Method and apparatus for determining the range, direction and velocity of an object

• US 5,748,295 A
• Filed: 07/31/1996
• Issued: 05/05/1998
• Est. Priority Date: 07/31/1996
• Status: Expired due to Fees

• Current Assignee: Instrumentation Corporation, The United States of America As Represented By The Secretary of Agriculture
• Original Assignee: The University of Georgia Research Foundation, Inc.
• Inventors: Farmer, Brian W.
• Primary Examiner(s): Hellner, Mark
• Application Number:
  US08/688,706
• Time in Patent Office: 643 Days
• Field of Search: 356/5.09, 356/28.5
• US Class Current: 356/5.09
• CPC Class Codes: G01S 17/34 using transmission of continuous, frequency-modulated waves while heterodyning the received signal, or a signal derived therefrom, with a locally-generated signal related to the contemporaneously transmitted signal
• CPC Class Codes: G01S 17/58 Velocity or trajectory determination systems; Sense-of-movement determination systems
• CPC Class Codes: G01S 7/4916 using self-mixing in the laser cavity

Abstract

The present invention provides a range finding system which utilizes feedback to induce mode hopping in a standing wave generator and which determines the range, direction and velocity of an object with respect to the range finding system based on the mode hop rate. The present invention provides a novel sampling technique which prevents aliasing by modulating the standing wave generator at a frequency equal to or greater than π times the frequency at which the optical path between the object and the standing wave generator is oscillating or vibrating. In order to account for Doppler effect, the standing wave interference coupling is analyzed over one period of the frequency modulation rate of the standing wave generator. A spectral analysis algorithm is implemented which determines the mode hop rate of the standing wave generator and the range, direction and velocity of the object with respect to the range finding system based on the determined mode hop rate. Implementation of the spectral analysis algorithm improves signal-to-noise ratio by avoiding thresholding to provide a range finding system which is substantially amplitude independent.

Claims

1. An apparatus for determining the range, direction and velocity of an object, said apparatus comprising:

a standing wave generator for generating standing waves and for projecting the standing waves onto the object;

means for detecting the standing waves generated by the standing wave generator and standing waves resulting from interference between the standing waves generated by the standing wave generator and standing waves reflected from the object and for producing electrical signals in response to the standing waves detected;

means for processing the electrical signals produced by said means for detecting the standing waves, said means for processing being electrically coupled to said means for detecting the standing waves for receiving the electrical signals produced by said means for detecting the standing waves, wherein a frequency modulated electrical signal is supplied to said standing wave generator for causing said standing wave generator to generate a frequency modulated standing wave having a frequency which is equal to or greater than π

times the highest frequency at which the optical path between the object and said apparatus is modulating, and wherein causing said standing wave generator to generate a frequency modulated standing wave having a frequency equal to or greater than π

times the highest frequency at which the optical path between the object and said apparatus is modulating prevents aliasing; and

means for determining a mode hop rate of said standing wave generator and for determining the range, direction and velocity of the object with respect to said apparatus based on the determined mode hop rate of said standing wave generator.

2. An apparatus according to claim 1 wherein said means for processing the electrical signals determines the mode hop rate of said standing wave generator over at least one period of the frequency modulated electrical signal.

3. An apparatus according to claim 1 wherein said means for processing the electrical signals converts the electrical signals into digital information and performs a spectral analysis algorithm on the digital information which transforms the digital information into frequency domain information and determines the mode hop rate of said standing wave generator based on peaks occurring in the frequency domain information.

4. An apparatus according to claim 1 wherein said means for processing the electrical signals comprises a digital signal processor.

5. An apparatus according to claim 4 wherein said spectral analysis algorithm comprises the Burg algorithm.

6. An apparatus according to claim 2 wherein said means for processing the electrical signals converts the electrical signals into digital information and performs a spectral analysis algorithm on the digital information which transforms the digital information into frequency domain information and determines the mode hop rate of said standing wave generator based on peaks occurring in the frequency domain information.

7. An apparatus according to claim 6 wherein said means for processing the electrical signals comprises a digital signal processor.

8. An apparatus according to claim 7 wherein said spectral analysis algorithm comprises the Burg algorithm.

9. An apparatus for determining the range, direction and velocity of an object with respect to said apparatus, said apparatus comprising:

a laser comprising an optical cavity having a first end and a second end, said first and second ends each comprising a surface which is partially transmissive and partially reflective, said optical cavity having a laser medium therein which is excitable to produce radiation emissions;

means for detecting light and for producing electrical signals in response to the light detected, said means for detecting light disposed in proximity to said second end for receiving light transmitted through said second end of said laser;

means for collimating light disposed in proximity to said first end of said laser for collimating light transmitted through said first end of said laser and for projecting a collimated beam of light onto the object;

means for processing the electrical signals produced by said means for detecting light, said means for processing being electrically coupled to said means for detecting light for receiving the electrical signals produced by said means for detecting light, wherein a frequency modulated electrical signal is supplied to said laser for causing said laser to generate a frequency modulated standing light wave having a frequency which is equal to or greater than π

times the highest frequency at which the optical path between the object and said laser is modulating, wherein causing said standing wave generator to generate a frequency modulated standing wave having a frequency equal to or greater than π

times the highest frequency at which the optical path between the object and said laser is modulating prevents aliasing, and wherein said means for processing the electrical signals determines the mode hop rate of said laser; and

means for determining the range, direction and velocity of the object with respect to said apparatus on the basis of the mode hop rate.

10. An apparatus according to claim 9 wherein said means for processing the electrical signals determines the distance, direction and velocity of the object with respect to said apparatus independently of each other on the basis of the mode hop rate.

11. An apparatus according to claim 9 wherein said means for processing the electrical signals converts the electrical signals into digital voltage signals and performs a signal processing algorithm on the digital voltage signals which transforms the digital voltage signals into frequency domain information and determines where any peaks in the frequency domain information occur, and wherein the determination as to the occurrence of peaks is used to determine the mode hop rate of said laser.

12. An apparatus according to claim 10 wherein said means for processing the electrical signals converts the electrical signals into digital signals and performs a signal processing algorithm on the digital voltage signals which transforms the digital voltage signals into frequency domain information and determines where any peaks in the frequency domain information occur, and wherein the determination as to the occurrence of peaks is used to determine the mode hop rate of said laser.

13. A method for determining the range, direction and velocity of an object, said method comprising the steps of:

(1) frequency modulating a standing wave generator with a frequency modulated electrical signal to produce standing waves wherein the frequency of the frequency modulated electrical signal is equal to or greater than π

times the highest frequency at which the optical path between the object and the standing wave generator is modulating such that aliasing is prevented;

(2) projecting the standing waves onto the object;

(3) detecting interference coupling between the standing waves generated by the standing wave generator and standing waves reflected from the object;

(4) determining a mode hop rate of the standing wave generator resulting from the interference coupling of the standing waves; and

(5) determining the distance, direction of motion and velocity of the object based on the determined mode hop rate.

14. A method according to claim 13 wherein step (4) comprises the steps of performing a spectral analysis technique on the digital representation which transforms the digital representation into frequency domain information and determines the mode hop rate based on the occurrence of peaks in the frequency domain information.

15. A method according to claim 13 wherein step (3) comprises the steps of:

(1) sensing the interference coupling with a detector which produces electrical signals in response to standing waves impinging on the detector; and

(2) sampling the electrical signals and converting the electrical signals into digital information.

16. A method according to claim 14 wherein step (3) comprises the steps of:

(1) sensing the interference coupling with a detector which produces electrical signals in response to standing waves impinging on the detector; and

(2) sampling the electrical signals and converting the electrical signals into digital information.

17. A method for determining the range, direction and velocity of an object which is modulating with respect to a laser of a laser range finding system, said method comprising the steps of:

(1) frequency modulating the laser of the laser range finding system with a frequency modulated electrical signal to produce a standing wave of laser light having a frequency which is equal to or greater than π

times the highest frequency at which the optical path between the object and the laser is modulating, wherein frequency modulating the laser to produce a standing wave of laser light having a frequency equal to or greater than π

times the highest frequency at which the optical path between the object and the laser is modulating prevents aliasing;

(2) projecting the laser light onto the object, wherein light reflected from the object is reflected back into the laser as external optical feedback, and wherein at least some of the external optical feedback constructively interferes with the standing wave of laser light produced by the laser to create a plurality of standing light waves of different modes;

(3) detecting the plurality of standing light waves of different modes;

(4) determining the mode hop rate of the laser over a predetermined period of time; and

(5) determining the distance, direction and velocity of the object independently of each other based on the mode hop rate determined over the predetermined period of time.

18. A method according to claim 17 wherein step (4) includes performing a spectral analysis algorithm to determine frequency peaks from which the determination is made as to the mode hop rate.

19. A method according to claim 17 wherein step (3) comprises the steps of:

(1) sensing the plurality of standing light waves of different modes with a detector which converts the light waves into electrical signals; and

(2) sampling the electrical signals and converting the electrical signals into digital information.

20. A method according to claim 19 wherein step (4) comprises the steps of:

(1) performing a spectral analysis algorithm on the digital information which transforms the digital information into frequency domain information and determines how many different frequencies exist in the light waves detected by the detector over a predetermined period of time; and

(2) using the determination obtained by performing the spectral analysis algorithm to determine the mode hop rate of the laser.