Optics: measuring and testing – Range or remote distance finding – With photodetection
Reexamination Certificate
1998-12-22
2001-09-11
Buczinski, Stephen C. (Department: 3662)
Optics: measuring and testing
Range or remote distance finding
With photodetection
C342S127000, C342S195000, C356S005090
Reexamination Certificate
active
06288777
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a system and a method for determining at least one physical quantity, usable preferably in active generation of corresponding depth and reflectivity images by means of a laser, and utilization thereof for environment detection.
2. Discussion of Background
It is known to use an active AMCW, or amplitude modulation continuous-wave laser measurement system, for range finding and/or reflectivity measurement. In the case of such a measuring system there exists, for example, the possibility of creating a sinusoidal transmission signal having the form TRM(t)=sin(&ohgr;
1
t) e.g. by means of a semiconductor laser diode and emitting it over a to be measured signal path of the geometrical length D.
Owing to the signal path, the reception signal having the form REC(t)=B sin(&ohgr;
1
t−&phgr;) exhibits both an attenuation and a phase shift with respect to the original transmission signal TRM(t). The reception signal may, for example, be received by means of an avalanche photodiode.
It is an essential aspect of such a system to determine the existing phase shift as well as the existing attenuation with high precision, i.e. a relative error of approximately 0.01%, and with a very high measurement rate, i.e. up to 1×10
6
measurements/second. Herein the reception signal may have high signal dynamics or attenuation of up to 80 dB (1:10,000).
The geometrical length D of the measuring sticks and of the signal path in the system may be calculated directly for example by means of the following equation (1).
D
=
ϕ
c
ω
1
wherein, c designates the propagation velocity of the transmission signal TRM(t), &ohgr;
1
designates the measurement frequency used, &phgr; designates the phase shift of the reception signal REC(t) with respect to the transmission signal TRM(t), and D designates the geometrical length of the signal path.
The following is a description of a principle of measurement used in the above described system and method.
The original sinusoidal transmission signal TRM(t)=sin(&ohgr;
1
t) is compared to the reception signal REC(t)=B sin(&ohgr;
1
t−&phgr;) received at the end of the signal path, which is now attenuated and phase shifted relative to the transmission signal TRM(t). In order to obtain the phase shift &ohgr; of interest, the following equation (2) is used, which is obtained by application of the general mathematical interrelation sine sin &bgr;=1/2[cos(&agr;−&bgr;)−cos(&agr;+&bgr;)].
sin(&ohgr;
1
t
)·
B
·sin(&ohgr;
1
t
−&phgr;)=
B
/2[cos(−&phgr;)−cos(2&ohgr;
1
t
−&phgr;)] (2)
As can be seen from the left-hand member of equation (2), the original transmission signal TRM(t) is multiplied by the reception signal REC(t) to be evaluated. Following this multiplication, the double-frequency signal component (cos(2&ohgr;
1
t−&phgr;)) resulting from the multiplication is filtered out. In this way the phase angle to be determined is indirectly available in the value B/2 cos(−&phgr;). This term, however, contains two unknown quantities (which are to be determined), i.e., B and &phgr;, so that the following equation (3) is required which is obtained by application of the general mathematical interrelation sine cos &bgr;=1/2[sin(&agr;−&bgr;)+sin(&agr;+&bgr;)].
cos(&ohgr;
1
t
)·
B
·sin(&ohgr;
1
t
−&phgr;)=
B
/2[sin(−&phgr;)+sin(2&ohgr;
1
t
−&phgr;)] (3)
Here, as well, the double-frequency signal component (sin(2&ohgr;
1
t−&phgr;)) resulting from the multiplication is filtered out, so that ultimately the two intermediate results B/2 cos(−&phgr;) and B/2 sin(−&phgr;) containing the two unknown quantities B and &phgr; are obtained. By applying the following equations (4) and (5) it is now possible to calculate the two values B and &phgr; it to be determined.
φ
*
=
arctan
(
B
sin
(
-
φ
)
B
cos
(
-
φ
)
)
(
4
)
B
*
=
2
·
(
B
2
sin
(
-
φ
)
)
2
+
(
B
2
cos
(
-
φ
)
)
2
(
5
)
Herein B* and &phgr;* designate the calculated values for differentiation from the physical measurement values B and &phgr;.
It is now possible to calculate the geometrical length D of the signal path by applying the value &phgr;* thus determined in accordance with equation (1), and to calculate the intensity, or reflectivity, of the reception signal REC(t) by applying the attenuated amplitude thus determined of the reception signal REC(t).
Although the above described principle of measurement is absolutely exact in mathematical terms, there nevertheless result the following problems in technical implementation.
The signals to be multiplied by each other are located within a range of several tens of MHZ. This multiplication is generally performed by means of analog mixers. Such analog mixers are, however, not absolutely linear and their parameters are moreover temperature-dependent. This brings about errors in multiplication which falsify the final result, i.e., induce errors of measurement.
Moreover the signal cos(&ohgr;
1
t) in equation (3), which is necessary for calculation, is generated from the original transmission signal TRM(t)=sin(&ohgr;
1
t). The analog phase shifter usually employed for this purpose is, however, not an ideal phase shifter and accordingly causes equally temperature-dependent amplitude and/or phase errors resulting in errors of measurement.
Finally, the calculations required for obtaining B* and &phgr;* are performed in large-scale integrated digital signal processors, which requires that prior to these calculations the two intermediate results B/2 cos(−&phgr;) and B/2 sin(−&phgr;) obtained must be filtered and subsequently converted from an analog signal into a corresponding digital signal by means of an analog-to-digital converter. Due to the fact that these two intermediate results are processed through different, non-ideal filters and analog-to-digital converters, in turn temperature-dependent errors of measurement are introduced.
SUMMARY OF THE INVENTION
In view of the above described problems in the prior art, it is an object of the present invention to furnish a system and method for determining at least one physical quantity from a transmission signal and a reception signal, whereby non-corrupted measurement results may be obtained, or whereby it is possible to eliminate or compensate for errors of measurements.
This object is attained according to the present invention by providing a novel system for determining at least one physical quantity by means of a relation between a transmission signal and a reception signal, including a transmitter which generates and emits a transmission signal having a predetermined frequency; a receiver which receives the transmission signal, which has passed through a signal path, as a reception signal; a reference signal generator which generates a reference signal having a frequency which has a predetermined fixed relation to the frequency of the transmission signal; a first mixer which mixes the reception signal with the reference signal and produces an intermediate frequency signal; and a processor coupled to the intermediate frequency signal and configured to determine the at least one physical quantity based at least in part on the intermediate frequency signal. The invention is, of course, not limited to single-frequency methods but also encompasses multifrequency systems.
Owing to the fact that the reference signal is directly and separately derived, or generated, from the transmission signal, it is not necessary to derive the reference signal from the transmission signal by mixing, whereby errors owing to this mixing process may be avoided, errors brought about by the fact that the apparatus is not ideal.
Pre
Froehlich Christoph
Mettenleiter Markus
Buczinski Stephen C.
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
Z+F Zoller & Fröhlich GmbH
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