DE3347442A1 - Device for measuring distance, particularly for motor vehicles - Google Patents

Abstract

A device for measuring distance in motor vehicles is described in which several electroacoustical transducers are activated successively in time. In this arrangement, only a single controllable switching element, via which both the output signal of an ultrasonic generator and the echo signal received by the transducer can be transmitted, is allocated to each transducer. Compared with a known system having separate switches for the transmitted and received signal, the circuit expenditure is considerably reduced. The order in which these transducers are selected can be varied as a function of certain parameters. If a transducer detects an echo signal having a short transit time, this transducer is selected more frequently than the remaining transducers. Due to special measures for damping, the transducers have a short settling time so that short distances can also be measured.

Description

Device for distance measurement, in particular for motor vehicles

The invention is based on a device for distance measurement according to the features of the preamble of claim 1.

Such a distance measuring device for motor vehicles is from DE-OS 31 35 282 known. Thereby electroacoustic transducers, the ultrasonic signals can both transmit and receive, in multiplex operation individually one after the other controlled. Two controllable switching elements are assigned to each converter, wherein in each case one switching element is the transmission circuit and the other switching element controls the receiving circuit. To control these switching elements is a relative high effort is required, since the two switching elements assigned to a converter not at the same time or. turned off.

The invention is based on the object of a device for distance measurement to create an operationally reliable data acquisition with the least possible effort permitted by a variety of transducers.

According to the invention, this object is achieved with the characterizing features of claim 1 solved.

The advantage of the device according to the invention over the prior art technology is the greater, the greater the number of a control unit controlled converter is.

Compared to the prior art, there is a switching element in each case saved per converter. This also reduces the control effort for the individual Switching elements considerably reduced. Each converter thus leads to the invention Establishing only one signal transmission path, while in the prior art to each Converter two transmission paths are connected. This simplifies the device according to the invention the cabling effort, because at The transducers are used for the distance measurement device in motor vehicles usually in the bumper, but the control unit is near the dashboard housed.

An embodiment is particularly preferred in which the switching element is arranged immediately in front of the transducer and to a sufficient amplitude amplified output signal of the generator switches. As a result, compared to the known circuit saved additional components, because that is where the switching element is located in front of a transformer that increases the amplitude of the generator signal. at the known device so a transformer is necessary for each converter.

Since the switching elements are arranged directly in front of the converters, you only need a filter for the echo signals, which is based on the resonance frequency the converter is tuned, while in the prior art each converter has one Filter is assigned. However, then of course these converters must all be the same Have resonance frequency, which is however easily manageable nowadays.

The switching elements that can be selectively activated one after the other by the control device are connected in parallel to a circuit point to which the generator signal is supplied and at which the echo signal for the signal receiving stage can be tapped is. So that the stepped-up output signal of the generator does not affect the amplifier Overdrive inadmissibly, this switching point with the signal receiving stage is over connected to a circuit that has a different transmission behavior for the generator signal than for the echo signal. In the simplest case, you could just as with the execution Provide limiter diodes according to the state of the art. However, it is used as a signal receiving stage an amplifier whose gain factor is inversely proportional to the The amplitude of the input signal changes automatically, can be achieved simply by switching on of limiter diodes no operationally safe switching behavior. During airtime then namely the gain factor is small and weak echo signals that shortly after End of the transmission signal are received, are not sufficiently amplified, because changing the gain factor takes a certain amount of time. Therefore is proposed according to an advantageous development of the invention that this the circuit upstream of the signal receiving stage has a controllable analog switch has, which is blocked for the duration of the generator signal and during a Measurement time for the Echosignaldurobge is controlled. This becomes the gain factor turned up to a high value during the transmission time, so that briefly later incoming echo signals can be safely evaluated.

However, it must be taken into account that it is not ideal analog switches gives. That means that the stepped up output signal of the generator too when the analog switch is blocked, to a certain extent on the input of the signal receiving stage worked through. The input voltage at the signal receiving stage depends on the Ratio of the input resistance to the blocking resistance of the analog switch. To improve the switching behavior, according to an advantageous development the invention of the circuit upstream of the signal receiving stage a further Have controllable switch that switches in phase opposition to the analog switch and the input of the signal receiving stage for the duration of the generator signal low-resistance switching path in parallel. This ensures that the input signal fed to the signal receiving stage during the switch-on time of the generator is negligibly small and consequently the gain factor is regulated to the highest possible value.

As such, the generator could remain switched on continuously if one more switching element whose output signal from the common circuit point blocks all switching elements during the measuring time. But then these could be high-frequency Scatter generator signals into the receiving circuit of the measuring amplifier, whereby its Sensitivity would be reduced again. This disadvantage does not occur when according to a further advantageous embodiment of the invention, the generator by a switching pulse of the control unit is excited in pulses and during the measuring time is switched off.

Overall, these measures achieve safe switching behavior, because there are also weak echo signals shortly after the end of the transmission signal occur, evaluated properly. Compared to known systems, this can create obstacles can be properly detected at a smaller distance from the transducer.

Both for the analog switch and for the further switch in the circuit upstream of the signal receiving stage and the switching element that is assigned to the converter, MOS field effect transistors would be best suited, because they have the lowest possible on-resistance and a high blocking resistance exhibit. Such a transistor can only have higher voltages in one direction block effectively.

For switching the generator signal with a peak amplitude of about 100 volts two transistors would be necessary. Therefore, be specific with a Embodiment of the invention of the analog switch and that associated with the converter Switching element implemented by a self-conducting junction field effect transistor and only the switch that should short-circuit the input of the signal receiving stage, is realized by a MOS field effect transistor. At this switch, come on it mainly depends on a low-ohmic forward resistance, which is the case with a transistor of this type is on the order of 10 ohms.

The individual components listed so far are controlled by control pulses of the control unit switched as follows: Before the start of each transmission and measurement cycle If the generator is switched off, the switching elements assigned to the converters are switched off locked and the signal receiving stage is ineffective in which the analog switch blocks and the other switch is controlled conductive. At the start of a cycle the generator is excited and at the same time the switching element of a certain converter steered through. This emits an ultrasonic signal. After a pulse time of about 300-500 microseconds the generator is switched off again and after one Delay time of about one millisecond, the signal receiving stage is activated, i.e. the analog switch is activated and the other switch is blocked. A Any echo signal picked up by the transducer is thus the signal receiving stage supplied After the end of the measuring time, the switching element is blocked and at the same time the signal reception stage is deactivated again. This cycle is then repeated, with a different switching element after each cycle in the case of a first embodiment is controlled by the control unit and thus another converter is activated. When using several converters, you can monitor and monitor a large area Obstacles that could stand in the way of a moving vehicle in good time recognize. Through the selective control of the converter and evaluation of the corresponding Echo signals can also be recognized in which direction an obstacle can be expected This can be done in a suitable manner by signal lamps or, for example, by a Liquid crystal display.

In practice it has been shown that the immediately successive Activation of the individual transducers to inaccurate measurement results due to external Can lead to interference. In order to improve the evaluation accuracy proposed according to an advantageous development of the invention that you each Converters several times in a row controls and from the respectively recorded Averages the measured values.

However, this has the consequence that the time span for selective control of all transducers is enlarged. This can lead to critical situations. Will namely when using this system in a motor vehicle during the first run detects no obstacle on a transducer at a sufficiently large distance, can it can happen with a fast moving vehicle that this obstacle is already is in an impermissible close range when this converter is activated again will.

Therefore, according to a particularly preferred embodiment of the invention suggested that, depending on certain parameters, certain transducers are preferred are controllable. Independent protection is claimed for this thought, there it can also be used in the known system mentioned at the beginning.

Various sizes can be considered as parameters.

For example, there is a vehicle in the front and rear bumper Equipped with several converters, you could insert the reverse gear only the rear converters and with the engagement of the forward gear only the front ones Activate converter. The repetition time to control a specific converter is thereby significantly reduced.

In another embodiment, the running time is used as the parameter of the echo signals selected. So it is preferred to drive the converter to which an echo signal with the shortest transit time was measured. This converter could can now be controlled alternately with one of the other transducers, so that an approximation to the obstacle with the shortest possible distance to the vehicle very often and thus is closely monitored.

In a specific embodiment, the converters are with a longer echo propagation times are controlled less often in direct succession than the transducer with the shortest echo delay. This means that the number of packets of ultrasound pulses is reduced and thus the repetition time for controlling a specific converter is shortened. It is consciously accepted that the measurement accuracy is reduced is because a mean value is now formed from a smaller number of measured values will. However, this is not critical as it is the signals from the converters acts that already did not detect an obstacle or an obstacle at a greater distance, while the echo signals of the transducer with the shortest echo propagation time continue to be very high be evaluated precisely.

For the functional reliability and measuring accuracy of the entire facility the training of the converter is one of the decisive factors.

In a known embodiment, a piezo disc is in a cup-shaped Housing arranged, with a lead wire approximately centrally on a coating of the piezo disk is soldered on.

This has the consequence that during the transmission of this lead wire absorbs and stores mechanical energy. This is saved after switching off Transferring energy back to the piezo disk so that it oscillates. While Naturally, echo signals cannot be correctly detected during this post-oscillation period so that distances of less than 20 cm are not detected with known transducers can be.

To eliminate this disadvantage, it is proposed that at least an electrical lead near the edge area with a coating of the piezo disk electrically conductive connects. The oscillation amplitude of the piezo disc in this The edge area is much smaller and less energy is stored in the wire The post-oscillation period can be further reduced if thin, preferably highly flexible ones are used Lead wires used. A further improvement is achieved in that the lead wire is mechanically damped m cn by at least vanishing the lead wire embeds the piezo disk adjoining section in a foam. if the piezo disk on a membrane-like face of a pot-shaped housing rests, which is held in a carrier, one is the housing vibrations dampen a silicone mass by placing the cup-shaped housing above the foam and fill the space between the housing and the carrier with this material. Also with that the post-oscillation period of the converter is reduced without the Sound pressure is reduced inadmissibly.

For these features, the design of the converter is used Independent protection is claimed, as such a type of converter is also used at the beginning mentioned embodiment according to the prior art can be used with advantage. By this design has succeeded for the first time in creating a compact converter, which works in a frequency range between 30 and 40 kilohertz and a measuring range from a distance of less than 20 cm to more than 4 m.

The invention and further advantageous embodiments are described below explained in more detail using the embodiment shown in the drawing.

1 shows a block diagram, FIG. 2 shows a circuit diagram of the receiving part, 3 shows a circuit diagram of a switching element connected upstream of the converter, FIG. 4 a circuit diagram of the transmitting part, FIG. 5 a basic circuit diagram of the part of a control device and 6 shows a converter in section.

In Fig. 1, 10 denotes a control device which is at the outputs 11 to 17 different control impulses, the function of which will be described later. The control device 10 also has two inputs 18 and 19 for certain measured values. In addition, outputs 20 to 25 are provided for controlling display elements. A display 26 is controlled serially via the output 20, on which the shortest measured distance can be read.

Indicator lamps 27 are controlled via the remaining outputs 21 to 25.

An electrical generator 30 supplies 31 pulses at its output with a frequency of 37 kilohertz running in parallel with the primary windings of two Transformers 32 and 33 are supplied. The transformer increases the peak amplitude of the generator signal to about 100 volts. This signal is on the secondary windings can be tapped off and is connected to the circuit points 36 or 37 supplied. Details of this circuit part are shown in FIG.

In the embodiment of FIG. 1, five are electroacoustic Converters 40 to 44 are provided. A switching element 45 to 49 is assigned to each converter. These switching elements can be controlled via control pulses at outputs 13 to 17. The switching elements 45, 46 and 47 are parallel to the switching point 36, the switching elements 48 and 49 connected in parallel to circuit point 37. The concrete training this switching element is shown in FIG.

Circuits 50 and 51 are connected to circuit points 36 and 37 two signal receiving stages 52 and 53 connected, their output signals to the inputs 18, 19 of the control device 10 are supplied.

From the block diagram of FIG. 1 it can be seen that the converters 40, 41 and 42 are combined to form a first converter group, and this converter group a first signal receiving stage 52 is assigned. These converters are at the rear of one Vehicle arranged on the left, in the middle and on the right. The other two transducers 43 and 44 are combined to form a second group of transducers, which is a further signal receiving stage 53 is assigned. These converters are arranged on the front left and right of the vehicle. All transducers are fed by a single signal generator 30, however the transmission signals are galvanically decoupled from one another by the transformers 32 and 33, respectively are. Even if converters of the first and second group are activated at the same time, can be recognized on the basis of the selective evaluation via the two signal reception stages whether there is an obstacle in the way at the front or at the back. This is especially true when Parking a vehicle in a space in the vehicle is important because it often happens when turning a collision occurs straight ahead.

With reference to FIG. 2, that of a signal receiving stage will now be described below 52 upstream circuit 50 explained in more detail. This circuit 50 contains a controllable analog switch 60 in the form of a normally on field effect transistor with a protective circuit through a Zener diode 61 and a resistor 62. The Analog switch 60 is in series between common node 36 and the input of the signal receiving stage 52 and must therefore the high transmission voltage of the Switch generator 30. The circuit 50 includes a further switch 63 in the form a MOS field effect transistor, which is connected to the output of the analog switch 60 and can switch a low-resistance switching path to ground. The analog switch 60 and the further switch 63 are activated by a control pulse at the output 12 of the Control device 10 jointly, but controlled in phase opposition. Located at output 12 of the Control unit ground potential, the transistors 64 and 65 are conductive and the The control input of the junction field effect transistor 60 is a negative voltage supplied by about 50 volts, so that the analog switch 60 blocks or is very high resistance. In the case of ground potential at output 12 of the control unit, however, it is the transistor 66 blocked and thus the MOS field effect transistor 63 conductive. the Signal receiving stage 52 is thus switched ineffective in two ways. On the one hand a signal from the input is blocked via the analog switch 60, and on the other hand the input of the signal receiving stage 52 is practical via the further switch 63 shorted. In this switching state, the switching circuit 50 is outside the measuring time, in particular also during the duration of the generator signal, the so that the signal receiving stage 52 is not supplied.

If a positive potential can be measured at output 12 of the control unit, are the transistors 64 and 65 blocked and the analog switch 60 is turned on. At the same time, the transistor 66 becomes conductive and the further switch 63 is blocked or switched off. high resistance. This allows an echo signal from the node 36 to the signal receiving stage 52 arrive. This switching state exists during the measuring time (M).

An integrated module is used as the signal receiving stage 52, from by the Valvo company under the name TDA 3047 is sold in series. the the exact wiring of this integrated module is given in the technical information 830304 of this manufacturer exactly described, so that more details about this can be found superfluous. In the present case it is wired as a narrowband amplifier, with the filter 67 at the input is matched to the transmission or resonance frequency.

The integrated module contains an amplifier, its gain factor is regulated depending on the amplitude of the input signal. With small input signal voltages this gain factor is higher than with high signal voltages. Becomes the signal reception level 52 an input signal is supplied, the switching state changes at its output, d is connected to input 18 of control device 10. So this is the one Case if one of the Connection point 36 connected Transducer an echo signal is received.

Overall, it should be noted that the signal receiving stage 52 is a Circuit 50 is connected upstream, which has a different transfer behavior for the generator signal than for the echo signal. This could also be done simply by using limiter diodes reach, but would not achieve the required sensitivity, as already described at the beginning.

3 shows the specific embodiment of a converter 40 assigned to it Switching element 45. This switching element 45 in turn contains a junction field effect transistor 70, via a control pulse at output 13 of the control unit and via the Transistors 71 and 72 is controlled. If ground potential is present at output 13, are the two transistors 71 and 72 conductive and the control input of the field effect transistor 70, a blocking potential of approximately minus 50 volts is applied. The field effect transistor 70 is blocked or

very high resistance, so that the converter 40 is not activated.

If there is a positive potential at output 13, the transistors are 71 and 72 blocked and the field effect transistor switches through, so that now the Converter 40 is activated.

By comparing FIGS. 2 and 3, it can be seen that the analog switch 60 and the switching element 45 are constructed and controlled in the same way. Even the field effect transistor 60 is connected to a Zener diode to protect it The transistor protects against destruction if the transmission voltage is impermissibly high Should achieve values.

Fig. 4 shows a transmitting part. The generator 30 includes a conventional one Wise structured multivibrator 80 with the NAND gate 81 and the frequency determining Resistors and a capacitor. This multivibrator oscillates with a frequency of 37 kilohertz as soon as at output 11 of the Control unit positive Potential is present. The multivibrator is blocked at ground potential. The output signal of the multivibrator 80 is fed to the output 31 via an amplifier 83 in an amplified manner. At this output 31 there is a switching element 84 in the form of a MOS field effect transistor connected in series with the primary winding of transformer 32 is. A pulse shaper stage is located at the control input of this field effect transistor 85 from the parallel connection of a resistor and a capacitor. To the secondary winding of the transformer 32, the filter 34 is connected, which has a variable inductance 86 in series with a resistor 87, which is used to balance the parallel capacitance of the converter connected to node 36 and the reduction in Serve post-oscillation.

Two Zener diodes 88 connected in series with opposite polarity serve as protective diodes and limit the amplitude of the transmission voltage.

The modules 34 and 35 are constructed identically. This also applies to the circuits 50 and 51 and the amplifiers 52 and 53. In addition, all are switching elements 45 to 49 wired in the manner shown in FIG.

Referring to Fig. 1, the function of the device will now be described for distance measurement explained in more detail, initially only the control of the converter 40, 41 and 42 should be considered. For this, reference is made to the pulse diagrams, which are drawn in the block of the control unit 10. In this context it should be noted that the control device 10 is a microprocessor can act, the signal processing and the triggering of the individual control pulses carried out under software control.

Before the start of a measuring cycle, there is a ground signal at all outputs, see above that the generator 30 is switched off, the switching elements 45,46 and 47 block and the signal receiving stage 52 is disabled because - as described - the Analog switch 60 blocks and the further switch 63 is activated. At the time T1 begins a first transmission and measurement cycle. At output 11 is during the period I a control pulse can be measured, which activates the generator. At the same time is over a Control pulse at output 15, the switching element 47 is controlled. This becomes the converter 42 activates and sends out an ultrasonic signal. During the pulse time I, the is about 300-500 microseconds, the generator is activated. After that, the Generator 30 switched off again. After a delay time V, the signal receiving stage 52 activated by a control pulse at output 12. The analog switch 60 is turned on and the further switch 63 of the circuit 50 is blocked. Since the switching element 47 is still closed, one of the converter 42 received echo signal via the node 36 and the circuit 50 to Signal receiving stage 52 arrive so that after a certain transit time L of the echo signal a signal change can be measured at input 18. The runtime can be used to determine the distance of an obstacle are calculated and shown on the display 26. At the same time will the indicator lamps 27 associated with the transducer 42 light up, so that the user recognizes in which direction the obstacle is.

After the measuring time M has elapsed, the signal receiving stage 52 is via the Signal at output 12 is again ineffective and at the same time also the switching element 47 of the converter 42 blocked. This ends the first cycle and a short time later At time T2, a control pulse for activating generator 30 is again generated triggered. The process is now repeated, but now there is a control pulse on Output 14 appears and the converter 41 is activated.

A new transmission and measurement cycle begins at time T3, with now The converter 40 is activated via a control pulse at the output 13. After that begins the entire process from the beginning and the converter 42 is activated again.

The delay time V between the end of the activation control pulse of the generator and the start of the measurement time is in the order of one millisecond. During this time, neither a signal is sent nor a received one Signal evaluated. This means that such signals are faded out that are caused by a The converter could oscillate and lead to a false display would.

The measuring time M is of the order of 18 milliseconds, see above that the time span for a transmission and measurement cycle is about 20 milliseconds. In this embodiment according to FIG. 1, a different switching element is used after each cycle controlled and thus another converter activated.

As shown in FIG. 1, the control pulse at output 12 shows the two Circuits 50 and 51 are switched simultaneously, and thus also the associated signal receiving stages 52 and 53 affected at the same time. By control pulses at outputs 16 and 17 the converters 43 and 44 of this converter group are triggered one after the other, however, a certain synchronicity must be given. The switching element 49 is initially controlled simultaneously with the switching element 47, then the switching elements 48 and 46 are activated and finally the switching elements 49 and 45 controlled. So there is always one converter of the first at the same time Converter group and a converter of the second converter group activated, whereby the However, the signals emitted by these transducers do not influence each other can, as the transducers of the two groups signals in opposite directions radiate. So you can say that for the converter on the front of the vehicle and for the Converter is provided at the rear of the vehicle each a multiplex system, but this both systems via a common control unit 10 and a common generator 30 are linked. Compared to a likewise conceivable solution with two completely separate multiplex systems, the circuit complexity is reduced. Of course, it would also be conceivable that the front and rear converters move one after the other a multiplex system can be controlled.

With a large number of converters, however, there is then the risk that because of the excessive repetition time, i.e. the sum of the cycle times for all converters, an obstacle may not be recognized in time.

Fig. 5 shows a block diagram of another embodiment, wherein For the sake of simplicity, only two converters 40 and 41 are to be controlled. With 90 and 91 are two pulse generators, each one after activation emit a certain number of impulses, the function of these building blocks has to be understood Imagine that the pulse generator 90 for example emits pulses when it is controlled at its input 92. When the fourth impulse has ended, appears at the output an overflow signal which is fed to the input 93 of the other pulse generator is activated and activated. This impulse generator then triggers four impulses and the overflow signal finally reactivates the pulse generator via a timing element 94 90. This process repeats itself over and over again. With every impulse there should be a complete The measuring cycle can be carried out as described above. From the pulse generator 90 the switching element 45 of the converter 40 is thus activated four times in succession. The four measured values are averaged and stored in a memory 95.

Then the pulse generator 91 is activated and thereby the switching element 46 and the converter 41 activated four times, so that another four measured values are recorded, the mean value of which is stored in the memory 96. Essential in this version is therefore that each switching element is controlled several times in succession before is switched to another switching element. By recording several measured values a converter increases the accuracy of the measured values and reduces the susceptibility to interference.

This means that the repetition time of the control of a converter is increased by a factor of 4 compared to the embodiment according to FIG. This may then lead to problems if the number of converters is relatively large. In order to remedy this disadvantage, certain parameters are now determined as a function of parameters Converter preferably driven. In Fig. 5 an embodiment is shown as an example, in which the echo propagation time is evaluated as a parameter. In addition is a comparator 97 is provided which stores the averaged measured values in the memories 95 and compares 96 with each other. The comparator 97 controls inputs 98 and 99 the impulse generators. A change in potential at these inputs results in 98.99 the number of pulses is reduced to two, for example. The comparator 97 becomes triggered by the output signal of the timing element 94.

The function of this switching arrangement can be imagined as follows: First, the converter 40 is activated four times and the averaged measured value is stored in memory 95. Then the converter 41 is activated four times and the averaged measurement value is stored in the memory 96. This gets over the timer 94 the comparator 97 triggered. If the measured value in memory 95 is now less than in the memory 96, the control input 99 of the pulse generator 91 is switched over, so that this pulse generator only emits two pulses in the following cycle. Since the smaller measured value corresponds to a smaller distance from the obstacle and therefore should preferably be observed, the converter 40 will turn again in the following cycle four times, but the converter 41 is activated only twice. The repetition time is thus decreased. Should the averaged measured value in memory 96 now be smaller than that Measured value in the memory 95, the control input 98 of the other is via the comparator 97 Pulse generator 90 activated so that it emits two pulses while now the pulse generator 91 emits four pulses again. Are in none of the stores When measured values are stored, the comparator 97 does not emit any control signals and the pulse generator does not trigger four control impulses each time. The same applies if the measured values in the memories 95 and 96 are the same size. Alternatively, you could in this case however, also reduce the number of pulses generated by the pulse generator.

Overall, it can be said that the converter with the shorter Echo delay is preferably controlled. The transducers with a longer echo delay are controlled less often. In a specific embodiment are on Rear of a vehicle four transducers arranged so that the repetition time 16 control pulses corresponds to the individual pulse generator. If an obstacle is detected by a transducer, the other converters are only activated twice in direct succession, so that the repetition time corresponds to the pulse duration of 10 pulses. This will the measurement accuracy significantly improved.

Of course, further embodiments of this basic idea are conceivable the preferred control of certain converters. For example, only certain Transducers are controlled when one of these transducers sends an echo signal with a certain Runtime recorded. The other converters would not be activated for this period of time. It would also be conceivable to change the order in which the converters are activated. For example, if the transducer 40 in FIG. 1 detects an echo signal with a short transit time, the converters could be controlled in the order 40, 41, 40, 42, 40, 41 ... In each case between two other transducers, the transducer 40 then becomes the short one Echo delay preferred; controlled.

6 shows a section through an electroacoustic transducer. A cup-shaped housing 100 made of aluminum has a membrane-like surface on its end face Section 101 on which a piezo disk 102 is fixed. The housing 100 sits in a carrier 103 made of plastic, on which an electrical line connector 104 can be latched is. The converter should have a resonance frequency of 35 to 40 kilohertz, see above that larger distances can also be measured. On the other hand, the post-oscillation period should of the transducer must be as small as possible, so that distances of less than 20 cm can be measured. To achieve this, the converter has the following inventive features: From a pin 105 leads an electrical Lead wire i06 to an edge area of the piezo disk 102. The lead wire is therefore not in the middle, in contrast to designs according to the prior art the piezo disk set.

Since the piezo disk 102 oscillates with a smaller amplitude in the edge area, less energy is transferred to the lead wire and stored there. The lead wire 1 o6 should also be as thin and flexible as possible.

Of course, deformations of this lead wire are also present in this one Execution not entirely avoidable. This lead wire is therefore used for damping at least in a section adjoining the piezo disk 102 in a foam 107 embedded.

The vibrations of the piezo disk 102 are over the membrane-like Section 101 is also transferred to the cup-shaped housing 100 I3n. these housing vibrations to dampen, the cup-shaped housing above the foam 107 with a special silicone mass 108 poured out. Also the space between the case and the carrier is filled with this silicone compound.

Claims (25)

Device for distance measurement, in particular for motor vehicles Patent claims: Device for distance measurement, in particular for motor vehicles with several electroacoustic Converters for sending and receiving ultrasonic signals, an electrical one Generator to activate this converter, a signal reception stage for the from the transducers recorded echo signals as well as a control unit for controlling switching elements, via which the transducers successively with the generator or the signal receiving stage are connectable, characterized in that each converter (40,41,42,43,44) each only a single switching element (45,46,47,48,49) is assigned and that via this Switching element both the output signal of the generator (30) and that from the converter received echo signal is transmittable. 2. Device according to claim 1, characterized in that the switching element (45,46,47,48,49) arranged immediately in front of the corresponding transducer (40,41,42,43,44) and directly the output signal of the amplified to a sufficient amplitude Generator (30) switches. 3. Device according to claim 1 or 2, characterized in that the switching elements (45, 46, 47) that can be selectively activated one after the other by the control unit (10) are connected in parallel to a node (36) to which the signal of Generator (30) is supplied and to which the echo signal for the signal receiving stage (52) can be tapped, this circuit point (36) with the signal receiving stage (52) is connected via a circuit (50) which is responsible for the generator signal has a different transmission behavior than for the echo signal. 4. Device according to claim 3, characterized in that the the Signal receiving stage (52) upstream circuit (50) a controllable analog switch (60) which is blocked for the duration of the generator signal and during a measuring time (M) is controlled for the echo signal. 5. Device according to claim 4, characterized in that the circuit (50) has a further controllable switch (63) which is out of phase with the Analog switch (60) switches and the input of the signal receiving stage (52) during the duration of the generator signal connects a low-resistance switching path in parallel. 6. Device according to at least one of the preceding claims, characterized in that the generator (30) is triggered by a switching pulse from the control device (10) is excited in pulses. 7. Device according to claim 5, characterized in that the analog switch (60) through a junction field effect transistor and the further switch (63) a MOS field effect transistor is implemented. 8. Device according to at least one of the preceding claims, characterized in that the switching elements (45,46,47,48,49) assigned to the converters are realized by junction field effect transistors (70). 9. Device according to at least one of the preceding claims, characterized in that by the control unit during a transmission and measurement cycle (10) Control pulses are emitted after the following time lapse: a) before the start of a Cycle, the generator (30) is switched off, all switching elements (45,46,47) block, the signal receiving stage (52) is deactivated; b) at the beginning of a cycle the generator (30) is excited and at the same time a switching element (45) of a converter (40) controlled; c) after the generator (30) has been switched off, the signal receiving stage is delayed (52) activated; c) after the end of the measuring time (M), the switching element (45) blocked and the signal receiving stage (52) switched ineffective. 10. Device according to claim 9, characterized in that this Sending and measuring cycle repeated continuously and a different one after each cycle Switching element (41) or (42) or (40) is controlled by the control device (10). 11. Device according to claim 9, characterized in that this Sending and measuring cycle. it is continuously repeated that each switching element (40) or (41) controlled several times in succession and a mean value from the measured values is formed and that only then another switching element (41) or (40) is controlled (Fig. 5). 12. Device, in particular according to at least one of the claims 9 to 11, characterized in that depending on certain parameters certain converters are preferably controllable. 1. Device according to claim 12, characterized in that the running times the echo signals of the transducers are compared with each other and the transducer with the shortest echo propagation time is preferred. 14. Device according to claim 12 or 13, characterized in that that for a certain period of time only certain converters are controlled and other converters cannot be activated. 15. Device according to claim 12 or 13, characterized in that that the order in which the individual transducers are activated can be changed. 16. Device according to at least one of claims 12 to 15, characterized characterized in that individual transducers with a longer echo delay time are less frequent are controlled immediately one after the other as the transducer with the shortest echo propagation time. 17. Device according to at least one of the preceding claims, characterized in that all converters are sequentially from a single generator are fed and the echo signals of all transducers of a single signal receiving stage are fed. 18. Device according to at least one of claims 1 to 16, characterized characterized in that all converters (40,41,42,43, 44) from a single generator (30) are fed that the echo signals of a first transducer group (40,41,42) a first signal receiving stage (52), the echo signals of a second transducer group (43,44), however, a second signal receiving stage (53) are fed. 19. The device according to claim 18, characterized in that the Switching elements (45,46,47) of the first converter group to a first Switching point (36) and the switching elements (43,44) of the second converter group a second node (35) are connected and that the generator signal (30) is galvanically decoupled, fed to both circuit points (36,37) and that the two signal reception stages (52s53) are activated simultaneously. 20. Device according to claim 19, characterized in that the Output signal of the generator (30) the primary windings of two transformers (32933) is supplied, the secondary windings of which at the first and second switching point (36,37) are connected. 21. Device according to at least one of claims 1 to 16, characterized characterized in that several transducers are combined into groups that each transducer group a generator and a signal receiving stage are assigned and that all generators, Converter and signal receiving stages can be controlled via a common control unit. 22. Device, in particular according to the preceding claims, characterized in that the transducer is one in a pot-shaped housing (100) arranged piezo disk (102), wherein at least one electrical lead wire (106) electrically conductive near the edge area with a coating on the piezo disk (102) connected is. 23. Device according to claim 20, that a thin, preferably more flexible Lead wire (106) is used. 24. Device according to claim 20 or 21, characterized in that that the lead wire (106) at least in that adjoining the piezo disk (102) Section is embedded in a foam (107). 25. Device according to at least one of claims 22 to 24, characterized characterized in that the piezo disk (102) is on a membrane-like end face (101) of the cup-shaped housing (100) rests in a carrier (103) is held, and that housing vibrations dampened by a silicone mass (108) be, with which the cup-shaped housing (100) above the foam (107) and the space between the housing (100) and the carrier (103) is filled.