Transmitting-end device for direct time-of-flight sensor and control method thereof

Abstract

The present invention provides a transmitting end device for a direct time-of-flight (DTOF) sensor and a control method thereof, including: a transmitting laser source configured to emit light; a transmitting end optical element configured to allow a first part of the light emitted by the transmitting laser source to transmit through the transmitting end optical element and to reflect a second part of the light emitted by the transmitting laser source by the transmitting end optical element; and at least one single-photon avalanche diode (SPAD) photosensitive unit, the at least one SPAD photosensitive unit receiving the second part of the light reflected by the transmitting end optical element and outputting an optical signal related to the returned light, wherein the start time of the light emitted by the transmitting laser source is determined according to the optical signal output by the at least one SPAD photosensitive unit.

Description

Technical Field

[0001] The present invention relates to the field of 3D depth sensing, and particularly to a method and system for protecting human eye safety and measuring start time of a direct time of flight (DToF) sensor. Background Art

[0002] With the development of lidar technology, the direct time of flight ranging method has received increasing attention. The principle of DTOF is to continuously emit light pulses to the object to be measured, and then use a sensor to receive the light reflected from the object to be measured. The distance to the object to be measured is obtained by detecting the flight time of the light pulse.

[0003] A direct time of flight (DToF) sensor is an active optical sensor, which at least includes two main parts: a transmitting end Tx and a receiving end Rx. Tx emits short pulse lasers, which irradiate the object to be measured, and part of the lasers are reflected by the object to be measured and received by Rx.

[0004] In a direct time of flight (DToF) sensor, the distance to the object to be measured is calculated by recording the time difference between the received signals of Tx and Rx. Commonly, by controlling the circuit signal, the start time is recorded when the light emitting signal is transmitted, and the stop time is recorded when Rx receives the measured light signal. The time difference is the time t for the laser to fly back and forth in the air. Given the speed of light C, the distance d of the object can be obtained as d = 1 / 2 * C * t.

[0005] However, due to the inductance effect of the VCSEL laser itself, after the electrical signal is transmitted, there is a certain jitter in the light emitting time of the driven VCSEL laser. The system only records the start time of the signal drive, resulting in inaccurate recording of the light emitting time, and further inaccurate distance detection results.

[0006] The vertical cavity surface emitting laser (VCSEL) of the transmitting end Tx emits discrete light spots in the far field after passing through a diffractive optical element. Since the light spot of the laser is diffused by the optical element, the laser beam will not cause damage to the human eye under normal operation. However, if the optical element is broken, it cannot play the role of light splitting, and the non-split laser beam will cause irreversible damage when it irradiates the human eye. The solution is to make a metal circuit on the diffractive optical element. The system real-time tests the resistance of the circuit. If the diffractive optical element is broken, the circuit is disconnected or the resistance changes significantly. After the system detects it, the laser output of the Tx end is turned off to achieve the effect of protecting human eye safety.

[0007] However, by adding structures or components to record the signal changes after the optical element breaks and controlling whether the Tx emits laser to protect human eye safety, the problem of improving the detection distance accuracy has not been solved.

[0008] In view of the above technical problems, a direct time-of-flight (DToF) sensor is needed to improve the detection distance accuracy and control whether the transmitting end Tx emits laser to protect human eye safety. Summary of the Invention

[0009] One aspect of the present invention provides a transmitting end device for a direct time-of-flight (DTOF) sensor, including: a laser emission source configured to emit light; a transmitting end optical element configured to allow a first part of the light emitted by the laser emission source to transmit through the transmitting end optical element and reflect a second part of the light emitted by the laser emission source by the transmitting end optical element; and at least one single photon avalanche diode (SPAD) photosensing unit that receives the second part of the light reflected by the transmitting end optical element and outputs an optical signal related to the returned light, wherein the start time of the light emission by the laser emission source is determined according to the optical signal output by the at least one SPAD photosensing unit.

[0010] One aspect of the present invention provides a transmitting end device for a direct time-of-flight (DTOF) sensor, wherein the start time of the light emission by the laser emission source is determined by determining the peak value of the optical signal.

[0011] One aspect of the present invention provides a transmitting end device for a direct time-of-flight (DTOF) sensor, further including a laser emission source driving circuit configured to turn off the transmitting end device when it is determined that the peak value change of the calibrated optical signal exceeds a first threshold.

[0012] One aspect of the present invention provides a transmitting end device for a direct time-of-flight (DTOF) sensor, wherein the first threshold is ±20%.

[0013] One aspect of the present invention provides a transmitting end device for a direct time-of-flight (DTOF) sensor, wherein the transmitting end optical element includes a collimating lens.

[0014] One aspect of the present invention provides a transmitting end device for a direct time-of-flight (DTOF) sensor, wherein the transmitting end optical element includes a diffractive optical element for generating an interference effect spot.

[0015] One aspect of the present invention provides a transmitting end device for a direct time-of-flight (DTOF) sensor, wherein the diffractive optical element includes an optical microstructure and an optical substrate, and the optical microstructure and the optical substrate are transparent materials in the working wavelength band.

[0016] One aspect of the present invention provides a transmitting end device for a direct time-of-flight (DTOF) sensor, wherein the transmitting end optical element includes a light homogenizing sheet.

[0017] One aspect of the present invention provides a transmitting end device for a direct time-of-flight (DTOF) sensor, wherein the light homogenizing sheet includes an optical microlens array and an optical substrate, and wherein the optical microlens array and the optical substrate are transparent materials in the working wavelength band.

[0018] One aspect of the present invention provides a transmitting end device for a direct time-of-flight (DTOF) sensor, wherein the light homogenizing sheet includes an optical microlens array and an optical substrate, and wherein the optical microlens array and the optical substrate are transparent materials in the working wavelength band.

[0019] One aspect of the present invention provides a transmitting end device for a direct time-of-flight (DTOF) sensor, wherein the transmitting end optical element further includes a band-pass filter configured to allow only light near the working wavelength of the DTOF sensor to pass through.

[0020] One aspect of the present invention provides a direct time-of-flight (DTOF) sensor device including the transmitting end device as described above. The DTOF sensor device includes: a transmitting end device configured to emit a short pulse laser to irradiate an object to be measured and record the start time of emitting the short pulse laser; and a receiving end device configured to receive a part of the laser reflected by the object to be measured and record the end time of receiving the measured light signal, so as to obtain the distance of the object to be measured through the time difference between the start time and the end time.

[0021] One aspect of the present invention provides a method for controlling a transmitting end device for a direct time-of-flight (DTOF) sensor. The transmitting end device includes a laser emission source, a transmitting end optical element, at least one single-photon avalanche diode (SPAD) photosensitive unit, and a laser emission source driving circuit. The method includes: emitting light through the laser emission source; transmitting a first part of the light emitted by the laser emission source through the transmitting end optical element and reflecting a second part of the light emitted by the laser emission source through the transmitting end optical element; and receiving the second part of the light reflected by the transmitting end optical element through at least one SPAD photosensitive unit and outputting an optical signal related to the returned light, wherein the start time of the light emitted by the laser emission source is determined according to the optical signal output by the at least one SPAD photosensitive unit; and turning off the transmitting end device through the laser emission source driving circuit when it is determined that the peak change of the calibrated optical signal exceeds a first threshold.

[0022] Beneficial effects

[0023] The direct time-of-flight (DToF) sensor device proposed according to the present invention is a system solution that can protect the safety of the human eye, improve the accuracy of start time recording, and thus improve the detection distance result. It completes multiple functions through only one device, enriching the functional integration of the system, improving the detection accuracy of the device, and enhancing the safety performance of the system. Brief Description of the Drawings

[0024] Figure 1 is a schematic diagram of the transmitting end of a direct time-of-flight DToF sensor according to an embodiment of the present invention;

[0025] Figure 2a is a schematic diagram of the far-field speckle distribution of Tx under normal operating conditions according to an embodiment of the present invention;

[0026] Figure 2b is a schematic diagram of the far-field speckle distribution of Tx when the diffractive optical element fails according to an embodiment of the present invention;

[0027] Figure 3a is a schematic diagram of the SPAD histogram output under normal operating conditions according to an embodiment of the present invention;

[0028] Figure 3b is a schematic diagram of the SPAD histogram output when the diffractive optical element fails according to an embodiment of the present invention;

[0029] Figure 4 is a schematic diagram of the transmitting end of a direct time-of-flight DToF sensor according to an embodiment of the present invention;

[0030] Figure 5a is a schematic diagram of the far-field spot distribution of Tx under normal operating conditions according to an embodiment of the present invention;

[0031] Figure 5b is a schematic diagram of the far-field spot distribution of Tx when the light homogenizing film fails according to an embodiment of the present invention;

[0032] Figure 6a is a schematic diagram of the SPAD histogram output under normal operating conditions according to an embodiment of the present invention;

[0033] Figure 6b is a schematic diagram of the SPAD histogram output when the light homogenizing film fails according to an embodiment of the present invention;

[0034] Figure 7 is a schematic diagram of the transmitting end of a direct time-of-flight DToF sensor with a filter according to an embodiment of the present invention; and

[0035] Figure 8 is a schematic diagram of the spectral transmittance of the filter according to an embodiment of the present invention. Detailed Implementation Modes

[0036] Before proceeding with the following detailed description, it may be advantageous to set forth definitions of certain words and phrases used throughout this patent document. The terms "coupled," "connected," and their derivatives refer to any direct or indirect communication or connection between two or more elements, whether or not those elements are in physical contact with each other. The terms "transmit," "receive," and "communicate," and their derivatives, encompass both direct and indirect communication. The terms "comprise" and "include," and their derivatives, mean including but not limited to. The term "or" is inclusive, meaning and / or. The phrase "associated with," and its derivatives, refer to including, being included within, interconnecting, containing, being contained within, connecting or being connected with, coupling or being coupled with, communicating with, cooperating with, interlacing, juxtaposing, being adjacent to, binding or being bound with, having, having an attribute, having a relationship or being related to, etc. The term "driver" refers to any device, system, or part thereof that controls at least one operation. Such a driver may be implemented in hardware, or a combination of hardware and software and / or firmware. The functions associated with any particular driver may be centralized or distributed, whether local or remote. The phrase "at least one," when used in conjunction with a list of items, means that different combinations of one or more of the listed items may be used, and only one item from the list may be required. For example, "at least one of A, B, C" includes any one of the following combinations: A, B, C, A and B, A and C, B and C, A and B and C.

[0037] Definitions of other specific words and phrases are provided throughout this patent document. Those of ordinary skill in the art should understand that in many cases, if not most cases, such definitions apply to the prior and future use of the words and phrases so defined.

[0038] In this patent document, the application combination of modules and the division of sub-modules are for illustrative purposes only. Without departing from the scope of the present disclosure, the application combination of modules and the division of sub-modules may have different ways.

[0039] Figure 1 is a schematic diagram of the transmitting end of a direct time-of-flight (DToF) sensor according to an embodiment of the present invention.

[0040] The components of the transmitting end Tx 100 of a direct time-of-flight (DToF) sensor according to an embodiment of the present invention include: a transmitting laser source VCSEL 103, a VCSEL driving circuit 102, a collimating lens 105, a diffractive optical element 110, at least one single-photon avalanche diode (SPAD) photosensing unit 104, and a SPAD control circuit and a SPAD readout circuit (not shown). Refer to Figure 1, the VCSEL light source 103 is placed inside the cavity 101, and its lower part is connected to the relevant driving circuit 102. At least one SPAD photosensitive unit 104 is arranged near the VCSEL light source 103. The light emitted by the VCSEL light source 103 passes through the collimating lens 105 and reaches the diffractive optical element 110. The diffractive optical element 110 includes two parts, namely the optical microstructure 111 and the optical substrate 112, both of which are transparent materials in the working wavelength band. Among them, the optical microstructure 111 is a kind of grating-like structure, and usually the overall structure thickness is within 100 μm. The surface of the optical microstructure 111 is composed of a series of stepped structures with undulating heights. The specific structure design varies according to the actual achieved spot pattern, but the principle is to decompose an incident beam of light into multiple beams through interference, achieving the effect of increasing the number of final spots. The optical substrate 112 is generally a flat glass, serving as the base material of the optical structure and playing a role in support and protection.

[0041] After the light passes through the diffractive optical element 110, the spots are replicated due to the interference effect, and the spot distribution as shown in Figure 2a is formed in the far field. Since the spots are replicated, the light intensity of each spot is weakened. At the same time, since a part of the light is reflected on the surface of the diffractive optical device 110 and returns to the cavity 101, the SPAD photosensitive unit 104 receives the returned optical signal. The histogram result of a frame for ranging formed after passing through the readout circuit and signal processing is as shown in Figure 3a . It can be considered that the position with the highest histogram is the starting time of light emission. Among them, those skilled in the art should be clear that a frame for ranging can vary according to the range to be measured.

[0042] When the diffractive optical element 110 is damaged (such as the optical substrate 112 is broken, or the optical microstructure 111 is scratched or adsorbed with water vapor, etc.), its optical function fails and it cannot play the role of replicating spots. Then the light emitted by the light source VCSEL 103 forms the spot distribution as shown in Figure 2b in the far field after passing through the collimating lens 105. Since the spots are not replicated, the light intensity of each spot will become stronger, and the laser intensity in this case will cause damage to the human eye. At this time, the histogram of the signal received by the SPAD photosensitive unit 104 after processing is as shown in Figure 3b . Since the diffractive optical element 110 is damaged, the light intensity reflected back to the cavity 101 will become weaker, resulting in fewer photons received by the SPAD photosensitive unit 104 and fewer triggering times. The histogram result formed after passing through the readout circuit and signal processing is as shown in Figure 3b . It can be seen that the peak value of the histogram has a significant decrease. Those skilled in the art should be clear that the above-described embodiments described with reference to Figures 1 - 3b are only used to illustrate the concept of the present invention, and it is not intended to limit the present invention toFigures 1 - 3b In the illustrated embodiment, for example, when the diffractive optical element 110 is damaged, the peak value of the histogram of the signal received by the SPAD photosensing unit 104 after processing may also increase significantly (when the damaged diffractive optical element 110 reflects most of the collimated light back to the SPAD photosensing unit 104). Therefore, after calibration, when it is determined that the change in the peak value in a frame exceeds a certain range (such as ±20%) relative to the calibration value, it can be considered that the diffractive optical element has failed, and the driver needs to turn off the Tx module.

[0043] So far, the entire Tx module has completed two functions: measuring the signal emission time and protecting the human eye safety.

[0044] Figure 4 It is a schematic diagram of the transmitting end of a direct time-of-flight (DToF) sensor according to an embodiment of the present invention.

[0045] The direct time-of-flight (DToF) sensor according to an embodiment of the present invention is an active optical sensor. The components of its transmitting end Tx include: a transmitting laser source VCSEL 403, a VCSEL driving circuit 402, a light homogenizing plate 410, at least one SPAD photosensing unit 404, and a SPAD control circuit and a SPAD readout circuit (not shown). Referring to Figure 4 , the VCSEL light source 403 is arranged inside the protection cavity 401, and its lower part is connected to the relevant driving circuit 402. At least one SPAD photosensing unit 404 is placed around the VCSEL light source 403. The light emitted by the VCSEL light source 403 reaches the light homogenizing plate 410. The light homogenizing plate 410 includes two parts, namely an optical microlens array 411 and an optical substrate 412, both of which are transparent materials in the working wavelength band. The optical microlens array 411 is composed of a series of sub-lenses arranged in a certain order on the optical substrate, and its diameter ranges from several hundred nanometers to several millimeters. The optical substrate 412 is generally a flat glass, which serves as the substrate of the optical structure and has the effect of support and protection.

[0046] After the light passes through the light homogenizing plate 410, the light source is diffused and shaped due to the action of the microlenses, and a spot distribution as shown in Figure 5a is formed in the far field as required. Since the spot is diffused, the light intensity per unit angle is weakened. At the same time, since a part of the light is reflected on the surface of the light homogenizing plate 410 and returns to the cavity 401, the SPAD photosensing unit 401 receives the returned light signal. The histogram result formed after passing through the readout circuit and signal processing is as shown in Figure 6a , and it can be considered that the position with the highest histogram is the starting time of light emission.

[0047] When the light homogenizing sheet 410 is damaged (such as the light substrate 412 is broken, or the optical microlens array 411 falls off or is scratched, etc.), its optical function fails and it cannot play the role of diffusing the light spot. Then, the light emitted by the light source VCSEL 403 forms the light spot distribution as shown in Figure 5b after passing through the failed light homogenizing sheet 410, that is, the original emission light spot distribution of the VCSEL. Since the light spot is not diffused, the light intensity of the light spot will become stronger, and the laser intensity in this case will cause damage to the human eye. At this time, the histogram of the signal received by the SPAD photosensitive unit 404 after being processed is as shown in Figure 6b . Since the light homogenizing sheet 410 is damaged, the light intensity reflected back to the cavity 101 will become weaker, resulting in fewer photons received by the SPAD photosensitive unit 404 and a decrease in the number of trigger times. The histogram result formed after passing through the readout circuit and being processed by the signal is as shown in Figure 6b . It can be seen that the peak value of the histogram has a significant decrease. After calibration, it is determined that the change in the peak value in one frame relative to the calibration value exceeds a certain range (such as ±20%), and it can be considered that the light homogenizing sheet fails, and the driver needs to turn off the Tx module.

[0048] So far, the entire Tx module has completed two functions: measuring the signal emission time and protecting the human eye safety.

[0049] Figure 7 FIG. is a schematic diagram of the transmitting end of a direct time-of-flight DToF sensor with a filter according to another embodiment of the present invention.

[0050] The direct time-of-flight (DToF) sensor according to the embodiment of the present invention is an active optical sensor. The components of its transmitting end Tx include: a transmitting laser source VCSEL 703, a VCSEL driving circuit 702, a light homogenizing sheet 710, at least one SPAD photosensitive unit 704, a SPAD control circuit and a SPAD readout circuit not shown, and an optional band-pass filter 720. Referring to Figure 7 shown, the VCSEL light source 703 is arranged inside the protection cavity 701, and its lower part is connected to the relevant driving circuit 702. At least one SPAD photosensitive unit 704 is placed around the VCSEL light source 703. The light emitted by the VCSEL light source 703 reaches the light homogenizing sheet 710. The light homogenizing sheet 710 includes two parts, namely an optical microlens array 711 and an optical substrate 712, both of which are transparent materials in the working wavelength band. At the same time, a band-pass filter 720 is attached to the upper surface of the light homogenizing sheet 710, and its spectral transmittance curve is as shown in Figure 8 shown (here, the working wavelength is selected as 940 nm).

[0051] Since the band - pass filter 720 only allows light near the operating wavelength of the system to pass through, it neither affects the transmission of the laser emitted by the VCSEL light source 703 nor restricts the interference of the spectrum of external ambient light, such as the visible light band of the sun, on the SPAD photosensitive unit 704. Its advantage lies in greatly reducing the interference of ambient light on the SPAD photosensitive unit in different light and dark environments, avoiding the situation where false alarms cause the transmitter Tx to be shut down by the system. In this case, after calibration, the change threshold range of the output histogram can be reduced (such as ±10%). When this value is exceeded, it can be considered that the optical component fails and the driver needs to shut down the Tx module.

[0052] Adding an optional narrow - band band - pass filter can improve the robustness of the system, reduce the probability of false alarms, and better achieve the function of eye - safety protection.

[0053] According to an embodiment of the present invention, there is provided a direct - time - of - flight DTOF sensor device including the transmitter device as described above. The DTOF sensor device includes: a transmitter device configured to emit a short - pulse laser to irradiate an object to be measured and record the start time of emitting the short - pulse laser; and a receiver device configured to receive a part of the laser reflected by the object to be measured and record the end time of receiving the measured optical signal, so as to obtain the distance of the object to be measured through the time difference between the start time and the end time.

[0054] According to an embodiment of the present invention, there is provided a method for controlling a transmitter device for a direct - time - of - flight DTOF sensor. The transmitter device includes a laser light source, transmitter - side optical elements, at least one single - photon avalanche diode SPAD photosensitive unit, and a laser light source drive circuit. The method includes: emitting light through the laser light source; transmitting a first part of the light emitted by the laser light source through the transmitter - side optical elements and reflecting a second part of the light emitted by the laser light source through the transmitter - side optical elements; and receiving the second part of the light reflected by the transmitter - side optical elements through at least one SPAD photosensitive unit and outputting an optical signal related to the returned light, wherein the start time of the light emitted by the laser light source is determined according to the optical signal output by the at least one SPAD photosensitive unit; and shutting down the transmitter device through the laser light source drive circuit when it is determined that the peak change of the calibrated optical signal exceeds a first threshold.

[0055] Although the present disclosure has been described with exemplary embodiments, various changes and modifications can be suggested to those skilled in the art. The present disclosure is intended to cover such changes and modifications that fall within the scope of the appended claims.

[0056] No description in this invention shall be construed as implying that any particular element, step, or function is an essential element that must be included within the scope of the claims. The scope of the patented subject matter is defined only by the claims.

Claims

1. A transmitting-end device for a direct time-of-flight (DTOF) sensor, comprising: A transmitting laser source configured to emit light; Transmitting-end optical elements configured to allow a first portion of the light emitted by the transmitting laser source to pass through the transmitting-end optical elements and to reflect a second portion of the light emitted by the transmitting laser source by the transmitting-end optical elements; And At least one single-photon avalanche diode (SPAD) photosensing unit that receives the second portion of the light reflected by the transmitting-end optical elements and outputs an optical signal related to the returned light, wherein the start time of the light emission by the transmitting laser source is determined based on the optical signal output by the at least one SPAD photosensing unit, wherein the start time of the light emission by the transmitting laser source is determined by determining the peak value of the optical signals within a frame, The transmitting-end device further includes a transmitting laser source driving circuit configured to turn off the transmitting-end device when it is determined that the peak value of the optical signals within a frame changes by more than a first threshold relative to a calibration value, wherein the peak value of the optical signals changing by more than a first threshold relative to the calibration value includes the peak value changing by more than the first threshold relative to the calibration value.

2. The transmitting-end device according to claim 1, wherein, The first threshold is ±20%.

3. The transmitting end device according to claim 1, wherein The transmitting-end optical elements include a collimating lens.

4. The transmitting end device according to claim 1, wherein The transmitting-end optical elements include a diffractive optical element for generating an interference effect spot.

5. The transmitting end device according to claim 4, wherein, The diffractive optical element includes an optical microstructure and an optical substrate, wherein the optical microstructure and the optical substrate are transparent materials in the working wavelength band.

6. The transmitting end device according to claim 1, wherein, The transmitting-end optical elements include a light homogenizing sheet.

7. The transmitting end device according to claim 6, wherein, The light homogenizing sheet includes an optical microlens array and an optical substrate, wherein the optical microlens array and the optical substrate are transparent materials in the working wavelength band.

8. The transmitting-end device according to claim 6, wherein, The transmitting-end optical elements further include a band-pass filter configured to allow only light near the working wavelength of the DTOF sensor to pass through.

9. A direct time-of-flight (DTOF) sensor device including the transmitting-end device according to any one of claims 1 to 8, the DTOF sensor device comprising: A transmitting-end device configured to emit a short-pulse laser to irradiate an object to be measured and record the start time of the emission of the short-pulse laser; And A receiving-end device configured to receive a portion of the laser reflected by the object to be measured and record the end time of receiving the measured optical signal, so as to obtain the distance of the object to be measured through the time difference between the start time and the end time.

10. A method for controlling a transmitting-end device for a direct time-of-flight (DTOF) sensor, the transmitting-end device including a transmitting laser source, transmitting-end optical elements, at least one single-photon avalanche diode (SPAD) photosensing unit, and a transmitting laser source driving circuit, the method comprising: Emitting light through the transmitting laser source; Transmitting a first portion of the light emitted by the transmitting laser source through the transmitting-end optical elements and reflecting a second portion of the light emitted by the transmitting laser source by the transmitting-end optical elements; And Receiving, by at least one SPAD photosensitive unit, a second portion of the light reflected by the transmitting end optical element, and outputting an optical signal related to the returned light, wherein the start time of the light emitted by the transmitting laser source is determined according to the optical signal output by the at least one SPAD photosensitive unit; and When it is determined that the peak value of the optical signal within a frame changes by more than a first threshold relative to the calibration value, turning off the transmitting end device through the transmitting laser source driving circuit, wherein the start time of the light emitted by the transmitting laser source is determined by determining the peak value of the optical signal within a frame, and wherein the change of the peak value of the optical signal by more than a first threshold relative to the calibration value includes that the peak value changes by more than the first threshold relative to the calibration value.

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