Heating Wire Circuit Switching for Capacitive Sensing Isolation
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Solution Overview
Problem
Existing systems for alternating heating and capacitive measuring operations using a common heating wire face challenges in efficiently utilizing heating current for heating while ensuring reliable and improved capacitive approach detection, which is affected by parasitic capacitances and thermal loads.
Innovation Solution
A circuit configuration utilizing a pair of first and second switching elements, preferably MOSFETs, to manage the heating and measuring operations, including a detection circuit for determining measuring capacitance and a control circuit for switching between heating and measuring modes, while incorporating a test phase to check the integrity and calibration of the detection circuit.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If additional impedances (diodes) are used to support the insulating effect of blocking switching devices during measuring operation, then the insulation effect is improved, but the heating current efficiency deteriorates due to ohmic losses
Solution Approach 1:
The patent extracts the additional impedances (diodes) from the circuit by using only the blocking switching devices themselves to provide the necessary insulation effect during measuring operation. The switching devices are configured to block current in both directions without requiring additional diodes, thereby eliminating the ohmic losses while maintaining the insulation effect.
2Adaptability or versatility
If the heating wire is used as an electrode for capacitive approach detection during non-heating phases, then the detection function is enabled, but the reliability deteriorates due to thermal load and thermally induced expansion fluctuations
Solution Approach 1:
The patent implements periodic switching between heating operation and measuring operation. During heating phases, the heating wire generates thermal energy; during measuring phases, the heating wire serves as a capacitive electrode. This periodic alternation allows the system to achieve both heating and detection functions while managing thermal load through controlled cycling.
Solution Approach 2:
The patent performs preliminary cooling or stabilization of the heating wire between heating and measuring phases. The control circuit manages the timing and duration of heating pulses to allow the heating wire to return to a stable thermal state before capacitive measurement, thereby reducing thermally induced expansion fluctuations and improving measurement reliability.
3Ease of operation
If pulse-width-modulated heating current is used for heating operation, then the heating control flexibility is improved, but the measuring operation reliability deteriorates due to interference in measuring capacitances from all poles
Solution Approach 1:
The patent segments the heating wire connections to different poles by using blocking switching devices that isolate the heating wire from both poles during measuring operation. This segmentation prevents interference from all-pole potentials during capacitive measurement while allowing full heating operation when switching devices are conductive.
Solution Approach 2:
The blocking switching devices act as intermediaries between the heating wire and the poles. During measuring operation, these switching devices block the connection to both poles, preventing interference in the measuring capacitances. The switching devices mediate between the heating function (requiring connection to both poles) and the measuring function (requiring isolation from both poles).
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
The proposed solution enables efficient heating operation while ensuring reliable capacitive approach detection by minimizing parasitic capacitances and thermal fluctuations, thereby improving the overall performance and longevity of the heating wire and detection circuit.
Implementation Method 1
the heating wire (2) has electric heating current, which is fed from two poles at two different heating potentials, passed through it, wherein a heating voltage drops across the heating wire
Implementation Method 2
Switching devices, in particular field-effect transistors, have parasitic capacitances, which can interfere with the determination of the actual measuring capacitance
Data Source
AI summary
The present disclosure relates to a circuit and a method for carrying out an alternating heating and capacitive measuring operation by means of a common heating wire, comprising the following steps: carrying out a heating operation, during which a pair of first switching elements and a pair of second switching elements are in a conducting state and the heating wire is conductively connected to one of two different heating potentials such that a heating current is applied to the heating wire; initiating a change from heating operation to measuring operation of the first switching elements and the second switching elements by way of a control circuit; carrying out the measuring operation in which a measuring capacitance of the heating wire is determined with respect to a reference potential by way of a detection circuit; and carrying out a test phase falling within the time frame of the measuring operation.