Optical Sensor LED Current Control via PWM Feedback
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Solution Overview
Problem
Mail processing systems face challenges in maintaining the operating characteristics of optical sensors due to factors like LED aging and dust build-up, which can lead to reduced sensitivity and increased costs with existing solutions.
Innovation Solution
Adjusting the LED current of optical sensors using a high frequency pulse-width modulated signal generated from a microcontroller, with feedback from the photo-detector to adjust the duty cycle, and using a low pass filter to convert the signal into a DC voltage for a current amplifier circuit, eliminating the need for expensive digital to analog converters.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If the LED current is increased to ensure sufficient light detection, then the photo-detector can reliably detect light when not blocked, but the sensor's ability to detect thinner material is reduced and the overall LED operating life is reduced
Solution Approach 1:
The patent applies periodic action by using pulse width modulation (PWM) to drive the LED instead of continuous DC current. The LED is switched on and off at high frequency, with the duty cycle adjusted to provide the required average current. This periodic operation reduces the LED's operating life impact while maintaining sufficient light output for reliable detection, as the LED spends part of the time off, reducing cumulative stress and heat generation.
Solution Approach 2:
The patent implements dynamics by making the LED current adjustable through PWM duty cycle control. Instead of using a fixed high current to ensure reliable detection, the system dynamically adjusts the average current level based on operational needs, allowing optimization between detection reliability and LED longevity. The microcontroller can modify the duty cycle to adapt to different detection requirements and operating conditions.
2Reliability
If the LED current is increased to overcome LED aging and dust build-up, then the sensor maintains detection capability, but the light intensity may pass through thinner materials causing false unblocked states
Solution Approach 1:
The patent applies feedback by using the photo-detector's output to monitor the light level and adjust the PWM duty cycle accordingly. The system continuously monitors the detected light intensity and modifies the LED drive current to maintain optimal detection conditions. This feedback mechanism allows the system to compensate for LED aging and dust accumulation while preventing excessive light intensity that could cause false readings with thin materials.
Solution Approach 2:
The patent implements parameter changes by dynamically adjusting the LED drive current through PWM duty cycle modification. Instead of using a fixed high current to overcome contamination effects, the system varies the current parameter based on feedback from the photo-detector, allowing optimization of detection accuracy for different material thicknesses while maintaining reliability under contaminated conditions.
3Measurement precision
If more sensitive detectors are used to maintain detection capability with lower LED current, then the sensor can detect thinner material better, but false triggers due to external light sources occur
Solution Approach 1:
The patent applies periodic action by using high-frequency PWM to drive the LED and synchronizing the detection to this periodic signal. The photo-detector is configured to detect light at the specific PWM frequency, allowing the use of sensitive detectors without false triggers from external light sources. The synchronous detection at the known PWM frequency filters out ambient light interference while maintaining high sensitivity to the modulated LED signal.
4Reliability
If current amplifiers with digital to analog converters are used for each sensor to adjust LED current, then the sensor can compensate for variations and aging, but the machine cost significantly increases
Solution Approach 1:
The patent applies mechanics substitution by replacing the traditional digital-to-analog converter (DAC) hardware with a PWM-based control system. Instead of using expensive DAC chips to generate analog control voltages for current amplifiers, the system uses a microcontroller to generate PWM signals that directly control the LED current through a simple pulse amplifier. This substitution maintains the compensation capability while dramatically reducing hardware complexity and cost.
Solution Approach 2:
The patent implements this principle by using a simple, low-cost PWM control architecture instead of expensive precision DAC hardware. The system uses inexpensive components (microcontroller, pulse amplifier, and basic filter) to achieve current control that would otherwise require costly dedicated DAC chips for each sensor. This approach prioritizes cost-effectiveness while maintaining adequate compensation capability through software-based PWM duty cycle adjustment.
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 maintains sensor effectiveness and operating life while reducing costs by compensating for variations in LED brightness and contamination without requiring complex hardware, ensuring reliable operation in mail processing systems.
Implementation Method 1
a light-emitting diode (LED) 14. When the optical sensor 10 is active, the LED 14 is activated to produce light to illuminate the photo-detector 12
Implementation Method 2
the light from the LED 14 is detected by the photo-detector 12
Implementation Method 3
a low pass filter converting this signal into a DC voltage
Data Source
AI summary
A system and method for adjusting the LED current of an optical sensor that does not decrease the effectiveness of the optical sensor or the length of its operating life, or significantly increase the cost due to hardware requirements. The LED current of an optical sensor is adjusted using a high frequency pulse-width modulated signal generated from a microcontroller. Based on feedback provided by the photo-detector, the duty cycle of the signal can be adjusted by the microcontroller. The signal passes through a low pass filter which averages the modulated signal into a DC voltage, which is then used to control a current amplifier circuit that provides current to the LED of the optical sensor. This adjustability enables the system to compensate for variations in sensor LED's and the LED brightness reduction to due aging and/or build-up of contaminants on the photo-detector and/or LED.