Method and device for optoelectronic distance measurement

• US 20010013929A1
• Filed: 01/23/2001
• Published: 08/16/2001
• Est. Priority Date: 02/14/2000
• Status: Active Grant

• Current Assignee: Hilti Aktiengesellschaft (Hilti Martin Family Trust)
• Original Assignee: Hilti Aktiengesellschaft (Hilti Martin Family Trust)
• Inventors: Torsten, Gogolla
• Granted Patent: US 6,633,367 B2
• Time in Patent Office: Days
• US Class Current: 356/5.01
• CPC Class Codes: G01C 3/08 Use of electric radiation detectors
• CPC Class Codes: G01S 17/36 with phase comparison between the received signal and the contemporaneously transmitted signal
• CPC Class Codes: G01S 7/497 Means for monitoring or calibrating

Abstract

The invention is directed to a method for optoelectronic distance measurement and a measurement device based on this method, in which two light emitters, particularly laser emitters and two photodiode receivers are used for calibration. Part of the modulated output of the main emitter reaches the measurement object and then arrives at the main photoreceiver in the form of scattered light and another part of the output of the main emitter travels directly to a reference photoreceiver, while a part of the modulated output of the reference light emitter is guided directly to the main photoreceiver and another part is guided directly to the reference photoreceiver. According to the invention, the light intensities of the main emitter and the reference emitter are simultaneously modulated with different modulation frequencies (f₁, f₂), and a signal mixture is formed in the main receiver as well as in the reference receiver, which signal mixture contains a signal with the intensity modulation frequency (f₁) of the main emitter and a signal with the intensity modulation frequency (f₂) of the reference emitter. The phases of the two signals of the signal mixture are measured simultaneously and the separation of the two phases is carried out by the different frequencies in a subordinate intermediate frequency range and by the different intensity modulation frequencies. At least two sequential measurement processes are preferably carried out, wherein the intensity modulation frequencies of the reference emitter and main emitter are exchanged in the second measurement process. Phase errors depending upon temperature, aging and reception power can be entirely eliminated by the invention in electro-optic distance measuring devices.

Claims

1. A method for optoelectronic distance measurement, wherein an intensity-modulated main light beam emitted by a main light emitter is directed to a measurement object at a distance, wherein the distance (D₀) of a measurement object from an observation point is to be measured, and a light scattered at the observation point reaches a main photoreceiver via reception optics, and a branched part of the main light beam is simultaneously directed to a reference photoreceiver via a first known reference distance (D₁), an intensity-modulated reference light beam emitted by a reference light emitter reaches the reference photoreceiver via a second known reference distance (D₂), and a part of the reference light beam reaches the main photoreceiver via a third reference distance (D₃), and wherein the signals delivered by the main photoreceiver and the reference photoreceiver are subjected to a comparative signal evaluation for obtaining a corrected measurement signal, wherein the light intensities of the main emitter and the reference emitter are simultaneously intensity-modulated at different frequencies, wherein the signal mixtures supplied by the main receiver and the reference receiver, each of the signal mixtures contains a signal component with the intensity modulation frequency of the main emitter and a signal component with the intensity modulation frequency of the reference receiver, are converted to an intermediate frequency range containing two frequency components, wherein one frequency component is formed by the signal of the reference emitter and the other frequency component is formed by the signal of the main emitter, and the separation of the phase information contained in the two simultaneously occurring intermediate frequency signals is based on the different frequencies in the intermediate frequency range and the different modulation frequencies for the intensity modulation of the main light beam and reference light beam for purposes of comparative signal evaluation.

2. The method of

claim 1

, wherein a plurality of measurement processes are successively carried out to achieve a definite distance measurement and to improve the measurement results, wherein the modulation frequencies for the intensity modulation are exchanged for one another and changed equally according to a set pattern.

3. The method of

claim 1

, wherein the delivery power of at least one of the main emitter and the reference emitter is varied for adapting to different dynamic requirements.

4. The method according to

claim 1

, wherein the main light emitter comprises a first laser and the reference light emitter comprises a second laser.

5. The method of

claim 4

, wherein the fundamental wavelengths (λ

₁, λ

₂) for the first and second lasers are selected differently.

6. The method of

claim 5

, wherein crosstalk between the light signal paths associated with the main beam and reference beam is reduced by optical filtering.

7. The method according to

claim 4

, wherein the phases of the signal components of the signal mixtures in the intermediate frequency range are determined by digital Fourier transformation with evaluation of the real and imaginary parts of the Fourier-transformed signal mixtures in the frequency domain at the respective intermediate frequencies (f_ZF1 and f_ZF2) for comparative signal evaluation.

8. The method of

claim 7

, wherein the signal mixtures to be supplied to the digital Fourier transformation are subjected to a low-pass filtering.

9. The method of

claim 8

, wherein the intermediate frequencies (f_ZF1 and f_ZF2) and the sampling times of an analog-to-digital conversion preceding the Fourier transform are selected such that an integral number of cycles of the signal component with the first intermediate frequency (f_ZF1) and the signal component with the second intermediate frequency (f_ZF2) lie in the measurement window of the digital sampling.

10. The method according to

claim 1

, wherein the component of the reference light beam reaching the main receiver is initially diffusely scattered and is guided to the main receiver only as a scattered component together with the backscattered component of the main light beam impinging via the reception optics.

11. The method according to

claim 1

, wherein the component of the reference light beam reaching the main receiver is initially diffusely reflected and is guided to the main receiver only as a scattered component together with the backscattered component of the main light beam impinging via the reception optics.

12. The method according to

claim 1

, wherein components of the reference light beam and main light beam reaching the reference receiver are initially diffusely scattered and are guided to the reference receiver only as a scattered light components.

13. The method according to

claim 10

, wherein components of the reference light beam and main light beam reaching the reference receiver are initially diffusely scattered and are guided to the reference receiver only as a scattered light components.

14. A device for optoelectronic distance measurement with two light transmitters each having a light beam that is intensity-modulated, wherein the light beam of the first light transmitter being a main emitter, is directable to a measurement object at a distance, wherein the distance (D₀) of the measurement object from an observation point is to be measured, and a separate light component reaches one of two photoreceivers, via a first reference distance (D₁), and wherein the light beam of the second light transmitter, reaches one of the reference receivers via a second reference distance (D₂), and a beam component separated from the light beam of the second light transmitter reaches a second photoreceiver being a main receiver, via a third reference distance (D₃), the second photoreceiver being acted upon in addition by a component of the light beam of the main emitter backscattered from the measurement object, with two signal mixers each being associated with the reference receiver and the main receiver and transforming the receiver signal mixtures into an intermediate frequency range, and an evaluating device for determining the measurement distance (D₀) from the output signals of the two mixers, wherein a device by which the light beams emitted by the main emitter and reference emitter can be simultaneously intensity-modulated with different frequencies (f₁, f₂).

15. The device of

claim 14

, wherein at least one of a transimpedance amplifier and impedance-monitored HF amplifier is arranged between at least one of the reference receiver and the main receiver and the respective associated mixer.

16. The device of

claim 14

, wherein at least one the main receiver and the reference receiver is an avalanche photodiode.

17. The device of

claim 16

, wherein the avalanche photodiode is a direct mixer, wherein a reception signal mixture is converted directly into the intermediate frequency range by modulation of the avalanche gain via a local oscillator signal (f₁₀) generated by a local oscillator.

18. The device of

claim 17

, wherein the local oscillator is an LC oscillator, the oscillation-determining elements of the LC oscillator being formed by the capacity of the avalanche photodiode.

19. The device of

claim 14

, wherein at least one of the main receiver and the reference receiver is a PIN photodiode.

20. The device of

claim 14

, wherein device is present in the intensity modulation device for mutual exchange of frequencies (f₁, f₂) of the modulation signals reaching the respective light emitter.

21. The device of

claim 20

, wherein the device is used for time-sequential mutual exchange of the frequencies of the intensity modulation signals reaching the respective light emitter.

22. The device of

claim 20

, wherein the device is used for uniformly changing the frequency of the intensity modulation signals reaching the respective light emitter.

23. The device of

claim 14

, wherein a scattering device is arranged in the beam path of a beam component traveling from the reference emitter to the main receiver and wherein a scattered light component of the beam component proceeding from the reference emitter travels from the scattering device to the main receiver.

24. The device of

claim 14

, wherein a scattering device is arranged in the beam path of beam components traveling from the reference emitter and from the main emitter to the reference receiver and wherein scattered light components of the beam components proceeding from the reference emitter and main emitter travel from the scattering device to the reference receiver.

25. The device of

claim 14

, wherein the device varies the light output of at least one of the reference emitter and the main emitter.

26. The device of

claim 14

, wherein the optical wavelengths (λ

₁, λ

₂) of the reference emitter and the main emitter are different.

27. The device of

claim 26

, wherein at least one of (i) an optical filter tuned to the wavelength emitted by the main emitter is arranged in the beam path from the main emitter to the reference receiver and (ii) an optical filter tuned to the wavelength emitted by the reference emitter is arranged in the beam path from the reference emitter to the main receiver.

28. The device of

claim 27

, wherein at least one of the main emitter and the reference emitter is one of an edge emitting laser diode, a VCSEL (Vertical Cavity Surface Emitting Laser Diode) and a light diode.

29. The device of

claim 14

, further comprising a mother oscillator to be used for both the transmitting part and reception part.

30. The device of

claim 14

or

claim 17

, wherein the two mixers are controllable by the same local oscillator with a frequency (f_LO) selected such that there is a signal mixture present in the intermediate frequency range whose signal components with both intermediate frequency components (f_ZF1, f_ZF2) contain phases in the modulation frequencies (f₁, f₂).