Sensing arrangement

Abstract

A heat flux sensor for sensing a heat flux of a target entity. The heat flux sensor comprises: a probe configured to make an electrical connection with a surface of the target entity, the probe comprising first and second probe members which each make an electrical connection with said surface, the first and second probe members defining therebetween a sensing area of said surface; a pulse generator configured to produce a periodic electric signal and supply the electric signal to the target entity via the probe; a controller configured to: determine a current of the electric signal; determine a voltage of the electric signal; determine a frequency of the electric signal; determine a measure of heat flux of said sensing area based on a change in said current, a change in said voltage and the frequency of the electric signal.

Description

[0001] M&C PM365833GB

[0002] 1

[0003] Sensing arrangement

[0004] The present invention relates to a sensing arrangement. In particular, the invention relates to a heat flux sensor for sensing a heat flux of a target entity and an associated method. The sensing arrangement and method may be applied to a braking system.

[0005] Electronic temperature sensors are well-known devices which are used to produce an electronic signal which is indicative of a temperature to which the sensor is exposed.

[0006] One example of a well-known electronic temperature sensor is a thermistor. Thermistors are resistors which undergo a large, predictable change in electrical resistance when exposed to a change in temperature. Positive Temperature Coefficient (PTC) thermistors increase in electrical resistance when exposed to an increase in temperature and Negative Temperature Coefficient (NTC) thermistors decrease in electrical resistance when exposed to an increase in temperature.

[0007] However, temperature sensors only produce an electric signal which is indicative of a temperature to which the sensor is exposed. In some applications it would be desirable to be able to measure the heat flux of a particular surface of a target entity. Heat flux is the rate of heat transfer per unit area through a surface of a target entity, indicating how much heat is flowing through the surface of the target entity at a given time.

[0008] Braking systems for vehicles usually comprise one or more rotors attached to a respective wheel, and one or more respective stators. Frictional engagement (of a rubbing type) between the rotor and the stator produces a braking force to brake the wheel to which the rotor is attached. In disk type braking systems the rotor is a brake disk and the stator is a calliper which urges brake pads into engagement with (a radial surface of) the brake disk to generate the braking force. In shoe type braking systems the rotor is a brake drum and the stator is a shoe arrangement which urges a surface of the shoe into engagement with (a circumferential surface of) the brake drum to generate the braking force.

[0009] Given the frictional engagement which produces the braking force within a braking system, the components of the braking system experience a temperature rise when the brakes are applied. The reduction in kinetic energy of a vehicle when it is braked by a

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[0012] braking system is primarily dissipated as heat, with the energy being shared across all braked wheels on the vehicle.

[0013] Whilst it is understood that measuring the temperature of components of a braking system can be used as a proxy for brake activity, measuring the temperature of a component of a braking system is a function of the aggregated kinetic energy dissipated by the braking system and of and the other mechanical elements in thermal contact with the braking system (e.g. wheel bearings, axle stub). As such, the results of measuring the temperature of components of a braking system to determine brake activity are slow and vague, giving very limited discernment between the efficacy of braking on different wheels of a vehicle and having restricted ability to consider braking events in real time.

[0014] It is an object of the present invention to provide an apparatus and method capable of providing an electronic signal indicative of heat flux of a target entity. Such apparatus and method may be utilised as part of a braking system which obviates or mitigates problems with performance monitoring within known braking systems, whether discussed above or otherwise.

[0015] According to a first aspect of the invention there is provided a heat flux sensor for sensing a heat flux of a target entity, the heat flux sensor comprising: a probe configured to make an electrical connection with a surface of the target entity, the probe comprising first and second probe members which each make an electrical connection with said surface, the first and second probe members defining therebetween a sensing area of said surface; a pulse generator configured to produce a periodic electric signal and supply the electric signal to the target entity via the probe; a controller configured to: determine a current of the electric signal; determine a voltage of the electric signal; determine a frequency of the electric signal; determine a measure of heat flux of said sensing area based on a change in said current, a change in said voltage and the frequency of the electric signal.

[0016] The frequency of the periodic electric signal may be greater than at least one of: about 1kHz, about 5kHz, about 10kHz, about 50kHz, about 100kHz, about 500kHz, about 1 MHz, about 5MHz, about 10MHz, about 50MHz, about 100MHz and about 500MHz.

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[0019] Said determination of a measure of heat flux may be also be based on one of, a combination of, or all of: a temperature coefficient of resistance of the target entity, a resistivity of the target entity, the surface area of the sensing area of said surface, a specific heat capacity of the target entity and a density of the target entity.

[0020] The pulse generator may produce said electric signal at a known current, thereby enabling said determination of the current of the electric signal.

[0021] The heat flux sensor may comprise a four-wire resistance measurement circuit which electrically connects the pulse generator to the probe.

[0022] The pulse generator may be electrically connected in series with an inductor and the current of the electric signal may be determined as a function of a measurement time, said voltage of the electric signal and an inductance of the inductor.

[0023] According to a second aspect of the invention there is provided a braking system comprising a static portion, a movable, braked portion, and a heat flux sensor according to the previous aspect of the invention, wherein the target entity is an element of the static portion or the movable, braked portion.

[0024] According to a third aspect of the invention there is provided a method of measuring a heat flux of a target entity using a heat flux sensor, the heat flux sensor comprising: a probe electrically connected with a surface of the target entity, the probe comprising first and second probe members which are each electrically connected with said surface, the first and second probe members defining therebetween a sensing area of said surface; a pulse generator; and a controller; the method comprising: the pulse generator producing a periodic electric signal; supplying the electric signal to the target entity via the probe; the controller determining a current of the electric signal; the controller determining a voltage of the electric signal; the controller determining a frequency of the electric signal; the controller determining a measure of heat flux of said sensing area based on a change in said current, a change in said voltage and the frequency of the electric signal.

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[0027] The frequency of the periodic electric signal may be greater than at least one of: about 1kHz, about 5kHz, about 10kHz, about 50kHz, about 100kHz, about 500kHz, about 1 MHz, about 5MHz, about 10MHz, about 50MHz, about 100MHz and about 500MHz.

[0028] Said determination of a measure of heat flux may also be based on one of, a combination of or all of: a temperature coefficient of resistance of the target entity, a resistivity of the target entity, the surface area of the sensing area of said surface, a specific heat capacity of the target entity and a density of the target entity.

[0029] The pulse generator may produce said electric signal at a known current, thereby enabling said determination of the current of the electric signal.

[0030] The heat flux sensor may comprise a four-wire resistance measurement circuit which electrically connects the pulse generator to the probe.

[0031] The pulse generator may be electrically connected in series with an inductor and the current may be determined as a function of a measurement time, said voltage and an inductance of the inductor.

[0032] According to a fourth aspect of the invention there is provided a method of measuring a heat flux of a portion of a braking system, wherein the braking system comprises a static portion and a movable portion, and wherein the method comprises the method of the previous aspect of the invention, wherein said target entity is an element of the static portion or the movable portion.

[0033] A detailed description of one or more exemplary embodiments of the invention is provided below with reference to the accompanying drawings, in which:

[0034] Figure 1 is a schematic view of a portion of a braking arrangement which includes a heat flux sensor probe in accordance with an embodiment of the present invention;

[0035] Figures 2 and 3 show schematic perspective and cross-sectional views respectively of a portion of a brake disc and heat sensor probe as shown in the arrangement in Figure 1; and

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[0038] Figure 4 shows a schematic circuit diagram of a heat flux sensor in accordance with an embodiment of the present invention.

[0039] Figure 1 shows a schematic view of a braking arrangement which includes a heat flux sensor probe in accordance with an embodiment of the present invention. The braking arrangement itself is entirely conventional and includes a rotor in the form of a brake disc 12 which is connected to a wheel of a vehicle, and a stator in the form of a brake calliper 14. In use the brake disc 12 rotates (as indicated by the arrow within the Figure) and when braking is required the calliper 14 urges brake pads (not shown) in to contact with the brake disc 12 so as to apply a braking force to the brake disc 12, thereby reducing the speed of rotation of the brake disc 12 and hence of the connected wheel. The urging of the brake pads into contact with the brake disc 12 so as to apply the braking force results in frictional engagement between the brake pads and brake disc which produces heat. The process of braking is entirely conventional and is not critical to the understanding of the present invention - as such further detail regarding the braking process is omitted for the purposes of conciseness.

[0040] A probe 16 of a heat flux sensor in accordance with an embodiment of the present invention is mounted to the brake disc 12. Electrical signals are provided to the probe 16 and obtained from the probe 16 via an electrical cable 18.

[0041] Operation of the heat flux sensor in accordance with an embodiment of the present invention is now discussed in more detail with reference to Figures 2, 3 and 4.

[0042] Figure 2 shows a schematic perspective view of a portion of the brake disc 12 and probe 16 of the heat flux sensor. Figure 3 shows a schematic cross section through said portion of the brake disc shown in Figure 2 and the probe 16. Finally, Figure 4 shows a circuit diagram which illustrates the main electrical components of a heat flux sensor in accordance with an embodiment of the present invention.

[0043] The probe 16 is configured to make an electrical connection with a surface 12a of the brake disc 12. The probe 16 comprises a first probe member 16a and a second probe member 16b which each make an electrical connection with the surface 12a. The first and second probe members 16a, 16b define between them a sensing area 18 of the surface 12a. Within Figure 2, the sensing area 18 is bounded by inner edges of the

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[0046] first and second probe members 16a, 16b and, to aid clarity, upper and lower dashed lines. The sensing area 18 is also indicated within Figure 2 by the presence of hatching.

[0047] Electrical signals are provided to and obtained from the first and second probe members via wires 18a and 18b respectively.

[0048] Referring now to Figure 4, a pulse generator 20 is electrically connected to the probe 16 (and in particular the first and second probe members 16a, 16b of the probe 16) such that the pulse generator can produce a periodic electric signal and supply said electric signal to the target entity 12 via the probe 16. A controller (indicated schematically by the dashed box 22) is configured to determine a current of the electrical signal and determine a voltage of the electrical signal.

[0049] In the embodiment shown in Figure 4, the determination of the current of the electric signal is represented schematically by the presence of ammeter 24 and the determination of the voltage of the electric signal is represented schematically by the voltmeter 26.

[0050] The controller 22 is further configured to determine a frequency of the electric signal and to determine a measure of heat flux of the sensing area 18 of the target entity 12 based on a change in said current, a change in said voltage and the frequency of the electric signal. The way in which the controller determines a measure of heat flux of the sensing area is discussed in more detail further below within the present specification.

[0051] As seen in the schematic circuit diagram shown in Figure 4, the heat flux sensor comprises a 4-wire resistance measurement circuit which electrically connects the pulse generator 20 to the probe 16. The four wires used as part of the 4-wire resistance measurement circuit are indicated by 28a, 28b, 28c and 28d. Each of the four wires 28a-28d has its own, relatively low, resistance which is indicated within the Figure as 30a-30d respectively - that is to say the resistors 30a-30d in the Figure do not exist as separate entities, they are merely indicative of the resistance inherently present in each of the wires.

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[0054] A 4-wire resistance measurement circuit (which is also sometimes known as a Kelvin resistance measurement circuit) is well known in the art as an accurate way of measuring resistance. Full details of the operation of the 4-wire measurement circuit are omitted for the purpose of conciseness. However, it is noted that the power supply 20 is connected (via the first and second probe members 16a, 16b) across the target entity 12 by the first and second wires 28a, 28d. These wires are sometimes known as Force wires. Current flowing through the first wire (between the power supply 20 and target entity 12) is measured (as represented schematically by the ammeter 24) at the first wire 28a. The voltage across the target entity 12 is measured (via first and second probe members 16a, 16b) via the second and third wires 28b, 28c. These wires are sometimes known as Sense or Kelvin wires. The measurement of the voltage between the second and third wires 28b, 28c is represented schematically by the voltmeter 26.

[0055] The heat flux sensor further includes an inductor 32 connected in series with the pulse generator 20. As discussed in more detail further below, the current of the electrical signal maybe determined as a function of a measurement time, the voltage of the electrical signal and an inductance of the inductor. It will be appreciated that, in the present embodiment, the inductor is utilised in order to help determine the current of the electrical signal (as a function of time, voltage and inductance). However, in other embodiments any appropriate way of determining the current of the electrical signal may be used. For example, the current of the electrical signal may be measured by the controller or another component, or the current of the electrical signal may be set by the controller or another component.

[0056] Within the embodiment of the invention described above, the first and second probe members may take any appropriate form. What is critical is that the first and second probe members each make an electrical connection with the surface of the target entity. In the case where the target entity is moving whilst the heat flux sensor is in use, the first and second probe members allow an electrical connection with the surface of the target entity whilst the target entity is moving. In one embodiment, the first and second probe members are each formed generally of a strip of metal. The strip of metal has a surface, a portion of which is generally planar, and which contacts the surface of the target entity. The metal strip is also bent so that the strip itself provides a resilient biasing force, which urges the metal strip into contact with the surface of the target entity.

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[0059] General structure and components making up a heat flux sensor in accordance with the present invention have now been described. What follows below is an example calculation which shows how a heat flux sensor according to the present invention may be used to determine the measure of heat flux of a target entity.

[0060] Referring once more to the circuit shown in Figure 4, the pulse generator 20 (which may also be referred to as the power source of the circuit) produces an electric signal of known voltage Vk. The electric signal is a transient voltage with a frequency fw, whose current changes at a rate di / dt (also referred to as irate). At a measurement time tmeas, there are two instantaneous signals of interest, measured Voltage Vmeas and determined or measured current Imeas-

[0061] The current imeas is the drive current through the first and second probe members 16a, 16b (and hence through a portion of brake disk 12 between the first and second probe members 16a, 16b). The current imeas is driven by the pulse generator 20 and flows via the first and fourth wires 28a, 28d. Measurement or determination of this current is represented by the ammeter 24.

[0062] The determined or measured voltage Vmeas is the voltage developed across the first and second probe members 16a, 16b, as measured via the second and third wires 28b, 28c. Measurement or determination of this voltage is represented by the voltmeter 26.

[0063] The current imeas may either be measured by means of Analogue to Digital Converter or calculated from a known current rate of rise D (with units A / s (Amps per second)).

[0064] If the pulse generator 20 circuit applies a voltage Vk, through the inductor 32, having inductance L, then (assuming zero circuit resistance)

[0065] D = Vk / L (1)

[0066] By way of example, if Vk is 12V, and L is 75pH, then D is 160,000 A / s.

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[0069] The determined current imeas is related to current rate of rise D and the time tmeas after which the voltage is applied as:

[0070] imeas=D . tmeas (2)

[0071] By way of example at a time 10ps after the voltage is applied, the current imeas is determined to be 1.6A.

[0072] Alternatively, the current may be measured directly as imeas.

[0073] At the surface 12a of the brake disk 12, the first and second probe members 16a, 16b are spaced apart from one another (in a direction generally parallel to the direction of current flow / electron movement between the first and second probe members) by a contact separation distance Kd. Likewise, at the surface 12a of the brake disk 12, the width of the first and second probe members 16a, 16b (in a direction generally perpendicular to the direction of current flow / electron movement between the first and second probe members) is a contact width Kw.

[0074] The effect of these dimensions is to define a surface area Kaof the sensing area 18.

[0075] Ka= Kw. Kd(3)

[0076] By way of example, if Kwis 11.5 mm and Kd is 15 mm, then Kais 1.725 E-04 m2.

[0077] The current of the electrical signal in the brake disk disc flows in accordance with the known skin effect. This means that the electrons travel through the surface 12a of the brake disk 12 primarily near the surface, to a depth defined by the skin effect.

[0078] The transient electric signal produced by the pulse generator has a frequency fw, which determines a signal skin depth Ks, within the brake disk, between the first and second probe members. The signal skin depth Ks is generally perpendicular to the surface 12a of the brake disk 12.

[0079] For a given material of brake disk

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[0082] Ks = > / ( 2 . p / (2 . TT . fw . |Jr. |Jo)) (4)

[0083] where: p is the resistivity of the material at frequency fw, pris the relative permeability of the material at frequency fwand o is the permeability of free space.

[0084] By way of example, if the brake disk is formed from steel, at a frequency of 1 E06 Hz, p is 14.3 E-08 Qm and pris 100. This results in a signal skin depth Ksof 1.903 E-05 m.

[0085] The creation of a system with known skin depth lies at the heart of the invention, whereby the very low but finite mass of the sensing volume (discussed below) may be defined. The sensing element so defined lacks any hindrance to heat flow and has homogeneity with the remaining material. Such arrangement enables more intimate contact than would be the case by introducing foreign temperature detecting materials as part of the sensor. Put another way, the present invention utilises the material of the target entity itself as part of the heat flux sensor. This improves accuracy of the sensor by eliminating the need for a sensor portion, separate to the target entity, which needs to be thermally coupled to the target entity.

[0086] The signal skin depth Ksand surface area Kaof the sensing area 18 define a sensing volume Kvoi of the target entity as follows:

[0087] Kvol = Ka. Ks (5)

[0088] Using the example values above, Kvoiis 3.283 E-09 m3.

[0089] In order to determine a resistance Ro of the volume of the target entity between the first and second probe members 16a, 16b and as defined by the skin depth of the target entity, this can be approximated as:

[0090] Ro = (p . Kd) / (Kw. Ks) (6)

[0091] Using the example values above, Ro = 9.801 E-03 Q.

[0092] This is the base resistance at a single point in time at 20°C.

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[0095] However, when the brake is operating the brake disk will be subject to rubbing friction, applying heat energy.

[0096] The resistance R of the target entity is related to the temperature T of the target entity as follows:

[0097] R = Ro . [1 + a . (T - To)] (7)

[0098] where a is the temperature coefficient of resistance of the material of the target entity, and To is the temperature at which the resistance of the target entity is Ro - in this case 20°C. (T - To) may be expressed as AT (the temperature change of the area of the target entity) and the expression above can be reformulated as:

[0099] AT = ((R / Ro) - 1) / a (8)

[0100] The resistance of the portion of the target entity between the first and second probe members can be determined as a function of the determined current and the determined voltage in accordance with Ohm's law:

[0101] R = V / 1 (9)

[0102] Using expression 8, a determined R of 15 mQ and assuming that the brake disk is formed of steel, having an a of 0.003, then AT is about 177°C.

[0103] The heat flux q transferred through the target entity can then be determined as follows:

[0104] q = AT. (SHC . Kmass) / Ka(10)

[0105] where SHC is the specific heat capacity of the material of the target entity and Kmass is the mass of the sensing volume Kvoigiven by:

[0106] Kmass=CP ■ Kvol (11)

[0107] where (p is the density of the material of the target entity.

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[0110] Given the density (p of steel is 7.85 E3 kg / m3and the specific heat capacity SHC of steel is 420 J / kg °C, this gives a heat flux q of 11.07 E03 W / m2.

[0111] If it is assumed that the heat flux of the sensing area is approximately the same for the whole surface area of the target entity then the total heat power Ptot emitted by the target entity is approximated by:

[0112] Ptot = q . SA (12)

[0113] where SA is the total surface area of the target entity. In an example of brake disc having an outside diameter of 430 mm and an inside diameter of 220 mm, ignoring the thickness of the disk, the total surface area of the disc is approximated as 0.214m2. It follows that, in this example the total heat power Ptot emitted by the target entity is approximately 2.37 kW.

[0114] The sensor above can be used to measure the heat flux of a surface of a portion of a braking system. As previously discussed, the measurement of heat flux in certain applications is advantageous as compared to the measurement of temperature.

[0115] Heat flux is a measure of energy being transferred at an instant in time for a given area. This area would ideally have:

[0116] • an infinitely small depth

[0117] • no thermal mass

[0118] • intimate contact with the remainder of the material body being heated

[0119] Utilising the skin depth effect, and using a relatively high frequency of electric measurement signal, a very small dimension of depth results in a very low, (approaching zero), thermal mass. Since the sensing current travels in the surface of the material body, the intimate contact of the sensing area / volume with the rest of the material body is assured.

[0120] It is therefore important to understand that the instantaneous temperature recorded for the sensing area as part of the method of measuring heat flux is not the same as a regular temperature reading. If a temperature detector (e.g. RTD, thermistor or thermocouple) were to be used the following would apply:

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[0123] • The detector material would not be homogenous with the body being heated, although a level of intimate contact might be created

[0124] • The detector would be in the direct path of the heat flow, thus disturbing it

[0125] • The detector would have thermal mass, even if this could be minimised

[0126] A temperature detector that is inserted in the body of the material could provide an aggregate of total energy, which is not the same as the rate of flow. A detector that had thermal lag or its own thermal capacity would give an integral form of the energy transfer, but the transient detail of the energy transfer would be lost.

[0127] On this basis, the instantaneous temperature of a given area as part of the present invention must be understood in the context of thermal flux (power transfer / area) rather than equated with a regular measurement of temperature which is aggregated over time.

[0128] The heat flux sensor above can be used to determine the heat flux of a surface of a brake disk. This can provide quantitative information about braking activity of the brake disk and its respective calliper.

[0129] However, the heat flux sensor may also be used to qualitatively determine brake performance of separate wheels of a vehicle by comparing the heat flux measurements made at each wheel - this may be used to determine under or over performing brakes.

[0130] A typical brake application follows the profile set out below, which compares surface temperature of the brake disk and heat flux for the brake disk:

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[0133]

[0134] In light of the above, the proposed invention not only provides an apparatus and method for measuring heat flux, but also enables, in a particular application, both quantitative and qualitative measurement of performance of a braking system, and, in particular, separate wheels of a braking system. The results of this process can identify sticking brakes, ineffective brakes and give a meaningful value of actual brake performance during a braking event whilst in situ on the vehicle.

[0135] One benefit of measuring heat flux in the manner discussed above is that, as skin depth is generally very small for frequencies in the megahertz range, the sensing volume is generally small and hence the thermal mass of the sensing volume is also small. This means that the sensing volume is able to change temperature relatively rapidly, such that it is sensitive to changes in temperature of the brake disk during a braking operation or thereafter.

[0136] For completeness, it should be mentioned that, when the heat flux sensor discussed above is used to detect the heat flux of the brake disk, the brake disk will be moving. As such, the sensing volume Kvoi discussed above is a virtual element within the brake disk which lies beneath the probe and is swept by the rotating disk.

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[0139] The present embodiment of the invention relates to a braking system (or braking arrangement) 10 in which the target entity is a moveable, braked portion of the braking system (i.e. the brake disc 12). However, in other embodiments, the target entity may not be a moveable braked portion of the braking system - instead, it may be a static portion of the braking system, such as (part of) the brake calliper 14.

[0140] In the above-discussed embodiment, the heat flux sensor is configured to sense a heat flux of a target entity in the form of a brake disc of a braking system. However, the heat flux sensor may be used to sense a heat flux of any appropriate target entity. Such target entities may be static or moving. In addition, such target entities may be solid or liquid. Examples of other potential target entities for the present invention are molten or solid steel during steel manufacture, molten or solid glass during glass manufacture or mixtures which occur during the manufacture of food - for example, yoghurt, marmalade, jam or soup.

[0141] Within the discussion above, mention is made of determining various quantities - for example: determining a current of the electric signal, determining a voltage of the electric signal, and determining a frequency of the electric signal. It will be appreciated that such determination covers both a measurement of the relevant property or a setting of the relevant property to a particular value. Such measurement or setting of the relevant property may be carried out by the controller directly or may be carried out indirectly by one or more separate components which are in communication with the controller.

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Claims

M&C PM365833GB16CLAIMS:

1. A heat flux sensor for sensing a heat flux of a target entity, the heat flux sensor comprising:a probe configured to make an electrical connection with a surface of the target entity, the probe comprising first and second probe members which each make an electrical connection with said surface, the first and second probe members defining therebetween a sensing area of said surface;a pulse generator configured to produce a periodic electric signal and supply the electric signal to the target entity via the probe;a controller configured to:determine a current of the electric signal;determine a voltage of the electric signal;determine a frequency of the electric signal;determine a measure of heat flux of said sensing area based on a change in said current, a change in said voltage and the frequency of the electric signal.

2. A heat flux sensor according to claim 1, wherein said determination of a measure of heat flux is also based on one of, a combination of, or all of: a temperature coefficient of resistance of the target entity, a resistivity of the target entity, the surface area of the sensing area of said surface, a specific heat capacity of the target entity and a density of the target entity.

3. A heat flux sensor according to claim 1 or claim 2, wherein the pulse generator produces said electric signal at a known current, thereby enabling said determination of the current of the electric signal.

4. A heat flux sensor according to any preceding claim, wherein the heat flux sensor comprises a four-wire resistance measurement circuit which electrically connects the pulse generator to the probe.

5. A heat flux sensor according to any preceding claim wherein the pulse generator is electrically connected in series with an inductor and wherein the current of the electric signal is determined as a function of a measurement time, said voltage of the electric signal and an inductance of the inductor.69636459-1M&C PM365833GB176. A braking system comprising a static portion, a movable, braked portion, and a heat flux sensor according to any proceeding claim, wherein the target entity is an element of the static portion or the movable, braked portion.

7. A method of measuring a heat flux of a target entity using a heat flux sensor, the heat flux sensor comprising:a probe electrically connected with a surface of the target entity, the probe comprising first and second probe members which are each electrically connected with said surface, the first and second probe members defining therebetween a sensing area of said surface;a pulse generator; anda controller;the method comprising:the pulse generator producing a periodic electric signal;supplying the electric signal to the target entity via the probe;the controller determining a current of the electric signal;the controller determining a voltage of the electric signal;the controller determining a frequency of the electric signal;the controller determining a measure of heat flux of said sensing area based on a change in said current, a change in said voltage and the frequency of the electric signal.

8. A method of measuring a heat flux according to claim 7, wherein said determination of a measure of heat flux is also based on one of, a combination of or all of: a temperature coefficient of resistance of the target entity, a resistivity of the target entity, the surface area of the sensing area of said surface, a specific heat capacity of the target entity and a density of the target entity.

9. A method of measuring a heat flux according to claim 7 or claim 8, wherein the pulse generator produces said electric signal at a known current, thereby enabling said determination of the current of the electric signal.69636459-1M&C PM365833GB1810. A method of measuring a heat flux according to any of claims 7 to 9, wherein the heat flux sensor comprises a four-wire resistance measurement circuit which electrically connects the pulse generator to the probe.

11. A method of measuring a heat flux according to any of claims 7 to 10, wherein the pulse generator is electrically connected in series with an inductor and wherein the current is determined as a function of a measurement time, said voltage and an inductance of the inductor.

12. A method of measuring a heat flux of a portion of a braking system, wherein the braking system comprises a static portion and a movable portion, and wherein the method comprises the method of any of claims 7 to 11, wherein said target entity is an element of the static portion or the movable portion.69636459-1

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