Digital PLL Inductive Sensing at Fixed Frequency for Metal Detection
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Solution Overview
Problem
Conventional inductive sensing technologies face challenges in detecting different types of objects using a single device due to the need for distinct sense elements and circuits, which is not feasible in terms of cost or space, especially in small form factor devices, and suffer from limitations in sensitivity and accuracy due to temperature drift and clock jitter.
Innovation Solution
The implementation of an inductance-sensing circuit with a digital control loop that maintains a fixed frequency, allowing the detection of both ferrous and non-ferrous metals using a single inductive sensing circuit by selectively coupling capacitors into the resonant circuit, thereby improving electromagnetic compliance and reducing the need for additional logic and circuits.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Adaptability or versatility
If distinct sense elements and circuits are used for different types of objects, then detection capability for various objects is improved, but device complexity and cost increase
Solution Approach 1:
The patent implements a universal inductive sensing circuit that can detect both ferrous and non-ferrous metals using the same sense element and circuitry. By operating at a fixed resonant frequency and measuring inductance changes, the circuit achieves multi-functionality without requiring separate detection circuits for different object types, thereby reducing device complexity while maintaining versatility
Solution Approach 2:
The patent changes the measurement parameter from frequency variation to inductance measurement at a fixed frequency. By maintaining a constant resonant frequency and measuring the inductance of the sense element, the system can differentiate between object types through inductance values rather than requiring separate circuits, thus reducing complexity while preserving detection capability
2Reliability
If conventional inductive sensing is used, then simple circuit design is maintained, but sensitivity and accuracy are limited due to temperature drift and clock jitter
Solution Approach 1:
The patent employs a feedback mechanism where the resonant circuit's frequency is maintained at a fixed value through continuous monitoring and adjustment. The system measures the inductance at this stabilized frequency, compensating for temperature drift effects and reducing sensitivity to clock jitter, thereby improving reliability without significantly increasing circuit complexity
Solution Approach 2:
The patent replaces traditional mechanical or analog frequency-tuning mechanisms with a digital inductance measurement approach. By using a fixed-frequency resonant circuit and measuring inductance through digital signal processing, the system achieves higher sensitivity and accuracy while reducing the complexity associated with mechanical tuning components
3Measurement precision
If multiple sense elements are used for different object types, then detection accuracy is improved, but area and volume requirements increase
Solution Approach 1:
The patent measures inductance at a fixed resonant frequency rather than sweeping through multiple frequencies or using multiple sense elements. This parameter change allows a single sense element to provide accurate detection for different object types by analyzing inductance variations at the fixed frequency, thereby maintaining measurement precision while minimizing the required area
Solution Approach 2:
The patent makes the single sense element universal by operating it at a fixed frequency and measuring inductance changes caused by different object types. This approach enables one sense element to perform the function of multiple specialized elements, achieving detection accuracy for various objects without increasing the area occupied by sense elements
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 enables high dynamic range inductance sensing with improved sensitivity and reduced temperature drift, allowing for the detection of various objects without requiring separate sense elements or circuits, and is scalable and self-calibrating, thus suitable for small form factor devices.
Implementation Method 1
a resonant circuit including an inductance sensor electrode and a capacitor. The digital PLL is coupled to the resonant circuit and maintains a fixed frequency of the resonant circuit
Implementation Method 2
Inductive sensing may allow for linear encoding, rotary encoding, linear variable differential transformer (LVDT) sensing, and the like. Inductive sensing technology may enable touch and proximity detection for human interface on a wide variety of materials including both ferrous and non-ferrous metals. Detection occurs by measuring small deflections of conductive targets.
Data Source
AI summary
Front-end circuits that combine inductive and capacitive sensing are described. In one embodiment, an apparatus includes a plurality of inductive elements, an inductive measurement circuit, and a frequency divider circuit. The inductive measurement circuit is to output a first signal with a first frequency. The first signal is associated with an inductance change of one of the inductive elements. A feedback circuit can maintain the sinusoidal operation of the first signal. The frequency divider circuit can generate a second signal with a second frequency that is lower than the first frequency.