Time-of-flight distance measurement using dual modulation frequencies
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Solution Overview
Problem
Conventional optical distance measurement devices using the indirect time-of-flight (iTOF) technique face a trade-off between signal-to-noise ratio (SNR) and detectable distance range, as increasing the light modulation frequency improves SNR but reduces the detectable distance range, and vice versa.
Innovation Solution
The proposed solution involves a time-of-flight distance measurement device that uses two-stage measurement with two light modulation frequencies, where the second modulation frequency is N-times higher than the first. This allows for increased detectable distance range and noise immunity by sequentially using the two frequencies for measurement.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If the light modulation frequency is increased to improve signal-to-noise ratio, then the SNR is improved, but the detectable distance range is shortened
Solution Approach 1:
The measurement process is segmented into two distinct stages: a first measurement stage using a first modulation frequency to obtain a first phase, and a second measurement stage using a second modulation frequency (N-times higher than the first) to obtain a second phase. This segmentation allows each stage to serve its specific purpose - the first stage provides coarse distance information with longer exposure, while the second stage provides fine distance information with higher SNR, thereby resolving the contradiction between detectable distance range and signal-to-noise ratio.
Solution Approach 2:
The system dynamically switches between two different modulation frequencies based on the measurement requirements. The light source is controlled to modulate at the first frequency for the first measurement, then switches to the second frequency (N-times higher) for the second measurement. This dynamic adjustment allows the system to optimize both detectable distance range and SNR by using appropriate frequencies for appropriate measurement stages.
2Length of stationary object
If the exposure interval is extended to increase detectable distance range, then the detectable distance range is improved, but the light modulation frequency decreases and SNR deteriorates
Solution Approach 1:
The measurement process is divided into two segments with different exposure intervals and modulation frequencies. The first measurement uses a longer exposure interval with lower modulation frequency to maximize detectable distance range, while the second measurement uses a shorter exposure interval with higher modulation frequency (N-times higher) to maximize SNR. This segmentation resolves the contradiction by allowing each segment to optimize for its specific goal.
Solution Approach 2:
The system employs periodic modulation of the light source at two different frequencies in sequence. The light source is modulated at the first frequency during the first measurement period, then switched to the second frequency (N-times higher) during the second measurement period. This periodic action with varying frequencies enables the system to achieve both extended detectable distance range and high SNR by utilizing the appropriate frequency-exposure interval combination for each measurement stage.
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 effectively enhances the detectable distance range while maintaining high noise immunity, overcoming the limitations of conventional iTOF techniques by achieving both a larger range and higher SNR simultaneously.
Implementation Method 1
The light source is configured to illuminate an object using a first modulation frequency and a second modulation frequency to modulate a light driving signal
Implementation Method 2
The light detector is configured to detect reflected light from the object to generate a first detection signal corresponding to the light source being illuminated at the first modulation frequency
Data Source
AI summary
There is provided a distance measurement device including a light source, a light detector, a time control circuit and a processor. In first measurement, the time control circuit controls the light source to illuminate at a low modulation frequency, and the processor calculates a rough flying time according to a first detection signal of the light detector to determine an operating phase zone and a delay time. In second measurement, the time control circuit controls the light source to illuminate at a high modulation frequency and causes a light driving signal of the light source and a detecting control signal of the light detector to have a difference of the delay time, and the processor calculates a fine flying time according to a second detection signal of the light detector.