Metal Detectors – Part 4: The Pulse Induction Method
In the previous: Part 3, we are talking about the Metal Detectors using The Induction Balance Method
Today we will talk about next used the Metal Detector’s working principle.
We are going to describe next principle of work in the metal detectors – namely, The Pulse Induction Method.
The searching head consists a coil L1 – need to solve, not simultaneously, two tasks: to transmit electromagnetic field in the transmitting circuitry, and also used as the antenna in the receiving amplifier stage.
The coil of inductor L1, unlikely the described in previous parts methods, there isn’t necessary to shield the coil with an aluminum foil for minimizing the ground influence.
It is too important to say, that the inductor needs to be constructed with great attention.
When to the leads of L1 is applied a Hi-Current DC impulse, from it constant amplitude state ( we are signing this as a forward polarity) there is “charging” the Coil of Inductor L1 and produce the electromagnetic field, proportionally to a number of turns and the current through the coil. The charging process, generally, lasts some milliseconds (e.g. 10 ms) and until it is continuing, the simple metal detecting systems don’t do anything. The most complicated metal detecting systems are measuring the rise of current, sampling its amplitude till whole charge pulse lasts. From the software point of view, it can be used later for the searching coil self-diagnosis and to do some kind of a ground effect minimizing activities.
Reverse pulse: A basic meaning to the described method has the self or pulse induction process.
When the system stops the charging Pulse, recently applied to the Inductor L1, on the leads of the inductor L1 is starting the specific “transitional process”.
This self-induction “transitional process” starts with the U L1 voltage quickly go to zero, but don’t stop on the neutral – equal to the Negative power source terminal. The Reverse Self-induction Voltage is going to rise (As a magnitude or in an absolute value) with a reverse polarity (or with a negative sign), and often, the voltage value reaches -100V.
When is used a high impedance schematic to create an appropriate damping оr an attenuation of this self-induction pulse, it the attenuated in the time by the exponential law, tending the neutral potential.
If near the inductor L1 have the metallic object, dependent its ferrous or nonferrous characters, the Reverse Induction pulse – in time of “transitional processes”, the pulse magnetize the object or on its surface are creating Eddy’s currents. In Both cases, the Self-induced pulse has an increased duration.
As we mention in the text above Coil have to solve two tasks:
Already charged and properly high impedance circuitry attenuated reverse voltage can be carefully clamped by amplitude, not allowing to destroy the input parts of the differential amplifier circuit. After a proper attenuation and scaling via an Automatic Gain Control schematic, the”scaled small signal of the reverse pulse” – limited to 3 Volts, which is the normal MCU ADC or Analog comparator circuit level, for the further processing – here is the place to be time stamped the first strobe taken ST1.
MCU based System starts the High Resolution 16 or 32-bit Capturing Timer. (In only discrete hardware realizations, this task is done by the integrator circuit).
In absence of metal near the searching Coil – The MCU system takes its final activity – strobe and store the second sample ST2, near the zero, e.g. 50 mV (to minimizing the noise and AC line interference influence) and this is applying to be as ground relative calibration procedure.
In only discrete hardware realizations, the second strobe stops the integrator. The Output Voltage is proportional to the metallic object volume, and reverse proportional to its distance and a searching coil. The Integrator output voltage is controlling the signal generator, which signal is applied to Low-Frequency Amplifier, and then to the Headphones.
MCU based metal detectors representing the virtual “Rectangle” – the vector “constructed” with ST1 and ST2. The “shape of the rectangle” or the magnitude of ST1 and ST2 represented the vector, carrying the information, how is large or the ground depth where the metallic object is.
The Pulse Induction method is often used by metal detectors manufacturers.
Although the described method, by nature, does not accurately determine the different metals – one from another, the very different metals are distinguishable.
Practical usage of the PI-Pulse Induction Metal Detectors:
PIC. 1 Practical Example: The PI MD Garrett ATX – Parameters and Modes of work
PIC. 2 Practical Example: The PI MD Garrett ATX – Technical Specifications
In the practice, the typical pulse induction metal detector is generating near 100 pulses per second.
If you are going to use the metal detector on the highly mineralized ground or in the areas where are magnetite hot-rocks, a Pulse Induction or PI Metal Detector is very suitable.
Searching for the “treasure” into a salt water beach is easier if you are using a pulse induction metal detector.
On a salt water beach the Pulse Induction Metal Detector can be seen in an action – (See Video 1.):
VIDEO 1. CSCOPE CS4PI – PI Metal Detector Review and Test
Don’t Stop To Reinvent Yourself, Dear Explorers!
Another Garret Metal Detectors:
National Geographic Pro Metal Detector: