KR20090097416A - Pixel of cmos sensor for cancelling ambient light and method thereof - Google Patents

Abstract

A pixel of a CMOS sensor for removing the ambient-light and an operation method thereof are provided to be effectively used in a place in which the change of brightness is serious. A pixel of a CMOS sensor(400) includes a light sensing unit(410), an input transistor(M3), first and second capacitors(C0, C1), a amplifier(-A), and first and second feedback transistors(M6, M5). The light sensing unit outputs the light as an electric signal. The input transistor receives the outputted signal. The first capacitor is connected to the input transistor and accumulates electric charges generated due to the ambient light. In order to maintain the electric charge accumulated in the first capacitor, the amplifier provides a virtual ground node in which current does not flow.

Description

Pixel of CMOS sensor for canceling ambient light and its operation method

The present invention relates to a CMOS sensor, and more particularly to a pixel structure of a CMOS sensor used in a system for measuring distance using structured light.

Mobile robots, such as sweeping robots and guide robots, find out their own location and create a surrounding map. In particular, creating a two-dimensional or three-dimensional distance map of the surroundings is an essential element for the robot to generate a movement path and avoid unexpected obstacles during the movement.

To this end, various methods using a visual sensor, an ultrasonic sensor, or a touch sensor are applied. A method using a structured light and a camera is an effective method that can be used even in a place where a small amount of calculation and a change in brightness are severe. Known.

The present invention provides a pixel of a CMOS sensor for accurate distance measurement by removing the ambient light caused by sunlight when measuring the distance using the structured light.

According to an aspect of the present invention, a pixel includes a light sensing unit for outputting light as an electrical signal; An input transistor for receiving the output signal; A first capacitor coupled to the input transistor and accumulating charge generated by ambient light; An amplifier providing a virtual ground node through which no current flows to maintain charge accumulated in the first capacitor; A first feedback transistor that generates a first feedback path between the output node of the amplifier and the virtual ground node when the laser light source is off; A second feedback transistor for generating a second feedback path between the output node of the amplifier and the virtual ground node when the laser light source is on; And a second capacitor positioned in the second feedback path, the second capacitor accumulating charge corresponding to the difference voltage between the voltage delivered to the input transistor when the laser light source is off and the voltage delivered to the input transistor when the laser light source is on. do.

According to another aspect of the present invention, a method of operating a pixel of a CMOS sensor includes: performing a reset operation of emptying charges accumulated in a second capacitor; When the laser light source is off, turning on the input transistor and the first feedback transistor and accumulating charges generated by the ambient light transmitted through the input transistor in the first capacitor; When the laser light source is on, the input transistor and the second feedback transistor are turned on, and the charge corresponding to the difference voltage between the voltage delivered to the input transistor when the laser light source is off and the voltage delivered to the input transistor when the laser light source is on. Is accumulated in the second capacitor, and as the number of on / off times of the laser light source increases, charges corresponding to the differential voltage are accumulated in the second capacitor in proportion to the number of on / off times.

Hereinafter, with reference to the accompanying drawings will be described in detail an embodiment of the present invention. In the following description of the present invention, if it is determined that detailed descriptions of related well-known functions or configurations may unnecessarily obscure the subject matter of the present invention, the detailed description thereof will be omitted. In addition, terms to be described below are terms defined in consideration of functions in the present invention, which may vary according to intention or custom of a user or an operator. Therefore, the definition should be made based on the contents throughout the specification.

1A to 1C are diagrams illustrating a principle of distance measurement using structured light.

Light is emitted to the obstacle 30 by using an active light source 10 such as a laser, and image information reflected by the obstacle 30 is obtained by using the camera sensor 20. In this case, the camera sensor 20 is positioned on the light source 10 while maintaining a constant distance d from the light source 10 to obtain image information. The light source 10 may use a near infrared line laser beam. The near-infrared line laser beam can be used to acquire image information even in the absence of illumination.

Referring to FIG. 1B, the laser light is emitted from the light source 10 onto the obstacle 30 in the form of a plane while having a constant viewing angle α.

1C illustrates a camera image 40 in the form of a line profile obtained by the camera sensor 20. In FIG. 1B, light reflected from points a and b of the obstacle 30 is represented as a and b of the camera image 40, respectively, and a value in the Y-axis direction is a distance between the camera sensor 20 and the obstacle 30. Is proportional to

The distance between the camera sensor 20 and the obstacle 30 obtained from the coordinates of the camera image 40 and the angle θ of the camera sensor 20 toward the obstacle 30, the camera sensor 20 and the light source 10. By the distance d between the distance data between the light source 10 and the obstacle 30 can be obtained by the triangular method (triangular method). Since the triangulation method is a known technique, detailed description thereof will be omitted.

2 is a diagram illustrating a camera image and distance data when ambient light is included.

As described with reference to FIG. 1, when the distance is measured using the structured light, the line laser may not be accurately detected as shown in FIG. 2 when there is high ambient light, for example, sunlight. The present invention proposes a CMOS sensor to which the difference image integration principle as shown in FIG. 3 is applied to solve such a problem.

3 is a diagram illustrating a principle of differential image integration applied to a CMOS sensor according to an exemplary embodiment of the present invention.

When the laser light source is on, the image signal 310 by the ambient light and the laser light source is obtained. In addition, when the laser light source is off, an image signal 312 is obtained by only ambient light. In the difference block 314, the difference image signal 316 between the image signal 310 and the image signal 312 is obtained. The differential image signal 316 is an image generated by a laser. When the ambient light is large in illumination such as sunlight, fine images are obtained compared to the ambient light.

Therefore, the above process is repeated by turning the laser light source on and off again. That is, the image signal 320 by the laser light source and the ambient light when the laser light source is on and the image signal 322 only by the ambient light when the laser light source is off are obtained, and the image signal 320 by the difference block 324. ) And the image signal 322 are differentially generated to generate a differential image signal 326.

When the on-off operation of the laser light source is repeatedly performed N times for a short time, the cumulative operation 350 is performed on the differential image signals such as the differential images 316 and 326 to generate an integrated integral integral image signal 360. do. At this time, the noise signal generated in the circuit is also integrated. When the difference between the differential integrated image signal 360 and the integrated noise signal becomes large enough to be detected by a winner exclusive circuit or an analog-to-digital converter, the differential integrated image signal 360 may be used as an image signal by a laser light source.

4 is a circuit diagram illustrating a pixel structure of a CMOS sensor for removing ambient light according to an exemplary embodiment of the present invention.

4 shows a circuit corresponding to one pixel of a CMOS sensor. The CMOS sensor 400 includes a light detector 410 and a difference image signal integrator 420.

The photo detector 410 includes a photodiode PD and three transistors M0, M1, and M2. The photodetector 410 receives a photodiode PD to receive light to generate photocharge, a reset transistor M0 to discharge photocharge generated to initialize, and a source to amplify and convert the generated photocharge into a voltage signal. It may have a three-transistor structure including a follower transistor M1 and a row select transistor M2 that selectively outputs a voltage signal.

The photo detector 410 may be configured of various types of conventional photo sensors as long as the photo detector PD has a function of outputting an amount of charge received by the photodiode PD as a voltage.

The reset voltage (V _R) and a reset signal (RXn) photoelectric downward from a signal transmitted from the photodiode (PD) for when the off state of when in the photo-sensing unit 410, a reset signal (RXn) of the transistor (MO) on Output the voltage V _SIG .

In FIG. 4, the voltage V _PIXOUT output from the light detector 410 indicates that the reset voltage V _R and the signal voltage V _SIG are output to the correlated double sampling circuit 415. The voltage V _CDSOUT represents the difference voltage between the reset voltage V _R and the signal voltage V _SIG . As such, the double sampling method for measuring the received voltage is known as a method for removing reset noise, fixed pattern noise (FPN), and the like.

As described with reference to FIG. 3, the difference image signal integrating unit 420 is a difference between an image signal by ambient light when the laser light source is turned off and an image signal by ambient light and laser light source when the laser light source is turned on. By accumulating the image signal, an operation for detecting the image signal by the laser light source is performed.

The difference image signal integrating unit 420 includes an input transistor M3, a first capacitor CO, an amplifier (-A), a first feedback transistor M6, a second feedback transistor M5, and a second capacitor C1. And an output transistor M7.

The input transistor M3 receives the voltage signal output from the light detector 410. In more detail, the voltage detection unit 410 receives the input voltage signal via the CDS circuit. A transistor M4 serving as a dummy switch may be selectively added and connected to the input transistor M3 in order to remove noise caused by clock feedthrough generated when the transistor is turned on and off.

The first capacitor C0 is connected to the input transistor M3 and accumulates charges generated by ambient light when the laser light source is turned off.

The amplifier (-A) provides a virtual ground node through which no current flows to maintain charge by ambient light accumulated in the first capacitor CO. The amplifier (-A) is not limited to its configuration as long as it is an amplifier operated by an inverter having one input and one output. According to an embodiment of the present invention, the amplifier (-A) may be configured as an inverter of the cascode form as shown in FIG. In the following, it is assumed that the amplifier (-A) is composed of an inverter.

When the inverter (-A) is connected to the input node (ⓐ) and the output node (ⓑ) by the first feedback transistor M6 or the second feedback transistor M5 as described below, the input path is generated, The node ⓐ is provided with a virtual ground node through which no current flows, and takes an offset voltage V _{amp _ offset of the} inverter.

When the laser light source is off, the first feedback transistor M6 becomes the gate input PHP3 high to generate a feedback path between the output ⓑ of the amplifier and the virtual ground node ⓐ. The first feedback transistor M6 may be turned on at the same time as the second feedback transistor M5 so that an initial reset operation for emptying the charge accumulated in the second capacitor may be performed.

The second feedback transistor M5 has the gate input PHP2 high when the laser light source is turned on, thereby generating a feedback path between the output ⓑ of the inverter and the virtual ground node ⓐ.

The second capacitor C1 is positioned in the feedback path generated when the laser light source is turned on. The second capacitor C1 is the voltage transferred to the first capacitor C0 by the input transistor M3 when the laser light source is off, and the voltage transferred to the input transistor M3 when the laser light source is turned on. Accumulate charge corresponding to the differential voltage.

As the number of integrations increases while the laser light source is turned on and off, the second capacitor C1 may have the ambient light voltage V _amb and the laser light source delivered to the input transistor when the laser light source is turned off in proportion to the on-off frequency. When the charge corresponding to the difference voltage between the ambient light transferred to the input transistor and the voltage V _{amb _ laser} by the laser light source may be accumulated. Every time the difference voltage is accumulated, a noise signal generated by a circuit constituting the pixel is integrated together with the second capacitor C1.

The output transistor M7 outputs the accumulated charge to the second capacitor C1 whenever the on-off frequency of the laser light source reaches a predetermined number of times. The predetermined number of times is the number of times the laser light source is turned on and off when the difference between the amount of charge by the laser light source and the amount of charge due to the noise signal generated by the circuit reaches a level detectable by the winner monopoly circuit or the analog-to-digital converter accumulated in the second capacitor. Can be determined. For example, the predetermined number may be about 50 times.

On the other hand, the inverter offset voltage (V _{amp _ offset} ) formed at the input node of the amplifier in the pixel circuit of FIG. 4 is a little for each pixel when the pixel circuit of FIG. 4 is arranged in an n × n matrix and implemented as a CMOS sensor chip. It will have different values. Therefore, according to one embodiment of the present invention, the voltage signal output from the output transistor M7 is offset voltage removal circuit for removing the inverter offset voltage (V _{amp _ offset} ) generated at the input node of the amplifier by the feedback path The inverter offset voltage V _{amp _ offset} may be removed through (not shown). In this way, the output voltage by the laser light source can be accurately measured by shifting the inverter offset voltage V _{amp _ offset} different from each pixel to the reference voltage.

5 is a circuit diagram illustrating an amplifier configuration of FIG. 4 according to an embodiment of the present invention.

According to an embodiment of the present invention, the configuration of the amplifier (-A) may be configured as an inverter of the cascode amplifier type as shown in FIG.

By configuring the amplifier circuit with an inverter circuit as shown in FIG. 5, a virtual ground node in which a current having an inverter offset voltage V _{amp _ offset} does not flow when forming a feedback path between an input and an output may be provided.

As a result, the number of transistors can be reduced compared to conventional amplifiers using an OP amplifier, and a voltage line for providing a reference voltage (V _ref ) to an input terminal of the OP amplifier, that is, a metal line on a pixel, is removed. can do. Thus, the pixel size can be reduced.

6 is a timing diagram illustrating operations of a transistor and a laser light source of the CMOS sensor illustrated in FIG. 4.

6 shows the gate input RXn of the reset transistor M0, the gate input ROWSELn of the row select transistor M2, the laser light source Laser, the gate input PHP1n of the input transistor M3, and the first feedback transistor. The operation timings of the gate input PHP3n of M6, the gate input PHP2n of the second feedback transistor M5, and the gate input PHP4n of the output transistor M7 are shown.

The operation of the CMOS sensor according to the exemplary embodiment of the present invention will be described in detail below with reference to FIGS. 6, 7, and 8.

FIG. 7 is a circuit diagram showing an operating state of the circuit of FIG. 4 when the laser light source is turned off, and showing the operation timing. FIG.

Fig. 7 shows a circuit operation state when the laser light source is off and only ambient light is present. When the laser light source is turned off, only the gate input PHP1 of the input transistor M3 and the gate input PHP3 of the first feedback transistor M6 are turned on.

Then, due to the operation of the first feedback transistor M6, the input terminal ⓐ and the output terminal ⓑ of the inverter -A are connected to the input offset ⓐ of the inverter -A by generating a feedback path. V _{amp _ offset} ), and an input terminal ⓐ creates a virtual ground where no current flows. The charge generated by the ambient light is accumulated in the first capacitor CO.

Therefore, the capacitor CO is charged to a voltage of V _amb -V _{amp _ offset} . This may be expressed as Equation 1 below.

[Equation 1]

V _CO = V _amb -V _{amp _ offset}

FIG. 8 is a diagram showing a circuit diagram and an operation timing showing an operating state of the circuit of FIG. 4 when the laser light source is turned on.

8 shows a circuit operation when the laser is turned on and both the ambient light and the laser signal are present at the same time. When the laser light source is turned on, only the gate input PHP1 of the input transistor M3 and the gate input PHP2 of the second feedback transistor M5 are turned on.

As the second feedback transistor M5 is turned on, the inverter offset voltage V _{amp _ offset} is maintained at the input terminal ⓐ of the inverter -A, and the input terminal ⓐ is a virtual ground where no current flows. ground) is maintained.

If only PHP1 and PHP2 are turned on, the charge (Q _INT ) transferred to the feedback path may be represented by Equation 2 below.

[Equation 2]

Q _INT = C0 * (V _{amb + laser} -V _{amp _ offset} -V _CO )

Since V _CO = V _amb -V _{amp _ offset} ,

Thus, Q _INT =-CO * (V _amb -V _{amb + laser} )

Therefore, V _C1 can be expressed as follows.

[Equation 3]

V _C1 = C0 / C1 * (V _amb - V _{amb + laser} )

If CO = C1, V _C1 = V _amb -V _{amb + laser}

Therefore, the second capacitor C1 receives a voltage by the laser light source. That is, only the PHP1 and the PHP2 are turned on to charge the second capacitor C1 with the voltage V _amb -V _{amb + laser} .

When the operation illustrated in FIG. 7 is performed again, the voltage V _CO of the first capacitor becomes V _amb -V _{amp_offset} . When performing the operation illustrated in FIG. 8, the voltage V _C1 of the second capacitor C1 is performed. ) _Has the value 2 * (V _amb -V _{amb + laser} ).

If the above operation is repeated N times, the output voltage can be expressed by the following equation.

[Equation 4]

V _out = V _{amp _ offset} + N (V _{amb -} V _{amb + laser} )

That is, the difference video signal of (V _amb -V _{amb + laser} ) is amplified by N times.

9 is a diagram illustrating an output voltage according to on and off of a laser light source.

FIG. 9 shows that as the laser light source is repeatedly turned on and off, the voltage caused by the laser light source, that is, the difference voltage between the ambient light voltage and the voltage when the laser light source is on increases in proportion to the on-off repetition frequency (N).

10 is a circuit diagram illustrating a pixel structure of a CMOS sensor for removing ambient light according to another exemplary embodiment of the present invention.

The circuit of FIG. 10 is the same except for the configuration of the light detector 1010. That is, the configuration of the difference image signal integrator 1020 is the same as that of the difference image signal integrator 420 of FIG. 4. Compared to the circuit structure shown in FIG. 4, the circuit of FIG. 10 is a structure in which a global shutter is applied to simultaneously start and finish light exposure time of all pixels.

As illustrated in FIG. 10, the photo detector 1010 may include a photodiode PD, a reset transistor M10, a source follower transistor M11, and a bias transistor M12 that applies a bias voltage to the source follower transistor. It may be configured to include.

In this configuration, the voltage output from the light sensing unit 1010 is not affected by noise such as fixed pattern noise due to the dispersion of the threshold voltage of the source follower transistor M11 without passing through the CDS circuit (not shown). It may be transmitted to the difference image signal integrating unit 1020. The reason is that when the voltage caused by the ambient light is charged in the first capacitor CO when the laser is off, the threshold voltage of the source follower transistor M11 is charged together, and the laser light source and the ambient light when the laser is on. It is removed together when finding the difference with the voltage. Therefore, since the exposure time can be reduced by the configuration as shown in FIG. 10, the circuit can be operated in high ambient light.

11 is a flowchart illustrating a method of operating pixels of a CMOS sensor according to an exemplary embodiment of the present disclosure.

When the difference image integration operation is started, the first feedback transistor M6 and the second feedback transistor M5 are turned on to perform a reset operation of emptying the charge accumulated in the second capacitor.

The laser light source is turned off to obtain an ambient light (sunlight) image signal from the light detector 410 (S 1110). Specifically, when the laser light source is off, the input transistor M3 and the first feedback transistor M6 are turned on and the charge generated by the ambient light transmitted through the input transistor M3 is transferred to the first capacitor C0. Accumulate).

By turning on the laser light source, the optical sensor 410 acquires an image signal by the ambient light and the laser light source (S 1120), and the voltage V _amb transmitted by the ambient light and the voltage transmitted when the laser light source is on. The difference image voltage of (V _{amb + laser} ) is stored in the second capacitor C1 (S 1130).

Specifically, when the laser light source is on, the input transistor M3 and the second feedback transistor M5 are turned on, and when the laser light source is off, the voltage V _amb transmitted to the input transistor and the input transistor when the laser light source is on. The charge corresponding to the difference voltage of the voltage (V _{amb + laser} ) transferred to is accumulated in the second capacitor C1.

It is determined whether the on-off frequency n of the laser light source reaches a predetermined number N (S 1140). If the predetermined number N has not been reached, the number of on-offs n is increased (S1150) to repeat the operations of steps S1110 to S1130. As the on-off frequency of the laser light source increases, charges corresponding to the difference image voltage may accumulate in the second capacitor C1.

Whenever the on-off frequency of the laser light source reaches a predetermined number of times, the output transistor M7 is turned on, and the pixel value is read by outputting the charge accumulated in the second capacitor (S 1160).

The voltage signal output from the output transistor may be input to the offset voltage removing circuit for removing the offset voltage generated at the input node of the amplifier by the feedback path.

According to one embodiment of the present invention, even in the presence of ambient light such as high-light sunlight, it is possible to accurately measure the distance by the structured light. Therefore, when the CMOS sensor is applied to the mobile robot according to an embodiment of the present invention, the mobile robot can improve performance of map construction, obstacle avoidance, intruder detection, and the like.

The above description is only one embodiment of the present invention, and those skilled in the art to which the present invention pertains may implement it in a modified form without departing from the essential characteristics of the present invention. Therefore, the scope of the present invention should not be limited to the above-described examples, but should be construed to include various embodiments within the scope equivalent to those described in the claims.

1A to 1C are diagrams illustrating a distance measuring principle using structured light;

2 is a diagram illustrating a camera image and distance data when ambient light is included;

3 is a diagram illustrating a principle of difference image integration applied to a CMOS sensor according to an exemplary embodiment of the present invention.

4 is a circuit diagram illustrating a pixel structure of a CMOS sensor for removing ambient light according to an exemplary embodiment of the present invention.

5 is a circuit diagram illustrating an amplifier configuration of FIG. 4 according to an embodiment of the present invention.

FIG. 6 is a diagram illustrating an operation timing of a transistor and a laser light source of the CMOS sensor illustrated in FIG. 4.

FIG. 7 is a diagram showing a circuit diagram and an operation timing showing an operating state of the circuit of FIG. 4 when the laser light source is turned off;

8 is a diagram showing a circuit diagram and an operation timing showing an operating state of the circuit of FIG. 4 when the laser light source is turned on,

9 is a view showing an output voltage according to the on and off of the laser light source,

10 is a circuit diagram illustrating a pixel structure of a CMOS sensor for removing ambient light according to another exemplary embodiment of the present invention.

11 is a flowchart illustrating a method of operating pixels of a CMOS sensor according to an exemplary embodiment of the present disclosure.

Claims (10)

A light detector for outputting light as an electrical signal; An input transistor for receiving the output signal; A first capacitor connected to the input transistor and accumulating charge generated by ambient light; An amplifier providing a virtual ground node through which no current flows to maintain charge accumulated in the first capacitor; A first feedback transistor for generating a first feedback path between an output node of the amplifier and the virtual ground node when a laser light source is turned off; A second feedback transistor that generates a second feedback path between an output node of the amplifier and the virtual ground node when the laser light source is turned on; And Positioned in the second feedback path and accumulating charge corresponding to a differential voltage between a voltage delivered to the input transistor when the laser light source is off and a voltage delivered to the input transistor when the laser light source is on; A pixel of a CMOS sensor, comprising two capacitors. The method of claim 1, And a charge corresponding to the difference voltage is accumulated in the second capacitor in proportion to the on / off frequency of the laser light source as the on / off frequency of the laser light source increases. The method of claim 1, The light detector A photodiode that receives light to generate photocharges; A reset transistor for discharging the generated photo charges; The source follower transistor for amplifying and converting the optical charge into a voltage signal; And And a bias transistor configured to apply a bias voltage to the source follower transistor for operation of a global shutter method. The method of claim 1, And the amplifier comprises an inverter circuit providing a virtual ground node through which no current flows when a feedback path is generated between an input and an output. The method of claim 1, And turning on the first feedback transistor and the second feedback transistor at the same time to perform an initial reset operation to empty the charge accumulated in the second capacitor. The method of claim 1, And an output transistor for outputting the charge accumulated in the second capacitor whenever the on-off frequency of the laser light source reaches a predetermined number of times. The method of claim 6, The electric charge output from the output transistor is input to an offset voltage removing circuit for removing an offset voltage generated at an input node of the amplifier. A pixel operation method of a CMOS sensor according to claim 1, Performing a reset operation to empty the charge accumulated in the second capacitor; When the laser light source is off, turning on the input transistor and the first feedback transistor and accumulating charges generated by ambient light transmitted through the input transistor in the first capacitor; The input transistor and the second feedback transistor are turned on when the laser light source is on, the voltage delivered to the input transistor when the laser light source is off, and the voltage delivered to the input transistor when the laser light source is on. Accumulating charge in the second capacitor corresponding to the differential voltage with And the charge corresponding to the difference voltage accumulates in the second capacitor in proportion to the on-off frequency as the on-off frequency of the laser light source increases. The method of claim 8, And outputting the charge accumulated in the second capacitor whenever the on-off frequency of the laser light source reaches a predetermined number of times. The method of claim 8, And the charge output from the output transistor is input to an offset voltage removing circuit for removing an offset voltage generated at an input node of the amplifier by the feedback path.

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