Optical distance calculation device and distance calculation method

Abstract

The present application discloses an optical distance calculation device and a method for distance calculation. An optical distance calculation device (100) includes: a light source configured to emit light according to the indirect time-of-flight measurement technique. A photon mixer unit (102) configured to generate and store a plurality of electrical output signals (112) respectively corresponding to a plurality of predetermined phase offset values applied according to the indirect time-of-flight measurement technique. A signal processing circuit (110, 124) configured to process the plurality of electrical output signals according to the indirect time-of-flight measurement technique so as to calculate a measurement vector and a measured phase angle from the measurement vector. The signal processing circuit (110, 124) is configured to calculate a phase angle correction value using reference illumination data and apply the calculated phase angle correction value so as to correct the measured phase angle; and the signal processing circuit is configured to calculate a distance using the corrected measured phase angle.

Description

Technical Field

[0001] The present invention relates to an optical distance calculation device, which is configured to emit light and process received reflected light according to indirect time-of-flight measurement technology, for example. The present invention also relates to a method of distance calculation, which is of the type that uses indirect time-of-flight measurement technology, for example. Background Art

[0002] In so-called time-of-flight sensing systems and other systems (such as game console vision systems), it is known to use an illumination source to illuminate the surrounding environment (sometimes referred to as the "scene") within the field of view of the illumination source, and to process the light reflected by the features of the scene. Such so-called LiDAR (light detection and ranging) systems use an illumination source to illuminate the scene with light, and use a detection device (such as a photodiode array, some optical elements, and a processing unit) to detect the light reflected from an object in the scene. The light reflected from an object in the scene is received by the detection device and converted into an electrical signal, which is then processed by the processing unit by applying time-of-flight (ToF) calculations to determine the distance of the object from the detection device. Although different types of LiDAR systems are known based on different operating principles, such systems basically illuminate the scene and detect the reflected light.

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

[0004] Another known LiDAR system uses the so-called "indirect time-of-flight" (iToF) ranging technology. The iTOF system emits a continuous-wave light signal, and the reflection of the continuous-wave light signal is received and analyzed by a detector device. Multiple samples (such as four samples) of the light reflected from the features of the scene are acquired, and each sample is phase-stepped, for example, by 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 an iToF system, high-frequency signal processing (demodulation) occurs at the pixel level, and thus the post-pixel signal bandwidth required to integrate a large number of pixels on the same chip is low. Therefore, compared with a direct ToF system, an iToF system can support a larger number of channels and thus support higher spatial resolution measurements. However, an iToF system has limited distance measurement capabilities. In this regard, to achieve low random distance measurement errors, an iToF system requires a high modulation frequency, which in turn reduces the distance range that can be clearly measured. For example, a 100 MHz modulation frequency results in an approximate clear measurement range of 1.5 m. In addition, traditional iToF systems are vulnerable to errors caused by multiple reflections and multiple propagation paths.

[0006] As explained above, an iToF system samples with respect to different applied phases. A typical iToF system includes a buffer that stores analog signals for m phases generated by a so-called photon mixer device for subsequent signal processing. A discrete Fourier transform unit calculates the fundamental frequency of the complex signal stored in the buffer based on the 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 calculated, and the distance to the object can be solved using the phase angle information.

[0007] However, such amplitude modulation continuous wave type systems suffer from inherent depth measurement errors due to aliasing of the correlation function of the transmitted optical signal and the reference signal. This is because high-order harmonics exist in both the transmitted optical signal and the reference signal, and these high-order harmonics are not considered in the modeling or measurement. This error is called the wobbling error or the circle 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 errors, it is known to use a phenomenological method to compensate for circular errors. In this regard, correction methods based on B-splines or lookup tables are known, for example 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, such as temperature and frequency for modulating the optical illumination signal emitted by the light source and demodulating the optical signal received by the photon mixer device). SUMMARY OF THE INVENTION

[0009] According to a first aspect of the present invention, there is provided an optical distance calculation device, comprising: a light source configured to emit light according to an indirect time-of-flight measurement technique; a photon mixer unit configured to generate and store a plurality of electrical output signals respectively corresponding to a plurality of predetermined phase offset values applied according to the indirect time-of-flight measurement technique; a signal processing circuit configured to process the plurality of electrical output signals according to the indirect time-of-flight measurement technique so as to calculate a measurement vector and a measured phase angle from the measurement vector; wherein the signal processing circuit is configured to calculate a phase angle correction value using reference illumination data and apply the calculated phase angle correction value so as to correct the measured phase angle; and the signal processing circuit is configured to calculate a distance using the corrected measured phase angle.

[0010] The reference illumination data may be prior knowledge of the waveform model of the light emitted by the light source.

[0011] The device may further comprise: a data storage configured to store a plurality of input phase angles and a plurality of phase angle correction values respectively associated with the plurality of input phase angles; wherein the plurality of phase angle correction values may be pre-calculated using the waveform model of the light emitted by the light source; and the signal processing circuit may be configured to access the data storage and provide the phase angle correction value corresponding to the measured phase angle, and apply the phase angle correction value to the measured phase angle to produce the corrected measured phase angle.

[0012] The data storage may be configured to store a lookup table comprising the plurality of input phase angles and the plurality of phase angle correction values.

[0013] The waveform model of the light emitted by the light source can be used to calculate the estimated phase angle corresponding to a predetermined phase angle, and a phase correction value can be calculated from the estimated phase angle and the associated predetermined phase angle.

[0014] The signal processing circuit can be configured to employ a first phase angle calculation technique and a second phase angle calculation technique; the signal processing circuit can be configured to, in response to the total number of the plurality of predetermined phase offset values applied in a time frame according to the indirect time-of-flight measurement technique, select the first phase angle calculation technique to calculate a first phase angle correction as the phase angle correction value, or select the second phase angle calculation technique to calculate a second phase angle correction as the phase angle correction value.

[0015] The signal processing circuit can be configured to measure the slope time of the cycle of the emitted light and use the measured slope to configure the waveform model of the emitted light.

[0016] The signal processing circuit can be configured to calculate the measured amplitude from a measurement vector that calculates the measured phase angle; and the signal processing circuit can be configured to apply an amplitude correction value to the measured amplitude to provide a corrected measured amplitude.

[0017] The waveform model of the light emitted by the light source can be used to calculate the estimated phase angle corresponding to a predetermined phase angle; and the waveform model of the light emitted by the light source can be used to calculate the estimated amplitude value of the measurement vector, and the amplitude correction value is derived from the estimated amplitude value.

[0018] The amplitude correction value can also be derived from the average value of the estimated amplitude values and the estimated amplitude values.

[0019] The signal processing circuit can be configured to employ a first amplitude calculation technique and a second amplitude calculation technique; the signal processing circuit can be configured to, in response to the total number of the plurality of predetermined phase offset values applied in a time frame according to the indirect time-of-flight measurement technique, select the first amplitude calculation technique to provide a first amplitude correction value as the amplitude correction value, or select the second amplitude calculation technique to provide a second amplitude correction value as the amplitude correction value.

[0020] The data storage can be configured to store the plurality of input phase angles and a plurality of amplitude correction values respectively associated with the plurality of input phase angles; the plurality of amplitude correction values can be pre-calculated using the waveform model of the light emitted by the light source; and the signal processing circuit can be configured to access the data storage and provide the amplitude correction value corresponding to the measured phase angle, and apply the amplitude correction value to the measured amplitude to generate the corrected measured amplitude.

[0021] The signal processing circuit can be configured to measure transient values corresponding to the light emitted by the light source during a calibration phase.

[0022] The transient value can be an average transient measured over multiple periods of the emitted light.

[0023] The signal processing circuit can be configured to use another indirect time-of-flight measurement technique to calculate multiple measurement vectors as reference measurements, and to calculate the multiple phase angles respectively from the multiple measurement vectors; the associated accuracy of the another indirect time-of-flight measurement technique can be greater than the accuracy of the indirect time-of-flight measurement technique used to calculate the measurement vectors; and the signal processing circuit can be configured to use the indirect time-of-flight measurement technique to measure another multiple of vectors, and to calculate another multiple of phase angles respectively from the another multiple of vectors.

[0024] A reference measurement regarding a substantially static reference scene can be obtained using another indirect time-of-flight measurement technique, and the another multiple of vectors can be generated regarding the substantially static reference scene using the indirect time-of-flight measurement technique.

[0025] The apparatus can further include: a range of measurable distances having an associated data set of phase angle errors; wherein the signal processing circuit can be configured to use the multiple phase angles and the another multiple of phase angles to calculate multiple phase angle errors; the multiple phase angle errors can provide a correction regarding a part of the range of the measurable distances; and the phase angle correction value calculated using the indirect time-of-flight measurement technique can be related to distances outside and within the part of the measurable distances, and can help to complete the data set of the phase angle errors.

[0026] The data set of the phase angle errors can be completed using the calculated phase angle correction value, the multiple phase angles and an interpolation technique.

[0027] The another indirect time-of-flight measurement technique can use a greater number of phase shift samples than the indirect time-of-flight measurement technique, and the number of phase shift samples employed by the another indirect time-of-flight measurement technique can be odd.

[0028] The reference illumination data can be difference data calculated between an image of a substantially static reference scene obtained in a reference mode and another image of the substantially static reference scene obtained in a standard mode.

[0029] The standard mode can employ the indirect time-of-flight measurement technique, and the reference mode can employ the another time-of-flight measurement technique; the accuracy of the another indirect time-of-flight measurement technique can be greater than the accuracy of the indirect time-of-flight measurement technique.

[0030] The apparatus may include an array of photon mixer units, the array including the photon mixer units. Each of the photon mixer unit arrays may be configured to contribute to the calculation of the plurality of phase angle correction values stored in the data store.

[0031] According to a second aspect of the present invention, there is provided a distance calculation method, including: a light source emits light according to indirect time-of-flight measurement technology; a photon mixer unit generates and stores a plurality of electrical output signals respectively corresponding to a plurality of predetermined phase offset values applied according to the indirect time-of-flight measurement technology; a signal processing circuit processes the plurality of electrical output signals according to the indirect time-of-flight measurement technology so as to calculate a measurement vector and a measured phase angle from the measurement vector; the signal processing circuit uses reference illumination data to calculate a phase angle correction value and applies the calculated phase angle correction value so as to correct the measured phase angle; and the signal processing circuit uses the corrected measured phase angle to calculate a distance.

[0032] According to a third aspect of the present invention, there is provided a distance calculation method, including: emitting light according to indirect time-of-flight measurement technology; generating and storing a plurality of electrical output signals respectively corresponding to a plurality of predetermined phase offset values applied according to the indirect time-of-flight measurement technology; processing the plurality of electrical output signals according to the indirect time-of-flight measurement technology so as to calculate a measurement vector and a measured phase angle from the measurement vector; using reference illumination data to calculate a phase angle correction value and applying the calculated phase angle correction value so as to correct the measured phase angle; and using the corrected measured phase angle to calculate a distance.

[0033] Therefore, it is possible to provide an apparatus and a method, each of which can generate a calculated angle with improved accuracy when calculating according to indirect time-of-flight measurement technology. The calculated angle is less susceptible to harmonics in the continuous wave illumination signal, especially in the case of a large harmonic content. BRIEF DESCRIPTION OF THE DRAWINGS

[0034] Now, with reference to the drawings, at least one embodiment of the present invention will be described only by way of example, in which:

[0035] Figure 1 is a schematic diagram of an optical distance calculation apparatus constituting an embodiment of the present invention;

[0036] Figure 2 is performed by Figure 1 the apparatus and is a flowchart of a distance calculation method constituting another embodiment of the present invention;

[0037] Figure 3 is a flowchart of a method for calculating a phase angle error correction value constituting a further embodiment of the present invention;

[0038] Figure 4 shows a series of graphs of waveforms involved in calculating a phase angle error correction value using the method of Figure 3 ;

[0039] Figure 5 is a graphical representation of a look-up table for phase angle correction employed by the apparatus and method of Figure 1 and Figure 2 respectively;

[0040] Figure 6 is a schematic diagram of another optical distance calculation apparatus constituting another embodiment of the present invention;

[0041] Figure 7 is a flowchart of another method of distance calculation performed by the apparatus of Figure 6 and constituting a further embodiment of the present invention;

[0042] Figure 8 is a flowchart of a method for calculating an amplitude error correction value constituting yet a further embodiment of the present invention;

[0043] Figure 9 is a graphical representation of another look-up table for amplitude correction employed by the apparatus and method of Figure 6 and Figure 7 respectively. DETAILED DESCRIPTION

[0044] Throughout the following description, the same reference numerals will be used to identify the same components.

[0045] Referring to Figure 1 , the first indirect time-of-flight distance calculation apparatus 100 includes an electromagnetic radiation source (not shown), such as a laser diode (LD) or a light-emitting diode (LED). In this example, the electromagnetic radiation source is infrared light that is amplitude-modulated according to the indirect time-of-flight measurement technique so as to be emitted as a continuous-wave optical signal. The detection and ranging module of the apparatus 100 includes an optical receiver photon mixer pixel device 102, which includes a photodiode 104 having an anode operatively coupled to a ground potential and a cathode coupled to a first input of a photon mixer 106, the output of which is coupled to an input of an integrator 108. In this example, for the sake of brevity and clarity of description, a single photon mixer pixel device 102 is being described. However, those skilled in the art will understand that the detection and ranging module includes an array of photon mixer pixel devices of the type described above.

[0046] The phase signal generator 112 is configured to generate a continuous wave electrical signal. The phase offset of the continuous wave signal can be selected via the control input 114, and the phase of the continuous wave signal can be selected from a set of phase offsets: [θ0, θ1, …, θ m-1 . The output of the phase signal generator 112 is coupled to the second input of the optical mixer 106.

[0047] The output of the integrator 108 is coupled to the input of the digital Fourier transform (DFT) unit 110. In this regard, the phase angle measurements are serially transmitted to the DFT unit 110, thereby reducing the memory requirements for the detection and ranging module. The DFT unit 110 includes an internal buffer (not shown) to support the serial transmission of measurements from the integrator 108. To support this arrangement, the DFT unit 110 is operatively coupled to the timing control unit 116 to maintain synchronization of data processing.

[0048] The timing control unit 116 has a synchronization output 118 that is operatively coupled to the timing input 120 of the DFT unit 110. The control output 122 of the timing control unit 116 is coupled to the control input 114 of the phase signal generator 112.

[0049] The DFT unit 110 has a plurality of digital in-phase (I) / quadrature (Q) outputs 125. In this example, the DFT unit 110 includes b pairs of digital I / Q outputs corresponding to different harmonics of the measured signal. Since the output of the integrator 108 is the accumulated charge and, in this example, in the analog domain, the output of the integrator 108 needs to be converted to the digital domain. This can be achieved, for example, by employing a photon counter as the integrator 108 or by providing an analog-to-digital converter prior to the DFT unit 110.

[0050] The first pair of I / Q outputs among the plurality of digital I / O outputs 125 related to the first harmonic of the received reflected optical signal is coupled to a phase angle calculation unit, such as the arctangent unit 124. The output of the arctangent unit 124 is coupled to the first input 126 of the summing unit 128. The summing unit 128 constitutes a combiner, and the summing unit 128 has an output for providing a corrected phase angle. The output of the arctangent unit 124 is also operatively coupled to the input of a look-up table unit 130 supported by data storage, and the output of the look-up table unit 130 is operatively coupled to the second input 132 of the summing unit 128. In this example, the DFT unit 110, the arctangent unit 124, the look-up table unit 130, the data storage, and the summing unit 128 constitute a signal processing circuit.

[0051] In operation ( Figure 2) In this case, the light source emits a continuous-wave optical signal that illuminates the scene (step 200). For example, the light signal emitted is reflected by an object in the scene. The phase signal generator 112 generates a continuous-wave electrical signal, and the timing control unit 116 controls the cycling through a set of phase offsets with respect to the continuous-wave optical signal for the electrical signal. The synchronization output 118 also applies a synchronization signal to the DFT unit 110.

[0052] To calculate the corrected phase angle, the phase angle is calculated by applying the electrical signal generated by the phase signal generator 112 to the photon mixer 106, and the phase offset of the electrical signal cycles through the set of phase offsets mentioned above, and the digital representation of the charge stored in the integrator 108 measured (step 202) for each phase offset in the set of phase offsets, and the digital representation of the charge is serially received by the DFT unit 110 and converted into a pair of I / Q outputs to form an I / Q vector (step 204) V, which represents a complex-valued analog electrical measurement with respect to the fundamental frequency. In this regard, the integrator 108 provides a plurality of phase-separated amplitude measurement outputs in series, and the plurality of phase-separated amplitude measurement outputs represent the respective cumulative charge levels with respect to the applied phase offset values of the photon mixer pixel device 102. The DFT unit 110 calculates the intermediate I value and Q value of the phase-separated amplitude measurements received serially respectively for each frame period, and the intermediate I value and Q value are accumulated in the frame cycle to generate the final I value and Q value results. The operation of such an arrangement includes using the DFT unit 110 to iteratively calculate vectors for each incoming phase angle measurement.

[0053] The DFT unit 110 can also generate other I / Q vectors for the harmonics of the charge measured by the integrator 108. After the electrical measurement signal is converted to the frequency domain, the DFT unit 110 provides the I value and Q value for the fundamental frequency at its output. In this example, the synchronization signal ensures that the arctangent unit 124 synchronously receives the fundamental frequency I / Q output of the current measurement frame of the DFT unit 110. Then, the arctangent unit 124 calculates (step 206) the angle of the vector V according to the indirect time-of-flight measurement technique, which constitutes the calculated phase angle extracted (measured) from the fundamental frequency I value and Q value in the complex plane

[0054] The extracted phase angle is received by the summing unit 128 and the look-up table unit 130. In response to receiving the extracted phase angle the look-up table unit 130 accesses (step 208) the phase angle correction value corresponding to the value of the received extracted phase angle and outputs the phase angle correction value The phase angle correction value Received by summing unit 128 and applied (step 210) (e.g., added) to the received and extracted phase angle corresponding to the phase angle correction value retrieved by lookup table unit 130 The extracted phase angle The extracted phase angle And the phase angle correction value Combined to produce a corrected phase angle The corrected phase angle Is provided (step 212) at the output of summing unit 128. The corrected phase angle can then be used to calculate the distance to the reflection source of the emitted light.

[0055] Repeat (step 213) the above steps (step 202 to step 212) until no further correction of the measured angle is required.

[0056] Go to lookup table unit 130. It has been found that, in the case of having prior knowledge of the transient of the emitted optical signal, it is possible to calculate the phase angle error correction value to be applied to the estimated phase angle measurement in order to correct the estimated phase angle measurement for circular error. Refer to Figure 3 And Figure 4 , select the model s(t) of the emitted optical signal, which constitutes the reference illumination data. In this example, the model is based on the convolution of a pair of rectangular functions:

[0057]

[0058] s(t) is the convolution of an ideal rectangular waveform with pulse duration t p And another ideal rectangular waveform with a duration that can vary between 0 and 100% of the slew time t s Of the waveform (e.g., the rise time or fall time of the waveform). However, it should be understood that a person skilled in the art can further develop this model. For example, the above model assumes that the waveform has equal rise time and fall time, while different models can consider the possibility that the waveform has different rise times and fall times.

[0059] To characterize the model specific to the light emitted by the light source, during, for example, a calibration phase (e.g., during manufacturing, or during on-site use) during an end-of-line test of device 100, measure the slew time t s (step 300) once of the emitted optical signal in order to account for operating parameter drift and / or temperature effects that occur during the lifetime of device 100. The slew time can be the average of multiple slew times measured over multiple cycles of the emitted optical signal. By measuring the slew time, it is possible to obtain an indication of the bandwidth of the optical signal and the harmonics of the optical signal that cause circular error. In this regard, the bandwidth-time product Γ is given by the following expression:

[0060] Γ = f mod t s (2)

[0061] The bandwidth time product Γ represents the ratio of the tilt time to the optical signal period.

[0062] Using the model of the above equation (1) for the theoretically calculated time-of-flight distance system and characterized as by the tilt time measured above (step 300) (step 302), m measurements are made within the period of the optical signal s(t) 400. This results in m different integration periods for the integrator 108 and sampling points are obtained every 2π / m over the entire illumination period of the optical signal 400. Each sampling point P k (where k = 0,..., m - 1) is characterized within the period of the optical signal 400 as:

[0063] P k (φ) = s p (φ) * MIX k (φ) (3)

[0064] where and Φ is the phase angle introduced by the time-of-flight path from the light source of the device 100 to an object in the scene and back to the photon mixer pixel device 102 of the optical receiver. Using the above expression (2), for each of the k (= 0,..., m - 1) phase angle shifted mixed signals 402, 404, 406, 408, the sampling point P k (Ф) is calculated (step 304). As those skilled in the art will recognize, the m sampling points within a given period of the optical signal 400 are used to calculate (step 306) the in-phase and quadrature vector components using a Fourier transform, in this example for the first harmonic of the optical signal 400:

[0065]

[0066]

[0067] where

[0068] For example, taking a four-phase system (m = 4) as an example, the first sampling point value P0, 410, the second sampling point value P1, 412, the third sampling point value P2, 414, and the fourth sampling point value P3, 416 are calculated as follows:

[0069] I(Φ) = P0(Φ) - P2(Φ) (6)

[0070] Q(Φ) = P1(Φ) - P3(Φ) (7)

[0071] Based on the in-phase vector component and the quadrature vector component, an estimated value Φ of the current phase angle value Φ can be calculated using any one of the following: est (Ф)418 (i.e., an estimate of the actual or ideal solution (reference true value) of the phase angle value Φ of the in-phase component and the quadrature component of the vector) (step 308): a first phase angle calculation technique (e.g., the atan2 method); or a second phase angle calculation technique (e.g., the trigonometric method), where the number of phase offsets employed by the phase signal generator 112 is four:

[0072]

[0073]

[0074] Where:

[0075]

[0076]

[0077] In the case of a four-phase system, it is possible to select between the atan2 or trigonometric method to estimate the phase angle based on a configuration-based model according to the value of the bandwidth-time product Γ. For example, if the bandwidth-time product Γ is less than about 0.2, it is more beneficial to use the trigonometric method than the atan2 method. Then, using the knowledge of the actual or predetermined phase angle Φ used in calculating the in-phase component and the quadrature component of the above vector, for the estimated phase angle Ф est Calculate the phase angle error value Ф err 420 (step 310):

[0078] φ err =φ est -φ (12)

[0079] Apply the configured model to equation (3), which can be used together with the expressions for calculating the above in-phase value and quadrature value, and the appropriate equation for obtaining the estimated phase angle from the in-phase component and the quadrature component can be used together with the above expression (equation (12)) to calculate the phase angle error value Ф err . This calculation process constitutes the calculation (step 312) of the phase angle error value Ф err as a function. Then this function is used to calculate the estimated phase angle Ф with respect to a predetermined number of "reference true value" phase angles est and the corresponding phase angle error value Ф err . In this regard, the predetermined number of "reference true value" phase angles can be quantization values, such as 12-bit quantized phase angle values. In this example, the phase angle error value Ф err for the corresponding estimated phase angle Ф estConvert (e.g., invert (-Ф err )) before storing the values together in the look-up table unit 130.

[0080] Reference Figure 5 , it can be seen that when plotting the estimated phase angle error value Ф est , with respect to the calculated phase angle error value -Ф err , the calculated phase angle error value -Ф err and the estimated phase angle error value Ф est vary cyclically. In this regard, since the circular error is periodic, with a period of 2π in this example, the number of calculated values stored in the look-up table unit 130 can optionally be reduced to a quarter of the original. However, those skilled in the art should understand that if the model of the transmitted optical signal is asymmetric (e.g., the model has different rise and fall times and / or more generally an asymmetric envelope), then usually an alternative to this simplification is required. In this regard, due to the lack of symmetry in the model, all calculated values based on the asymmetric model need to be stored for subsequent access, and thus all calculated phase angle error values -Ф err need to be stored.

[0081] In another example ( Figure 6 ), an amplitude calculation path 134 is added to Figure 1 the first indirect time-of-flight distance calculation device 100 to produce a second indirect time-of-flight distance calculation device 150. In this regard, and as already described above with respect to Figure 1 , a phase angle calculation path 136 is provided, with an arctangent element 124, a look-up table unit 130, and a summing unit 128 in the phase angle calculation path 136. The amplitude calculation path 134 is arranged in parallel with the phase angle calculation path 136 and receives the output from the look-up table unit 130. Similar to the arctangent unit 124, the first pair of I / Q outputs of a plurality of digital I / Q outputs 125 related to the first harmonic of the received reflected optical signal are also coupled to the input of an amplitude calculation unit 138, and the output of this amplitude calculation unit 138 is operably coupled to the first input 140 of a multiplier unit 142. In this example, the look-up table unit 130 includes a first output 144 operably coupled to the second input 132 of the summing unit 128 and a second output 146 operably coupled to the second input 148 of the multiplier unit 142. The multiplier unit 142 also includes an output for providing the product of the first signal and the second signal respectively applied to its two inputs.

[0082] In operation ( Figure 7 ), the second indirect time-of-flight distance device 150 operates in the same way as Figure 1operates in a similar manner to the first indirect flight time calculation device 100, which is related to the generation of the I / Q components of the vector V (steps 200 to step 204) and is actually related to the generation of the corrected phase angle value However, in addition to using the I / Q components of the vector V to calculate the corrected phase angle value The amplitude calculation unit 138 also uses the I / Q components of the vector V to calculate (step 214) the amplitude of the vector V according to the indirect flight time measurement technique, and this amplitude constitutes the amplitude Lx extracted (measured) from the fundamental frequency in-phase value and quadrature value in the complex plane meas The multiplier unit 142 receives the extracted amplitude Lx meas In response to receiving the extracted angle The lookup table unit 130 also accesses (step 208) the amplitude correction value C corresponding to the received extracted angle value Lx and outputs this amplitude correction value C at the second output 146 of the lookup table unit 130 Lx This amplitude correction value C Lx is received by the multiplier unit 142 at its second input 148 and is multiplied (step 216) with the extracted amplitude Lx received at its first input 140 meas This amplitude Lx meas corresponds to the amplitude correction value C retrieved by the lookup table unit 130 Lx The product of the extracted amplitude Lx meas and the amplitude correction value C Lx produces the corrected measured amplitude Lx cor which is provided (step 218) at the output of the multiplier unit 142 cor .

[0083] Repeat (step 220) the above steps (steps 202 to step 218) until no further correction of the measured angle and amplitude is required.

[0084] Go back to the lookup table unit 130 again. In this example as explained above, the phase angle error value -Ф is retrieved with reference to the extracted phase angle err and the amplitude correction value C Lx . Similar to the phase angle error value -Ф err it is possible to calculate the amplitude correction value C for the lookup table based on the model of the optical signal emitted by the light source (listed in equation (1) above). In this regard, the in-phase component and quadrature component of the vector V calculated above with respect to equations (4) to (7) are also used to calculate the amplitude correction value C Lx . Lx。Similar to the phase angle, there are more than one technique available for calculating the amplitude of vector V: a first amplitude calculation technique (e.g., Euclidean norm (L2) technique) and a second amplitude calculation technique (e.g., the so-called taxi norm (Manhattan distance) (L1)), with the latter technique being more suitable for a four-phase system where piecewise linear segments of the in-phase component (I(Φ)) and the quadrature component (Q(Φ)) are used for phase angle extraction. For completeness, using the Euclidean norm L2 technique, the amplitude of vector V is calculated as follows:

[0085]

[0086] However, in this example, using the taxi norm L1 technique, the amplitude of vector V is calculated as follows:

[0087] L1 est = |I(φ)| + |Q(φ)| (14)

[0088] It has been calculated for it with respect to the triangular phase angle calculation technique described above (equations (9), (10), (11)).

[0089] In a manner similar to the way the above calculation of the phase angle error value Ф err is performed, an amplitude correction value C is calculated using the measured vector V and the in-phase value and quadrature value corresponding to the nominal true phase angle Ф Lx :

[0090]

[0091] where x indicates the amplitude calculation technique used to calculate the amplitude of the measured vector V and the amplitude corresponding to the nominal true phase angle Ф.

[0092] Reference Figure 8 ., in order to characterize the model specific to the light emitted by the light source, as for the foregoing example, for example, during manufacturing or during on-site use, the skew time t of the period of the optical signal emitted during the end-of-line test of the device 100 is measured (step 500) s once, in order to take into account the operating parameter drift and / or temperature effects that occur during the lifetime of the device 100. The skew time can be the average of multiple skew times measured over multiple periods of the emitted optical signal. By measuring the skew time, it is possible to obtain an indication of the bandwidth of the optical signal and the harmonics of the optical signal that cause circular error.

[0093] Using the model of Equation (1) for the theoretical time-of-flight distance calculation system as characterized by the measured tilt time (step 500) as described above (step 502), m measurements are made during the period of the optical signal s(t) 400. This results in m different integration periods for the integrator 108, and sampling points are obtained every 2π / m over the entire illumination period of the optical signal 400. Each sampling point P k (where k = 0, …, m−1) is characterized by Equation (3) as described above during the period of the optical signal 400.

[0094] Using Equation (3) above, for each of the k (= 0, …, m−1) phase-shifted mixed signals 402, 404, 406, 408, the sampling point P k (Ф) is calculated (step 504). The m sampling points during a given period of the optical signal 400 are used to calculate (step 506) the in-phase vector component and the quadrature vector component using Fourier transform, which in this example is the first harmonic of the optical signal.

[0095] Adopting the four-phase system (m = 4) used above, according to Equations (6) and (7) above, the in-phase vector component I(Φ) and the quadrature vector component Q(Φ) are calculated using the first sampling point value P0, 410, the second sampling point value P1, 412, the third sampling point value P2, 414, and the fourth sampling point value P3, 416.

[0096] Based on the in-phase and quadrature vector components, the estimated phase angle Φ est (Ф) 418 and the estimated amplitude value Lx(Ф) are calculated (step 508) using, for example, the atan2 method or the trigonometric method and the taxi norm or the Euclidean norm method.

[0097] Depending on the value of the bandwidth-time product Γ as described above, the atan2 method or the trigonometric method can be used to estimate the phase angle based on the configured model. Similarly, depending on whether the number of measurements is odd or even, the taxi norm or the Euclidean norm method can be used to estimate the amplitude based on the configured model. Then, according to Equation (12) above, using the knowledge of the actual phase angle Φ used in calculating the in-phase component and the quadrature component of the above vectors, for the estimated phase angle Ф est the phase angle error value Ф err 420 is calculated (step 510). Similarly, according to Equation (15) above, for the estimated phase angle Ф est the amplitude correction value C Lx is calculated (step 510).

[0098] Apply the configured model to Equation (3), which can be used together with the expressions for calculating the above-mentioned in-phase value and quadrature value, and the appropriate equation and the above expression (Equation (15)) for obtaining the estimated amplitude from the in-phase component and the quadrature component can be used to calculate the amplitude correction value C Lx This calculation process constitutes another function for calculating (step 512) the amplitude correction value C Lx Then use the function already mentioned above to calculate the estimated phase angle Ф for a predetermined number of "ground truth" phase angles est and the corresponding phase angle error value Ф err . In this regard, the predetermined number of "ground truth" phase angles can be quantization values, such as 12-bit quantization phase angle values. In this example, the phase angle error value Ф err is converted (e.g., inverted (-Ф est )) before being stored in the look-up table unit 130 together with the corresponding estimated phase angle Ф err value

[0099] Reference Figure 9 , it can be seen that when plotting the estimated phase angle value Ф est , with respect to the calculated amplitude correction value C Lx , the calculated phase angle correction value C Lx also varies cyclically with the estimated phase angle error value Ф est . However, as mentioned above, those skilled in the art should understand that if the model of the transmitted optical signal is asymmetric (e.g., the model has different rise times and fall times and / or more generally an asymmetric envelope), then usually an alternative to this simplification needs to be adopted. In this regard, due to the lack of symmetry in the model, it is necessary to store all the calculated values based on the asymmetric model for subsequent access, and thus it is necessary to store all the calculated phase angle error values -Ф err as well as all the calculated amplitude correction values C Lx

[0100] In another embodiment, for example, during manufacturing, a reference measurement can be performed to calibrate the device 100 in order to generate look-up table data. In this regard, another indirect time-of-flight measurement technique can be employed to perform a reference measurement of a substantially reference static scene using a higher-precision measurement mode of the devices 100, 150, and this precision is higher than the precision level of using the indirect time-of-flight measurement technique described above during the standard operation of the devices 100, 150. In this regard, the higher precision provided by the higher-precision measurement mode can be attributed in this example to obtaining a larger number of sampling points P through the higher-precision indirect time-of-flight measurement technique k ​, such as the number of measurements at equidistant phase offsets within a time frame is greater than the number of measurements made during the standard operation of devices 100, 150. In such an operating mode, a lower circular error is expected. Optionally, the number of sampling points can be odd.

[0101] Once obtained, the reference image is used in combination with the measurements of the same static reference scene in the standard operating mode to calculate the data points of the look-up table. In this regard, the difference between the amplitude values and / or phase angle values calculated using the higher precision mode and those obtained using the standard precision mode is used to generate the data points for the look-up table. For example, using such reference illumination data, the phase angle error value can be calculated as follows:

[0102] Φ err (Φ ref ) = Φ 标准 (Φ ref ) - Φ ref (16)

[0103] where, Ф err (Ф ref ) is the phase angle value error with respect to the reference measurement, particularly the phase angle value calculated with respect to the reference measurement, Ф 标准 (Ф ref ) is the phase angle value calculated with respect to the measurement made using the standard precision mode, and Ф ref is the phase angle value calculated using the higher precision mode. Regarding the amplitude error value, it can be calculated as follows:

[0104]

[0105] where, Lx err (Ф ref ) is the amplitude error value with respect to the reference measurement with the corresponding reference phase angle measurement value Ф ref , Lx 标准 (Ф ref ) is the amplitude value calculated with respect to the amplitude measurement made using the standard precision mode, and Lx ref (Ф ref ) is the amplitude measurement made using the higher precision mode.

[0106] In a further example, such as during manufacturing, reference measurements can be taken to calibrate the device 100 in order to generate look-up table data. However, for some applications, the look-up table data can benefit from supplementary data regarding one or more distances, e.g., where the reference scene is missing one or more objects at the one or more distances and thus data cannot be acquired. In this regard, the techniques previously described for generating error values using a model of the configuration of the light emitted by the light source can be employed to provide additional error value data regarding the one or more distances.

[0107] In this regard, a higher precision mode is used to calculate a first plurality of measurement vectors as reference measurements regarding a substantially static reference scene, and a first plurality of phase angles are calculated respectively from the first plurality of measurement vectors. Similarly, using a standard precision mode, a second plurality of measurement vectors are calculated, and a second plurality of phase angles are calculated respectively from the second plurality of measurement vectors.

[0108] In this example employing the reference scene and the previous examples, the reference scene is substantially static, or in other words quasi-static, between measurements using the higher precision mode and the standard precision mode.

[0109] The device 100 is capable of measuring distances over a range (or span) of possible distances limited by aliasing associated with the modulation frequency of the light source used. As explained above, the first plurality of phase angles and the second plurality of phase angles are used to calculate a plurality of phase angle errors, constituting the use of reference illumination data, the plurality of phase angle errors providing correction regarding a part (but not all) of the range of measurable distances. Also as explained above, this may be due to the reference scene missing one or more objects at one or more distances covered by the range of measurable distances. Thus, the data set of phase angle error values associated with the range of measurable distances is incomplete. However, outside of this part of the range of measurable distances, one or more calculations of measurement vectors corresponding to distance values are performed using the techniques previously described for calculating phase angle error values using a model of the configuration of the light emitted by the light source. The one or more phase angle error values calculated using the latter technique help to complete the data set of phase angle error values.

[0110] Optionally, any suitable interpolation technique can be used in conjunction with the above techniques to generate a data set of phase angle error values. In this regard, one or more phase angle error values calculated using a model of the transient of the light emitted by the light source can be used to complete the data set of phase angle error values, the plurality of phase angle error values being calculated using the reference scene and the interpolation technique.

[0111] In this example and other examples set forth herein, when this measurement technique (and prior measurement techniques) are employed with respect to a pixel array, multiple amplitude error values and / or phase angle error values may be calculated to populate the data store for a look-up table. However, the number of error values calculated depends on the content of the static scene and thus, in some examples, known techniques (such as statistical techniques) are used to calculate missing data points, such as for fitting available data points to complete a curve representative of the calculated error values stored in the look-up table.

[0112] Those skilled in the art should recognize that the above implementations are merely examples of the various implementations conceivable within the scope of the appended claims. In fact, it should be recognized that, for example, the above-described techniques employing reference measurements can be used in combination with the above-described techniques for calculating parameters of the emitted light during standard operation of apparatuses 100, 150.

[0113] It should be recognized that, unless otherwise explicitly stated, references to "light" herein are intended as references to the optical range of the electromagnetic spectrum, e.g., between approximately 350 nm and approximately 2000 nm, such as between approximately 550 nm and approximately 1400 nm, or between approximately 600 nm and approximately 1000 nm.

[0114] Alternative embodiments of the invention may be implemented as a computer program product for use with a computer system, the computer program product being, for example, a series of computer instructions stored on a tangible data recording medium (such as a disk, CD-ROM, ROM, or fixed disk) or embodied in a computer data signal that is transmitted through a tangible medium or a wireless medium (such as microwave or infrared). The series of computer instructions may constitute all or part of the above-described functionality and may also be stored in any memory device, volatile or non-volatile, such as a semiconductor device, a magnetic device, an optical device, or other memory device.

Claims

1. An optical distance calculation device, comprising: A light source configured to emit light according to an indirect time-of-flight measurement method; A photon mixer unit configured to generate and store a plurality of electrical output signals respectively corresponding to a plurality of predetermined phase offset values applied according to the indirect time-of-flight measurement method; A signal processing circuit configured to process the plurality of electrical output signals according to the indirect time-of-flight measurement method so as to calculate a measurement vector and a measured phase angle from the measurement vector; Wherein The signal processing circuit is configured to calculate a phase angle correction value and apply the calculated phase angle correction value so as to correct the measured phase angle; The signal processing circuit is configured to use prior knowledge of a model of the waveform of the light emitted by the light source as reference illumination data to calculate the phase angle correction value, and the model is pre-characterized by a measured tilt time of the light emitted by the light source; and The signal processing circuit is configured to use the corrected measured phase angle to calculate the distance.

2. The device according to claim 1, further comprising: A data storage configured to store a plurality of input phase angles and a plurality of phase angle correction values respectively associated with the plurality of input phase angles; Wherein The plurality of phase angle correction values are pre-calculated using the waveform model of the light emitted by the light source; And The signal processing circuit is configured to access the data storage and provide the phase angle correction value corresponding to the measured phase angle, and apply the phase angle correction value to the measured phase angle to produce the corrected measured phase angle.

3. The device according to claim 2, characterized in that, The data storage is configured to store a look-up table, and the look-up table includes the plurality of input phase angles and the plurality of phase angle correction values.

4. The device according to claim 1 or claim 2 or claim 3, characterized in that, The waveform model of the light emitted by the light source is used to calculate an estimated phase angle corresponding to a predetermined phase angle, and the phase correction value is calculated from the estimated phase angle and the associated predetermined phase angle.

5. The device according to claim 1, wherein The signal processing circuit is configured to adopt a first phase angle calculation method and a second phase angle calculation method, and the signal processing circuit is configured to select the first phase angle calculation method to calculate a first phase angle correction as the phase angle correction value, or select the second phase angle calculation method to calculate a second phase angle correction as the phase angle correction value in response to the total number of the plurality of predetermined phase offset values applied in a time frame according to the indirect time-of-flight measurement method.

6. The device according to any one of claims 1 to 3, characterized in that, The signal processing circuit is configured to measure the tilt time of the cycle of the emitted light and use the measured slope to configure the waveform model of the emitted light.

7. The device according to claim 1, wherein The signal processing circuit is configured to calculate a measured amplitude from the measurement vector for calculating the measured phase angle; and The signal processing circuit is configured to apply an amplitude correction value to the measured amplitude to provide a corrected measured amplitude.

8. The device according to claim 7, wherein The waveform model of the light emitted by the light source is used to calculate an estimated phase angle corresponding to a predetermined phase angle; and The waveform model of the light emitted by the light source is used to calculate an estimated amplitude value of the measurement vector, and the amplitude correction value is derived from the estimated amplitude value.

9. The apparatus according to claim 7, wherein the signal processing circuit is configured to employ a first amplitude calculation method and a second amplitude calculation method, and the signal processing circuit is configured to select the first amplitude calculation method to provide a first amplitude correction value as the amplitude correction value or select the second amplitude calculation method to provide a second amplitude correction value as the amplitude correction value in response to the total number of the plurality of predetermined phase offset values applied in a time frame according to the indirect time-of-flight measurement method.

10. The apparatus according to claim 7, wherein the apparatus further comprises a data storage configured to store a plurality of input phase angles and a plurality of amplitude correction values respectively associated with the plurality of input phase angles; using the waveform model of the light emitted by the light source to pre-calculate the plurality of amplitude correction values; and the signal processing circuit is configured to access the data storage and provide the amplitude correction value corresponding to the measured phase angle, and apply the amplitude correction value to the measured amplitude to generate the corrected measured amplitude.

11. The device according to any one of claims 1-3, characterized in that, The signal processing circuit is configured to measure transient values corresponding to the light emitted by the light source during a calibration phase.

12. The apparatus according to claim 4, wherein the signal processing circuit is configured to employ another indirect flight measurement method to calculate a plurality of measurement vectors as reference measurements, and calculate a plurality of phase angles respectively from the plurality of measurement vectors, the associated accuracy of the another indirect time-of-flight measurement method being greater than the accuracy of the indirect time-of-flight measurement method used to calculate the measurement vectors; and the signal processing circuit is configured to use the indirect time-of-flight measurement method to measure another plurality of vectors and calculate another plurality of phase angles respectively from the another plurality of vectors.

13. The apparatus according to claim 12, further comprising: a measurable distance range having an associated data set of phase angle errors; wherein the signal processing circuit is configured to use the plurality of phase angles and the another plurality of phase angles to calculate a plurality of phase angle errors, the plurality of phase angle errors providing correction for a part of the measurable distance range; and the phase angle correction value calculated using the indirect time-of-flight measurement method is related to distances outside and within the part of the measurable distance range, and helps to complete the data set of the phase angle errors.

14. The device according to claim 1, characterized in that, The reference illumination data is difference data calculated between an image of a substantially static reference scene acquired in a reference mode and another image of the substantially static reference scene acquired in a standard mode.

15. A method for distance calculation, comprising: The light source emits light according to the indirect time-of-flight measurement method; The photon mixer unit generates and stores a plurality of electrical output signals respectively corresponding to a plurality of predetermined phase offset values applied according to the indirect time-of-flight measurement method; The signal processing circuit processes the plurality of electrical output signals according to the indirect time-of-flight measurement method so as to calculate a measurement vector and a measured phase angle from the measurement vector; The signal processing circuit calculates a phase angle correction value and applies the calculated phase angle correction value so as to correct the measured phase angle; The signal processing circuit uses prior knowledge of a model of the waveform of the light emitted by the light source as reference illumination data to calculate the phase angle correction value, and the model is pre-characterized by the measured tilt time of the light emitted by the light source; And The signal processing circuit uses the corrected measured phase angle to calculate the distance.

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