no patent

[moodz]

Good analysis .. though the latest version of the CC transmitter patent has the TX coil grounded on one side and terminated by an impedance on the other side with each alternation of the bridge switches.
So yes the TX coil sees a low DC impedance during transmit times ( flat tops ) but a high AC impedance R during simultaneous recieve times ( flat tops ).

Thus we can send and recieve from the one coil even though the current in the coil is never zero. ( but it is effectively constant during flatops ). If L1 is very large the current does not want to "change" easily.

The V1 and L1 being effectively a constant current source ( which has a high output impedance for AC but not DC ) and D1 ( switch ) ensuring continuity of the constant current source during the brief flyback times.
The target responses occur across the R1 impedance.

In the minelab patent they drive the coil with a voltage source not a current source so the coil is grounded ( AC wise ) on both ends and there is no opportunity to extract a target signal.

[Teleno]

I’ve been digging deep into the LTSpice simulations for this ZP (Zero Point) servo and a few major things have clicked. The defining feature of this design is tying the coil to a low impedance at the end of the flyback, which is a double edged sword.

The impedance the coil sees is just the Rds_on of the damping MOSFET, which is tied to a virtual ground via a transimpedance amplifier. Because of this, the residual discharge of the coil is trapped in a closed loop, stretching its decay time constant (L/R) out to a lazy 10us or more.

When a fast target is present it acts like a rogue transformer core. Its eddy currents steal energy right at the transition, distorting the very beginning of the decay and pushing its amplitude upward. This early energy theft actually allows us to detect these targets as a DC offset in the coil decay long after their own eddy currents have died out.

The downside: Because the target's unique signature is forced to ride on the coil's long coil' decay, its own time constant gets completely overwritten. You're left with a pure amplitude shift, which means zero discrimination. Every target and the ground collapse into a DC offset and that's it.

I've simulated two modes of operation.

Mode 1: Fast servo, early sampling.

The servo locks (more or less) onto the zero point moment, where the target steals the most energy, and pins the voltage to the reference rail. From there on the waveform has a bump that depends on the target's tau before decaying with the coil's tau.

The Difficulty: This zero-point is a moving target. Because it's different for every target, a loop sampling at a fixed delay will lead or lag missing the zero-point for most targets.

Solution: To make the loop lock correctly you need to pinpoint that exact peak. Standard peak detectors can’t handle nanoseconds and millivolts, so you need a Constant Fraction Discriminator (CFD). It turns the detector into something worthy of a CERN physics lab, but I’ve implemented it in LTSpice with promising results.

Mode 2: Slow servo, later sampling.

Sampling past the zero point gives a smooth coil decay to wait on. When a target appears, you just measure the amplitude change.

Advantage: Simple. The coil's low impedance acts as a passive time stretcher. An ultra-fast target (0.5us) alters the decay at the very beginning and that amplitude bump is preserved all the way down the tail. This makes tiny gold highly detectable late in the decay, whereas in mode 1 such targets remain practically invisible.

Disadvantage:This mode is basically a super sensitive beach beeper, but you lose all hope of ground compensation or target discrimination.


We're left with a classic trade-off: you either go the complex CFD route to get proper discrimination and ground balance, or you build a beach beeper that finds tiny gold. A dual mode gets you the best of both worlds.

By the way, moodz, the signal in your bipolar CC design will behave just like this ZP setup. Because your design keeps the coil on a low impedance all the time, you're going to run into these same physics and trade-offs.

[moodz]

...they are persistant .. Nokta is battling them here :

https://ipsearch.ipaustralia.gov.au/patents/2023204592

this patent is trying to lock up bipolar constant current sensing till 2044.

[Teleno]

Its hidden away in patent US9829598B2. .... note the expiry date of this patent

The patent lists alternatives/combinations of the damping element that do not improve on the constant current sink, which is optimal, and therefore they defeat the stated technical effect.

Only a current sink gives non-exponential discharge. It forces the back-emf voltage at any given time to whatever level sustains that current, and the coil current ramps down linearly: dI/dt roughly constant. A linear ramp reaches zero at a calculable finite time. That's the physical reason a current sink gives a faster and more deterministic zero than than any other alternative or combination. It's a different decay law rather than an engineering preference.

Looking at what the patent lists (resistor-only, current source only, resistor+current source in parallel, various nonlinear admittances), the resistor-only case is exponential and slower, the parallel R+current sink combination isn't a different mechanism as it's still using a current sink element to do the fast part of the work, with the resistor handling a secondary/residual and exponential delayed role (post-decay damping). So the "alternatives" aren't such and can't achieve the speed of a current sink.

Contrary to the claims, the residual (which is mentioned as a prior art unsolved problem in Moody's) cannot be eliminated and the document admits this itself. It states outright tat a jitter of ~10 ps alone produces several μA of residual current and several mV of residual voltage, which after preamp gain becomes hundreds of mV. Any servo loop nulls the average or expected error over many cycles, but cannot achieve a nonzero residual on any given cycle. So a claim phrased as "achieving zero residual" should really be read as "minimizing residual" and that's true of both yours and this patent. The "improvement" is not clear.

This patent's broader language reduces to the same core physics of a current sink draining the coil's stored energy at roughly constant current, governed by a feedback loop that minimizes (not eliminates) residual error. Then the technical substance underlying both patents is the same, the "different" claim language in the later filing maps to functionally the same physical mechanism, just with the feedback loop's control variable redescribed: adjusting the Tx on-tine instead of the sink. Yet the current sink needs to be previously adjusted to a near optimal value, and the patent doesn't disclose this.

The document doesn't disclose or that K (the magnitude of the current sink) needs to be prest close to optimal for the Tx-on-time-control embodiment to converge in a well-behaved (low-sensitivity, fast-settling) regime. It shows the sensitivity problem near optimal K only in the context of the other control variable (switch timing), and seems to implicitly assume the reader will carry that caution over to the Tx-side case without stating it.

This is a real insufficiency of disclosure. If the only way to make "adjust Tx on-time, leave K fixed" work robustly is to first set K near its optimal value by some separate, undisclosed means (manual calibration, a startup auto-tune sequence, characterizing C in production) then the claimed single-feedback-loop architecture is incomplete as written. It's presenting one knob as sufficient while quietly depending on the other knob already being in the right neighborhood, with no disclosed method for getting it there. That's a legitimate "enablement" question (does the patent teach someone how to actually build the thing, or does it skip an essential step?) which is exactly the kind of issue a patent attorney would want flagged when assessing whether the later claims hold up against the earlier disclosure.

[Teleno]

Its hidden away in patent US9829598B2. .... note the expiry date of this patent

They basically repackaged your patent, to put it mildly.
After al the obscurong legalese what remains in clear engineering terma is your idea:

The Active Element: A Controllable T/R Switch
Instead of relying solely on a static resistor, this invention uses the transmit/receive (T/R) switch itself as an active, controllable damping element. Rather than treating the T/R switch as a simple on/off gate (open during high voltage, closed during receive), it's operated as a variable-impedance device — it can behave as:
a constant-current sink,
a resistor,
a parallel combination of both,
or some other nonlinear current-voltage relationship
— and its exact behavior is tuned in real time.

The feedback loop:

Samples the signal coming out of the T/R switch right at the end of the back-emf decay,
Compares it against a target (ideally zero residual current/voltage),
Feeds back to adjust either the switch's drive (current magnitude, turn-on timing) or the duration/voltage of the preceding transmit pulse,
This self-corrects cycle-to-cycle, so it automatically compensates for coil inductance changes (e.g., from magnetic soil) without needing to know exactly when the back-emf actually ends.

[moodz]

Its hidden away in patent US9829598B2. .... note the expiry date of this patent
No I am not using a TIA just a straight 40 db low noise amp .. the bipolar TEM relies on reciprocal energy conservation .. or "ping pong" using a high inertial factor ( in this case a big inductor ).
The big inductor is a clamp for DC current ( so the ground cant tilt the pulses ) but a high impedance for target signals ( which are not DC ) so we can simultaneously recieve and transmit from one coil with no TX "off" time.
On the bench MAGPI4 is 4 times simpler and 4 times more sensitive than MAGPI3 .

The bipolar monocoil TEM patents will surpass the minelab patents that are trying to lock up bipolar PI technology till nearly 2040.
My reason for patenting is not to make money its to make sure the idea does not get hijacked.

[Teleno]

... the original purpose of the current damping servo loop was to improve the damping speed of a monopolar PI over a conventionally damped PI and it does this ( there is at least one active patent still covering this today ... not mine )

Interesting. Care to share the patent number?

However the damping loop can become messy and the servo action interferes with the target and ground balance requirements.

I think ground balance is better left to DSP. The loop keeping the coil in a "superdamped" state is already challenging enough.

The dropout voltage across the diode is what causes problems in MAGPI 3 and the requirement for a ringing cleanup resistor ( residual damping ).

These problems can be avoided as in the schematic above.

At the end of damping the coil "wants" a low impedance (inverting configuration with input resistance of 25 ohms) and the gain of 10x is low enough to prevent saturating the LM6171 during the transition of the diode from ON to OFF. The op amp remains in total control of virtual ground never letting lose of the coil. The output is enough for DAC. The higher gain for the sevo loop is added in a second stage.

Below is the transmit current in the MAGPI 4 monocoil with the RX voltage ( from the same coil ) and then the RX voltage x 100 ( 40 dB ) post preamp. No TX tilt ( approx 40 uA ) No resistor at all. No ringing. One microsecond flybacks.

Looks good. Are you using a TIA?

[moodz]

... the original purpose of the current damping servo loop was to improve the damping speed of a monopolar PI over a conventionally damped PI and it does this ( there is at least one active patent still covering this today ... not mine )

However the damping loop can become messy and the servo action interferes with the target and ground balance requirements.
There is a solution though ... MAGPI 4 - the next generation .. uses alot less parts than version 3 because it drops the requirement for a servo loop and utilises bipolar pulsing so we have twice the signal to noise of a monopolar PI.
We dont even need the ringing clean up resistance as the damping is almost perfect in MAGPI 4. ( this is the basis of my patent TEM monocoil ).
There is a clue in the orginal patent that is behind MAGPI3 ..... the patent specifies a switch not a diode as the damping current element because whereas a diode can be a switch .. not all switches are diodes.
The dropout voltage across the diode is what causes problems in MAGPI 3 and the requirement for a ringing cleanup resistor ( residual damping ).
Below is the transmit current in the MAGPI 4 monocoil with the RX voltage ( from the same coil ) and then the RX voltage x 100 ( 40 dB ) post preamp. No TX tilt ( approx 40 uA ) No resistor at all. No ringing. One microsecond flybacks.

[Teleno]

When the coil is optimally balanced, its inductance L is virtually grounded through a low R, with a time constant

In the absence of a target this is a slowly decaying exponential (about 12us with L=300uH and R=25)

In the presence of a target the signal is a subtraction of two exponentials: the coil's and target's This is a family of functions that rise from zero to a maximum as in a sine, and then decay.

The time to the maximum depends on the relative amplitudes and taus:

[Teleno]

A fast servo loop will reflect the target in the gate control signal. The control pulses are passed to a second leaky integrator with high gain to produce the amplified target signal. It will be quasi-static if the leak factor is small. A depletion type damping mosfet will settle to a Vgs not far from zero, which is good for integration (more balanced + and - pulses) .

Red: no target.
Blue: 1us target
Green 10us target

The bump on the left is the servo transient before reaching the steady state.