Time-of-Flight Laser Circuitry for Echo Walk Error Correction
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Solution Overview
Problem
Time-of-flight laser distance devices suffer from walk error due to varying signal strengths and fixed gain settings, leading to inaccurate distance measurements, especially with weak return pulses.
Innovation Solution
The implementation of a laser distance device with multiple comparators and analog-to-digital converters to measure time-of-flight values at different trip levels, along with timed-gain control and automatic gain control circuitry, allows for accurate determination of the leading edge of return pulses and correction of walk error.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If a fixed gain setting is used in the photodiode circuit, then the circuit is simple and stable, but walk error increases for weak return pulses resulting in inaccurate distance measurements
Solution Approach 1:
The patent implements a dynamic gain control system that automatically adjusts the gain of the photodiode circuit based on the detected signal strength. The system transitions from a fixed gain setting to a variable gain setting, where the gain is modified in real-time to optimize the detection of return pulses with varying amplitudes, thereby reducing walk error while maintaining manageable circuit complexity through automated control.
Solution Approach 2:
The patent changes the gain parameter of the photodiode circuit dynamically based on the detected signal conditions. By adjusting the gain parameter according to the return pulse strength, the system optimizes the comparator trip point detection for both strong and weak signals, resolving the contradiction between measurement precision and device complexity through adaptive parameter modification.
2Reliability
If the electronic gain is increased to detect weak return pulses, then detection sensitivity improves, but saturation of the photodiode circuits occurs and signal-to-noise ratio deteriorates for selected targets
Solution Approach 1:
The patent employs dynamic gain adjustment where the electronic gain is modified based on the detected signal strength. For weak return pulses, the gain is increased to improve detection sensitivity, while for strong signals, the gain is reduced to prevent saturation and maintain optimal signal-to-noise ratio. This dynamic adaptation resolves the contradiction between detection reliability and harmful effects of saturation and noise.
Solution Approach 2:
The system changes the gain parameter dynamically based on signal conditions. By monitoring the return pulse amplitude and adjusting the gain accordingly, the patent prevents photodiode saturation for strong signals while enhancing detection capability for weak signals, thereby eliminating the harmful effects of both saturation and insufficient detection sensitivity.
3Measurement precision
If a single comparator with fixed reference level is used, then the device complexity is low, but walk error occurs for pulses with varying amplitude resulting in time measurement inaccuracies
Solution Approach 1:
The patent divides the detection process into multiple stages by implementing multiple comparators with different reference levels. Each comparator is optimized to detect specific amplitude ranges of return pulses, allowing accurate time-of-flight measurement across varying signal strengths. This segmentation approach reduces walk error by ensuring that at least one comparator operates in its optimal detection range for any given pulse amplitude.
Solution Approach 2:
The patent adds an amplitude dimension to the detection system by implementing multiple comparators with different reference levels. Instead of relying on a single fixed reference level, the system uses multiple reference levels corresponding to different pulse amplitudes, thereby extending the detection capability across a wider dynamic range and eliminating walk error caused by amplitude variations.
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 significantly reduces walk error by accurately measuring time-of-flight for both strong and weak return pulses, improving the accuracy and resolution of distance measurements.
Implementation Method 1
a collimated infrared pulsed laser at a nominal pulse repetition rate of several hundred Hertz or lower
Implementation Method 2
a processing circuit to convert the lapsed time between the transmitted pulse and the received pulse
Implementation Method 3
a receiving photo detector
Data Source
AI summary
A time-of-flight laser distance device includes a laser transmitter, a laser pulse return detector to receive reflected laser pulses, and comparators with respective first and second trip levels. A preliminary distance to a target is based on a linear ramp as a function of lapsed time from laser pulse generation through an effective time period wherein an associated return pulse is detected. Time-of-flight values associated with a leading edge of the return pulse are obtained at each of the first and second trip levels. A time difference is determined between the obtained values, and a correction factor is applied with respect to the preliminary measured distance based on the determined time difference. A dual ADC may be used wherein the leading edge of the return pulse is calculated by measuring a slope associated with the time-of-flight values and extrapolating a slope origin to a zero crossing point.