Patent Publication Number: US-2023156356-A1

Title: Solid-state image sensor, imaging device, and method of controlling solid-state image sensor

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This Application is a Continuation Application of U.S. patent application Ser. No. 16/964,676, filed Jul. 24, 2020, which is a 371 National Stage Entry of International Application No.: PCT/JP2018/046045, filed on Dec. 14, 2018, which in turn claims the benefit of Japanese Priority Patent Application No.: 2018-014228 filed Jan. 31, 2018, the entire contents of each of which are incorporated herein by reference. 
    
    
     TECHNICAL FIELD 
     The present technology relates to a solid-state image sensor, an imaging device, and a method of controlling a solid-state image sensor. Specifically, the present technology relates to a solid-state image sensor that detects a change in luminance, an imaging device, and a method of controlling a solid-state image sensor. 
     BACKGROUND ART 
     Conventionally, a synchronous solid-state image sensor that captures image data (frame) in synchronization with a synchronization signal such as a vertical synchronization signal has been used in an imaging device or the like. With this general synchronous solid-state image sensor, image data can be acquired only at every synchronization signal cycle (e.g., 1/60 second). Hence, it is difficult to deal with requests for higher-speed processing in fields such as traffic and robots. Against this background, an asynchronous solid-state image sensor has been proposed that detects a change in luminance as an address event in real time for each pixel address (see Patent Document 1, for example). Such a solid-state image sensor that detects an address event for each pixel is called a dynamic vision sensor (DVS). 
     CITATION LIST 
     Patent Document 
     
         
         Patent Document 1: Japanese Patent Application Laid-Open No. 2016-501495 
       
    
     SUMMARY OF THE INVENTION 
     Problems to be Solved by the Invention 
     The asynchronous solid-state image sensor (i.e., DVS) described above can generate and output data at a much higher speed than the synchronous solid-state image sensor. For this reason, in the traffic field, for example, processing of recognizing an image of a person or an obstacle can be executed at high speed, and safety can be improved. However, in the above-mentioned solid-state image sensor, it is difficult to control the detection sensitivity for an address event to an appropriate value. For example, if the detection sensitivity for the address event is too low, there is a possibility that the obstacle detection may fail in the image recognition. On the other hand, if the detection sensitivity for the address event is too high, the address event may be excessively detected when the luminance of all pixels changes due to a change in illumination or the like, and power consumption may increase. 
     The present technology has been made in view of such a situation, and an object of the present technology is to control the detection sensitivity for an address event to an appropriate value in a solid-state image sensor that detects an address event. 
     Solutions to Problems 
     The present technology has been made to solve the above-mentioned problems, and a first aspect thereof is a solid-state image sensor including a pixel array unit in which multiple pixel circuits are arranged, each pixel circuit detecting a change in luminance of incident light occurring outside a predetermined dead band as an address event, and a control unit that controls a width of the dead band according to the number of times the address event is detected in the pixel array unit within a fixed unit cycle, and a method of controlling the solid-state image sensor. This brings about the effect that an address event is detected outside a dead band having a width corresponding to the number of times of detection. 
     Additionally, in the first aspect, the control unit may widen the dead band as the number of times of detection increases. This brings about the effect that an address event is detected outside a wider dead band as the number of times of detection increases. 
     Additionally, in the first aspect, each of the multiple pixel circuits may compare each of the upper limit and the lower limit of the dead band with the amount of change in the luminance, and detect the address event on the basis of the comparison result. This brings about the effect that an address event is detected on the basis of the result of comparison between each of the upper limit and the lower limit of the dead band and the amount of change in the luminance. 
     Additionally, in the first aspect, the control unit may control the width of the dead band in a case where the number of times of detection is outside a predetermined allowable range. This brings about the effect that an address event is detected outside a dead band having a width corresponding to the number of times of detection outside the allowable range. 
     Additionally, in the first aspect, the pixel array unit may be divided into multiple areas, and the control unit may control the width of the dead band for each of the multiple areas. This brings about the effect that an address event is detected outside a dead band having a width controlled for each area. 
     Additionally, in the first aspect, each of the multiple pixel circuits may include a photoelectric conversion element that photoelectrically converts the incident light to generate a photocurrent, and a current-voltage conversion circuit that converts the photocurrent into a voltage. The photoelectric conversion element may be arranged on a light receiving chip, and the current-voltage conversion circuit may be arranged on a detection chip laminated on the light receiving chip. This brings about the effect that an address event is detected by the circuits arranged in a distributed manner on each of the light receiving chip and the detection chip. 
     Additionally, a second aspect of the present technology is an imaging device including: a pixel array unit in which multiple pixel circuits are arranged, each pixel circuit detecting a change in luminance of incident light occurring outside a predetermined dead band as an address event; a control unit that controls a width of the dead band according to the number of times the address event is detected in the pixel array unit within a fixed unit cycle; and a recording unit that records data obtained from a detection result of the address event. This brings about the effect that an address event is detected outside a dead band having a width corresponding to the number of times of detection, and data obtained from the detection result is recorded. 
     Effects of the Invention 
     According to the present technology, in a solid-state image sensor that detects an address event, an excellent effect that the detection sensitivity for an address event can be controlled to an appropriate value can be obtained. Note that the effect described herein is not necessarily limited, and the effect may be any of those described in the present disclosure. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG.  1    is a block diagram showing a configuration example of an imaging device of a first embodiment of the present technology. 
         FIG.  2    is a diagram showing an example of a laminated structure of a solid-state image sensor of the first embodiment of the present technology. 
         FIG.  3    is a block diagram showing a configuration example of the solid-state image sensor of the first embodiment of the present technology. 
         FIG.  4    is a block diagram showing a configuration example of a pixel circuit of the first embodiment of the present technology. 
         FIG.  5    is a circuit diagram showing a configuration example of a current-voltage conversion circuit of the first embodiment of the present technology. 
         FIG.  6    is a circuit diagram showing a configuration example of a buffer, a subtractor, and a quantizer of the first embodiment of the present technology. 
         FIG.  7    is a block diagram showing a configuration example of a signal processing unit of the first embodiment of the present technology. 
         FIG.  8    is graphs showing an example of changes in a voltage signal, a differential signal, and a detection signal before widening a dead band in the first embodiment of the present technology. 
         FIG.  9    is graphs showing an example of changes in the voltage signal, the differential signal, and the detection signal after widening the dead band in the first embodiment of the present technology. 
         FIG.  10    is a diagram showing an example of the number of times of detection before and after changing the dead band width in the first embodiment of the present technology. 
         FIG.  11    is a flowchart showing an example of the operation of the solid-state image sensor of the first embodiment of the present technology. 
         FIG.  12    is a block diagram showing a configuration example of a solid-state image sensor of a second embodiment of the present technology. 
         FIG.  13    is a diagram showing an example of information held in a memory in the second embodiment of the present technology. 
         FIG.  14    is a block diagram showing a schematic configuration example of a vehicle control system. 
         FIG.  15    is an explanatory diagram showing an example of an installation position of an imaging unit. 
     
    
    
     MODE FOR CARRYING OUT THE INVENTION 
     Hereinafter, modes for carrying out the present technology (hereinafter referred to as embodiments) will be described. The description will be given in the following order. 
     1. First embodiment (example of controlling dead band width according to the number of times of detections) 
     2. Second embodiment (example of controlling dead band width according to the number of times of detections for each area) 
     3. Example of application to movable body&gt; 
     1. First Embodiment 
     Configuration Example of Imaging Device 
       FIG.  1    is a block diagram showing a configuration example of an imaging device  100  of a first embodiment of the present technology. The imaging device  100  includes an imaging lens  110 , a solid-state image sensor  200 , a recording unit  120 , and an imaging control unit  130 . As the imaging device  100 , a camera mounted on an industrial robot, an in-car camera, or the like is assumed. 
     The imaging lens  110  collects incident light and guides it to the solid-state image sensor  200 . The solid-state image sensor  200  photoelectrically converts incident light to detect an address event, and executes predetermined processing such as object recognition on the basis of the detection result. The solid-state image sensor  200  supplies data indicating the execution result to the recording unit  120 . 
     The recording unit  120  records the data from the solid-state image sensor  200 . The imaging control unit  130  controls the solid-state image sensor  200  and causes the solid-state image sensor  200  to start the detection of an address event. 
     Configuration Example of Solid-State Image Sensor 
       FIG.  2    is a diagram showing an example of a laminated structure of the solid-state image sensor  200  of the first embodiment of the present technology. The solid-state image sensor  200  includes a detection chip  202  and a light receiving chip  201  laminated on the detection chip  202 . These chips are electrically connected through a connection part such as a via. Note that other than vias, Cu—Cu bonding or bumps can be used for connection. 
       FIG.  3    is a block diagram showing a configuration example of the solid-state image sensor  200  of the first embodiment of the present technology. The solid-state image sensor  200  includes a row drive circuit  211 , a bias voltage supply unit  212 , a pixel array unit  213 , a column drive circuit  214 , a signal processing unit  220 , and a memory  215 . 
     Additionally, in the pixel array unit  213 , multiple pixel circuits  300  are arranged in a two-dimensional lattice shape. Hereinafter, a set of pixel circuits  300  arranged in the horizontal direction is referred to as a “row”, and a set of pixel circuits  300  arranged in a direction perpendicular to the row is referred to as a “column”. 
     The pixel circuit  300  detects a change in luminance that occurs outside a predetermined dead band as an address event, and generates a detection signal indicating the detection result. Here, a dead band indicates a range of the amount of change in luminance where no address event is detected. A bias voltage Vbon indicating the upper limit of the dead band and a bias voltage Vboff indicating the lower limit of the dead band are supplied to each of the multiple pixel circuits  300 . 
     Additionally, an address event includes an on event and an off event. An on event is detected when the amount of change in luminance is larger than the upper limit (Vbon) of the dead band. On the other hand, an off event is detected when the amount of change in luminance is smaller than the lower limit (Vboff) of the dead band. A detection signal includes a 1-bit signal indicating the on-event detection result and a 1-bit signal indicating the off-event detection result. Note that while the pixel circuit  300  detects both an on event and an off event, the pixel circuit  300  may detect only one of them. 
     The row drive circuit  211  drives each of the rows to generate a detection signal. When the pixel circuit  300  in a driven row detects an address event, the pixel circuit  300  supplies a request for transmitting a detection signal to the column drive circuit  214 . 
     The column drive circuit  214  deals with each request of the column and returns a response on the basis of the dealt result. The pixel circuit  300  that has received the response supplies the detection signal to the signal processing unit  220 . 
     The signal processing unit  220  performs predetermined image processing such as image recognition on the detection signal. The signal processing unit  220  supplies data indicating the processing result to the recording unit  120 . 
     Additionally, for every fixed unit cycle, the signal processing unit  220  counts the number of times of detection, which is the number of times an address event is detected in the pixel array unit  213  within the cycle, and the number of times of detection is held in the memory  215 . In a case where both the on event and the off event exist, the number of times of detection is counted every time either the on event or the off event is detected. For example, in a case where an on event is detected in 10 pixels, an off event is detected in 15 pixels, and no address event detected in the remaining pixels within a unit cycle, the number of times of detection is 25. 
     Then, the signal processing unit  220  reads the number of times of detection from the memory  215 , and controls the difference between the bias voltages Vbon and Vboff (i.e., dead band width) by transmitting a control signal according to the number of times of detection. For example, the signal processing unit  220  widens the dead band as the number of times of detection increases. Here, the control signal is a signal for instructing the bias voltage supply unit  212  to increase or decrease each of the bias voltages Vbon and Vboff. Note that the signal processing unit  220  is an example of a control unit described in “CLAIMS”. 
     The bias voltage supply unit  212  generates the bias voltages Vbon and Vboff according to the control signal from the signal processing unit  220 , and supplies them to all the pixel circuits  300 . The memory  215  holds the number of times of detection and the upper and lower limits of the dead band. 
     Configuration Example of Pixel Circuit 
       FIG.  4    is a block diagram showing a configuration example of the pixel circuit  300  of the first embodiment of the present technology. The pixel circuit  300  includes a photoelectric conversion element  301 , a current-voltage conversion circuit  310 , a buffer  320 , a subtractor  330 , a quantizer  340 , and a transfer circuit  350 . 
     The photoelectric conversion element  301  photoelectrically converts incident light to generate an optical signal. The photoelectric conversion element  301  supplies the generated photocurrent to the current-voltage conversion circuit  310 . 
     The current-voltage conversion circuit  310  converts the photocurrent from the photoelectric conversion element  301  into a logarithmic voltage signal. The current-voltage conversion circuit  310  supplies the voltage signal to the buffer  320 . 
     The buffer  320  corrects the voltage signal from the current-voltage conversion circuit  310 . The buffer  320  outputs the corrected voltage signal to the subtractor  330 . 
     The subtractor  330  lowers the level of the voltage signal from the buffer  320  according to a row drive signal from the row drive circuit  211 . The subtractor  330  supplies the signal with lowered level to the quantizer  340  as a differential signal. 
     The quantizer  340  quantizes the differential signal from the subtractor  330  into a digital signal and outputs it as a detection signal to the transfer circuit  350 . 
     The transfer circuit  350  transfers the detection signal from the quantizer  340  to the signal processing unit  220 . The transfer circuit  350  supplies a request for transmitting a detection signal to the column drive circuit  214  when an address event is detected. Then, when the transfer circuit  350  receives a response to the request from the column drive circuit  214 , the transfer circuit  350  supplies the detection signal to the signal processing unit  220 . 
     Configuration Example of Current-Voltage Conversion Circuit 
       FIG.  5    is a circuit diagram showing a configuration example of the current-voltage conversion circuit  310  of the first embodiment of the present technology. The current-voltage conversion circuit  310  includes N-type transistors  311  and  313  and a P-type transistor  312 . Metal-oxide-semiconductor (MOS) transistors are used as these transistors, for example. 
     The N-type transistor  311  has a source connected to the photoelectric conversion element  301  and a drain connected to a power supply terminal. The P-type transistor  312  and the N-type transistor  313  are connected in series between the power supply terminal and the ground terminal. Additionally, the connection point of the P-type transistor  312  and the N-type transistor  313  is connected to the gate of the N-type transistor  311  and an input terminal of the buffer  320 . Additionally, a predetermined bias voltage Vbias is applied to the gate of the P-type transistor  312 . 
     The drains of the N-type transistors  311  and  313  are connected to the power supply side, and such a circuit is called a source follower. These two source followers connected in a loop convert the photocurrent from the photoelectric conversion element  301  into a logarithmic voltage signal. Additionally, the P-type transistor  312  supplies a constant current to the N-type transistor  313 . 
     Additionally, in each of the pixel circuits  300 , the photoelectric conversion element  301  is arranged on the light receiving chip  201 . On the other hand, circuits and elements other than the photoelectric conversion element  301  are arranged on the detection chip 
     Configuration Example of Buffer, Subtractor, and Quantizer 
       FIG.  6    is a circuit diagram showing a configuration example of the buffer  320 , the subtractor  330 , and the quantizer  340  of the first embodiment of the present technology. 
     The buffer  320  includes P-type transistors  321  and  322  connected in series between the power supply and the ground terminal. For example, MOS transistors are used as these transistors. The gate of the P-type transistor  322  on the ground side is connected to the current-voltage conversion circuit  310 , and a bias voltage Vbsf is applied to the gate of the P-type transistor  321  on the power supply side. Additionally, the connection point of the P-type transistors  321  and  322  is connected to the subtractor  330 . With this connection, impedance conversion is performed on the voltage signal from the current-voltage conversion circuit  310 . 
     The subtractor  330  includes capacitors  331  and  333 , P-type transistors  332  and  334 , and an N-type transistor  335 . For example, MOS transistors are used as these transistors. 
     One end of the capacitor  331  is connected to the buffer  320 , and the other end is connected to one end of the capacitor  333  and the gate of the P-type transistor  334 . The gate of the P-type transistor  332  receives input of a row drive signal from the row drive circuit  211 , and the source and drain of the P-type transistor  332  are connected to both ends of the capacitor  333 . The P-type transistor  334  and the N-type transistor  335  are connected in series between the power supply terminal and the ground terminal. Additionally, the other end of the capacitor  333  is connected to the connection point of the P-type transistor  334  and the N-type transistor  335 . A bias voltage Vba is applied to the gate of the N-type transistor  335  on the ground side, and the connection point of the P-type transistor  334  and the N-type transistor  335  is also connected to the quantizer  340 . With such a connection, a differential signal indicating the amount of change in luminance is generated and output to the quantizer  340 . 
     The quantizer  340  includes P-type transistors  341  and  343 , and N-type transistors  342  and  344 . For example, MOS transistors are used as these transistors. 
     The P-type transistor  341  and the N-type transistor  342  are connected in series between the power supply terminal and the ground terminal, and the P-type transistor  343  and the N-type transistor  344  are also connected in series between the power supply terminal and the ground terminal. Additionally, the gates of the P-type transistors  341  and  343  are connected to the subtractor  330 . The bias voltage Vbon is applied to the gate of the N-type transistor  342 , and the bias voltage Vboff is applied to the gate of the N-type transistor  344 . 
     The connection point of the P-type transistor  341  and the N-type transistor  342  is connected to the transfer circuit  350 , and the voltage at the connection point is output as a detection signal VCH. The connection point of the P-type transistor  343  and the N-type transistor  344  is also connected to the transfer circuit  350 , and the voltage at the connection point is output as a detection signal VCL. With such a connection, the quantizer  340  outputs the high-level detection signal VCH in a case where the differential signal exceeds the bias voltage Vbon, and outputs the low-level detection signal VCL in a case where the differential signal falls below the bias voltage Vboff. The detection signal VCH indicates the on-event detection result, and the detection signal VCL indicates the off-event detection result. 
     Note that while only the photoelectric conversion element  301  is arranged on the light receiving chip  201  and the other elements are arranged on the detection chip  202 , the circuit to be arranged on each chip is not limited to this configuration. For example, the photoelectric conversion element  301  and the N-type transistors  311  and  313  may be arranged on the light receiving chip  201 , and the others may be arranged on the detection chip  202 . Alternatively, the photoelectric conversion element  301  and the current-voltage conversion circuit  310  may be arranged on the light receiving chip  201 , and the others may be arranged on the detection chip  202 . Alternatively, the photoelectric conversion element  301 , the current-voltage conversion circuit  310 , and the buffer  320  may be arranged on the light receiving chip  201 , and the others may be arranged on the detection chip  202 . Alternatively, the photoelectric conversion element  301 , the current-voltage conversion circuit  310 , the buffer  320 , and the capacitor  331  may be arranged on the light receiving chip  201 , and the others may be arranged on the detection chip  202 . Alternatively, the photoelectric conversion element  301 , the current-voltage conversion circuit  310 , the buffer  320 , the subtractor  330 , and the quantizer  340  may be arranged on the light receiving chip  201 , and the others may be arranged on the detection chip  202 . 
     Configuration Example of Signal Processing Unit 
       FIG.  7    is a block diagram showing a configuration example of the signal processing unit  220  of the first embodiment of the present technology. The signal processing unit  220  includes an image processor  221 , a detection counter  222 , and a bias controller  223 . 
     The image processor  221  executes predetermined processing such as object recognition on the image data including the detection signal from the pixel array unit  213 . The image processor  221  supplies the execution result to the recording unit  120 . Note that the processing may be executed on the image data by a digital signal processor (DSP) or the like outside the solid-state image sensor  200 , instead of by the image processor  221 . 
     For every fixed unit cycle, the detection counter  222  counts the number of times an address event is detected by the pixel array unit  213  within the cycle. For every unit cycle, the detection counter  222  sets the number of times of detection in the memory  215  to an initial value at the start of the cycle. Then, each time an address event is detected, the detection counter  222  increments the number of times of detection and updates the number of times of detection with the incremented value. That is, the detection counter  222  counts up. Note that while the detection counter  222  counts up, it may count down instead. 
     The bias controller  223  controls the bias voltage according to the number of times of detection. The bias controller  223  sets the upper limit and the lower limit of the dead band in the memory  215  to the initial value when the imaging control unit  130  gives an instruction to start detection of an address event. Then, for every unit cycle, the bias controller  223  reads the number of times of detection from the memory  215  at the end of the cycle and determines whether or not the number of times of detection is a value within a predetermined allowable range. 
     In a case where the number of times of detection is outside the allowable range and is greater than the upper limit of the range, the bias controller  223  widens the dead band. For example, the bias controller  223  raises the upper limit of the dead band by a predetermined value, lowers the lower limit of the dead band by a predetermined value, and updates the memory  215  with the changed value. Additionally, the bias controller  223  controls the bias voltages Vbon and Vboff to values corresponding to the upper limit and the lower limit of the updated dead band by the control signal. 
     On the other hand, in a case where the number of times of detection is less than the lower limit of the allowable range, the bias controller  223  narrows the dead band. For example, the bias controller  223  lowers the upper limit of the dead band by a predetermined value, raises the lower limit of the dead band by a predetermined value, and updates the memory  215  with the changed value. Additionally, the bias controller  223  controls the bias voltages Vbon and Vboff to values corresponding to the upper limit and the lower limit of the updated dead band by the control signal. 
     Additionally, in a case where the number of times of detection is within the allowable range, the bias controller  223  does not control the width of the dead band and maintains the current value. 
     Note that while the bias controller  223  controls the width of the dead band only in a case where the number of times of detection is outside the allowable range, the dead band may be widened as the number of times of detection increases without providing the allowable range. 
     Additionally, while the bias controller  223  increases or decreases both the upper limit and the lower limit of the dead band, the width of the dead band can be controlled by increasing or decreasing only one of them. 
     Additionally, while the bias controller  223  does not limit the control amount of the dead band width, the dead band width can be controlled within a certain control range. For example, if the width of the dead band reaches the upper limit of the control range, the bias controller  223  does not widen the dead band any further even if the number of times of detection is greater than the upper limit of the allowable range. Additionally, if the width of the dead band reaches the lower limit of the control range, the bias controller  223  does not narrow the dead band any further even if the number of times of detection is less than the lower limit of the allowable range. 
       FIG.  8    is a graph showing an example of changes in a voltage signal, a differential signal, and a detection signal before widening the dead band in the first embodiment of the present technology. Here, a of  FIG.  8    is a graph showing an example of changes in a voltage signal of a pixel, and b of  FIG.  8    is a graph showing an example of changes in a differential signal of the pixel. Here, c of  FIG.  8    is a graph showing an example of changes in a detection signal of the pixel. In a of  FIG.  8   , the vertical axis indicates the level of the voltage signal from the current-voltage conversion circuit  310 , and the horizontal axis indicates time. In b of  FIG.  8   , the vertical axis indicates the level of the differential signal from the subtractor  330 , and the horizontal axis indicates time. In c of  FIG.  8   , the vertical axis indicates the level of the detection signal from the quantizer  340 , and the horizontal axis indicates time. In c of  FIG.  8   , an upward arrow indicates a detection signal when an on event is detected, and a downward arrow indicates a detection signal when an off event is detected. 
     When the luminance of light incident on a certain pixel changes, the voltage signal changes according to the change. Additionally, the differential signal indicating the amount of change in luminance also changes. Then, at timings TO and T 1 , the level of the differential signal falls below the lower limit of the dead band, for example. Additionally, at timings T 2 , T 3 , and T 4 , the level of the differential signal exceeds the upper limit of the dead band. Hence, an off event is detected at the timings TO and T 1 , and an on event is detected at the timings T 2 , T 3 , and T 4 . Additionally, in a case where the level of the differential signal is within the dead band, no address event is detected. 
     Here, it is assumed that the number of times of detection is greater than the upper limit of the allowable range, and the bias controller  223  widens the dead band. 
       FIG.  9    is a graph showing an example of changes in a voltage signal, a differential signal, and a detection signal after widening the dead band in the first embodiment of the present technology. Here, a of  FIG.  8    is a graph showing an example of changes in a voltage signal of a pixel, and b of  FIG.  8    is a graph showing an example of changes in a differential signal of the pixel. Here, c of  FIG.  8    is a graph showing an example of changes in a detection signal of the pixel. In a of  FIG.  9   , the vertical axis indicates the level of the voltage signal, and the horizontal axis indicates time. In b of  FIG.  9   , the vertical axis indicates the level of the differential signal, and the horizontal axis indicates time. In c of  FIG.  9   , the vertical axis indicates the level of the detection signal, and the horizontal axis indicates time. 
     It is assumed that after the dead band width is changed, changes in luminance similar to those as before the change occur. After the change, at the timing TO, the level of the differential signal falls below the lower limit of the dead band. Additionally, at the timings T 1  and T 2 , the level of the differential signal exceeds the upper limit of the dead band. Hence, an off event is detected at timing the TO, and an on event is detected at timings the T 1  and T 2 . As described above, since the detection sensitivity for an address event is lowered than that before widening the dead band, the number of times of detecting the address event becomes smaller. 
       FIG.  10    is a diagram showing an example of the number of times of detection before and after changing the dead band width in the first embodiment of the present technology. Here, a of  FIG.  10    is a histogram showing an example of the number of times of detection in each unit cycle before and after widening the dead band. Here, b of  FIG.  10    is a histogram showing an example of the number of times of detection in each unit cycle before and after narrowing the dead band. 
     In a case where the number of times of detection is greater than the upper limit of the allowable range, the bias controller  223  widens the dead band. As a result, the detection sensitivity for the address event is reduced, and the number of times of detection becomes smaller than that before changing the dead band width. 
     On the other hand, in a case where the number of times of detection is less than the lower limit of the allowable range, the bias controller  223  narrows the dead band. As a result, the detection sensitivity for the address event is increased, and the number of times of detection becomes greater than that before changing the dead band width. 
     As described above, since the bias controller  223  increases or decreases the width of the dead band according to the number of times of detection, the width of the dead band can be set to an appropriate range. 
     For example, assume a case where the brightness of the entire screen changes due to a change in illumination. In this case, since the brightness of all pixels changes, if the dead band is too narrow, an address event may be detected in all pixels. As the number of times of detecting the address event increases, the load of the circuit that transfers the detection signal and the circuit that processes the detection signal increases, which may increase the power consumption of the solid-state image sensor  200  as a whole. However, since the bias controller  223  widens the dead band as the number of times of detection increases, it is possible to curb excessive detection of address events and reduce power consumption. 
     Additionally, consider a case where a change in luminance occurs in some of all pixels and the change amount is small. In this case, if the dead band is too wide, there is a possibility that the address event cannot be detected in the pixel where the change has occurred, and the address event is missed. However, since the bias controller  223  narrows the dead band as the number of times of detection decreases, it is possible to prevent the address event from being missed. 
     Operation Example of Solid-State Image Sensor 
       FIG.  11    is a flowchart showing an example of the operation of the solid-state image sensor  200  of the first embodiment of the present technology. The operation is started when a predetermined application for detecting an address event is executed. 
     The signal processing unit  220  in the solid-state image sensor  200  initializes the upper and lower limits of the dead band and the number of times of detection (step S 901 ). Then, the signal processing unit  220  counts the number of times of detecting the address event (step S 902 ), and determines whether or not the unit cycle has passed (step S 903 ). If the unit cycle has not passed (step S 903 : No), the signal processing unit  220  repeats step S 902 . 
     On the other hand, if the unit cycle has passed (step S 903 : Yes), the signal processing unit  220  determines whether or not the number of times of detection is greater than the upper limit of the allowable range (step S 904 ). If the number of times of detection is greater than the upper limit of the allowable range (step S 904 : Yes), the signal processing unit  220  raises the dead band upper limit and lowers the dead band lower limit to widen the dead band (step S 905 ). 
     If the number of times of detection is equal to or less than the upper limit of the allowable range (step S 904 : No), the signal processing unit  220  determines whether or not the number of times of detection is less than the lower limit of the allowable range (step S 906 ). If the number of times of detection is less than the lower limit of the allowable range (step S 906 : Yes), the signal processing unit  220  lowers the dead band upper limit and raises the dead band lower limit to narrow the dead band (step S 907 ). 
     If the number of times of detection is a value within the allowable range (step S 906 : No), the signal processing unit  220  initializes the number of times of detection (step S 908 ), and repeats step S 902  and subsequent steps. Additionally, the signal processing unit  220  also executes step S 908  after step S 905  or S 907 . 
     As described above, according to the first embodiment of the present technology, since the signal processing unit  220  controls the width of the dead band according to the number of times of detecting the address event, the detection sensitivity for the address event can be controlled to an appropriate value. 
     2. Second Embodiment 
     In the above-described first embodiment, the signal processing unit  220  controls the bias voltage of all pixels to the same value. However, with this configuration, the detection sensitivity for the address event may be inappropriate for some scenes. For example, in a case where the brightness of a part of the pixel array unit  213  changes due to a change in illumination, address events are excessively detected in that part. A solid-state image sensor  200  of a second embodiment is different from the first embodiment in that a pixel array unit  213  is divided into multiple areas and the bias voltage is controlled for each area. 
       FIG.  12    is a block diagram showing a configuration example of the solid-state image sensor  200  of the second embodiment of the present technology. The solid-state image sensor  200  of the second embodiment is different from the first embodiment in that the pixel array unit  213  is divided into M (M is an integer of 2 or more) unit areas  305 . In each of the unit areas  305 , pixel circuits  300  of I rows×J columns (I and J are integers) are arranged. 
     Additionally, a signal processing unit  220  of the second embodiment counts the number of times of detection for each unit area  305 , and controls the bias voltage according to the number of times of detection. Additionally, a bias voltage supply unit  212  of the second embodiment supplies bias voltages Vbon 1  to VbonM and bias voltages Vboff 1  to VboffM. A bias voltage Vbonm and a bias voltage Vboffm (m is an integer from 1 to M) are supplied to the m-th unit area  305 . 
       FIG.  13    is a diagram showing an example of information held in a memory  215  in the second embodiment of the present technology. The memory  215  holds the number of times of detection, the dead band upper limit, and the dead band lower limit for each of the M unit areas  305 . 
     For example, assume that the number of times of detection in a unit cycle of an area whose area identification number for identifying the unit area  305  is “01” is “15”, and the number of times of detection is within the allowable range. Additionally, assume that the number of times of detection in a unit cycle of an area whose area identification number is “02” is “0”, and the number of times of detection is less than the lower limit of the allowable range. In this case, the signal processing unit  220  does not change the dead band upper limit and lower limit of the area whose area identification number is “01”. On the other hand, as for the area whose area identification number is “02”, the signal processing unit  220  lowers the dead band upper limit “U02” and raises the dead band lower limit “L02”. 
     As described above, according to the second embodiment of the present technology, since the signal processing unit  220  controls the dead band width according to the number of times of detecting the address event for each unit area, the detection sensitivity of each unit area can be controlled to an appropriate value. 
     3. Example of Application to Movable Body 
     The technology of the present disclosure (present technology) can be applied to various products. For example, the technology of the present disclosure may be implemented as a device mounted on any type of movable bodies including a car, an electric car, a hybrid electric car, a motorcycle, a bicycle, a personal mobility, an airplane, a drone, a ship, a robot, and the like. 
       FIG.  14    is a block diagram showing a schematic configuration example of a vehicle control system which is an example of a mobile control system to which the technology according to the present disclosure can be applied. 
     A vehicle control system  12000  includes multiple electronic control units connected through a communication network  12001 . In the example shown in  FIG.  14   , the vehicle control system  12000  includes a drive system control unit  12010 , a body system control unit  12020 , an outside information detection unit  12030 , an inside information detection unit  12040 , and an integrated control unit  12050 . Additionally, as a functional configuration of the integrated control unit  12050 , a microcomputer  12051 , an audio image output unit  12052 , and an in-car network interface (I/F)  12053  are illustrated. 
     The drive system control unit  12010  controls the operation of devices related to the drive system of the vehicle according to various programs. For example, the drive system control unit  12010  functions as a controller of devices including a drive force generation device for generating a drive force of a vehicle such as an internal combustion engine or a drive motor, a drive force transmission mechanism for transmitting the drive force to wheels, a steering mechanism that adjusts the steering angle of the vehicle, and a braking device that generates a braking force of the vehicle. 
     The body system control unit  12020  controls the operation of various devices equipped on the vehicle body according to various programs. For example, the body system control unit  12020  functions as a controller of a keyless entry system, a smart key system, a power window device, or a controller of various lamps such as a headlamp, a back lamp, a brake lamp, a blinker, or a fog lamp. In this case, the body system control unit  12020  may receive input of radio waves transmitted from a portable device substituting a key or signals of various switches. The body system control unit  12020  receives input of these radio waves or signals, and controls a door lock device, a power window device, a lamp, and the like of the vehicle. 
     The outside information detection unit  12030  detects information on the outside of the vehicle equipped with the vehicle control system  12000 . For example, an imaging unit  12031  is connected to the outside information detection unit  12030 . The outside information detection unit  12030  causes the imaging unit  12031  to capture an image of the outside of the vehicle, and receives the captured image. The outside information detection unit  12030  may perform object detection processing or distance detection processing of a person, a vehicle, an obstacle, a sign, characters on a road surface, or the like on the basis of the received image. 
     The imaging unit  12031  is an optical sensor that receives light and outputs an electrical signal corresponding to the amount of light received. The imaging unit  12031  can output an electric signal as an image or can output the electrical signal as distance measurement information. Additionally, the light received by the imaging unit  12031  may be visible light or non-visible light such as infrared light. 
     The inside information detection unit  12040  detects information on the inside of the vehicle. For example, a driver state detection unit  12041  that detects a state of a driver is connected to the inside information detection unit  12040 . The driver state detection unit  12041  includes a camera for capturing an image of the driver, for example, and the inside information detection unit  12040  may calculate the degree of fatigue or concentration of the driver or determine whether or not the driver is asleep, on the basis of the detection information input from the driver state detection unit  12041 . 
     The microcomputer  12051  can calculate a control target value of the drive force generation device, the steering mechanism, or the braking device on the basis of the information on the outside or the inside of the vehicle acquired by the outside information detection unit  12030  or the inside information detection unit  12040 , and output a control command to the drive system control unit  12010 . For example, the microcomputer  12051  can perform coordinated control aimed to achieve functions of an advanced driver assistance system (ADAS) including collision avoidance or shock mitigation of a vehicle, follow-up traveling based on an inter-vehicle distance, vehicle speed maintenance traveling, vehicle collision warning, vehicle lane departure warning, or the like. 
     Additionally, the microcomputer  12051  can control the drive force generation device, the steering mechanism, the braking device, or the like on the basis of the information around the vehicle acquired by the outside information detection unit  12030  or the inside information detection unit  12040 , to perform coordinated control aimed for automatic driving of traveling autonomously without depending on the driver&#39;s operation, or the like. 
     Additionally, the microcomputer  12051  can output a control command to the body system control unit  12020  on the basis of the information on the outside of the vehicle acquired by the outside information detection unit  12030 . For example, the microcomputer  12051  can control the headlamp according to the position of the preceding vehicle or oncoming vehicle detected by the outside information detection unit  12030 , and perform coordinated control aimed for glare prevention such as switching from high beam to low beam. 
     The audio image output unit  12052  transmits an output signal of at least one of audio or image to an output device capable of visually or aurally notifying a passenger or the outside of a vehicle of information. In the example of  FIG.  14   , an audio speaker  12061 , a display unit  12062 , and an instrument panel  12063  are illustrated as examples of the output device. The display unit  12062  may include at least one of an onboard display or a head-up display, for example. 
       FIG.  15    is a diagram showing an example of the installation position of the imaging unit  12031 . 
     In  FIG.  15   , imaging units  12101 ,  12102 ,  12103 ,  12104 , and  12105  are included as the imaging unit  12031 . 
     For example, the imaging units  12101 ,  12102 ,  12103 ,  12104 , and  12105  are provided in positions such as a front nose, a side mirror, a rear bumper, a back door, and an upper portion of a windshield in the vehicle interior of the vehicle  12100 . The imaging unit  12101  provided on the front nose and the imaging unit  12105  provided on the upper portion of the windshield in the vehicle interior mainly acquire images of the front of the vehicle  12100 . The imaging units  12102  and  12103  provided on the side mirrors mainly acquire images of the side of the vehicle  12100 . The imaging unit  12104  provided on the rear bumper or the back door mainly acquires an image of the rear of the vehicle  12100 . The imaging unit  12105  provided on the upper portion of the windshield in the vehicle interior is mainly used to detect a preceding vehicle or a pedestrian, an obstacle, a traffic light, a traffic sign, a lane, or the like. 
     Note that  FIG.  15    shows an example of imaging ranges of the imaging units  12101  to  12104 . An imaging range  12111  indicates the imaging range of the imaging unit  12101  provided on the front nose, imaging ranges  12112  and  12113  indicate the imaging ranges of the imaging units  12102  and  12103  provided on the respective side mirrors, and an imaging range  12114  indicates the imaging range of the imaging unit  12104  provided on the rear bumper or the back door. For example, by superimposing the pieces of image data captured by the imaging units  12101  to  12104 , a bird&#39;s eye view image of the vehicle  12100  as viewed from above can be obtained. 
     At least one of the imaging units  12101  to  12104  may have a function of acquiring distance information. For example, at least one of the imaging units  12101  to  12104  may be a stereo camera including multiple imaging elements, or may be an imaging element having pixels for phase difference detection. 
     For example, the microcomputer  12051  can measure the distance to each three-dimensional object in the imaging ranges  12111  to  12114  and the temporal change of this distance (relative velocity with respect to vehicle  12100 ) on the basis of the distance information obtained from the imaging units  12101  to  12104 , to extract, as a preceding vehicle, the closest three-dimensional object on the traveling path of the vehicle  12100  in particular, the three-dimensional object traveling at a predetermined speed (e.g., 0 km/h or more) in substantially the same direction as the vehicle  12100 . Moreover, the microcomputer  12051  can set an inter-vehicle distance to be secured in advance with respect to the preceding vehicle, and perform automatic brake control (including follow-up stop control), automatic acceleration control (including follow-up start control), and the like. As described above, it is possible to perform coordinated control aimed for automatic driving of traveling autonomously without depending on the driver&#39;s operation, or the like. 
     For example, on the basis of the distance information obtained from the imaging units  12101  to  12104 , the microcomputer  12051  can extract three-dimensional object data regarding three-dimensional objects by classifying the data into three-dimensional objects such as two-wheeled vehicle, ordinary vehicle, large vehicle, pedestrian, and telephone pole, and use the data for automatic avoidance of obstacles. For example, the microcomputer  12051  identifies obstacles around the vehicle  12100  into obstacles visible to the driver of the vehicle  12100  and obstacles hardly visible to the driver of the vehicle  12100 . Then, the microcomputer  12051  can determine the collision risk indicating the degree of risk of collision with each obstacle, and when the collision risk is a setting value or more and there is a possibility of a collision, can perform driving support for collision avoidance by outputting a warning to the driver through the audio speaker  12061  or the display unit  12062 , or by performing forcible deceleration or avoidance steering through the drive system control unit  12010 . 
     At least one of the imaging units  12101  to  12104  may be an infrared camera that detects infrared light. For example, the microcomputer  12051  can recognize a pedestrian by determining whether or not a pedestrian is present in the images captured by the imaging units  12101  to  12104 . Such pedestrian recognition is performed by a procedure of extracting feature points in images captured by the imaging units  12101  to  12104  as infrared cameras, and a procedure of performing pattern matching processing on a series of feature points indicating the outline of an object to determine whether or not the object is a pedestrian, for example. When the microcomputer  12051  determines that a pedestrian is present in the images captured by the imaging units  12101  to  12104  and recognizes the pedestrian, the audio image output unit  12052  causes the display unit  12062  to superimpose a square outline for emphasis on the recognized pedestrian. Additionally, the audio image output unit  12052  may cause the display unit  12062  to display an icon or the like indicating a pedestrian in a desired position. 
     Hereinabove, one example of the vehicle control system to which the technology of the present disclosure can be applied has been described. Of the above-described configuration, the technology according to the present disclosure is applicable to the imaging unit  12031 , for example. Specifically, the imaging device  100  of  FIG.  1    can be applied to the imaging unit  12031 . By applying the technology according to the present disclosure to the imaging unit  12031 , it is possible to control the detection sensitivity for the address event to an appropriate value and improve the reliability of the vehicle control system. 
     Note that the above-described embodiments are an example for embodying the present technology, and the matters in the embodiments and the matters specifying the invention in “CLAIMS” have a corresponding relationship. Similarly, the matters specifying the invention in “CLAIMS” and the matters having the same names in the embodiments of the present technology have a correspondence relationship. Note, however, that the present technology is not limited to the embodiments, and can be embodied by variously modifying the embodiments without departing from the gist of the present technology. 
     Additionally, the processing procedure described in the above embodiments may be regarded as a method including a series of these procedures, or may be regarded as a program for causing a computer to execute the series of procedures or a recording medium storing the program. As the recording medium, for example, a compact disc (CD), a minidisc (MD), a digital versatile disc (DVD), a memory card, a Blu-ray (registered trademark) disc, or the like can be used. 
     Note that the effect described in the present specification is merely an illustration and is not restrictive, and other effects can be obtained. 
     Note that the present technology can also be configured in the following manner. 
     (1) A solid-state image sensor including: 
     a pixel array unit in which multiple pixel circuits are arranged, each pixel circuit detecting a change in luminance of incident light occurring outside a predetermined dead band as an address event; and 
     a control unit that controls a width of the dead band according to the number of times the address event is detected in the pixel array unit within a fixed unit cycle. 
     (2) The solid-state image sensor according to (1) above, in which 
     the control unit widens the dead band as the number of times of detection increases. 
     (3) The solid-state image sensor according to (1) or (2) above, in which 
     each of the multiple pixel circuits compares each of an upper limit and a lower limit of the dead band with an amount of change in the luminance, and detects the address event on the basis of the comparison result. 
     (4) The solid-state image sensor according to any one of (1) to (3) above, in which 
     the control unit controls the width of the dead band in a case where the number of times of detection is outside a predetermined allowable range. 
     (5) The solid-state image sensor according to any one of (1) to (4) above, in which 
     the pixel array unit is divided into multiple areas, and 
     the control unit controls the width of the dead band for each of the multiple areas. 
     (6) The solid-state image sensor according to any one of (1) to (5) above, in which 
     each of the multiple pixel circuits includes 
     a photoelectric conversion element that photoelectrically converts the incident light to generate a photocurrent, and 
     a current-voltage conversion circuit that converts the photocurrent into a voltage, 
     the photoelectric conversion element is arranged on a light receiving chip, and 
     the current-voltage conversion circuit is arranged on a detection chip laminated on the light receiving chip. 
     (7) An imaging device including: 
     a pixel array unit in which multiple pixel circuits are arranged, each pixel circuit detecting a change in luminance of incident light occurring outside a predetermined dead band as an address event; 
     a control unit that controls a width of the dead band according to the number of times the address event is detected in the pixel array unit within a fixed unit cycle; and 
     a recording unit that records data obtained from a detection result of the address event. 
     (8) A method of controlling a solid-state image sensor, including: 
     a counting procedure of counting the number of times an address event is detected within a fixed unit cycle in a pixel array unit in which multiple pixel circuits are arranged, each pixel circuit detecting a change in luminance of incident light occurring outside a predetermined dead band as the address event; and 
     a control procedure of controlling a width of the dead band according to the number of times of detection. 
     REFERENCE SIGNS LIST 
     
         
           100  Imaging device 
           110  Imaging lens 
           120  Recording unit 
           130  Imaging control unit 
           200  Solid-state image sensor 
           201  Light receiving chip 
           202  Detection chip 
           211  Row drive circuit 
           212  Bias voltage supply unit 
           213  Pixel array unit 
           214  Column drive circuit 
           215  Memory 
           220  Signal processing unit 
           221  Image processor 
           222  Detection counter 
           223  Bias controller 
           300  Pixel circuit 
           301  Photoelectric conversion element 
           305  Unit area 
           310  Current-voltage conversion circuit 
           311 ,  313 ,  335 ,  342 ,  344 N-type transistor 
           312 ,  321 ,  322 ,  332 ,  334 ,  341 ,  343  P-type transistor 
           320  Buffer 
           330  Subtractor 
           331 ,  333  Capacitor 
           340  Quantizer 
           350  Transfer circuit 
           12031  Imaging unit