Digital Time of Flight Circuit Phase Comparison
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Solution Overview
Problem
Existing time of flight measurement circuits face challenges in achieving high resolution and stability due to temperature drift and ambient light interference, particularly when aiming for millimeter precision and handling photons that arrive outside expected envelopes or from parasitic elements.
Innovation Solution
A fully digital time of flight measurement circuit is developed, utilizing direct digital equivalents for phase comparison and integration, along with delta-sigma modulation and adaptive threshold techniques to enhance resolution and reduce drift, and incorporating dithering to manage pulse width and ambient light effects.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If an analog circuit with variable delay line and charge pump is used for time of flight measurement, then the measurement can be performed, but the circuit drifts with temperature changes reducing stability
Solution Approach 1:
The patent replaces the analog circuit (variable delay line, charge pump, integrator) with a fully digital implementation. The phase comparator and integrator are implemented as digital circuits that count pulses and calculate averages, eliminating temperature-dependent analog components while maintaining measurement functionality. This substitution resolves the drift issue by using digital logic that is inherently more stable against temperature variations.
2Quantity of substance
If photons are detected outside the expected envelope or from parasitic elements, then more photons are captured, but measurement accuracy deteriorates due to ambient light interference
Solution Approach 1:
The patent employs periodic gating synchronized to the laser excitation signal. The digital circuit opens the measurement window only during expected photon arrival times (synchronized with laser flashes) and closes it during ambient light periods. This periodic action allows capture of signal photons while rejecting out-of-window photons from ambient light or parasitic reflections, maintaining accuracy while collecting sufficient signal photons.
Solution Approach 2:
The patent uses the measured phase information to dynamically adjust the timing window for photon detection. The system continuously refines the measurement window based on previous measurements and the known time of flight, creating a feedback mechanism that optimizes the detection window to track the expected photon arrival times even as conditions change, thereby maintaining precision while capturing adequate photons.
3Use of energy by moving object
If the measurement window is widened to capture more photons, then signal strength increases, but the impact of ambient light and parasitic reflections increases
Solution Approach 1:
The patent applies local quality by making the detection window time-dependent and position-specific. Instead of a uniformly open window, the circuit creates a localized measurement window that is open only during the specific time interval when photons from the target are expected to arrive. This localized approach concentrates detection sensitivity in the relevant time region while excluding ambient light periods, achieving strong signal capture without proportional increase in ambient light interference.
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 solution achieves high-resolution time of flight measurements with improved stability and accuracy, effectively compensating for temperature drift and ambient light interference, enabling precise distance determination even under challenging conditions.
Implementation Method 1
single photon avalanche diode (SPAD)... When a SPAD is reached by a photon, it is set in an avalanche mode and produces an electric pulse
Implementation Method 2
The device emits periodic infrared laser flashes toward a target. The photons reflected from the target return to a single photon avalanche diode (SPAD) array
Implementation Method 3
phase comparator 10 that receives the pulses generated by a SPAD array 12, and a half-wave signal H produced by a variable delay line 14... The delay of signal H relative to signal Href is the sought time of flight ToF
Data Source
AI summary
A method of measuring the phase of a response signal relative to a periodic excitation signal, comprises the steps of producing for each cycle of the response signal two transitions synchronized to a clock and framing a reference point of the cycle; swapping the two transitions to confront them in turns to the cycles of the response signal; measuring the offsets of the confronted transitions relative to the respective reference points of the cycles; performing a delta-sigma modulation of the swapping rate of the two transitions based on the successive offsets; and producing a phase measurement based on the duty cycle of the swapping rate.