SPAD Sensor Pixel Selection for Ambient Light Noise
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Solution Overview
Problem
Conventional SPAD-based distance sensors face challenges with high ambient light interference and noise, leading to reduced sensitivity and increased measurement times due to the need for multiple time-of-flight measuring units and excessive data processing, especially in industrial environments.
Innovation Solution
An optoelectronic sensor with a light receiver comprising a plurality of pixel elements configured as avalanche photo diodes, where a switching circuit connects only selected pixel elements to time-of-flight measuring units based on intensity measurements, excluding ambient light and dark count events, and utilizing a pixel selection unit to determine which elements contribute to the measurement, thereby reducing the number of time-of-flight measuring units and improving signal-to-noise ratio.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If multiple SPADs are combined by OR-gates to form macro pixels to deal with ambient light, then the sensor can manage ambient light interference, but the measurement period becomes long and real-time measurement capability is lost
Solution Approach 1:
The sensor array is divided into multiple independently controllable zones, allowing selective activation of only those zones needed for the current measurement, thereby reducing the total measurement time while maintaining ambient light rejection capabilities
Solution Approach 2:
The sensor enables dynamic switching between different operational modes (single pixel mode, macro pixel mode, zone mode) depending on ambient light conditions and measurement requirements, allowing real-time adaptation to optimize both reliability and speed
2Measurement precision
If each individual SPAD is connected to a TDC for distance measurement, then measurement precision is improved, but the wafer area consumption becomes prohibitive and fill factor is impaired
Solution Approach 1:
A single TDC is designed to serve multiple SPADs through time-multiplexed operation, allowing the same hardware resource to be shared across different pixels by sequentially measuring their signals, thereby reducing the total number of TDCs needed while maintaining precision measurement capability
Solution Approach 2:
The system uses periodic time-multiplexed switching to sequentially connect different SPADs to the shared TDC, enabling each pixel to be measured in its own time window while using the same hardware resource, thus reducing area consumption while preserving measurement precision
3Measurement precision
If SPADs are operated in Geiger mode for high sensitivity, then detection sensitivity is improved, but dark noise and ambient light interference trigger false avalanche events
Solution Approach 1:
The system extracts and identifies pixels affected by dark noise or ambient light through statistical analysis of avalanche events, then excludes these specific pixels from distance measurements, thereby removing the harmful effect while maintaining high sensitivity operation in Geiger mode
Solution Approach 2:
The system continuously monitors avalanche event rates and uses this feedback to dynamically identify and exclude noisy pixels from measurements, allowing the sensor to adapt to changing ambient conditions while maintaining high detection sensitivity through Geiger mode operation
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 results in more robust, precise, and faster distance measurements even in adverse lighting conditions, with reduced bandwidth and power consumption, as only relevant pixel elements contribute to the measurement, enhancing the signal-to-noise ratio and allowing for real-time data processing.
Implementation Method 1
an avalanche photo diode element biased with a bias voltage greater than a breakthrough voltage in order to trigger an avalanche event upon light reception
Implementation Method 2
the incident light triggers a controlled avalanche breakthrough (avalanche event). This multiplies the charge carriers generated by incident photons
Implementation Method 3
A short light pulse is transmitted and the time until detection of the remitted or reflected light pulse is measured
Implementation Method 4
the amplitude of transmitted light is modulated and a phase shift between the transmitted light and the received light is determined
Data Source
AI summary
An optoelectronic sensor (10) for measuring a distance of an object (18) in accordance with a time of flight principle comprises a light transmitter (12) for transmitting a light signal (14), a light receiver (22) for receiving the light signal (20) after reflection or remission by the object (18), the light receiver (22) having a first plurality of pixel elements (24, 24a) each configured as an avalanche photo diode element biased with a bias voltage greater than a breakdown voltage and thus operated in a Geiger mode in order to trigger an avalanche event upon light reception, a distance measuring unit (34) having a second plurality of time of flight measuring units (34a) connected to pixel elements (24a) for determining a time of flight between transmission and reception of a light signal, the second plurality being less than the first plurality, switching means (32, 32a) for connecting selected pixel elements (24a) to time of flight measuring units (34a) in a one-to-one fashion, and a pixel selection unit (28, 30) for determining pixel elements (24a) to be connected by the switching means (32, 32a) based on an intensity measurement.