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 increasing the signal to noise ratio of the signal waveforms generated by an apparatus for optically sensing a remote object in relative motion along the line of sight of said apparatus, comprising the steps of:
(a) providing said apparatus for optically sensing said object, said apparatus including means for
(a1) sending a predetermined number N.sub.S of optical pulses towards said object,
(a2) receiving a number N.sub.S of optical signals reflected by said object,
(a3) converting said number N.sub.S of optical signals into a number N.sub.S of digital signal waveforms S.sub.k, k=1, 2, 3, . . . , N.sub.S, each of the said digital signal waveforms being formed of data sampled at a predetermined number N.sub.p of range values R.sub.i, i=1, 2, 3, . . . , N.sub.p, and
(a4) numerically processing said digital signal waveforms,
(b) selecting a set of range shift parameter values RSP.sub.j, j=1, 2, 3, . . . , N.sub.R,
(c) performing a range shift of each said waveform S.sub.k by the value (k−1).times.RSP.sub.j,
(d) generating a waveform SA.sub.j by computing the average of the N.sub.S said waveforms S.sub.k range shifted according to step (c),
(e) finding the peak signal amplitude comprised in the averaged signal waveform SA.sub.j,
(f) repeating steps (c) to (e) for the N.sub.R values of said range shift parameter RSP.sub.j as selected in step (b),
(g) finding the values of said range shift parameter that give averaged signal waveforms having peak signal amplitudes exceeding a predetermined threshold,
(h) combining the averaged signal waveforms determined in step (g) and updating a previously measured signal waveform having the maximum peak signal amplitude with said waveform combination.
2. A method according to claim 1 wherein said provided apparatus for optically sensing a remote object is a laser rangefinder.
3. A method according to claim 1 wherein said provided apparatus for optically sensing a remote object is a lidar.
4. A method according to claim 1 wherein said provided apparatus for optically sensing a remote object 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 determining the range to said object from the range value R.sub.i at which the maximum signal amplitude occurs in the waveform combination determined in step (h).
6. A method according to claim 1 further including the step of measuring the relative velocities of said object along the line of sight of said provided apparatus by computing the ratio of the sign-reversed range shift parameters determined in step (g) by the time delay between detections of two consecutive optical signals reflected by said object.
7. A method according to claim 1 wherein said remote object consists of a plurality of distinct objects, each said distinct object moving independently from each other along the line of sight of said apparatus.
8. A method according to claim 7 further including the step of measuring the relative velocities of said distinct objects along the line of sight of said provided apparatus by computing the ratio of the sign-reversed range shift parameters determined in step (g) by the time delay between detections of two consecutive optical signals reflected by said distinct objects.
9. A method according to claim 7 wherein said interval includes range shift parameter values associated to said object moving at a relative velocities comprised between a predetermined minimum velocity and a predetermined maximum velocity.
10. A method according to claim 1 wherein said set of range shift parameter values RSP.sub.j is determined from a series of N.sub.R equally-spaced values comprised within a predetermined interval.
11. A method according to claim 1 further including the step of recording the maximum signal amplitude of said averaged signal waveform obtained from each value of said range shift parameter, and displaying said maximum signal amplitude as a function of said range shift parameter, whereby the motion of said object at more than one velocity value can be determined from said displayed maximum signal amplitude.
12. An apparatus for optically sensing a remote object in relative motion along a line of sight of said apparatus, said apparatus comprising:
an optical emitter for sending a predetermined number Ns of optical pulses towards said object;
an optical receiver for receiving a number Ns of optical signals reflected by said object and for converting said number N.sub.s of optical signals into a number N.sub.s of digital signal waveforms S.sub.k, k=1, 2, 3, . . . , N.sub.s, each of the digital signal waveforms being formed of data sampled at a predetermined number N.sub.p of range values R.sub.i, i=1, 2, 3, . . . , N.sub.p, and
a control and processing unit for numerically processing said digital signal waveforms, said control and processing unit being further adapted to:
(a) select a set of range shift parameter values RSP.sub.j, j=1, 2, 3, . . . , N.sub.R,
(b) perform a range shift of each said waveform S.sub.k by the value (k−1).times.RSP.sub.j,
(c) generate a waveform SA.sub.j by computing the average of the N.sub.S said waveforms S.sub.k range shifted according to step (b),
(d) find the peak signal amplitude comprised in the averaged signal waveform SA.sub.j,
(e) repeat steps (b) to (d) for the N.sub.R values of said range shift parameter RSP.sub.j as selected in step (a),
(f) find the values of said range shift parameter that give averaged signal waveforms having peak signal amplitudes exceeding a predetermined threshold,
(g) combine the averaged signal waveforms determined in step (f) and update a previously measured signal waveform having the maximum peak signal amplitude with said waveform combination.