Road sensor assembly for measuring a road surface condition

Abstract

A road sensor assembly (100) comprises: a road sensor (1) suitable for being installed in the road surface, a control unit (2) suitable for being connected on the roadside, and temperature probes (5) arranged in the road surface to detect the temperature of the road surface; the road sensor (1) comprises a box (6) that contains a resin body (60), a capacitance probe (3) and a conductivity probe (4).

Description

[0001] ROAD SENSOR ASSEMBLY FOR MEASURING A ROAD SURFACE CONDITION

[0002] DESCRIPTION

[0003] The present invention relates to a road sensor assembly used for measuring a road surface condition.

[0004] Road sensors installed in the surface of the road surface are known to measure the road surface condition. The purpose of the sensors is to provide road maintenance managers with information on the state of the road surface, such as: temperature at various levels of the road surface, presence and thickness of a water film, presence of ice, and measurement of salt concentration in the case of anti-icing treatments.

[0005] The sensor is installed inside a special recess formed into the road surface, flush with the surface of the road surface in order to avoid being damaged by tires. In fact, such sensors are subject to traffic-induced stresses with loads up to 20 kg / cm2.

[0006] However, the installation of the sensor in the road surface results in mechanical stresses on the sensor due to two main factors:

[0007] - traffic that can reach peaks of more than 14, ,000 thousand vehicles / day, and

[0008] - internal stresses due to meteorological factors.

[0009] In addition, the sensor is exposed to chemicals that can degrade its performance.

[0010] As a result, issues related with the periodic replacements of the sensors arise, originating costs and traffic interruption issues on the roadway.

[0011] A frequent cause of a degraded performance or even of a complete failure of the sensor is due to the thermal and mechanical stresses experienced by the electronics contained inside the sensor, particularly in the PCB where the components are soldered.

[0012] Such stresses are worsened when the PCB is immersed in high-density resins that increase the internal stress and generate microcracks in the soldered components that may render the sensor useless.

[0013] The purpose of the present invention is to eliminate the drawbacks of the prior art by providing a road sensor assembly for measuring a road surface condition that is capable of minimizing the risk of failure, extending the period of use as much as possible, without imposing any constraints on the installation position in the road surface.

[0014] Another purpose is to provide such a road sensor assembly that is durable, reliable, efficient, and easy to install.

[0015] These purposes are achieved in accordance with the invention with the features of the attached independent claim 1 .

[0016] Advantageous realizations of the invention appear from the dependent claims.

[0017] Further features of the invention will appear clearer from the following detailed description, which refers to a purely illustrative and therefore non-limiting embodiment, illustrated in the accompanying drawings, wherein:

[0018] Fig. 1 is a perspective block diagram that illustrates the road sensor assembly according to the invention;

[0019] Fig. 2 is a view of a capacity probe of the road sensor of the road sensor assembly in Fig. 1 ;

[0020] Fig. 2A is a sectional view that diagrammatically illustrates a water layer on the capacitance probe of Fig. 2;

[0021] Fig. 3 is a circuit block diagram that illustrates a capacitance measurement circuit;

[0022] Fig. 4 is a block circuit diagram that illustrates a phase-shifting measurement circuit of the capacitance measurement circuit of Fig. 3;

[0023] Fig. 5 is a circuit diagram that illustrates an implementation of the capacitance measurement circuit of Fig. 3;

[0024] Fig. 6A is a graph that illustrates the two sinusoidal signals generated by the sinusoidal signal generator of the capacitance measurement circuit;

[0025] Fig. 6B is a graph that illustrates the two squared sinusoidal signals coming out of the squarers of the phase-shifting measurement circuit;

[0026] Fig. 6C is a graph that illustrates a pulse waveform coming out of the OR circuit of the phase shift measurement circuit;

[0027] Fig. 6D is a graph that illustrates an output voltage from a low-pass filter of the phase-shift measurement circuit;

[0028] Fig. 7 is a graph that illustrates a measurement of the phase shift and output voltage from the low-pass filter as a function of the capacitance detected by the capacitance probe; and Fig. 8 is a graph that illustrates the relationship between the in-air capacitance and the frequency of the sinusoidal signals generated by the sinusoidal generator in such a way to verify the phase shift between the two sinusoidal signals.

[0029] With the aid of the Figures, the road sensor assembly according to the invention is described, which is comprehensively indicated with reference numeral 100.

[0030] Referring to Fig. 1 , the road sensor assembly (100) comprises a road sensor (1 ) suitable for being installed in the road surface.

[0031] The road sensor (1 ) does not contain any active electronic components and delegates all measurement and processing functions to a control unit (2) located at the roadside, which is therefore not subject to stress.

[0032] The road sensor (1 ) contains

[0033] - a capacitance probe (3) suitable for detecting a capacitance value detected on the surface of the road surface that is indicative of a water thickness on the road surface, and

[0034] - a conductivity probe (4) suitable for detecting an electrical conductivity value that is indicative of a concentration of chemicals on the road surface.

[0035] The road sensor assembly (100) comprises temperature probes (5) separated from the road sensor (1 ) and arranged at various positions and levels of the road surface in order to detect the temperature of the road surface. The temperature probes (5) are connected to the control unit (2).

[0036] The road sensor (1 ) comprises a box (6) that contains a body (60). By way of example, the box has a cylindrical shape that is open at the top and is made of a metal material, such as stainless steel. The body (60) is made of resin, such as epoxy resin, with high density, that is, with a density greater than 1 .5 g / cm3.

[0037] The body (60) of the road sensor may comprise a plurality of protrusions (32) of discoidal shape that protrude superiorly from the body.

[0038] The capacitance probe (3) comprises two electrodes (30, 31 ) embedded in the body (60) of the road sensor. The first electrode (30) has a discoidal shape and the second electrode (31 ) has an annular shape and is arranged around the first electrode (30).

[0039] The conductivity probe (4) comprises a plurality of metal elements (40) embedded in the body (60) of the road sensor. The electrodes (30, 31 ) of the capacitance probe and the metal elements (40) of the conductivity probe are connected to the control unit (2) by means of respective transmission lines (35, 45) protected by a pipe or sheath (63) connected to the box (6) of the road sensor and suitable for being inserted inside the road surface.

[0040] The temperature probes (5) are connected with appropriate cables (55) to the control unit (2), in appropriate positions are not constrained to the position of the road sensor (1 ).

[0041] The road sensor assembly (100) eliminates the risks of internal failure of the road sensor (1 ) because the road sensor (1 ) does not contain any active electronic components, and a failure of the temperature probes (5) does not require the costly replacement of the entire road sensor (1 ), but only that of the temperature probes (5) that are installed separately to the road sensor (1 ).

[0042] The fact that the control unit (2) is removed from the body (60) of the sensor requires the use of transmission lines (35, 45), the length of which can reach a few tens of meters, to connect the capacitance and conductivity probes (3, 4) to the control unit (2). This originates limitations on the type of capacitance and conductivity probes (3, 4) and the relevant mode of operation, as the transmission lines (35, 45) are home to circulating currents and their presence may affect the measurement of the parameters detected by the capacitance and conductivity probes (3, 4). This is the case of the capacitance probe (3), wherein the capacitance varies depending on the thickness of the water layer on the capacitance probe and on the relative dielectric constant of the water (E ), which is about 80 at a temperature of 25°C.

[0043] Referring to Fig. 2, the capacitance probe (3) has the structure of a planar capacitor made, for example, with interdigital technique or with planar electrodes of any shape. In such a case, the capacitance probe (3) has a first circular metal electrode (30) surrounded by a second metal electrode (31 ) in the shape of a circular ring or annulus that form a capacitor having an in-air capacitance (Co). The in-air capacitance (Co) is taken as the reference value when no water layer is disposed on the capacitance probe (3).

[0044] Fig. 2A illustrates a water layer (W) having a thickness (h) disposed on the capacity probe (3). The presence of the water layer (W) on the capacity probe (3) causes an increase of the capacity (C) of the capacity probe according to the following relationship:

[0045] C( / r) = Cof h) F1 where f(h) is a function of the geometry of the capacitance probe (3) and of the thickness (h) of the water layer and varies between 1 and EH « 80.

[0046] The innovative aspect of the road sensor assembly (100) consists in the configuration of the control unit (2) capable of detecting the capacitance variation of the capacitance probe (3) due to the presence of the water layer (W), measuring the corresponding variation of an appropriate electrical quantity and thus eliminating the presence of active electronic components inside the road sensor (1 ).

[0047] For such a purpose, the control unit (2) has a circuit to which the capacitance probe (3) is connected with a transmission line (35) of characteristic impedance, for example 75 Q. The length of the transmission line (35) must be an integer multiple of a half wavelength, at the chosen operating frequency. This is to prevent the distributed capacitance of the transmission line (35) from adding to that of the capacitance probe (3), altering the capacitance values detected by the capacitance probe.

[0048] A convenient way to make such a capacitance measurement is to detect the current variation in the transmission line (35) generated by a sinusoidal source (s(t)) of appropriate frequency (F), with a capacitance measurement circuit (7) shown in Fig. 3.

[0049] In order to avoid any interference, the transmission line (35) should be made with a shielded coaxial cable having a characteristic impedance usually in the range of 50 -100 Q.

[0050] In order to ensure an adequate sensitivity of the capacitance measurement, it is necessary that the variation of the capacitive reactance of the capacitance probe (3) as a function of the dielectric constant of the water (EH) is of the same order of magnitude as the characteristic impedance of the transmission line (35). For this reason, advantageously, the characteristic impedance of the transmission line (35) is 80 Q.

[0051] On the other hand, the in-air capacitance (Co) value of the capacitance probe (3) is constrained by the geometry of the capacitance probe (3) and by its size. Practical values of Coare typically comprised between 2 and 8 pF. Consequently, the maximum values of C(h) vary between 160 and 640 pF. This places constraints on the frequency values of the sinusoidal generator as will be discussed below.

[0052] Fig. 3 shows the diagram of the capacitance measurement circuit (7). The capacitance measurement circuit (7) comprises a sinusoidal signal generator (70) that generates a sinusoidal signal s(t) = Asin(2nFt) of frequency F.

[0053] The sinusoidal signal generator (70) is connected to the transmission line (35) which has a characteristic impedance (Ro) comprised between 50 and 100 Q.

[0054] The transmission line (35) is connected to the capacity probe (3) that detects the capacity C(h).

[0055] A block (35a) of Roimpedance was placed at the output of the sinusoidal signal generator (70) to simulate the transmission line (35). Another block (35b) of Ro impedance was placed in parallel with the capacitance probe (3) to simulate the transmission line (35).

[0056] The variation in the thickness (h) of the water layer (W) results in a corresponding variation in the capacitance (C(h)) of the capacitance probe (3), which in turn produces a mismatch in the transmission line (35) and a consequent variation in the phase and in the amplitude of the current (1(h)) circulating in the transmission line (35).

[0057] By measuring a phase variation of the current (1(h)) circulating in the transmission line (35), an indirect measurement of the capacitance (C(h)) of the capacitance probe and thus of the thickness (h) of the water layer can be obtained.

[0058] The preferred way to measure the phase variation of the current (1(h)) circulating in the transmission line (35) is to compare two voltage sinusoidal waveforms (VA1 ; VA2) respectively detected upstream and downstream of the block (35a) of Roimpedance that simulates the transmission line (35). A phase shift measurement circuit (8) is used for this purpose.

[0059] Various circuit configurations of the phase shift measurement circuit (8) can be used either with discrete components or using the commercially available integrated circuits.

[0060] A possible realization of the phase shift measurement circuit (8) is shown in Fig. 4. The phase shift measurement circuit (8) may comprise discrete components in order not to be constrained to a specific supplier and to a specific technical solution, also in view of the contingent difficulty to find microelectronic components on the market. The phase shift measurement circuit (8) uses only two types of microcircuits, i.e. operational amplifier and comparator, which can be easily replaced with equivalent and commercially available ones if needed.

[0061] With reference to the block diagram in Fig. 4, the two sinusoidal waveforms (VA1 , VA2) upstream and downstream of the block (35a) of Roimpedance that simulates the transmission line are applied to two squarer circuits (80). Each squarer circuit (80) can be realized with an operational amplifier.

[0062] Square waves (01 , 02) come out of the squarer circuits (80) and are sent as input to an OR circuit (81 ). The OR circuit (81 ) can be realized with a signal comparator.

[0063] A pulse waveform (O) having a duty-cycle (d) that depends on the phase shift between the two sinusoidal waveforms (VA1 , VA2) comes out at the output of the OR circuit (81 ), according to the relationship: d = - p + AT = 0.5 + — (F2)

[0064] To2 J ToV’ where AT is the delay between the two sinusoidal waveforms (VA1 , VA2) caused by the phase shift.

[0065] The pulse waveform (O) coming out of the OR circuit (81 ) is sent to a low- pass filter (82). The low-pass filter (81 ) can be realized with an R-C circuit.

[0066] After an appropriate filtering of the pulse waveform (O) with the low-pass filter (82), an output voltage (Vu) equal to: is obtained, where Vurepresents the indirect measurement of the thickness (h) of the water layer, E is the voltage swing of the OR circuit (81 ); whereas the output voltage (Vu) of the low-pass filter (82) varies between E / 2 and 3E / 4.

[0067] Fig. 5 illustrates a possible implementation of the capacitance measurement circuit (7) used to characterize its performance with a SPICE simulator.

[0068] The capacitance measurement circuit (7) comprises the sinusoidal signal generator (70) that operates at a certain frequency (F) whose value depends on the system parameters. By way of example, the frequency (F) may be comprised between 10 and 20 MHz. The value of a capacitance (C4) in parallel with the termination impedance (R2) of the transmission line (35) detects the presence of a water layer on the sensor surface.

[0069] Fig. 6A illustrates the two sinusoidal waveforms (VA1 , VA2) detected upstream and downstream of an impedance block that simulates the transmission line.

[0070] Fig. 6B illustrates the square waves (01 , 02) coming out of the squarers

[0071] (80).

[0072] Fig. 6C illustrates the pulse waveform (O) coming out of the OR circuit

[0073] (81 ).

[0074] Fig. 6D illustrates the voltage (Vu) coming out of the low-pass filter (82).

[0075] Fig. 7 illustrates a measurement of the phase shift and of the voltage (Vu) coming out of the low-pass filter, as a function of the capacitance detected by the capacitance probe (3), with E=3.3V and F=12 MHz.

[0076] The maximum phase shift value to obtain a dynamics capable of guaranteeing a good resolution is 60°, compared with the nominal value of 90°.

[0077] The corresponding voltage variation is approximately 80% of the nominal voltage, that is E / 4.

[0078] Fig. 7 shows the trend of the phase shift between the sinusoidal waveforms (VA1 and VA2) and the voltage at the output of the low-pass filter (82), as a function of the value of the capacitance variation (C(h)) of the capacitance probe (3), due to the presence of the water layer (W) with thickness (h), with E=3.3V.

[0079] The in-air capacitance value (Co) of the capacitance probe (3) is a project datum, being determined by the dimensions and geometry of the capacitance probe. The frequency (F) of the sinusoidal signal generated by the sinusoidal signal generator (70) required to obtain a dynamics of Vu equal to that defined by formula F4 is expressed by the relationship: where Cmaxis the capacity value that corresponds to the thickness of water layer, so that:

[0080] ^max=^H2O o ~ 80 Co(F6) Fig. 8 represents the relationship between the in-air capacitance (Co) and the frequency (F) of the sinusoidal signal, so that the formula F4, to which a phase shift of 60° corresponds, is verified.

[0081] The curve in Fig. 8 is a relationship of the type: where Cmax is the capacitance value of the capacitance probe (3) that satisfies the formula (F6), whereas a is a dimensionless parameter.

[0082] Table 1 shows the values of Cmax and Cothat result in the value of R0 / Xc=0.25 in accordance with formula F5.

[0083] Tab e 1

[0084] The project data are:

[0085] Rois the characteristic impedance of the transmission line

[0086] Co is a function of the dimensional requirements of the capacitance probe (3)

[0087] Cmax depends on Coaccording to the relationship F6

[0088] The frequency value of the sinusoidal signal generator (70) is expressed by the formula (F5), which is a function of the above parameters.

[0089] The length of the transmission line (35) must be an integer multiple of half the wavelength of the sinusoidal signal (s(t)), so that the no-load phase shift results in « 0.

[0090] Therefore, the length of the transmission line (35) must satisfy the relationship:

[0091] L=n / 2 kc / F n=1 ,2,.... (F8) where:

[0092] L is the length of the transmission line in meters; c is the speed of light in vacuum k is the nominal speed correction factor in the transmission line so that the no-load phase shift is « 0. Variations and equivalent changes may be made to the present embodiment of the invention, within the scope of a technician of the field, and nevertheless within the scope of the invention as expressed by the appended claims.

Claims

CLAIMS1. Road sensor assembly (100) comprising:- a road sensor (1 ) suitable for being installed in the road surface; said road sensor (1 ) being internally devoid of electronic components,- a control unit (2) suitable for being connected on the roadside, and- temperature probes (5) that are separate from the road sensor (1 ), and are arranged in the road surface to detect the temperature of the road surface; said temperature probes (5) being connected to the control unit (2) by means of a transmission line (55) of the temperature probes; wherein said road sensor (1 ) comprises:- a box (6) containing a resin body (60),- a capacitance probe (3) suitable for detecting a capacitance value of the road surface indicative of a water thickness on the road surface, and- a conductivity probe (4) suitable for detecting an electrical conductivity value of the road surface that is indicative of a concentration of chemical agents on the road surface; wherein the capacitance probe (3) comprises two electrodes (30, 31 ) embedded in the body (60) of the road sensor; the conductivity probe (4) comprises a plurality of metal elements (40) embedded in the body (60) of the road sensor; the electrodes (30, 31 ) of the capacitance probe and the metal elements (40) of the conductivity probe are connected to the control unit (2) by means of respective transmission lines (35, 45) protected by a pipe or sheath (63) connected to the box (6) of the road sensor and suitable for being disposed in the road surface.

2. The road sensor assembly (100) according to claim 1 , wherein said control unit (2) comprises a capacitance detection circuit (7) connected to said transmission line (35) connected to said capacitance probe (3).

3. The road sensor assembly (100) according to claim 2, wherein said capacitance detection circuit (7) comprises:- a sinusoidal signal generator (70) generating a sinusoidal signal (s(t)); said sinusoidal signal generator (70) being connected to said transmission line (35) of the capacitance probe; and- a phase shift measurement circuit (8) to detect a phase shift of a current(1(h)) circulating in the transmission line (35) of the capacitance probe.

4. The road sensor assembly (100) according to claim 3, wherein said transmission line (35) of the capacitance probe has a characteristic impedance (Ro) between 50Q and 100Q; and said phase shift measurement circuit (8) is configured to detect a phase shift between two sinusoidal waveforms (VA1 , VA2) upstream and downstream of a block (35a) having a characteristic impedance equal to the characteristic impedance (Ro) of said transmission line of the capacitance probe.

5. The road sensor assembly (100) according to claim 4, wherein said phase shift measurement circuit (8) comprises:- two squarers (80) that receive said sinusoidal waveforms as input (VA1 , VA2) and output square waves (01 , 02),- an OR circuit (81 ) that receives said square waves (01 , 02) from the squarers (80) as input and outputs a pulse waveform (O) having a duty-cycle (d) that depends on the phase shift between the two sinusoidal waveforms (VA1 , VA2),- a low-pass filter (82) that receives said pulse waveform (O) as input and outputs a voltage (Vu) representing an indirect measurement of a thickness (h) of a water layer (W) on said capacitance probe (3).

6. The road sensor assembly (100) according to any one of the preceding claims, wherein the transmission line (35) of the capacitance probe comprises a shielded coaxial cable.

7. The road sensor assembly (100) according to any one of claims 3 to 6, wherein the transmission line (35) of the capacitance probe has a length equal to an integer multiple of a half wavelength of the sinusoidal signal (s(t)) emitted by the sinusoidal signal generator (70).

8. The road sensor assembly (100) according to any one of the preceding claims, wherein the box (6) of the road sensor has a cylindrical shape open at the top and is made of stainless steel, and the body (60) of the road sensor is made of epoxy resin having a density greater than 1 .5 g / cm3.

9. The road sensor assembly (100) according to any one of the preceding claims, wherein the body (60) of the road sensor comprises a plurality of protrusions (32) with discoidal shape projecting superiorly from the body.

10. The road sensor assembly (100) according to any one of the preceding claims, wherein said capacitance probe has a structure of a planar capacitor andcomprises a first circular metal electrode (30) surrounded by a second metal electrode (31) in the shape of a circular ring or annulus.

Images