The present application relates to distance measuring and depth sense techniques, particularly, a time-of-flight distance measuring method and a related time-of-flight distance measuring system using structured light to detect the distance with respect to a target.
Time-of-flight distance measuring is done by measuring the phase difference between the incident and reflected light through a sensor, which has high complexity, high power consumption, and long computation time. In conventional art, the frequency of the light signal is increased to increase the accuracy. However, increasing the frequency of the light signal causes the wavelength to decrease, so that the phase ambiguity problem becomes serious due to the shorter wavelength for the same measurement distance. Therefore, the existing technology senses multiple light signals with different frequencies to ensure phase accuracy, and accordingly, the power consumption and operation time increase again. Therefore, how to solve the above-mentioned issues without compromising accuracy has become an important work task in this field.
One purpose of the present application is to disclose a time-of-flight (TOF) distance measuring method and a related system to address the technical problem of time-consuming and power-consuming of conventional TOF distance measuring methods.
One embodiment of the present application discloses a TOF distance measuring method, configured to measure a distance to a target. The TOF distance measuring method includes the following steps: sensing, by an optical sensor; a reflected light signal reflected by the target and obtaining a phase shift between the reflected light signal and an incident light signal, wherein the phase shift is smaller than 2π; obtaining a plurality pieces of first depth information according to the phase shift; obtaining, by a processing unit, a disparity of the reflected light signal corresponding to the incident light signal; obtaining a second depth information according to the disparity; and obtaining the distance from the plurality pieces of first depth information according to the second depth information.
One embodiment of the present application discloses a TOF distance measuring system, configured to measure a distance to a target. The TOF distance measuring device includes an optical sensor and a processing unit. The optical sensor is configured to sense a reflected light signal reflected by the target. The processing unit is configured to obtain a phase shift between the reflected light signal and an incident light signal and a disparity of the reflected light signal corresponding to the incident light signal according to the reflected light signal sensed by the optical sensor, obtain a plurality pieces of first depth information according to phase shift, and obtain a second depth information according to the disparity, wherein the phase shift is smaller than 2π. The processing unit is further configured to obtain the distance from a plurality pieces of first depth information according to the second depth information.
Specifically, the TOF distance measuring method disclosed in the present application solves the issue of phase ambiguity of TOF distance measuring in a novel way, thereby reducing the power consumption and processing time.
The following disclosure provides many different embodiments or examples for implementing different features of the present disclosure. Specific examples of components and arrangements are described below to simplify the present disclosure. These are, of course, merely examples and are not intended to be limiting. For example, the formation of a first feature over or on a second feature in the description that follows may include embodiments in which the first and second features are formed in direct contact and may also include embodiments in which additional features may be formed between the first and second features, such that the first and second features may not be in direct contact. In addition, the present disclosure may repeat reference numerals and/or letters in the various embodiments. This repetition is for the purpose of simplicity and clarity and does not in itself dictate a relationship between the various embodiments and/or configurations discussed.
There are three main implementations of existing three-dimensional (3D) image sensors: stereo binocular, structured light, and time-of-flight (TOF). Usually, in TOF implementation, the sensor is used to measure the time-of-flight of the light signal and then combined with the speed of light to calculate the TOF distance, i.e., the distance between the sensor and the object to be measured, where the time-of-flight of the light signal is inferred from the phase delay between the transmitted laser and the returned laser.
However, the length of the distance to the target (i.e., the object to be measured) may cause the above phase delay to exceed one cycle (i.e., 2π), i.e., the phase blurring caused by phase mixing.
Therefore, it is necessary to use the disparity of the light signal to help determine how many cycles of the above phase delay overlap to obtain the distance to the target. The details are described below.
The TOF distance measuring system 100 includes a light-emitting device 120, an optical sensor 140, and a processing unit 160. The light-emitting device 120 is configured to transmit the incident light signal LT to the target 101. The optical sensor 140 is configured to receive the reflected light signal LR reflected from the target 101 and generate an electronic signal based on the reflected light signal LR that it receives. The processing unit 160 is coupled to the light-emitting device 120 and the optical sensor 140 and configured to control the operation of the light-emitting device 120 and the optical sensor 140, for example, using a clock signal to control the on and off of the light-emitting device 120 and the optical sensor 140. The processing unit 160 is further configured to process the electronic signal generated by the optical sensor 140. In some embodiments, the processing unit 160 includes a microcontroller unit (MCU), central processing unit (CPU), or graphics processing unit (GPU).
Since the phase of the incident light signal Li transmitted by the light-emitting device 120 is known, after the optical sensor 140 receives the reflected light signal LR, it can determine the phase shift θ. For clarity, the incident light signal LT and the reflected light signal LR can be expressed as the following signal Equations 1 and 2.
LT:Q
T
=A cos(ft)+B (Equation 1)
LR:Q
R(t)=A′=cos(ft+θ+2nπ)+B′ (Equation 2),
where, A and A′ represent the amplitude, f represents the frequency, t represents the time, and B represents the noise generated in the TOF distance measuring system 100. Specifically, when the light-emitting device 120 generates the incident light signal LT, it collaterally generates a system noise B. When the system noise B is reflected by the target into B′, it is reflected back to the optical sensor 140 together with the reflected light signal LR.
Referring to
To ease of calculation, the time points t1 and t3 are chosen to be half a cycle apart. Converted into a phase change, the phase changes by π from time point t1 to t3, for example, from 0° to 180°. Correspondingly, the time points t2 and t4 are also chosen to be half a cycle apart, for example, from 90° to 270°. Under this condition, Equation 2 of the reflected light signal LR can be solved as follows.
where, k is the phase change between time points t2 and t1, and parameters A1-A4 can be expressed as the following Equations.
A1=(Q1−B)×cos θ (Equation 6)
A2=(Q2−B)×cos(θ+k) (Equation 7)
A3=(Q3−B)×cos(θ+π) (Equation 8)
A4=(Q4−B)×cos(θ+π+k) (Equation 9)
In some embodiments, k is set as 0.57π to further simplify the parameters of the Equation of the reflected light signal LR as follows.
The phase shift θ can be calculated by using light intensities Q1-Q4, and hence, a plurality of possible values (a plurality of different n) of the phase delay θ+2nπ can be obtained. The processing unit 160 uses the phase delay θ+2nπ to obtain a plurality pieces of possible depth information Z(n). The depth information Z(n) can be expressed as the following Equations.
wherein c is the speed of light.
However, the TOF distance measuring system 100 cannot determine the actual n value of the phase delay θ+2nπ based on the incident light signal LT and the reflected light signal LR alone. Therefore, the TOF distance measuring system 100 needs to use the disparity d between the incident light signal LT and the reflected light signal to obtain the actual distance Z from the plurality pieces of possible depth information Z(n). The details are discussed below.
Reference is made to
The light-emitting device 120 includes a light source 122 and an optical microstructure 124. The light source 122 generates a laser signal LS to irradiate the optical microstructure 124, and the optical microstructure 124 changes the path of the laser signal LS to generate the incident light signal LT. In the present embodiment, the incident light signal LT has a predetermined pattern to that the energy is concentrated on the predetermined pattern. In the embodiment shown in
For example (however, the present disclosure is not limited thereto), the optical microstructure 124 includes a diffractive optical element (DOE) or refractive optical element (ROE), configured to conically diffract (or conically refract) the laser signal LS to generate the incident light signal LT, so that the projection of the incident light signal LT on the target 101 forms a plurality of light spots that are separated from each other. In certain embodiments, a collimating lens is further arranged between the light source 122 and the optical microstructure 124, wherein the collimating lens is configured to direct the laser signal LS into parallel light.
The number of light spots S1-S5 shown in
After the optical sensor 140 receives the reflected light signal Lift having the pattern of the plurality of light spots S1-S5, the processing unit 160 uses epipolar geometry to calculate the disparity d of the pattern of each light spot S1-S5, and then uses the disparity d to calculate the depth information Z′, which can be expressed as the following Equations.
wherein b represents a baseline distance in the epipolar geometry, and F represents the focal length of the optical sensor 140. For example, referring to
Since the epipolar geometry performs the calculation based on geometrical optics, the computed disparity d is free of phase ambiguity. However, because the geometric optics itself has errors after all, the calculation of the depth information Z′ has a certain degree of error. In this case, the processing unit 160 may determine which of the depth information Z(n) is the actual distance Z according to the depth information Z′.
During the process of obtaining the true distance Z, the processing unit 160 first compares the depth information Z′ to a plurality of depth information Z(n) with different n values, then selects one of the plurality of depth information Z(n) that is closest to the depth information Z′, and finally decides that the selected depth information Z(n) is the actual distance Z. In some embodiments, the processing unit 160 subtracts the plurality of depth information Z (n) from the depth information Z′ respectively to take the absolute values, then selects the smallest one from the plurality of absolute values, and then decides that the depth information Z(n) corresponding to the selected absolute value is the actual distance Z.
In addition, the disparity d varies with the distance Z of the target. From Equation 13, it is known that the disparity d is inversely proportional to the distance Z′. For the same incident light signal LT and the target 101, the disparity d is larger when the distance Z is closer, and smaller when the distance Z is further. Taking
In some embodiments, the light-emitting device 120 controls the incident light signal UT to have a pattern of regularly arranged light spots. In some embodiments, the light-emitting device 120 controls the incident light signal LT to have a pattern of irregularly arranged light spots, wherein the spacing of all light spots from adjacent light spots conforms to the limit of the shortest measurement distance Linin of the TOF distance measuring system 100.
In summary, compared with the conventional art, the TOF distance measuring system 100 does not need to additionally emit a light signal with a different frequency f to assist in determining the n value, thus reducing the number of times the incident light signal LT is irradiated, and obtaining the actual distance Z with respect to the target with the aid of the disparity d to solve the problem of phase ambiguity. Under a condition that the accuracy of sensing is not reduced, the TOF distance measuring system 100 reduces the system's power consumption and does not reduce the frame rate because it reduces the number of times the incident light signal LT is irradiated.
In view of the foregoing, the TOF distance measuring system 100 of the present application can use the TOF distance measuring method 500 in 5 to sense the distance Z from the target 101. First, the light-emitting device 120 generates an incident light signal LT, which irradiates the target 101 to generate a reflected light signal LR (S510). The optical sensor 140 then senses the reflected light signal LR, so that the processing unit 160 obtains the phase shift θ between the reflected light signal LR and the incident light signal LT according to the sensed reflected light signal LR (S520) and obtains a plurality pieces of depth information Z(n) based on the phase shift θ (S530). The processing unit 160 then uses the epi polar geometry to obtain the disparity d between the reflected light signal LR and the incident light signal LT (S540) and obtain the depth information Z′ based on the disparity d (S550). Finally, the distance Z with respect to the target 101 is obtained from the plurality pieces of depth information Z(n) based on depth information Z′ (S560).
This application is a continuation of International Application No. PCT/CN2020/096791, filed on Jun. 18, 2020, the disclosure of which is hereby incorporated by reference in its entirety
Number | Date | Country | |
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Parent | PCT/CN2020/096791 | Jun 2020 | US |
Child | 17474901 | US |