NL8601891A - Road traffic monitoring installation - detects magnetic effect as vehicles pass over buried induction loop installation - Google Patents

Abstract

An a.c. signal from an oscillator (7) is fed via cables (6) to the primary loop (2) of a loop installation (1) buried in the surface of the roadway. The secondary loop (3) is connected (8) to a detector circuit (9). The detector output is fed to a signal processor which determines data about a vehicle (4) moving over the loop installation (1), e.g. size of vehicle, speed, number of vehicles per time unit, etc. When the vehicle passes, the a.c. in the secondary loop (3) undergoes phase and amplitude variations. These changes are detected by an amplitude or phase demodulator.

Description

Y.

86.5065 / AA / MvC *

Short designation: Detection system for detecting the presence and movement of a means of transport.

The invention relates to a detection system for detecting the presence and displacement of a transport means relative to a measuring location, comprising a loop-5 system arranged in the area of the measuring location, which magnet is substantially perpendicular to the displacement direction of the transport means is polarized and comprises a multi-turn transmit loop, a detection circuit connected to the loop system, which generates and passes a transmit signal having an alternating current component through the transmit loop, so that an alternating magnetic field is generated in the transmit loop, and the detects the influence of the means of transport on the field and supplies a measuring signal in response thereto, and a processing device connected to the detection chain for receiving and processing the measuring signal.

A detection system of this kind is known in the field of traffic engineering, in which the loop system is arranged in a groove of the road surface of a roadway. The known system comprises a single loop per measuring point, which forms part of an LC circuit of an oscillator, so that an alternating current 20 passes through the loop acting as a transmission loop and the magnetic field is generated in the region of the measuring place. As a vehicle approaches the loop, the LC circuit will become more detuned, changing the frequency of the transmit signal passed through the loop. This frequency is measured to obtain a corresponding indication of the presence of a vehicle.

The known system has the major drawback that the LC circuit is also detuned by changes in parasitic capacities and parasitic inductions. These parasites can arise, for example, through the use of long connecting cables between the actual loop in the road surface and the detection chain. Weather influences, in particular temperature, humidity, precipitation and brine, can greatly change these parasitic capacities and inductions. This adversely affects the accuracy and sensitivity of the detection.

8601891 - 2 - *

Moreover, when using some of these detection systems for respective side-by-side lanes, the settings of the different LC circuits must be such that the circuits oscillate at different frequencies in the absence of a vehicle. Due to the different frequency ranges used for the different detection systems, the sensitivities for the detection systems will also be different. Because of the relatively small sensitivity of the known detection system 10, the loop will have to have relatively large dimensions and consist of a relatively large number of turns, which, however, also increases the influence of neighboring loops of other detection systems and the cost of manufacturing and installing the loop. loop is relatively high.

Because the known detection system works insufficiently stable and is easily disturbed by external parasitic influences, the error in detecting and counting vehicles can be 10%. Another drawback is that the determination of the length of a passing vehicle by measuring the covering time of the loop by the vehicle is unreliable. Although the speed of a vehicle can be measured by using a number of detection systems with a common processing device, the speed cannot be determined sufficiently accurately below a certain lower limit. In addition, the vehicle length and speed measurements are strongly influenced by the lateral position of the vehicle relative to the center of the loop. Another drawback of the known detection system is that small vehicles, such as motorcycles, cannot be detected.

Because the known detection system cannot measure the quantities of a passing vehicle desired for good traffic control in a sufficiently stable and insufficiently accurate manner, such detection systems find less extensive application than is considered desirable for good traffic control.

Due to the unreliable operation, it is not possible, for example, to apply such a detection system for detection of wrong-way drivers, with an acceptably small risk of false alarms.

The object of the invention is to eliminate the drawbacks of the known system 8601891 * - 3 - »The object of the object is achieved in that, according to the invention, in the system of the type mentioned in the preamble, the loop system has a reception loop system with a number, each consisting of a number of winding reception loops, wherein the field generated by the transmission signal induces a reception signal in each reception loop by mutual induction, that the reception loop system is connected to the detection circuit and that the detection circuit outputs a measurement signal output 10 dependent on the reception signals to the processing device supplies. Detection takes place purely by magnetic coupling. A change in the dielectric constant in the area of the measuring site, for example by water and brine, has no influence on the detection. The detection is also insensitive to temperature changes. The signal received from a receive loop is directly proportional to the amount of magnetic symmetry disturbance around the loops. The frequency of the transmission signal is fixed and can be selected independently of the dimensions of the loop system.

Because a very high sensitivity can be achieved, the dimensions of the loop system can be chosen small.

A signal-to-noise ratio of 105 can be achieved.

By choosing suitable dimensions and numbers of turns of loop arrays of different detection arrays arranged in a row in different rows of strips, the loops will have negligible mutual inductance and yet high sensitivity. As a result, the frequencies of the transmission signals of the different detection systems can be chosen equally, so that they all have the same> Ö sensitivity. All the reception loops can divide the detection chain with time division by means of a multiplexer, and with more detection systems also the processing device.

In an embodiment of the single receive loop detection system, the passage time along a point of the loop system, the coverage time of the loop system and, using statistical information on the shape of the signal received from the receive loop and the length of an associated vehicle , the speed of the transport means can be estimated.

8601891 * - 4 -

Moreover, in an embodiment of the loop system with two receiving loops, which follow each other along the displacement path of the transport means, the speed and the displacement direction can easily be determined with the absolute value of the angle 5 with respect to the normal displacement path.

When the two receiving loops are coupled in series such that these loops act as a single lemniscate receiving loop, the two partial loops of which are polarized in opposite directions, the speed of the means of transport can be obtained, for example by means of sampling and numerical analysis of the receiving loop measuring signal received.

With a suitable shape of the two receiving loops, for instance as two opposite triangles, which together form a rectangle, the lateral position of the transport means with respect to the receiving loop system can be measured.

In an embodiment of the loop system with four square-shaped receiving loops, the lateral position and the angle with respect to the normal displacement path can be determined.

By suitable magnetic orientation, in which the polarization of adjacent loops is reversed in direction, the different receiving loops can be connected in series, so that the receiving loop system only needs to be connected to the detection circuit via one conductor pair 25, so that the connection costs of the loop system are the detection chain and the susceptibility to interference are reduced. The detection chain can also be carried out more simply and therefore cheaper.

Preferably, the serially connected receiving loops are formed by a single conductor.

The invention will be elucidated with reference to the figures.

In the figures shows:

Fig. 1 a first embodiment of the detection system according to the invention, used for detecting cars;

Figures 2a, 2b waveforms of the system of Fig.

1 occurring signals;

Fig. 3 a second embodiment of the detection system according to the invention; 860 1 8 9 1 - 5 -

Pig. 4a through 4d waveforms from the system of FIG.

3 occurring signals;

Fig. 5 a third embodiment of the detection system according to the invention; FIG. 6a, 6b waveforms of signals occurring in the system of FIG. 5;

Fig. 7 shows the waveform of an element shown in the system of FIG.

5 shows an alternative to the waveform of FIG. 6b; FIG. 8 is an embodiment of the detection circuit of the system of FIG. 5;

Fig. 9 another embodiment of the loop system; and

Fig. 10 is yet another embodiment of the loop system.

The detection system according to the invention will be explained for the application in which vehicles must be detected. The loop system can herein be arranged in the road surface to be driven by the vehicle, wherein the loops of the loop system will usually be rectangular, in connection with their simple installation,

However, the loops can have any geometrical shape and can even be moved above the road surface next to the roadway. The detection system according to the invention is therefore also suitable for detecting passing vessels from shore, for example.

The detection system shown in Fig. 1 comprises a loop system 1 having a solid loop or transmit loop 2 shown by a thick line and a secondary loop or receive loop 3 shown by a thinner line. The loop system 30 1 is arranged in the road surface of a roadway over which a vehicle 4, such as a car, drives. Vehicle 4 moves in the direction indicated by arrow 5. The loops 2 and 3 of the loop system 1 can be arranged in the same groove in the road surface. The transmit loop 2 is connected via a pair of conductors 6 to the output of an oscillator 7, which supplies a transmit signal with an alternating current component of a certain frequency to the transmit loop 2. The receiving loop 3 is connected via a conductor pair 8 to an input of a detection circuit 9, which supplies at least one measuring signal 8601891% 6 to a processing device 11 via a signal path 10.

The transmission signal passed through the transmission loop 2 generates an alternating magnetic field of a certain strength and with the frequency of the transmission signal. By means of mutual inductance, this magnetic field in the reception loop 3 generates a reception signal fed via the conductor pair 8 to the detection circuit 9, the frequency of which is equal to that of the transmission signal. The more the vehicle 4 approaches the loop system 1, the more energy of the field generated by the transmit loop 2 will be absorbed by the vehicle 4, generating paint currents therein. The amplitude of the reception signal / shown in Fig. 2a decreases thereby. As the vehicle 4 moves away from the loop system 1, the amplitude of the reception signal increases again. The amplitude of the reception signal is thus actually modulated by passing a vehicle. The reception signal can therefore easily be converted by means of an amplitude demodulator and level shifter of the detection circuit 9 into a measuring signal, as shown in Fig. 2b, the shape of which corresponds to the envelope of the reception signal shown in Fig. 2a.

At approximately the time t indicated in fig. 2b, the front A of the vehicle 4 reaches the left side of the loop system 1 shown in fig. 1. At about time t2 the rear side of the vehicle 4 leaves the right side of the fig. 1 loop system shown 1. The time between t ^ and t2 is called covering time and depends on the length (type) and the speed of the vehicle 4.

Fig. 3 shows an embodiment of the detection system 30 according to the invention with a loop system 12 consisting of a transmission loop 13 corresponding to the transmission loop 2 of FIG.

1, and two receiving loops 14a, 14b which are arranged successively in non-overlapping manner in the displacement direction 5. Each receiving loop 14a, 14b is connected to a separate input of the detection circuit 9 with a separate conductor pair 15a, 15b.

When approaching and passing a vehicle 4 in the direction indicated by the arrow 5, the energy of the field in the vicinity of the receiving loop 14a will first be absorbed 860 1 8 9 1 I »♦ - 7 - and then the energy of the field in the vicinity of the receiving loop 14b. The receiving loop 14a thereby supplies the receiving signal shown in Fig. 4a and the receiving loop 14b supplies the receiving signal shown in Fig. 4c. If processing by the detection circuit 9, for example by means of amplitude demodulation and level shifting of each reception signal as in the system of Fig. 1, for the reception loop 14a the measuring signal shown in Fig. 4b is obtained and for the reception loop 14b the measurement signal shown in Fig. 4d. The times t ^ and t2 of Fig. 4b and t ^ 'and t2' of Fig. 4d correspond to the times tj and t2 of Fig. 2b, ie the reaching of the respective receiving loop through the front A of the vehicle 4 respectively leaving the respective receiving loop through the rear side B of the vehicle 4. From each of the measuring signals shown in Figs. 4b and 4d, the covering time of the loop system 12 and the speed and the length of the vehicle 4 are determined.

The measuring signals according to Figs. 4b and 4d obtained from the two receiving loops 14a, 14b have been displaced by a distance D in time. The distance D is directly proportional to the speed of the vehicle, so that by measuring the distance D, a possibility for determining the speed 25 is obtained. The distance D can be between the time points t ^ and t ^ shown in Fig. 4. * are measured, but also between two other corresponding times of the different measurement signals, for example the times where the angle of inclination is greatest or the times where the measurement signals reach a maximum jQ.

An important difference from that shown in fig.

The detection system shown in I is also that in the detection system according to Fig. 3 the direction of movement of the vehicle 4 can also be determined. Depending on the direction of movement 35, the measuring signal according to Fig. 4b obtained from the receiving loop 14a will occur earlier or lacer than the measuring signal obtained from the receiving loop 14b according to Fig. 4d.

The maxima of the measuring signals obtained from the two receiving loops 14a, 14b according to figures 4b and 4d will have 860 1 89 t - 8 - * different values when the vehicle 4 loop system 12 is inclined with respect to the normal direction of movement 5 and moving with its longitudinal axis offset from the center of the loop assembly 12. With this, the processing device 11 can determine the absolute deviation angle or weaving angle with respect to the normal displacement direction.

Fig. 5 shows a third embodiment of the detection system according to the invention with a loop system 16 consisting of a transmit loop 17, such as the transmit loop 2 of FIG. 1 and the transmit loop 13 of FIG. 3, and a composite receive loop 18, which consists of two series-connected receiving loops 18a, 18b, which are arranged one after the other in the direction of movement of the vehicle 4 with opposite magnetic polarity direction. The receiving loops 18a and 18b thereby have shifted magnetic diameters, which are perpendicular to the plane of the drawing and pass through the center of the respective receiving loops. The polarity direction of the receiving loops 18a, 18b is indicated by the circular arrows within these loops.

The receiving loops 18a and 18b can be made separately and then connected in series or can be made of a single continuous wire. The composite lemniscate-shaped receiving loop 18 is connected to an input of the detection circuit 9 via a pair of conductors 19.

In the absence of a vehicle 4, the receive signals generated in the receive loops 18a and 18b cancel each other, so that the composite receive signal fed to the detection circuit 9, as shown in Fig. 6a, will be zero. When the vehicle 4 approaches the loop system 16, the energy in the vicinity of the receiving coil 18a will be absorbed more than in the vicinity of the receiving coil 18b, so that the receiving signal generated in the receiving loop 18a will be smaller than the receiving signal generated in the receiving loop 18b. through which a measurably composite reception signal with the frequency of the transmission signal is fed to the detection circuit 9.

When the receiving loops 18a and 18b are equally covered by the vehicle 4, the composite receiving signal will also be zero.

When the vehicle 4 moves away from the loop system 16 in the direction indicated by the arrow 5, the reception signal generated in the reception loop 18a will be greater than the reception signal generated in the reception loop 18b, so that a detectable composite measurement signal with the frequency of the transmission signal the detection chain 9 is conducted.

Since the reception loops 18a and 18b are oppositely polarized, the phase of the reception signal when shifted from loop system 16 to offset loop system 16 will be shifted 180 °. The phase shift occurs suddenly, as indicated in the center of the waveform of Fig. 6a.

By means of suitable amplitude demodulation, in which the phase of the reception signal also fulfills a function, the measuring signal shown in Fig. 6b, which is the envelope of the waveform shown in Fig. 6a, can be obtained.

The measurement signal of Figure 6b for the detection system 20 of Figure 5 contains the same information as the measurement signal of Figures 4b and 4d for the detection system of Figure 3, so that with the aid of the detection system of Figure 5 the same quantities for the vehicle 4 can be determined. The front h of the vehicle 4 reaches the left side 25 of the receiving loop system 16 of Fig. 5 at time t ^.

The rear B of the vehicle 4 leaves the receiving loop system 16 at time t ^,

Since the reception signals of the two reception loops 18a, 18b are composed, the times of occurrence 30 of the maximum and the minimum of the waveform of the measurement signal shown in Fig. 6b depend on the length of the vehicle 4. With a long vehicle 4 will the waveform in it. center, as shown in Fig. 7 for a relatively long time. The front of a relatively long vehicle 15 reaches the left side of the receiving loop 18a of Fig. 5 at time t t and the left side of the receiving loop 18b at time tg. The rear of the relatively long vehicle leaves the receiving loop 18a at time t ^ and the receiving loop 18b at time t7 · As the vehicle is longer, the maximum and minimum of the waveforms shown in Figures 6a and 7 will are further apart and the time between t ^ and t ^ that the amplitude of the waveform is zero will be longer. From this, the processing device 11 can possibly determine the speed and the length of the vehicle with the aid of correction data.

If the length of the loop system 16 of Fig. 5 in the direction of movement of the vehicle L ^, which is known, is the speed of the vehicle v and the length of the vehicle 10 Lv and (Fig * 7), then: v = Ld / T1 or v = Lj / T2 or, on average, v = Lj (1 / T ^ + and L = V.T0 + L, or v 3d 15 Lv = v. (T ^ + T2 + T2) - or, at peak detection, \ = ν · ν

Speed and length of the vehicle can also be determined by sampling the signal received from the lemniscate-shaped composite receiving loop 18 and vectorial analysis of the samples obtained.

The detection circuit 9 of the detection system according to FIG.

5 may consist of a synchronous amplitude demodulator with a balance modulator, such as a ring modulator.

Fig. 8, however, shows an embodiment of the detection circuit 9 for the detection system of FIG. 5 with a synchronous amplifier ("lock-in amplifier"). Fig. 8 also shows the oscillator 7, which is connected via a current source circuit 20 to a pair of output terminals 21a, 21b for connection to the conductor pair 6 for the transmit loop.

The detection circuit 9 with the synchronous amplifier comprises a set of input terminals 22a, 22b for connection to the conductor pair 19 for receiving the reception signal. The input terminals 22a, 22b are connected via a band-pass filter 23 to the positive and negative terminals of an operational amplifier 24 and to the negative and positive terminals of an operational amplifier 25, respectively. The outputs of the operational amplifiers 24 and 25 are connected with inputs of an electronic switch 26, of which a control input 8601891 * - - 11 - receives a switching signal with the frequency of the transmission signal supplied to the transmission loop via a phase-shifting circuit 27 of the oscillator 7. The output signal of the switch 26 is supplied via an integrator 28 and an amplifier 29 to an output terminal 30 connected to the processing device 11.

The adjustment of the phase shift circuit 27 is such that the zero crossings of the switching signal received from the oscillator 7 coincide with the zero crossings of the receiving signal received from the receiving loop 19 with the frequency. of the transmission signal. During the first half of each period. from the switching signal, the switch 26 passes the signal received from the operational amplifier 24 to the integrator 28, and during the second half of each period of the switching signal, the switch 26 passes the signal received from the operational amplifier 25 to the integrator 28.

When the phase of the receive signal received at the input terminals 22a, 22b changes, the polarity of the output signal of the switch 20 also changes as a result. The synchronous operation of the switch 26 removes noise and interfering signals from the receive signal.

The bandpass filter 23 serves to limit the frequency range for which the detection circuit must operate, so that the influence of noise and interference signals is also limited thereby.

Due to the presence of the current source 20, the strength of the current through the transmit loop connected to the terminals 21a, 21b is independent of the resistance of the connection conductors 6 to the transmit loop and of the resistance of the transmit loop itself, so that a change in these resistances due to change of these resistances due to change of the ambient temperature does not affect the strength of the generated magnetic field.

Fig. 9 shows an alternative embodiment of a loop system 31 of the detection system according to the invention 35 with a transmit loop 32, such as the transmit loop 2 of the system of FIG. 1, and two serially connected receive loops 33a, 33b, which together form a composite receive loop 33 . The polarization directions of the reception loops 33a and 33b are opposite. As shown, the size of each receiving loop 33a, 33b measured transverse to the displacement path of the vehicle 4 changes such that it increases for one loop and decreases for the other loop. This makes it possible to determine the distance from the longitudinal axis of the vehicle 4 to the symmetry line running parallel to the direction of displacement 5.

Fig. 10 shows yet another embodiment of a loop system 34 of the detection system according to the invention with a transmit loop 35, as the transmit loop 2 of the detection system 10 of FIG. 1, and four square-mounted receive loops 36a, 36b, 36c, 36d, which connected in series and thereby form a composite receiving loop 36. Each two side-by-side receiving loops 36a through 36d are oppositely magnetically polarized. Using the loop-15 system 34 it is possible to determine the displacement speed of a vehicle 4 perpendicular to its normal displacement direction and the angle (weaving angle) between the actual displacement path of the vehicle 4 and its normal displacement direction 5.

.20 As with the loop system 16 of FIG. 5, the receiving loops 33a, 33b of FIG. 9 and also the receiving loops 36a through 36d of FIG. 10 may consist of a single continuous wire.

Each loop of the loop system of a detection system according to the invention can consist of a number of turns.

For the detection of vehicles and installed in the road surface, this number can be limited to about three. With relatively small dimensions of the loops and with a large distance between the plane of the loops and the displacement plane of the means of transport to be detected, the number of turns for each loop can be increased without any problem.

In order to counteract changes in capacitive coupling between the windings and between the loops, it is preferable to bond the windings and the loops together, for example by means of synthetic resin.

The loop system of a detection system according to the invention may comprise any number of receive loops. The reception loops can be connected in series in groups, for example in the manner shown in Figures 5, 9 and 10, and each non-series connected reception loop and each group of 40 reception loops can be separately connected to the detection circuit 860 1 8 9 1 s ψ - 13 - 9 are connected for detecting the different reception signals, so that the information about a passing means of transport can be determined in different ways.

An important advantage here is that only a single transmission loop with a single transmission signal with one transmission frequency has to be used. The receiving loops are preferably within the area enclosed by the transmit loop. However, a receive loop can also be arranged wholly or partly outside the transmit loop. The functions of the embodiment 10 with series-connected reception loops can be performed by means of separate reception loops which are separately connected to the detection circuit 9. The reception loops. may be connected in series in the detection circuit 9 or the signals from these reception loops may be electronically counted or subtracted from each other.

The conductors connecting the receiving loops to the detection circuit are preferably individually shielded conductors.

- conclusions - 860 1 83 1

Claims (25)

1. Detection system for detecting the / displacement of a transport means with respect to a measuring location, comprising a loop system arranged in the region of the measuring location, which is magnetically polarized substantially perpendicular to the displacement direction of the transport means and which a multi-turn transmission loop, a detection circuit connected to the loop system, which generates a transmission signal with an alternating current component and passes it through the transmission loop, so that an alternating magnetic field 10 is generated in the transmission loop, and that the influence of the transport means on the field and provides a measurement signal in response thereto, and a processing device connected to the detection circuit for the reception and processing of the measurement signal, characterized in that the loop system comprises a reception loop system with a number of reception loops each consisting of several turns the field generated by the transmit signal in each receive loop d By means of mutual induction, a reception signal induces that the reception loop system is connected to the detection circuit and that the detection circuit supplies a measuring signal output dependent on the reception signals to the processing device. Detection system according to claim 1, characterized in that the receiving loop system comprises two or more receiving loops and in that the magnetic diameters of the receiving loops are offset from each other. Detection system according to claim 1 or 2, characterized in that the receiving loops are arranged side by side within the surface enclosed by the transmission loop. Detection system according to claim 3, characterized in that the receiving loop system comprises two receiving loops and in that the receiving loops are arranged successively in the expected direction of movement of the transport means. 8601891. 5 f - 15 - Detection circuit according to claim 4, characterized in that the size of one receiving loop measured perpendicular to the expected displacement direction decreases going in this direction and the corresponding size of the other receiving loop increases thereby. Detection circuit according to claim 3, characterized in that the receiving loop system comprises four receiving loops and that the receiving loops are arranged in square. Detection circuit according to any one of the preceding claims, characterized in that the detection circuit demodulates each reception signal by means of an amplitude demodulator and in response thereto supplies a group of respective measuring signals or a composite measuring signal to the processing device. Detection circuit according to any one of claims 2 to 6, characterized in that the receiving loops are arranged with such orientation and connected in series such that adjacent receiving loops 20 are magnetically opposed polarized, so that the detection circuit has a single reception signal of the receive loop system, and that the sensing circuit detects an amplitude change and a phase change of the receive signal and delivers two or more respective measurement signals or a composite measurement signal to the processor in response to these changes. Detection circuit according to claim 8, characterized in that the receiving loop system consists of a number of receiving loops formed by a single conductor. 30 1C. Detection circuit according to claim 7, 8 or 9, characterized in that the detection circuit comprises a synchronous amplitude demodulator for the reception signal. Detection circuit according to claim 10, characterized in that the synchronous amplitude demodulator comprises a balance modulator. Detection circuit according to claim 8 or 9, characterized in that the detection circuit comprises a synchronous amplifier which receives the reception signal synchronously with a switching signal, the frequency of which is equal to the frequency of the 860 1 89 1 - 16 transmission signal, with inversion of the receive signal amplifies during every second half period of the switching signal. Detection circuit according to claim 10, 11 or 12f, characterized in that the measuring signal supplied by the detection circuit is supplied to the processing device via an integrator circuit. Detection circuit according to any one of the preceding claims, characterized in that the reception signal received from a reception loop is supplied to the detection circuit via a band-pass filter whose center frequency is chosen equal to the frequency of the transmission signal. Detection circuit according to one of the preceding claims, characterized in that each loop is connected to the detection circuit 15 by means of separately shielded conductors. Detection circuit according to any one of the preceding claims, characterized in that the detection circuit comprises a current source which supplies the transmission signal. Detection circuit according to any one of the preceding claims, wherein the transmission loop is arranged in a closed slot in a carrier, characterized in that the receiving loop system is arranged in the carrier such that the slot for the transmission loop is also occupied by the receiving loop system. Detection circuit according to any one of the preceding claims, characterized in that the windings of each loop are fastened together over its entire length. Detection circuit according to claim 18, characterized in that overlapping parts of different loops are attached to each other. 20. Detection circuit according to claim 8, characterized in that the detection circuit has an intervalometer for determining the duration of a first period during which the envelope of the reception signal deviates from a reference value and in that a sub-circuit one with the reciprocal value of the first period duration provides proportionate output as a measure of the speed of the means of transport. 21. Detection circuit as claimed in claim 20, characterized in that the interval meter determines the duration of a second period following the first period, during which the envelope of the reception signal deviates from a reference value and in that the sub-circuit one with the sum of the reciprocal values of the first period duration and second period duration provides proportionate output as a measure of the speed of the means of transport. Detection circuit according to claim 20, characterized in that the interval meter determines the duration of a third period following the first period, during which the envelope of the reception signal is approximately equal to the reference value, and that a multiplier supplies the product of the output of the sub-chain with the third period duration as a measure of the length of the means of transport. A detection circuit according to claim 22, characterized in that an adder provides the sum of the product and a value corresponding to the length of the loop system as a measure of the length of the transport means. 24. Detection circuit according to claim 21, characterized in that the interval meter determines the duration of the third period occurring between the first and second periods, and that a multiplier supplies the product of the output of the sub-chain by the sum of the first t / m third period durations as mset for the length of the means of transport. 25. Detection circuit as claimed in claim 24, characterized in that a subtractor provides the difference of the product and a value corresponding to the length of the loop system as a measure of the length of the transport means. 26. Detection circuit according to claim 20 or 21, characterized in that an interval meter determines a duration of a peak-peak period between the occurrence of a maximum absolute value in the first period and the occurrence of a maximum absolute value in one on the first period following period in which the receive signal deviates from the reference value and esi provides multiplier product of the output 35 of the sub-chain with the peak-peak period as a measure of the length of the means of transport. 86 (M '