DE4427547C2 - Inductive vehicle detector - Google Patents

Description

The invention relates to an inductive vehicle detector an induction loop (DE 93 09 414 U1).

Known inductive vehicle detectors usually exist from at least one induction loop, the z. B. in corresponding recesses of a roadway arranged and via a connecting cable to a central Evaluation device is connected. Here is the Induction loop part of a resonant circuit LC oscillator, the capacitor of which is part of the Evaluation device is. When the loop is damped driving over with a vehicle changes the Inductance of the induction loop and thus the frequency of the resonant circuit. This frequency change is then through the electronics downstream of the oscillator are evaluated.

In these known inductive vehicle detectors affect the ohmic resistances of the loop and before all of the loop feed cables the sensitivity of the Detectors disadvantageous because the one to be evaluated Frequency change is smaller, the larger this Resistance is. So the longer the connecting line between induction loop and evaluation device, depending  As a result, the signal-to-noise ratio becomes smaller. Usually are therefore only lengths of the connecting lines of a maximum of 200 to 300 m can be realized.

Another disadvantage of the known vehicle detectors is that when using multiple Induction loops (multichannel detectors) that only spatially operated little separately and / or by a common supply cable connected to the evaluation unit are, despite different frequencies of the corresponding Resonant circuits undesirable coupling or interference between the channels of the detectors arise. Even more serious occurs this problem when multiple detectors u. U. with each several induction loops are coupled together.

Finally, it is extremely disadvantageous that the known inductive vehicle detectors usually with must be connected to a loop output stage has relatively low output resistance because of it otherwise not for vibration training of the corresponding resonant circuit would come. Such low-resistance output stage significantly affects the Sensitivity of the induction loop and Evaluation system of the existing system.

Inductive vehicle detectors are also known become where the respective induction loop Is part of an adjustable frequency generator. Also in these detectors form the loops and one in the Evaluation unit existing capacity C an LC resonant circuit.

In contrast to the detectors described above in this case, however, the resonance frequency will not automatically set by an oscillator and  maintained, but the frequency is processor controlled generated and changed in the comparison until the Resonance frequency is reached. When damping the Induction loop in turn changes its inductance and causes a phase shift from that Evaluation device is evaluated.

In essence, these detectors also - due to the high ohmic resistances of the connecting lines, the required low-resistance loop output stages and the Influencing sensitivity through unwanted coupling neighboring induction loops - the same disadvantages as the detectors described above, so that also at these vehicle detectors the length of the connecting lines strong between induction loop and evaluation device is limited.

From DE 93 09 414 U1 a circuit arrangement for Monitor one in anti-lock braking systems, transmission and Motor controls usable inductive encoder known. In order to achieve that the error detection times of the Circuit arrangement are small, is in this document proposed the AC voltage portion of the Inductive encoder signal whose frequency is less than the frequency of a test signal is in a synchronous rectifier Convert signal whose frequency is higher than the frequency of the AC component of the inductive sensor signal.

DE 41 42 680 A1 describes an evaluation circuit for one Inductive sensor for determining adjustment paths (e.g. Steering angles etc.) known. The evaluation circuit includes one Oscillator, the signals of the coil of the inductive sensor  be fed. The resulting output signals are in a synchronous rectifier evaluated so that, depending on Phase relationship, either ohmic or inductive Proportion of the coil is measured. Since both shares of both the measured variable as well as the temperature depend on suitable phase relationship, temperature compensation possible, so that an output signal is obtained that is only from the to measuring size depends.

From the article by P. Lingenfelser and P. Thilo, "Moderne Inductive loop detectors measure road traffic ", Road Traffic Technology, issue 9/10/1968, pages 121 to 123, a loop detector is known in which an oscillator a complex resistor and one with this resistor connected induction loop feeds. The measurement signals are fed to a phase-measuring rectifier, the Shift of the phase position of the high-frequency Loop current versus the applied oscillator voltage measures when the loop inductance is changed. A measure for the phase shift or the loop detuning that occurring at the output of the phase-measuring rectifier DC voltage. This is amplified in an amplifier and fed to a differentiator. The time constant of the Differentiator is dimensioned so that only fast Changes in the loop inductance can be evaluated. Slow loop upsets, e.g. B. by Temperature changes, however, are not evaluated.

GB 2 231 188 A is an inductive vehicle detector known with an induction loop and an oscillator, at which is the result of damping the induction loop Frequency shift of the oscillator using a Microprocessor is evaluated.

The invention has for its object an inductive Vehicle detector specifying the length of the Connection lines between induction loop and Evaluation device can be much longer than in the known detectors. It should therefore be a driving in particular witness detector are created in which an impairment of Sensitivity of the detector by coupling in External signals such as neighboring loops and one Impairment of the measuring sensitivity by the ohmic Lead resistance and through the loop output stage (Signal-to-noise ratio) is largely avoided.

This object is achieved by the features of Claim 1 solved. Further  particularly advantageous embodiments of the invention disclose the subclaims.

The invention is essentially based on the idea that the induction loop is not the inductance of a oscillating LC oscillator, but the test object an impedance measuring device, the respective change the impedance of the induction loop measured frequency-selectively becomes. The frequency-selective measurement is carried out with a contained in the evaluation device Synchronous rectifier with downstream low-pass filter existing circuit arrangement, which only the amplitudes of those vibrations, the frequency of the voltage of which Measuring voltage source corresponds.

Because the induction loop is no longer part of an oscillator is, the loop output stage in the invention Detector with high impedance. Due to the high-resistance loop output stage and the high Frequency selectivity makes it possible to use the detectors closely operate adjacent loops without interference from External signals and loss of sensitivity due to mutual Energy deprivation occur.

The high achievable with the detectors according to the invention S / N ratio also allows motorcycles to be used in the Usually only a slight change in the impedance of the loop cause to detect.

The use of a is particularly advantageous Analog multiplier as a synchronous rectifier, because as Control voltage directly the sinusoidal measuring voltage (smaller  Distortion factor) is used. Since ideally only one Frequency is contained in the control frequency (measuring frequency), only the frequency mix of the input voltage contained amplitude of the voltage with the measuring frequency evaluated and thus suppresses all other interference signals.

Synchronous rectifier, in which the polarity of the Input voltage via a generated from the control voltage Square wave signal is switched over, are basically also in connection with the detectors according to the invention applicable, but have the disadvantage that according to the Shares in the Fourier series of the controlling square wave signal even odd multiples of the measuring frequency make a contribution can supply to the DC output voltage. Because there Harmonic components of a rectangle only slightly compared to that Are damped, this can lead to interference in the DC voltage by demodulating z. B. Long-wave transmitters or measuring frequencies of the neighboring loops to lead.

Since with the usual synchronous rectifiers Output voltage depends on the cosine of the Phase shift between control voltage and measuring voltage (phase sensitive synchronous rectifier) has proven to be proven advantageous by the induction loop and / or the phase shifts caused by the leads an adjustable - preferably processor-controlled - phases to compensate the slide. This phase shifter can for example, can be integrated into the loop output stage. By compensating for the phase shift using the Phase shifter causes the overall circuit that the Synchronous rectifier works almost phase-independent, because at a set phase of 0 ° with damping  Phase changes of less than 5 ° result. For that small changes in the phase can cos Φ in good approximation be set equal to 1.

Instead of a phase sensitive synchronous rectifier with A separate phase shifter can of course also be used phase-independent synchronous rectifier can be used. However, such synchronous rectifiers are relative elaborate to manufacture and therefore with correspondingly high costs connected.

For further evaluation of the output at the Low-pass filter signal values can do this advantageously via an analog / digital converter Microcontrollers are fed.

If the measured values of several inductive vehicle detectors should be evaluated with the same microcontroller, can the detectors in a conventional manner via a Multiplexer with the microcontroller or the upstream A / D converter can be connected.

Further details and advantages of the invention emerge from the following embodiment explained with reference to figures examples. Show it:

Fig. 1 shows the block diagram of an inductive vehicle detector according to the invention and

FIG. 2 shows the block diagram of a loop output stage for operating the vehicle detector according to FIG. 1.

In Fig. 1, 1 denotes a vehicle detector according to the invention, which essentially consists of an induction loop 2 , a measuring voltage source (sine generator) 3 with a downstream loop output stage 4 and an evaluation device 5 . Loop output stage 4 and induction loop 2 are connected to one another via connecting lines 6 , 7 , which can be up to 1000 m long, for example.

The evaluation device 5 is connected via an analog / digital converter 8, a microcontroller 9 , which in turn is connected to a measured value display 10 .

The evaluation device 5 essentially contains a signal amplifier 50 , the output signals of which are fed to a phase-sensitive synchronous rectifier (analog multiplier) 51 , which in turn is followed by a low-pass filter 52 . The respective control input 53 of the synchronous rectifier 51 is also connected to the measuring voltage source 3 via an electrical line 11 .

As can be seen from FIG. 2, the loop output stage 4 consists of a signal amplifier 40 with a high-resistance output 41 , which amplifies the frequency and amplitude-stable sinusoidal signals generated by the measuring voltage source 3 and then forwards them to the induction loop 2 . In addition, the loop output stage 4 contains an integrated phase shifter, which is shown in FIG. 2 as a separate block 42 and is implemented, for example, in a manner known per se by an all-pass filter of the 1st order.

The mode of operation of the detector 1 according to the invention is discussed in more detail below:
While at the input 54 of the synchronous rectifier (analog multiplier) 51 without a source of interference, the voltage measured on loop 2 and amplified by means of amplifier 50 (hereinafter also referred to as measuring voltage):

U _e (t) = û _{e *} sin (2πf _{st *} t + Φ)

is located, the voltage at the control input 53 of the synchronous rectifier 51 has the following profile:

U _st (t) = û _{st *} sin (2πf _{st *} t),

where f _{st is} the frequency of the control voltage U _st generated by the measurement voltage source 3 and Φ the phase shift between the control voltage U _st and the measurement voltage U _e , which can be compensated for with the aid of the phase shifter 42 ( FIG. 2).

While the quantities û _st and f _{st are} constant, û _e and Φ change when damping, ie when a vehicle passes the induction loop 2 .

The following curve thus results for the voltage U _a at the output 55 of the multiplier 51 :

U _a (t) = (U _e (t) _* U _st (t)) / E,

where E is a computing unit of the multiplier (e.g. 10 V) is.

With

sinα _* sinβ = 0.5 _* [cos (α-β) - cos (α + β)]

follows:

U _a (t) = 0.5 _* û _{e *} û _st / E _* [cosΦ - cos (4πf _{st *} t + Φ] = U ₌ + U _≈

Accordingly, a DC voltage is generated, which is superimposed on an AC voltage with twice the measuring frequency. Since phase Φ is compensated with phase shifter 42 , ie

Φ = 0, cos (Φ) = 1,

surrendered:

U _a (t) = 0.5 _* Û _{e *} û _st / E _* [1 - cos (4πf _{st *} t] = U ₌ + U _≈

In this case, the DC voltage U ₌ is independent of the phase Φ when damping.

Does the measuring voltage contain an interference frequency of z. B. f = f _st + 5 kHz, the output voltage U _a (t) contains two additional frequencies:

f₁ = 5 kHz and f₂ = 2f _st + 5 kHz.

The low-pass filter 52 connected downstream of the synchronous rectifier now has the task of filtering out all spectra, apart from the direct voltage U ₌ caused by the measuring frequency, by averaging. In the case of a low-pass filter with the cut-off frequency f _gTP = 0 Hz, there would (theoretically) be an output voltage

U _a (t) = U ₌ = 0.5 _* û _{e *} û _st / E

surrender. It therefore supplies only the voltage U _e a portion of the output voltage of the low-pass filter, the frequency of which is equal to the control frequency and thus, as desired, also to the measurement frequency. Due to the infinitely large time constant of the low-pass filter, no changes in the output voltage over time would be possible at this cut-off frequency.

For a cut-off frequency f _gTP <0 Hz, a certain frequency band is no longer filtered out of the mixed signal of U _e , but rather a frequency band. With regard to the measurement frequency, the circuit behaves like a bandpass filter with the bandwidth B = 2 f _gTP and the center frequency f _M = f _st = f _measurement .

The low-pass filter 52 accordingly determines the properties of the overall circuit and must be designed so that it meets the requirements with regard to rise time, step response, interference suppression, etc. (e.g. 4th order Bessel TP with f _gTP = 100 Hz).

Consider the one used for the detection DC voltage component

U ₌ = 0.5 _* û _{e *} û _st / E,

you can see that the output voltage is proportional to the amplitude û _{e of} the loop voltage U _e (t). The induction loop 2 is driven by a large resistor (high-resistance output of the loop output stage), so that the current is almost constant. It follows that the direct voltage component U ₌ proportional to the magnitude of the loop impedance | Z | (Z = U / I).

Reference list

1 inductive vehicle detector
2 induction loop
3 measuring voltage source
4 loop power stage
40 signal amplifiers
41 high impedance amplifier output
42 phase shifters
5 evaluation device
50 signal amplifiers
51 synchronous rectifier, analog multiplier
52 low pass filter
53 input, control input
54 entrance
55 exit
6 , 7 connecting lines
8 A / D converter
9 microcontrollers
10 Measured value display
11 electrical connection line

Claims (6)

1. Inductive vehicle detector with an induction loop ( 2 ), which is connected via a connecting line ( 6 , 7 ) to a loop output stage ( 4 ), which has a signal amplifier ( 40 ) with a high-impedance output resistor and a phase shifter ( 42 ) for changing the Contains phase of the voltage applied to the induction loop ( 2 ) and that with a measuring voltage source ( 3 ) for generating an alternating voltage with a predeterminable frequency, which is connected to the phase shifter ( 42 ), and with a synchronous rectifier ( 51 ) with a downstream low-pass filter ( 52 ), the measuring input ( 54 ) of the synchronous rectifier ( 51 ) being connected to the signal amplifier ( 40 ) and the control input ( 53 ) being connected to the measuring voltage source ( 3 ) and the output of the low-pass filter ( 52 ) via an analog / digital converter ( 8 ) is connected to a microcontroller ( 9 ) and which consist of the synchronous rectifier ( 51 ) and the low-pass filter ( 52 ) the circuit arrangement only measures the amplitudes of those vibrations whose frequency corresponds to the voltage of the measuring voltage source ( 3 ). 2. Inductive vehicle detector according to claim 1, characterized in that the phase shifter ( 40 ) is controllable by a processor. 3. Inductive vehicle detector according to claim 1 or 2, characterized in that the synchronous rectifier ( 51 ) is designed essentially as an analog multiplier. 4. Inductive vehicle detector according to one of claims 1 to 3, characterized in that it is in the synchronous rectifier ( 51 ) is a phase-independent synchronous rectifier. 5. Inductive vehicle detector according to one of claims 1 to 4, characterized in that a signal voltage amplifier ( 50 ) is arranged between the induction loop ( 2 ) and the synchronous rectifier ( 51 ). 6. Inductive vehicle detector according to one of claims 1 to 5, characterized in that the measuring voltage source ( 3 ) is a sine wave generator.