Dynamic Threshold Adjustment for SPAD Distance Sensors
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Solution Overview
Problem
SPAD-based light receivers, such as SiPMs, face challenges in accurately measuring light transit time due to dynamic compression, after-pulsing, and extraneous light interference, which limits their dynamic range and measurement accuracy, especially in distinguishing between useful and noise signals.
Innovation Solution
A distance-measuring optoelectronic sensor with a light transmitter and receiver that dynamically adjusts the threshold based on level information from the received signal, allowing for fast and adaptive separation of useful light from noise, using analog threshold adjustments and multi-channel evaluation structures to enhance measurement accuracy and retain multi-echo capability.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Speed
If Geiger mode avalanche photodiodes (SPADs) are used to achieve high sensitivity and fast response, then measurement speed and sensitivity are improved, but dynamic range is reduced due to inherent dead time and signal saturation
Solution Approach 1:
The light receiver is divided into multiple independently operable SPAD cells or groups of cells. Each cell can be independently controlled, allowing selective activation based on signal strength requirements. This segmentation enables the system to handle both weak and strong signals effectively, extending the dynamic range while maintaining fast response characteristics.
Solution Approach 2:
The system dynamically adjusts operating parameters such as bias voltage, integration time, and cell activation based on ambient light conditions and signal strength. This dynamic adaptation allows the sensor to optimize its performance for different measurement scenarios, effectively expanding the usable dynamic range while preserving high-speed measurement capability.
2Reliability
If multiple SPADs are connected together to form SiPM for increased sensitivity, then signal detection capability is improved, but measurement accuracy deteriorates due to dynamic compression and loss of signal strength information
Solution Approach 1:
Instead of activating all SPAD cells simultaneously, the system selectively activates only the necessary number of cells based on the expected signal strength. This partial activation approach prevents signal saturation while maintaining sufficient detection sensitivity, thereby preserving information about signal strength and improving measurement accuracy.
Solution Approach 2:
The system incorporates feedback mechanisms that monitor the output signal level and dynamically adjust the number of active cells or integration parameters. This feedback control ensures optimal utilization of the SPAD array, maintaining both high detection capability and accurate signal strength measurement across varying light conditions.
3Device complexity
If a fixed threshold is used for signal evaluation to simplify processing, then device complexity is reduced, but measurement precision worsens due to inability to adapt to varying ambient light and noise levels
Solution Approach 1:
The evaluation threshold is made dynamic rather than fixed. The system automatically adjusts the threshold based on measured ambient light levels, noise characteristics, and signal strength. This dynamic thresholding maintains high measurement precision across varying conditions while using computationally efficient algorithms that do not significantly increase device complexity.
Solution Approach 2:
The system performs self-calibration by automatically adapting its evaluation parameters based on the current operating conditions. Through built-in reference measurements and noise characterization, the sensor autonomously optimizes its signal detection thresholds without requiring external calibration or complex processing, thereby maintaining precision while minimizing added complexity.
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 enables improved measurement accuracy and dynamic range by effectively distinguishing between useful and noise signals, reducing the impact of after-pulsing and extraneous light, and maintaining multi-echo capability, thus enhancing the precision of light transit time measurements.
Implementation Method 1
a light beam is emitted into the monitored area and the light beam reflected by objects is received again
Implementation Method 2
In an avalanche photodiode (APD), the incoming light triggers a controlled avalanche breakdown (avalanche effect). This multiplies the charge carriers generated by incoming photons, creating a photocurrent that is proportional to the light reception intensity
Implementation Method 3
For optical distance determination, the travel time of a light signal is often measured, which corresponds to the distance via the speed of light
Data Source
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AI summary
A distance-measuring optoelectronic sensor (10) for detecting and determining the distance to an object (16) in a monitoring area (14) is described. The sensor comprises a light transmitter (12) for emitting a light signal (34), a light receiver (18) with at least one light receiving element (20) for generating a received signal from the light signal (38) emitted by the object (16), and a control and evaluation unit (22). The control and evaluation unit (22) is configured to determine the light travel time and thus the distance to the object (16) from the received signal and to evaluate the received signal with a threshold to determine the time of reception. Furthermore, the control and evaluation unit (22) is configured to determine the level of the received signal and to dynamically adjust the threshold using this level information.