CN110412607B - TOF pixel circuit with high dynamic range and ranging system - Google Patents
TOF pixel circuit with high dynamic range and ranging system Download PDFInfo
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- CN110412607B CN110412607B CN201810404994.1A CN201810404994A CN110412607B CN 110412607 B CN110412607 B CN 110412607B CN 201810404994 A CN201810404994 A CN 201810404994A CN 110412607 B CN110412607 B CN 110412607B
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01S—RADIO DIRECTION-FINDING; RADIO NAVIGATION; DETERMINING DISTANCE OR VELOCITY BY USE OF RADIO WAVES; LOCATING OR PRESENCE-DETECTING BY USE OF THE REFLECTION OR RERADIATION OF RADIO WAVES; ANALOGOUS ARRANGEMENTS USING OTHER WAVES
- G01S17/00—Systems using the reflection or reradiation of electromagnetic waves other than radio waves, e.g. lidar systems
- G01S17/02—Systems using the reflection of electromagnetic waves other than radio waves
- G01S17/06—Systems determining position data of a target
- G01S17/08—Systems determining position data of a target for measuring distance only
- G01S17/10—Systems determining position data of a target for measuring distance only using transmission of interrupted, pulse-modulated waves
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01S—RADIO DIRECTION-FINDING; RADIO NAVIGATION; DETERMINING DISTANCE OR VELOCITY BY USE OF RADIO WAVES; LOCATING OR PRESENCE-DETECTING BY USE OF THE REFLECTION OR RERADIATION OF RADIO WAVES; ANALOGOUS ARRANGEMENTS USING OTHER WAVES
- G01S17/00—Systems using the reflection or reradiation of electromagnetic waves other than radio waves, e.g. lidar systems
- G01S17/88—Lidar systems specially adapted for specific applications
- G01S17/89—Lidar systems specially adapted for specific applications for mapping or imaging
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01S—RADIO DIRECTION-FINDING; RADIO NAVIGATION; DETERMINING DISTANCE OR VELOCITY BY USE OF RADIO WAVES; LOCATING OR PRESENCE-DETECTING BY USE OF THE REFLECTION OR RERADIATION OF RADIO WAVES; ANALOGOUS ARRANGEMENTS USING OTHER WAVES
- G01S17/00—Systems using the reflection or reradiation of electromagnetic waves other than radio waves, e.g. lidar systems
- G01S17/88—Lidar systems specially adapted for specific applications
- G01S17/93—Lidar systems specially adapted for specific applications for anti-collision purposes
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01S—RADIO DIRECTION-FINDING; RADIO NAVIGATION; DETERMINING DISTANCE OR VELOCITY BY USE OF RADIO WAVES; LOCATING OR PRESENCE-DETECTING BY USE OF THE REFLECTION OR RERADIATION OF RADIO WAVES; ANALOGOUS ARRANGEMENTS USING OTHER WAVES
- G01S7/00—Details of systems according to groups G01S13/00, G01S15/00, G01S17/00
- G01S7/48—Details of systems according to groups G01S13/00, G01S15/00, G01S17/00 of systems according to group G01S17/00
- G01S7/483—Details of pulse systems
- G01S7/486—Receivers
- G01S7/4865—Time delay measurement, e.g. time-of-flight measurement, time of arrival measurement or determining the exact position of a peak
- G01S7/4866—Time delay measurement, e.g. time-of-flight measurement, time of arrival measurement or determining the exact position of a peak by fitting a model or function to the received signal
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- Transforming Light Signals Into Electric Signals (AREA)
- Solid State Image Pick-Up Elements (AREA)
Abstract
The invention provides a TOF pixel circuit with a high dynamic range, which comprises a photosensitive control unit, a first reading circuit and a second reading circuit. The first reading circuit and the second reading circuit are symmetrical circuits and share the photosensitive control unit. The photosensitive control unit comprises a photodiode and a transmission transistor. The first reading circuit and the second reading circuit comprise reset transistors, gain control units, signal storage control units and first output units. The first reading circuit and the second reading circuit respectively comprise a global exposure transmission unit, and the global exposure transmission unit comprises a global exposure storage unit and a second output unit. The TOF pixel circuit provided by the invention can realize output modes of rolling exposure and global exposure. The invention also provides a ranging system with a high dynamic range TOF image sensor.
Description
Technical Field
The present invention relates to image sensing devices, and more particularly to an image sensor pixel circuit and a ranging system for TOF applications with high dynamic range.
Background
Tof (time of fly) is mainly applied to a system for acquiring a 3D image in an image sensor device. The system measures the distance of the imaging target to the image sensing device using the time of arrival of light from the light source to the object and reflection back to the image sensor based on optical time of flight. Each pixel of the image sensor participates in ranging to obtain a depth image with high accuracy.
With the wide application of 3D images, such as applications of AR (augmented reality), VR (virtual reality), unmanned aerial vehicles, robots, digital cameras, and the like, TOF pixel circuits and sensing devices of the pixel circuits will be further developed. The method can be applied to acquiring high-precision images, and can also realize the functions of object identification, obstacle detection and the like. And the depth calculation of the TOF is not influenced by the surface gray scale and the characteristics of the target object, so that the target three-dimensional image can be detected very accurately.
Disclosure of Invention
The present invention aims to provide a TOF pixel circuit with a high dynamic range, the pixel circuit comprising:
a light sensing control unit including a photodiode for accumulating charges generated by a photoelectric effect in response to incident light and a transfer transistor; the two transmission transistors are respectively connected to the photodiodes and used for respectively transferring and outputting the charges generated by the photodiodes according to transmission control signals during exposure;
the first reading circuit and the second reading circuit are respectively connected to the photosensitive control unit, are symmetrical circuits and share the photosensitive control unit; the first reading circuit and the second reading circuit are respectively connected to the photodiode through one of the transfer transistors; the first reading circuit and the second reading circuit respectively comprise:
a reset transistor connected to a first voltage source, resetting the floating diffusion point voltage according to a reset control signal;
a gain control unit connected between the reset transistor and the floating diffusion point; the gain control unit comprises a gain control transistor and a capacitor, one end of the gain control transistor is connected to the reset transistor and the capacitor, the other end of the gain control transistor is connected to the floating diffusion point, and the grid of the gain control transistor is connected to a control signal; the other end of the capacitor is connected to a specified voltage;
the signal storage control unit comprises a signal storage control transistor and a capacitor and is used for storing charges generated by photoelectric effect after the photodiode is exposed; the signal storage control transistor is connected between the floating diffusion point and the capacitor; the other end of the capacitor is connected with a specified voltage;
optionally, the number of the signal storage control transistors is two, a first signal storage control transistor is connected between the output end of the photosensitive control unit and a second signal storage control transistor, and the second signal storage control transistor is connected to the floating diffusion point; one end of the capacitor is connected to a connection point of the first signal storage control transistor and the second signal storage control transistor, and the other end of the capacitor is connected to a specified voltage;
optionally, the number of the signal storage control transistors is two, the first signal storage control transistor is connected between the output end of the photosensitive control unit and the capacitor, and the other end of the capacitor is connected to a specified voltage; the second signal storage control transistor is connected between the output end of the photosensitive control unit and the floating diffusion point;
a first output unit connected to the floating diffusion point for amplifying a voltage signal of the floating diffusion point and outputting the amplified voltage signal to a column line in a rolling exposure mode;
optionally, the first output unit comprises a source follower transistor and a row select transistor, the source follower transistor having a gate connected to the floating diffusion point and a drain connected to a second voltage
A source; the source output end of the transistor is connected to a column line through the row selection transistor;
optionally, the first reading circuit and the second reading circuit further include a global exposure transmission unit respectively connected between the source output end of the source follower transistor of the first output unit and the column line, and configured to store, read, and output a signal in a global exposure mode, where the global exposure transmission unit includes a global exposure storage unit and a second output unit;
the global exposure storage unit comprises a first global exposure transmission control transistor, an image signal storage capacitor, a second global exposure transmission control transistor and a reset signal storage capacitor, wherein the first global exposure transmission control transistor is connected between the source output end of the source follower transistor of the first output unit and the second global exposure transmission control transistor; the second global exposure transfer control transistor is connected to the second output unit; one end of the image signal storage capacitor is connected to a connection point of the first global exposure transmission control transistor and the second global exposure transmission control transistor, and the other end of the image signal storage capacitor is connected to a ground end; one end of the reset signal storage capacitor is connected to a connection point of the second global exposure transmission control transistor and the second output unit, and the other end of the reset signal storage capacitor is connected to a ground end;
the second output unit comprises a source electrode following transistor and a row selecting transistor, is connected between the global exposure storage unit and a column line, and is used for amplifying and outputting signals of the global exposure storage unit; the gate of the source follower transistor is connected to the output end of the global exposure storage unit, and the drain of the source follower transistor is connected to a third voltage source; the source output end of the transistor is connected to a column line through the row selection transistor;
the first voltage source and the second voltage source are variable voltage sources. The capacitor in the TOF pixel circuit may be a parasitic capacitor of a node, a Poly capacitor, a mim (metal insulator metal) capacitor, a mom (metal oxide metal) capacitor, or a MOS capacitor.
The invention also provides a ranging system of a TOF, comprising:
a TOF image sensor including an array of TOF pixel circuits supporting mixed exposure with high dynamic range as set forth in the above summary of the invention;
the control signal processing unit is used for controlling the working process of the system and processing the image data acquired by the TOF pixel array circuit supporting the mixed exposure;
the modulatable light source is used for receiving the modulation signal to generate a modulation light signal and feeding the received modulation signal back to the TOF pixel array circuit supporting the mixed exposure;
the TOF image sensor comprises a signal phase locking module for locking a modulation signal and a modulatable light
The signal fed back by the source is phase adjusted and locked.
The TOF pixel circuit provided by the invention supports two reading modes of rolling exposure and global exposure, can select different output modes according to application, and can output images with High Dynamic Range (HDR).
The design of different types of TOF pixel circuits provided by the invention can effectively isolate the signal storage control unit circuit from other circuits, reduce leakage current in the circuits, and has small equivalent capacitance and higher circuit operation speed.
The TOF pixel circuit and the ranging system provided by the invention can accurately measure the distance from an imaging target to the image sensing device, are applied to obtaining high-precision images, and can realize the functions of object identification, obstacle detection and the like.
Drawings
FIG. 1 is a schematic diagram of a TOF pixel circuit according to a first embodiment of the present disclosure;
FIG. 2 is a timing diagram of a pixel circuit in a rolling exposure mode according to a first embodiment of the present invention;
FIG. 3 is a timing diagram of a pixel circuit in a global exposure mode according to a first embodiment of the present invention;
FIG. 4 is a schematic diagram of a TOF pixel circuit according to a second embodiment of the present disclosure;
FIG. 5 is a timing diagram of a pixel circuit in a rolling exposure mode according to a second embodiment of the present invention;
FIG. 6 is a timing diagram of a pixel circuit in a global exposure mode according to a second embodiment of the present invention;
FIG. 7 is a schematic diagram of a TOF pixel circuit according to a third embodiment of the present disclosure;
FIG. 8 is a timing diagram of a pixel circuit in a rolling exposure mode according to a third embodiment of the present invention;
FIG. 9 is a timing diagram of a pixel circuit in a global exposure mode according to a third embodiment of the present invention; and
fig. 10 is a basic block diagram of a ranging system according to the present invention.
Detailed Description
The following describes the embodiments of the present invention in detail with reference to the drawings provided by the present invention.
Fig. 1 is a block diagram of a TOF pixel circuit with high dynamic range according to an embodiment of the present invention.
In this embodiment, the sensing control unit of the TOF pixel circuit includes a photodiode PD and transmission transistors TXa and TXb. TXa and TXb share a photodiode PD. The first read circuit is connected to the photodiode PD through the transfer transistor TXa, and the second read circuit is connected to the photodiode PD through the transfer transistor TXb.
The first reading circuit and the second reading circuit are mutually symmetrical circuits. The first reading circuit and the second reading circuit respectively comprise a reset transistor, a gain control unit, a signal storage control unit and a first output unit. Taking the first reading circuit as an example:
the signal storage control unit employs a signal storage control transistor INTa and a capacitor Cina. The signal storage control transistor INTa is connected between the floating diffusion point fda and the capacitor Cina. The other end of the capacitor Cina is connected with a designated voltage Vrm.
The reset transistor RSTa is connected to a variable voltage source Vrab, and resets a voltage at the floating diffusion point fda according to a reset control signal rst. The gain control transistor DCGa has one terminal connected to the reset transistor RSTa and the capacitor Cdcga, one terminal connected to the floating diffusion point fda, and a gate connected to the control signal dcg. The other terminal of the capacitor Cdcga is connected to a specified voltage, which may be pixvdd, ground or other specified voltage value. The source follower transistor SFa has a gate connected to the floating diffusion point fda and a drain connected to the variable voltage source Vrsf. The source output of the source follower transistor SFa is connected to the output column line pixa through the rolling exposure row select transistor RS _ Sa.
The global exposure transfer unit includes a first global exposure transfer control transistor GSSGa, an image signal storage capacitor Csiga, a second global exposure transfer control transistor GRSTa, and a reset signal storage capacitor Crsta. The second output unit includes a pole-follower transistor GSFa and a row selection transistor GS _ Sa.
The second reading circuit structure is symmetrical to the first reading circuit structure, and corresponding devices and connection relations in the circuit are also consistent, so that the description of the structure and connection relation of the second reading circuit is omitted.
Fig. 2 shows a timing diagram of a TOF pixel circuit with a high dynamic range in a rolling exposure mode, and in conjunction with the TOF pixel circuit shown in fig. 1, a specific implementation process of a first embodiment of the invention is as follows:
rolling exposure mode:
a. firstly, initializing a circuit, wherein each control signal in the circuit is shown as a process in figure 2;
and (3) an exposure process:
b. turning on a light source pulse to expose the PD, and according to control signals gs _ TXa and gs _ TXb, transmitting transistors TXA and TXB are conducted, and the TXA and TXB are conducted alternately in sequence with a phase difference of pi, so that charges accumulated by the photodiode PD are transmitted to capacitors Cina and Cinb; when the exposure is finished, setting the control signal int to be low level, and respectively storing the charges accumulated by the photodiode PD to the capacitors Cina and Cinb;
and (3) reading:
c. the row selection transistors RS _ Sa and RS _ Sb are conducted, the control signals rst and dcg are set to be high level, the transistors RSTa, RSTb, DCGa and DCGb are conducted, and the floating diffusion points fda and fdb are reset to the voltage Vrab;
d. initial signal voltages VaL0 and VBL0 output by the first reading circuit and the second reading circuit when the low conversion gain LCG is read respectively;
e. setting the control signal rst to be high level, and resetting the floating diffusion points fda and fdb again;
f. the control signal dcg is set to low level, and the initial signal voltages Vah0 and Vbh0 output by the first reading circuit and the second reading circuit when the high conversion gain HCG is read;
g. the control signal int is set to a high level, the transistors INTa and INTb are turned on, and charges stored in the capacitors Cina and Cinb are transferred to the floating diffusion points fda and fdb, respectively;
h. the control signal int is set to a low level, and the signal voltages Vah1 and Vbh1 output by the first reading circuit and the second reading circuit when the high conversion gain HCG is read;
i. the control signals int and dcg are set to high level, and partial charges are transferred to the capacitors Cdcga and Cdcgb;
j. the control signal int is set to a low level, and the signal voltages VaL1 and VbL1 output by the first reading circuit and the second reading circuit at the time of reading a low conversion gain are read.
Correlation calculations were performed for VaL1 and VaL0, Vah1 and Vah0, VbL1 and VbL0, Vbh1 and Vbh0, respectively, yielding VaL-VaL 1-VaL0, Vah-Vah 1-Vah0, VbL-VbL 1-VbL0, and Vbh-Vbh 1-Vbh 0. Two frame signals at different gains (high conversion gain/low conversion gain) can be combined into an HDR signal, Va ═ f (Vah, VaL), Vb ═ f (Vbh, VbL).
If the pulse width of the signal source is T, the time of flight of the light source in the air is Ttoft Vb/(Va + Vb), so that the distance d between the object and the pixel array is Ttoff/2C 1/2C T Vb/(Va + Vb), wherein C is the propagation speed of light in vacuum.
In the timing shown in fig. 2, the hatched portions of the control signals gs _ txa and gs _ txb represent the effective charge accumulation time domain.
Global exposure mode:
the implementation of the global exposure mode of the TOF pixel circuit with a high dynamic range according to an embodiment of the present invention is described in detail with reference to the pixel circuit of fig. 1 and the circuit timing of the global exposure mode shown in fig. 3. The realization of the high dynamic range of the pixel circuit is that a frame of image is respectively read in a high conversion gain mode and a low conversion gain mode, and then a frame of image with the high dynamic range is synthesized through operation, and the realization is as follows:
a. firstly, initializing a circuit, wherein each initialization signal in the circuit is shown as a process in figure 3;
and (3) an exposure process:
b. turning on a light source pulse to expose the PD; the transmission transistors TXa and TXb are turned on according to the control signals gs _ TXa and gs _ TXb, and the TXa and TXb are alternately turned on in sequence with a pi phase difference, so that the charges accumulated in the photodiode PD are transmitted to the capacitors Cina and Cinb; when exposure is finished, setting the control signal int to be low level, and respectively storing the charges accumulated by the photodiode PD to the capacitors Cina and Cinb;
and (3) a storage process:
c. setting the control signals rst and dcg to be high level, enabling the transistors RSTa, RSTb, DCGa and DCGb to be conducted, resetting Crsta, Crstb, Csiga, Csigb, and fda and fdb, and respectively storing initial signals to Crsta and Crstb;
int is set to be high level, charges stored in the capacitors Cina and Cinb are respectively transferred to fda and fdb, and when the control signal sa is set to be low level, signal voltage is respectively stored in the capacitors Csiga and Csigb;
and (3) reading:
setting the GS _ sel signal to be at a high level, turning on the row selection transistors GS _ Sa and GS _ Sb, reading an initial signal at a high conversion gain HCG, and reading signal voltages Vah0 and Vbh0 of the first reading circuit and the second reading circuit, respectively;
setting the sb signal to high level, turning on transistors GRSTa and GRSTb, and redistributing the charges stored in capacitors Csiga and Csigb to the charges stored in Crsta and Crstb, respectively;
g. voltage signals Vah1 and Vbh1 of the first reading circuit and the second reading circuit at the time of reading the high conversion gain HCG, respectively;
as shown in fig. 3, the above operation is repeated, and one frame of the image of the low conversion gain LCG is read. The initial signal and signal voltages for low conversion gain are VaL1 and VaL0, VbL1 and VbL0, respectively.
Operations on VaL1 and VaL0, Vah1 and Vah0, VbL1 and VbL0, Vbh1 and Vbh0, yield VaL-VaL 1-VaL0, Vah-Vah 1-Vah0, VbL-VbL 1-VbL0, and Vbh-Vbh 1-Vbh 0.
Two frame signals at different gains can be synthesized into an HDR signal, Va ═ f (Vah, VaL), Vb ═ f (Vbh, VbL). If the width of the signal source pulse is T, the time of flight of the light source is Ttof T × Vb/(Va + Vb), and thus the distance d of the object from the pixel array is Ttof/2 × C1/2 × C × T Vb/(Va + Vb), where C is the propagation speed of light in vacuum.
In the timing circuit of fig. 3, hatched portions in the gs _ txa and gs _ txb signals represent effective charge accumulation time domains.
The TOF pixel circuit with a high dynamic range provided by the embodiment of the invention can respectively realize the flight time of a light source in the air and the distance between a target object and a pixel array in rolling exposure and global exposure modes.
Example two:
fig. 4 is a circuit diagram of a TOF pixel with high dynamic range according to a second embodiment of the invention. The different part of the circuit from the embodiment of the present invention is a signal storage control unit. The signal storage control units in the first and second reading circuits described in this embodiment respectively include first signal storage control transistors INa and INb, second signal storage control transistors RDa and RDb, and capacitors Cina and Cinb.
Taking the first reading circuit as an example, the first signal storage control transistor INa is connected between the transmission transistor TXa and the second signal storage control transistor RDa, the second signal storage control transistor is connected to the floating diffusion point fda, and the capacitor Cina has one end connected to a connection point of the transistors INa and RDa and the other end connected to the designated voltage Vrm. The second reading circuit and the first reading circuit are symmetrical circuits, and the device arrangement and connection mode are the same, and no additional description is provided.
Fig. 5 is a timing diagram of a TOF pixel circuit implementing the second proposed rolling exposure mode of the invention.
Rolling exposure mode:
a. initializing the circuit;
b. turning on a light source pulse, exposing the photodiode PD, and sequentially and alternately conducting the transistors TXA and TXB with a pi phase difference according to control signals gs _ TXa and gs _ TXb so that charges accumulated by the photodiode PD are transferred to the capacitors Cina and Cinb; when exposure is finished, setting the control signal int to be low level to store the charges accumulated by the photodiode PD to the capacitors Cina and Cinb respectively;
c. the control signal RS _ sel is set to be at a high level, and the transistors RS _ Sa and RS _ Sb are turned on; rst and dcg are set to high level, transistors RSTa, RSTb and DCGa, DCGb are turned on, floating diffusion points fda and fdb are reset to voltage Vrab;
d. reading initial signal voltages VaL0 and VBL0 of the first reading circuit and the second reading circuit at low conversion gain, respectively;
e. setting the control signal rst to be high level, and resetting the floating diffusion points fda and fdb again;
f. setting the control signal dcg to be low level, reading the initial signal voltage of the high conversion gain HCG, and respectively setting the output voltages of the first reading circuit and the second reading circuit to be Vah0 and Vbh 0;
g. when the control signal rd is set to high level, the transistors RDa and RDb are turned on, and the charges stored in the capacitors Cina and Cinb are transferred to the floating diffusion points fda and fdb, respectively;
h. setting the control signal rd to be at a low level, reading the signal voltage at the time of high conversion gain, wherein the output voltages of the first reading circuit and the second reading circuit are Vah1 and Vbh1 respectively;
i. the control signals rd and dcg are set to high level, and partial charges are transferred to the capacitors Cdcga and Cdcgb;
j. the control signal rd is set to be at a low level, the signal voltage with low conversion gain is read, and the outputs of the first reading circuit and the second reading circuit are VaL1 and VBL1 respectively.
Correlation calculations were performed for VaL1 and VaL0, Vah1 and Vah0, VbL1 and VbL0, Vbh1 and Vbh0, respectively, yielding VaL-VaL 1-VaL0, Vah-Vah 1-Vah0, VbL-VbL 1-VbL0, and Vbh-Vbh 1-Vbh 0. Two frame signals at different gains (high conversion gain/low conversion gain) can be combined into an HDR signal, Va ═ f (Vah, VaL), Vb ═ f (Vbh, VbL).
If the pulse width of the signal source is T, the time of flight of the light source in the air is Ttoft Vb/(Va + Vb), so that the distance d between the target object and the pixel array is Ttoff/2C 1/2C T Vb/(Va + Vb), wherein C is the propagation speed of light in vacuum.
Global exposure mode:
fig. 6 is a timing diagram of a TOF pixel circuit in a global exposure mode according to a second embodiment of the invention.
a. Initializing the circuit;
and (3) an exposure process:
b. turning on a light source pulse to expose the photodiode PD; the transistors TXa and TXb are alternately turned on in sequence with a phase difference of pi in accordance with the control signals gs _ TXa and gs _ TXb, so that the charges accumulated in the photodiode PD are transferred to the capacitors Cina and Cinb; when exposure is finished, setting the control signal int to be low level, and respectively storing the charges accumulated by the photodiode PD to the capacitors Cina and Cinb;
and (3) a storage process:
c. the control signals rst and dcg are set to be high level, the transistors RSTa and RSTb, DCGa and DCGb are conducted, and the voltages of the floating diffusion points fda and fdb are reset respectively; setting the control signal sb to be low level, and respectively storing the reset signals in the capacitors Crsta and Crstb;
d. when the control signal rd is set to high level, the transistors RDa and RDb are turned on, and the charges stored in the capacitors Cina and Cinb are transferred to the floating diffusion points fda and fdb, respectively; the control signal sa is set to be at a low level, and the signal voltage is respectively stored in the capacitors Csiga and Csigb;
and (3) reading:
e. setting the control signal GS _ sel to be at a high level, turning on the row selection transistors GS _ Sa and GS _ Sb, reading an initial signal with high conversion gain, and respectively reading voltage signals Vah0 and Vbh0 at the moment;
f. the control signal sb is set to high level, the transistors GRSTa and GRSTb are conducted, and the signal charges stored in the capacitors Csiga and Csigb are redistributed with the charges stored in Crsta and Crstb respectively;
g. reading voltage signals Vah1 and Vbh1 at high conversion gain respectively;
as shown in fig. 6, the above operation is repeated, and an image at the time of one frame of low conversion gain LCG is read. The initial and voltage signals for low conversion gain are VaL0 and VbL0, VaL1 and VbL1, respectively. Operations are performed on VaL1 and VaL0, Vah1 and Vah0, VbL1 and VbL0, Vbh1 and Vbh0, yielding VaL-VaL 1-VaL0, Vah-Vah 1-Vah0, VbL-VbL 1-VbL0, and Vbh-Vbh 1-Vbh 0.
Two frame signals at different gains can be synthesized into an HDR signal, Va ═ f (Vah, VaL), Vb ═ f (Vbh, VbL).
If the width of the signal source pulse is T, the time of flight of the light source is Ttof T × Vb/(Va + Vb), and thus the distance d of the object from the pixel array is Ttof/2 × C1/2 × C × T Vb/(Va + Vb), where C is the propagation speed of light in vacuum.
In this embodiment, the hatched portions of gs _ txa and gs _ txb in fig. 5 and 6 represent the effective charge accumulation time domain.
In the second embodiment, the signal storage control unit includes first and second signal storage control transistors INa and INb and RDa and RDb, respectively. Taking the first read circuit as an example, the first signal storage control transistor INa is connected between the transmission transistor TXa and the second signal storage control transistor RDa, and the second signal storage control transistor RDa is connected to the floating diffusion point fda. The arrangement mode can form the isolation of the capacitor Cina from other circuits and reduce the leakage current in the circuit. The capacitor of the signal storage control unit is isolated from other capacitors in the circuit, so that the equivalent capacitance of the circuit is small, and the speed is also increased.
Fig. 7 is a circuit diagram of a TOF pixel with high dynamic range according to a third embodiment of the present invention. The signal storage control unit includes first signal storage control transistors INa and INb, second signal storage control transistors RDa and RDb, and capacitors Cina and Cinb, respectively. Unlike the first and second embodiments, the first read circuit is taken as an example, the first signal storage control transistor INa is connected between the output terminal of the transmission transistor TXa of the light sensing control unit and the capacitor Cina, and the second signal storage control transistor RDa is connected between the output terminal of the transmission transistor TXa of the light sensing control unit and the floating diffusion point fda. The other end of the capacitor Cina is connected to a specified voltage Vrm.
Fig. 8 is an operation timing chart of a TOF pixel circuit in the rolling exposure mode according to the third embodiment. As shown in the figure:
a. initializing the circuit;
and (3) an exposure process:
b. turning on a light source pulse to expose the photodiode PD; the transistors TXa and TXb are alternately turned on in sequence with a phase difference of pi in accordance with the control signals gs _ TXa and gs _ TXb, so that the charges accumulated in the photodiode PD are transferred to the capacitors Cina and Cinb; when exposure is finished, setting the control signal int to be low level, and respectively storing the charges accumulated by the photodiode PD to the capacitors Cina and Cinb;
and (3) reading:
c. the control signal RS _ sel is set to be at a high level, the transistors RS _ Sa and RS _ Sb are turned on, the control signals rst and dcg are set to be at a high level, and the transistors RSTa, RSTb, DCGa and DCGb are turned on to reset the voltages of the floating diffusion points fda and fdb;
d. initial signal voltages VaL0 and VBL0 of the first reading circuit and the second reading circuit at the time of low conversion gain are read, respectively;
e. setting the control signal rst to be high level, and resetting the floating diffusion points fda and fdb again;
f. the control signal dcg is set to low level, the initial signal voltage when the conversion gain HCG is high, the output voltages Vah0 and Vbh0 of the first reading circuit and the second reading circuit are read;
g. the control signal int is set to a high level, the transistors INTa and INTb are turned on, and charges stored in the capacitors Cina and Cinb are transferred to the floating diffusion points fda and fdb, respectively;
h. setting the control signal int to be low level, reading the signal voltage of the high conversion gain HCG, and setting the output voltages of the first reading circuit and the second reading circuit to be Vah1 and Vbh1 respectively;
i. the control signals rd and dcg are set to high level, and partial charges are transferred to Cdcga and Cdcgb;
j. the control signal int is set to low level, the signal voltage with low conversion gain is read, and the outputs of the first reading circuit and the second reading circuit are VaL1 and VbL1 respectively.
Correlation calculations were performed for VaL1 and VaL0, Vah1 and Vah0, VbL1 and VbL0, Vbh1 and Vbh0, respectively, yielding VaL-VaL 1-VaL0, Vah-Vah 1-Vah0, VbL-VbL 1-VbL0, and Vbh-Vbh 1-Vbh 0. Two frame signals at different gains (high conversion gain/low conversion gain) can be combined into an HDR signal, Va ═ f (Vah, VaL), Vb ═ f (Vbh, VbL).
If the pulse width of the signal source is T, the time of flight of the light source in the air is Ttoft Vb/(Va + Vb), so that the distance d between the target object and the pixel array is Ttoff/2C 1/2C T Vb/(Va + Vb), wherein C is the propagation speed of light in vacuum.
Fig. 9 is an operation timing diagram of a TOF pixel circuit in the global exposure mode according to the third embodiment. The working process is as shown in the figure:
a. initializing the circuit;
and (3) an exposure process:
b. turning on a light source pulse to expose the photodiode PD; the transistors TXa and TXb are alternately turned on in sequence with a phase difference of pi in accordance with the control signals gs _ TXa and gs _ TXb, so that the charges accumulated in the photodiode PD are transferred to the capacitors Cina and Cinb; when exposure is finished, setting the control signal int to be low level, and respectively storing the charges accumulated by the photodiode PD to the capacitors Cina and Cinb;
and (3) a storage process:
c. setting the control signals rst and dcg to be high level, conducting the transistors RSTa and RSTb and DCGa and DCGb, and resetting the voltages of the floating diffusion points fda and fdb; setting the control signal sb to be low level, and respectively storing the reset signals to the capacitors Crsta and Crstb;
d. the control signals int and rd are set to high level, the transistors INa and INb, RDa and RDb are turned on, and the charges stored in the capacitors Cina and Cinb are transferred to the floating diffusion points fda and fdb, respectively; setting the control signal sa to be low level, and storing the signal voltage at the moment to Csiga and Csigb;
and (3) reading:
e. the control signal GS _ sel is set to be at a high level, the row selection transistors GS _ Sa and GS _ Sb are turned on, an initial signal at the time of reading the high conversion gain HCG is read, and voltage signals Vah0 and Vbh0 of the first reading circuit and the second reading circuit are read respectively;
f. the control signal sb is set to high level, the transistors GRSTa and GRSTb are conducted, and the signal charges stored in the capacitors Csiga and Csigb are redistributed with the charges stored in Crsta and Crstb respectively;
g. the voltage signals Vah1 and Vbh1 at high conversion gain are read, respectively.
As shown in fig. 9, the above steps are repeated, and one frame of image with low conversion gain is read. The initial signal and signal voltages for low conversion gain are VaL1 and VaL0, VbL1 and VbL0, respectively.
Correlation calculations were performed for VaL1 and VaL0, Vah1 and Vah0, VbL1 and VbL0, Vbh1 and Vbh0, respectively, yielding VaL-VaL 1-VaL0, Vah-Vah 1-Vah0, VbL-VbL 1-VbL0, and Vbh-Vbh 1-Vbh 0. Two frame signals at different gains (high conversion gain/low conversion gain) can be combined into an HDR signal, Va ═ f (Vah, VaL), Vb ═ f (Vbh, VbL).
If the width of the signal source pulse is T, the time of flight of the light source is Ttof T × Vb/(Va + Vb), and thus the distance d of the object from the pixel array is Ttof/2 × C1/2 × C × T Vb/(Va + Vb), where C is the propagation speed of light in vacuum.
In the third embodiment, the first signal transistors INa and INb and the second signal storage control transistors RDa and RDb can reduce the contact of the signal storage unit capacitors Cina and Cinb with other circuits, thereby playing the role of isolation. The leakage current in the circuit can be effectively reduced, the capacitors Cina and Cinb of the signal storage control unit are isolated from other capacitors in the circuit, the equivalent capacitance is small, and the circuit speed can be accelerated.
In this embodiment, the hatched areas of gs _ txa and gs _ txb in fig. 8 and 9 represent the effective charge accumulation time domain.
Fig. 10 is a basic block diagram of a ranging system of a pixel circuit of a TOF capable of supporting a rolling exposure mode and a global exposure mode according to the present invention. As shown, the ranging system includes a TOF image sensor, a modulatable light source, and a control and signal processing unit.
A TOF image sensor being a sensing device comprising an array of TOF pixels as set forth in any one of the above embodiments of the invention. Which senses the illumination intensity and quantizes the light signal into a digital signal. The image sensor circuit generates a modulation signal for controlling the exposure process of the TOF pixel array on the one hand and for sending the modulation signal to the modulatable light source for generating a modulated light source signal on the other hand. The TOF image sensor further comprises a signal phase locking module which can perform phase adjustment and locking on the generated modulation signal and the signal which can modulate the light source feedback.
The modulatable light source receives the modulation signal, generates a modulated light signal, and feeds the received modulation signal back to the TOF image sensor.
And the control and signal processing unit controls the working process of the whole distance measuring system and processes the image data acquired by the TOF image sensor.
The distance measuring system provided by the invention can be applied to acquiring high-precision images and can realize the functions of object identification, obstacle detection and the like. And the depth calculation of the TOF is not influenced by the surface gray scale and the characteristics of the target object, so that the target three-dimensional image can be detected very accurately.
The present invention includes, but is not limited to, the embodiments set forth in this patent. The corresponding revisions or modifications of the embodiments according to the present invention by those skilled in the art are all within the protection scope of the present invention.
Claims (24)
1. A TOF pixel circuit having a high dynamic range, the pixel circuit comprising:
a light sensing control unit including a photodiode for accumulating charges generated by a photoelectric effect in response to incident light and a transfer transistor; the two transmission transistors are respectively connected to the photodiodes and used for respectively transferring and outputting the charges generated by the photodiodes according to transmission control signals during exposure;
a first reading circuit and a second reading circuit respectively connected to the photosensitive control unit, the first reading circuit and the second reading circuit respectively including:
a reset transistor connected to a first voltage source, for resetting the voltage of the floating diffusion point according to a reset control signal;
a gain control unit connected between the reset transistor and the floating diffusion point;
the signal storage control unit comprises one or more signal storage control transistors and a capacitor and is used for storing the charges generated by the photoelectric effect of the photodiode;
a first output unit connected to the floating diffusion point, for amplifying a voltage signal of the floating diffusion point and outputting the amplified voltage signal to a column line;
the first reading circuit and the second reading circuit are symmetrical circuits and share the photosensitive control unit; the first reading circuit and the second reading circuit are connected to the photodiode through one of the transfer transistors, respectively.
2. The TOF pixel circuit with high dynamic range of claim 1 wherein the gain control unit comprises a gain control transistor and a capacitor, the gain control transistor having one end connected to the reset transistor and the capacitor and another end connected to the floating diffusion point; the other end of the capacitor is connected to a specified voltage.
3. The TOF pixel circuit with high dynamic range of claim 1 wherein said signal storage control unit comprises a signal storage control transistor and a capacitor, said signal storage control transistor being connected between said floating diffusion point and said capacitor, said capacitor being connected at another end to a specified voltage.
4. A TOF pixel circuit with high dynamic range according to claim 1 wherein the signal storage control unit comprises two signal storage control transistors and a capacitor, the first signal storage control transistor being connected between the output of the photosensing control unit and the second signal storage control transistor; the second signal storage control transistor is connected to the floating diffusion point; the capacitor has one end connected to a connection point of the first signal storage control transistor and the second signal storage control transistor, and the other end connected to a prescribed voltage.
5. The TOF pixel circuit with high dynamic range of claim 1 wherein said signal storage control unit comprises two signal storage control transistors and a capacitor, a first signal storage control transistor being connected between the output of said light sensing control unit and said capacitor, the other end of said capacitor being connected to a specified voltage; the second signal storage control transistor is connected between the output end of the photosensitive control unit and the floating diffusion point.
6. The TOF pixel circuit with high dynamic range of claim 1 wherein said first output cell comprises a source follower transistor and a row select transistor, said source follower transistor having its gate connected to said floating diffusion point and its drain connected to a second voltage source; the source output terminal thereof is connected to a column line through the row select transistor.
7. A TOF pixel circuit with high dynamic range according to claim 1, 3, 4, 5 or 6, wherein the first and second reading circuits respectively comprise a global exposure transmission unit connected between a source follower transistor source output terminal of the first output unit and a column line for storing, reading and outputting signals in global exposure mode.
8. The TOF pixel circuit with high dynamic range of claim 7 wherein said global exposure transfer unit comprises a global exposure storage unit and a second output unit.
9. The TOF pixel circuit with high dynamic range of claim 8 wherein the global exposure storage unit comprises a first global exposure transmission control transistor, an image signal storage capacitor, a second global exposure transmission control transistor and a reset signal storage capacitor, the first global exposure transmission control transistor being connected between the source output of the source follower transistor of the first output unit and the second global exposure transmission control transistor; the second global exposure transfer control transistor is connected to the second output unit; one end of the image signal storage capacitor is connected to a connection point of the first global exposure transmission control transistor and the second global exposure transmission control transistor, and the other end of the image signal storage capacitor is connected to a ground end; one end of the reset signal storage capacitor is connected to a connection point of the second global exposure transmission control transistor and the second output unit, and the other end of the reset signal storage capacitor is connected to a ground end.
10. The TOF pixel circuit with high dynamic range of claim 8 wherein said second output cell comprises a source follower transistor and a row select transistor connected between said global exposure storage cell and a column line for amplifying output of a signal of said global exposure storage cell; the gate of the source follower transistor is connected to the output end of the global exposure storage unit, and the drain of the source follower transistor is connected to a third voltage source; the source output terminal thereof is connected to a column line via the row select transistor.
11. A TOF pixel circuit having a high dynamic range according to claim 1 or claim 6 wherein the first and second voltage sources are variable voltage sources.
12. A TOF pixel circuit with high dynamic range according to claim 1, 2 or 9 wherein the capacitance is a parasitic capacitance, a Poly capacitance, a MIM capacitance, a MOM capacitance or a MOS capacitance.
13. A ranging system, comprising:
a TOF image sensor including an array of TOF pixel circuits arranged in rows and columns having a high dynamic range, each pixel circuit including:
a light sensing control unit including a photodiode for accumulating charges generated by a photoelectric effect in response to incident light and a transfer transistor; the two transmission transistors are respectively connected to the photodiodes and used for respectively transferring and outputting the charges generated by the photodiodes according to transmission control signals during exposure;
a first reading circuit and a second reading circuit respectively connected to the photosensitive control unit, the first reading circuit and the second reading circuit respectively including:
a reset transistor connected to a first voltage source, for resetting the voltage of the floating diffusion point according to a reset control signal;
a gain control unit connected between the reset transistor and the floating diffusion point;
the signal storage control unit comprises one or more signal storage control transistors and a capacitor and is used for storing the charges generated by the photoelectric effect of the photodiode;
a first output unit connected to the floating diffusion point for amplifying a voltage signal of the floating diffusion point and outputting the amplified voltage signal to a column line;
the first reading circuit and the second reading circuit are symmetrical circuits and share the photosensitive control unit; the first reading circuit and the second reading circuit are respectively connected to the photodiode through one of the transfer transistors;
the control signal processing unit is used for controlling the working process of the system and processing the image data acquired by the TOF pixel circuit array;
and the modulatable light source is used for receiving the modulation signal, generating a modulation light signal and feeding back the received modulation signal to the TOF pixel circuit array.
14. The ranging system according to claim 13, wherein the gain control unit comprises a gain control transistor and a capacitor, one end of the gain control transistor is connected to the reset transistor and the capacitor, and the other end is connected to the floating diffusion point; the other end of the capacitor is connected to a specified voltage.
15. The range finding system of claim 13, wherein the TOF image sensor comprises a phase locking module for phase adjusting and locking the modulated signal and the signal fed back by the modulatable light source.
16. The ranging system according to claim 13, wherein the signal storage control unit comprises a signal storage control transistor and a capacitor, the signal storage control transistor is connected between the floating diffusion point and the capacitor, and the other end of the capacitor is connected to a specified voltage.
17. The range finding system of claim 13, wherein the signal storage control unit comprises two signal storage control transistors and a capacitor, the first signal storage control transistor being connected between the output of the light sensing control unit and the second signal storage control transistor; the second signal storage control transistor is connected to the floating diffusion point; the capacitor has one end connected to a connection point of the first signal storage control transistor and the second signal storage control transistor, and the other end connected to a prescribed voltage.
18. The distance measuring system of claim 13, wherein said signal storage control unit comprises two signal storage control transistors and a capacitor, the first signal storage control transistor being connected between the output of said light sensing control unit and said capacitor, the other end of said capacitor being connected to a specified voltage; the second signal storage control transistor is connected between the output end of the photosensitive control unit and the floating diffusion point.
19. The ranging system according to claim 13, wherein the first output unit comprises a source follower transistor and a row select transistor, the source follower transistor having a gate connected to the floating diffusion point and a drain connected to a second voltage source; the source output terminal thereof is connected to a column line via the row select transistor.
20. A ranging system according to claim 13, 16, 17, 18 or 19, wherein the first and second reading circuits respectively comprise a global exposure transfer unit connected between the source output terminal of the source follower transistor of the first output unit and the column line for storing, reading and outputting signals in the global exposure mode.
21. The ranging system according to claim 20, wherein the global exposure transfer unit comprises a global exposure storage unit and a second output unit.
22. The range finding system of claim 21, wherein the global exposure storage unit comprises a first global exposure transmission control transistor, an image signal storage capacitor, a second global exposure transmission control transistor, and a reset signal storage capacitor, the first global exposure transmission control transistor being connected between the source output terminal of the source follower transistor of the first output unit and the second global exposure transmission control transistor; the second global exposure transfer control transistor is connected to the second output unit; one end of the image signal storage capacitor is connected to a connection point of the first global exposure transmission control transistor and the second global exposure transmission control transistor, and the other end of the image signal storage capacitor is connected to a ground end; one end of the reset signal storage capacitor is connected to a connection point of the second global exposure transmission control transistor and the second output unit, and the other end of the reset signal storage capacitor is connected to a ground end.
23. The ranging system according to claim 21, wherein the second output unit comprises a source follower transistor and a row select transistor connected between the globally exposed memory cell and a column line for amplifying the signal output of the globally exposed memory cell; the gate of the source follower transistor is connected to the output end of the global exposure storage unit, and the drain of the source follower transistor is connected to a third voltage source; the source output terminal thereof is connected to a column line via the row select transistor.
24. Ranging system according to claim 13, 14 or 22, characterized in that the capacitance is a parasitic capacitance, a Poly capacitance, a MIM capacitance, a MOM capacitance or a MOS capacitance.
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