Method of multi-phase correlations vector synthesis ranging by fractional correlation

Abstract

In N-phase correlations vector synthesis time-of-flight (ToF) ranging employing N correlators, the correlation time at each signal cycle is reduced to mitigate pixel saturation by sun light or strong reflected light as well as to minimize the influence of external noise. Typically, the correlation time, during which the received signal is correlated with the transmitting signal, is set to be one full cycle in each transmitting signal period. In this invention, reducing the correlation time to $\frac{1}{N},\frac{1}{2N},\mathrm{or}\frac{1}{kN}$
of a full cycle period in each transmitting signal period is disclosed, where k is a real number greater than 1, but k is not 2. Depending on the intensity of the ambient light, the correlation time is flexibly and optimally selected. Multiple fractional correlations produced by a reduced correlation time are integrated over multiple signal periods to obtain more reliable signals of the correlation vectors.

Claims

1. A method for measuring a distance between a transmitter and an object based on a phase delay between a transmitting signal and a reflected and received signal by a receiver in N-phase correlations time-of-flight (ToF) ranging, where the receiver includes an N-phase correlation vector controller, a correlator array, a zero-force synthesizer, and a signal processor, the N-phase correlation vector controller generating N delay-tap signals that control N-phase correlators in the correlator array, and each of the N-phase correlators performing an integration by accumulating photons in image sensor pixels of the receiver, where N is an odd number greater than or equal to 3, the method comprising: (a) setting, by the N-phase correlation vector controller of the receiver, an integration start time of each of the N-phase correlators, at which time the N-phase correlators start to accumulate photons in the image sensor pixels, where the integration start time of each of the N-phase correlators is sequentially and equally time delayed by one period of the transmitting signal divided by N; (b) adjusting, by the N-phase correlation vector controller of the receiver, an integration time of the N-phase correlators, during which time the N-phase correlators accumulate the photons in the image sensor pixels, from one transmitting signal period to $\frac{1}{N}$ of the one transmitting signal period; (c) obtaining, by the N-phase correlators of the receiver, N correlation vectors over one or more periods of the transmitting signal, where each of the N correlation vectors is sequentially and equally time delayed by one period of the transmitting signal divided by N; (d) synthesizing by the zero-force synthesizer of the receiver, two-phase orthogonal signals from the N correlation vectors, where the zero-force synthesizer uses pre-determined synthesis coefficients; and (e) determining by the signal processor of the receiver, the phase delay or the distance from the two-phase orthogonal signals.

2. The method according to claim 1, wherein in the step (b), the adjusting is performed from one transmitting signal period to $\frac{1}{2N}$ of the one transmitting signal period.

3. The method according to claim 1, wherein in the step (b), the adjusting is performed from one transmitting signal period to $\frac{1}{kN}$ of the one transmitting signal period, where k is a real number greater than 1, but k is not 2.

4. The method according to claim 1, wherein in the step (e), the distance is further compensated for by pre-estimated phase errors with a period over 360° based on a phase difference between a perfect circle and a 2N-gon Lissajous graph resulting from the two-phase orthogonal signals that are synthesized from the N correlation vectors.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Embodiments of the claimed subject matter are understood by referring to the figures in the attached drawings, as provided below.

(2) FIG. 1A illustrates a block diagram of a multi-phase correlations time-of-flight (ToF) ranging apparatus for a five-phase case (N=5).

(3) FIG. 1B draws a correlation timing diagram of the multi-phase correlations ToF ranging apparatus in FIG. 1A.

(4) FIG. 1C draws zero-force (ZF) synthesized orthogonal signals by the multi-phase correlations ToF ranging in FIG. 1A via a computer simulation.

(5) FIG. 1D draws a Lissajous graph of the ZF synthesized orthogonal signals in FIG. 1 C.

(6) FIG. 1E draws the arctangent of the ratio between the ZF synthesized orthogonal signals in FIG. 1C.

(7) FIG. 1F draws the phase delay measurement error for the ZF synthesized orthogonal signals in FIG. 1C in comparison with that of the conventional four-phase correlations (four bucket) algorithm.

(8) FIG. 2A draws a correlation timing diagram when the correlation time is Ts/N in each clock cycle of the transmitting signal for the five-phase case (N=5) in FIG. 1A according to the present invention.

(9) FIG. 2B draws the phase delay measurement error for the ZF synthesized orthogonal signals when the Ts/N correlation time is applied for the case of N=5 in comparison with that of the conventional four-phase correlations (four-bucket) algorithm.

(10) FIG. 3A draws a correlation timing diagram when the correlation time is Ts/(2N) in each clock cycle of the transmitted signal for the five-phase case (N=5) in FIG. 1A according to the present invention.

(11) FIG. 3B draws the phase-delay measurement error for the ZF synthesized orthogonal signals when the Ts/(2N) correlation time is applied for the case of N=5 in comparison with that of the conventional four-phase correlations (four-bucket) algorithm.

(12) FIG. 4A draws a correlation timing diagram when the correlation time is Ts/(kN) in each clock cycle of the transmitting signal for the five-phase case (N=5) in FIG. 1A according to the present invention.

(13) FIG. 4B draws zero-force (ZF) synthesized orthogonal signals when the correlation time Ts/(kN) is applied for the case of N=5 and k=7.2 in FIG. 4A.

(14) FIG. 4C draws a Lissajous graph of the ZF synthesized orthogonal signals in FIG. 4B.

(15) FIG. 4D draws the arctangent of the ratio between the ZF synthesized orthogonal signals in FIG. 4B.

(16) FIG. 4E draws the phase error between the perfect circle and the Lissajous graph in FIG. 4C.

(17) FIG. 4F illustrates a block diagram compensating for the phase delay measurement error in FIG. 4C according to the present invention.

DETAILED DESCRIPTION OF THE INVENTION

(18) In the following, numerous specific details are set forth to provide a thorough description of various embodiments of the claimed subject matter. Certain embodiments may be practiced without these specific details or with some variations in detail. In some instances, certain features are described in less detail so as not to obscure other aspects of the disclosed embodiments. The level of detail associated with each of the elements or features should not be construed to qualify the novelty or importance of one feature over the others.

(19) In what follows, the principle of a fractional correlation method in multi-phase correlations time-of-flight (ToF) ranging is explained for the case of N=5—that is , five-phase correlations vector synthesis—that is disclosed in U.S. Pat. No. 11,435,455 B2 by the present inventor. A block diagram of the five-phase correlations vector synthesis ToF ranging is illustrated in FIG. 1A. In U.S. Pat. No. 11,435,455 B2, the starting time of the correlation is shifted by 72° at each of the five correlators, and correlation time is a whole cycle at the five correlators. The effective correlation time, during which a non-zero correlation is performed, is half a period (T.sub.s/2=180°) of the transmitting signal, as illustrated in FIG. 1B, where T.sub.s is the period of the transmitting signal. The computer simulation results for the correlation time T.sub.s/2 are drawn in FIG. 1C˜FIG. 1F. The transmitting signal used in the computer simulation is a 50% duty cycle square-waveform signal.

(20) In the present invention, a flexible correlation time is applied that is less than half a period (<T.sub.s/2) of the transmitting signal. Each fractional correlation at each signal cycle is accumulated when the total integration time is extended to multiple signal cycles.

(21) In FIG. 2A, the correlation time of 1/15.sup.th of a period (T.sub.s/5=72°) is applied to correlators #1˜#5. The output signals (V.sub.1, V.sub.2, V.sub.3, V.sub.4, and V.sub.5) of the correlators (#1, #2, #3, #4, and #5), respectively, are the same as those in FIG. 1A. Therefore, the phase delay measurement error (FIG. 2B) of the orthogonal signals ZF synthesized by correlation vectors V.sub.1˜V.sub.5 is the same as that in FIG. 1F.

(22) Even though the correlation time is reduced to T.sub.s/5 from T.sub.s/2, the maximum measurement error is ±0.25°. This implies that, instead of applying the correlation time of half a period, an equivalent result is obtained by applying

(23) $\frac{1}{N}$
of a period. The advantage of reducing the correlation time is to keep the pixels from being saturated as well as reduce noise by decreasing the correlation time when sun light or strong reflected light is present.

(24) In another aspect, a correlation time of 1/10.sup.th of a period

(25) $\left(\frac{T_s}{2N}=36^{\circ }\right)$
is applied to correlators #1˜#5, as shown in FIG. 3A . The correlation starting time at each of five correlators is shifted by 72° (T.sub.s/5=72°) as before; however, the correlation time is further reduced to

(26) 0$\frac{1}{2N}$
of a period of the transmitting signal.

(27) The Lissajous graph of the ZF synthesized orthogonal signals for the 50% duty cycle square-waveform input signals is a 2N-gon as disclosed in U.S. 11,221,237 B2 by the present inventor. For the case of N=5, the Lissajous graph for the signals in FIG. 1C is a 10-gon, as drawn in FIG. 1D. Since the angle of each side of the 10-gon is 36°, the correlation time of 36° in FIG. 3A implies physically that the correlation is performed for one side of the total of 10 sides. In this case, the phase delay measurement error of the ZF synthesized orthogonal signals is shown to be ±0.27°, which is very close to the ±0.25° that was achieved with a 180° correlation time of a period in FIG. 1F. By shortening more of the correlation time, pixel saturation can be greatly reduced and the amount of noise infiltrating the pixels can also be lowered.

(28) In a further development, the correlation time can be reduced

(29) $\frac{1}{kN}$
or one period of the transmitting signal, where k is a real number greater than 1, but k is not 2. As long as the correlation starting time of each of the five correlators is equally spaced by 72° (T.sub.s/5=72°), the correlation time of

(30) $\frac{1}{kN}$
of a period of the transmitting signal yields legitimate five correlation vectors. In FIG. 4B, orthogonal signals are drawn that are ZF synthesized from the five-phase correlations vectors when the correlation time k=7.2—correlation time is 10° of a 360° period—is applied. In FIG. 4C and FIG. 4D, a Lissajous graph and the arctangent of the ratio between the ZF synthesized orthogonal signals are drawn, respectively. The Lissajous graph still holds a 10-gon shape; however, the phase estimation shows a periodic phase error in the arctangent graph (FIG. 4D). The periodic phase estimation errors have a certain repetitive pattern every 36° over one period of the transmitting signal. The repetitive error pattern of phase estimation is due to an intrinsic characteristic of the ZF synthesis. On the whole, the estimated phase asymptotically approaches an ideal circle, as drawn in FIG. 4E.

(31) When the number of phases in the N-phase correlations increases such that N=7, 9, and 11, the repetitive phase error pattern becomes a 14-gon, 18-gon, and 22-gon, respectively. Therefore, even though the phase error has the same pattern, the error amount gets smaller as the number of phases increases. As the 2N-gon and an ideal circle is known a priori, the phase error is pre-calculated over the Lissajous circle via piecewise approximation and can be stored in a look-up table to compensate for the error, as illustrated in FIG. 4F. Accordingly, while reducing the correlation time by

(32) $\frac{1}{kN}$
of a period of the transmitting signal in order to mitigate pixel saturation, accurate phase estimation is feasible by compensating for the pre-calculated error in real time.

(33) The method of reducing the correlation time presented in the invention enables the selection of an optimal correlation time in order to mitigate pixel saturation by sun light as well as to minimize the influence of external noise.

(34) While the exemplary methods of the present disclosure described above are represented as a series of operations for clarity of description, it is not intended to limit the order in which the steps are performed, and the steps may be performed simultaneously or in different order as necessary. In order to implement the method according to the present disclosure, the described steps may further include other steps, may include remaining steps except for some of the steps, or may include other additional steps except for some of the steps.

(35) While the present disclosure has been described with reference to embodiments thereof, it will be apparent to those of ordinary skill in the art that various changes and modifications may be made thereto without departing from the spirit and scope of the present disclosure as set forth in the following claims.