Design of Pulse Coil Detectors for Metal Detection
Pulse coil detectors are a popular choice among hobbyists and professionals alike for detecting metallic objects. While various designs and methods exist, the core functionality remains consistent: these devices utilize a coil to generate magnetic pulses, which are then used to detect metal objects in the vicinity.
Coil Specifications and Detection Depth
The effectiveness of a pulse coil detector largely depends on the coil's size. The depth at which an object can be detected is influenced by the coil's diameter and the size of the target. Generally, the maximum detection depth is estimated to be up to five times the coil's diameter, while the smallest detectable object should be about 5% of the coil's diameter. For practical applications, such as searching for coins or jewelry, coils ranging from 25 cm to 40 cm in diameter are typically sufficient. However, for deeper targets, such as a 20 cm iron cover buried at a depth of two meters, a larger coil of about 1 meter in diameter may be necessary.
The design of the coil can vary in shape, with round coils being the most common, although square and elliptical shapes can also be utilized. Inductance values for these coils usually fall between 300 to 500 μH, and for this design, we will assume an inductance of 400 μH.
Power Supply Considerations
A reliable power supply is crucial for the operation of a pulse coil detector. A dual power supply providing ±10 volts or ±12 volts is often ideal. However, using a single battery pack of 10 or 12 volts along with a DC/DC converter to generate the second voltage rail can simplify the design. Care must be taken, as the output from the DC/DC converter may introduce ripple voltage, which can affect detector performance, especially at higher frequencies.
The pulse coil is energized via a high-speed transistor or MOSFET, which allows current to build up until limited by the circuit's resistance. A stronger magnetic field can penetrate deeper into the ground, but if the pulse duration is too long, it may lead to ground saturation and hinder the detection of smaller objects. A pulse duration of around 250 microseconds is often optimal.
Inductance and Resistance Calculations
When designing the coil, both the inductance and resistance must be carefully calculated. The resistance of the coil influences the maximum current that can flow through it, which in turn affects the strength of the magnetic field generated. For instance, a coil with an overall resistance of around 3 ohms can allow for a current of approximately 4 amps when powered by a 12-volt source over a pulse duration of 250 microseconds.
The physical dimensions of the coil, including wire diameter and number of turns, are also pivotal. A table summarizing these parameters can help in achieving the desired inductance and resistance values, ensuring that the coil operates effectively within the specified limits.
Discharge Curve and Signal Processing
The discharge curve of the coil can be divided into three distinct phases:
Phase One: Breakdown Effect on the Drive MOSFET
In this initial phase, the MOSFET controls the current pulse through the coil. If the MOSFET is switched off, the current flowing through a parallel resistor will induce a voltage in the coil. The duration of this phase can be calculated based on the coil's inductance and the resistance in the circuit.
Phase Two: High Current Decay
Once the induced voltage surpasses the breakdown voltage of the MOSFET, the current begins to decay exponentially. This phase is characterized by a rapid drop in voltage, making it challenging to perform reliable measurements. Most metal detectors bypass this phase to focus on the next stage.
Phase Three: Final Current Decay and Eddy Currents
In the final phase, the current continues to decay, influenced by the presence of nearby metals. This phase is crucial for detection, as it provides information about the type of metal present. However, the signal levels are low, requiring significant amplification for accurate analysis.
Signal Discrimination Challenges
The analysis of signals during detection presents challenges. The low signal levels necessitate high gain amplification, which can also amplify noise. Traditional pulse-sensing metal detectors often average the signals instead of analyzing them in detail, leading to poor discrimination capabilities. Advanced designs may employ digital signal processing to enhance detection accuracy and provide more detailed information about the detected metals.
In summary, the design and functionality of pulse coil detectors revolve around the careful selection of coil size, power supply considerations, and effective signal processing techniques. By understanding these principles, enthusiasts can create efficient and effective metal detection devices tailored to their specific needs.
