Digital signal processing in optical systems used for ranging applications
Methods and apparatuses for reducing the response time along with increasing the probability of ranging of optical rangefinders that digitize the signal waveforms obtained from the pulse echoes returned from various types of objects to be ranged, the pulse echoes being too weak to allow successful ranging from a single waveform or the objects being possibly in motion during the capture of the pulse echoes. In a first embodiment of the invention, the response time at close range of a digital optical rangefinder is reduced by using a signal averaging process wherein the number of data to be averaged varies with the distance according to a predetermined function. In a second embodiment of the invention, the probability of ranging objects in motion along the line of sight of a digital optical rangefinder is increased and the object velocity measured by performing a range shift of each acquired signal waveform prior to averaging. In a third embodiment of the invention, the signal waveforms acquired in the line of sight of a digital optical rangefinder are scanned over a predetermined zone and range shifted and averaged to allow for early detection and ranging of objects that enter in the zone.
1. A method for optically sensing a remote object using an optical sensing apparatus, said object moving in a zone covered by said apparatus, the method comprising the steps of:
(a) angularly scanning the line of sight of said optical sensing apparatus to cover a predetermined zone,
(b) sending at least one optical pulse at each predetermined line of sight tilted by an angle θ j relative to a reference direction,
(c) receiving for each predetermined line of sight θ j at least one optical signal,
(d) converting said optical signals into digital signal waveforms S, each of the said digital signal waveforms being formed of data sampled at a predetermined number N p of range values R i , i=1, 2, 3, . . . , N p , and
(e) storing into memory said signal waveforms S(R i , θ j , t k ) acquired for each line of sight θ j and for each acquisition time t k ,
(f) numerically processing said digital signal waveforms, said numerical processing step comprising:
(f1) retrieving from said memory the signal waveforms S acquired for the lines of sight θ j enclosed in an angular subregion delimited by predetermined boundary angles θ REF and θ LIM , so that θ LIM ≦θ j ≦θ REF ,
(f2) selecting intervals over which the values of the object velocity V, the object direction of travel θ V , the distance of intersection D REF of said object with the reference line of sight θ REF and the time t REF of intersection with said reference line of sight θ REF will be varied,
(f3) computing for a first combination of parameters V, θ V , D REF , t REF the time t j at which said object could have intersected a first line of sight θ j enclosed in said angular subregion, and retrieving the signal waveform S(R i , θ j , t k ) acquired at the time t k which is the closest to said computed time t j ,
(f4) computing for said first combination of parameters V, θ V , D REF , t REF the distance D j from said apparatus at which the object could have intersected said first line of sight θ j ,
(f5) performing a range shift by the quantity D REF -D j of said signal waveform S(R i , θ j , t k ) retrieved in step (d),
(f6) repeating step (f3) to (f5) for each said line of sight θ j enclosed in said angular subregion,
(f7) generating a waveform SA by computing the average of the set of signal waveforms that have been range-shifted according to step (g),
(f8) repeating steps (f3) to (f7) for each different combination of said parameters V, θ V , D REF , t REF ,
(f9) finding the specific combination of said parameters V, θ V , D REF , t REF , that gives an averaged signal waveform SA having the maximum signal amplitude,
(f10) updating a previous signal waveform having the maximum signal amplitude with the one determined in step (f9),
(f11) updating motion parameters of said object with the said combination of parameters V, θ V , D REF , t REF determined in step (f9);
wherein the signal to noise ratio of the generated signal waveform generated by the optical sensing apparatus is increased by said numerical processing.
2. A method according to claim 1 wherein said optical sensing apparatus comprises a laser rangefinder.
3. A method according to claim 1 wherein said optical sensing apparatus comprises a lidar.
4. A method according to claim 1 wherein said optical sensing apparatus uses at least one light-emitting diode as means for sending optical pulses towards said object.
5. A method according to claim 1 further including the step of repeating steps (f1) to (f11) for a set of angular subregions, each of the said angular subregions being delimited by predetermined values of the boundary angles θ REF and θ LIM .
6. A method according to claim 5 wherein the boundary angle θ REF of the first angular subregion points along the maximum right-sided deviation of the line of sight of said apparatus, whereby an object coming from the right is ranged as soon as it enters in the zone covered by said apparatus.
7. A method according to claim 5 wherein the boundary angle θ REF of the second angular subregion points along the maximum left-sided deviation of the line of sight of said apparatus, whereby an object coming from the left is ranged as soon as it enters in the zone covered by said apparatus.
8. A method according to claim 5 wherein said first combination of parameters V, θ V , D REF , t REF for each angular subregion is given by the specific combination of parameters that maximized the signal amplitude comprised in the averaged signal waveform obtained from the preceding angular subregion, whereby an object in regular motion is ranged using a reduced set of combination parameters V, θ V , D REF , t REF .
9. A method according to claim 1 further including the step of measuring the range to said object from the distance D REF determined in step (f11).
10. An apparatus for optically sensing a remote object moving in a zone covered by said apparatus and for increasing the signal to noise ratio of the signal waveforms generated by said apparatus, said apparatus comprising:
means for angularly scanning a line of sight of said apparatus to cover a predetermined zone,
an optical emitter module for sending at least one optical pulse at each predetermined line of sight tilted by an angle θ j relative to a reference direction,
an optical receiver module for receiving for each predetermined line of sight θ j at least one optical signal,
means for converting said optical signals into digital signal waveforms S, each of the said digital signal waveforms being formed of data sampled at a predetermined number N p of range values R i , i=1, 2, 3, . . . , N p , and
a control and processing unit for processing said received optical pulses, wherein said control and processing unit is further adapted to:
(a) store into memory said signal waveforms S(R i , θ j , t k ) acquired for each line of sight θ j and for each acquisition time t k ,
(b) retrieve from said memory the signal waveforms S acquired for the lines of sight θ j enclosed in an angular subregion delimited by predetermined boundary angles θ REF and θ LIM , so that θ LIM ≦θ j ≦θ REF ,
(c) select intervals over which the values of the object velocity V, the object direction of travel θ V , the distance of intersection D REF of said object with the reference line of sight θ REF and the time t REF of intersection with said reference line of sight θ REF will be varied,
(d) compute for a first combination of parameters V, θ V , D REF , t REF the time t j at which said object could have intersected a first line of sight θ j enclosed in said angular subregion, and retrieving the signal waveform S(R i , θ j , t k ) acquired at the time t k which is the closest to said computed time t j ,
(e) compute for said first combination of parameters V, θ V , D REF , t REF the distance D j ;
from said apparatus at which the object could have intersected said first line of sight θ j ,
(f) perform a range shift by the quantity D REF -D j of said signal waveform S(R i , θ j , t k ) retrieved in step (d),
(g) repeat step (d) to (f) for each said line of sight θ j enclosed in said angular subregion,
(h) generate a waveform SA by computing the average of the set of signal waveforms that have been range-shifted according to step (g),
(i) repeat steps (d) to (h) for each different combination of said parameters V, θ V , D REF , t REF ,
(j) find the specific combination of said parameters V, θ V, D REF , t REF , that gives an averaged signal waveform SA having the maximum signal amplitude,
(k) update a previous signal waveform having the maximum signal amplitude with the one determined in step (j), and
(l) update motion parameters of said object with the said combination of parameters V, θ V , D REF , t REF determined in step (j).
11. An apparatus according to claim 10 , comprising a laser rangefinder.
12. An apparatus according to claim 10 , comprising a lidar.
13. An apparatus according to claim 10 , said optical emitter module comprising at least one light-emitting diode.
14. An apparatus according to claim 10 , further including the step of repeating steps (b) to (II) for a set of angular subregions, each of the said angular subregions being delimited by predetermined values of the boundary angles θ REF and θ LIM .
15. An apparatus according to claim 14 , wherein the boundary angle θ REF of the first angular subregion points along the maximum right-sided deviation of the line of sight of said apparatus, whereby an object coming from the right is ranged as soon as it enters in the zone covered by said apparatus.
16. An apparatus according to claim 14 , wherein the boundary angle of the angle θ REF of the second angular subregion points along the maximum left-sided deviation of the line of sight of said apparatus, whereby an object coming from the left is ranged as soon as it enters in the zone covered by said apparatus.
17. An apparatus according to claim 14 , wherein said first combination of parameters V, θ V , D REF , t REF for each angular subregion is given by the specific combination of parameters that maximized the signal amplitude comprised in the averaged signal waveform obtained from the preceding angular subregion, whereby an object in regular motion is ranged using a reduced set of combination parameters V, θ V , D REF , t REF .
18. An apparatus according to claim 10 , further including the step of measuring the range to said object from the distance D REF determined in step (1).