GB2533920A - Circuit and method for measuring a resistance value of a glow plug - Google Patents

Abstract

A method and circuit for measuring a resistance of a glow plug 360, in order to determine its temperature, which comprise connecting a shunt resistor 510 in series to the glow plug to create a voltage divider 600, applying a voltage to the free end of the shunt resistor, grounding the free end of the glow plug, monitoring alternatively the voltage drop on the glow plug and the voltage drop on the shunt resistor, integrating over time the first and the second voltage signal obtaining an output signal, using the output signal for generating a pulse width modulated (PWM) signal having a high value and low value, and determining the resistance value of the glow plug using the value of the shunt resistor and the variation over time of the PWM signal. The measurement is taken while the glow plug is in situ in a diesel engine.

Description

Intellectual Property Office Application No. GII1423058.5 RTM Date:9 June 2015 The following terms are registered trade marks and should be read as such wherever they occur in this document: WiFi Intellectual Property Office is an operating name of the Patent Office www.gov.uk/ipo

S CIRCUIT AND METHOD FOR MEASURING A RESISTANCE VALUE

OF A GLOW PLUG

TECHNICAL FIELD

The present disclosure relates to a circuit and a method for measuring the resistance of a ceramic glow plug, in order to determine an actual operating temperature value of it. BACKGROUND It is known that Diesel engine are provided with ceramic or metallic glow plugs for allowing cold-start of the engine and for optimizing combustion performance during engine operation.

Glow plugs are located in a combustion chamber of the engine and are electrically connected to a battery of the vehicle by means of an electric switch controlled and drived by an ECU.

During operation of the engine the temperature of glow plugs must be accurately controlled to keep the operating temperature in a predetermined operating range. Indeed, an excessive heating of the glow plugs can stress them, shortening their service life, while a too low temperature can lead to an increase of polluting substances in the environment.

The temperature of a ceramic glow plug is controlled by monitoring its electrical resistance since the temperature depends on the electrical resistance of the glow plug. According to a known prior art the electrical resistance of a glow plug is determined by measuring directly both current and voltage of the glow plug and calculating the resistance by means of the Ohm law.

The measurement precision of the electric current is strictly depending on the technical implementation and usually is insufficient to be compliant with the accuracy requirements, therefore a compensation al the end of the line of production is needed, which increases the production cost, In any case the determination of the electrical resistance of a glow plug made according to the prior art can have, as a optimistic case, a tolerance equal or greater of the 4%, which involves a temperature tolerance of ± 60° C, considering a glow plug working temperature of 1250°C.

In view of the above, an object of an embodiment of the present invention is to improve the resolution accuracy of the determination of a resistance value of a ceramic glow plug. 15 Another object is that of accomplish the above-mentioned goals with a simple, rational and rather inexpensive solution.

SUMMARY

These and other objects are achieved by the embodiments of the invention as defined in the independent claims. The dependent claims includes preferred and/or advantageous 20 aspects of said embodiments.

More particularly, an embodiment of the invention provides a method of measuring a resistance value of a glow plug which provides for: a) connecting a shunt resistor, having a known resistance value, in series to the glow plug to create a voltage divider, b) connecting a free end of the shunt resistor with a voltage power source and a free end of the glow plug to a ground pole, c) supplying a first voltage value to the free end of the shunt resistor, d) monitoring alternatively a first and a second voltage signal indicative respectively of a voltage drop value on the glow plug and on the shunt resistor, e) integrating over time the first and the second voltage signal obtaining an output signal, f) using the output signal for generating a pulse width modulated signal having a high value and low value, g) determining the resistance value of the glow plug on the basis of the value of the shunt resistance and of a variation over time of the pulse width modulated signal.

Thanks to this solution, it is possible to obtain a high monitoring resolution of the glow plug resistance value being the measured resistance value depending on the tolerances of the shunt resistor. The error in determining the resistance of the glow plug, using the method above, can be of the order of 1%.

According to an aspect of the invention, the resistance value of the glow plug is determined by the equation: Itoff RGP RsH X (K2) X) on wherein RGp is the resistance value of the glow plug, RsH is the resistance value of the shunt resistor, ton is a time interval wherein the pulse width modulated signal has the high value, ton is a time interval wherein the pulse width modulated signal has the low value, K1 and K2 are a first and a second predetermined resistance value.

This aspect if the invention provides a reliable and easy way to determine a resistance value of the glow plug.

According to an aspect of the invention, the phase of monitoring alternatively a first and a second voltage signal is performed at an oscillating frequency of the pulse width modulated 25 signal.

This aspect if the invention provides a reliable and economic way for monitoring the first and the second voltage signal.

According to an aspect of the invention, the oscillating frequency of the pulse width modulated signal is calculated by means of the following equation: VBAT Rep K3 * K4 * RsH f f VL Rep + RsH RsH * K3 + Rep * K4 wherein f is the oscillating frequency of the pulse width modulated signal, RGp is the resistance value of the glow plug, RsH is the resistance value of the shunt resistor, VBAT is the power source voltage, VH and 17 are voltage values, K3 and K4 are constant predetermined coefficients.

This aspect of the invention provides a reliable and easy way to determine the oscillating frequency of the pulse width modulated signal.

According to an aspect of the invention, the constant predetermined coefficients k3 and 1(4 are chosen in order to have a duty cycle of the of the pulse width modulated signal VPWM 15 closed to 50%.

This aspect of the invention allows a good accuracy of the measure of the resistance value of the glow plug.

An aspect of the invention provides to determine a plurality of resistance values of the glow plug and then to calculate an arithmetic mean value.

This aspect of the invention allows to minimize measurement errors due to alternating component of eventual ground signals.

A different embodiment of the invention provides for a circuit for measuring a resistance value of a glow plug provided with a first end connected to a ground pole, the circuit comprising a shunt resistor connected, at a common node, to the glow plug for creating an 25 electric divider, and having a free end connectable to a voltage power source, a pulse width modulated signal generator circuit for generating a pulse width modulated signal, having a first input connected to the common node and a second input connected to an electronic switch, controlled by the pulse width modulated signal, and configured for monitoring alternatively a first and a second voltage signal indicative respectively of a voltage drop value on the glow plug and on the shunt resistor, a digital logic unit being connected to an output of the pulse width modulated signal generator circuit.

This circuit has the advantage to allow ratiometric measures, i.e. the measured values of the resistance of the glow plug are independent from the voltage value supplied to the circuit. Thanks to this circuit the measure of the resistance of the glow plug is performed 10 with a very high accuracy depending on the shunt resistor tolerance.

Furthermore, this circuit does not influence the normal operation of supplying voltage to the glow plug.

According to an aspect of the invention, the pulse width modulated generator circuit comprises a voltage integrator and a commutator with input hysteresis.

This solution allows to create the pulse width modulated generator circuit in a cheap way.

According to an aspect of the invention, between the free end of the glow plug and the electric switch is interposed a resistor, and between the free end of the shunt resistor and the electric switch is interposed a resistor.

Thanks to this solution it is possible to choose the electrical resistance value of the two 20 resistors so to have a high accuracy of the measure of the electrical resistance of the glow plug.

According to an aspect of the invention, the voltage integrator comprises an amplifier and a capacitor which is connected between an output and an input of the amplifier.

This aspect of the invention allows, in combination with the previous aspect of the invention 25 to improve the accuracy of the measure of the electrical resistance of the glow plug.

According to an aspect of the invention, the free end of the shunt resistor is connectable to the voltage power source by means of an electric switch.

This aspect of the invention provides a reliable and economic way to connect the shunt resistor with the voltage power source The method of the invention can be carried out with the help of a computer program comprising a program-code for carrying out all the steps of the method described above, and in the form of a computer program product comprising the computer program. The method can be also embodied as electromagnetic signals, said signal being modulated to carry a sequence of data bits which represent a computer program to carry out all steps of the method.

Another embodiment of the invention provides an engine comprising a glow plug connected in series to a shunt resistor, having a known resistance value, a free end of the shunt resistor being connected with a vokage power source and a free end of the glow plug being connected with a ground pole, and an electronic control unit configured to: -supply a first voltage value to the free end of the shunt resistor, -monitor alternatively a first and a second voltage signal indicative respectively of a voltage drop value on the glow plug and on the shunt resistor, -integrate over time the first and the second voltage signal obtaining an output signal, -use the output signal for generating a pulse width modulated signal having a high value and low value, -determine the resistance value of the glow plug on the basis of the value of the shunt resistor and of a variation over time of the pulse width modulated signal.

Another embodiment of the invention provides an automotive system comprising an engine provided with a glow plug connected in series to a shunt resistor, having a known resistance value, a free end of the shunt resistor being connected with a voltage power source and a free end of the glow plug being connected with a ground pole, - means for supplying a first voltage value to the free end of the shunt resistor, - means for monitoring alternatively a first and a second voltage signal indicative respectively of a voltage drop value on the glow plug and on the shunt resistor, 5 -means for integrating over time the first and the second voltage signal obtaining an output signal, -means for using the output signal for generating a pulse width modulated signal having a high value and low value, - means for determining the resistance value of the glow plug on the basis of the value of the shunt resistor and of a variation over time of the pulse width modulated signal.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will now be described, by way of example, with reference to the accompanying drawings.

Figure 1 schematically shows an automotive system belonging to a motor vehicle.

Figure 2 is the section A-A of an internal combustion engine belonging to the automotive system of figure 1.

Figure 3 is a scheme of a circuit for measuring a resistance value of a glow plug according to an embodiment of the invention.

Figure 4a, 4b, 4c show the variation over time of some signals used in an embodiment 20 invention.

DETAILED DESCRIPTION

Some embodiments may include an automotive system 100, as shown in Figures 1 and 2, that includes an internal combustion engine (ICE) 110 having an engine block 120 defining at least one cylinder 125 having a piston 140 coupled to rotate a crankshaft 145. A cylinder 25 head 130 cooperates with the piston 140 to define a combustion chamber 150.

A fuel and air mixture (not shown) is disposed in the combustion chamber 150 and ignited, resulting in hot expanding exhaust gasses causing reciprocal movement of the piston 140. The fuel is provided by at least one fuel injector 160 and the air through at least one intake port 210. The fuel is provided at high pressure to the fuel injector 160 from a fuel rail 170 in fluid communication with a high pressure fuel pump 180 that increase the pressure of the fuel received from a fuel source 190. Each of the cylinders 125 has at least two valves 215, actuated by a camshaft 135 rotating in time with the crankshaft 145. The valves 215 selectively allow air into the combustion chamber 150 from the port 210 and alternately allow exhaust gases to exit through a port 220. In some examples, a cam phaser 155 may selectively vary the timing between the camshaft 135 and the crankshaft 145.

In the combustion chamber 150 is located a glow plug 360 which is a heating element which is electrically activated for cold starting of the engine and also for improving the combustion performance within the combustion chamber.

The air may be distributed to the air intake port(s) 210 through an intake manifold 200. An air intake duct 205 may provide air from the ambient environment to the intake manifold 200. In other embodiments, a throttle body 330 may be provided to regulate the flow of air into the manifold 200. In still other embodiments, a forced air system such as a turbocharger 230, having a compressor 240 rotationally coupled to a turbine 250, may be provided. Rotation of the compressor 240 increases the pressure and temperature of the air in the duct 205 and manifold 200. An intercooler 260 disposed in the duct 205 may reduce the temperature of the air. The turbine 250 rotates by receiving exhaust gases from an exhaust manifold 225 that directs exhaust gases from the exhaust ports 220 and through a series of vanes prior to expansion through the turbine 250. This example shows a variable geometry turbine (VGT) with a VGT actuator 290 arranged to move the vanes to alter the flow of the exhaust gases through the turbine 250. In other embodiments, the turbocharger 230 may be fixed geometry and/or include a waste gate.

The exhaust gases exit the turbine 250 and are directed into an exhaust system 270. The exhaust system 270 may include an exhaust pipe 275 having one or more exhaust aftertreatment devices 280. The aftertreatment devices may be any device configured to 5 change the composition of the exhaust gases. Some examples of aftertreatment devices 280 include, but are not limited to, catalytic converters (two and three way), oxidation catalysts, lean NOx traps, hydrocarbon adsorbers, selective catalytic reduction (SCR) systems, and particulate filters. Other embodiments may include an exhaust gas recirculation (EGR) system 300 coupled between the exhaust manifold 225 and the intake 10 manifold 200. The EGR system 300 may include an EGR cooler 310 to reduce the temperature of the exhaust gases in the EGR system 300. An EGR valve 320 regulates a flow of exhaust gases in the EGR system 300.

The automotive system 100 may further include an electronic control unit (ECU) 450 in communication with one or more sensors and/or devices associated with the ICE 110. The ECU 450 may receive input signals from various sensors configured to generate the signals in proportion to various physical parameters associated with the ICE 110. The sensors include, but are not limited to, a mass airflow and temperature sensor 340, a manifold pressure and temperature sensor 350, a combustion pressure sensor that may be integral within the glow plugs 360, coolant and oil temperature and level sensors 380, a fuel rail pressure sensor 400, a cam position sensor 410, a crank position sensor 420, exhaust pressure and temperature sensors 430, an EGR temperature sensor 440, and an accelerator pedal position sensor 445. Furthermore, the ECU 450 may generate output signals to various control devices that are arranged to control the operation of the ICE 110, including, but not limited to, the fuel injectors 160, the throttle body 330, the EGR Valve 320, the VGT actuator 290, and cam phaser 155 and the glow plug 360. Note, dashed lines are used to indicate communication between the ECU 450 and the various sensors and devices, but some are omitted for clarity.

Turning now to the ECU 450, this apparatus may include a digital central processing unit (CPU) in communication with a memory system and an interface bus. The CPU is 5 configured to execute instructions stored as a program in the memory system 460, and send and receive signals to/from the interface bus. The memory system 460 may include various storage types including optical storage, magnetic storage, solid state storage, and other non-volatile memory. The interface bus may be configured to send, receive, and modulate analog and/or digital signals to/from the various sensors and control devices. 10 The program may embody the methods disclosed herein, allowing the CPU to carryout out the steps of such methods and control the ICE 110.

The program stored in the memory system 460 is transmitted from outside via a cable or in a wireless fashion. Outside the automotive system 100 it is normally visible as a computer program product, which is also called computer readable medium or machine readable medium in the art, and which should be understood to be a computer program code residing on a carrier, said carrier being transitory or non-transitory in nature with the consequence that the computer program product can be regarded to be transitory or non-transitory in nature.

An example of a transitory computer program product is a signal, e.g. an electromagnetic signal such as an optical signal, which is a transitory carrier for the computer program code. Carrying such computer program code can be achieved by modulating the signal by a conventional modulation technique such as OPSK for digital data, such that binary data representing said computer program code is impressed on the transitory electromagnetic signal. Such signals are e.g. made use of when transmitting computer program code in a wireless fashion via a WiFi connection to a laptop.

In case of a non-transitory computer program product the computer program code is embodied in a tangible storage medium. The storage medium is then the non-transitory carrier mentioned above, such that the computer program code is permanently or non-permanently stored in a retrievable way in or on this storage medium. The storage medium can be of conventional type known in computer technology such as a flash memory, an Asic, a CD or the like.

Figure 3 illustrates a circuit 500 for measurIng a resistance value Rep of the glow plug 360.

The glow plug 360 is connected, at a common node 515, in series to a shunt resistor 510 10 having a known resistance value RSH in order to make a voltage divider 600.

A free end 365 of the glow plug 360 is connected to a ground pole 370 of the engine block 120, while a free end (terminal) 520 of the shunt resistor 510 is connected to a voltage power source 530, providing voltage value VBAT, by means of a switch 540 controlled by the ECU 450.

According to an embodiment of the invention the voltage power source 530 is a battery of the vehicle.

The switch 540 can be a MOS-FET transistor having a drain pole connected to the voltage power source 530, a gate pole connected to the ECU 450, and a source pole connected to the shunt resistor 510.

The divider 600, at the common node 515 of the glow plug 360 and of the shunt resistor 510, is connected to a first input of a generator pulse width modulated circuit 700, which generates a pulse width modulated signal VPWM.

A second input of the generator pulse width modulated circuit 700 is connected to an electric switch 570 which alternatively connects, to said second inputs, the free end 365 25 of the glow plug 360 and the free end 520 of the shunt resistor 510. *11

Between the free end 520 of the shunt resistor 510 and the electric switch 570 is interposed an electrical resistor 580 having a first resistance value k1, while between the free end 365 of the glow plug 360 and the electric switch 570 is interposed an electrical resistor 550 having a second resistance value kz.

As told above, the electric switch 570 alternatively switches between the free end 520 of the shunt resistor 510 and the free end 365 of the glow plug 360 for monitoring alternatively a first and a second voltage signal Vi, Vz indicative respectively of a voltage drop value on the glow plug 360 and on the shunt resistor 510.

In detail, the first and the second voltage signal Vi, V2 can be represented by means of the following formulas: Vi= (Vo-Vup) V2= (Vo-Vomni) wherein the Vo is the voltage value at the common node 515, Vup is the voltage value at the free end 520 of the shunt resistor 510, Vowm is the voltage value at the free end 365 of 15 the glow plug 360.

The frequency of switching of the electric switch 570 is controlled by means of the pulse width modulated signal Vpwm, as it will be disclosed in the following.

According to the disclosed embodiment, the pulse width modulated circuit 780 comprises a voltage integrator 710 which calculate the integral over time of the first and the second voltage signal Vi, V2 obtaining an output voltage signal VINTOUT, Fig. 4a illustrates the variation over time of the voltage signal indicative of the first and the second voltage signal V1, V2, while Fig. 4b illustrates the variation over time of the output voltage signal VINT,OUT* The voltage integrator 710 comprises an amplifier 715 and a capacitor 730, having a 25 capacity value C, which is connected between an output and an input of the amplifier 715.

According another embodiment, the voltage integrator 710 may also comprise the resistors 550, 580.

The output of the voltage integrator 710 is also connected to an input of a commutator 720, with input hysteresis, which commutates between a high voltage value VH and a low voltage value VL respectively generating the pulse width modulated signal VpWM commutating between a low value, equal to 0, and a high value, equal to 1.

In detail, when the voltage value of the output signal VINT,OUT exceeds the high voltage threshold value VH the commutator 720, with input hysteresis, switches the pulse width modulated signal Vpwm value from the high value to the low value. On the contrary, when the voltage value of the output signal VINT,con falls below the voltage threshold value VL the commutator 720, with input hysteresis, switches the pulse width modulated signal Vpwm value from the low value to the high value, as illustrated in Fig. 4c.

In the present embodiment of the invention the commutator 720, with input hysteresis, 8 is a Schmidt trigger.

An output of the commutator 720, with input hysteresis, is connected to the electric switch 570, realizing a close-loop feed-back of the pulse width modulated signal Vpwm, which is used as control signal to control the switching of the electric switch 570. The output of the commutator 720 is also connected to a digital logic unit 800 which determines the resistance value Rcp of the glow plug 360 on the basis of the resistance value RsH of the shunt resistor 510 and of a variation over time of the pulse width modulated signal Vpwm. In detail, the digital logic unit 800 determines the resistance value RpG of the glow plug 360 by means of the following formula: R" = R" x (172;1( )x (=fit \ (-on wherein RGp is the resistance value of the glow plug, RsH is the resistance value of the shunt resistor, ton is an time interval wherein the pulse width modulated signal has the high value, toff is a time interval wherein the pulse width modulated signal has the low value, IC, and K2 are the first and the second predetermined resistance values respectively of the resistor 580, 550.

The frequency of switching of the electric switch 570 is equal to the frequency of the pulse width modulated signal VpwM, which can be determined by means of the following formula: VBAT RCP K3 * K4 * RsH f VH - RGp RsH RsH * K3 + RGp * Rit wherein f is the oscillating frequency of the pulse width modulated signal, RGp is the resistance value of the glow plug, RsH is the resistance value of the shunt resistor, VBAT is the power source voltage, VH and FL are voltage values, IC3 and 1(4 are constant predetermined coefficients.

In detail 1(3 and IC, are determined according to the fomulas: K3 = K1 X C K4 -K2 X C wherein lc, is the resistance value of the resistor 580, IQ is the resistance value of the resistor 550, and C is the capacity of the capacitor 730.

According to a preferred embodiment, the values k3 and Ics are chosen in order to have a duty cycle of the of the pulse width modulated signal Vpwm equal to 50%, which is a value that allows a good accuracy of the measure.

Furthermore, according to an embodiment of the invention the ratio VBAT-is used to select the best frequency range in order to maximize immunity to ground shift effects of the engine ground pole 365 having, at the same time, a high measurement accuracy. According to a preferred embodiment, when the voltage power source 530 supplies the voltage value VEAT at the free end of the shunt resistor 510, the digital logic unit 800 determines a plurality of resistance values RGpj,..., RGIDA of the glow plug 360 and then it calculates an arithmetic mean value RGR. which is used to determine the temperature of the glow plug 360, according to its physical properties. Around a nominal operating temperature value of the glow plug, the glow plug temperature is linear correlated to its resistance, and it can be expressed according the following the formula: TGp = To + aRGpim wherein To and a are specific coefficients of the physical properties of the glow plug.

Determining the arithmetic mean value RGR" allows to minimize measurement errors due to alternating component of eventual ground signals entering in the circuit 1 through the ground pole 365.

A determined temperature value TGp of the glow plug 360 is then used by the ECU, which provides to compare the determined temperature value TGp with a target temperature value TTARGET. On the basis of the result of the comparison the ECU controls a conduction or interdiction state of the switch 540 in order to regulate a glow plug temperature value equal to the target temperature value TTARGET.

While at least one exemplary embodiment has been presented in the foregoing summary and detailed description, it should be appreciated that a vast number of variations exist. It should also be appreciated that the exemplary embodiment or exemplary embodiments are only examples, and are not intended to limit the scope, applicability, or configuration in any way. Rather, the foregoing summary and detailed description will provide those skilled in the art with a convenient road map for implementing at least one exemplary embodiment, it being understood that various changes may be made in the function and arrangement of elements described in an exemplary embodiment without departing from the scope as set forth in the appended claims and their legal equivalents.

REFERENCES

100 motor vehicle automotive system internal combustion engine engine block 125 cylinder 130 cylinder head camshaft piston crankshaft 147 gearbox 148 clutch combustion chamber cam phaser fuel injector fuel rail 180 fuel pump 190 fuel source 200 intake manifold 205 air intake pipe 210 intake port 215 valves 220 exhaust port 225 exhaust manifold 230 turbocharger 240 compressor 250 turbine 260 intercooler 270 aftertreatment system 275 exhaust pipe 290 VGT actuator 300 exhaust gas recirculation system 305 EGR conduit 310 EGR cooler 320 EGR valve 330 throttle body 340 mass airflow and temperature sensor 350 manifold pressure and temperature sensor 360 glow plug 365 glow plug free end 370 ground pole 380 coolant and oil temperature and level sensors 400 fuel rail pressure sensor 410 cam position sensor 420 crank position sensor 430 speedometer 440 EGR temperature sensor 445 accelerator pedal position sensor 450 ECU 460 memory system 500 circuit 510 shunt resistor 515 common node 520 shunt resistor free end 530 voltage power source 550 electrical resistor 570 electric switch 580 electrical resistor 600 voltage divider 700 generator pulse width modulated circuit 710 voltage integrator 715 amplifier 730 capacitor 720 commutator 800 digital logic unit

Claims (14)

1. CLAIMS1. A method for measuring a resistance value (Rpe)of a glow plug (360) which provides for: a) connecting a shunt resistor (510), having a known resistance value (Rsm), in series to the glow plug (360) to create a voltage divider (600), b) connecting a free end (520) of the shunt resistor (510) with a voltage power source (530) and a free end (365) of the glow plug (360) to a ground pole (370), c) supplying a first voltage value ( \Ism) to the free end (520) of the shunt resistor (510), d) monitoring alternatively a first and a second voltage signal (VI. V2) indicative respectively of a voltage drop value on the glow plug (360) and on the shunt resistor (520), e) integrating over time the first and the second voltage signal (V1, V2) obtaining an output signal (Virmour), f)using the output signal (VINT,ouT) for generating a pulse width modulated signal (Vpwm) having a high value and low value, g) determining the resistance value (Rpe) of the glow plug on the basis of the value of the shunt resistor and of a variation over time of the pulse width modulated signal (Vpwm).
2. 2. A method according to claim 1, wherein the resistance value (RGp) of the glow plug is determined by the equation: RGp = R" x PIC)x \K11 \t" wherein RGP is the resistance value of the glow plug, RsH is the resistance value of the shunt resistor, ton is an time interval wherein the pulse width modulated signal has the high value, toff is a time interval wherein the pulse width modulated signal has the low value, Ki and K2 are a first and a second predetermined resistance values.
3. 3. A method according to claim 1, wherein the phase of monitoring alternatively a first and a second voltage signal (Vi, V2) is performed at an oscillating frequency of the pulse width modulated signal (Vpwm).
4. 4. A method according to claim 3, wherein the oscillating frequency of the pulse width modulated signal (Vpwm)is calculated by means of the following equation: VBAT Rap K3 K4 * R$H f = VH VL Rap + RsH RsH * K3 + Rap * K4 wherein f is the oscillating frequency of the pulse width modulated signal, Rap is the resistance value of the glow plug, RsH is the resistance value of the shunt resistor, VBAT is the power source voltage, VH and VL are voltage values, K3 and K4 are predetermined constant coefficients.
5. 5. A method according to claim 4, wherein constant predetermined coefficients k3 and 1(4 are chosen in order to have a duty cycle of the pulse width modulated signal VPWM closed to 50%.
6. 6. A method according to claim 1, which provides to determine a plurality of resistance values (RGp., Rep,n) of the glow plug (360) and then to calculate an arithmetic mean value (Rep.m).
7. 7. A circuit (500) for measuring a resistance value (RGp)of a glow plug(360) provided with a first end (365) connected to a ground pole (370), the circuit comprising a shunt resistor(510) connected, at a common node (515), to the glow plug (360) for creating an electric divider (600), and having a free end (520)connectable to a voltage power source (530), a pulse width modulated signal generator circuit (700) for generating a pulse width modulated signal (Vpwm), having a first input connected to the common node (515) and a second input connected to an electric switch (570), controlled by the pulse width modulated signal (Vpwm), and configured for monitoring alternatively a first and a second voltage signal (VI, V2) indicative respectively of a voltage drop value on the glow plug (360) and on the shunt resistor (520), a digital logic unit (800) being connected to an output of the pulse width modulated signal generator circuit (700).
8. 8. A circuit according to claim 7, wherein the pulse width modulated generator circuit (700) comprises a voltage integrator (710) and a commutator (720), with input hysteresis.
9. 9. A circuit according to claim 8, wherein between the free end (365) of the glow plug (360) and the electric switch (570) is interposed a resistor (550), and between the free end (520) of the shunt resistor (510) and the electric switch (570) is interposed a resistor (580).
10. 10. A circuit according to claim 8, wherein the voltage integrator (710) comprises an amplifier (715) and a capacitor (730) which is connected between an output and an input of the amplifier (715).
11. 11. A circuit according to claim 7, wherein the free end (520) of the shunt resistor (510) is connectable to the voltage power source (530) by means of an electric switch (540).
12. 12. A computer program comprising a computer-code for performing the method according to any claim from 1 to 6.
13. 13. Computer program product on which the computer program according to claim 10 is stored.
14. 14. An electromagnetic signal modulated as a carrier for a sequence of data bits representing the computer program according to claim 10.