Capacitive Touch Control Panel with PWM-Aware Reference Updating
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Solution Overview
Problem
The use of electrostatic capacitance type touch switches in air conditioning control panels for vehicles experiences erroneous judgments due to noise generated by PWM control of light-emitting diodes, leading to increased costs when trying to mitigate these errors by relocating LEDs or adjusting their brightness.
Innovation Solution
A control panel design that includes a memory device for storing reference values and a judging unit to determine touch operations based on detected electrostatic capacitance differences, with a reference value updating mechanism that adjusts based on the state of PWM control for both brightness adjustment and indicator LEDs, reducing noise-induced errors.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Use of energy by moving object
If light-emitting diodes are disposed proximate to electrodes for brightness adjustment through PWM control, then illumination efficiency is improved, but noise is generated causing erroneous touch operation judgments
Solution Approach 1:
The system updates reference values for touch detection at specific timing (when PWM control is not executed or when indicator LED brightness is at maximum), before noise from PWM control affects the electrostatic capacitance measurements. This preliminary updating ensures accurate reference values are established when noise interference is minimal or predictable.
Solution Approach 2:
The system changes the timing parameter for reference value updates based on the PWM control state. Reference values are updated selectively at moments when PWM dimming is inactive or at peak brightness, avoiding periods when noise from PWM switching would corrupt the electrostatic capacitance measurements used for touch detection.
2Reliability
If light-emitting diodes are relocated away from electrodes to reduce noise, then touch operation detection accuracy is improved, but device complexity and cost increase
Solution Approach 1:
Instead of physically relocating components, the system preliminarily updates reference values at optimal timing before noise interference occurs. This temporal separation allows components to remain in their original compact positions while achieving accurate touch detection by avoiding noisy measurement periods.
Solution Approach 2:
The system changes the temporal parameter of reference value updates rather than altering the spatial arrangement of components. By updating references only when PWM noise is absent or minimal, the system maintains component proximity for simplicity while achieving noise-resistant touch detection.
3Reliability
If amplitude control is used instead of PWM for brightness adjustment, then noise is reduced, but the number of parts increases and cost increases
Solution Approach 1:
The system maintains PWM control for cost-effectiveness but preliminarily updates reference values at moments when PWM switching noise is minimal. This timing-based approach achieves noise reduction without changing the control method or adding circuit components.
Solution Approach 2:
The system changes the timing parameter of reference value updates to coincide with low-noise periods of PWM operation, rather than changing the PWM control method itself. This preserves the simple, cost-effective PWM architecture while achieving accurate touch detection.
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 effectively reduces the risk of erroneous judgments caused by noise from PWM control, maintaining accurate touch operation detection while minimizing the number of components and costs.
Implementation Method 1
Touch switches of electrostatic capacitance type include electrodes that are disposed proximate of operating portions that are subject to touch operations. Touch operations are detected based on changes in electrostatic capacitances in accordance with a conductive body such as a human body approaching these electrodes.
Data Source
AI summary
Techniques reduce erroneous judgment due to effects of noise accompanying PWM control. LEDs light operating portions and function as indicators. Circuitry adjusts brightness of the LEDs through PWM control. A detecting circuit outputs detected values in accordance with electrostatic capacitances of the electrode. A CPU senses a touch operation when a difference between a detected value of the detecting circuit and a reference value is more than a prescribed value. The CPU stores a detected value as a reference value which is the first value detected by the detecting circuit after the PWM control execution state of the LEDs has been changed or the first value that is detected by the detecting circuit, which is comprised by a touch switch that is in a specified positional relationship with respect to the touch switch which executing state of PWM control of the LEDs has changed, after transition of the executing state.