Variable Gain Amplifier for Bar Code Readers
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Solution Overview
Problem
Bar code readers face interference from ambient light and back-scatter noise, leading to signal misinterpretation and reading failures, especially with the transition to LED illumination and the ambiguity in amplification gain switching during scanning.
Innovation Solution
A method to control the amplifier gain in a bar code reader by adjusting feedback resistance based on signal thresholds and scan direction changes, ensuring accurate signal amplification and interpretation.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If the feedback resistance is increased to amplify weak bar code signals, then the signal-to-noise ratio is improved, but the output signal reaches saturation level and becomes distorted
Solution Approach 1:
The feedback resistance is made dynamically adjustable rather than fixed. The amplifier controller continuously monitors the output signal level and adjusts the feedback resistance accordingly - using high resistance values to amplify weak signals and maintain good signal-to-noise ratio, and switching to low resistance values when the output approaches saturation to prevent distortion and maintain reliable signal output.
Solution Approach 2:
The electrical parameter (feedback resistance) is changed based on the operating conditions. The amplifier controller switches between different resistance values (e.g., 10kΩ to 1MΩ) depending on the signal strength and output level, allowing the amplifier to adapt its gain characteristic to maintain optimal performance across varying signal conditions.
2Reliability
If the feedback resistance is decreased to lower the output signal level below saturation, then the signal distortion is reduced, but the signal-to-noise ratio deteriorates
Solution Approach 1:
The feedback resistance is dynamically adjusted based on real-time monitoring of the output signal level. When the output approaches saturation, the controller decreases the resistance to lower the gain and prevent distortion. When the output is well below saturation, the controller increases the resistance to maximize signal-to-noise ratio, thus adaptively optimizing both reliability and measurement precision.
Solution Approach 2:
The amplifier controller uses feedback from the output signal level to automatically adjust the feedback resistance. The controller monitors the output voltage and compares it against predetermined thresholds, then switches the feedback resistance accordingly to maintain the output within the optimal operating range, preventing both saturation and excessive noise amplification.
3Measurement precision
If the amplifier gain is switched during bar code scanning, then the signal level is optimized, but the signal interpretation becomes ambiguous
Solution Approach 1:
The amplifier gain switching is synchronized with the periodic scanning operation of the bar code reader. The controller monitors the scan motor direction changes and only switches the feedback resistance during the reverse scanning portions when no bar code signal is being read. During forward scanning when signals are present, the gain remains constant to avoid signal interpretation ambiguity, thus maintaining both optimized signal levels and clear signal interpretation.
Solution Approach 2:
The amplifier controller anticipates the need for gain adjustment by monitoring scan motor direction changes in advance. The feedback resistance switching is timed to occur just before or during the reverse scan transition, ensuring that gain changes are complete before the next forward scan begins, thereby preventing any period where gain switching could interfere with signal interpretation.
4Measurement precision
If the feedback resistance remains at a larger value to maintain high signal-to-noise ratio, then the signal-to-noise ratio is improved, but the output signal may reach saturation point
Solution Approach 1:
The amplifier controller continuously monitors the output signal level and uses this feedback to determine when to switch the feedback resistance. When the output voltage approaches the saturation level (e.g., within a predetermined threshold), the controller switches to a lower resistance value to reduce the gain and bring the output back into the linear operating range, thus preventing saturation while maintaining high signal-to-noise ratio during normal operation.
Solution Approach 2:
The feedback resistance parameter is dynamically changed based on the output signal level. The system transitions between different resistance states (high resistance for maximum signal-to-noise ratio, low resistance to prevent saturation) according to real-time operating conditions, allowing the amplifier to maintain optimal performance across the full dynamic range of input signal levels.
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 reduces ambiguity in signal interpretation and enhances the accuracy of bar code reading by dynamically adjusting the amplifier gain in response to changing signal levels and scan direction, effectively mitigating the impact of ambient light and back-scatter noise.
Implementation Method 1
converting the received light into an initial electrical signal
Data Source
AI summary
A system and method for controlling an amplifier in a bar code reader are disclosed, wherein the method may include receiving light at a photodiode within the bar code reader from a bar code being scanned by a scan mirror powered by a scan motor; converting the received light into an initial electrical signal; determining whether the scan motor is undergoing a change in direction; establishing a gain value for the amplifier based on an outcome of the determining step; and amplifying the initial electrical signal into an amplified signal using the established gain value.