TOF Range Imaging Noise Subtraction for Accurate Distance Sensing
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Solution Overview
Problem
Existing range imaging devices face accuracy deterioration due to noise charges integrated in charge storage units, which are not effectively distinguished from actual measurement charges, leading to inaccurate distance calculations.
Innovation Solution
A range imaging device and method that includes a light source, light-receiving unit, and distance calculation unit, where noise charges are identified and subtracted from integrated charges using a noise charge acquisition mode followed by a ranging charge acquisition mode, ensuring accurate distance calculations by correcting for noise interference.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If charge integration is performed in charge storage units during the measurement period, then the measurement precision is improved, but noise charges are also integrated which deteriorates the measurement precision
Solution Approach 1:
The measurement period is segmented into multiple integration periods corresponding to different time bins. Each charge storage unit integrates charge for a specific time window, allowing the system to distinguish between noise charges (integrated throughout the entire period) and actual measurement charges (integrated only during specific time windows when light pulses are emitted). This temporal segmentation enables selective noise rejection while preserving measurement accuracy.
Solution Approach 2:
The system performs preliminary charge integration during a noise measurement period before the actual ranging measurement. By first integrating charge during a period when no light pulses are emitted, the system captures only noise charges in each charge storage unit. This preliminary noise charge data is then subtracted from the total integrated charge during the measurement period, effectively removing noise interference from the final distance calculation.
2Measurement precision
If multiple charge storage units are used for integration, then the accuracy is improved through noise subtraction, but the device complexity increases
Solution Approach 1:
Each pixel circuit is divided into multiple charge storage units, with each unit responsible for integrating charge during a specific time bin. This segmentation allows parallel processing of different time windows, improving measurement accuracy through temporal resolution while keeping each individual charge storage unit relatively simple in structure.
Solution Approach 2:
The charge storage units serve multiple functions: they integrate charge during different time bins for ranging measurements, store noise charge data for subtraction, and enable temporal filtering of ambient light. This multi-functionality reduces the need for separate dedicated circuits for each function, thereby managing device complexity while achieving high measurement precision.
3Measurement precision
If transfer transistors are switched to distribute charge to charge storage units, then the integration accuracy is improved, but switching noise is introduced
Solution Approach 1:
The transfer transistors are switched periodically at specific timing intervals that synchronize with the light pulse emission. By opening the transfer transistors only during specific time windows when light pulses are emitted and keeping them closed during other periods, the system integrates charge with high temporal precision while minimizing the duration of switching operations, thereby reducing switching noise interference.
Solution Approach 2:
The system performs preliminary switching operations during a noise measurement period to establish baseline switching noise characteristics. This preliminary data allows the system to distinguish between switching noise and actual measurement signals, enabling accurate noise subtraction and improving overall measurement precision despite the presence of switching noise.
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
The method enhances the accuracy of distance measurements by isolating and subtracting noise charges, thereby improving the precision of range imaging devices.
Implementation Method 1
a photoelectric conversion element and multiple charge storage units, wherein the photoelectric conversion element generates charge according to light incident from a measurement space
Implementation Method 2
a light source that emits light pulses to a measurement space
Implementation Method 3
time of flight (hereinafter referred to as TOF) type range imaging devices which measure the distance to an object based on the time of flight of light, using the speed of light
Data Source
AI summary
A range imaging device includes a light source unit that emits light pulses to a measurement space; a light-receiving unit including a photoelectric conversion element that generates charge according to light incident from the space, a pixel circuit including charge storage units in which the charge is integrated in a frame cycle, and a pixel drive circuit that performs switching operation of transfer transistors to distribute the charge to the storage units for integration at integration timing synchronizing with emission of the pulses; and a distance calculation unit that calculates a distance between object in the space and the light-receiving unit based on charge determined by a first charge integrated in each storage unit. The calculation unit calculates the distance by subtracting a second charge from each first charge, the second charge being noise charge as an integrated charge other than the charge distributed and integrated by the switching operation.