In the earlier types of electronic detectors the reflected signal separated from the emitted, or geometrically - by mutual arrangement the receiving and the emitting coil, or by means of special compensation schemes. It is obvious that there can be and temporary method of separation of emitted and the reflected signals. This method is widely used, for example, pulsed echo and radar. When the locations of the delay mechanism of the reflected signal due to the significant time of signal propagation to the object and back.
In relation to the detectors, this may be a phenomenon self-induction in the conductive object. How to use this in practice? After the effects of pulse magnetic induction in a conducting object occurs and some time supported (due to the phenomenon of self-induction) fading the current pulse causing the detainee at the time the reflected signal. He carries useful information, and should be registered.
Thus, you may propose another scheme for the construction of the detector, fundamentally different from that considered previously by the method of separation signals. Such a detector has received the name of pulse. It consists of generator of current pulses, the receiving and radiating coils, which can be combined into one, switching devices and the processing unit of the signal.
The pulse generator current generates short pulses of current millisecond range entering the emitting coil, where they are converted into pulses the magnetic induction. Since the emitting coil - load pulser - has a pronounced inductive, on the fronts of the pulses of the generator arise overload in the form of spikes. These spikes can reach the amplitude of tens-hundreds (!) the volt, however, the use of protective limiters is unacceptable as it would lead to the delay of the pulse front current and magnetic induction, and, ultimately, the complexity of separation the reflected signal.
Foster and radiant coil can be positioned relative to each other quite arbitrarily, since direct penetration of the radiated signal in the take-up spool and the influence of the reflected signal separated by time. In principle, one coil can act as a foster and radiant, however, in this case it is more complicated to unleash high voltage output the circuit of the generator of current pulses and sensitive input circuits.
The switching device is intended to produce the above-mentioned separation of the emitted and reflected signals. It blocks the input circuit of the device for a certain the time is determined by the duration of the current pulse radiating in the coil, the discharge time of the coil and the time during which it is possible the appearance of short responses of the device from the massive low-conductive objects soil. After that time, the switching device must provide transmitting a signal from the receive coil to the block of signal processing.
The HV unit is designed to convert input electric the signal in readable human form. It can be constructed based on the solutions used in the detectors of other types.
The disadvantages of pulse detectors include the complexity of the implementation in practice, discrimination of objects by type of metal, the complexity of the equipment generating and switching pulses of voltage and current of large amplitude, high the level of interference.
Author: A. I. Shchedrin