Optics: measuring and testing – Range or remote distance finding – With photodetection
Reexamination Certificate
2003-04-15
2004-12-07
Gregory, Bernarr E. (Department: 3662)
Optics: measuring and testing
Range or remote distance finding
With photodetection
C356S005070, C356S005080
Reexamination Certificate
active
06829043
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention is directed to distance measurement technology.
2. Description of the Related Art
People undertaking construction and repair projects frequently need to measure distances. Traditional tape measures are inconvenient. They can require the use of two people in many instances. Tape measurements can lack accuracy. A user may align the tape on a slant or bend the tape when making a measurement against a fixed object.
Optical measuring systems exist for making more accurate distance measurements. However, many of these systems have drawbacks that make them undesirable to users. Some systems require the use of expensive precision components that drive the price of the measuring device beyond the purchase point of many consumers. Other systems suffer from inaccuracy due to noise and other extraneous effects.
One traditional type of system is the narrowband ranging system. This system emits one or more modulated optical signals that produce reflections on an incident target. The system captures the reflections and determines the distance to the target based on phase shifts detected in the captured reflections. These systems typically require the use of an expensive high precision receiver, such as an avalanche gain photodiode. The performance of these systems can also erode as the signal to noise ratio falls. This can be a significant drawback, because environmental conditions in the working area can provide substantial signal attenuation.
Another traditional type of system is the wideband pulsed system. This system also emits one or more optical signals that produce reflections on an incident target. The system captures the reflections and measures the round trip signal delay to obtain the distance to the target. The system determines the time difference between the time a signal pulse departs the system and the time that the system receives a reflection of the pulse. Traditional systems identify pulse departure and arrival through threshold detection—comparing the signals to a threshold level. One typical technique is half-maximum detection, which establishes a reference threshold based on the peak intensity of the signal pulses. Unfortunately, this technique does not operate well in low signal to noise ratio environments. The system has difficulty establishing a consistent detection point, because the low signal to noise ratio increases estimation errors in the measurement of signal amplitude. Challenges also arise when trying to measure time delay between signal pulses. When an asynchronous clock is employed to measure the time between pulses, significant inaccuracies can occur unless the system employs measurement intervals with impractically long durations. In order to avoid such measurement intervals, the system can employ expensive high-speed components with substantial power consumption.
SUMMARY OF THE INVENTION
The present invention, roughly described, pertains to technology for measuring distances. A measurement device emits a beam that reflects on the surface of an object. The measurement device captures the return beam and determines the distance to the object, based on the time of flight of the beam from transmission to capture by the measurement device.
One implementation of the measurement device enhances accuracy by deriving feedback reference pulses from pulses in the emitted beam and injecting them into the device's receive path. This creates a receive waveform that includes one or more feedback reference pulses in the emitted beam and corresponding return pulses in the return beam. This enables the measurement device to directly measure time delay between a return pulse and a reference pulse that lead to the generation of the return pulse. In some implementations, the measurement device also attenuates the feedback reference pulses, so that they have intensities similar or equal to the intensities of the return pulses.
One embodiment of the measurement device digitizes the receive waveform and processes it to obtain clean versions of the feedback reference pulses and return pulses. This enables the device to accurately identify corresponding points in a feedback reference pulse and return pulse, so that reliable time of flight measurements can be made. One implementation of the measurement device includes a histogram processor that collects waveform samples at varying comparison thresholds. The device uses the most accurate information at each threshold to create a digitized composite waveform that corresponds to the analog waveform received by the measurement device. This functionality allows accurate waveform reconstruction in environments with low signal to noise ratios. In one embodiment, signal processing within the measurement device also removes noise, scales reference pulses, and removes distortions caused by pulse trailing edges running into subsequent pulses.
In one implementation, the histogram processor generates a waveform histogram at each waveform sampling threshold. The histogram includes intervals, and each interval reflects the results of waveform samples taken in a time window. The histogram processor converts each interval's contents into an additional amplitude offset from the corresponding sampling threshold. The histogram processor adds the additional amplitude to the sampling threshold to obtain an amplitude component for the interval and weights the reliability of the amplitude component. The weighted amplitude components for an interval in each histogram are combined to obtain a composite amplitude component for the interval.
The histogram processor utilizes the characteristics of the waveform noise to accurately determine composite waveform amplitudes. The histogram processor determines the additional amplitude offset and weights reliability based on an inverse error function derived from the crossing statistics of the random noise in the waveform. A histogram interval's additional amplitude decreases and reliability increase as the waveform samples in the interval move closer to being equally distributed above and below the sampling threshold. This condition indicates that the waveform amplitude is close to the sampling threshold and random noise is driving the sampling results to oscillate above and below the sampling threshold.
Aspects of the present invention can be accomplished using hardware, software, or a combination of both hardware and software. The software used for the present invention is stored on one or more processor readable storage media including hard disk drives, CD-ROMs, DVDs, optical disks, floppy disks, tape drives, RAM, ROM or other suitable storage devices. In alternative embodiments, some or all of the software can be replaced by dedicated hardware including custom integrated circuits, gate arrays, FPGAs, PLDs, and special purpose computers. In one embodiment, software implementing the present invention is used to program one or more processors. The processors can be in communication with one or more storage devices, peripherals and/or communication interfaces.
These and other objects and advantages of the present invention will appear more clearly from the following description in which the preferred embodiment of the invention has been set forth in conjunction with the drawings.
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patent: 6288775 (2001-09-01), Tanaka
Lewis Robert
Thompson Chad
Varian George
Andrea Brian
Gregory Bernarr E.
Toolz, Ltd.
Vierra Magen Marcus Harmon & DeNiro LLP
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