Rangefinder with reduced noise receiver

• US 20050110976A1
• Filed: 11/23/2004
• Published: 05/26/2005
• Est. Priority Date: 11/26/2003
• Status: Active Grant

• Current Assignee: Meade Instruments Corporation
• Original Assignee: Meade Instruments Corporation
• Inventors: LaBelle, John
• Granted Patent: US 7,508,497 B2
• Time in Patent Office: Days
• US Class Current: 356/5.10
• CPC Class Codes: G01C 3/08 Use of electric radiation detectors
• 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 7/4813 Housing arrangements
• CPC Class Codes: G01S 7/4873 by deriving and controlling a threshold value
• CPC Class Codes: G01S 7/497 Means for monitoring or calibrating

Abstract

A rangefinder and method are provided for measuring a distance between the rangefinder and a target by measuring the flight time of a beam reflected from the target. In an embodiment, a receiver having reduced noise detects weak reflections from distant targets or targets with poor reflectivity. The receiver generates a voltage signal proportional to a reflected beam, clamps the voltage signal to remove a portion of the noise therefrom, and compares the voltage signal to a threshold. In some embodiments, the receiver also filters the voltage signal to remove a portion of the noise therefrom. In an embodiment, the sensitivity of the receiver is increased as a function of time to reduce the likelihood of detecting stray reflections from objects other than the intended target. In some embodiments, the rangefinder collects calibration data with each range measurement or group of range measurements. The calibration data comprise a plurality of simulated range measurements.

Claims

1. A rangefinder configured to determine a range to a target by measuring a flight time of a beam traveling between the rangefinder and the target, the rangefinder comprising:

a transmitter configured to emit the beam towards the target;

a receiver configured to detect a reflection of the beam from the target, the receiver comprising;

a photodiode configured to generate a current signal proportional to the received reflection;

a transimpedance amplifier configured to receive the current signal and to generate a voltage signal proportional thereto;

noise reduction circuitry configured to reduce noise present in the voltage signal by limiting the voltage signal relative to a first voltage level; and

a comparator configured to indicate when the voltage signal exceeds a second voltage level; and

a processor configured to convert the flight time of the beam into a range measurement when the comparator indicates the voltage signal has exceeded the second voltage level.

2. The rangefinder of claim 1, wherein the noise reduction circuitry comprises clamping circuitry and is further configured to substantially reduce about half the noise present in the voltage signal.

3. The rangefinder of claim 1, wherein the receiver further comprises:

one or more filters configured to remove noise components from the voltage signal; and

an amplifier configured to amplify the voltage signal.

4. The rangefinder of claim 3, wherein the amplifier is further configured to set a mean value of the voltage signal to the first voltage level.

5. The rangefinder of claim 4, wherein the noise reduction circuitry is configured to remove portions of the voltage signal below the first voltage level.

6. The rangefinder of claim 5, wherein the first voltage level comprises substantially zero volts.

7. The rangefinder of claim 3, wherein the one or more filters comprise a first filter coupled between the transimpedance amplifier and the amplifier, the first filter configured to remove low frequency noise components from the voltage signal.

8. The rangefinder of claim 7, wherein the one or more filters further comprise a second filter coupled between the noise reduction circuitry and the comparator, wherein the second filter is configured to remove high-frequency noise components from the voltage signal.

9. The rangefinder of claim 1, wherein the second voltage level is configured to decrease as a function of time.

10. The rangefinder of claim 9, wherein the second voltage level is configured to decrease exponentially.

11. The rangefinder of claim 1, further comprising timing circuitry configured to measure the flight time between the emission of the beam by the transmitter and the detection of the beam by the receiver.

12. The rangefinder of claim 11, wherein the timing circuitry comprises a capacitor configured to charge at a first rate when the transmitter emits the beam and to discharge at a second rate in response to the reflected beam detected by the receiver.

13. The rangefinder of claim 12, wherein the timing circuitry further comprises a timing comparator configured to indicate when the capacitor is discharged.

14. The rangefinder of claim 12, wherein the timing circuitry further comprises a counter configured to measure a discharge time of the capacitor.

15. A method for detecting a reflected beam received from a target, the method comprising:

generating a voltage signal proportional to the reflected beam;

removing a first portion of noise from the voltage signal;

comparing the voltage signal to a first voltage level to detect the reflected beam, wherein the first voltage level is above a second portion of the noise present in the voltage signal; and

indicating that the voltage signal is at least as great as the first voltage level.

16. The method of claim 15, wherein removing the first portion of noise comprises filtering low frequency noise components from the voltage signal.

17. The method of claim 16, further comprising amplifying the voltage signal.

18. The method of claim 17, wherein amplifying the voltage signal comprises setting the mean value of the voltage signal to a second voltage level.

19. The method of claim 18, wherein removing the first portion of noise comprises clamping the voltage signal at the second voltage level.

20. The method of claim 19, wherein setting the mean value of the voltage signal to the second voltage level comprises adjusting the mean value to approximately zero volts.

21. The method of claim 19, further comprising filtering high frequency noise components from the voltage signal after the amplifying.

22. The method of claim 15, wherein generating the voltage signal comprises:

receiving the reflected beam;

converting the reflected beam into a current signal;

buffering the current signal; and

converting the current signal into the voltage signal.

23. The method of claim 15, wherein comparing the voltage signal to the first voltage level comprises decreasing the first voltage level as a function of time.

24. A rangefinder receiver comprising:

means for generating a voltage signal proportional to a beam reflected from a target;

means for removing noise from the voltage signal; and

means for comparing the voltage signal to a first voltage level.

25. The rangefinder receiver of claim 24, wherein the means for generating the voltage signal comprises:

means for receiving the beam;

means for generating a current signal proportional to the beam; and

means for converting the current signal into the voltage signal.

26. The rangefinder of claim 24, wherein the means for removing noise comprises means for limiting the voltage signal to a second voltage level to thereby remove a portion of noise present in the voltage signal.

27. The rangefinder receiver of claim 26, wherein the means for limiting the voltage signal comprises:

means for removing high frequency noise components from the voltage signal;

means for amplifying the voltage signal and substantially centering the voltage signal with respect to the second voltage level; and

means for clamping the voltage signal at the second voltage level.

28. The rangefinder receiver of claim 24, further comprising means for removing low frequency noise components from the voltage signal.

29. The rangefinder receiver of claim 24, further comprising means for varying the first voltage level as a function of time.

30. A receiver comprising:

a photodiode configured to generate a current signal proportional to a received light pulse;

a transimpedance amplifier configured to receive the current signal and to generate a voltage signal proportional thereto, wherein the voltage signal comprises noise and a voltage pulse proportional to the light pulse;

noise reduction circuitry configured to remove noise present in the voltage signal; and

a threshold that is adjustable in a search pattern.

31. The receiver of claim 30, wherein the received light pulse is detected when the voltage signal exceeds the threshold.

32. The receiver of claim 31, further comprising a comparator configured to indicate when the voltage signal exceeds the threshold.

33. The receiver of claim 30, wherein the search pattern ranges from about zero volts to about 2.5 volts.

34. The receiver of claim 30, wherein the transimpedance amplifier comprises:

an amplifier;

a voltage buffer configured to substantially match the impedance of the photodiode to the impedance of the amplifier; and

a resistive element coupled between the output of the amplifier and the input of the voltage buffer.

35. The receiver of claim 30, wherein the noise reduction circuitry comprises circuitry configured to clamp the voltage signal.

36. The receiver of claim 35, wherein the circuitry configured to clamp the voltage signal comprises:

a high-pass filter;

an amplifier coupled to the high-pass filter, wherein the high-pass filter and the amplifier are configured to adjust the mean value of the voltage signal;

a first diode configured to remove portions of the voltage signal below the mean value; and

a second diode configured to bias the first diode.

37. The receiver of claim 35, further comprising a low-pass filter.