DE202012100303U1 - Signal propagation time measurement - Google Patents

Abstract

Measuring core (10) for a signal propagation time measurement with a transmit pulse input for supplying a transmit pulse (26a), a receive pulse input for supplying a receive pulse (26b), a coarse measurement unit (24), in which a main signal delay between a particular from the transmit pulse (26a) transmission time and a a measuring unit (20, 22) in which a start counter offset between the transmission time and the count clock and a Endzählerversatz between the reception time and the count clock can be determined from the received pulse (26b) specific time, and an evaluation unit (18 ) configured to calculate the signal propagation time from the main signal delay, the start counter offset and the final counter offset, characterized in that the fine measuring unit (20, 22) generates a start pulse generator (20) to generate a start pulse whose duration corresponds to the start counter offset, and an echo pulse generator (22) to produce an echo pulse whose duration is equal to the final counter offset and that the start pulse generator (20), the final pulse generator (22) and the coarse measuring unit (24) are implemented on an FPGA (12).

Description

The invention relates to a measuring core for a signal propagation time measurement with a coarse measurement based on a clock and a fine measurement based on the respective offset between transmitting or receiving pulse and the clock according to the preamble of claim 1.

Numerous sensors use a signal transit time principle in which the time interval between transmission and reception of a signal is converted into a distance over the signal propagation time. In this way, different frequency ranges of the electromagnetic spectrum are utilized, such as microwaves and light.

In the case of optoelectronic sensors based on the principle of the time of flight method, a short pulse of light is emitted in a pulse transit time method and the time taken to receive a remission or reflection of the light pulse is measured. Alternatively, in a phase method, transmitted light is amplitude modulated and a phase shift between transmitted and received light is determined, wherein the phase shift is also a measure of the light transit time.

Optoelectronic distance measurement can be required for example in vehicle safety, logistics or factory automation or safety technology. In particular, a rangefinder based on a reflected light beam may respond to a change in the distance of the reflector or the reflecting or remitting target. A special application is a reflection light barrier, in which the distance between light transmitter and reflector is monitored. The light transit time method is also the principle according to which distance-measuring laser scanners operate whose travel beam measures out a plane or a spatial area.

One area of application for microwaves is level measurement. In this case, the signal propagation time is determined until reflection at an interface of the medium whose level is to be measured. The radiated microwaves are guided in a probe (TDR, time domain reflectometry), or alternatively radiated freely as in a radar and reflected from the interface.

A challenge with runtime methods is to achieve the measurement resolution. At a sampling of the received signals with, for example, 1 GHz, after all, a temporal resolution of 1 ns is achieved, but this only corresponds to an accuracy of 15 cm in the distance measurement. Accordingly, a temporal resolution in the range of picoseconds is ultimately desirable. However, such a fast signal sampling for digital measurement evaluation is difficult and at least not compatible with low production costs.

A conventional solution is to perform the measured value acquisition mixed digital and analog. In this case, a coarse quantization is performed as a digital measurement by means of a counter in the relatively slow MHz range. At 100MHz, for example, this gives a rough measurement with a resolution of about 1.5m. An analog measured value acquisition evaluates the offset between the clock and the actual transmit or receive pulse at the beginning and at the end of the signal delay. Thus, the analog evaluation only has to cope with a much smaller measuring range, so that smaller, cheaper analog components with clearly better linearity properties can be used.

However, such circuits must be specially developed with high investment costs and are then still completely inflexible.

It is therefore an object of the invention to implement such a coarse and fine measurement divided signal propagation time measurement easier.

This object is achieved by a measuring core according to claim 1. The invention is based on the basic idea of subdividing the transit time measurement as explained in a rough measurement with a relatively slow cycle and an accurate fine measurement of the remaining time remaining with respect to this cycle. These residual maturities occur at the beginning as well as at the end of the signal runtime and are referred to as start counter offset and end counter offset. For their measurement generates an FPGA from the signals on the measurement path, ie a transmit pulse and a receive pulse, an electrical start pulse and an electrical echo pulse, such as a square wave signal whose length represents the respective offset to the clock.

Counting cycles in the range of a few MHz to a few hundred MHz are therefore sufficient, since the required accuracy is achieved via the fine measurement. Even compared to the required distance resolution, even 1 GHz sampling would be slow, as this still only equates to 15cm of measurement accuracy. A count clock of the required high frequency makes but hardly a digital component, at least no cost-effective device and no FPGA.

The invention has the advantage that a cost-effective and adaptable signal propagation time measurement is made possible due to the flexibility of the FPGA. Unlike a special one analogue circuit can be used for standard components with high availability and interchangeability.

The fine-measuring unit preferably has at least one integrator downstream of the FPGA in order to convert the duration of the start pulse signals and echo pulse signals into a current or voltage value. Thus, the start and echo pulses provided by the FPGA are evaluated with a very simple component.

The integrator preferably has a capacitor and a current source, and the current source is connectable to the capacitor for a duration that depends on an applied pulse signal. Such an integrator circuit is simple. At the same time, it is highly linear and precise in the small measuring range to which the fine measurement can be limited thanks to the coarse measurement.

The integrator preferably has a differential connection in order to be driven differentially with a pulse and a negation of the pulse. This provides additional advantages in the measurement resolution because the integrator detects the start counter offset and the final counter offset even more accurately.

The integrator is preferably followed by an A / D converter to convert the current or voltage value into a digital value for further processing in the evaluation unit. Thus, the start pulses and echo pulses become a digital value, which can be easily offset to the desired signal delay.

The evaluation unit is preferably implemented on a digital component following the FPGA, in particular a microprocessor. There, the necessary clearing capacities are provided

The start pulse generator is preferably designed to generate a first start pulse whose associated start counter offset is related to the beginning of the transmit pulse, wherein the echo pulse generator is adapted to generate a first echo pulse whose associated end counter offset is related to the beginning of the receive pulse, such that in the evaluation unit, a first signal delay related to the beginning of the pulse can be determined. The pulse start of transmit pulse and receive pulse is very easy to recognize, for example, by a threshold evaluation or a comparator stage. Thus, the corresponding to the beginning related first signal delay is easily measurable.

Similarly, the start pulse generator is preferably configured to generate a second start pulse whose associated start counter offset is related to the end of the transmit pulse, wherein the echo pulse generator is configured to generate a second echo pulse whose associated end counter offset is related to the end of the receive pulse. so that in the evaluation unit, a second signal transit time based on the end of the pulse can be determined.

Although the pulse end or the beginning of the pulse can be easily determined, they are level-dependent because one must decide on a level from which the pulse is defined. As a result, level-dependent distortions of the measured transit time can arise. For this reason, in a preferred development, the evaluation unit is designed to determine from the first signal propagation time and the second signal propagation time a signal propagation time related to the center of gravity of the pulse. The center of gravity, which usually also corresponds to the vertex of the pulse, does not show the aforementioned level dependence, so that a more accurate time-of-flight measurement becomes possible.

The start pulse generator is preferably designed to generate a third start pulse whose pulse width corresponds to the duration of the transmission pulse. Similarly, the echo pulse generator is preferably designed to generate a third echo pulse whose pulse width corresponds to the duration of the received pulse. These pulse widths can be used for additional evaluations, for example for a pulse shape-dependent correction of the measured signal propagation times or for a power measurement.

The coarse measuring unit and the fine measuring unit preferably have at least one further evaluation channel in order to determine the signal propagation time for at least one further received pulse. Thus, the measuring core is multi-capable, so it can evaluate several received pulses in parallel or sequentially. This is useful in situations where the transmit pulse is reflected multiple times, such as particles in the measurement range such as rain, snow or dust, or when a transparent object is placed in front of the actual measurement object, as in the case of a glass panel, which in particular can act around the windscreen of a sensor.

Preferably, an optoelectronic sensor, in particular range probe or laser scanner, is equipped with a measuring core according to the invention. Such sensors are very common. As a further example of an alternative application possibility, the measuring core is used in a microwave sensor, in particular a free-radiating fill level sensor according to the radar principle or TDR fill level sensor.

The invention will be described below with regard to further advantages and features with reference to the accompanying drawings of exemplary embodiments explained. The figures of the drawing show in:

1 a very simplified sectional view of a sensor with a measuring core according to the invention;

2 a block diagram with an overview of the elements of the measuring core;

3 a block diagram of an FPGA of the measuring core;

4 a block diagram of a start or echo pulse generator, which is implemented on the FPGA;

5 a representation of the signal waveforms in different processing stages of the measuring core; and

6 a circuit diagram of an integrator for the evaluation of pulses generated in the FPGA.

1 shows a very simplified sectional view of an optoelectronic sensor 100 with a measuring core according to the invention 10 , The sensor 10 includes a light emitter 102 , about the light pulses on a transmission beam 104 into a surveillance area 106 to be sent out. The one there on an object 108 reflected light beam 110 returns to the sensor 100 back and is from a light receiver 112 detected.

The measuring core 10 is part of a control unit 114 , which among other things controls the transmission behavior and the measuring core 10 the transmit pulse and the returning beam of light 110 in the light receiver 112 generates received pulses. In the measuring core 10 the light transit time between the transmission of the transmission pulse and the reception of the reception pulse is determined in a manner explained in more detail below, by the distance of the object 108 to investigate. Such distance values or derived variables is the sensor 100 then at an exit 116 to disposal.

Instead of the single-beam optoelectronic distance meter just described 100 can the measuring core 10 but also be used in other sensors that measure a signal propagation time, in particular a laser scanner, wherein the light beam 104 is deflected periodically by a deflection unit to scan a plane or a space area, or in a level sensor which measures the level in a container with freely radiated in the manner of the radar or guided by a probe microwaves.

2 shows an overview of the elements of the measuring core 10 , The measuring core receives as input variables except one clock the transmit pulse and the received pulse. In a multi-channel extension, it is also conceivable to supply a plurality of receiving pulses in order to set up a multi-capable measuring system in which a plurality of objects arranged one behind the other on the measuring beam can be evaluated. The output variable is the time of flight designated by the time of flight (TOF), which can already be offset by means of the signal velocity given by the speed of light to a distance value.

The measuring core 10 includes an FPGA 12 to which said input quantities are applied, an integrator 14 which is in the FPGA 12 evaluated for fine measurement pulses, an A / D converter 18 for digitizing the integrated currents or voltages and a downstream evaluation unit 18 , which is shown here as a microprocessor and which executes the final calculations of the individual intermediate results for the desired signal propagation time. On an additional connection passes the FPGA 12 the evaluation unit counter values of a coarse measurement of the signal delay.

3 put the FPGA 12 once again in a block diagram. The implementation in FPGA structures uses appropriate elements to avoid errors due to run-time differences. The signal delay measurement is divided into a coarse measurement, which counts whole clocks, and a fine measurement for the offset of transmit pulse and receive pulse with respect to the clock. The FPGA 12 has a star pulse generator for fine measurement 20 , an echo pulse generator 22 and for coarse measurement, a counter unit 24 on. The transmit pulse becomes the start pulse generator 20 and the counter unit 24 , the receive pulse to the echo pulse generator 22 and the counter unit 24 fed. In addition, all elements shown get 20 . 22 . 24 of the FPGA 12 the clock and a reset line. The of start pulse generator 20 and echo pulse generator 22 for the fine measurement are pulses denoted by I1 to I6 from the integrator 14 or a corresponding multiplicity of integrators. The counter unit 24 gives the rough measurement counter values.

4 shows one more level the interconnection of the start pulse generator 20 , The echo pulse generator 22 is preferably identical and therefore will not be explained separately. In 5 Different signal curves are shown in order to better understand the individual processing stages.

The incoming transmission pulse 26 is initially in a differential input stage 28 to a square pulse 30 changed. The square pulse 30 can be tapped directly as pulse width. In addition, the square pulse 30 each one arrangement 32a Supplied by two flip-flops and an AND gate, in which the clock is used to generate a pulse which is connected to one edge of the rectangular pulse 30 begins and ends with a clock edge. The difference between the arrangements 32a -B is an inversion of the incoming square pulse 30 , so once a pulse 34a outputting the start counter offset between transmit pulse 26a and the clock relative to the beginning of the pulse and once a pulse 36a outputting the start counter offset between transmit pulse and clock relative to the end of the pulse.

In 5 are also the analogue receiving side pulses shown, so the received pulse width 30b , a pulse 34b , which evaluates the final counter offset between receive pulse 26b and pulse relative to the beginning of the pulse, and a pulse 36 , which evaluates the final counter offset between receive pulse 26b and measures clock relative to the end of the pulse. The pulses 30a -b, 34a -Federation 36a -B still carry a second name as currents I1 to I6, their assignment from the 5 can be read and already in 2 are used.

In the counter unit 24 becomes the coarse measurement with the transmission pulse 26a started and with the received pulse 26b completed. This produces counter values 38 which roughly measure the signal propagation time in accordance with a counter clock of, for example, several tens or hundreds of MHz.

6 shows an exemplary circuit diagram of an integrator 14 to which the different pulses 30a -b, 34a -b, 36a -B are supplied. In this case, the signal, here denoted by S, preferably differentially applied once directly and once inverted. The capacitor 40 is caused by the power source 42 charged to a level corresponding to the duration of the applied pulse. Thereafter, the resulting integrated signal in the A / D converter can 16 be converted into a digital numerical value.

I1

die Pulsweite des Sendepulses 26a

I2

der Startzählerversatz zwischen Sendepuls 26a und dem Takt, bezogen auf den Beginn des Sendepulses 26a

I3

der Startzählerversatz zwischen dem Sendepuls 26a und dem Takt, bezogen auf das Ende des Sendepulses 26a

I4

die Pulsweite des Empfangspulses 26b

I5

der Endzählerversatz zwischen dem Empfangspuls 26b und dem Takt, bezogen auf den Beginn des Empfangspulses 26b,

I6

der Endzählerversatz zwischen dem Empfangspuls 26b und dem Takt, bezogen auf das Ende des Empfangspulses 26b,

ZVV

die vollständigen gezählten Takte der Signallaufzeit, bezogen auf den Beginn von Sendepuls und Empfangspuls, und

ZHH

die vollständigen gezählten Takte der Signallaufzeit, bezogen auf das Ende von Sendepuls und Empfangspuls.

The evaluation unit 18 Now, the required individual values for calculating the signal propagation time are present, which for the sake of simplicity are referred to below with their already introduced symbols I1 to I6. So it is:

I1

the pulse width of the transmission pulse 26a

I2

the start counter offset between transmit pulse 26a and the clock, based on the beginning of the transmission pulse 26a

I3

the start counter offset between the transmit pulse 26a and the clock relative to the end of the transmit pulse 26a

I4

the pulse width of the received pulse 26b

I5

the final counter offset between the received pulse 26b and the clock, based on the beginning of the received pulse 26b .

I6

the final counter offset between the received pulse 26b and the clock relative to the end of the received pulse 26b .

ZVV

the complete counted clocks of signal propagation time relative to the beginning of transmit pulse and receive pulse, and

ZHH

the complete counted clocks of signal propagation time relative to the end of transmit pulse and receive pulse.

From this, first of all, a signal propagation time t _B with respect to the beginning of the pulse and a signal propagation time t _E with respect to the end of the pulse are calculated as t _B = I 2 - I 5 + ZVV and t _E = I 3 - I 6 + ZHH.

These signal propagation times can be used for themselves, but still carry a certain degree of inaccuracy, because it is very difficult to define the start of the pulse and the pulse end level-independent. Therefore, an averaging or focusing is preferably carried out finally to the level-independent signal delay t with respect to the vertex of the pulses 26a -B, namely t = (t _B + t _E ) / 2.

The pulse widths known by I1 and I4 can be used for additional evaluations, such as corrections of the signal propagation time t or a power measurement.

Claims (12)

Measuring core ( 10 ) for a signal propagation time measurement with a transmit pulse input for supplying a transmit pulse ( 26a ), a receiving pulse input for supplying a received pulse ( 26b ), a coarse measuring unit ( 24 ), in which a main signal transit time between one of the transmission pulse ( 26a ) certain transmission time and one from the received pulse ( 26b ) particular receiving time in clock periods of a count clock can be determined, a fine measuring unit ( 20 . 22 ), in which a start counter offset between the transmission time and the count clock and a final counter offset between the reception time and the count clock can be determined, and an evaluation unit ( 18 ), which is designed to calculate the signal propagation time from the main signal delay, the start counter offset and the final counter offset, characterized in that the fine measurement unit ( 20 . 22 ) a start pulse generator ( 20 ) to generate a start pulse whose duration corresponds to the start counter offset, and an echo pulse generator ( 22 ) to apply an echo pulse whose duration corresponds to the end counter offset and that the start pulse generator ( 20 ), the final pulse generator ( 22 ) and the coarse measuring unit ( 24 ) on an FPGA ( 12 ) are implemented. Measuring core ( 10 ) according to claim 1, wherein the fine measuring unit ( 20 . 22 ) at least one the FPGA ( 12 ) subordinate integrator ( 14 ) to convert the duration of the start pulse signals and echo pulse signals into a current or voltage value. Measuring core ( 10 ) according to claim 2, wherein the integrator ( 14 ) a capacitor ( 40 ) and a power source ( 42 ) and the power source ( 40 ) with the capacitor ( 42 ) is connectable for a duration that is dependent on an applied pulse signal ( 30 . 34 . 36 ) depends. Measuring core ( 10 ) according to claim 2 or 3, wherein the integrator ( 14 ) has a differential terminal to connect to a pulse ( 30 . 34 . 36 ) and a negation of the pulse ( 30 . 34 . 36 ) to be driven differentially. Measuring core ( 10 ) according to one of claims 2 to 4, wherein the integrator ( 14 ) an A / D converter ( 16 ) is subordinate to the current or voltage value in a digital value for further processing in the evaluation unit ( 18 ) to transform. Measuring core ( 10 ) according to one of the preceding claims, wherein the evaluation unit ( 18 ) on a the FPGA ( 12 ) downstream digital module, in particular a microprocessor is implemented. Measuring core ( 10 ) according to any one of the preceding claims, wherein the start pulse generator ( 20 ) is adapted to a first start pulse ( 34a ), whose associated start counter offset (I2) to the beginning of the transmission pulse ( 26a ), and wherein the echo pulse generator ( 22 ) is adapted to a first echo pulse ( 34b ) whose associated final counter offset (I5) to the beginning of the received pulse ( 26b ), so that in the evaluation unit a first signal delay time (t _B ) relative to the beginning of the pulse can be determined. Measuring core ( 10 ) according to any one of the preceding claims, wherein the start pulse generator ( 20 ) is adapted to a second start pulse ( 36a ) whose associated start counter offset (I3) to the end of the transmit pulse ( 26a ), and wherein the echo pulse generator ( 22 ) is adapted to a second echo pulse ( 36b ) whose associated end counter offset (I6) to the end of the received pulse ( 26b ), so that in the evaluation unit ( 18 ) A second signal delay time (t _E ) based on the end of the pulse can be determined. Measuring core ( 10 ) according to claim 7 and 8, wherein the evaluation unit ( 18 ) is designed to determine from the first signal delay time (t _B ) and the second signal delay time (t _E ) a pulse transit time related to the signal (t). Measuring core ( 10 ) according to any one of the preceding claims, wherein the start pulse generator ( 20 ) is adapted to a third start pulse ( 30a ) whose pulse width (I1) the duration of the transmission pulse ( 26a ), and / or wherein the echo pulse generator ( 22 ) is adapted to a third echo pulse ( 30b ) whose pulse width (I4) the duration of the received pulse ( 26b ) corresponds. Measuring core ( 10 ) according to one of the preceding claims, wherein the coarse measuring unit ( 24 ) and the fine measuring unit ( 20 . 22 ) have at least one further evaluation channel in order to determine the signal propagation time to at least one further received pulse. Optoelectronic sensor ( 100 ), in particular distance scanners or laser scanners, with a measuring core ( 10 ) according to one of the preceding claims, or microwave sensor, in particular free radiating level sensor according to the radar principle or TDR level sensor, with a measuring core ( 10 ) according to any one of the preceding claims.

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