Traffic monitoring equipment

Abstract: 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.

Description: 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. 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. 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 cne or a combination of the factors which will be discussed below. 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. 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. 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. 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. 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. 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 66/0934) must pass through a minimal signal level before being detected as a valid trigger for time pulses. Patent No. 66/0934 describes circuitry incorporated within the instrument which is adapted to make the instrument impervious to "weak" signals. This is done by the provision of, for each detector (or cable) an amplifier to amplify the signal received, a trigger circuit to shape the pulse and chop the voltage to a standard value, and preferably a monostable multivibrator within a given time constant. The amplifier is advantageously adapted to pass only a chosen polarity pulse, and is furthermore carefully designed to minimise its sensitivity to stray pulses. Once the pulse is received from the detector, an amplification of 400 may be employed to amplify the pulse, and a Schmidt trigger circuit is used to shape and chop the pulse to a square 3 volt wave. The monostable multivibrators preferably have a time constant of about 100 milliseconds. During this time period any pick-up pulse or stray pulse coming into the circuit would have no effect. There is thus a minimum threshold level below which signals will be ignored, in order to avoid rise time errors 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, reference is made to South African patent No. 76/4959. According to this patent, the instrument is preferably designed to be unaffected by the initial polarity of the pulse from each cable by being adapted to divide the positive and negative phases of the electronic signal from each cable, process them separately in a manner including inverting one of the phases and then to recombine them. Preferably, the phase dividing and recombining circuit must comprise active devices such as transistors so as not to cause a substantial voltage drop in the pulse derived by passage of a vehicle over it. 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 measurements 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. 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 capacitance 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. 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. 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. 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. 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. Patent no. 88/2312 provides a preferred cable in which the piezo-electric effect predominates over any others, and this can be achieved by the employment of a formulation comprising or consisting of a pulverised piezo-electric crystalline material provided as a filter in a synthetic polymer which itself may also exhibit piezo-electric properties. Preferably a coaxial type cable is employed of two core type where the insulation between the inner core and the concentric outer core exhibits the preferred electrical effect as the outer conductor may then serve against electrical noise from extraneous sources. This patent further elaborates techniques to reduce unwanted cross cable interference and noise picked up from external sources. Patent no. 88/6666 relates not only to speed measurement for law enforcement purpose but also for capturing all traffic data and analysing it for other purposes. In particular, this patent discusses the means to automatically accommodate inductance drift, in the cables, over long periods of time or related to a particular installation or other factors. The invention will be more fully described by way of examples with reference to the accompanying diagrammatic 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 shown 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; and Figure 6 is a block diagram of electronic processing means disclosed in South African Patent no. 76/14959. 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". 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 "V b ". The pulse reaches the peak at the position of the wheel 1' as indicated by the broken line 8 at "V p ". 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 attenuating oscillation 11. Time T for the type footprint to first touch the cable to the moment when it leaves the cable can be regarded as composed of the two parts t 1 and t 2 and under normal conditions: t 1 = t 2 and T = t 1 + t 2 Thus for a vehicle with a tread contact area of 20cm : at 100km/h --- T 100 = 0,2 x 3600 100 = 7,2ms 30 km/h --- T 30 = 0,2 x 3600 30 = 24ms 160 km/h --- T 160 = 0,2 x 3600 160 =4,5ms 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: speed (km/h) = distance (in metres x 3600) time (in milliseconds) The instrument is preferably designed to be unaffected by the initial polarity of the pulse from each cable. The electronic processing means to achieve this result is illustrated in part of Figure 6. The electronic circuitry described is only by way of example, as any other suitable circuitry can also be implemented to achieve the desired effect. As shown in Figure 6, cable 41, as indicated schematically is a three core cable, one core 41a being a common earth, the second core 41b transmitting the signal from the cable 37 and the third core 41c transmitting the signal from cable 36. The cable 41 first transmits the signals to the main instrument 42 as indicated here within the broken lines 42. There is provided a phase divider and combiner 43 for the signal from the cable 26 and a phase divider and a combiner 44 for the signal from the cable 37. Each phase divider and combiner also incorporates further amplification of the signal. Apart from amplification suitable refinement of the signal is also carried out both in the impedance converters 39 and 40 and in the phase dividers and combiners 43 and 44. One of the phase dividers and combiners also performs an inverting operation on the signal and this is indicated in this diagram by the block 45. The outputs go to a gating unit 17 and of course one will be a start signal and the other a stop signal. A master oscillator 47 feeds to the gating circuitry as well. The master oscillator provides a single at 2 megahertz frequency, the high frequency being selected to provide a low gating error. The gated signal then representing an analogue of the measured time passed from the gating circuitry 46 to a frequency dividing circuitry 18 which reduces the frequency of the signal to the counting signal of 10 kilohertz. This is fed to a time counting circuitry 48 which passes its output in turn to the calculator 49. The calculator 49 performs the operation on the time signal of converting it to a speed measurement, related naturally to the measured distance 38. The speed measurement is fed out of the calculator 20 to decoders 50 which provide a decimal read-out as at 51 and may if desired also provide a stored speed reading at 52. The speed read-out naturally may be in kilometres per hour or miles per hour or any other measurement as desired. 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 v out as shown. This is the equivalent circuit for the generative type of piezo-electrical or tribo-electrical polymer which is used in the insulating material 17 between the core 13 and shield 14. 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 t d 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. A vehicle travelling 100 km/h t 1 = 3,6 milliseconds and t d < t 1 , with NORMAL OR GOOD SIGNAL but with weak signal td 1 = t 1 = 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 100km/h T r =1,5 x 3600 100 = 54 ms 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, % error = 3,6 w 100 54 = 6,6% NOTE: The measuring distance should be smaller than shortest wheelbase of a vehicle to avoid start being triggered by back wheel. 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%). 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. 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. 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.

Claims: Apparatus for validation checks applied to traffic monitoring equipment including traffic speed detection equipment characterised by the provision of at least one selected from 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 by means of at least one selected from devices known in the art to pass through a minimum signal level before being accepted as a valid trigger for time pulses, to be applied to check the correct sequences 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, 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, to further validate by ongoing checking of the level of insulation resistance between conductors in a particular cable and monitoring for low impedance, to validate against degradation of the inner coaxial conductor or the screen in the case of a coaxial conductor by means of capacitance checking, the apparatus being designed to monitor for any spurious signal which is not in accordance with a 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 the signal must fall are prescribed and every signal produced by the cable is checked against these parameters, for the purpose of enhancing the reliability, accuracy and convenience of operating such equipment. Apparatus for validation checks applied to traffic monitoring equipment including traffic speed detection equipment as claimed in Claim 1, characterised by a device designed according to known technology to detect the attainment by the said pulse (18) of a value (19) within a specified rise time in order for the said pulse to be accepted as valid. Apparatus for validation checks applied to traffic monitoring equipment including traffic detection equipment as claimed in either one of Claims 1 and 2, characterised by a device designed according to known technology to detect the attainment by the said pulse (18) of a pulse validation level (20) which is higher than the pulse trigger level (19), and to generate a validation signal (24) if the pulse attains the said validation level, failing which the equipment aborts the speed detection. Apparatus for validation checks applied to traffic monitoring equipment including traffic detection equipment as claimed in any one of Claims 1 to 3, characterised in that speed checking is done by independent time measuring devices, one device measures the time from start one to stop one independently of the other device which measures from start two to stop two and means known in the art is provided to check the sequence of these signals. Apparatus for validation checks applied to traffic monitoring equipment including traffic detection equipment as claimed in any one of Claims 1 to 4, characterised in that apparatus provided for radio frequency interference check uses means known in the art such that traffic monitoring cables act as antennas for electro-magnetic energy and provide radio signals which are processed to provide a validation signal (33) only if below a predetermined amplitude and otherwise an abort signal. Apparatus for validation checks applied to traffic monitoring equipment including traffic detection equipment as claimed in any one of Claims 1 to 5, characterised in that further validation is provided by ongoing checking of the level of insulation resistance between conductors in a particular cable and devices provided for monitoring for low impedance, in which memory means according to known technology will memorise the initial impedance of the cable and/or be set to a prescribed minimum limit impedance for monitoring the degrading of cable resistance by means of impedance measurements. Apparatus for validation checks applied to traffic monitoring equipment including traffic detention equipment as claimed in any one of Claims 1 to 6, characterised in that the apparatus provided for capacitance checking, the apparatus being designed to monitor at least one selected from frequency changes, phase changes, changes in natural frequency, the apparatus comprising means known in the art to measure the said changes. Apparatus for validation checks applied to traffic monitoring equipment including traffic detection equipment as claimed in Claim 7, characterised in that the cable is detected to have a certain capacitance per meter length and an error signal is produced by means indicating a faulty cable should a break occur in the inner core conductor and/or of the coaxial screen conductor. Apparatus for validation checks applied to traffic monitoring equipment including traffic detection equipment as claimed in one of Claims 1 to 8, characterised in that the apparatus 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 (32) are prescribed and every signal produced by the cable is checked against these parameters.

Inventors: GEBERT FRANZ JOSEF, GEBERT RALF DIETER HEINRICH, GEBERT RUDIGER HEINZ, GEBERT, FRANZ JOSEF, GEBERT, RALF DIETER HEINRICH, GEBERT, RUDIGER HEINZ
Applicants: GEBERT FRANZ JOSEF, GEBERT RALF D H, GEBERT RUEDIGER HEINZ, GEBERT, FRANZ JOSEF, GEBERT, RALF DIETER HEINRICH, GEBERT, RUDIGER HEINZ
Application year: 1990
Main classification: G08G1
IPC codes: G08G1