Method for measuring the distance to at least one target

• US 6,516,286 B1
• Filed: 04/04/2000
• Issued: 02/04/2003
• Est. Priority Date: 04/06/1999
• Status: Expired due to Term

• Current Assignee: Leica Geosystems AG (Hexagon AB)
• Original Assignee: Leica Geosystems AG (Hexagon AB)
• Inventors: Ramseier, Ernst, Orawez, Georg, Rutishauser, Esther, Aebischer, Beat, Betschon, Christian, Gächter, Bernhard
• Primary Examiner(s): Hilten, John S.
• Assistant Examiner(s): Washburn, Douglas N
• Application Number:
  US09/542,879
• Time in Patent Office: 1,036 Days
• Field of Search: 356/1, 356/5, 356/373, 364/569, 396/104, 280/561, 354/403, 250/201, 250/560, 738/612.8
• US Class Current: 702/158
• CPC Class Codes: G01S 17/10 using transmission of interrupted, pulse-modulated waves determination of distance by phase measurements G01S17/32
• CPC Class Codes: G01S 7/497 Means for monitoring or calibrating

Abstract

To measure the distance to targets, local maxima exceeding a threshold (ε) and being associated with a correlation function (f) are determined, the reference values of which are obtained from scalar products of a received signal (e), produced by reflections of an optical pulse by the targets, and number of comparison functions, copies of a reference function (r) which are shifted as a function of time, and the transit times of the reflections are derived from the time-related shifts of the corresponding comparison functions. By interpolation with respect to the time-related shifts or optimization thereof, for example by adjusting them so that the orthogonal distance of the received signal (e) from the vector space which is spanned by the corresponding comparison functions is a minimum, the transit times can be determined even more accurately.

Claims

1. A method for measuring the distance to at least one target comprising:

sending, by a transmitter, at least one optical pulse toward the target;

picking up, by a receiver, reflections of the at least one pulse as input signals;

converting the reflections of the at least one pulse into a received signal (e);

storing the reflections of the at least one pulse

determining at least one reception time by comparing the received signal (e) with comparison functions, which are copies, shifted by various time intervals, of a previously determined and stored reference function (r), and approximating the at least one reception time from the time intervals which correspond to the most similar comparison functions;

determining a transit time from the reception time of the input signal; and

calculating the distance from said reception time, wherein said reference function (r) is previously determined by sending at least one optical reference pulse toward a reference target and scanning a received reference signal corresponding to the at least one reference pulse to determine reference values of the reference function (r), and thereafter, the reference function (r) is stored.

2. The method as claimed in claim 1 wherein the at least approximate determination of the at least one reception time is based on a set of comparison functions which correspond to predetermined fixed time intervals.

3. The method as claimed in claim 2, wherein the predetermined fixed time intervals are integral multiples of a fixed basic interval.

4. The method as claimed in claim 2 wherein only those comparison functions where in each case the similarity function (f) representing a measure of similarity with the received signal (e) forms a local maximum which exceeds a specific threshold (ε

) are used.

5. The method as claimed in claim 4, wherein the similarity function used is a correlation function (f).

6. The method as claimed in claim 5, wherein the function values of the correlation function are calculated in each case as the scalar product of the received signal (e) and one of the comparison functions.

7. The method as claimed in claim 2 wherein exact reception times are determined starting from the approximately determined reception times, by interpolation between the time intervals.

8. The method as claimed in claim 4, wherein the interpolation is effected by determining the local maximum of the correlation function (f) expanded between reference values to give a continuous function.

9. The method as claimed in claim 7, wherein the interpolation is carried out at least in part by minimizing the distance of the received signal (e) from a vector space which is spanned by the comparison functions corresponding to the reception times, starting from the vector space which is spanned by the comparison functions which correspond to the approximately determined reception times.

10. The method as claimed in claim 1, wherein the received signal (e) is scanned at times which in each case follow one another at intervals of a fixed scanning increment (Δ

).

11. The method as claimed in claim 1, wherein the reference function (r) is determined at a number of points which follow one another at intervals of reference increments (δ

).

12. The method as claimed in claim 11, wherein the reference increment (δ

) corresponds in each case to the scanning increment (Δ

) divided by an integer>

1.

13. The method as claimed in claim 1, wherein, to improve the resolution as a function of time, the reference input signal is shifted as a function of time by various delay values.

14. The method as claimed in claim 13, wherein the shifting of the reference input signal as a function of time is effected by delaying the emission of the optical pulse by the transmitter (6) or of the reference input signal.

15. The method as claimed in claim 13, wherein the shifting of the reference input signal as a function of time is effected by changing the distance to the reference target (10).

16. The method as claimed in claim 1, wherein, to improve the signal-to-noise ratio, the emission of the optical pulse and the scanning of the reference input signal are repeated and, for the formation of the reference value of the reference function (r), the scanning values corresponding to identical transit times are summed or averaged.

17. The method as claimed in claim 10, wherein, starting from reference values, the reference function (r) is expanded by interpolation to give a continuous function preferably continuously differentiable at least once.

18. The method as claimed in claim 13, wherein, a formulation according to which the reference function (r) can be fixed by a finite number of parameters and the parameters are determined from a comparison of the reference input signal with copies of the reference function which are shifted as a function of time by the delay values is postulated for the reference function (r).

19. The method as claimed in claim 18, wherein, to determine the parameters, the distance of the reference received signal from a superposition of the copies of the reference function (r) which are shifted as a function of time by the delay values is minimized.