CN109870677B - Frequency modulation continuous wave interference signal amplitude normalization method - Google Patents
Frequency modulation continuous wave interference signal amplitude normalization method Download PDFInfo
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Abstract
An amplitude normalization method of frequency modulation continuous wave interference signals is used for correcting the interference signals, correcting the interference signals into constant amplitude oscillation signals with constant amplitude, and simultaneously not changing the frequency and phase attributes of the interference signals. The method comprises the steps of sampling interference signals and converting the interference signals to obtain digital signals, demodulating the amplitude of the interference signals according to the PI characteristics of the semiconductor laser, and simultaneously carrying out adaptive gain on the signals according to the effective values of the interference signals to obtain constant-amplitude oscillation interference signals with constant amplitude. The method ensures that the amplitude of the interference signal is not influenced by factors such as current modulation of the semiconductor laser, detection distance and the like, and can improve the stability of the frequency modulation continuous wave interference system and the accuracy of phase discrimination of the interference signal.
Description
Technical Field
The invention relates to the technical field of signal processing and analysis, in particular to an amplitude normalization method, and further relates to an amplitude normalization method of a frequency modulation continuous wave interference signal.
Background
The Frequency Modulated Continuous Wave (FMCW) laser interference technology originates from a frequency modulated continuous wave radar, takes an optical path as a measurement object, and can directly or indirectly realize measurement of various physical quantities such as displacement, strain, stress, temperature, angular velocity, current, magnetic field and the like. The frequency modulation continuous wave optical fiber displacement sensor is based on the frequency modulation continuous wave interference principle, a semiconductor laser with linear modulation of frequency is adopted to emit light waves with linear modulation of optical frequency, and through the optical fiber Fabry-Perot interferometer structure, because the transmission optical paths of reference light and signal light are different, time delay is generated, so that frequency difference exists between the two beams of light, and heterodyne interference is generated to form interference signals. The frequency of the interference signal is related to the absolute optical path difference, and the change of the initial phase (i.e. phase shift) of the interference signal is related to the change of the optical path difference. The optical path difference can be obtained by carrying out frequency discrimination and phase discrimination on the interference signals, so that the measurement of various physical quantities of the interference signals is realized.
Because the semiconductor laser adopts the method of driving current internal modulation to realize frequency modulation, but the output optical power of the semiconductor laser also changes along with the driving current, the amplitude of the frequency modulation continuous wave interference signal is unequal in one modulation period and changes along with the modulation current. Meanwhile, the amplitude of the interference signal also changes with the change of the detection distance.
In signal frequency and phase discrimination, amplitude modulation and amplitude fluctuation of interference signals can cause large errors in frequency and phase discrimination. Therefore, before signal frequency discrimination and phase discrimination, amplitude normalization is carried out on the interference signal, so that the interference signal is a constant amplitude oscillation signal with constant amplitude, and the precision of the signal frequency discrimination and the phase discrimination can be improved.
When the modulation current varies linearly, the amplitude of the interference signal also varies linearly. In this case, the modulation slope of the amplitude of the interference signal can be calculated by peak-finding of the interference signal, linear fitting of the peak point, thereby realizing amplitude demodulation. However, the peak searching and line fitting in the method have large calculated amount, and the real-time performance of the phase discrimination of the interference signals is influenced. For the frequency modulation continuous wave displacement sensor, if the displacement change exceeds 1/4 wavelength in two adjacent measurement periods, the direction of displacement and the displacement increment cannot be judged. Therefore, the conventional method has a problem of poor real-time performance when measuring a fast moving object. The above method fails when the modulation current is not linearly varied. And the peak positioning sensitivity is poor and is influenced by signal noise, the peak calculation error is large, the requirement on the signal-to-noise ratio of an interference signal is high, and the general type is poor.
Disclosure of Invention
The invention provides a frequency modulation continuous wave interference signal amplitude normalization method, which aims to solve the problems of poor real-time performance and high requirement on the signal-to-noise ratio of an interference signal in the prior art.
In order to achieve the purpose, the invention provides the technical scheme that: a method for normalizing the amplitude of frequency modulated continuous wave interference signals comprises the following steps:
step 1, sampling interference signals and obtaining digital signals through analog-to-digital conversion;
step 2, carrying out amplitude demodulation on the frequency modulation continuous wave interference signal according to the PI characteristic of the semiconductor laser: the formula for signal amplitude demodulation is:
wherein S t ' is the demodulated interference signal, S t For interference signals before demodulation, I mean Is the average value of the drive current, I t For driving current transients, I th Is the threshold current of the laser;
and 3, step 3: calculating the effective value of the interference signal after amplitude demodulation;
and 4, step 4: calculating the signal amplitude of the interference signal after amplitude demodulation;
and 5: and calculating a gain coefficient and performing adaptive gain on the signal.
Calculating the effective value of the interference signal after amplitude demodulation in the step 3, wherein the calculation formula of the effective value of the signal is as follows:
wherein U is the effective value of the signal,
is the mean value of the signal, n is the number of sample points, and i is the sample point number.
In the step 4, the amplitude A of the signal and the amplitude A of the frequency modulation continuous wave interference signal are calculated p The relation with the effective value U is:
compared with the prior art, the invention has the following beneficial effects:
1. the amplitude demodulation of the invention removes the amplitude modulation of the interference signal, and corrects the interference signal into a sine wave of constant amplitude oscillation, which is beneficial to improving the phase discrimination precision of the interference signal.
2. According to the invention, the amplitude demodulation is carried out on the digital interference signal according to the PI characteristic of the semiconductor laser, the amplitude modulation slope of the interference signal does not need to be calculated in the amplitude correction process, the calculated amount is reduced, and the real-time performance of a displacement measurement algorithm and the detectable target speed upper limit are improved.
3. The invention adds digital automatic gain control after the amplitude adjusting step, obtains the signal amplitude according to the interference signal effective value, further accurately calculates to obtain the signal gain, amplifies the interference signal to a fixed amplitude, ensures that the amplitude of the interference signal is not changed by the change of factors such as measuring distance, target reflectivity, alignment and the like, improves the stability and the adaptability of the system, and simultaneously improves the precision of the subsequent frequency discrimination and phase discrimination algorithms.
4. The invention has the advantages of universality: the method has no requirement on the waveform of the modulation current, the amplitude demodulation effect is not influenced by the noise of the interference signal, and the universality is strong.
Drawings
FIG. 1 is a plot of the PI characteristic curve and frequency modulated continuous wave interference signal of a laser;
fig. 2 is an amplitude normalization of the frequency modulated continuous wave interference signal, with the normalized amplitude set to 3V.
The specific implementation mode is as follows:
the present invention will be described in further detail below with reference to the accompanying drawings.
The invention provides a method for normalizing the amplitude of a frequency modulation continuous wave interference signal, which is characterized in that a sampling unit is used for sampling the interference signal in a modulation period and converting the interference signal into a digital signal, and the amplitude of the frequency modulation continuous wave interference signal is demodulated according to the PI characteristic of a semiconductor laser.
The method is characterized in that amplitude modulation is carried out on the amplitude modulation of the interference signal according to the modulation current of the semiconductor laser, and meanwhile, self-adaptive gain is carried out according to the effective value of the interference signal, so that the interference signal is converted into a constant amplitude oscillation signal with constant amplitude.
Referring to fig. 1 and fig. 2, a method for normalizing the amplitude of an interference signal of a frequency modulated continuous wave specifically includes the following steps:
step 1: sampling interference signals in a modulation period and obtaining digital signals through analog-to-digital conversion;
step 2, carrying out amplitude demodulation on the frequency modulation continuous wave interference signal according to the PI characteristic of the semiconductor laser, and specifically comprising the following steps:
according to the PI characteristics of a semiconductor laser, the output power of the laser can be expressed as:
p＝k(I t -I th )，
where k is the slope of the laser PI characteristic curve, I t For driving current transients, I th Is the threshold current of the laser.
According to the principle of optical heterodyne detection, the photocurrent output by the photodetector is:
where R is the detector sensitivity, p s Is the signal optical power, p r As reference optical power, p s 、p r Proportional to the output power of the laser, f is the frequency difference between the signal light and the reference light.
The power of the laser is in a linear relation with the modulation current; the photocurrent (i.e., the interference signal) output by the photodetector is proportional to the power of the laser. So that the interference signal amplitude A s Is modulated by the drive current of the laser, A s The relationship to the drive current is:
A s ＝k s (I t -I th )，
wherein k is s For the scaling factor, the equation for the available signal amplitude demodulation is:
wherein S t For the demodulated interference signal, S t For interference signals before demodulation, I mean Is the average value of the drive current.
Step 3, calculating the effective value of the interference signal after amplitude demodulation:
the effective value of the signal is calculated by the formula
Wherein U is the effective value of the signal,
is the average value of the signal, n is the number of sample points, and i is the serial number of the sample points.
And 4, step 4: calculating the amplitude of the interference signal after amplitude demodulation, wherein the frequency modulation continuous wave interference signal is a cosine signal, and the signal amplitude A thereof p The relationship with the valid value U is:
and 5: calculating signal gain with a set amplitude of A r E.g. amplitude A r Can be set to 3V with a gain of
And amplifying the amplitude-demodulated signal by a gain L to realize automatic gain control of the frequency-modulated continuous wave interference signal.
All other embodiments, which can be obtained by a person skilled in the art without making any creative effort based on the embodiments in the present invention, belong to the protection scope of the present invention.
Claims (1)
1. A method for normalizing the amplitude of frequency modulated continuous wave interference signals comprises the following steps:
step 1, sampling interference signals and obtaining digital signals through analog-to-digital conversion;
step 2, carrying out amplitude demodulation on the frequency modulation continuous wave interference signal according to the PI characteristic of the semiconductor laser: the formula for signal amplitude demodulation is:
wherein S t ' is the demodulated interference signal, S t For interference signals before demodulation, I mean Average value of drive current, I t For driving current transients, I th Is the threshold current of the laser;
and 3, step 3: calculating the effective value of the interference signal after amplitude demodulation;
and 4, step 4: calculating the signal amplitude of the interference signal after amplitude demodulation;
and 5: calculating a gain coefficient, and performing self-adaptive gain on the signal;
the calculation formula of the effective value of the signal in the step 3 is as follows:
wherein U is the effective value of the signal,
is the average value of the signal, n is the number of sample points, and i is the serial number of the sample points.
In the step 4, the amplitude A of the signal and the amplitude A of the frequency modulation continuous wave interference signal are calculated p The relation with the effective value U is:
and 5: calculating signal gain with a set amplitude of A r E.g. amplitude A r Can be set to 3V with a gain of
And amplifying the amplitude-demodulated signal by a gain L to realize automatic gain control of the frequency-modulated continuous wave interference signal.
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| Publication number | Priority date | Publication date | Assignee | Title |
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| JP2006105669A (en) * | 2004-10-01 | 2006-04-20 | Niigata Univ | Method and apparatus for measuring laser interference displacement |
| CN108444381A (en) * | 2018-05-15 | 2018-08-24 | 西安工业大学 | One kind is for eliminating the nonlinear bearing calibration of semiconductor laser frequency modulation interference signal |
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| DE102013210256A1 (en) * | 2013-06-03 | 2014-12-04 | Robert Bosch Gmbh | INTERFERENCE SUPPRESSION ON AN FMCW RADAR |
| EP3574337B1 (en) * | 2017-01-24 | 2022-11-23 | HRL Laboratories, LLC | Dual frequency fmcw lidar and method |
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| Publication number | Priority date | Publication date | Assignee | Title |
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| JP2006105669A (en) * | 2004-10-01 | 2006-04-20 | Niigata Univ | Method and apparatus for measuring laser interference displacement |
| CN108444381A (en) * | 2018-05-15 | 2018-08-24 | 西安工业大学 | One kind is for eliminating the nonlinear bearing calibration of semiconductor laser frequency modulation interference signal |
Non-Patent Citations (2)
| Title |
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| Identification of FMCW radar in mutual interference environments using frequency ramp modulation;Youngwook Kim;《2016 10th European Conference on Antennas and Propagation (EuCAP)》;20161231;全文 * |
| 高精度双干涉光路调频连续波激光绝对测距系统;时光等;《红外与激光工程》;20160831;第45卷(第8期);全文 * |
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