Spread-Spectrum TOF Camera Phase Anti-Aliasing
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Solution Overview
Problem
Existing 3D optical time of flight (TOF) systems face ambiguity in distance determination due to objects at periodic distances, and struggle to handle highly reflective objects without reducing phase sensitivity and increasing complexity.
Innovation Solution
The system transmits a signal with a frequency domain characteristic approximating a sinc function, causing the correlation function to resemble a box/square wave, which reduces the return signal's correlation magnitude to zero beyond the desired range, thus eliminating ambiguity without trading off distance precision or increasing complexity.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If conventional TOF systems use single-frequency light signals, then the system is simple to implement, but distance determination becomes ambiguous for objects at periodic distances
Solution Approach 1:
The patent applies periodic action by modulating the light source at multiple discrete frequencies over an integration time period. The transmitter generates electrical signals at different frequencies (e.g., fundamental frequency and harmonic frequencies) and the light source modulates accordingly. This periodic variation in frequency allows the system to distinguish between objects at different distances by analyzing the phase-shifted return signals at each frequency, thereby eliminating distance ambiguity while maintaining reasonable system complexity.
Solution Approach 2:
The patent segments the measurement process by dividing the integration time period into multiple exposure time periods, each corresponding to a different frequency. The receiver integrates signals separately for each frequency component and then combines the results. This segmentation allows independent processing of multiple frequency signals, enabling unambiguous distance determination through correlation analysis without requiring excessive system complexity.
2Measurement precision
If the system transmits at multiple frequencies to eliminate distance ambiguity, then distance determination becomes unambiguous, but the system complexity increases
Solution Approach 1:
The patent applies universality by using a single light source that can be modulated at multiple frequencies, and a single receiver that can detect and integrate signals at different frequencies. The same hardware components perform multiple functions: the light source serves both as the optical transmitter and as the carrier for multiple frequency modulations, while the receiver handles both the optical detection and the multi-frequency signal processing. This multi-functionality eliminates the need for separate systems for each frequency, thereby reducing overall system complexity while maintaining unambiguous distance measurement capability.
Solution Approach 2:
The patent transitions from a single-frequency measurement approach to a multi-frequency approach by adding the frequency dimension to the measurement process. Instead of measuring only at one frequency, the system measures at multiple discrete frequencies within the integration time period. This dimensional expansion in the frequency domain allows the correlation function to provide unambiguous distance information through the pattern of phase shifts across different frequencies, effectively resolving the ambiguity problem without proportionally increasing system complexity.
3Length of stationary object
If the integration time period is extended to improve distance measurement range, then measurement range increases, but noise susceptibility increases
Solution Approach 1:
The patent applies periodic action by transmitting signals at multiple discrete frequencies throughout the extended integration time period. This periodic frequency variation creates a correlation function with multiple zero-crossings that provide unambiguous distance information. The periodic modulation allows the system to maintain high signal-to-noise ratio over extended measurement ranges by distributing the measurement across multiple frequency cycles, thereby reducing the impact of noise while extending the measurement range.
Solution Approach 2:
The patent maintains continuity of useful action by continuously integrating signals from multiple frequency components throughout the entire integration time period. Rather than using intermittent or pulsed measurements, the system continuously collects data at different frequencies, ensuring that the correlation function is built from continuous, complementary information. This continuous multi-frequency integration maximizes the useful signal accumulation while minimizing noise susceptibility, as the noise does not correlate across the different frequency components.
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 unambiguous distance determination even in the presence of highly reflective objects, maintaining precision and reducing noise susceptibility, while avoiding the need for extra processing or high power penalties.
Implementation Method 1
a light source, and an illumination driver coupled to the signal generator and the light source... The light source is configured to generate a transmit optical waveform corresponding with the electrical transmit signal
Implementation Method 2
a receiver... The receiver is configured to receive a reflected optical waveform that is the transmit optical waveform reflected off of an object
Implementation Method 3
determine a time of flight (TOF) for the light pulse to travel from the light source (e.g., a laser or light emitting diode (LED)) to a target object and return by analyzing the phase shift between the reflected light signal and the transmitted light signal
Data Source
AI summary
A three dimensional time of flight (TOF) camera includes a transmitter and a receiver. The transmitter is configured to generate an electrical transmit signal at a plurality of frequencies over an integration time period and generate a transmit optical waveform corresponding with the electrical transmit signal. The receiver is configured to receive a reflected optical waveform that is the transmit optical waveform reflected off of an object, integrate the reflected optical waveform over the integration time period, and determine a distance to the target object based on a TOF of the optical waveform. The integration time period includes exposure time periods. A length of each of the exposure time periods corresponds to one of the frequencies. The TOF is determined based on a correlation of the electrical transmit signal and the return optical waveform utilizing a correlation function with respect to the integration time period.