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(11) | EP 0 387 092 A2 |
| (12) | EUROPEAN PATENT APPLICATION |
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| (54) | Traffic monitoring equipment |
| (57) Traffic monitoring equipment including traffic speed measuring equipment has a signal
validation facility incorporated to enhance reliability of the equipment. This facility
permits the monitoring operation, for example speed measurement, to proceed only if
the validation is positive, if negative the operation is aborted and/or a warning
signal or shutdown is activated. Validation is made of pulse sequence, pulse strength,
radio frequency interference, insulation breakdown, cable integrity (where cables
are used) and the like. |
[0001] This invention relates to validation checks applied to traffic monitoring equipment including traffic speed detection equipment. The invention is, however, in principle applicable to any traffic data collection equipment subject to what will be said herein below. An area of primary application which is envisaged, for example, is traffic monitoring equipment based on a cable or cables extending across a road or other surface and providing electrical signals. As is known such electrical signals can have one or more of several physical origins including piezo-electric effects whether resistive and/or generative piezo-electric effects, capacitive effects and tribo-electric effects, for example.
[0002] The validation checks or security checks are to be provided to enhance the reliability, accuracy and convenience of operating such equipment. In the use of such equipment for law enforcement this can have favourable legal implications. Generally less reliance may be placed on the operator of the equipment to ensure the integrity of the equipment and the security of the measurements it takes.
[0003] Thus there is a problem of improvement of measuring the speed of vehicles on the base that speed equals distance divided by time more accurately and reliably by means of error checking and automatic signal rejection, equipment shutdown and error signals for defective or below standard features. The improvement of accuracy lies within checking one or a combination of the factors which will be discussed below.
[0004] The apparatus may be adapted to check the correct sequence of impulses derived from several cables a single cable and/or a sensor pad or other sensor according to the set up of the equipment. This can be a security against swopping errors, for example, connection of cables incorrectly to the equipment after the cables have been set out in a suitable array with or without sensors or the like on a road surface, for example in the case of a duplicated pair of cables extended across the full width of a road in a spaced parallel array for speed law enforcement. Here the correct sequence of pulses will be from start cable 1, then start cable 2 followed by stop cable 1 and then stop cable 2 or, the reverse sequence for traffic in the opposite direction on the other side of the road being stop cable 2 then stop cable 1, then start cable 2 and then start cable 2. Any sequence other than these will be rejected by the instrument as a result of this validity checking facility. In this way the speed checking is done by independant time measuring devices, i.e. instrument 1 function measures the time from start 1 to stop 1 independently of the instrument 2 function which measures from start 2 to stop 1, each on either the front or the back axles or any other axles in physical contact with the road. Such cables can also be used and the verification can have application in measurement of deceleration and acceleration.
[0005] This sequence checking verification can be generalised as stated to any desired or required arrays of cable, cables and/or detector pads, magnetic loop detectors or others.
[0006] A RFI radio frequency interference check can be implemented in the instrument to monitor any radio interference during and in between measurements in order to ensure interference free measurements. To the extent thus that the cables act as antennas for electro-magnetic energy and provide radio signals any risk of this causing measurement errors can be excluded by such radio interference checks and validations provided in the instrument.
[0007] A EMI (Electromagnetic Interference) check from sources such as two-way radios, high frequency communication, high tension cables or lighting, can be implemented in the instrument to monitor any RFI (Radio Frequency Interference) or other EMI interference during and in between measurements in order to ensure interference free measurements. To the extent thus that the cables act as antennas for electro-magnetic energy fields and any risk of this causing measurement errors can be excluded.
[0008] Further validation can be provided by ongoing checking of the level of insulation resistance between conductors in a particular cable and monitoring for low impedance, that is below the prescribed threshold value. Thus the instrument will memorise the initial impedance of the cable and/or be set to a prescribed minimum limit impedance for monitoring the degrading of cable insulation resistance by means of impedance measurements. Thus the gradual or abrupt deterioration of a cable in service conditions leading to degrading of the insulation resistance and which can lead to erroneous measurements and degraded accuracy or reliability can be monitored for and the equipment rendered inoperative and/or warning signals displayed when the cable deteriorates below a certain prescribed state. It will be appreciated that the degrading of the impedance of insulation material between conductors of the cable (be it a coaxial cable, for example or a two core cable) can seriously degrade the signal strength to be generated between the conductors, for example, such as is generated by piezo-electric effects in such material. Thus low impedance may arise due to physical contact of the conductors due to mechanical damage of the insulating material between them. Low impedance can also result due to moisture and water becoming present in the cable which, for example, can arise during rainy weather where the outside sheath of the cable has become pervious to water, for example, due to mechanical damage again or water has access to the interior of the cable via its ends or in other ways. Degradation of the cable insulation can also occur under conditions of service due to heat, solar radiation, mechanical impulses of the vehicles and other conditions of use.
[0009] The instrument can be provided with further validation by means of a facility for signal strength monitoring. Thus each individual pulse, be it piezo-electric or tribo-electric or both in origin (for example, refer to South African patent number 66/0493) must pass through a minimal signal level before being detected as a valid trigger for time pulses. There is thus a minimum threshold level below which signals will be ignored, in order to avoid rise time error due to weak signals. The instrument is given this validation facility designed to prescribe that the minimum signal strength must arise within a certain minimum time measured from a starting threshold in order to be validated for time measurement. Thus in reference to a curve of the signal drawn on a time base the first pulse (be it positive or negative) must have a minimum steepness, i.e. a minimum value of the first differential of magnitude of the pulse with respect to time. For example, the minimum rise time to the prescribed minimum signal level may be of the order of 1 ms to 20 ms depending on the application and the features required in various circumstances. As stated this validation test will be applied typically to both positive and negative going signals, whichever occurs first. In the context of positive and negative starting signals preference is made to South African patent No. 76/4959 whose content is incorporated herein by reference. The discovery and observation of this phenomenon and the design of suitable circuitry to provide accurate measurement in its context is described in this patent. The addition of the signal strength monitoring or validation features described will thus further enhance the security, reliability and convenience of use of the apparatus. The instrumentation will be so designed that if the time measurement start signal has triggered positive the validation signal must be of the same polarity, i.e. positive and vice versa if the start signal has triggered negative. Failing this check again the start signal will not be accepted.
[0010] Degradation of the inner coaxial conductor or the screen in the case of a coaxial conductor by means of capacitance checking can be a further validation of checking features supplied to an instrument. Here the instrument will be designed to monitor frequency changes, phase changes, changes in natural frequency, for example, in particular ringing of the cable and any other means of detecting changes in capacitances can be employed. For example, any particular cable will be detected to have a certain capacitance per metre length and should a break occur of course the capacitance will change and an error signal can be produced indicating a faulty cable. Thus the integrity of the inner core conductor and/or of the coaxial screen conductor can be monitored in this way again to provide more reliable operation of the instrument without dependance upon the operator.
[0011] The instrument can also be designed to monitor for any spurious signal which is not in accordance with the typical signal produced by the cable under normal circumstances and required for detection of traffic and/or speed measurements. Thus parameters describing a category of signal within which the signal must fall can be prescribed and every signal produced by the cable can be checked against these parameters. If such a spurious signal should occur, for example, more than a prescribed number of times the cable can be flagged by means of a suitable error message displayed by the instrument for checking.
[0012] The instrument can be made "fail safe" in this or any of the other aspects by being adapted to stop functioning and to await correction if any prescribed threshold is exceeded.
[0013] The monitoring of cables as described can be adapted to coaxial cables, triaxial cables, screened pair cables or any other cable construction or array. The coaxial cables referred to herein have been referred to merely by way of example.
[0014] These validity checks and security features can furthermore be provided in the context of apparatus described in South African patent 81/6666 and in the context of patent No. 88/2312. Reference to the contents of these patents and the others referred to above is hereby made and their contents incorporated in this disclosure by reference.
[0015] The invention will be more fully described by way of examples with reference to the accompanying drawings, in which :
Figure 1 is a series of schematic side views of a vehicle wheel passing over a cable with below that an indication of the pulse generated in the cable at the various positions of the wheel shown,
Figure 2 is a schematic representation of an equivalent circuit for the cable,
Figure 3 is a graphical representation of respectively normal and weak signals with the resulting trigger signal down and verification signals shown below in corresponding positions,
Figure 4 is a graphical representation of respectively normal and interference signals with the trigger signal shown below in relation to RFI and EMI checking,
Figure 5 is a graphical representation of a signal with a trigger signal shown below in corresponding positions relating to insulation resistance checks.
[0016] As shown in figure 1 a vehicle wheel 1 is rotating as shown by the arrow 2 and moving forward as shown by the arrow 3 over a road surface 4 which has laid on it a cable 5 of piezo-electric shielded cable type. The wheel 1 thus moves to the position fully over the cable as shown at 1′ and then to the position where it is just leaving the cable as shown at 1˝.
[0017] The graph 6 below shows the voltage pulse produced in the piezo-electric cable 5 by the passage of the wheel and the three positions shown above the broken line 7, 8 and 9 indicating correspondence between the successive positions on the graph 6 and the successive positions of the wheel 1. Thus at the position 1 of the wheel where it first touches the cable 5 is indicated by the broken line 7 the pulse begins to rise from the zero line 10 at "Vb". The pulse reaches the peak at the position of the wheel 1′ as indicated by the broken line 8 at "Vp". The pulse then declines to a voltage of zero at the position of the wheel 1˝ indicated by the broken line where the pulse crosses the zero line 10 and then moves into a negative pulse portion or "under shoot "U" followed by an atenuating oscillation 11. Time T for the tyre footprint to first touch the cable to the moment when it leaves the cable can be regarded as composed of the two parts t₁ and t₂ and under normal conditions :
t₁ = t₂ and T = t₁ ÷ t₂
[0019] In normal speed timing equipment using such cables two parallel spaced cables are set on the road in an array and the time is measured between a pulse being generated in the first cable and a pulse generated in the second cable by the same wheel set of a vehicle. On this basis speed is calculated by the formula distance divided by time or :
[0020] In figure 2 a concentric shielded cable 12 is shown, the central core 13 carrying the signal and the outer shield 14 being earthed. The electrical equivalent of the cable is shown as a capacitance 15 which can be located in an equivalent circuit comprising a series connection of the capacitor 15 with a piezo-electrical or tribo-electrical generator 16 to provide the output voltage at Vout as shown. This is the equivalent circuit for the generative type of piezo-electrical or tribo-electrical ponomo which is used in the insulating material 17 between the core 13 and shield 14.
[0021] As shown in figure 3 the minimum threshold level detection with polarity verification is an important validation check in accordance with this invention. In figure 3 the graph 18 shows the normal pulse of the kind shown in figure 1 as compared with a weak signal 18′. The broken lines 19 show the trigger level and the broken lines 20 the verification level to which the instrument is set in accordance with this invention. The broken lines 21 show the commencement of the pulse 18 and the broken lines 22 the commencement of the trigger signal 23 which follows after a time delay td from the commencement of the signal. The verification signal 24 commences at the instant indicated by the broken lines 25. By contrast in the case of a weak signal no verification signal as indicated at 26 occurs and the measurement is aborted.
[0022] A vehicle travelling 100 km/h t₁ = 3,6 milliseconds
and td < t₁, with NORMAL OR GOOD SIGNAL
but with weak signal td₁ = t₁ = 3,6 ms.
therefore consider the speed being measured over a distance of 1,5 m (see note) the error occurance if minimum threshold detection is not applied can be :
true time at
Maximum error can be 3,6 ms if start triggered at peak signal strength (delay 3,6 ms)
and stop being a good signal triggered with wheel touching the cable (delay 0 ms) approximately zero,
NOTE: The measuring distance should be smaller than shortest wheelbase of a vehicle to avoid start being triggered by back wheel.
[0023] Minimum threshold verification can be applied using another criteria, namely that the verification signal must occur within a minimum specified time, e.g. 1 ms or an adjustable delay being calculated once the speed of the vehicle has been established, e.g. 3 ms if 30 km/h or 0,3 ms at 160 km/h - this allows a maximum constant signal error over the whole speed range (e.g. 1%).
[0024] Figure 4 illustrates techniques used in radio frequency interference and electro-magnetic interference checking. The graph 27 shows a normal pulse and the graph 28 a pulse which has been degraded by RFI or EMI intereference. As will be seen this interference can produce a trigger signal 29 which is premature as compared with the correct trigger signal as indicated at 30. This type of interference can arise with deteriorating sensors making them prone to external signals. RFI and EMI signals receiving components in the instrument can then provide a necessary instruction to disallow measurement where interference on these sources is detected.
[0025] Figure 5 shows an example graph of a pulse arising where insulation resistance results in a weaker signal which leads to greater triggering uncertainty. Here, with the triggering voltage at the level indicated by the broken line 32 the triggering pulse 33 only occurs at the instant indicated by the broken lines 44 when it should have occured at the instant indicated by the broken lines 35 implying the delay period shown. Hence the insulation resistance check will be carried out as described above.
[0026] The capacitance check implies monitoring breakage of the inner or outer conductors of the coaxial cable which could lead to false triggering by vehicles moving across or in close proximity of the cable. This occurs as a result of compression or tension waves which move the cables and cause the broken cable ends to make and break contact thus causing an impulse which cannot be reliably related to the occurance of physical contact with the tyre of a vehicle. In general spurious signal monitoring will be resorted to as described above.
1. Apparatus for validation checks applied to traffic monitoring equipment including
traffic speed detection equipment adapted to enhance the reliability, accuracy and
convenience of operating such equipment, to provide at least one selected from error
checking and automatic signal, rejection, equipment shutdown and error signals for
defective or below standard features, in which an electrical pulse produced by passage
of a vehicle wheel is detected and monitored to pass through a minimal signal level
before being accepted as a valid trigger for time pulses.
2. Apparatus for validation checks applied to traffic monitoring equipment including
traffic speed detection equipment as claimed in claim 1, in which a curve of the signal
drawn on a time base the first pulse (be it positive or negative) must have a minimum
steepness, i.e. a minimum value of the first differential of magnitude of the pulse
with respect to time.
3. Apparatus for validation checks applied to traffic monitoring equipment including
traffic speed detection equipment as claimed in either one of claims 1, in which a
pulse validation level is provided which is higher than a pulse triggering level,
adapted to generate a validation signal of the pulse attains the pulse validation
level, failing which the equipment aborts the speed detection.
4. Apparatus for validation checks applied to traffic monitoring equipment including
traffic speed detection equipment as claimed in any one of claims 1 to 3, which is
adapted to check the correct sequence of impulses derived from at least two selected
from several cables, a single cable a sensor pad and other sensors set up for traffic
monitoring.
5. Apparatus for validation checks applied to traffic monitoring equipment including
traffic speed detection equipment adapted to enhance the reliability, accuracy and
convenience of operating such equipment, to provide at least one selected from error
checking and automatic signal, rejection, equipment shutdown and error signals for
defective or below standard features, in which an electrical pulse produced by passage
of a vehicle wheel is detected and monitored to be adapted to check the correct sequence
of impulses derived from at least two selected from several cables, a single cable,
a sensor pad and other sensors set up for traffic monitoring.
6. Apparatus for validation checks applied to traffic monitoring equipment including
traffic speed detection equipment as claimed in either one of claims 4 or 5, in which
speed checking is done by independant time measuring devices instrument one function
measures the time from start one to stop one independantly of the instrument two function
which measures from start two to stop two and sequence of these signals is checked.
7. Apparatus for validation checks applied to traffic monitoring equipment including
traffic speed detection equipment as claimed in one of claims 1 to 6, in which a radio
frequency interference check and electro-magnetic interference check is implemented
in the instrument to monitor any radio interference during and in between measurements
in order to ensure interference free measurements, using means adapted so that traffic
monitoring cables act as antennas for electro-magnetic energy and provide radio signals
which are processed to provide a validation signal only if below a predetermined amplitude
and otherwise an abort signal.
8. Apparatus for validation checks applied to traffic monitoring equipment including
traffic speed detection equipment adapted to enhance the reliability, accuracy and
convenience of operating such equipment, to provide at lest one selected from error
checking and automatic signal, rejection, equipment shutdown and error signals for
defective or below standard features, in which an electrical pulse produced by passage
of a vehicle wheel is detected and monitored to carry out a radio frequency interference
check implemented in the instrument to monitor any radio interference during and in
between measurements in order to ensure interference free measurements, using means
adapted so that traffic monitoring cables act as antennas for electro-magentic energy
and provide radio signals which are processed to provide a validation signal only
if below a predetermined amplitude and otherwise an abort signal.
9. Apparatus for validation checks applied to traffic monitoring equipment including
traffic speed detection equipment as claimed in any one of claims 1 to 8, in which
further validation is provided by ongoing checking of the level of insulation resistance
between conductors in a partcular cable and monitoring for low impedance, in which
the instrument will memorise the initial impedance of the cable and/or be set to a
prescribed minimum limit impedance for monitoring the degrading of cable insulation
resistance by means of impedance measurements.
10. Apparatus for validation checks applied to traffic monitoring equipment including
traffic speed detection equipment adapted to enhance the reliability, accuracy and
convenience of operating such equipment, to provide at least one selected from error
checking and automatic signal, rejection, equipment shutdown and error signals for
defective or below standard features, in which an electrical pulse produced by passage
of a vehicle wheel is detected and monitored to be further validated by ongoing checking
of the level of insulation resistance between conductors in a particular cable and
monitoring for low impedance, in which the instrument will memorise the initial impedance
of the cable and/or be set to a prescribed minimum limit impedance for monitoring
the degrading of cable insulation resistance by means of impedance measurements.
11 Apparatus for validation checks applied to traffic monitoring equipment including
traffic speed detection equipment as claimed in any one of claims 1 to 10, in which
degradation of the inner coaxial conductor or the screen in the case of a coaxial
conductor by means of capacitance checking is a further validation of checking features
supplied to an instrument, instrument being designed to monitor at least one selected
from frequency changes, phase changes, changes in natural frequency.
12. Apparatus for validation checks applied to traffic monitoring equipment including
traffic speed detection equipment as claimed in claim 11, in which the cable is detected
to have a certain capacitance per metre length and an error signal is produced indicating
a faulty cable should a break occur in the inner core conductor and/or of the coaxial
screen conductor.
13. Apparatus for validation checks applied to traffic monitoring equipment including
traffic speed detection equipment adapted to enhance the reliability, accuracy and
convenience of operating such equipment, to provide at least one selected from error
checking and automatic signal, rejection, equipment shutdown and error signals for
defective or below standard features, in which an electrical pulse produced by passage
of a vehicle wheel is detected and monitored to validate against degradation of the
inner coaxial conductor or the screen in the case of a coaxial conductor by means
of capacitance checking is a further validation of checking features supplied to an
instrument, instrument being designed to monitor at least one selected from frequency
changes, phase changes, changes in natural frequency.
14. Apparatus for validation checks applied to traffic monitoring equipment including
traffic speed detection equipment as claimed in any one of claims 1 to 13, in which
the instrument is designed to monitor for any spurious signal which is not in accordance
with the typical signal produced by the cable under normal circumstances and required
for detection of traffic and/or speed measurements, in which parameters describing
a category of signal within which the signal must fall are prescribed and every signal
produced by the cable is checked against these parameters.
15 Apparatus for validation checks applied to traffic monitoring equipment including
traffic speed detection equipment adapted to enhance the reliability, accuracy and
convenience of operating such equipment, to provide at least one selected from error
checking and automatic signal, rejection, equipment shutdown and error signals for
defective or below standard features, in which an electrical pulse produced by passage
of a vehicle wheel is detected and monitored, the instrument being designed to monitor
for any spurious signal which is not in accordance with the typical signal produced
by the cable under normal circumstances and required for detection of traffic and/or
speed measurements, in which parameters describing a category of signal within which
the signal must fall are prescribed and every signal produced by the cable is checked
against these parameters.