Optics: measuring and testing – Range or remote distance finding – With photodetection
Reexamination Certificate
2003-06-16
2004-10-05
Gregory, Bernarr E. (Department: 3662)
Optics: measuring and testing
Range or remote distance finding
With photodetection
C356S005010, C356S005100
Reexamination Certificate
active
06801305
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention relates generally for optical distance measurement.
PRIOR ART
Optical distance measuring devices per se have been known for a long time. These devices emit a modulated light beam, which is aimed at a desired target surface, whose distance from the device is to be ascertained. The light reflected from or scattered by the target face aimed at is in part detected again by the device and used to ascertain the distance sought. A distinction is made between so-called phase measuring methods and pure transit time methods for determining the distance sought. In the transit time measuring method, for instance, a light pulse of the shortest possible duration is emitted by the device, and its transit time to the target and back again into the measuring device is ascertained. From this, because the value of the speed of light is known, the distance from the device to the target can be calculated.
In the phase measuring methods, the variation in phase with the travel distance is utilized to determine the distance. Via the phase displacement of the returning light, compared to the light emitted, the route traveled by the light and thus the spacing from the target object can be determined.
The range of application of such distance measuring devices includes differences in the range from a few millimeters up to several hundred millimeters.
Depending on the travel distances to be measured and the capability of the target object to return a beam, different demands are made of the light source, the quality of the measurement beam, and the detector.
At relatively short distances—in the range up to several meters—from a target that furthermore has good retro-reflectivity, a well-collimated light beam of limited diffraction will advantageously be used, in order to obtain good resolution and a correspondingly strong returning signal.
At great distances, the problem is meeting the desired target with a fine light beam, such as a focused, limited-diffraction laser beam, so that for this type of application, it is advantageous to use a measuring beam with a larger diameter.
The optical distance measuring devices known from the prior art can fundamentally be divided into two categories, depending on the disposition of transmission conduits and reception conduits. First, there are apparatuses in which the transmission conduit is disposed next to the reception conduit at a certain distance from it, so that the respective optical axes extend parallel to one another. Second, monoaxial measuring devices exist in which the reception conduit extends coaxially with the transmission conduit.
From German Patent DE 198 40 049 C2, a monoaxial device for optical distance measurement is known for geodetic, construction and industrial uses, with which both highly retro-reflective and poorly retro-reflective target objects can be measured. This apparatus furthermore has high resolution, even for poorly retro-reflective objects.
The apparatus of DE 198 40 049 C2 has a transmitter unit, with one or two optical radiation sources for generating two separable beams. One beam is limited in its diffraction and includes light in the visible wavelength range, while conversely the other beam is divergent and is in the visible or infrared wavelength range.
A disadvantage of the apparatus of DE 198 40 049 C2 is the complex generation of the various measuring beams. Depending on the type of embodiment, the device uses two separate laser diodes, or dual-wavelength lasers, to generate the two measuring beams. The different beam divergences are generated by complicated telescope lens elements in the two beam paths of this apparatus. The apparatus of DE 198 40 049 C2 furthermore requires selection means for detecting the different, coaxially extending types of radiation and beams. The selection means mentioned in DE 198 40 049 C2, such as optical filters, controllable frequency doublers, and Q switchers are complicated and expensive optical components that make the measuring system quite complex, inconvenient to handle, and expensive.
Biaxial measuring systems, by comparison, have the advantage of not requiring complicated beam splitting, so that optical crosstalk from the transmission conduit directly into the reception conduit can be better suppressed.
On the other hand, in biaxial distance measuring devices there is the disadvantage that for the range of short measurement distances, parallax can cause detection problems. The projection of the target object onto the detector surface, which for great target distances is still located unequivocally on the detector, increasingly migrates away from the optical axis of the receiving branch as the measurement distance becomes shorter, leading to a variation in the beam cross section in the plane of the detector.
SUMMARY OF THE INVENTION
The apparatus of the invention for optical distance measurement, has the advantage over the prior art that the beam generated in the distance meter can be adapted to the various measurement tasks (that is, different target objects or target distances that have to be measured) with merely a single optical means. In particular, the divergence and the direction of the measuring beam can be adapted to the particular measurement requirement with this single optical means.
Two separate, different beam paths for furnishing beams of different divergence are unnecessary in the apparatus of the invention. This advantageously makes a compact, handheld measuring device possible, for instance.
The different beam divergences of the measuring beam can be achieved by simply varying the relative disposition of the light source and an optical means.
In a first advantageous embodiment of the distance measuring device of the invention, the optical means used is a lens or lens combination that serves as a lens element and is placed for instance in the beam path of the transmitter unit of the measuring device. Other optical means can also be imagined and will be described later.
Advantageously, the apparatus of the invention for optical distance measurement can be realized by using a laser as the light source. Various kinds of miniaturized lasers that can be built into such measuring devices are presently available. Because of their small size and high power, semiconductor lasers are especially advantageous for the purpose; by now, they are available with suitable quality even for the visible part of the spectrum of electromagnetic waves.
To generate a collimated, limited-diffraction measuring beam, the light source is placed in the focus of the lens element on the object. The divergent beam emerging from the light source is collimated after passing through the lens element and forms a largely parallel measuring beam, which can for instance be used for the distance measurement with high resolution.
If a measuring beam of increased divergence is required, as a result of the need to measure relatively great measuring distances, then in the apparatus of the invention, the lens element lens can be moved in the direction of the optical axis of the measurement signal, in order to vary and adjust the desired divergence of the measuring beam.
Thus in the apparatus of the invention, it is possible for instance first to use a divergent beam, for aiming the device at a target object located far away, and after the measuring beam strikes the target object, the measuring beam can be recollimated with limited diffraction, in order to achieve good resolution and a clearly measurable returning signal in the measuring device.
To vary the beam divergence, the lens element lens is adjusted in its position by way of various actuators that are controlled by a closed-loop control mechanism.
In another embodiment of the apparatus of the invention, the position of the light source can be varied via corresponding actuators and an associated closed-loop control circuit relative to what is then optionally a fixed collimating lens element.
The control of the actuators that are capable of varying the relative disposition of the lens element and the light source is
Stierle Joerg
Wolf Peter
Andrea Brian
Gregory Bernarr E.
Robert & Bosch GmbH
Striker Michael J.
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