Inductive Proximity Switch Circuit for Temperature-Stable Metal Detection
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Solution Overview
Problem
Inductive proximity switches face challenges in maintaining consistent response behavior across a wide temperature range and distinguishing between ferrous and non-ferrous metals, with existing solutions experiencing temperature-dependent signal changes and requiring complex coil configurations.
Innovation Solution
The implementation of a current-fed oscillating circuit with a complex voltage divider or impedance converter to generate an auxiliary voltage signal, which is used in series with the mutual induction voltage to produce a differential voltage that is temperature-independent and sensitive to metal presence, allowing for consistent response across temperatures and metal types.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If a current-fed oscillating circuit with a single coil is used, then the device complexity is reduced, but the temperature dependence of the signal increases
Solution Approach 1:
The patent divides the single coil into two separate coils (transmitting coil and receiving coil) that are inductively coupled. This segmentation allows the system to measure coupling changes between the coils, which are less susceptible to temperature variations compared to the impedance of a single coil, thereby reducing temperature dependence while maintaining relatively simple device complexity.
2Length of stationary object
If the switching distance is increased, then the detection range is improved, but the relative change in transimpedance becomes extremely small
Solution Approach 1:
The patent introduces a metallic trigger as an intermediary object that enters the alternating magnetic field generated by the transmitting coil and induces currents in the receiving coil. This intermediary mechanism amplifies the coupling changes between the coils when a target is present, enabling detection at increased switching distances while maintaining sufficient measurement precision through the enhanced coupling effect.
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 solution achieves temperature independence and large relative changes in the differential voltage when a target approaches, enabling consistent performance and differentiation between metal types without the need for additional coils or complex coil configurations.
Implementation Method 1
the oscillating circuit transmitter coil generating an alternating magnetic field which is capable of inducing a mutual induction voltage in at least one receiver coil
Implementation Method 2
the eddy current losses caused by the trigger in an alternating magnetic field are evaluated
Data Source
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AI summary
The invention relates to an inductive proximity switch which has a resonant circuit (Ls, C) comprising a resonant circuit transmission coil (Ls) and a capacitance (C), wherein the resonant circuit transmission coil (Ls) generates an alternating magnetic field which induces a mutual induction voltage in at least one receiving coil (LE), and the oscillation state of the resonant circuit can be influenced by means of a metallic release which enters or moves away from the alternating field. An evaluation circuit is used to obtain a switching signal (uswitch) from the change in the oscillation state of the resonant circuit, wherein the change in the complex coupling between the at least two coils, the transmission coil (Ls) and the receiving coil (LE), can be evaluated, as a switching signal (uswitch), using an auxiliary voltage signal (UHSP) in the presence or absence of the release. The auxiliary voltage signal (UHSP) is obtained in a predefinable ratio (v) as a partial voltage from the resonant circuit voltage (USK) of the resonant circuit (Ls, C), wherein the auxiliary voltage signal (UHSP) is connected in series with the mutual induction voltage induced in the receiving coil (LE) in such a manner that a differential voltage (UD) whose magnitude has been reduced in comparison with the induced mutual induction voltage by the auxiliary voltage signal (UHSP) is obtained at the output of the receiving coil (LE), to earth (G) or to a potential. The differential voltage (UD) is supplied to the evaluation circuit (DG, SK) in a suitable manner in order to obtain the switching signal (Uswitch). The proximity switch may also be in the form of an oscillator.