Phase angle correction value calculation device and method of calculating a phase angle correction value

Abstract

The invention relates to a phase angle correction value calculation device and a method of calculating a phase angle correction value. A phase angle error calculation device (100) comprises a light source (102) which emits rays according to an indirect time-of-flight (iToF) measurement technique. The photonic mixer unit (104) generates and stores a plurality of electrical output signals corresponding to a plurality of predetermined phase offset values. A signal processing circuit processes the electrical output signals according to the iToF measurement technique in order to calculate therefrom a reference vector and a reference phase angle. The output signals correspond to measurements made at a first level of accuracy. The circuit processes a subset of the output signals according to the iToF measurement technique and calculates therefrom a measurement vector and a measurement phase angle. The subset of the output signals correspond to measurements made at a second level of accuracy which is lower than the first level of accuracy. The circuit calculates a phase angle correction value using the reference phase angle and the measurement phase angle.

Description

TECHNICAL FIELD

[0001] The present invention relates to a phase angle correction value calculation device of the type configured to emit rays and process received reflected light, for example according to an indirect time-of-flight measurement technique. The present invention also relates to a method of calculating phase angle correction values, for example of the type employing an indirect time-of-flight measurement technique. BACKGROUND

[0002] In so-called time-of-flight sensing systems and other systems, such as game console vision systems, it is known to employ an illumination source to illuminate the surrounding environment (sometimes referred to as a "scene") within the field of view of the illumination source, and to process the rays reflected by features of the scene. Such so-called LiDAR (Light Detection and Ranging) systems use an illumination source to illuminate the scene with rays, and a detection device (e.g. an array of photodiodes, some optical elements and a processing unit) to detect the rays reflected from objects in the scene. The rays reflected from objects in the scene are received by the detection device and converted into electrical signals, which are then processed by the processing unit by applying a time-of-flight (ToF) calculation in order to determine the distance of the objects from the detection device. Although different kinds of LiDAR systems are known based on different operating principles, such systems are fundamentally scene-illuminating and reflected light-detecting.

[0003] In this regard, so-called "flash LiDAR" techniques, which are direct ToF ranging techniques, employ a light source that emits pulses of rays that are subsequently reflected by features of the scene and detected by a detector device. In such techniques, the measured time of the round trip to and from a reflected feature is used to calculate the distance to the reflected feature, directly using the pulse of rays. The pulses of rays incident on the detector device are sampled at a very high sampling rate in the time domain. Therefore, the signal path in the processing circuitry implementing such techniques requires high bandwidth of the signal and large silicon "real estate", i.e. such implementations require a relatively large area on the silicon wafer, which in turn limits the number of channels that can be supported on the integrated circuit. Therefore, the actual spatial number of channels that can be supported by such flash LiDAR sensors is typically lower than 100. To overcome this limitation, mechanical scanning systems are implemented that require moving parts.

[0004] Another known LiDAR system employs a so-called "indirect time-of-flight" (iToF) ranging technique. An iToF system emits a continuous wave light signal and reflections of this continuous wave light signal are received by a detector device and analyzed. A plurality of samples (e.g. four samples) of the rays reflected from features of a scene are acquired, each sample being phase stepped by e.g. 90°. Using this illumination and sampling method, the phase angle between the illumination and the reflection can be determined and the determined phase angle can be used to determine the distance to the reflecting feature of the scene.

[0005] In iToF systems, high frequency signal processing (demodulation) occurs at pixel level and thus the post-pixel signal bandwidth required for integrating a large number of pixels on the same chip is low. As a result, iToF systems can support a larger number of channels and thus a higher spatial resolution measurement compared to direct ToF systems. However, iToF systems have a limited distance measurement capability. In this regard, to achieve low random distance measurement errors, iToF systems require a high modulation frequency which in turn reduces the range of distances that can be unambiguously measured. For example, a 100 MHz modulation frequency results in an approximate unambiguous measurement range of 1.5 m. Furthermore, conventional iToF systems are susceptible to errors due to multiple reflections and multiple propagation paths.

[0006] As explained above, iToF systems sample with respect to different applied phases. A typical iToF system comprises a buffer that stores analog signals with respect to m phases for subsequent signal processing generated by a so-called photonic mixer device. A discrete Fourier transform unit computes the fundamental frequency of the complex signal stored by the buffer in terms of in-phase and quadrature components of the signal. Using the values of the in-phase and quadrature components, the phase angle and amplitude of the complex signal can be computed and the phase angle information can be used to solve for the distance to the object.

[0007] However, such amplitude modulated continuous wave type systems suffer from an inherent depth measurement error due to aliasing of the emitted ray signal with the reference signal correlation function. This is due to the presence of high order harmonics in both the emitted ray signal and the reference signal, which are not accounted for in the modeling or measurement. This error is referred to as wiggling error or cyclic error (see "Modeling "wiggling" as a multi-path interference problem in AMCW ToF imaging", Feigin et al., OSA Optics Express, 2015).

[0008] To avoid or at least mitigate the error, it is known to employ phenomenological methods to compensate for the cyclic error. In this regard, B-spline based or lookup table based correction methods are known, e.g. as described in "Time-of-Flight sensor calibration for accurate range sensing" (Lindner et al., Computer Vision and Image Understanding, 114 (2010), pp. 1318-1328), for compensating for the aggregation of error phenomena. However, such techniques are sensitive to changes in system properties, e.g. temperature and frequency used for modulating the optical illumination signal emitted by the light source and demodulating the optical signal received by the photonic mixer device.

[0009] For some implementations, the application of a large number of phases reduces the cyclic error of a given phase angle measurement to a negligible level, in particular when the number of phase offset values employed is odd. However, a significant drawback of this approach is the amount of time required by the photonic mixer device to generate a large number of electrical output signals. In this regard, since the outcome of the scene is dynamic, the more time spent, the greater the chance that the phase angle measurement will be affected by so-called motion artifacts. Therefore, from a practical point of view, it is advantageous to employ a smaller number of phase offset values than the relatively high number of phase offset values required to minimize the cyclic error and to use a model predicting the amplitude of the cyclic error as suggested above to compensate for the cyclic error when measuring the phase angle.

[0010] Generally, the common approach for reducing the cyclic error is deployed in a one-off manner during a calibration phase at the end of the manufacturing process of the iToF system. During calibration, an object is placed in front of the iToF camera and is repositioned from the minimum measurable distance to the maximum measurable distance to the iToF camera in several steps. At each repositioning step, the distance to the object is measured using a relatively low number of phases, e.g. four phase offset values, employed during normal operation of the iToF system, which measured distance is recorded together with the known ideal distance to the object. After the measurement has been made with respect to each repositioning step, a curve is constructed and the cyclic error of each phase angle measurement can be estimated. The estimated cyclic error can then be recorded in a lookup table and used for cyclic error compensation in real-time operation of the iToF system. However, such calibration techniques are time-consuming as it requires physical object repositioning.

[0011] U.S. Patent No. 7,936,449 describes an improved calibration procedure for cycle error compensation, during which a subject is statically positioned at a known distance, rather than being repositioned to a plurality of different distances from an iToF camera, a measurement is performed on a set of known phase delays introduced into each distance measurement. This method reduces the time required for calibration, but requires a highly accurate delay line to be provided that introduces the known phase delays, which increases the production cost of the silicon die of the iToF system and / or the cost and time of calibration. Moreover, since this calibration is also only performed once at the end of the production cycle, it does not correct for drift in the cycle error over time due to aging or temperature changes in the iToF system. SUMMARY

[0012] According to a first aspect of the application, there is provided a phase angle error calculation apparatus for an optical distance measuring system, the apparatus comprising: a light source configured to emit a ray in accordance with an indirect time-of-flight measurement technique; a photon mixer unit configured to generate and store a plurality of electrical output signals corresponding respectively to a plurality of predetermined phase shift values applied during a measurement period in accordance with the indirect time-of-flight measurement technique; a signal processing circuit configured to process the plurality of electrical output signals in accordance with the indirect time-of-flight measurement technique in order to calculate a reference vector and a reference phase angle from the reference vector, the plurality of electrical output signals corresponding to measurements made at a first level of accuracy; wherein the signal processing circuit is configured to process a subset of electrical output signals from the plurality of electrical output signals in accordance with the indirect time-of-flight measurement technique in order to calculate a measurement vector and a measurement phase angle from the measurement vector, the subset of electrical output signals corresponding to measurements made at a second level of accuracy lower than the first level of accuracy; and the signal processing circuit is configured to use the reference phase angle and the measurement phase angle to calculate a phase angle correction value.

[0013] The apparatus can further comprise: a time-of-flight modification unit configured to introduce a phase delay into the ray emitted by the light source and reflected back to the photon mixer unit; wherein the signal processing circuit can be configured to perform repeated calculations of the phase angle correction value over a plurality of measurement periods following the measurement period in order to calculate a plurality of phase angle correction values.

[0014] The phase delay can be unknown.

[0015] The time-of-flight modification unit can be a delay network that provides a plurality of different time delays and can be configured to delay the phase of the ray emitted by the light source by different amounts between the plurality of measurement periods.

[0016] The photomixer unit can comprise a photodetector element. A substantially static object can be located in front of the photodetector element. The substantially static object can provide a substantially single distance to the photodetector element.

[0017] The delay network can comprise an analog delay line.

[0018] Application of a plurality of different time delays of the delay network can provide a series of phase measurements comprising respective phase errors; the series of phase measurements can range at least within a cyclic error period.

[0019] The apparatus can further comprise: a plurality of photomixer units comprising the photomixer unit, the plurality of photomixer units comprising a plurality of photodetector elements respectively; a time-of-flight modification unit configured to introduce a plurality of respective phase delays into the rays reflected back to the plurality of photodetector elements; wherein the signal processing circuitry can be configured to perform a calculation of phase angle correction values within a measurement period in order to calculate a plurality of phase angle correction values.

[0020] The time-of-flight modification unit can be an object located in a scene and arranged relative to the plurality of photodetector elements so as to extend sufficiently far away from the plurality of photodetector elements to enable calculation of a series of phase angle correction values with respect to and at least within a cyclic error period.

[0021] The signal processing circuitry can be configured to support an error modelling unit configured to analyse the plurality of phase angle correction values and to generate a model of the plurality of phase angle correction values.

[0022] The error modelling unit can be configured to generate a look-up table comprising phase error correction values for a predetermined series of calculated phase angles with respect to measurements made at a second level of accuracy.

[0023] The phase error correction values can correspond to a plurality of uncorrected measured phase angles; the range of uncorrected measured phase angles can comprise a cyclic error period. The plurality of uncorrected measured phase angles can comprise phase angle intervals therebetween; the phase angle intervals can be uniform.

[0024] The plurality of electrical output signals with respect to measurements made at the first level of accuracy can comprise a first set of electrical output signals with respect to measurements made at the second level of accuracy and a second set of electrical output signals with respect to measurements made at the second level of accuracy.

[0025] The plurality of predetermined phase offset values can comprise a first set of predetermined phase offset values and a second set of predetermined phase offset values; and the first set of electrical output signals relating to measurements made at the second level of accuracy can correspond to the first set of predetermined phase offset values, and the second set of electrical output signals relating to measurements made at the second level of accuracy can correspond to the second set of predetermined phase offset values.

[0026] The second set of phase offset values can be different to the first set of phase offset values. The first set of phase offset values can comprise a number of phase offset values in common with the second set of phase offset values. The first set of phase offset values can be identical to the second set of phase offset values.

[0027] The signal processing circuitry can be configured to support a motion detection unit configured to detect motion during generation of the plurality of electrical output signals, and to omit calculation of the phase angle correction value in response to detecting motion.

[0028] The motion detection unit is configured to detect motion by comparing a first electrical output signal relating to a first predetermined phase offset value of the plurality of predetermined phase offset values with a second electrical output signal relating to a second, subsequent predetermined phase offset value of the plurality of predetermined phase offset values, the second subsequent predetermined phase offset value can be subsequent to the application of a number of the plurality of predetermined phase offset values, and the second phase offset value can be substantially the same as the first phase offset value.

[0029] The signal processing circuitry can be configured to detect motion by calculating a difference between the first electrical output signal and the second electrical output signal.

[0030] According to a second aspect of the application, there is provided an optical distance measuring system, the system comprising a phase angle error calculation device as set out above in relation to the first aspect of the application, the system further comprising: signal processing circuitry configured to apply the calculated phase angle correction value to a measured phase angle in order to correct the measured phase angle; and the signal processing circuitry is configured to use the corrected measured phase angle to calculate a range.

[0031] According to a third aspect of the present application, there is provided a method of calculating a phase angle correction value, the method comprising: a light source emitting radiation in accordance with an indirect time-of-flight measurement technique; a photon mixer unit generating and storing a plurality of electrical output signals corresponding to a plurality of predetermined phase offset values applied in accordance with the indirect time-of-flight measurement technique over a measurement period; a signal processing circuit processing the plurality of electrical output signals in accordance with the indirect time-of-flight measurement technique to calculate a reference vector and a reference phase angle from the reference vector, the plurality of electrical output signals corresponding to measurements made at a first level of accuracy; wherein the signal processing circuit processes a subset of electrical output signals from the plurality of electrical output signals in accordance with the indirect time-of-flight measurement technique to calculate a measurement vector and a measurement phase angle from the measurement vector, the subset of electrical output signals being a smaller set than the plurality of electrical output signals and corresponding to measurements made at a second level of accuracy lower than the first level of accuracy; and the signal processing circuit uses the reference phase angle and the measurement phase angle to calculate the phase angle correction value.

[0032] According to a fourth aspect of the present application, there is provided a method of reducing errors in an indirect time-of-flight measurement system, the method comprising: illuminating a scene; repeatedly calculating a first phase angle at a first accuracy and a second phase angle at a second accuracy in respect of a single measurement period using electrical output signals in accordance with an indirect time-of-flight measurement technique, the calculation of the first phase angle employing a plurality of phase offset values and the calculation of the second phase angle employing a subset of the plurality of phase offset values; delaying the propagation of radiation reflected by the scene; calculating a plurality of phase errors using the calculated first and second phase angles; correcting subsequent phase angle measurements using a model of the plurality of phase errors.

[0033] Thus, there is provided an apparatus and method which is each able to calculate a cyclic error or so-called "wobble" without the need to know the distance to an object in a calibration scene. As a result of using a library of measurements in respect of a set of electrical output signals in respect of a measurement period, which can be used to calculate phase angles at two different levels of accuracy and compare the two, the need to rely on a priori distance data can be avoided. Furthermore, the reduction in the need to rely on a priori distance data enables calibration to be performed using an arbitrary scene. This also serves to improve the accuracy of the calculation of the phase error values, which hitherto have relied on mechanical movement of a reference object in the scene as part of the calibration process. In addition, the apparatus and method provide greater flexibility in the timing of the performance of the calibration, as the calibration can be performed in situ without the need for any special or dedicated calibration objects or arrangements. The apparatus and method also provide a degree of immunity to thermal variations (drift) and ageing of components in the apparatus. BRIEF DESCRIPTION OF DRAWINGS

[0034] At least one embodiment of the present application will now be described, by way of example only, with reference to the accompanying drawings in which:

[0035] Figure 1 is a schematic diagram of a phase angle error calculation device that forms an embodiment of the invention;

[0036] Figure 2 is a schematic diagram of a scene used by the device of Figure 1

[0037] Figure 3 and Figure 4 is a schematic diagram of one or more objects in a scene used by the device of Figure 1 and the method of Figure 2

[0038] Figure 5 is a plot modelling the compensation data and the use of the compensation data by the device of Figure 1 and the method of Figure 2

[0039] Figure 6 is a flowchart of a method of correcting the calculated phase angle obtained using the method of Figure 2

[0040] Figure 7 is a schematic diagram of another phase angle error calculation device that forms a further embodiment of the invention;

[0041] Figure 8 is a flowchart of another method of calculating phase angle correction values performed by the device of Figure 7 and forming yet another embodiment of the invention;

[0042] Figure 9 is a schematic diagram of an object in a scene used by the device of Figure 7 and the method of Figure 9

[0043] Figure 10 is a schematic diagram of a measurement frame used to detect motion in relation to the device of Figure 1 or the method of Figure 7 Figure 2 Figure 8 DETAILED DESCRIPTION

[0044] Throughout the following description identical reference signs will be used to identify identical parts.

[0045] Reference is made to Figure 1 ​​​​​​​​The first phase angle error calculation apparatus 100 includes a source 102 of electromagnetic radiation, such as a laser diode (LD) or a light emitting diode (LED). In this example, the source of electromagnetic radiation is an infrared ray that is amplitude modulated in accordance with an indirect time-of-flight measurement technique for emission as a continuous wave optical signal. The detection and ranging module of the apparatus 100 includes an optical receiver photonic mixer pixel device 104 that includes a photodetector element (e.g., a photodiode 106) having an anode that is operably coupled to a modulating voltage source and a cathode that is coupled to a first input of a photonic mixer 108, the output of which is coupled to an input of an integrator 110. Although a single photonic mixer pixel device 104 is being described for the sake of simplicity and clarity of description, those skilled in the art will appreciate that the detection and ranging module includes an array of photonic mixer pixel devices of the kind described above.

[0046] The phase signal generator 112 is configured to generate a continuous wave electrical signal. The phase of the continuous wave signal can be selected from a set of phase offsets [θ0, θ1,..., θm-1] via a control input 114. A first output of the phase signal generator 112 is coupled to a second input of the photonic mixer 108, and a second output of the phase signal generator 112 is operably coupled to the source 102 of electromagnetic radiation.

[0047] An output of the integrator 110 is coupled to an input of a digital Fourier transform (DFT) unit 116. In this regard, the phase angle measurements are serially transmitted to the DFT unit 116, thereby reducing the memory requirements for the detection and ranging module. The DFT unit 116 includes an internal buffer (not shown) to support the serial transmission of the measurements from the integrator 110. To support this arrangement, the DFT unit 116 is operably coupled to a timing control unit 118 to maintain synchronization of the data processing.

[0048] The timing control unit 118 has a synchronization output 120 that is operably coupled to a timing input 122 of the DFT unit 116. A control output 124 of the timing control unit 118 is operably coupled to the control input 114 of the phase signal generator 112.

[0049] In this example, the DFT unit 116 has a plurality of digital in-phase (I) / quadrature (Q) outputs 126. In this example, the DFT unit 116 includes b pairs of digital I / Q outputs corresponding to different harmonics of the measured signal. Since the output of the integrator 110 is a cumulative charge, and in this example, in the analog domain, the output of the integrator 110 needs to be converted to the digital domain. This can be accomplished, for example, by employing a photon counter as the integrator 110 or providing an analog-to-digital converter prior to the DFT unit 116.

[0050] A first pair of I / Q outputs of the plurality of digital I / O outputs 126 related to the first harmonic of the received reflected light signal is coupled to a phase angle calculation unit, such as an arctangent unit 128. A first output of the arctangent unit 128 is coupled to an input 130 of an angle error analysis unit 132, which in this example has a data modeling function. The angle error analysis unit 132 is provided to calculate a phase error value, and in this example, generate a look-up table that is saved in a data store. As a result, the angle error analysis unit 132 is operatively coupled to a data store supported by, for example, a digital memory. The data store stores a look-up table 134. A second output of the arctangent unit 128 is operatively coupled to an input 136 of an angle correction unit 138, a port 140 of the angle correction unit 138 having access to the look-up table 134. An output of the angle correction unit 138 is coupled to more downstream processing units that perform various other functions, such as distance calculations using the corrected measured angle. However, because the structure and operation of the downstream hardware is not central to understanding the embodiments set forth herein, the downstream hardware will not be further described herein for purposes of clarity and conciseness of the description. In this example, the DFT unit 116, the arctangent unit 128, the angle error analysis unit 132, the data store, and / or the angle correction unit 138 comprise a signal processing circuit.

[0051] In operation ( Figure 2 ), the source of electromagnetic radiation 102 emits a continuous wave light signal that illuminates (step 200) a scene. In order for the examples described herein to be operable, it is necessary to introduce a phase delay into the rays emitted by the source of electromagnetic radiation 102 and / or to apply a phase delay to the rays somewhere between emission and reception, including at emission and reception. In this example, an object is placed in the scene to act as a time-of-flight modifier that reflects the emitted light signal. In this regard, and with reference to Figure 3 , the object 300 can be intentionally placed relative to the source of electromagnetic radiation 102 so as to present the source of electromagnetic radiation 102 with a surface that extends away from the array of photon mixer pixel devices 104, thereby providing a plurality of reflections at a distance from the apparatus 100. In such examples, the object 300 Figure 3) can be a simple tilted object that provides a plurality of distances to the array of photon-mixer pixel devices 104 that span a sufficient range of distances to cover one cycle error period. In the case of a normal mode of operation that employs four phase offset values, the cycle error period is π / 2. In another example, object 300( Figure 4 ) can be any one or more objects that are randomly positioned in a scene but provide a sufficient number of different distances to the array of photon-mixer pixel devices 104 to span a range of distances corresponding to at least a cycle error period. However, it should be appreciated that the distance to any portion of the object is not known to or stored by the apparatus 100. In these examples, the object 300 constitutes a time-of-flight modification unit.

[0052] A light signal reflected by the object is received (step 202) by each photon-mixer pixel device 104 of the array of photon-mixer devices of the apparatus 100. The phase signal generator 112 generates a continuous wave electrical signal, and the timing control unit 118 controls the cycling through the set of phase offsets with respect to the electrical signal relative to the continuous wave light signal. In this example, for calibration purposes, the phase signal generator 112 cycles through a plurality of phase offset values corresponding to a high precision measurement, for example, this results in a measurement with a very low level of cycle error, e.g., eight phase offset values. However, a different number of phase offset values can be employed, e.g., a larger number of phase offset values. A synchronization signal is also applied to the DFT unit 116 by the synchronization output 120.

[0053] For the sake of brevity, the calculation of the phase error values will now be described with respect to a single pixel in the array of pixels, but it should be appreciated by those skilled in the art that the described functionality is also performed by the other pixels in the array of pixels. Thus, to calculate the phase angle error, a reference phase angle is calculated by applying the electrical signal generated by the phase angle generator 112 to the photon-mixer 108. The phase of the electrical signal is cycled through the set of phase offset values mentioned above, and a digital representation of the charge generated and stored (step 204) in the integrator 110 with respect to each of the set of phase offset values successively received by the DFT unit constitutes a measurement, and all of the electrical output signals generated by the photon-mixer 108 are used to convert (step 206) the digital representation of the charge into a reference I / Q output pair with respect to all of the phase offset values applied by the phase signal generator 112. In this regard, the plurality of electrical output signals generated during a measurement period or frame period constitute a measurement of a first level of precision. The reference I / Q output constitutes an I / Q reference vector VR that represents a complex-valued analog electrical measurement with respect to the fundamental frequency.

[0054] It will be appreciated that the integrator 110 provides a plurality of phase- separated amplitude measurement outputs in series, which represent respective accumulated charge levels with respect to the applied phase offset values of the photonic mixer pixel device 102. Thus, the DFT unit 116 calculates intermediate I and Q values of the serially received phase-separated amplitude measurements separately for each measurement cycle, which are accumulated with respect to the measurement cycle to generate final I and Q value results. In this example, the operation of such an arrangement comprises iteratively calculating vectors with respect to each incoming phase angle measurement using the DFT unit 116. Thus, as part of the calculated (step 206) reference I / Q output pair, the DFT unit 116 generates (step 208) an interim result constituting a measurement I / Q output with respect to a subset of the pairs of electrical input values and corresponding to a subset of the phase offset values applied by the phase signal generator 112. The subset of electrical output signals used constitutes a measurement result of a second level of precision. This measurement I / Q output pair constitutes an I / Q measurement vector VM, which represents a complex-valued analogue electrical measurement result with respect to the fundamental frequency. The DFT unit 116 can also generate other I / Q vectors with respect to harmonics of the charge measured by the integrator 108.

[0055] It will also be appreciated that, where a sufficient number of phase offset values are applied for a measurement result of a first level of precision, the plurality of electrical output signals can comprise a plurality of sets of electrical output signals corresponding to measurement results of a second level of precision. For example, where nine phase offset values are employed to measure at a first level of precision, the plurality of electrical output signals (eight) comprises a first set of electrical output signals (four) and a second set of electrical output signals (four), both corresponding to measurement results of a second level of precision. It follows that the plurality of phase offset values used to generate the plurality of electrical output signals can be conceptually divided into a plurality of sets of predetermined phase offset values, for example, a first set of phase offset values corresponds to the first set of electrical output signals and a second set of phase offset values corresponds to the second set of electrical output signals. In such an example, the different sets of predetermined phase offset values sets comprise different phase offset values, i.e. the intersection of the sets yields an empty set in terms of set theory. Thus, the first set of predetermined phase offset values is different to the second set of predetermined phase offset values. However, in other examples, the first set of predetermined phase offset values can comprise a plurality of phase offset values in common with the second set of predetermined phase offset values, i.e. the intersection is not an empty set. In some examples, the first set of predetermined phase offset values can be the same as the second set of predetermined phase offset values.

[0056] After converting the electrical measurement signal to the frequency domain, the DFT unit 116 provides the I and Q values ​​at its output regarding the fundamental frequency of the reference I / Q output pair and the measurement I / Q output pair. In this example, a synchronization signal ensures that the arctangent unit 128 synchronously receives the fundamental frequency I / Q output of the current measurement frame from the DFT unit 116. Then, the arctangent unit 128 calculates (step 210) the angle of the reference vector VR constituting the reference phase angle φref in the complex plane from the fundamental frequency reference I and Q values ​​using an indirect time-of-flight measurement technique. Similarly, the arctangent unit 128 also calculates (step 212) the measurement phase angle constituting the complex plane from the fundamental frequency measurement I and Q values ​​using an indirect time-of-flight measurement technique. The angle of the measurement vector VM. The reference phase angle φref corresponds to the measurement result of the first accuracy level, and the measured phase angle The measurement results corresponding to the second accuracy level are greater than those of the first accuracy level. The angle error analysis unit 132 receives the calculated reference phase angle φref and the calculated measured phase angle. And calculate (step 214) the error or correction value, for example, between the calculated reference phase angle φref and the calculated measured phase angle. The difference between them. The above process is performed on each of the photonic mixer pixel devices 104 in the array, and the angle error analysis unit 132 receives the calculated reference phase angle φref and the calculated measured phase angle from all the photonic mixer pixel devices 104 in the array of photonic mixer devices. (Unless data about one or more pixels in the array is ignored) and calculate (step 214) multiple correction / error values. In this respect, in this example, object 300 effectively applies different phase delays to the light emitted by source 102 of electromagnetic radiation, and therefore each photon mixer pixel device in the array is able to provide a pair of phase angles (reference) for calculating the different phase delays. and measurement The data. Therefore, one or more objects 300 extend sufficiently away from the photonic mixer display device to enable the calculation of a series of phase angle error values ​​with respect to the cyclic error period and within the cyclic error period.

[0057] Once the angle error analysis unit 132 has a sufficient number of calculated error values ​​(within the cyclic error period), the angle error analysis unit 132 analyzes multiple calculated phase errors and uses the corresponding calculated measured phase angle. To model the distribution of the multiple error values ​​(step 216), this distribution can typically be fitted to one or more sine curves. Modeling can be performed, for example, by any suitable sine fitting algorithm.

[0058] Turning to Figure 5 At the time of plotting, the measured phase angle The reference error values 310 have a sinusoidal shape. The modelled sinusoidal signal 312 follows the plot of the error values. In effect, when the modelled sinusoidal signal 312 is used to subtract the expected error values obtained directly from the model from the corresponding plotted phase angle error values 310, the resulting compensated error data 314 is greatly reduced compared to the corresponding plotted error values. In some examples, the error values can be binned prior to modelling.

[0059] It can be seen that when the measured phase angle error values φerrorcalculated phase angles are plotted, the plot is periodic. In this regard, since the cyclic error is periodic, in this example with a period of 2π, the number of calculated values stored in the look-up table 134 can optionally be reduced to a quarter for a four-phase offset value measurement (i.e. π / 2). This is the cyclic error period.

[0060] Referring back to Figure 2 Thereafter, in this example, the angle error analysis unit 132 selects the corresponding calculated measured phase angle comprising the predetermined phase angle interval and the corresponding modelled expected phase error values for these equally spaced points, and stores (step 218) them in the look-up table 134 for subsequent use when the apparatus 100 is not performing the correction process. In this example, the look-up table 134 comprises 64 equally spaced measured phase angles In other examples, the model can be accessed directly via the angle error analysis unit 132 rather than using the look-up table 134 and is instead used when a particular calculated measured phase angle is received. In such examples, the angle correction unit 138 is operatively coupled to the angle error analysis unit 132 since the look-up table 134 is not necessary.

[0061] During non-calibration operation Figure 6 ), the look-up table 134 is used as described below. In this regard, the operation will be described with reference to a single photonic mixer pixel device 104, but it will be appreciated by the skilled person that the described operation applies equally to the other pixels of the array.

[0062] The source of electromagnetic radiation 102 re-emits a continuous-wave light signal that illuminates (step 220) the scene. This could be light emitted by objects in the scene reflecting off the emitted light. A phase signal generator 112 generates a continuous-wave electrical signal, and a timing control unit 118 controls a loop through a set of phase offsets of the electrical signal relative to the continuous-wave light signal. In this example, a lower level of second precision is used, and therefore four phase offset values ​​are generated by the phase signal generator. A synchronization signal is also applied to the DFT unit 116 by a synchronization output 120.

[0063] To calculate the corrected phase angle, the phase angle is calculated by applying an electrical signal generated by phase signal generator 112 to photonic mixer 108, and the phase shift of the electrical signal is cyclically passed through the set of phase shifts mentioned above, and a digital representation of the charge constituting the measurement, generated and stored (step 222) in integrator 110 for each phase shift value in the set of phase shift values ​​is received by DFT unit 116, which is connected in series with an I / Q output pair and converted into an I / Q output pair, which constitutes an I / Q vector (step 224) V representing a complex value of analog electrical measurement with respect to the fundamental frequency.

[0064] As in the previous example relating to lookup table generation, DFT unit 116 can also generate additional I / Q vectors for the harmonics of the charge measured by integrator 110. After the electrical measurement signal is converted to the frequency domain, the I and Q values ​​for the fundamental frequency are provided at the output of DFT unit 116. In this example, a synchronization signal ensures that the fundamental frequency I / Q output of the current measurement frame of DFT unit 116 is received synchronously by arctangent unit 128. Subsequently, arctangent unit 128 calculates (step 226) the calculated phase angle extracted in the complex plane from the fundamental frequency I and Q values ​​based on indirect time-of-flight measurement techniques. The angle of vector V.

[0065] Extracted phase angle It is then passed to angle correction unit 138, which can then access a lookup table stored by lookup table 134. In response to the extracted phase angle... Upon receiving the data, the angle correction unit 138 accesses (step 228) lookup table 134 and obtains the value of the extracted phase angle relative to the received data. Corresponding phase angle correction value This angle correction value The angle correction unit 138 applies (step 230) (e.g., adds) to the obtained extracted phase angle. Extracted phase angle The combination with the phase correction value -φerr produces the corrected phase angle. the corrected phase angle is provided at the output of the angle correction unit 138. The corrected phase angle may then be used by the signal processing circuit to calculate the distance to the source of the reflection of the emitted ray.

[0066] The above steps (steps 220-230) are repeated (step 232) until no further correction of the measured angle is required.

[0067] In another example, the one or more objects 300 need not be specifically set up for calibration purposes, and can be sought by the user from naturally occurring scenes. In this regard, the calibration is performed in the field before, during or after the measurement application for which the device 100 is being used. As such, the calibration can be performed "on the fly" or in the field in real time. To collect a sufficient amount of data across a cycle of error periods, varying scenes can be selected. The degree of movement in the scene can vary from periodic and infrequent to more frequent. In the event that the variation in the scene is sufficiently fast to corrupt the data collected during the calibration, motion detection can be employed to ignore measurements made in the event that movement has occurred within the measurement period. An example of such motion detection is described later in this document.

[0068] With reference to Figure 7 , the phase delay is applied to the rays emitted by the source of electromagnetic radiation 102 in a different manner than described above with respect to the foregoing examples. Instead of or in addition to using one or objects extending away from the source of electromagnetic radiation 102, a plurality of phase-delayed sources are disposed between the source of electromagnetic radiation 102 and the phase signal generator 112. As such, Figure 7 The device of Figure 1 differs from the device of In this example, the delay line is a low-accuracy delay line. In this regard, a given delay generated by the delay line possesses an inherent error bound, and thus can vary by up to a fraction of the phase error period, for example, by up to about one-half to about one-quarter of that period. In other examples, the error bound can vary by more than about one-half of the phase error period. In this example, the plurality of phase-delayed sources are configured to generate a plurality of phase delays such that the phase delays are random. The plurality of phase delays thus includes different phase delays, each of which is not known to the device 100, and thus is not stored.

[0069] In operation Figure 8 ), the source of electromagnetic radiation 102 emits a continuous wave light signal that illuminates (step 240) the scene. However, the analog delay line 142 introduces a phase delay to the light signal that illuminates the scene. With reference to Figure 9Another object 300 can be placed opposite the source of electromagnetic radiation 102 (e.g., directly opposite the electromagnetic radiation 102) such that the source of electromagnetic radiation 102 does not present a surface extending away therefrom, but instead presents a surface that is substantially static and provides a substantially constant distance or a single distance to the photodiode 106 of the photon-mixer pixel device 104.

[0070] In this example, the analog delay line 142 is configured to provide a plurality of phase delays to the source of electromagnetic radiation 102 in order to span one cycle error period for each measurement period. In the normal mode of operation with four phase offset values, the cycle error period is π / 2. As a result, the application of the plurality of phase delays enables a series of measurements to be made, which series of measurements respectively include a plurality of phase errors. However, in this example, the series of phase measurements are within the cycle error period.

[0071] The optical signal reflected by the object 300 is received (step 242) by the photon-mixer pixel device 104. The phase signal generator 112 generates a continuous wave electrical signal, and the timing control unit 118 controls cycling through a set of phase offsets with respect to the phase of the electrical signal relative to the continuous wave optical signal. In this example, for the purposes of calibration, the phase signal generator 112 again cycles through a plurality of phase offset values corresponding to high precision measurements, e.g., eight phase offset values, which produce measurements with very low cycle error levels. The synchronization signal is also applied to the DFT unit 116 by the synchronization output 120.

[0072] To calculate the phase angle error, the reference phase angle The electrical signal generated by the phase signal generator 112 is applied to the photon-mixer 108, and the phase offset of the electrical signal is cycled through the set of phase offsets mentioned above, and a digital representation of the charge constituting the measurement is generated and stored (step 244) in the integrator 110 by the DFT unit 116 with respect to each of the set of phase offset values, which DFT unit 116 is in series with and converted to the reference I / Q output pair, which reference I / Q output pair uses all of the electrical output signals generated by the photon-mixer 108 with respect to all of the phase offset values applied by the phase signal generator 112. The reference I / Q output pair constitutes an I / Q reference vector V R .

[0073] It will be appreciated that integrator 110 provides a succession of phase-separated amplitude measurement outputs representing respective accumulated charge levels for applied phase offset values with respect to photonic hybrid pixel device 102. Thus, DFT unit 116 computes for each frame period or measurement period intermediate I and Q values for phase-separated amplitude measurements that are respectively received successively, which are accumulated over the frame period to generate final I and Q value results. Operation of such an arrangement includes iteratively computing vectors for each incoming phase angle measurement using the DFT unit. As a result, as part of the process for computing reference I / Q output pairs, DFT unit 116 generates (step 246) interim results that constitute measurement I / Q output pairs with respect to a subset of the phase offset values applied by phase signal generator 112. The measurement I / Q output pairs constitute I / Q measurement vectors V M representing analog electrical measurements of complex values with respect to the base frequency. As in the previous example, DFT unit 116 can also generate other I / Q vectors with respect to harmonics of the charge measured by integrator 108.

[0074] After the electrical measurement signals are converted to the frequency domain, I and Q values with respect to the base frequency of the reference I / Q output pairs and the measurement I / Q output pairs are provided at the output of DFT unit 116. In this example, the synchronization signal ensures that the base frequency I / Q outputs of the current measurement frame of DFT unit 116 are received synchronously by arctangent unit 128. Arctangent unit 128 then computes (step 250) an angle that constitutes a reference vector VR of reference phase angle in the complex plane from the base frequency reference I and Q values, in accordance with an indirect time-of-flight measurement technique. Similarly, and in accordance with an indirect time-of-flight measurement technique, arctangent unit 128 computes (step 252) an angle that constitutes a measurement vector VM of measurement phase angle in the complex plane from the base frequency measurement I and Q values. Reference phase angle constitutes a measurement at a first level of precision, and measurement phase angle constitutes a measurement at a second level of precision, the first level of precision being higher than the second level of precision. Angle error analysis unit 132 receives the computed reference phase angle and the computed measurement phase angle and computes (step 254) an error value, for example, the difference between the computed reference phase angle and the computed measurement phase angle

[0075] ​Since the process of applying a phase delay to the source of electromagnetic radiation 102 and subsequently calculating error values is with respect to a single pixel, the process has to be repeated until the angle error analysis unit 132 has collected a sufficient amount of data, for example, across a cyclic error period. The signal processing circuitry therefore determines (step 256) whether the angle error analysis unit 132 has calculated a sufficient amount of data to be able to analyse a plurality of calculated phase errors and model the calculated data. In the event that the angle error analysis unit 132 requires further error value data points, the random phase delay generated by the analogue delay line 142 is automatically changed and the above process is repeated (steps 240 to 254) for successive measurement periods, i.e. a plurality of measurement periods, until a sufficient amount of error correction values have been generated over the cyclic error period. In this regard, it will be appreciated that between successive measurement periods, different amounts of phase delay are applied to the rays emitted by the source of electromagnetic radiation 102.

[0076] Thereafter, the angle error analysis unit 132 models the distribution of the plurality of error values with respect to the calculated measurement phase angles As with the example described with respect to the reference Figures 1-4 The distribution can be fitted as one sine or more than one sine. The modelling can be, for example, by any suitable sine fitting algorithm. In this regard, Figure 5 A plot of the curve of the calculated measurement phase angles

[0077] Referring back to Figure 8 In this example, the angle error analysis unit 132 selects a plurality of equally spaced points corresponding to the calculated phase angles and their corresponding modelled error values and stores them in a look-up table 134 with respect to a predetermined range of measured phase angles calculated at the second level of accuracy, for example, over the cyclic error period, for subsequent use when the apparatus 100 is not performing the correction process. In this example, the look-up table 134 comprises 64 equally spaced measurement phase angles In other examples, the model can be accessed directly via the angle error analysis unit 132 rather than using the look-up table 134 and is instead used when a particular calculated measurement phase angle is received. In such examples, the angle correction unit 138 is operatively coupled to the angle error analysis unit 132 as the look-up table 134 is not necessary.

[0078] As a process of correcting calculated measured phase angles measured at the second level of accuracy using the data stored in the look-up table 134, it will be appreciated that, in accordance withFigure 6 The method description is related to the use of the above-mentioned... Figure 8 The method described is used to generate error correction data for application.

[0079] In the example above, it is desirable to detect motion relative to the scene, because motion within the scene would hinder attempts to accurately measure the phase angle during calibration. (Reference) Figure 10 The signal processing circuitry may include a motion detection unit, enabling motion to be detected optionally by comparing electrical output signals generated within consecutive frames (i.e., measurement periods), wherein the electrical output signals are associated with the application of the same phase offset value to the photonic mixer 108. For example, during a repetitive measurement period 310 employing 12 phase offset values, a first electrical output signal is generated by the mixer pixel 108 with respect to a first phase offset value 312. According to indirect time-of-flight measurement techniques, for the remainder of the duration of the measurement period 310, other phase offset values ​​314 are sequentially applied to the photonic mixer 108 until the pattern of applying phase offset values ​​is repeated at the start of a subsequent measurement period. In this respect, at the start of the next measurement period, the first phase offset value 312 is again applied to the photonic mixer 108, and a second electrical output signal is generated. In this example, this can be applied to any of the examples described above, comparing the first electrical output value with the second electrical output value using any suitable comparison method (e.g., subtraction and comparison with zero) to determine whether the first electrical output signal is substantially different from the second electrical output signal. In this respect, the degree of difference can be defined with reference to system noise, such as the standard deviation of system noise. When the difference between the first and second electrical output signals exceeds a predetermined motion threshold, the signal processing circuit ignores the phase angle values ​​calculated for calibration purposes, as they are considered to be contaminated by motion in the scene. Therefore, the discarded calculated phase measurements do not result in the calculation of the corresponding phase error value and do not contribute to the dataset subsequently modeled in the above example. When the motion threshold is not exceeded, the calculated phase angle values ​​can be used in the dataset for modeling purposes. However, it should be understood that the use of the above motion detection technique is purely exemplary, and other motion detection techniques can be employed, for example, as described in co-pending European Patent Application No. 20192948.6, the contents of which are hereby incorporated by reference.

[0080] The skilled person will appreciate that the above implementations are merely examples of the various implementations conceivable within the scope of the appended claims. Indeed, it will be appreciated that other techniques can be employed to calculate the above phase angles. For example, in relation to the second level of precision employing four phase offset values, a trigonometric calculation unit employing trigonometric calculation techniques to calculate the amplitude and / or phase can be employed instead of the above arctan unit 128. In this regard, the calculated phase angles can be corrected in real-time for the cyclic error using a "trigonometric" phase measurement made at the first level of precision based on a DFT-based measurement.

[0081] It will also be appreciated that, although in the above examples an analogue delay line is employed, this should not be taken to exclude a digital delay line.

[0082] In the above set out examples in relation to Figure 1 the phase signal generator 112 to the photomixer 108. However, the skilled person will appreciate that other embodiments are possible within the context of an indirect time-of-flight approach, for example the photomixer 108 can be provided with a continuous electrical signal and the plurality of phase offset values can be applied to the source of electromagnetic radiation 102 by the phase signal generator 112 instead of applying the plurality of phase offset values to the photomixer 108.

[0083] It will be appreciated that references herein to "radiation" are intended as references to the optical range of the electromagnetic spectrum, for example between about 350 nm and about 2000 nm, such as between about 550 nm and about 1400 nm, or between about 600 nm and about 1000 nm, unless explicitly stated otherwise.

Claims

1. A phase angle error computation apparatus for an optical distance measuring system, the apparatus comprising: a light source configured for emitting a radiation ray in accordance with an indirect time-of-flight measurement technique; a photomixer unit configured for generating and storing a plurality of electrical output signals corresponding respectively to a plurality of predetermined phase offset values applied during a measurement period in accordance with the indirect time-of-flight measurement technique; a signal processing circuit configured for processing the plurality of electrical output signals corresponding to measurements made at a first level of accuracy in accordance with the indirect time-of-flight measurement technique in order to compute a reference vector and a reference phase angle therefrom, the reference vector being constituted by a reference in-phase / quadrature, I / Q, output pair generated using the plurality of electrical output signals; wherein the signal processing circuit is configured for processing a subset of electrical output signals from the plurality of electrical output signals corresponding to measurements made at a second level of accuracy lower than the first level of accuracy in accordance with the indirect time-of-flight measurement technique in order to compute a measurement vector and a measurement phase angle therefrom, the measurement vector being constituted by a measurement I / Q output pair generated using the subset of electrical output signals; and the signal processing circuit is configured for computing a phase angle correction value based on a difference between the reference phase angle and the measurement phase angle.

2. The apparatus of claim 1, further comprising: a time-of-flight modification unit configured for introducing a phase delay into the radiation ray emitted and reflected back to the photomixer unit; wherein the signal processing circuit is configured for performing repeated computation of the phase angle correction value over a plurality of measurement periods following the measurement period in order to compute a plurality of phase angle correction values.

3. The apparatus of claim 2, wherein, the time-of-flight modification unit is a delay network providing a plurality of different time delays and configured for delaying the phase of the radiation ray emitted by the light source by different amounts between the plurality of measurement periods.

4. The apparatus of claim 3, wherein, the delay network comprises an analog delay line.

5. The apparatus of any one of claims 3 or 4, wherein, application of the plurality of different time delays to the delay network provides a range of phase measurements comprising respective phase errors, the range of phase measurements being at least over a cyclic error period.

6. The apparatus of claim 1, further comprising: a plurality of photomixer units including the photomixer unit, the plurality of photomixer units respectively comprising a plurality of photodetector elements; a time-of-flight modification unit configured for introducing a plurality of respective phase delays into the radiation ray reflected back to the plurality of photodetector elements; wherein the signal processing circuit is configured for performing computation of the phase angle correction value over the measurement period and with respect to the plurality of photomixer units in order to compute a plurality of phase angle correction values.

7. The apparatus of claim 3, wherein, The plurality of photon mixer units comprises a plurality of photodetector elements, the time-of-flight modification unit is an object located in a scene, and is arranged relative to the plurality of photodetector elements so as to extend substantially away from the plurality of photodetector elements, thereby enabling a series of phase angle correction values to be calculated in respect of and at least within a cycle error period.

8. The device of any one of claims 1 to 4, wherein, The signal processing circuitry is configured to support an error modelling unit configured to analyse the plurality of phase angle correction values and to generate a model of the plurality of phase angle correction values.

9. The apparatus of claim 8, wherein, The error modelling unit is configured to generate a look-up table comprising phase error correction values for a predetermined series of calculated phase angles in respect of measurements made at the second level of accuracy.

10. The apparatus of any one of claims 1 to 4, wherein, The plurality of electrical output signals in respect of measurements made at the first level of accuracy comprises a first set of electrical output signals in respect of measurements made at the second level of accuracy and a second set of electrical output signals in respect of measurements made at the second level of accuracy.

11. The apparatus of claim 10, wherein The plurality of predetermined phase offset values comprises a first set of predetermined phase offset values and a second set of predetermined phase offset values; and The first set of electrical output signals in respect of measurements made at the second level of accuracy corresponds to the first set of predetermined phase offset values, and the second set of electrical output signals in respect of measurements made at the second level of accuracy corresponds to the second set of predetermined phase offset values.

12. The device of any one of claims 1 to 4, wherein, The signal processing circuitry is configured to support a motion detection unit configured to detect motion during generation of the plurality of electrical output signals and to omit calculation of the phase angle correction values in response to detection of motion.

13. The apparatus of claim 12, wherein, The motion detection unit is configured to detect the motion by comparing a first electrical output signal in respect of a first predetermined phase offset value of the plurality of predetermined phase offset values with a second electrical output signal in respect of a subsequent second predetermined phase offset value of the plurality of predetermined phase offset values, the subsequent second predetermined phase offset value being applied after a number of the plurality of predetermined phase offset values, and the subsequent second predetermined phase offset value being substantially the same as the first predetermined phase offset value.

14. An optical distance measuring system, the system comprising the phase angle error calculation apparatus of any one of claims 1 to 4, the system further comprising: signal processing circuitry configured to apply calculated phase angle correction values to the measured phase angle so as to correct the measured phase angle; and the signal processing circuitry is configured to use the corrected measured phase angle to calculate a distance.

15. A method of calculating phase angle correction values, the method comprising: a light source emitting a ray in accordance with an indirect time-of-flight measurement technique; a photon mixer unit generating and storing a plurality of electrical output signals, the plurality of electrical output signals respectively corresponding to a plurality of predetermined phase offset values applied in a measurement period in accordance with the indirect time-of-flight measurement technique; The signal processing circuit processes the plurality of electrical output signals in accordance with the indirect time-of-flight measurement technique to calculate a reference vector and a reference phase angle from the reference vector; The plurality of electrical output signals correspond to measurements made at a first level of precision, the reference vector being constituted by a reference in-phase / quadrature, I / Q, output pair generated using the plurality of electrical output signals; wherein The signal processing circuit processes a subset of electrical output signals from the plurality of electrical output signals in accordance with the indirect time-of-flight measurement technique to calculate a measurement vector and a measurement phase angle from the measurement vector, the subset of electrical output signals being a smaller set than the plurality of electrical output signals and corresponding to measurements made at a second level of precision lower than the first level of precision, the measurement vector being constituted by a measurement I / Q output pair generated using the subset of electrical output signals; and The signal processing circuit calculates the phase angle correction value based on a difference between the reference phase angle and the measurement phase angle.

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