Capacitive Sensor Distance Measurement for Plasma Cutting
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Solution Overview
Problem
Existing methods for precisely adjusting the distance between a machine tool and an electrically conductive workpiece, such as in plasma cutting, are imprecise due to deformation of thin sheet metals and environmental influences affecting capacitive sensor resonance frequencies.
Innovation Solution
A method using a capacitive or inductive sensor with a non-linear resonance frequency curve, where the change in resonance frequency over time is used to accurately determine the distance, with comparison values and limit differences stored in a data processing unit to prevent incorrect measurements, and allowing for variable approach speed and calibration.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If the cutting torch is moved toward the workpiece until it touches the workpiece and then lifted by a mechanical path measurement device, then the ignition distance can be adjusted, but the method is imprecise for thin sheet metal because the sheet metal deforms and the torch penetrates into the indentations
Solution Approach 1:
The patent replaces the mechanical contact-based path measurement device with a capacitive sensor system. The capacitive sensor detects changes in capacitance as the torch approaches the workpiece, enabling non-contact measurement of the ignition distance. This eliminates the mechanical impact that causes sheet metal deformation while providing precise distance measurement through electrical field interaction between the sensor and workpiece.
2Measurement precision
If a ring-shaped capacitive sensor is used to measure the first location distance by monitoring resonance frequency changes, then the distance can be set, but the resonance frequency curve shifts due to ambient influences such as humidity or ambient temperature
Solution Approach 1:
The patent implements a feedback mechanism where the capacitive sensor continuously monitors the distance between the torch and workpiece during the approach. The control system uses this real-time feedback to adjust the torch position and maintain the predetermined first location distance, compensating for environmental influences on the resonance frequency through dynamic adjustment rather than relying on a fixed frequency-curve relationship.
Solution Approach 2:
The patent changes the measurement parameter from absolute resonance frequency to the rate of change of resonance frequency or capacitance with respect to distance. By focusing on the derivative or change in the electrical parameter rather than the absolute value, the system becomes insensitive to environmental shifts that cause uniform frequency curve displacement, while still detecting the non-linear distance relationship.
3Measurement precision
If the torch is stopped at a predetermined first location distance above the workpiece and then brought to the ignition distance by a path measurement device, then the ignition distance can be set, but the accuracy depends on the accuracy of the first location distance setting
Solution Approach 1:
The patent performs preliminary calibration by moving the torch toward the workpiece at a constant speed while the capacitive sensor records the electrical parameter changes. The system identifies the point where the rate of change of the electrical parameter matches a predetermined value, automatically setting the first location distance without requiring manual intervention or complex multi-step procedures. This preliminary automatic positioning simplifies the overall process while maintaining high accuracy.
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
Enables precise setting of the first location distance, reducing wear on the machine tool and improving machining accuracy by accounting for environmental changes and workpiece deformation.
Implementation Method 1
A precise adjustment of the distance at which the plasma cutting torch is ignited
Implementation Method 2
A method using a capacitive or inductive sensor with a non-linear resonance frequency curve
Implementation Method 3
where the change in resonance frequency over time is used to accurately determine the distance, with comparison values and limit differences stored in a data processing unit
Data Source
AI summary
A method is for setting at least one predetermined distance between a machine tool (10) and a metallic or non-metallic electrically conductive workpiece (12) according to which a capacitive and/or inductive sensor (16) fixed to the machine tool (10) while facing the workpiece (12). The machine tool (10) is displaced with constant speed toward the workpiece (12) by means of a drive device, and a sensor signal is output at predetermined time intervals by means of a data processing unit (24). The sensor signal has a characteristic quantity that continuously changes in a non-linear manner according to the distance of the machine tool (10) from the workpiece (12). The data processing unit (24) compares each read out value of the characteristic quantity with a comparative value read out before at a predetermined number of time intervals and calculates the difference between the values that are compared with one another.