HK1222275B - Calibration circuitry and method for a time of flight imaging system - Google Patents

Abstract

The subject application relates to calibration circuitry and method for a time of flight imaging system. A time of flight imaging system includes a light source coupled to emit light pulses to an object in response a light source modulation signal generated in response to a reference modulation signal. Each pixel cell of a time of flight pixel cell array is coupled to sense light pulses reflected from the object in response a pixel modulation signal. A programmable pixel delay line circuit is coupled to generate the pixel modulation signal with a variable pixel delay programmed in response to a pixel programming signal. A control circuit is coupled to receive pixel information from the time of flight pixel array representative of the sensed reflected light pulses. The control circuit is coupled to vary the pixel programming signal during a calibration mode to synchronize the light pulses emitted from the light source with the pulses of the pixel modulation signal.

Description

Calibration circuit and method for time-of-flight imaging system

Technical Field

The present invention relates to an image sensor. In particular, embodiments of the present invention relate to three-dimensional image sensors.

Background

Interest in three-dimensional (3D) cameras has increased as 3D applications have become more prevalent in applications such as multimedia, gesture-based human-machine interfaces, automobiles, and the like. A typical passive way to create a 3D image is to use multiple cameras to capture a stereoscopic image or multiple images. Using the stereo image, objects in the image may be triangulated to create a 3D image. One disadvantage of this triangulation technique is that it is difficult to create 3D images using small devices because there must be a minimum separation distance between each camera in order to create a three-dimensional image. In addition, this technique is complex and therefore requires significant computer processing power in order to create 3D images in real time.

For applications requiring real-time acquisition of 3D images, active depth imaging systems based on optical time-of-flight measurements are sometimes utilized. Time-of-flight imaging systems typically employ a light source that directs light toward an object, a sensor that detects light reflected from the object, and a processing unit that calculates the distance to the object based on the round-trip time it takes for the light to travel to and return from the object. In a typical time-of-flight sensor, a photodiode is typically used because of the high conversion efficiency from the light detection region to the sensing node. Separate circuitry is coupled to the photodiode in each pixel cell to detect and measure the light reflected from the object.

Existing 3D Complementary Metal Oxide Semiconductor (CMOS) imagers typically use charge modulation with time-of-flight imaging systems to extract range information for a variety of imaged objects. To achieve high depth resolution, expensive laser trigger systems with steep rising/falling edges and extensive post-signal processing are typically utilized to compensate for delays inherent in electronic systems.

Disclosure of Invention

One embodiment of the invention relates to a time-of-flight imaging system. The system comprises: a light source coupled to emit light pulses to an object in response to respective pulses of a light source modulation signal, wherein the light source modulation signal is generated in response to a reference modulation signal; a time-of-flight pixel cell array including a plurality of pixel cells, wherein each of the plurality of pixel cells is coupled to sense a reflected light pulse reflected back from the object in response to a respective pulse of a pixel modulation signal; a programmable pixel delay line circuit coupled to generate the pixel modulation signal in response to the reference modulation signal, wherein the pixel modulation signal is coupled to have a variable pixel delay programmed in response to a pixel programming signal; and control circuitry coupled to receive pixel information from the time-of-flight pixel array representative of the reflected light pulses sensed by the time-of-flight pixel array, wherein the control circuitry is coupled to vary the pixel programming signal to synchronize the light pulses emitted from the light source with the pulses of the pixel modulation signal in response to the pixel information received from the time-of-flight pixel array during a calibration mode.

Another embodiment of the invention is directed to a method of calibrating a time-of-flight imaging system, the method comprising: initializing a pixel programming signal having N bits to an initial value, wherein the pixel programming signal includes a coarse resolution portion and a fine resolution portion; emitting light pulses from a light source to an object in response to a reference modulation signal; delaying the reference modulation signal in response to the pixel programming signal to generate a pixel modulation signal; sensing the light pulses reflected from the object with an array of time-of-flight pixel cells in response to the pixel modulation signals; in response to the light pulse sensed with the time-of-flight pixel cell array, performing a coarse resolution scan in a coarse range of the coarse resolution portion of the pixel programming signal to identify a maximum coarse resolution estimate of the coarse resolution portion of the pixel programming signal; setting the coarse resolution portion of the pixel programming signal equal to the maximum coarse resolution estimate; in response to the light pulse sensed with the time-of-flight pixel cell array, performing a fine resolution scan in a fine range of the fine resolution portion of the pixel programming signal to identify a maximum fine resolution estimate of the fine resolution portion of the pixel programming signal; and setting the fine resolution portion of the pixel programming signal equal to the maximum fine resolution estimate.

Drawings

Non-limiting and non-exhaustive embodiments of the present invention are described with reference to the following figures, wherein like reference numerals refer to like parts throughout the various views unless otherwise specified.

FIG. 1 is a block diagram showing an example of a time-of-flight imaging system including a control circuit having a calibration mode, according to the teachings of this disclosure.

Figure 2 is a block diagram illustrating another example of a time-of-flight imaging system including control circuitry in greater detail during calibration according to the teachings of this disclosure.

Figure 3 is a block diagram illustrating an example of a programmable delay circuit according to the teachings of this disclosure.

FIG. 4 is a timing diagram showing an example of a reference modulation signal, calibration light pulses, pixel modulation signals, and sensed charge during calibration of a time-of-flight image sensing system according to the teachings of this disclosure.

FIG. 5 is a flow chart illustrating an example of processing performed during calibration of an example time-of-flight imaging system according to teachings of this disclosure.

Corresponding reference characters indicate corresponding components throughout the several views of the drawings. Skilled artisans appreciate that elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For example, the dimensions of some of the elements in the figures may be exaggerated relative to other elements to help improve understanding of various embodiments of the present invention. Also, common but well-understood elements that are useful or necessary in a commercially feasible embodiment are often not depicted in order to facilitate a less obstructed view of these various embodiments of the present invention.

Detailed Description

Methods and circuits for calibrating time-of-flight pixel cells in a time-of-flight imaging system are disclosed. In the following description, numerous specific details are set forth to provide a thorough understanding of embodiments. One skilled in the relevant art will recognize, however, that the techniques described herein can be practiced without one or more of the specific details, or with other methods, components, materials, and so forth. In other instances, well-known structures, materials, or operations are not shown or described in detail to avoid obscuring aspects.

Reference throughout this specification to "one embodiment" or "an embodiment" means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the present invention. Thus, the appearances of the phrases "in one embodiment" or "in an embodiment" in various places throughout this specification are not necessarily all referring to the same embodiment. Furthermore, the particular features, structures, or characteristics may be combined in any suitable manner in one or more embodiments.

Throughout this description, several terms of art are used. These terms have the ordinary meaning in the art from which they originate unless explicitly defined herein or otherwise clearly indicated by the context of their use. For example, the term "or" is used in an inclusive sense (e.g., as in "and/or") unless the context clearly indicates otherwise.

As will be shown, examples of fast and accurate calibration circuits and methods are disclosed that provide low cost CMOS based 3D imaging systems, disclosing a light source such as an LED as an optical trigger source. Examples of the disclosed calibration circuits and techniques compensate for delays from the LEDs to the electronic shutter circuit in the overall system. In various examples, a time-of-flight imaging system includes a calibration architecture that utilizes an on-chip programmable delay line to perform flexible and very efficient calibration of delay differences between a light source and an image sensor. In various examples, the disclosed calibration circuits and techniques may achieve sub-nanosecond accuracy at the expense of minimal hardware cost and power consumption, in accordance with the teachings of this disclosure.

To illustrate, FIG. 1 is a block diagram showing an example of a time-of-flight imaging system 100 including a calibration circuit, according to the teachings of this disclosure. As shown, time-of-flight imaging system 100 includes a light source 102 that emits pulses of light, illustrated in fig. 1 as emitted light 104. As shown, the emitted light 104 is directed to an object 106. In an example, the emitted light 104 includes pulses of Infrared (IR) light. In accordance with the teachings of this disclosure, it should be appreciated that in other examples, the emitted light 104 may have a wavelength other than infrared, such as, for example, visible light, near infrared light, and the like. The emitted light 104 is then reflected back from the object 106, which is shown in FIG. 1 as reflected light 108. As shown, reflected light 108 is directed from object 106 through lens 110 and then focused onto time-of-flight pixel cell array 112. In an example, the time-of-flight pixel cell array 112 includes a plurality of time-of-flight pixel cells arranged in a two-dimensional array.

As shown in the depicted example, the light source 102 is coupled to emit light 104 in response to a light source modulation signal 114 received from a first delay circuit 118, the first delay circuit 118 coupled to a control circuit 116. The time-of-flight pixel cell array 112 is coupled to sense the reflected light 108 in response to a pixel modulation signal 122 received from a second delay circuit 120, the second delay circuit 120 also being coupled to the control circuit 116. As will be discussed in more detail below, the control circuit 116 includes a calibration mode that utilizes the first circuit 118 and the second delay circuit 120 to synchronize the light source modulation signal 114 and the pixel modulation signal 122 in accordance with the teachings of the present invention.

In the example depicted in FIG. 1, it should be noted that the time-of-flight pixel cell array 112 is positioned to the focal length f of the lens 110_lensTo (3). As shown in the example, the light source 102 and lens 110 are positioned at a distance L from the object. In one embodiment, it should be noted that the lens 110 may be implemented with a plurality of microlenses integrated into the time-of-flight pixel cell array 112.It should of course be understood that fig. 1 is not to scale and that in one example, the focal length f_lensSubstantially smaller than the distance L between the lens 110 and the object 106. Thus, it should be appreciated that for purposes of this disclosure, distance L and distance L + focal length f for purposes of time-of-flight measurement according to the teachings of this disclosure_lensAre substantially equal. Additionally, it should also be appreciated that for purposes of the present disclosure, both the distance between the light source 102 and the object 106 and the distance between the object 106 and the lens 110 are also substantially equal to L for purposes of time-of-flight measurements in accordance with the teachings of the present disclosure. Thus, the distance between the light source 102 and the object 106 (and/or the distance between the object 106 and the lens 110) is equal to half the round trip distance (e.g., D), so D is equal to 2 xL. In other words, assume that the distance L from the light source 102 to the object 106 plus the distance L from the object 106 back to the lens 110 is equal to the round trip distance D (or 2xL) according to the teachings of this disclosure.

In the depicted example, there is a delay time between the emission of a light pulse of the emitted light 104 and the receipt of the light pulse in the reflected light 108, caused by the amount of time it takes for the light pulse to travel the distance L from the light source 102 to the object 106, and then the additional time it takes for the corresponding reflected light pulse 108 to travel the distance L from the object 106 back to the pixel cell array 112. The delay time between the emitted light 104 and the reflected light 108 represents the time of flight for a light pulse to and from the light source 102 and the object 106. Once the time of flight (i.e., TOF) is known, the distance L from the light source 102 to the object 106 can be determined using the following relationships in equations 1 and 2 below:

where c is the speed of light, which is approximately equal to 3x 10⁸m/s and TOF for the light pulse traveling to the object as shown in FIG. 1The volume and the amount of time it takes to return from the object.

Figure 2 is a block diagram illustrating another more detailed example of a time-of-flight imaging system including control circuitry during calibration according to the teachings of this disclosure. In the example depicted in fig. 2, chip 240 includes control circuit 216 having a calibration mode that utilizes two programmable delay line circuits 218 and 220 to provide accurate synchronization between actual light pulses 204 emitted from light source 202 and charge modulated pulses in pixel modulation signal 222 that are used to control time-of-flight pixel array 212 to sense reflected light pulses 208 incident on time-of-flight pixel array 212 in accordance with the teachings of this disclosure.

In particular, as shown in the depicted example, during calibration of time-of-flight imaging system 200, calibration reflector 207 is positioned to the front of light source 202 and time-of-flight pixel cell array 212. As a result, light pulses 204 are reflected from calibration reflector 207 to time-of-flight pixel array 212 as reflected light pulses 208. In the illustrated example, calibration reflector 207 is positioned a relatively close distance (e.g., about 0.1 meters) from light source 202 and time-of-flight pixel cell array 212 during calibration. As a result, the optical signal incident on the reflected light pulse 208 of the time-of-flight pixel cell array 212 is relatively strong due to the close proximity to the collimating reflector 207.

In the example illustrated in fig. 2, time-of-flight pixel cell array 212 is a two-dimensional (2D) array of time-of-flight pixel cells (e.g., pixel cells P1, P2 … … Pn). As illustrated, each pixel cell P1, P2 … … Pn is arranged in rows (e.g., rows R1-Rx) and columns (e.g., columns C1-Cy) to sense the reflected light pulse 208 reflected back from the calibration reflector 207 in response to a respective pulse of the pixel modulation signal 222 generated by the programmable pixel delay line circuit 220.

As shown in the example, the programmable pixel delay line circuit 220 is coupled to generate the pixel modulation signal 222 by delaying the reference modulation signal 224 according to a pixel programming signal 228, the pixel programming signal 228 being generated by the control circuit 216. As discussed in more detail below, the programmable pixel delay line circuit 220 is coupled to provide a variable pixel delay that is programmed to synchronize the light pulses 204 emitted from the light source 202 with pulses in the pixel modulation signal 222 in response to a pixel programming signal 228, which are used to control the time-of-flight pixel array 212 to sense the reflected light pulses 208 in accordance with the teachings of the present invention. In an example, pixel control circuitry 232 is coupled to receive pixel modulation signal 222 to generate control signal 242, control signal 242 being coupled to control pixel cells included in time-of-flight pixel cell array 212 to sense reflected light pulses 208 in response to pulses of pixel modulation signal 222.

In the example depicted in fig. 2, the light source 202 is coupled to emit light pulses 204 to the calibration transmitter 207 in response to respective pulses of a light source modulation signal 214, the light source modulation signal 214 being coupled to be received from the programmable light source delay line circuit 218 through the driver circuit 230. As shown, the programmable light source delay line circuit 218 is coupled to generate the light source modulation signal 214 by delaying the reference modulation signal 224 according to a light source programming signal 226, the light source programming signal 226 being coupled to be received from the control circuit 216. In one example, the programmable light source delay line circuit 218 is capable of providing a variable pixel delay that is programmed in response to a light source programming signal 226.

In an example, control circuit 216 sets light source programming signal 226 to a fixed intermediate value and then changes pixel programming signal 228 to synchronize light pulses 204 emitted from light source 202 with pulses in pixel modulation signal 222, which are used to control time-of-flight pixel array 212 in accordance with the teachings of this disclosure. In particular, the control circuitry 116 is coupled to receive pixel information 238 from the time-of-flight pixel array 212, the pixel information 238 representing the reflected light pulse 208 observed by the time-of-flight pixel array 212.

In operation, the control circuit 216 is coupled to vary the pixel programming signal 228 in response to the pixel information 238 received from the time-of-flight pixel array 212 during the calibration mode to synchronize the light pulses 208 emitted from the light source 202 with the pulses of the pixel modulation signal 222, in accordance with the teachings of this disclosure.

In the depicted example, both the pixel programming signal 228 and the light source programming signal 226 are N-bit digital signals. For example, in one example, N-8, and the pixel programming signal 228 and the light source programming signal 226 are thus both 8-bit digital signals.

In the illustrated example, control circuitry 216 is coupled to receive pixel information 238 from time-of-flight pixel array 212 through readout circuitry 234. For example, in one example, pixel cells P1, P2 … … Pn in time-of-flight pixel cell array 212 are read out by readout circuitry 232 over bit lines. Sense circuit 234 may also include an amplifier to further amplify the signals received through the bit lines.

In an example, the pixel information 238 output by the readout circuitry 234 can be an M-bit digital signal. For example, in one example, M is 8, and the pixel information is thus a 10-bit digital signal generated by an analog-to-digital converter included in readout circuitry 234. In the depicted example, pixel information 238 read out of time-of-flight pixel array 212 represents the amount of charge Q optically generated in the pixel cell in response to reflected light pulses 208 sensed by time-of-flight pixel array 212 in accordance with the teachings of this disclosure.

In various examples, pixel information 238 may be read out after sensing the plurality of reflected light pulses 208 by time-of-flight pixel array 212. For example, in one example, 15 reflected light pulses 208 are sensed by time-of-flight pixel cell array 212 before pixel information 238 is read out each time by readout circuitry 234. By sensing multiple reflected light pulses 208 prior to each readout, additional charge Q is allowed to accumulate in the pixel cells of time-of-flight pixel cell array 212, which improves the signal-to-noise ratio of pixel information 238.

In various examples, the pixel information 238 read out of the time-of-flight pixel cell array 212 may be an average determined based on readings from two opposing edges of the time-of-flight pixel cell array 212. For example, in one example, the first row (e.g., row R1) and the last row (e.g., row Rx) are read out. The average charge for the first row (Q1 for example) is calculated and stored, and the average charge for the second row (Q2 for example) is calculated and stored. The purpose of this operation is to average out the top-to-bottom shift across the time-of-flight pixel cell array 212. The average of the readouts can then be determined by calculating the average of Q1 and Q2.

In one example, information read by sensing circuit 234 may also be subsequently passed to functional logic 238. In an example, function logic 238 may determine time-of-flight and distance information for each pixel cell. In an example, the functional logic 238 may also store time-of-flight information and/or even manipulate time-of-flight information (e.g., crop, rotate, adjust for background noise, or the like). In one example, the readout circuitry 234 may read out the entire row of time-of-flight information at once along the bit lines, or in another example, the time-of-flight information may be read out using a variety of other techniques (not illustrated), such as serial readout or readout of all pixel cells in full parallel at the same time.

In an example, the control circuit 216 is further coupled to the time-of-flight pixel cell array 212 to control operation of the time-of-flight pixel cell array 212 and synchronize operation of the time-of-flight pixel cell array 212 with the light source 202, as discussed above. It should be noted that in other examples, control circuit 216 may set pixel programming signal 228 to a fixed intermediate value and then change light source programming signal 226, or change both pixel programming signal 228 and light source programming signal 226 to synchronize light pulses 204 emitted from light source 202 with charge modulation pulses in pixel modulation signal 222 used to control time-of-flight pixel array 212, in accordance with the teachings of this disclosure.

Figure 3 is a block diagram illustrating an example of a programmable delay circuit 320 according to the teachings of this disclosure. It should be appreciated that the programmable delay circuit 320 of FIG. 3 may be the first delay circuit of FIG. 1118 and/or the second delay circuit 120, and/or an example of the programmable light source delay line circuit 218 and/or the programmable pixel delay line circuit 220 of fig. 2, and elements referenced below having similar names and numbers are thus coupled and function similarly to those described above. As shown in the described example, the programmable delay circuit 320 of FIG. 3 is coupled to receive the reference modulation signal 324 and to respond to the programming signal 328 by varying the delay t the reference modulation signal 324_dA delay is performed to produce a delayed output signal 322.

In an example, programming signal 328 is an n-bit digital signal coupled to provide a variable delay t during a calibration mode_d. In an example, the calibration pattern may be divided into a plurality of portions, including, for example, a first portion and a second portion. For example, during the first portion, a coarse adjustment may be made to the programming signal 328 to rapidly scan a large coarse range of delay values to rapidly synchronize the light pulses 204 emitted from the light source 202 with the pulses of the pixel modulation signal 222 in accordance with the teachings of this disclosure.

In an example, the Most Significant Bit (MSB)344 of the pass programming signal 328 is scanned during a first portion of calibration (e.g., a coarse portion) in accordance with the teachings of this disclosure. Thus, in the example where the programming signal 328 is an 8-bit digital signal, the most significant byte (i.e., the 4 high bits) is incremented in the coarse range as the calibrated coarse portion is scanned. For example, in one example, MSB 344 of programming signal 328 is incremented from binary value 0000 to binary value 1010 when scanning a calibrated coarse portion in accordance with the teachings of this disclosure. In the example, the lower bytes (i.e., the 4 lower bits) are not changed during the coarse scan.

During a second portion (e.g., a fine portion) of the calibration, fine adjustments are made to the programming signal 328 to closely scan a fine range of delay values at a higher resolution to more accurately synchronize the light pulses 204 emitted from the light source 202 with the pulses of the pixel modulation signal 222 in accordance with the teachings of this disclosure.

In an example, the Least Significant Bit (LSB)346 of the pass programming signal 328 is scanned during a fine portion of calibration in accordance with the teachings of this disclosure. Thus, in the example where the programming signal 328 is an 8-bit digital signal, the least significant byte (i.e., the 4 lower bits) is incremented in the fine range when scanning the fine portion through calibration. For example, in one example, the LSB 346 of the programming signal 328 is incremented from a binary value of 0000 to a binary value of 1010 when scanning the calibrated fine portion in accordance with the teachings of this disclosure. In the example, the upper byte (i.e., the 4 upper bits) is not changed during the fine scan.

According to the teachings of this disclosure, by scanning through multiple passes in the coarse and fine range values of the programming signal 328 during the coarse and fine portions of the calibration, the large range values can be scanned quickly and still provide fine resolution synchronization of the light pulses 204 emitted from the light source 202 with the pulses of the pixel modulation signal 222 during calibration.

In an example, programmable delay circuit 320 is implemented with multiple delay stages, including, for example, a delay stage as illustrated in fig. 3₁350A to delay stage_z350Z. In an example, decoder circuit 348 is coupled to receive N-bit programming signal 328 and to control each of a plurality of stages, including delay stages, in response to programming signal 328 in accordance with the teachings of this disclosure₁350A to delay stage_z350Z. In one example, a delay stage according to the teachings of this disclosure₁350A are coupled to respond to a pixel programming signal 328 versus a variable delay t_dPerforming a coarser adjustment, and a delay stage_z350Z are coupled to respond to a pixel programming signal 328 versus a variable delay t_dA finer adjustment is performed. In one example, a delay stage₁350A may provide a coarser variable delay t of about 10ns or more_dAnd a delay stage_z350Z may provide a finer variable delay t of about 0.1ns_d. In one example, a delay stage₁350A may be implemented with cascaded flip circuits or the like, and delay stages_z350Z may be implemented with cascaded inverter circuits or the like. In one example, according to the teachings of this disclosureIt should be appreciated that additional interstage delay circuits, not shown in fig. 3, may also be included, and may be implemented, for example, with cascaded inverter circuits having capacitive outputs or the like, which may provide a variable delay t of about 1ns_d。

To illustrate, FIG. 4 is a diagram showing a reference modulation signal 424, a light pulse 404 emitted during a scan operation of a calibration of a time-of-flight image sensing system, with a variable delay t, according to the teachings of this disclosure_dAnd an example of charge 438 sensed during calibration of the time-of-flight image sensing system. It should be appreciated that reference modulation signal 424 of fig. 4 may be an example of reference modulation signal 224 of fig. 2 and/or reference modulation signal 324 of fig. 3, light pulse 404 may be an example of light pulse 204 of fig. 2, pixel modulation signal 422 of fig. 4 may be an example of pixel modulation signal 222 of fig. 2 and/or pixel modulation signal 322 of fig. 3, and sensed charge 438 may represent pixel information 238 read out of time-of-flight pixel cell array 212 of fig. 2, and elements referenced below having similar names and numbering may therefore couple and function similarly to that described above. It should be appreciated that sensed charge 438, which represents pixel information 238 read out of the time-of-flight pixel array, represents the amount of charge Q optically generated in the pixel cells included in time-of-flight pixel cell array 212 in response to reflected light pulse 208 sensed by time-of-flight pixel cell array 212.

As shown in the depicted example, during a calibrated scanning operation of a time-of-flight imaging system, the first pulse 452 occurs at time t₀When the pixel modulation signal 422 has t_d1Resulting in a sensed charge 438 reading Q₁. During the second pulse 454, the pixel modulation signal 422 has t_d2Resulting in a sensed charge 438 reading Q₂. During the third pulse 456, the pixel modulation signal 422 has t_d3Resulting in a sensed charge 438 reading Q₃. During the fourth pulse 458, the pixel modulation signal 422 has t_d4Resulting in sensedCharge 438 reading Q₄. During the fifth pulse 460, the pixel modulation signal 422 has t_d5Resulting in a sensed charge 438 reading Q₅。

As illustrated in the depicted example, peak charge Q₄Is read out during a fourth pulse 458. As a result, the programming signal 228 is calibrated to equal the delay setting of the pixel modulation signal 422 at the pulse 458, which has t in the pixel modulation signal 422_d4The delay of (2). In other words, it may be determined that the value of the pixel programming signal 228 during the fourth pulse 458 is the value when the calibration light pulse 404 is most synchronized with the pixel modulation signal 422 compared to the other pulses in accordance with the teachings of the present invention. Thus, the value of the pixel programming signal 228 during the fourth pulse 458 is then saved as a result of a calibration operation of the time-of-flight sensor in accordance with the teachings of this disclosure.

FIG. 5 is a flow chart 562 illustrating an example of processing performed during calibration of an example time-of-flight imaging system according to teachings of this disclosure. Process block 564 shows that the light source programming signal and the pixel programming signal are initialized. In one example, the light source programming signal is initialized to set the delay of the light source modulation signal at an intermediate value. For example, in an example where the delay of the light source programming signal may be set to a value in the range of 0ns to 10ns, the light source programming signal is initialized to a value of, for example, 5 ns. In one example, the pixel programming signal is initialized to set the delay of the pixel modulation signal at one end of the delay range of the pixel programming signal. For example, in an example where the delay of the pixel programming signal may be set to a value in the range of 0ns to 10ns, the pixel programming signal is initialized to a value of, for example, 0 ns.

Processing block 566 shows delaying the reference modulation signal in response to the light source programming signal to generate the light source modulation signal. Accordingly, process block 568 shows emitting a light pulse from the light source in response to the light source modulation signal.

Processing block 570 shows delaying the reference modulation signal to generate the pixel modulation signal also in response to the pixel programming signal. Processing block 572 shows that the light pulses reflected from the calibration reflector are sensed by the time-of-flight pixel cell array in response to the pixel modulation signal.

Processing block 574 shows that in response to the sensed light pulse, the delay in the pixel modulation signal is then changed to perform a coarse resolution scan in a coarse range of a coarse resolution portion of the pixel programming signal to identify a maximum coarse resolution estimate. For example, if the pixel programming signal is an 8-bit digital signal, the most significant byte of the programming signal may be changed from binary 0000 to binary 1010 to scan in a coarse range of the coarse resolution portion of the pixel programming signal. In an example, the time-of-flight sensor may be sampled multiple times (e.g., 15 times) for each delay setting. Then, each average charge reading for each delay value may be stored and then compared to identify the delay value that results in the peak amount of charge observed to find the largest coarse resolution estimate. In one example, the average charge reading may be determined by reading out and storing charge values of the first and last rows of the time-of-flight pixel cell array to average out the dispersion of offsets across the time-of-flight pixel cell array. Processing block 576 shows that the coarse resolution portion (e.g., most significant 4 bits) of the pixel programming signal is then set equal to the maximum coarse resolution estimate corresponding to the maximum charge sensed by the time-of-flight pixel cell array in processing block 574 during the coarse calibration scan.

Process block 578 shows that in response to the sensed light pulse, the delay in the pixel modulation signal is then varied to perform a fine resolution scan in a fine range of a fine resolution portion of the pixel programming signal to identify a maximum fine resolution estimate. For example, continuing with the example pixel programming signal as an 8-bit digital signal, the least significant byte of the programming signal may be changed from binary 0000 to binary 1010 to scan in a fine range of the fine resolution portion of the pixel programming signal. In an example, the time-of-flight sensor may be sampled multiple times (e.g., 15 times) for each delay setting. Then, each average charge reading for each delay value can be compared to find the maximum fine resolution estimate. In an example, the average charge reading may also be determined by reading out and storing the charge values of the first and last rows of the time-of-flight pixel cell array to average out the dispersion of offsets across the time-of-flight pixel cell array. Processing block 580 shows that the fine resolution portion (e.g., the least significant 4 bits) of the pixel programming signal is then set equal to the maximum fine resolution estimate, which corresponds to the highest charge sensed by the time-of-flight pixel cell array in processing block 578 during the fine calibration scan.

Thus, according to the teachings of this disclosure, the coarse and fine resolution portions of the pixel programming signal are calibrated to the value at which the maximum charge can be observed during the coarse and fine calibration scans. According to the teachings of this disclosure, time-of-flight imaging systems may now enjoy improved efficiency and high depth resolution of the sensed object as the pixel programming signal is synchronized with the light source modulation signal.

The above description of illustrated embodiments of the invention, including what is described in the abstract, is not intended to be exhaustive or to limit the invention to the precise forms disclosed. While specific embodiments of, and examples for, the invention are described herein for illustrative purposes, various modifications are possible within the scope of the invention, as those skilled in the relevant art will recognize.

These modifications can be made to the invention in light of the above detailed description. The terms used in the following claims should not be construed to limit the invention to the specific embodiments disclosed in the specification. Rather, the scope of the invention is to be determined entirely by the following claims, which are to be construed in accordance with established doctrines of claim interpretation.

Claims (20)

1. A time-of-flight imaging system, comprising:

a light source coupled to emit light pulses to an object in response to respective pulses of a light source modulation signal, wherein the light source modulation signal is generated in response to a reference modulation signal;

a time-of-flight pixel cell array including a plurality of pixel cells, wherein each of the plurality of pixel cells is coupled to sense a reflected light pulse reflected back from the object in response to a respective pulse of a pixel modulation signal;

a programmable pixel delay line circuit coupled to generate the pixel modulation signal in response to the reference modulation signal, wherein the pixel modulation signal is coupled to have a variable pixel delay programmed in response to a pixel programming signal; and

control circuitry coupled to receive pixel information from the reflected light pulses sensed by the time-of-flight pixel array represented by the time-of-flight pixel array, wherein the control circuitry is coupled to vary the pixel programming signal during a calibration mode in response to pixel information received from the time-of-flight pixel array to synchronize the light pulses emitted from the light source with the pulses of the pixel modulation signal, wherein the pixel programming signal is an n-bit signal, wherein a most significant bit of the n-bit signal is varied during a first portion of the calibration mode, and wherein a least significant bit of the n-bit signal is varied during a second portion of the calibration mode.

2. The time of flight imaging system of claim 1, further comprising a programmable light source delay line circuit coupled to generate the light source modulation signal in response to the reference modulation signal, wherein the light source modulation signal is coupled to have a variable light source delay programmed in response to a light source programming signal.

3. The time of flight imaging system of claim 2, wherein the control circuit is coupled to set the light source programming signal.

4. The time-of-flight imaging system of claim 1, wherein changes to the most significant bits of the n-bit signal are coupled to perform coarse adjustments to the pixel programming signal, and wherein changes to the least significant bits of the n-bit signal are coupled to perform fine adjustments to the pixel programming signal.

5. The time-of-flight imaging system of claim 1, wherein the programmable pixel delay line circuit includes a plurality of stages, wherein a first stage of the plurality of stages is coupled to perform a coarse adjustment on the variable pixel delay in response to the pixel programming signal, and wherein a second stage of the plurality of stages is coupled to perform a fine adjustment on the variable pixel delay in response to the pixel programming signal.

6. The time-of-flight imaging system of claim 1, wherein the pixel information read out of the time-of-flight pixel array represents an amount of charge optically generated in response to the reflected light pulses sensed by the time-of-flight pixel array.

7. The time of flight imaging system of claim 1, further comprising readout circuitry coupled to the time of flight pixel cell array to readout pixel information from the pixel cells of the time of flight pixel cell array.

8. The time of flight imaging system of claim 1, wherein the object is a calibration reflector positioned in front of the light source and sensor to reflect the light pulses emitted from the light source back to the array of time of flight pixel cells during the calibration mode.

9. The time-of-flight imaging system of claim 1, further comprising a light source driver circuit coupled to drive the light source in response to pulses of the light source modulation signal.

10. The time of flight imaging system of claim 1, further comprising a pixel control circuit coupled to generate a control signal coupled to control the array of time of flight pixel cells to sense the reflected light pulses in response to a pulse of the pixel modulation signal.

11. A method of calibrating a time-of-flight imaging system, comprising:

initializing a pixel programming signal having N bits to an initial value, wherein the pixel programming signal includes a coarse resolution portion and a fine resolution portion;

emitting light pulses from a light source to an object in response to a reference modulation signal;

delaying the reference modulation signal in response to the pixel programming signal to generate a pixel modulation signal;

sensing the light pulses reflected from the object with an array of time-of-flight pixel cells in response to the pixel modulation signals;

in response to the light pulse sensed with the time-of-flight pixel cell array, performing a coarse resolution scan in a coarse range of the coarse resolution portion of the pixel programming signal to identify a maximum coarse resolution estimate of the coarse resolution portion of the pixel programming signal;

setting the coarse resolution portion of the pixel programming signal equal to the maximum coarse resolution estimate;

in response to the light pulse sensed with the time-of-flight pixel cell array, performing a fine resolution scan in a fine range of the fine resolution portion of the pixel programming signal to identify a maximum fine resolution estimate of the fine resolution portion of the pixel programming signal; and

setting the fine resolution portion of the pixel programming signal equal to the maximum fine resolution estimate.

12. The method of claim 11, further comprising delaying the reference modulation signal in response to a light source programming signal to generate a light source modulation signal, wherein the light pulse is emitted to an object in response to the light source modulation signal.

13. The method of claim 12, further comprising initializing the light source programming signal to an initial value, wherein the light source programming signal has N bits and includes a coarse resolution portion and a fine resolution portion.

14. The method of claim 11, wherein sensing the light pulse reflected from the object with the array of time-of-flight pixel cells in response to the pixel modulation signal comprises:

reading out pixel information from a first edge of the array of time-of-flight pixel cells and from a second edge of the array of time-of-flight pixel cells representing the reflected light pulses sensed at the first and second edges of the array of time-of-flight pixel cells, respectively; and

averaging the pixel information read out from the first and second edges of the time-of-flight pixel cell array.

15. The method of claim 14, wherein the array of time-of-flight pixel cells is arranged in a plurality of rows and columns of pixel cells, wherein the first and second edges of the array of time-of-flight pixel cells include a first and last row of the array of time-of-flight pixel cells.

16. The method of claim 14, wherein the sensing the light pulse reflected from the object with the array of time-of-flight pixel cells comprises sensing a plurality of light pulses reflected from the object with the array of time-of-flight pixel cells before each readout of pixel information from the first edge and the second edge of the array of time-of-flight pixel cells.

17. The method of claim 11, wherein the coarse resolution portion of the programming signal includes most significant bits of the pixel programming signal, and wherein the fine resolution portion of the programming signal includes least significant bits of the pixel programming signal.

18. The method of claim 17, wherein the performing the coarse resolution scan in the coarse range of the coarse resolution portion of the pixel programming signal comprises increasing the most significant bit from a minimum value of the coarse range to a maximum value of the coarse range.

19. The method of claim 17, wherein the performing the fine resolution scan in the fine range of the fine resolution portion of the pixel programming signal comprises increasing the least significant bit from a minimum value of the fine range to a maximum value of the fine range.

20. The method of claim 11, further comprising positioning a calibration reflector in front of the light source to reflect the light pulses from the light source to the array of time-of-flight pixel cells during calibration of the time-of-flight imaging system.