U.S. patent application number 09/834348 was filed with the patent office on 2002-02-07 for method of detecting and correcting non-linearities in a microwave radar system.
Invention is credited to Leinbaum, Stephan, Winner, Hermann, Winter, Klaus.
Application Number | 20020014989 09/834348 |
Document ID | / |
Family ID | 7638762 |
Filed Date | 2002-02-07 |
United States Patent
Application |
20020014989 |
Kind Code |
A1 |
Winter, Klaus ; et
al. |
February 7, 2002 |
Method of detecting and correcting non-linearities in a microwave
radar system
Abstract
A method of detecting and compensating for non-linearities in a
microwave radar system in which a transmitted signal, frequency
modulated according to a predefined function, is generated using a
transmit oscillator, and by mixing the transmitted signal with a
received signal reflected by an object, a distance of the microwave
radar system from the object is determined. In predefined time
windows a predefined constant test control voltage is applied to
the transmit oscillator instead of a control voltage effecting
frequency modulation, preferably using a test ramp, the
voltage/frequency characteristic of a module being updated during
the time window in order to generate the function characteristic
for frequency modulation taking into consideration the reference
signal.
Inventors: |
Winter, Klaus;
(Schwieberdingen, DE) ; Winner, Hermann;
(Karlsruhe, DE) ; Leinbaum, Stephan;
(Oberriexingen, DE) |
Correspondence
Address: |
KENYON & KENYON
One Broadway
New York
NY
10004
US
|
Family ID: |
7638762 |
Appl. No.: |
09/834348 |
Filed: |
April 12, 2001 |
Current U.S.
Class: |
342/174 ;
342/128; 342/70 |
Current CPC
Class: |
G01S 13/931 20130101;
G01S 7/4008 20130101; G01S 13/343 20130101 |
Class at
Publication: |
342/174 ; 342/70;
342/128 |
International
Class: |
G01S 007/40 |
Foreign Application Data
Date |
Code |
Application Number |
Apr 14, 2000 |
DE |
1 00 18 553.3 |
Claims
What is claimed is:
1. A method of detecting and correcting non-linearities in a
microwave radar system, comprising: generating a transmitted
signal, frequency modulated according to a predefined function,
using a transmit oscillator and by mixing the transmitted signal
with a received signal reflected by an object; determining a
distance of the microwave radar system from the object; testing
linearity characteristics of the microwave radar system using a
reference signal; using test results for a correction; in
predefined time windows, applying a predefined constant test
control voltage to the transmit oscillator instead of a control
voltage effecting frequency modulation; updating a
voltage/frequency characteristic of a module during a time window
in order to generate a function characteristic for frequency
modulation taking into consideration the reference signal; and
using the updated voltage/frequency characteristic for frequency
modulation outside the time window during operation of the
microwave radar system.
2. The method according to claim 1, further comprising: using a
stepped test ramp whose individual steps form the time window with
test control voltages of different values and whose steps each have
a predefined deviation; determining a test signal at each step; and
generating a corresponding frequency value of the transmit
oscillator.
3. The method according to claim 2, further comprising: adding up
absolute values of deviation differences to form a sum; and
comparing the sum to an error threshold.
4. The method according to claim 3, further comprising: determining
a linearity indicator by successive summations of the deviation
differences.
5. The method according to claim 3, further comprising: determining
the deviation differences by evaluating a count of a
microprocessor-controlle- d circuit arrangement, a minimum number
of required difference counts per step frequency to achieve a
minimum accuracy being determined.
6. The method according to claim 5, further comprising: determining
a step frequency from a total number of steps and a maximum sum of
the deviation differences for which an error response should
occur.
7. The method according to claim 6, further comprising: performing
a measurement over a plural number of periods in order to calculate
the step frequency; and dividing a measured difference count by the
number.
Description
Background Information
[0001] German Patent No. 197 13 967, for example, describes a
system for measuring distances in the surrounding area of motor
vehicles having a FMCW microwave radar sensor (FMCW=Frequency
Modulated Continuous Wave). This radar sensor has, as essential
elements, a transmit oscillator, a mixer, and an antenna system for
transmitting and receiving radar beams reflected by an object. The
transmitted signal is frequency modulated with a predefined ramp
function, for example, so that a frequency of the transmitted
signal modified in the meantime by the modulation ramp is present
due to the propagation time of the received reflected signal, and
this frequency difference is a measure for the distance
measurement.
[0002] The linearity of the above-mentioned ramp function, i.e., of
the transmitted frequency ramp, is of decisive importance for the
accuracy of measurement, the resolution, and the sensitivity of the
FMCW microwave radar. In order to ensure that a linear frequency
ramp is produced for modulation, the known device also has a
reference oscillator whose output signal is mixed with the
transmitted signal directly in the radar sensor simultaneously with
the actual distance measurement. A reference quantity can be
obtained by demodulating this signal, the reference quantity
containing any non-linearities in the transmitted branch of the
radar sensor, which can thus be taken into account accordingly in
the analysis.
[0003] In conventional microwave radar systems, frequency
regulation is also used (FLL or PLL control circuits) to achieve
sufficiently good linearities of the frequency ramp; this
regulation is implemented using appropriate additional, usually
expensive, circuit resources. Such a frequency regulation and the
respective hardware circuit components must be continuously
monitored in operation for proper functioning, since a faulty
response in the frequency regulating circuits results in modulation
with insufficiently linear frequency ramps.
[0004] Although using the known devices it is possible to perform
real time frequency regulation to the desired modulation frequency
with correction of non-linearities, this involves relatively high
circuit costs.
SUMMARY OF THE INVENTION
[0005] The present invention is based on the above-mentioned method
of detecting and correcting non-linearities in a microwave radar
system in which a transmitted signal, frequency modulated according
to a predefined function, is generated using a transmit oscillator,
and by mixing the transmitted signal with a received signal
reflected by an object, a distance of the microwave radar system
from the object is determined. According to the present invention,
in predefined time windows a predefined constant test control
voltage or a sequential series of such voltages is sent to the
transmit oscillator instead of a control voltage effecting
frequency modulation, provided in normal measuring cycles.
[0006] The reference signal determined for the respective constant
test control voltage which should correspond to a frequency value
of the transmit oscillator can now be used for correcting the
characteristic curve for frequency modulation and thus for
compensating for non-linearities in a simple manner. Thus, in
principle the present invention can be summarized as measuring the
voltage/frequency characteristic of the transmit oscillator
operating in the mm wavelength range using a test function.
[0007] Test control of the oscillator is achieved using the
constant control voltage which is applied to the transmit
oscillator (VCO) for the predefined time window. The test voltage
is converted into a transmitted frequency in the transmit
oscillator and the transmitted frequency is mixed with the
frequency of a reference oscillator (DRO), for example, with its
sixth harmonic. The mixed frequency is then proportional to the
emitted oscillator frequency, the frequency of the reference
oscillator being selected so that the mixed frequency is in the
range of <1 GHz.
[0008] For the usual type of ramp as a modulation function for the
transmit oscillator, a ramp function which is appropriately
pre-distorted in the voltage range is then generated from the
measurement results; this ramp function takes into account and
compensates for the non-linearities of the voltage/frequency
characteristic of the transmit oscillator. These non-linearities
may be caused, for example, by a non-linear frequency ramp in
normal operation due to a fault of the ramp generator or of a
regulating circuit (PLL or FLL), or also by a malfunction of a
reference oscillator (DRO) or a defective U/f conversion by the
transmit oscillator (GUNN oscillator).
[0009] According to a preferred embodiment, the method according to
the present invention is performed using a stepped test ramp, whose
individual steps form time window .DELTA.t each with test control
voltages of different magnitudes and whose steps each have a
predefined deviation, which may also be constant. The actual
frequency value of the transmit oscillator is then determined at
each step.
[0010] This linearity test is performed cyclically during the
operation of the microwave radar system, for example, once a
second. Advantageously, after a predefined number of measuring
cycles, preferably in every 10th measuring cycle and once at the
time of the initialization of the microwave radar, the stepped test
ramp is advantageously applied to the transmit oscillator and is
subsequently evaluated so that the difference of the deviations of
the frequency values with respect to the deviations of the test
ramp is determined from the measured frequency values. The absolute
values of the deviation differences of adjacent steps are added up
and the sum is compared to an error threshold.
[0011] In order to keep the memory requirements necessary for the
above-mentioned calculation low, a linearity indicator .delta.,
determined by successive summations of the deviation differences,
is formed. For this purpose, the transmit oscillator is stepped up
in constant steps from the lowest possible frequency over the
entire control range, whereby ideally a stepped curve with a
constant deviation is obtained for the intermediary frequency. In
the following step, the absolute value of the deviation difference
.vertline..DELTA.Hub.vertline. from step n to step n-1 is formed
for a total number of k steps and subsequently added up over all
deviation differences. The following equation is thus obtained for
linearity indicator .delta.: 1 = n = 1 k - 1 Hub n + 1 - Hub n ( 1
)
[0012] where
Hub.sub.n=f.sub.stufe(n)-f.sub.stufe(n-1) (2)
[0013] The frequency of the reference signal obtained by mixing can
be subdivided according to an advantageous embodiment using a
frequency divider until it can be measured during the predefined
time window with sufficient accuracy. A direct correspondence is
thus obtained between the value of the test voltage applied and the
oscillator frequency.
[0014] The present invention advantageously makes it possible to
update the voltage/frequency characteristic of the module during
the above-mentioned time window in order to generate the function
for frequency modulation taking into consideration the reference
signal, and to use the updated voltage/frequency characteristic for
frequency modulation of the transmit oscillator outside the time
window during operation of the microwave radar system. Thus the
entire voltage/frequency characteristic can be determined or
updated in a simple manner by varying the test control voltage in
consecutive time windows if it has changed, for example, due to
temperature influences. Subsequently the characteristic thus
determined can be taken into account in generating a modulation
ramp for the variation of the control voltage in normal operation,
so that a linear frequency ramp is transmitted by the microwave
radar as a result.
[0015] Overall, the method according to the present invention
allows for a cost-effective design of a microwave radar system
having the required frequency regulating components, since the
voltage/frequency characteristic can be updated, i.e. corrected,
via control commands integrated in a software program for ramp
generation in order to modulate frequency without any additional
circuit components. Furthermore, the proposed method is relatively
insensitive to fluctuations in the values of the control
electronics components in addition to being easily adjusted to new
operating conditions.
BRIEF DESCRIPTION OF THE DRAWINGS
[0016] FIG. 1 shows a block diagram of circuit and function
components of a microwave radar system having a test ramp for a
linearity test.
[0017] FIG. 2 shows a stepped curve of the test ramp for driving a
transmit oscillator in the microwave radar system according to FIG.
1.
[0018] FIG. 3 shows the stepped curve measured for the linearity
test in relation to the exact stepped curve.
DETAILED DESCRIPTION
[0019] FIG. 1 shows a block diagram with the elements that are
important for the present invention of an FMCW microwave radar
system. A voltage-controlled transmit oscillator 1 (GUNN VCO) in
the frequency range of 76.5 GHz, for example, is provided, whose
output signal is sent to a transceiver module 2. This transceiver
module 2 having an upstream lens 3 can be a component of a radar
sensor in a motor vehicle, for example, with which the distance to
an object such as a vehicle traveling ahead, for example, is to be
determined.
[0020] The transmitted signal is sent in transceiver module 2 to
three antennas 5 in this embodiment via coupling modules 4 for
lateral detection of an object, and the received signal reflected
by the object is mixed in a mixer 6 with transmitted signal
f.sub.GUNN, and the mixed signal at outputs 7 is used in the manner
described in the preamble for distance measurement. A reference
oscillator (DRO) 8 is also present, whose output signal is gated
with transmitted signal f.sub.GUNN resulting in an intermediate
frequency signal f.sub.ZF. There is also a mixer module 9, whose
output signal is passed through a frequency divider 10, resulting
in a reference signal which is explained in more detail below. The
signals are gated in module 2 in the way typically used in
microwave technology.
[0021] In the normal operating state, a linear ramp signal u(t) is
generated using a ramp generator 11; from this signal, a
characteristic voltage/frequency curve f(u) can be derived in a
module 12. The frequency of voltage-controlled transmit oscillator
1 can be modulated via a driver 14 according to the characteristic
of module 12 via a control module 13 which is usually implemented
in the software in the form of program control commands.
[0022] A test voltage generator 15 generates a predefined constant
test control voltage, here a test ramp 16, in predefined time
windows .DELTA.t which is applied in these time windows .DELTA.t to
transmit oscillator 1 via control module 13.
[0023] The test signal delivered by frequency divider 10 in these
time windows .DELTA.t, which corresponds to a certain frequency
value of transmit oscillator 1, is now used for voltage/frequency
characteristic correction in module 12 after passing through
control module 13. Thus a voltage ramp, appropriately pre-distorted
in the voltage range is generated from the test signals for the
modulation function of transmit oscillator 1; this voltage ramp
takes into account the non-linearity within the microwave radar
system.
[0024] FIG. 2 shows the ideal frequency F.sub.GUNN of transmit
oscillator 1 from f.sub.min to f.sub.max via a control voltage
U.sub.DAC compared to test ramp 16 of test generator 15 and having
steps S1 through S6 differing from one another by deviations H1
through H6. The sum of all deviation differences obtained from the
equation for a linearity indicator .delta. according to Formula (1)
2 = n = 1 k - L Hub n + 1 - Hub n
[0025] where
Hub.sub.n=f.sub.stufe(n)-f.sub.stufe(n--1)
[0026] is then compared to the fixed maximum value f.sub.max. If
the maximum value f.sub.max is exceeded, this is evaluated as a
positive error detection and the characteristic is corrected as
described with reference to FIG. 1. Since frequency deviations H1
through H6 may not be accurately determined in a time window
.DELTA.t, a minimum number of required difference counts per step
frequency must be established in order to obtain a corresponding
minimum accuracy in the deviation calculation and thus also in the
calculation of the sum of all deviation differences. Furthermore,
the total number of steps and the maximum sum of the deviation
differences for which an error response should occur is to be taken
into account.
[0027] Using a microprocessor-controlled electronic circuit
arrangement which is known heretofore, the required step frequency
can be determined from the measured signals. The number of level
changes that occur in a certain direction (i.e., high-low or
low-high) of the processor frequency within one period of the
reference signal is determined with the help of an interrupt
control (timer mode). Internally a counter is incremented with each
level change and an interrupt makes it possible to read this count
for each positive (or negative) edge of the reference signal. For a
processor frequency of 28.5 MHz, this results in a .DELTA.t of
approximately .+-.70 ns. In order to minimize the error in
calculating the step frequency, measurements can be performed over
n periods instead of a single period and then the difference count
can be divided correspondingly by n. Since the maximum error of
.+-.1 occurs in the difference count even when measuring over n
periods a maximum error of .+-.1/n is obtained by the subsequent
division.
[0028] FIG. 3 shows the exact stepped curve 16 having steps S1
through S5 as a dotted line and the measured stepped curve 17 as a
solid line. The positive and negative deviations .DELTA.s1 through
.DELTA.s5 are also shown for each deviation between steps S1
through S5. The systematic error for the linearity monitoring using
the sum of the deviation differences is maximum when the calculated
deviations have alternating maximum positive and maximum negative
errors. This occurs when the errors in the step frequency
calculation are alternately maximum positive and maximum negative
as shown in FIG. 3. Thus in the worst-case scenario the following
is obtained as the systematic error for linearity indicator
.delta.:
.delta.=(.DELTA.s1+2*.DELTA.s2+.DELTA.s3)+(.DELTA.s2+2*.DELTA.s3+.DELTA.s4-
)+(.DELTA.s3+2*.DELTA.s4+.DELTA.s5)+ (3)
[0029] or 3 = n = 2 k - 1 ( s n - 1 + 2 * s n + s n + 1 ) ( 4 )
[0030] or 4 = s 1 + 3 * s 2 + 4 * n = 3 k - 2 s n + 3 * s k - 1 + s
k ( 5 )
[0031] Taking into account the frequency values for step deviations
.DELTA.s1 through .DELTA.s5, the required number n of steps can now
be determined by simply substituting any desired maximum systematic
error in this equation. For example, for a lower ramp frequency of
approximately 400 MHz and a total deviation of approximately 200
MHz and a total of 10 steps for a maximum systematic error of 1
MHz, a minimum required n of 36 is obtained.
* * * * *