Time-of-flight Distance Measurement Using Multi-Frequency Phase Modulation
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Solution Overview
Problem
Time-of-flight distance measurement devices face challenges with aliasing and interference between sensors, particularly when increasing modulation frequency, which limits differentiation between near and distant targets and introduces errors due to common-mode components in intense background light.
Innovation Solution
The device employs a light emission control unit that modulates light at multiple frequencies, with a first and second modulation frequency, and changes phase polarity randomly at each subsequence transmission, distributing electric charges evenly across storage capacitances to minimize interference and aliasing effects.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If modulation frequency is increased to decrease distance error and increase SNR, then measurement precision is improved, but aliasing occurs at shorter distances making it difficult to differentiate between near and distant targets
Solution Approach 1:
The patent applies dynamics by making the modulation frequency variable rather than fixed. The light emission control unit dynamically changes the modulation frequency based on the measured distance: using higher frequencies for nearby targets and lower frequencies for distant targets. This dynamic adaptation resolves the contradiction by allowing high precision measurement without aliasing, as the system selects appropriate frequencies based on real-time distance conditions.
Solution Approach 2:
The patent changes the modulation frequency parameter adaptively according to distance measurements. By adjusting this critical parameter based on target distance, the system achieves high measurement precision for near targets using high frequencies while avoiding aliasing, and maintains reliable differentiation for distant targets using lower frequencies.
2Reliability
If spread spectrum technology with PN modulation is used to set delay time for each pulse, then interference resistance is improved, but exposure time becomes longer to obtain the same SNR
Solution Approach 1:
The patent uses periodic modulated light emission with specific repetition periods instead of spread spectrum technology. By emitting light in regular periodic pulses with controlled repetition periods, the system achieves both interference resistance and maintains short exposure times, resolving the contradiction between reliability and time loss.
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 allows for accurate distance measurement by reducing aliasing influence and minimizing interference from other sensors, enabling precise distance calculation over a wider range without increasing modulation frequency, thus improving measurement accuracy and range.
Implementation Method 1
a light emitting element emits light modulated at least at two or more different modulation frequencies including a first modulation frequency and a second modulation frequency
Implementation Method 2
a light receiving element divides and stores electric charge corresponding to the received incident light into a plurality of storage capacitances
Implementation Method 3
calculates the distance from the subject device to the target by using sampled values
Implementation Method 4
the phase polarity is changed in a forward or reverse direction at random with a 1⁄2 probability at each transmission of the sub sequence
Data Source
AI summary
A digital signal processing circuit measures a distance according to a plurality of modulation frequencies including a first modulation frequency and a second modulation frequency lower than the first modulation frequency. The digital signal processing circuit is configured such that, when measuring the distance at the first modulation frequency, a storage capacitance of a light receiving element 6 stores or discharges electric charges according to the timing when the polarity of a phase is controlled by a light emission control unit at each transmission of a sub sequence and the distance is measured according to the electric charges stored in the storage capacitance. The digital signal processing circuit corrects the distance measurement result based on the measurement result at the first modulation frequency and the measurement result at the second modulation frequency.