T/R Switch Back-EMF Decay Control for Metal Detector Signal Recovery
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Solution Overview
Problem
Time domain metal detectors face limitations in detecting fast time constant targets due to slow recovery of electronics after overload, saturation from overload, and the inclusion of unwanted spurious signals from magnetic soils, particularly when using T/R switches that do not allow for immediate synchronous demodulation after the high-voltage back-emf period.
Innovation Solution
A method is introduced to control the T/R switch to reduce the duration of back-emf decay and synchronize the coil current and voltage to zero at the end of the back-emf period, using a negative feedback loop to manage the T/R switch's state and admittance, allowing for earlier and more accurate demodulation of receive signals, thereby reducing unwanted signals from magnetic soils.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If T/R switch is used to protect receive electronics from high voltage during back-emf period, then receive electronics are protected from overload, but back-emf decay duration is extended causing delayed signal demodulation
Solution Approach 1:
A damping resistor is introduced as an intermediary element to accelerate back-emf decay. The resistor provides a controlled discharge path for the coil's stored energy, reducing the decay duration without requiring the T/R switch to remain in the protective state longer. This mediator enables the system to maintain electronics protection while reducing the time penalty associated with back-emf decay.
Solution Approach 2:
The system dynamically changes the coil circuit's resistance parameter by switching the damping resistor into the circuit during or after the back-emf period. This parameter change accelerates the decay of back-emf by increasing energy dissipation, thereby reducing the decay duration and enabling earlier signal demodulation while maintaining electronics protection.
2Measurement precision
If synchronous demodulation is delayed until after back-emf decay, then spurious signals from magnetic soils are reduced, but detection of fast time constant targets is compromised
Solution Approach 1:
The damping resistor enables the system to rush through the back-emf decay period more quickly, allowing synchronous demodulation to begin earlier. By accelerating the decay process, the system can skip through the problematic high-voltage period faster and transition to the demodulation phase sooner, thereby detecting fast time constant targets before their signals decay while still maintaining adequate signal quality.
3Reliability
If damping resistor is used to isolate high voltage back-emf from preamplifier, then receive electronics are protected, but signal-to-noise ratio of receive signal deteriorates
Solution Approach 1:
The damping resistor is activated periodically or conditionally - specifically during or after the back-emf period when high voltage is present - rather than continuously. This periodic activation provides protection precisely when needed during the high-voltage phase, while allowing the signal path to remain high-fidelity during the subsequent low-voltage receive period when the resistor would otherwise degrade the signal-to-noise ratio.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This approach enhances the detection of fast time constant targets by minimizing spurious responses and improving signal-to-noise ratio, allowing for earlier and more reliable detection without introducing significant interference from magnetic soil modulation.
Implementation Method 1
transmit electronics generating a repeating transmit signal cycle of a fundamental period, which is applied to an inductor, for example a transmit coil, which transmits a resulting varying magnetic field
Implementation Method 2
a high-voltage period and at least a low-voltage period within a repeating transmit signal cycle... during a high-voltage period when the voltage across the coil is relatively high (100-200V, for example)
Data Source
AI summary
A method for reducing unwanted signals, due to a back emf decay, within a receive signal received by a receive coil of a metal detector during a demodulation of the receive signal after a transmission by a transmit coil of the metal detector, including controlling a T/R switch to control a characteristic of the back emf decay to reduce a duration of the period of the back emf decay.