Abstract:
A device for distance measurement with the aid of electromagnetic waves includes a transmitting device for transmitting, in a measuring mode, electromagnetic waves as a transmitted signal to a measured object, a receiving device for receiving, in the measuring mode, the electromagnetic waves back-scattered by the measured object as a received signal, and an analyzer device for determining, in an analysis mode, the propagation time, and for outputting a measured distance, the analyzer device having a compensation unit for compensating distance measurements carried out during the analysis mode.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to distance measuring devices in general and, in particular, it relates to a device for distance measurement with the aid of electromagnetic waves. 
         [0002]    Although example embodiments of the present invention are utilizable for any type of distance measurements, example embodiments of the present invention are described below with reference to a warning device and a warning method for a motor vehicle. 
         [0003]    In particular, example embodiments of the present invention relate to a distance measuring device which uses electromagnetic waves and includes: a transmitting device for transmitting, in a measuring mode, electromagnetic waves as a transmitted signal to a measured object, the transmitting device also having a pulse generator for outputting a pulse control signal such that the electromagnetic waves are output as transmitted pulses as a function of an activation by a pulse generator; a receiving device for receiving, in the measuring mode, the electromagnetic waves back-scattered by the measured object as a received signal, the receiving device also having a delay unit for delaying in time the pulse control signal output by the pulse generator as a function of a ramp signal supplied to the delay unit and for outputting a delayed pulse control signal, and a mixing unit for mixing the received signal with transmitted pulses, time-delayed according to the delayed pulse control signal, and for outputting a measuring signal as a function of the measured distance, the measuring signal being output only if the time delay defined by the delay unit coincides with a propagation time of the transmitted pulses from the transmitting device to the measured object and back to the receiving device; and an analyzer device for determining, in an analysis mode, the propagation time, and for outputting the measured distance as a measurement result. 
       BACKGROUND INFORMATION 
       [0004]    In general, distance measuring systems which perform distance measurements on the basis of electromagnetic waves back-scattered by a measured object are used for distance measurement in motor vehicles. Electromagnetic waves having a base frequency of 24 gigahertz (GHz), for example, are transmitted as individual pulses to the measured object, i.e., an obstacle located in front of the vehicle, and reflected back by this object. The transmitted pulses reflected back by the measured object are detected in a receiving device of the measuring system where they are superimposed with the originally transmitted pulses, which are used as reference pulses. A mixing device and an analyzer circuit are responsible for a measuring signal being output from a mixing unit in which the reference pulse is mixed with the received transmitted pulses only if the reference pulses coincide in time with the corresponding transmitted pulses back-scattered by the measured object. Since the transmitted pulses back-scattered by the measured object require a propagation time from the transmitting device to the measured object and back to the receiving device of the measuring system, in order to achieve a time overlap, the received pulses in the receiving device of the measuring system are also time delayed by a delay unit. Normally a time delay is specified in the form of a ramp signal (voltage ramp) as  FIG. 4  shows for a conventional measuring system. In the graph of  FIG. 4 , the x axis corresponds to a signal variation over time, while the y axis denotes the signal delay of the transmitted pulses with respect to the reference pulses and is calibrated in distance values (25 cm . . . 2.5 m). If there is an obstacle in front of the transmitting device at a distance of 2.5 m, for example, the mixing unit situated in the receiving device outputs a measuring signal at a point in time corresponding to this measured distance as shown by the dashed line in  FIG. 4 . Furthermore,  FIG. 4  shows that a measurement is performed repetitively, i.e., the voltage ramp and thus the continuous time delay which is set by the delay unit is repeated multiple times. Furthermore,  FIG. 4  shows that a measuring pause between the individual voltage ramps is predefined in order to allow time for the analyzer unit of the measuring system (LF part) to analyze the pulses output by the mixing unit and to provide a measurement result. 
         [0005]      FIG. 3  shows a typical curve of the ramp signal in the scan phase and the analysis phase (A), the measured distance (ME) being plotted as a function of time. In the case of the conventional measurement method, as analysis time A the voltage ramp signal remains at a constant value, usually at a value between a minimum voltage value and a maximum voltage value of the voltage ramp. This value indicated in  FIG. 3  by Z corresponds to a specific distance which may be measured by the distance measuring system. Depending on the resolution of the measuring system, the distances associated with the voltage values of the voltage ramp are divided into “distance cells.” The distance cell corresponding to voltage value Z is thus measured during the analysis mode, since transmitted pulses are continuously transmitted to the measured object and are received from the measured object by the receiving device of the measuring system even during the analysis mode. 
         [0006]    If a measured object is located in front of the sensor within such a distance cell, a meaningful signal is obtained, which charges coupling capacitors situated between the HF part of the measuring system and the LF part of the measuring system, displacing the working point in the LF part of the measuring system. In a subsequent scan (reference symbol N in  FIG. 3 ) this disadvantageously results in a measuring error. 
       SUMMARY 
       [0007]    Example embodiments of the present invention provide a distance measuring device and a corresponding method in which a displacement of the working point which is caused by measurements performed during the analysis mode may be prevented. 
         [0008]    Example embodiments of the present invention provide, with the aid of the ramp generator provided in the receiving device of the measuring system, such a ramp signal to make it possible to address different distance cells even during the analysis mode, resulting in different distance measuring signals which mutually compensate one another during the analysis time. In this manner, it may be achieved that a displacement of the working point in the LF part of the measuring device is prevented by such a compensation. 
         [0009]    It is thus possible to retain coupling capacitors which are situated in the LF part, in order to implement a simple and cost-effective circuit arrangement. The coupling capacitors are charged during the measuring pause, i.e., during the analysis time, by distance measurements which are carried out in the individual distance cells; however, compensation is achieved by positive and negative chargings of the coupling capacitor canceling out one another due to the activation of different distance cells corresponding to different measured distances. In this manner, the state is achieved where no relevant distance measurement is performed during the analysis time, so that a measurement following a measurement pause may start in the LF part without the working point being displaced. 
         [0010]    The measuring device according to example embodiments of the present invention for distance measurement with the aid of electromagnetic waves has: a transmitting device for transmitting, in a measuring mode, electromagnetic waves as a transmitted signal to a measured object, the transmitting device also having a pulse generator for outputting a pulse control signal such that the electromagnetic waves are output as transmitted pulses as a function of an activation by a pulse generator; a receiving device for receiving, in the measuring mode, the electromagnetic waves back-scattered by the measured object as a received signal, the receiving device also having a delay unit for delaying in time the pulse control signal output by the pulse generator as a function of a ramp signal supplied to the delay unit and for outputting a delayed pulse control signal; and a mixing unit for mixing the received signal with transmitted pulses, time-delayed according to the delayed pulse control signal, and for outputting a measuring signal as a function of the measured distance, the measuring signal being output only if the time delay defined by the delay unit coincides with a propagation time of the transmitted pulses from the transmitting device to the measured object and back to the receiving device; and an analyzer device for determining, in an analysis mode, the propagation time and for outputting the measured distance as a measurement result, a compensation unit being provided for compensating distance measurements carried out during the analysis mode. 
         [0011]    The measurement result may be divided into different distance cells corresponding to the measured distance. 
         [0012]    The compensation unit for compensating distance measurements carried out during the analysis mode may include a processing and control unit (a microcontroller, for example) for processing the measuring signal output as a function of the measured distance, and a ramp generator, using the ramp signal, the ramp generator activating the delay unit during the analysis mode such that at least two different distance cells are set. 
         [0013]    The compensation unit may be arranged as a microcontroller, which predefines the ramp signal for the delay unit of the receiving device. 
         [0014]    In the measuring mode for measuring the distances, the distances may be divided into a predefinable number (n) of distance cells. The distance measurement carried out by the distance measuring device may be based on the use of optical radiation. 
         [0015]    Exemplary embodiments of the present invention are illustrated in the drawing and explained in greater detail in the description that follows. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
         [0016]      FIG. 1  shows a block diagram of a measuring system having a transmitting device, a receiving device, and an analyzer device according to an exemplary embodiment of the present invention. 
           [0017]      FIG. 2  shows the variation of a ramp signal during a measuring period according to an example embodiment of the present invention. 
           [0018]      FIG. 3  shows the variation of a ramp signal in a conventional measuring method. 
           [0019]      FIG. 4  shows a graph illustrating the generation of a measuring signal in a conventional measuring method. 
       
    
    
     DETAILED DESCRIPTION 
       [0020]    In the figures, the same reference symbols identify components or steps that are identical or have an identical function. 
         [0021]      FIG. 1  shows a block diagram of a distance measuring system according to an exemplary embodiment of the present invention. As illustrated in  FIG. 1 , the measuring system is divided into a transmitting device  100 , a receiving device  200 , and an analyzer device  400 . Normally, the block containing transmitting device  100  and receiving device  200  is referred to as the high-frequency part (HF part) of the circuit system, while analyzer device  400  forms the low-frequency part (LF part) of the analyzer device. 
         [0022]    The electromagnetic waves used for the distance measurement are generated in an oscillator unit  205 . The electromagnetic waves generated in oscillator unit  205  are supplied to transmitting device  100  as an oscillator output signal  207 , as well as further processed in receiving device  200 . 
         [0023]    In the following, the operation of transmitting device  100  will be briefly described. A reference numeral  102  denotes a transmission switching unit, which is responsible for the possibility of transmitting the electromagnetic waves provided as oscillator output signal  207  in a pulsed, rather than continuous, manner. For this purpose, transmission switching unit  102  is activated by a transmitted pulse generating signal output by a transmission driver unit  103 . A transmitted signal  104  having individual transmitted pulses is thus provided by transmission switching unit  102 . The base frequency of the transmitted pulses, i.e., the oscillator frequency of oscillator unit  205 , is typically 24 GHz. Such a frequency may be used for radar sensors. 
         [0024]    Transmitting device  100  also has a pulse generator  208 , which provides pulse control signals  210  for processing in transmitting device  100  and in receiving device  200 . Pulse generator  208  delivers pulse control signal  210  for activating transmission driver unit  103 , which switches oscillator output signal  207  through to a transmitting antenna  101  according to pulse control signal  210 . Transmitted signal  104  is thus emitted to a measured object  300  as a pulse signal and reflected/scattered/refracted by the object. Transmitted signal  104  back-scattered by measured object  300  is received by receiving device  200  in the form of a received signal  204 . The structure, i.e., base frequency and pulse modulation, of received signal  204  is exactly the same as that of transmitted signal  104  with the exception that the propagation time of the electromagnetic waves from transmitting antenna  101  of transmitting device  100  to measured object  300  and back to a receiving antenna  201  of receiving device  200  causes a time delay due to measured distance  301 . 
         [0025]    It should be pointed out that the measured distance, divided into different “distance cells” among other things, represents the desired measuring signal, which is to be obtained using the device for distance measurement. The propagation time difference between the point in time transmitted signal  104  is emitted from the transmitting antenna and the point in time the corresponding received signal  204  is received in receiving antenna  201  is approximately equal to twice the measured distance represented by reference numeral  301 , divided by the speed of light (c). 
         [0026]    In the following, the operation of receiving device  200  is briefly and schematically described with reference to the schematic block diagram of  FIG. 1 . 
         [0027]    Receiving device  200  has a reception switching unit  202  which is designed similarly to transmission switching unit  102  of transmitting device  100 , and is responsible for oscillator output signal  207  output by oscillator unit  205  being “chopped up” into individual pulses. A mixer input signal  216  is thus obtained, which, except for a time shift, corresponds to transmitted signal  104  of transmitting device  100 . As described above, taking into account the explanation for transmitting device  100 , reception switching unit  202  of receiving device  200  is also activated by an appropriate received pulse generation signal  212 . Received pulse generation signal  212  is obtained from a reception driver unit  203 , which is also activated via pulse generator  208  situated in transmitting device  100 . The pulse generator thus delivers pulse control signals  210  having identical repeat frequency and period length to transmitting device  100  and to receiving device  200 . 
         [0028]    Receiving device  200  also has a delay unit  209 , which makes it possible to time-delay pulse control signal  210  supplied to it in order to obtain a delayed pulse control signal  211 . The time delay provided by delay unit  209  may be set using a ramp signal  405 , which is described below. 
         [0029]    A reference numeral  213  denotes a mixing unit in which received signal  204  received from the measured object and the mixer input signal, which has the time delay provided by delay unit  209 , may be mixed. Mixing unit  213  is configured such that it outputs a measuring signal  215  only if the pulses contained in received signal  204  correlate exactly in time with the pulses contained in mixer input signal  216 . Such pulses have a curve such as was explained conventionally, with reference to  FIG. 4 , i.e., the output of an amplifier unit  401  (LF amplifier) corresponds to the lower curve shown in  FIG. 4 . 
         [0030]    In the following, analyzer device  400  provided in the measuring system is briefly explained. Amplifier unit  401  in the analyzer device is used for amplifying the output signal of mixing unit  213 , which represents measuring signal  215 . As shown above with reference to the explanation for receiving device  200 , the time of occurrence of measuring signal  215  corresponds to a measured distance  301 . The measuring signal output from amplifier unit  401  (output of the LF amplifier, see  FIG. 4 ) is supplied to a coupling capacitor  402 , which must be present in the distance measuring systems for decoupling the HF part from the LF part. Coupling capacitor  402  is connected to a processing and control unit  403 , in which measuring signal  215  is processed. Furthermore, processing and control unit  403 , which may be configured as a microcontroller, outputs a control signal  406  to a ramp generator  404 . Activating ramp generator  404  via control signal  406  causes a ramp signal  405  to be provided, which has a variation over time as described below with reference to  FIG. 2 . 
         [0031]    In the following, ramp signal  405 , which is output from ramp generator  404 , is described first in detail with reference to  FIG. 2 .  FIG. 2  shows the curve of a measured distance  301 , which corresponds to a time delay of the transmitted pulses due to the transmission from transmitting antenna  101 , reflection on measured object  300 , and subsequent receipt by receiving antenna  201 , plotted as a function of a time  501 . 
         [0032]    A reference numeral  502  in  FIG. 2  corresponds to a measuring period, which is repeated multiple times, i.e., repetitive measurements are provided. The measuring period is divided into a sampling period  503  and a signal analysis period  504 . As described above, the ramp-shaped signal during sampling period  503  results in a continuously increasing delay being provided by delay unit  209  to which ramp signal  405  is supplied. If the time delay set by the ramp during sampling period  503  and implemented by delay unit  209  corresponds to a specific measured distance  301 , in which there is a measured object  300  in front of the distance measuring system, a measuring signal  215  is output from mixing unit  213 , whereupon a corresponding distance may be calculated in processing device  403 . Such a determination of the distance is performed during signal analysis period  504  shown in  FIG. 2 . According to example embodiments of the present invention, ramp generator  404  delivers, during signal analysis period  504 , a compensation ramp signal  505  such that a predefinable number of ramps are run through, whereby different distance cells are settable. In this, manner coupling capacitor  402  receives different—positive and negative—voltage signals, which cancel out one another in the case of an appropriate number of set distance cells. The advantage according to example embodiments of the present invention is thus achieved in that the working point in the LF part of the distance measuring system is not displaced. 
         [0033]    Normally the potential of the voltage ramp, i.e., of ramp signal  405  generated by ramp generator  404 , is proportional to measured distance  301 . It may be provided to configure ramp signal  405  as a stepped function where each individual step corresponds to a distance cell. All distance cells are first addressed by the LF part of the measuring system from the beginning to the end of the measurement, whereupon the useful signal is stored. In the subsequent signal analysis period  504  a signal analysis is then performed, whereupon the next scan, i.e., the following sampling period  503 , begins. According to example embodiments of the present invention, different distance cells are constantly set even during signal analysis period  504 , since during signal analysis period  504  ramp signal  405  is configured as a compensation ramp signal  505 , as illustrated in  FIG. 2 . 
         [0034]    Not a single distance cell thus remains set, as described above with reference to the conventional method, but always new distance cells are set such that coupling capacitor  402  of analyzer device  400  cannot be charged to any substantial extent. It is thus achieved that a displacement of the working point in the LF part before a new measuring cycle, i.e., a new sampling period  503 , is prevented. 
         [0035]    Normally, the flanks of pulse control signal  210  trigger the transmitted pulse generation, i.e., the generation of mixer input signal  216 . The repeat frequency of pulse control signal  210  provided by pulse generator  208  is typically 2.5 megahertz (MHz). A range of 25 cm to 10 m may be settable for a measured distance  301 , the distance cells having a geometric length of 4 cm. The travel which is provided by compensation ramp signal  505  during a signal analysis period  504  of measuring period  502  may correspond to a travel corresponding to a geometric length of 100 cm. 
         [0036]    Regarding the conventional measuring method illustrated in  FIGS. 3 and 4 , reference is made to the preamble of the description. 
         [0037]    Although example embodiments of the present invention have been described above on the basis of certain exemplary embodiments, it is not limited thereto, but may be modified in multiple manners. 
         [0038]    Example embodiments of the present invention are also not limited to the above-mentioned application options in motor vehicles.