SPAD Macro-Cell Sensitivity Control for ToF Signal Distortion
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Solution Overview
Problem
In time-of-flight (ToF) cameras using time-correlated single-photon counting (TCSPC), signal distortion occurs due to dead time in SPAD and TDC components, especially at short ranges where the signal event rate approaches 10-20% of the laser pulse repetition frequency, leading to inaccurate distance measurements.
Innovation Solution
A method and device for controlling the sensitivity of SPAD macro-cells by dynamically enabling SPAD units in a time-periodic manner, using a control circuit to alternate the enablement of SPAD units across sub-cells, thereby reducing excess bias and maintaining spatial coverage while minimizing signal distortion.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Area of stationary object
If all SPAD units are enabled continuously to maintain spatial coverage, then the spatial coverage over the target scene is maintained, but signal distortion occurs due to dead time when signal event rate reaches 10-20% of laser pulse repetition frequency
Solution Approach 1:
The patent applies periodic action by enabling SPAD units in a time-periodic manner rather than continuously. The control circuit alternates between enabling different subsets of SPAD units in different time periods, creating a periodic pattern that reduces the average signal event rate per SPAD unit while maintaining overall spatial coverage over time. This periodic enabling/disabling pattern directly addresses the dead time issue by ensuring no single SPAD unit is overwhelmed by excessive signal events.
Solution Approach 2:
The patent segments the SPAD array into multiple sub-cells, with each sub-cell containing a subset of SPAD units. The control circuit independently controls the enabling of SPAD units within each sub-cell, allowing different segments to be activated at different times. This segmentation enables the system to distribute the detection load across multiple segments over time, maintaining spatial coverage while preventing any single segment from experiencing signal distortion.
2Measurement precision
If a set of SPADs is selectively enabled to avoid signal distortion, then signal distortion is reduced, but the probability that the target can be covered by the SPAD array decreases
Solution Approach 1:
The patent implements dynamics by making the SPAD unit enabling configuration change over time rather than being static. The control circuit dynamically adjusts which SPAD units are enabled in different time periods, creating a time-varying detection pattern. This dynamic approach allows the system to adapt to different spatial coverage requirements while maintaining signal accuracy, as different subsets of SPAD units are activated at different times to cover different regions of the target scene.
Solution Approach 2:
The patent ensures continuity of useful action by designing the time-periodic enabling pattern such that over multiple time periods, all or most SPAD units are enabled at least once. This continuous coverage approach ensures that the target scene is comprehensively monitored over time, even though only a subset of SPAD units is active at any given moment. The useful detection action continues uninterrupted across different time periods with different active SPAD subsets.
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 dynamic enabling of SPAD units in SPAD macro-cells effectively reduces signal distortion and maintains accurate distance measurements across a range of distances, including short ranges, by optimizing the number of enabled SPADs based on real-time signal rates and ambient conditions.
Implementation Method 1
A time-of-flight (ToF) camera employing time-of-flight techniques to determine depth information. Photons are emitted from a laser onto a target scene, and some of them that hit a target in the scene return to the dToF camera. The SPADs comprised in the dToF camera can be arranged in subgroups
Implementation Method 2
single-photo avalanche diode (SPAD) detectors
Data Source
AI summary
A SPAD macro-cell comprises an array of SPAD unit, each of which comprises a SPAD and a quenching circuit for the SPAD, a combination tree to combine output signals from the SPAD units and a time-to-digital converter (TDC) operably connected to an output of the combination tree. The SPAD macro-cell is divided to a plurality of sub-cells. The SPAD macro-cell further comprises a control circuit configured to enable at least one or some SPAD units in each sub-cell in a time period and enable another one or some other SPAD units in each sub-cell in the next time period.