WO2000043801A1

WO2000043801A1 - Sensor for measuring a distance from an object

Info

Publication number: WO2000043801A1
Application number: PCT/EP1999/007371
Authority: WO
Inventors: Martin Reiche
Original assignee: Valeo Schalter Und Sensoren Gmbh
Priority date: 1999-01-20
Filing date: 1999-10-05
Publication date: 2000-07-27
Also published as: EP1064563A1; DE19902076B4; DE19902076A1

Abstract

The invention relates to a sensor (1) for measuring a distance (a) from an object (2). According to the invention an oscillator (4) generates a carrier signal (5); a first modulation switch (17) modulates pulses on to the carrier signal (5) and generates a first pulse signal (3) which is emitted in the direction of the object (2); the first pulse signal (13) reflected by the object (2) and delayed by a propagation time is received; a power divider (16) positioned between the oscillator (4) and the first modulation switch (17) transmits the carrier signal (5) to a second modulation switch (19); the second modulation switch (19) modulates pulses onto the carrier signal (5) and generates a second pulse signal (9) which is delayed by a variable delay; by comparing the delay of the second pulse signal with the propagation time of the first pulse signal, means detect the propagation time and determine the distance (a) from the object (2). To increase the pass-band transmission loss of the modulation switches (17; 19) the invention provides for a third modulation switch (21) to be arranged between the oscillator (4) and the power divider (16).

Description

Title: Sensor for measuring a distance to an object

description

The present invention relates to a sensor for measuring a distance to an object, comprising: an oscillator for generating a carrier signal, a first modulation switch for modulating

Pulsing on the carrier signal and for generating a first pulse signal, a transmitting antenna for transmitting the first pulse signal in the direction of the object, a receiving antenna for receiving the first reflected by the object and delayed by a transit time

Pulse signal, a power divider, which is arranged between the oscillator and the first modulation switch, a second modulation switch for modulating

Pulse on the carrier signal, which over the

Power divider is guided to the second modulation switch, and to generate a second

Pulse signal, first means for delaying the second pulse signal by a variable delay, second means, which cooperate with the first means, for varying the delay of the second pulse signal, third means for comparing the transit time of the first

Pulse signal with the delay of the second pulse signal and fourth means for determining the delay set on the first means, in which the delay is equal to the transit time, and for determining the distance to the object from the determined delay and the

Propagation speed of the first pulse signal.

Such sensors are known from the prior art. They are used in a variety of different areas. A preferred area of application is the distance measurement between two motor vehicles. This allows, for example, the distance between two motor vehicles traveling in series by regulating the speed of the vehicle rear vehicle are kept constant. For this purpose, the rear motor vehicle has a sensor of the type mentioned at the beginning in its front area in the direction of travel.

The sensors for measuring the distance to an object usually determine the transit time of a periodic pulse signal from its emission by the sensor through the reflection on the object to the reception by the sensor. The distance to the object results from the transit time of the pulse signal multiplied by the propagation speed of the pulse signals and divided by two. Ultrasonic sensors or pulse radar sensors, for example, work on this basis.

The speed of propagation of the pulse signals in such sensors is approximately the speed of sound or the speed of light. In the case of pulse radar sensors in particular, the transit times to be determined are a assumption of a maximum measuring range of a few meters

Nanosecond range. Such short pulse transit times cannot be measured directly, for example by setting a timer when the pulse signal is sent with START and when the reflected pulse signal is received with STOP. Rather, indirect comparative methods are used to determine the pulse transit times. The comparative methods usually work according to the following principle: an oscillator generates a carrier signal, to which pulses are modulated in a first modulation switch. The output signal of the first modulation switch is referred to as the first pulse signal. A transmitting antenna transmits the first pulse signal towards the object. The first pulse signal is reflected by the object and received by a receiving antenna of the sensor. The received first pulse signal is delayed by a transit time compared to the transmitted first pulse signal.

The sensor also has a power divider, which is between the oscillator and the first

Modulation switch is arranged. The carrier signal is passed through the power divider to a second modulation switch. Pulses are also modulated onto the carrier signal in the second modulation switch. The output signal of the second modulation switch is referred to as the second pulse signal.

The second pulse signal is delayed by a variable delay by the first means. The first means are designed, for example, as a dead time element. Second means interact with the first means and serve to vary the delay of the second pulse signal. The second means are designed, for example, as a sweep voltage, by means of which the dead time element is controlled. By third means Running time of the first pulse signal compared with the delay of the second pulse signal. The third means are designed, for example, as a mixer, the output signal of which increases detectably precisely when the delay currently set on the first means by the second means is equal to the transit time of the first pulse signal.

If the delay of the second pulse signal is equal to the transit time of the first pulse signal, it must be determined which delay is set on the first means at this point in time. For this purpose, the sensor has fourth means. The fourth means also determines the distance to the object from the determined delay and the speed of propagation of the first pulse signal.

The known sensors have various disadvantages. The modulation switches of the sensors have only finite transmission loss in the open position, in which the transmission loss should ideally be infinite. Therefore, the amplitude of the pulse signal plotted in the time domain during the periods in which the modulation switch is open is also not zero, as should ideally be. Rather, the pulse signal has a clear amplitude value during these periods.

The amplitude values of the pulse signal during the periods when the modulation switch is open lead to the im Frequency range applied pulse signal to a fairly large continuous wave pointer on the carrier signal frequency. While the average real pulse power of the pulse signal is distributed over many spectral lines, the average continuous wave power is concentrated on a single spectral line.

The modulation switches of the known sensors achieve, for example, a pass loss of approximately -25 dB. Consequently, even with an open modulation switch with an assumed pause duty ratio of approximately 1, the pulse peak power P _pu ι becomes approximately 0.3% (10 -log x% = -25 dB; x% = iθ- ^25/1 ° = 0.3%) _SSp i _{tZe passed} ^by the switch as a continuous pointer. The power of the pointer of the carrier frequency, the so-called continuos wave power, is

^ CW ⁼ "pulse tip ~ ^ D ÜB.

The pulse duty cycle n _pulse (ie the ratio of the pulse duration T _pulse of the pulse signal to the pulse repetition period ^T _pu i _s i _ederho i) with an assumed pulse duration of T _pulse = 500 ps and a pulse repetition frequency of _pu ι _SW i _ederho i = ^{5 MHz} (pulse repetition period T _{pulswιederhol} = l / f _pU ι _swiederho i = 1/5 MHz = 200 ns) results from n _pulse = T _puls / T _{pulse repetition} = 500 ps / 200 ns = 2.5-10 ^"3. The average real is calculated Pulse power with a sufficient system bandwidth for P _average = P _{pulsSF tze} -26 dB (10- log n _pulse = -26 dB) and is therefore of a similar magnitude to P _cw . If one shows the spectrum of the average real pulse power and the average continuous wave power (see FIG. 3), it is noticeable that the average real pulse power is distributed over many spectral lines 30 (line spectrum), since it is a periodic signal while it is the mean continuous wave power is concentrated on a single spectral line 31. The power gap between the continuous wave pointer 31 and the pulse signal pointers 30 has an order of magnitude of ≥ 26 dB (P _{pulse line} - P _pu ι _SSp i _tze -52 dB) for the example introduced and is also dependent on the pulse shape of the pulse signal.

According to the ETSI (European Telecommunications Standards Institute) standard, there are 300,440 specifications by law which limit the maximum permissible power density of the sensor's transmission signals and limit it to a specific base frequency band (see Fig. 3). The base frequency band goes, for example, from f _x = 24 GHz to f ₂ = 24.25 GHz. Within this base frequency band, the maximum permissible power within a measurement window of 100 kHz bandwidth is p ₂ = 20 dBm and outside the base frequency band p _x = -30 dBm. The frequency spectrum of the pulse signal is now aligned such that the spectral lines at no frequency f exceed the maximum permissible power p _x or p ₂ . This requires that the continuous wave _pointer on the carrier signal _frequency f carrier _lies within this legally prescribed base frequency band. Often this exceeds Continuous wave power, however, even this relatively large maximum permissible power p ₂ within the base frequency band. To prevent this, the power of the entire pulse signal must then be reduced. However, this has the consequence that not only the power of the disturbing continuous wave pointer, but also the mean real pulse power is reduced, which leads to a reduction in the range of the sensor.

The known sensors which emit a pulse signal with a relatively large continuous wave pointer in the frequency spectrum also have the disadvantage that they are very susceptible to interference from interference signals which operate in the base frequency band in which the pulse signal is located. The sensor is particularly susceptible to interference with frequency-modulated interference signals, so-called frequency-modulated-continuos-wave (FMCW) - interference signals that sweep the carrier frequency of the continuous wave pointer at regular intervals.

Finally, due to the size of the continuous wave pointer on the carrier signal frequency, the known sensors have the disadvantage that they are very susceptible to coherent interference components or to undesired Doppler couplings.

An object of the present invention thus results from the aforementioned disadvantages of the prior art, to design and develop a sensor of the type mentioned in the introduction such that the size of the continuous wave pointer is reduced in the spectral power distribution of the pulse signals of the sensor on the carrier signal frequency.

To achieve this object, the invention proposes, starting from the sensor of the type mentioned, that a third modulation switch is arranged between the oscillator and the power divider.

According to the invention, it has been recognized that the size of the continuous wave pointer in the spectral power distribution of the pulse signals on the carrier frequency can best be reduced by increasing the forward loss of the first and second modulation switches. The forward attenuation of the modulation switch can be easiest increased according to the present invention by arranging further modulation switches in series with the first and second modulation switches. In principle, it would be possible to arrange one or more modulation switches in series with the first and second modulation switches. However, this would make the construction of the sensor relatively complex, and there would be problems with the synchronization of the modulation switches arranged in series with one another. The invention therefore proposes to arrange a third modulation switch in such a way that it is arranged in series with both the first and the second modulation switch. For this purpose, the third modulation switch is arranged between the oscillator and the power divider. The carrier signal is passed from the third modulation switch to the first and to the second modulation switch by the power divider.

In the sensor according to the invention, the transmission attenuations of the first and third modulation switches or the second and third modulation switches add up. If the third modulation switch has the same forward loss as the first and second modulation switches, the forward loss is doubled at the output of the first and second modulation switches. By increasing the forward attenuation of the modulation switch, the continuous wave pointer can be attenuated to the order of magnitude of the pulse signal pointer without great effort. In the example given at the beginning, the power of the pointer of the carrier frequency P ^ of the sensor _{according to the invention} can be reduced approximately to the size of the mean pulse line signal power P _pulse ι _inie . In particular, the size of the continuous wave pointer is reduced by increasing the forward attenuation of the first modulation switch so that the frequency spectrum of the first pulse signal, in particular the spectral line of the continuous wave pointer, remains below the maximum permissible power. Because of the small size of the continuous wave pointer, it is not necessary to reduce the power of the entire first pulse signal in order to ensure that the spectral line of the continuous wave pointer is within the legally prescribed power limit. Rather, the first pulse signal can be operated with the legally maximum possible power, so that the spectral lines of the mean real pulse power of the first pulse signal are just below the legally prescribed power limit. In this way, the legally permitted maximum power of the first pulse signal can be fully utilized.

By increasing the forward loss of the second modulation switch, the size of the continuous wave pointer of the second pulse signal is reduced. This can reduce the sensor's sensitivity to interference from other devices that emit signals at frequencies that lie within the sensor's base frequency band.

By increasing the forward loss of both the first and the second modulation switch, the Susceptibility of the sensor to coherent interference components or to undesired Doppler coupling can be reduced.

The sensor according to the invention thus has decisive advantages over the sensor known from the prior art.

According to an advantageous development of the present invention, it is proposed that the sensor have a pulse repetition frequency (PRF) generator which controls the first modulation switch and the second modulation switch via a trigger signal. The PRF generator generates, for example, a square-wave signal whose frequency corresponds to the pulse repetition frequency of the first and the second pulse signal. With each rising or falling edge of the square-wave signal, the first and the second modulation switch are activated by a trigger signal in such a way that they close for the duration of a pulse. In this way, pulses with a certain pulse duration can be modulated onto the carrier signal with a certain pulse repetition frequency. The first pulse signal is then present at the output of the first modulation switch and the second pulse signal is present at the output of the second modulation switch.

It is conceivable that the first means act directly on the second pulse signal and this by a variable Delay delay. According to an advantageous development of the invention, however, it is proposed that the trigger signal for driving the second

Modulation switch is delayed by the first means by the variable delay. As a result, the second pulse signal is delayed indirectly by delaying the corresponding trigger signal.

According to an advantageous embodiment of the present invention, it is proposed that the PRF generator control the third modulation switch via the trigger signal and a delayed trigger signal. As a result, the third modulation switch can be easily synchronized with the first and the second modulation switch.

The PRF generator advantageously controls the third modulation switch in such a way that it is closed at least when the first modulation switch or the second modulation switch is closed. This ensures that pulses can still be modulated onto the carrier signal by specifically closing the first or second modulation switch. The third modulation switch can be switched more slowly than the first and the second modulation switch. This allows a narrower band design of the lines from the third modulation switch to the first and second modulation switches and the power divider. Narrow band Components are easier to implement than broadband components and are therefore also much cheaper.

The PRF generator preferably controls the third modulation switch in such a way that it is closed by a multiple of the time for which the first modulation switch or the second modulation switch is closed.

The modulation switches can be of any design. According to a preferred embodiment of the invention, however, it is proposed that the modulation switches each have a transmission line with a variable reflection factor for carrying the carrier signal, a stub line opening into the transmission line and means for changing the terminating resistance of the stub line, the reflection factor of the transmission line being changed by changing the terminating resistance of the Stub line is adjustable. Modulation switches designed in this way are switched by changing the reflection factor of the transmission line. These modulation switches are particularly suitable for high-frequency technology because they have particularly short switching times. Their passage loss in the open state is compared to other types

However, modulation switches are relatively small. It is therefore particularly advantageous in the case of first and second modulation switches designed in this way to reduce the pass-through to increase the arrangement of further modulation switches in series.

Finally, according to another advantageous development of the invention, it is proposed that the sensor be operated in a frequency range which lies outside the basic frequency band for power limitation which is legally prescribed for the sensor according to ETSI 300 440. The known sensors must all be operated within this base frequency band, since otherwise the spectral line of the continuous wave pointer would exceed the legally prescribed performance limit. Therefore, in the known sensors it must be ensured that the frequency spectrum of the first pulse signal is aligned in such a way that the spectral line of the continuous wave pointer lies exactly within the basic frequency range. In the sensor according to the invention, however, the size of the continuous wave pointer could be reduced to such an extent that the sensor can also be operated outside this base frequency band without the range of the entire pulse signal having to be reduced. By operating the sensor outside the legally prescribed base frequency band, the sensor can avoid interference from other devices that emit signals with a frequency that is generally within the base frequency band. As a result, the susceptibility to failure of the sensor according to the invention can be decisively reduced. A preferred embodiment of the present invention is explained in more detail below with reference to the drawings. Show it:

1 shows a circuit of a sensor according to the invention in accordance with a preferred embodiment,

Fig. 2 shows the waveforms of the sensor according to the invention from Fig. 1, and

3 shows the spectral power distribution of a sensor known from the prior art.

In Fig. 1, a sensor according to the invention is identified in its entirety with reference number 1. The sensor 1 is used to measure the distance a to an object 2. A preferred area of use for the sensor 1 is the distance measurement between two motor vehicles. In this way, for example, the distance between two motor vehicles traveling behind one another can be kept constant by regulating the speed of the rear vehicle. For this purpose, the rear motor vehicle has sensor 1 in its front area in the direction of travel.

The sensor 1 determines the transit time of a first periodic pulse signal 3 from its transmission by the sensor 1 via the reflection on the object 2 to the reception by the Sensor 1. The distance a to the object 2 results from the transit time of the first pulse signal 3 multiplied by the propagation speed of the pulse signal 3 and divided by two. The first pulse signal 3 is designed, for example, as an ultrasound signal or as a radar signal.

The sensor 1 works according to the following principle: an oscillator 4 generates a carrier signal 5, to which 17 pulses are modulated in a first modulation switch. The output signal of the first modulation switch 17 is referred to as the first pulse signal 3. A transmitting antenna 7 transmits the first pulse signal 3 in the direction of the object 2. The first pulse signal 3 is reflected by the object 2, and the reflected pulse signal 13 is received by a receiving antenna 8 of the sensor 1. The received first pulse signal 13 is delayed due to the distance traveled from the sensor 1 to the object 2 and back to the sensor 1 by a transit time T _run compared to the transmitted first pulse signal 3. The transit time T _run depends on the distance a from the object and the propagation speed of the first pulse signal 3.

The sensor 1 also has a power divider 16 which is arranged between the oscillator 4 and the first modulation switch 17. The carrier signal 5 is passed through the power divider 16 to a second modulation switch 19. The carrier signal 5 are in the second modulation switch 19 also modulates pulses. The output signal of the second modulation switch 19 is referred to as the second pulse signal 9.

The second pulse signal is delayed by _delay means 11 first by a variable delay T. The first means 11 are designed as a dead time element. Second means 12 cooperate with the first means 11 and serve to vary the delay T _{verz of} the second pulse signal 9. The second means 12 are designed as a sweep voltage, by which the dead time element is controlled. The transit time T _{run of} the first pulse signal 3 is compared with the delay T _{ver2 of} the second pulse signal 9 by third means 14. The third means 14 are designed as a mixer whose output signal if and only detectable increases when the currently set by the second means 12 to the first means 11 delay T _delay equal to the period i ^τ _on the received first pulse signal 13 is.

If the delay T _{delay of} the second pulse signal 9 is equal to the transit time T _{run of} the first pulse signal 3, it must be determined which delay T delay is set on the first means 11 at this point in time when the mixer _{output signal} increases. For this purpose, the sensor 1 has fourth means. The fourth means also _delimits the distance a to the object 2 from the determined delay T and the Propagation speed of the first pulse signal 3 determined.

The present invention is particularly concerned with increasing the pass loss of the first modulation switch 17 and the second modulation switch 19. This is achieved in that a third modulation switch 21 is arranged between the oscillator 4 and the power divider 16. The third modulation switch 21 is thus arranged in series with both the first modulation switch 17 and the second modulation switch 19. In each case two modulation switches 17 and 21 or 19 and 21 arranged in series form a switching unit 6 or 16. The transmission loss of the switching units 6, 16 is formed by the sum of the transmission losses of the individual modulation switches 17 and 21 or 19 and 21 arranged in series .

For the periodic actuation of the modulation switches 17, 19, these are controlled by a pulse repetition frequency (PRF) generator 18 via a trigger signal 20. The PRF generator 18 generates a square-wave signal, the frequency of which corresponds to the pulse repetition frequency f _pu ι _swiederho i of the first pulse signal 3 and the second pulse signal 9. On each rising edge of the square-wave signal, the first modulation switch 17 and the second modulation switch 19 become such by the trigger signal 20 controlled that they close for the duration of a pulse T _pulse . In this manner, the carrier signal 5 may be modulated with a certain pulse repetition frequency _pu ι _SW i i _ederho pulses with a specific pulse duration T _pulse. The first pulse signal 3 is then present at the output of the first modulation switch 17 and the second pulse signal 9 is present at the output of the second modulation switch 19.

The trigger signal 5 is delayed by the first delay 11 by the variable delay T before driving the second modulation _{switch 19} . The second modulation switch 19 is then activated by the delayed trigger signal 22. The second pulse signal 9 is therefore delayed indirectly by delaying the trigger signal 5.

The PRF generator 18 controls the third modulation switch 19 via the trigger signal 20 and the delayed trigger signal 22 such that it is closed at least when the first modulation switch 17 or the second modulation switch 19 are closed. The third modulation switch is switched more slowly than the first modulation switch 17 and the second modulation switch 19.

2 shows the signal curves of the switching times of the modulation switches 17, 19, 21. The PRF generator 18 controls the third modulation switch 21 in such a way that it is closed by a multiple of the time T _switch for which the first modulation switch 17 or the second modulation switch 19 is closed (T _switch = n * T _pulse ).

With a bandwidth of the pulses that are modulated onto the carrier signal 5 of f _pulse = 2 GHz, a pulse duration of T _pulse = 500 ps results. With an assumed pulse repetition frequency of f _pu ι _swiederho i = ⁵ MHz, a pulse repetition period T _{pulse repetition} = 200 yields ns for the first pulse signal 3 and the second pulse signal 9. The first means 11 are driven such that the second pulse signal 9 by a delay T _delay = 30 ns compared to the first pulse signal 3. In its closed state, the third modulation switch 21 covers the closed states of the first modulation switch 17 and the second modulation switch 19. In the example from FIG. 2, the switching operation of the third modulation switch 21 takes ^τ _scha i _t = 4 ns, that is to say eight times the pulse duration T _pulse . This allows a narrower band design (250 MHz) of the lines from the third modulation switch 21 to the first modulation switch 17 and the second modulation switch 19 and the power divider 16.

The modulation switches 17, 19, 21 each have a transmission line with a variable reflection factor for carrying the carrier signal 5, a stub line opening into the transmission line and means for changing the terminating resistance the branch line. The reflection factor of the transmission line can be adjusted by changing the terminating resistance of the stub line. Modulation switches 17, 19, 21 designed in this way are switched by changing the reflection factor of the transmission line. Such modulation switches 17, 19, 21 are particularly suitable for high-frequency technology, since they have particularly short switching times.

Claims

claims

Sensor (1) for measuring a distance (a) to an object

(2), with: an oscillator (4) for generating a carrier signal (5), a first modulation switch (17) for modulating pulses onto the carrier signal (5) and for generating a first pulse signal (3), a transmitting antenna (7) for transmitting the first pulse signal (3) in the direction of the object (2), a receiving antenna (8) for receiving the first pulse signal (13) reflected by the object (2) and delayed by a transit time, a power divider (16) which is arranged between the oscillator (4) and the first modulation switch (17), a second modulation switch (19) for modulating pulses onto the carrier signal (5), which is led to the second modulation switch (19) via the power divider (16), and for generating a second pulse signal (9), first means (11) for delaying the second pulse signal (9) by a variable delay, second means (12), which cooperate with the first means (11), for varying the delay of the second pulse signal (9), third means (14) for comparing the transit time of the first pulse signal (3, 13) with the delay of the second pulse signal (19) and fourth means for determining the delay set on the first means (11), in which the delay is equal to the transit time, and for determining the distance (a) to the object (2) from the determined delay and the propagation speed of the first pulse signal (3, 13), characterized in that a third modulation switch (21) is arranged between the oscillator (4) and the power divider (16).

2. Sensor (1) according to claim 1, characterized in that the sensor (1) has a pulse repetition frequency (pulse repeat frequency, PRF) generator (18) which the first modulation switch (17) and the second modulation switch (19th ) controlled via a trigger signal (20).

3. Sensor (1) according to claim 2, characterized in that the trigger signal (20) for driving the second modulation switch (19) by the first means (11) is delayed by the variable delay.

4. Sensor (1) according to claim 3, characterized in that the PRF generator (18) the third modulation switch

(21) via the trigger signal (20) and a delayed trigger signal (22).

5. Sensor (1) according to one of claims 2 to 4, characterized in that the PRF generator (18) controls the third modulation switch (21) such that it is closed at least when the first modulation switch (17) or second modulation switches (19) are closed.

6. Sensor (1) according to claim 5, characterized in that the PRF generator (18) the third modulation switch

(21) controls such that it is closed by a multiple of the time for which the first modulation switch (17) or the second modulation switch (19) is closed.

7. Sensor (1) according to one of claims 1 to 6, characterized in that the modulation switches (17; 19; 21) each have a transmission line with variable passage attenuation for carrying the carrier signal (5), a spur line opening into the transmission line and means for Modify the terminating resistance of the stub line, the forward loss of