Time-of-Flight Distance Sensor FPN Correction
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Solution Overview
Problem
Time-of-flight distance image capturing devices face challenges in accurately calculating distances due to fluctuations in fixed pattern noise (FPN) caused by temperature changes and variations in measurement conditions, such as integration times, which can lead to ineffective FPN correction and impaired auto exposure functionality.
Innovation Solution
A distance image capturing device and method that incorporate a light source unit, a light receiving unit with a pixel circuit and charge accumulation units arranged in a two-dimensional matrix, and a pixel driving circuit that synchronizes charge distribution and accumulation with optical pulse irradiation. The device includes specific periods for accumulation, FPN measurement, and differential signal output within a frame cycle to accurately measure and correct for FPN, enabling precise distance calculation.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If FPN correction is performed using pre-measured FPN values, then distance calculation accuracy is improved under stable conditions, but measurement accuracy deteriorates when FPN fluctuates due to temperature changes or integration time variations
Solution Approach 1:
The patent implements periodic FPN measurement within each frame cycle by allocating specific periods (e.g., first period for initial FPN measurement, second period for updated FPN measurement). This allows the system to continuously update FPN values according to current temperature and integration conditions, resolving the contradiction between using pre-measured values and adapting to changing conditions.
Solution Approach 2:
The patent performs FPN measurement in advance within the same frame cycle before final distance calculation. By measuring FPN in the first period and using it for correction in subsequent periods, the system prepares correction data preliminarily while ensuring it reflects current device conditions, thus maintaining both accuracy and reliability.
2Measurement precision
If FPN is measured for each measurement condition, then correction accuracy is improved, but device complexity and operational burden increase significantly
Solution Approach 1:
The patent designs the pixel circuit to perform multiple functions: normal photoelectric conversion during accumulation periods and FPN measurement during dedicated measurement periods. The same pixel circuit structure handles both distance measurement and FPN characterization, eliminating the need for separate measurement systems and reducing overall device complexity.
Solution Approach 2:
The patent merges FPN measurement functionality into the existing distance measurement frame cycle. By combining FPN measurement periods with normal operation periods in a unified time-division multiplexed approach, the system achieves condition-specific FPN correction without requiring separate measurement systems or increasing operational complexity.
3Reliability
If FPN measurement is performed frequently to track fluctuations, then correction reliability is improved, but measurement time and frame cycle duration increase
Solution Approach 1:
The patent maintains continuous distance measurement functionality while integrating FPN measurement periods within the same frame cycle. By alternating between accumulation periods for distance measurement and measurement periods for FPN characterization, the system ensures both functions occur continuously without significant interruption, maintaining reliability while minimizing time loss.
Solution Approach 2:
The patent implements periodic FPN measurement at strategically chosen intervals within the frame cycle rather than continuously. This periodic approach updates FPN values frequently enough to track fluctuations reliably while keeping measurement periods discrete and time-limited, thus balancing correction reliability with frame cycle efficiency.
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 allows for accurate distance calculation even when FPN fluctuates, enhancing the device's performance in varying conditions and ensuring reliable operation during auto exposure adjustments.
Implementation Method 1
a photoelectric conversion element configured to generate a charge according to incident light
Implementation Method 2
measures the distance between a measuring instrument and an object based on a flight time of light in space
Data Source
AI summary
In a distance image capturing device, the frame cycle includes an accumulation period in which reflected light of the optical pulse is accumulated in the charge accumulation unit, a first output period in which a first voltage value corresponding to the amount of charges accumulated in the charge accumulation unit in the accumulation period is output, an FPN measurement period in which a fixed pattern noise accumulated in the charge accumulation unit is measured, a second output period in which a second voltage value corresponding to the amount of charges accumulated in the charge accumulation unit in the FPN measurement period is output, and a readout period in which a differential signal corresponding to a difference between the first voltage value and the second voltage value is output, and the distance calculation unit calculates the distance to the subject using a signal value of the differential signal.