Optical distance measurement

Details Optical distance measurement Optical distance measurement

2002-10-21

2004-06-22

Buczinski, Stephen C. (Department: 3662)

Optics: measuring and testing

Range or remote distance finding

With photodetection

C342S145000, C356S005010, C356S005100

Reexamination Certificate

active

06753950

ABSTRACT:

FIELD OF THE INVENTION
This invention relates to distance measurement, particularly an optical distance measurement system.
BACKGROUND OF THE INVENTION
One common approach to measuring distance to a far object is to measure the time of flight of a pulse of light from the measuring system to the far object and back again, and then to calculate the distance to the far object based upon the speed of light. Systems employing this method commonly employ a laser to generate the light pulse and so are known generically as “laser range finders” (LRF's) or “light detection and ranging” (LiDAR) systems. Typical applications are the measurement of altitude, target range or distance for survey applications in civil engineering and metrology. LRF's may be built as stand alone hand held units or embodied in larger systems.
A known LRF is shown in FIG.
1
and comprises a laser
1
, an optical transmission system
2
, an optical reception system
3
, a light sensitive detector
4
, pulse detection circuitry
5
, and timing calculation and display electronics
6
.
In operation, the user initiates a measurement of range using input
7
, which causes a laser fire pulse to be sent to the laser
1
and the laser to emit a pulse of light at time T0 as represented by the plot
10
. This pulse is focussed by the transmission optics
2
and travels to the remote object
8
where it is reflected. The receiving optics
3
collects a portion of the reflected light pulse illustrated as plot
12
and focuses the energy onto the light sensitive detector
4
. The detector
4
converts the received light pulse into an electrical signal and the pulse detector
5
discriminates against any electrical noise generated by the light sensitive detector to provide a clean, logic level pulse from the incoming light detector signal at time T1.
This pulse is passed to timing calculation and display electronics
6
which calculates and displays the range to the remote object based upon the time of flight of the laser pulse (T1-T0) and the speed of light (c) in the intervening medium.
Often there are multiple pulses apparent within the reflected signal captured by the detector
4
due to reflections from a number of different objects (e.g. vegetation) in the path of the light pulse or variations in the refractive index of the intervening atmosphere. LiDAR systems, instead of using a simple pulse discrimination system
5
, apply further signal processing and analysis to the signal output by detector
4
to calculate the position and strength of these additional reflections and hence enable various characteristics of the intervening objects or atmosphere to be studied.
To reduce cost, some LRF devices employ a single optical system with an optical beam splitter to separate the transmitted and reflected pulses.
The maximum range that can be measured by an LRF or LiDAR system is determined by the point at which the LRF can no longer discriminate between the incoming reflected pulse and any background illumination or effects inherent in the optical detector such as e.g. thermally generated dark current and shot noise.
Because of the losses in transmission and reflection, to achieve ranges of more than a few kilometers requires laser technologies such as Nd:YAG or Erbium:Glass which are relatively expensive. Lower cost systems have been built using solid state laser diodes but because the energy in each transmitted pulse is relatively low their range is limited to a few hundred meters.
Some systems extend the range by sending many (N=hundreds or thousands) pulses and summing the reflected signals to improve discrimination against the uncorrelated detector noise. Using this technique an LRF using solid state laser diodes can achieve a range of up to 2-4 Km. However, this process only provides a N improvement in discrimination at best. In addition, because a pulse cannot be transmitted until the reflection of the previous pulse has been detected to avoid ambiguity, at long ranges the pulse repetition rate is limited. For example to send and receive a pulse to a remote object at 5 Km takes ~30 &mgr;S and so to collect 1000 samples takes 0.03 S. In practice, it is found that over this time period slight movements in the line of sight or the remote object can substantially reduce the advantages of summing the received pulses.
There is also a trade off between the range and the light transmitting and light gathering capabilities of the optical systems used. Wide aperture optical systems will improve range but increase size and cost.
To overcome the disadvantages of these systems, alternative approaches have been developed.
One example is the system described in GB 1 585 054. In this system the output of a Carbon dioxide laser is passed through an acousto-optical modulator and output. Received infra-red signals are detected, and an electronic circuit using surface acoustic wave devices is provided that can determine the range and velocity of a target.
One particularly effective embodiment of the technique can be achieved using a signal known as the Maximal Length Sequence (MLS). This is a family of pseudo random noise binary signal (PRBS) which are typically generated using a digital shift register whose input is generated from appropriate feedback taps. The use of such a sequence is described in GB 1 585 054.
The maximal length sequence is the pseudo random noise sequence with the longest period which can be generated with a shift register of r sections. It has a length N=2′−1 shift register clock cycles and has good auto-correlation properties as the auto-correlation function has only two values; either −1/N or a peak of 1.0 at the point of correlation.
FIG. 2
illustrates one example of a maximal length sequence generated by a four stage shift register
20
. Alternative length sequences can be generated by using longer shift registers with the appropriate feedback taps.
This approach may also be combined with averaging techniques to improve the signal to noise ratio and hence range further.
Another document describing a similar approach is DE 199 48 803 which describes a rangefinder. A maximum length sequence (MLS) is transmitted and correlated with a received reflected signal. The MLS is a good choice because its binary nature allows efficient modulation of laser diodes. In addition, because the signal only takes values of +1 and −1, the cross-correlation can be computed simply only additions and subtractions, without the need for multiplications.
A further difficulty is that the distance precision is limited by the sample rate of the analogue to digital converter. For example, if the sample rate is 33 MHz, then the smallest time increment which can be measured is ~30 nS which equates to a distance precision of ~5 m. This is insufficient for many applications. To overcome this problem, the sample rate can be increased, but this increases system cost because more expensive components and more sophisticated circuitry are needed.
In the apparatus of DE 19948803 a controllable delay line is provided between the transmitted signal and the cross correlator; the length of the delay introduced by the delay line is controlled by the timing electronics. In operation, successive MLS signals are transmitted and the delay line is adjusted in small steps until the correlation peak is maximised. The total time of flight is calculated from the whole number of MLS clock cycles plus the small delay added by the delay line at which the correlation peak is maximised. In DE 199 49 803 the delay step size is set equal to one fifth of the MLS clock sample frequency and so the precision of the time and distance measurement is increased by a factor of five. A major disadvantage of this technique is that the total measurement time is increased by the number of steps required to find the correlation peak. This is problematic in many applications:
a) in low power, hand held applications transmitting more MLS signals wastes power, reducing battery life;
b) in covert applications, increasing the number of

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