Open-load diagnosis by an electrical system comprising an electronic control unit connected to a load that includes a capacitor

Abstract

The invention relates to an electrical system (1) comprising an electronic control unit (2) having at least one switch (5) with an output connected on one side to a load (3) and on the other side to a current source (10), the control unit (2) being configured to activate and deactivate the switch (5) and to establish an open-load diagnosis by measuring the voltage at the output of the switch (5) after the switch (5) has been deactivated. The invention also relates to a motor vehicle comprising such an electrical system (1). For the abstract: Figure 2.

Description

Title of the invention: Open-load diagnosis by an electrical system comprising an electronic control unit connected to a load that includes a capacitor. Technical field

[0001] This description relates to the field of electrical systems, particularly those installed in motor vehicles. It specifically relates to an electrical system comprising an electronic control unit with at least one switch having an output connected on one side to a load and on the other side to a current source. It also relates to a motor vehicle comprising such an electrical system. State of the art

[0002] An electronic control unit intended for controlling a load, for example an actuator in a motor vehicle, must be able to check its outputs to detect any malfunctions (e.g., an open load). Various means exist for this purpose, including integrated output circuits configured to measure the voltage at the output of the electronic control unit's switch (i.e., the low-side switch, also known as the LSD, or the high-side switch, also known as the HSD) after a short time interval (e.g., 100µs) following its deactivation, in order to quickly diagnose a fault. The measured voltage value is compared to a pre-established threshold value (e.g., 2.2V), and an open load diagnosis is made if the measured value is greater than the threshold value.This method allows for the efficient and rapid diagnosis of an open load when the electronic control unit is connected to a load containing only inductors and resistors. However, this diagnostic method no longer works when the load includes an input capacitor, as is the case with some modern actuators that incorporate such a component, notably to mitigate the effects of electrostatic discharge. Indeed, a capacitor discharges relatively slowly, and if the voltage at the switch output is measured shortly after its deactivation, the resulting voltage value remains above the threshold, leading to an erroneous diagnosis. This problem is illustrated by the graph shown in [Fig. 1].As can be seen, due to the slow discharge of the capacitor of the load, the voltage measured at the output of the switch of the electronic control unit after a short time interval (lOOus) which follows the . The switch deactivation value is greater than the threshold value (Vs), resulting in an erroneous open load diagnosis. Summary

[0003] The present description aims to overcome at least part of this drawback. In particular, its purpose is to provide a solution so that an electronic control unit connected to a load incorporating a capacitor can quickly diagnose an open load, while minimizing the cost of implementing such a solution.

[0004] According to a first aspect, an electrical system is described that includes an electronic control unit having at least one switch with an output connected to a load. The control unit is configured to turn the switch on and off and to perform an open-load diagnosis by measuring the voltage at the switch output after the switch has been turned off. The input impedance of the load is a capacitor. The switch is connected in series with an inductor. And the electronic control unit is configured to: - after deactivating the switch, apply a diagnostic command in the form of a low duty cycle pulse width modulation, comprising several pulses, so as to discharge the capacitor over the duration of the diagnostic command; - establish open load diagnosis after the last pulse of the diagnostic command.

[0005] According to one variant, the duty cycle can be between 5% and 15%.

[0006] According to another variant, the duty cycle can be equal to 10%.

[0007] According to yet another variant, the diagnostic command may include a number of pulses and the coil can have an inductance value, said number and said value being predefined according to the capacitance of the capacitor.

[0008] According to yet another embodiment, since the voltage on the load has a rise time when the switch is activated by an activation command, and the capacitor is likely to recharge during each pulse of the diagnostic command, the number of pulses of the diagnostic command and the inductive value of the coil can be optimized by taking into account one or more of the following constraints: - minimize capacitor charging during each diagnostic command pulse; - minimize the voltage rise time on the load when the switch activation command is given; - minimize the number of pulses in the diagnostic command.

[0009] According to yet another variant, the diagnostic command can include between 5 and 15 pulses.

[0010] According to yet another variant, the diagnostic command may include 10 pulses.

[0011] According to yet another variant, the switch may be a high-side switch and the load may be mounted between the output of the switch and a low voltage reference point, or the switch may be a low-side switch and the load may be mounted between the output of the switch and a high voltage reference point.

[0012] According to yet another variant, the inductance of the coil can be between 1 and 50mH.

[0013] According to a second aspect, a motor vehicle is described which includes an electrical system as defined above. Brief description of the figures

[0014] Other features and advantages will become apparent upon examination of the detailed description below, and the accompanying figures, in which:

[0015] [Fig-1] is a graph that illustrates the problem of open load diagnosis with a method according to the prior art;

[0016] [Fig.2] is a diagram of an example embodiment of an electrical system according to the present description; and

[0017] [Fig.3] is a graph that illustrates an open load diagnosis performed by an electrical system according to the present description. Detailed description

[0018] Figure 2 schematically illustrates an example of an embodiment of an electrical system 1 according to the present description. This system conventionally comprises an electronic control unit 2, referred to below by the acronym ECU, which is connected to a load 3, for example an actuator of a motor vehicle, by means of at least one cable 4. Conventionally, the ECU 2 comprises at least one switch 5, which can be, as in the illustrated example, a high-side switch, better known by the acronym HSD according to the English terminology "High Side Driver", or a low-side switch, better known by the acronym LSD according to the English terminology "Low Side Driver".

[0019] The ECU 2 is thus configured to activate and deactivate the switch 5 by means of an ON / OFF signal or, advantageously, by means of a pulse-width modulation signal, also known by the acronym PWM. Furthermore, as will be detailed below, the ECU 2 of the electrical system 1 according to this description is configured to establish an open load diagnosis by measuring the voltage at the output of switch 5.

[0020] Conventionally, the output of switch 5 is connected to ground via a low-value output capacitor 9, for example, a few nanofarads, primarily to mitigate the effects of electrostatic discharge. A current source 10 is also connected between the output of switch 5 and ground. Its role is to discharge the output capacitor 9 when switch 5 is open, by drawing current to ground. The size of the current source 10 is matched to that of the capacitor 9.

[0021] According to the present description, the input impedance of the load 3 is a capacitor 6, which is used, in particular, to mitigate the effects of electrostatic discharge. When the switch 5 of the ECU 2 is a high-side switch, as in the illustrated example, the load 3 is connected between the output of the switch 5 and a low voltage reference point. Conversely, when the switch 5 of the ECU 2 is a low-side switch, the load 3 is connected between the output of the switch 5 and a high voltage reference point.

[0022] The capacitance of the load capacitor 6 is generally much greater than that of the output capacitor 9 (in a non-limiting example, the capacitance of the output capacitor 9 may be 1 µF and the capacitance of the load capacitor 6 1 µF). As a result, the current source 10 is not sized to discharge the load capacitor 6 rapidly. Thus, according to the prior art, after switching off the switch 5, the discharge of the load capacitor 6 occurs slowly through the load resistors 3.

[0023] Advantageously, the ECU 2 incorporates a coil 7 connected in series at the input of the switch 5 and, in parallel, with a diode 8, also known by the terminology of freewheeling diode, well known to those skilled in the art. As will be seen below, the coil 7 advantageously limits the recharging of the capacitor 6 of the load 3 when a particular command is applied to the switch 5, in particular a diagnostic command in the form of pulse-width modulation which, according to the present description, is applied by the ECU 2 to the switch 5 after its deactivation, so as to discharge the capacitor 6 for the duration of the diagnostic command.

[0024] Indeed, the UCE 2 of the electrical system 1 according to the present description is advantageously configured to, after deactivation of the switch 5, apply to it a diagnostic command in the form of a pulse-width modulation with a low duty cycle, comprising several pulses, so as to discharge the charging capacitor 6 over the duration of the diagnostic command, and to establish the open load diagnosis after the last pulse of the diagnostic command.

[0025] Thanks to the pulse-width modulation signal, the discharge rate of the charging capacitor 6 is advantageously maximized. Indeed, when a capacitor receives a PWM signal, it undergoes repeated charging and discharging cycles depending on the ON and OFF states of the PWM signal. During the ON (or high) periods, the capacitor charges. During the OFF (or low) periods, the capacitor discharges through the circuit. With each pulse of the diagnostic control PWM signal, the current source 10 draws current to ground, which discharges capacitors 9 and 6. As explained above, the current source is not sized to discharge capacitor 6. Applying several pulses has the effect of repeating the discharge phenomenon by the current source 10 and thus reduces the discharge time of the charging capacitor 6.

[0026] Furthermore, the use of a low duty cycle makes it possible to maximize the OFF time of the diagnostic control signal so that the capacitor spends more time discharging and the discharge is faster.

[0027] According to the present description, the duty cycle is preferably between 5% and 15%, for example, 10%. Furthermore, the number of pulses in the diagnostic command and the inductance value of coil 7 are predefined based on the capacitance of capacitor 6 in load 3. For example, coil 7 has an inductance value preferably between 1 and 50 mH, and the number of pulses is 10.

[0028] More specifically, the voltage across load 3 has a rise time when switch 5 is activated by an activation command, and capacitor 6 is likely to recharge during each pulse of the diagnostic command. The number of pulses in the diagnostic command and the inductive value of coil 7 are optimized by taking into account one or more of the following constraints: minimizing the recharging of capacitor 6 during each pulse of the diagnostic command, minimizing the rise time of the voltage across load 3 when switch 5 is activated, and / or minimizing the number of pulses in the diagnostic command. Preferably, the diagnostic command applied by ECU 2 thus comprises between 5 and 15 pulses, for example, 10 pulses.

[0029] Figure 3 shows a graph illustrating the implementation of an open-load diagnostic by the ECU 2 of the electrical system 1 according to the present description. In the illustrated example, switch 5 is deactivated at time td. From this time, the ECU 2 applies a diagnostic command to it in the form of pulse-width modulation with a low duty cycle, for example 10%, repeating this process. Several pulses are applied periodically. The discharge of capacitor 6 then occurs in such a way that it is possible to measure the output voltage of switch 5 after the tenth pulse, at time tm, and to obtain at this instant a voltage value which, when the load is correctly connected, is lower than the threshold value Vs, which is, for example, set at 2.2V. It is at this instant that the ECU 2 can therefore establish the open load diagnosis, concluding in the illustrated case that the load 3 is correctly connected since the measured voltage is lower than the threshold value Vs.

[0030] It is noted that the voltage measured at the output of switch 5 logically increases with each activation of switch 5. This is because capacitor 6 of load 3 recharges during the high periods of the PWM signal. However, these rechargings of capacitor 6 are advantageously limited by the coil 7, which is connected in series with switch 5. This coil slows the current flowing through it, thereby slowing the charging of capacitor 6 and allowing it to reach the threshold value Vs after a very short time interval, on the order of 200 μs.

[0031] In practice, it is desirable for the ECU 2 to be able to establish an open-load diagnosis as quickly as possible; in other words, the aim is to obtain the lowest possible number of pulses to allow the voltage to drop below the threshold value Vs. To achieve this, the fact that the higher the inductance of coil 7, the more the voltage rise will be delayed will be taken into account. Furthermore, the capacitance value of capacitor 6, which also constrains the number of pulses required, will also be considered. Similarly, the duty cycle constrains the inductance value of coil 7, because the higher the duty cycle, the higher the inductance of coil 7 must also be, which increases its cost and, by extension, the manufacturing cost of the electrical system 1 as described herein.A balance will therefore be sought to fix the duty cycle, the value of the number of pulses and the value of the inductance of coil 7 so as to be able to minimize the charging of the capacitor during each pulse of the diagnostic command, to minimize the time of rise of the voltage on the load 3 during the activation command of switch 5 and to minimize the number of pulses of the diagnostic command.

[0032] Furthermore, given that the PWM modulation causes abrupt interruptions in the current flowing through coil 7 during successive activation and deactivation commands of switch 5, it should be noted that diode 8, connected in parallel with coil 7, advantageously protects against an overvoltage that is inevitably generated and which could damage the components. System 1 electronics. Indeed, such an overvoltage is generated due to the physical impossibility of a current stopping in a coil. Thus, thanks to diode 8, the energy accumulated in coil 7 will be able to flow through diode 8, which will allow a gradual decrease in the energy stored in coil 7.

[0033] Therefore, thanks to the electrical system according to the present description described above, a solution is provided so that an electronic control unit connected to a load which incorporates a capacitor can quickly diagnose an open load, while minimizing the cost of implementing such a solution.

Claims

Demands

1. Electrical system (1) comprising an electronic control unit (2) having at least one switch (5) having an output connected on the one hand to a load (3) and on the other hand to a current source (10), the control unit (2) being configured to turn the switch (5) on and off and to establish an open load diagnosis by measuring the voltage at the output of the switch (5) after the switch (5) has been turned off, characterized in that the input impedance of the load is a capacitor (6), the switch is connected in series with an inductor (7), and the electronic control unit (2) is configured to: • after the switch (5) has been turned off, apply a diagnostic command to it in the form of a low duty cycle pulse-width modulation, comprising several pulses, so as to discharge the capacitor over the duration of the diagnostic command;• establish open load diagnosis after the last pulse of the diagnostic command.;

2. Electrical system (1) according to claim 1, characterized in that the duty cycle is between 5% and 15%.

3. Electrical system (1) according to any one of the preceding claims, characterized in that the duty cycle is equal to 10%.

4. Electrical system (1) according to any one of the preceding claims, characterized in that the diagnostic control comprises a number of pulses and the coil (7) has an inductance value, said number and said value being predefined as a function of the capacitance of the capacitor (6).

5. Electrical system (1) according to claim 4, characterized in that the voltage on the load (3) has a rise time when the switch (5) is activated by an activation command, and the capacitor (6) is capable of recharging during each pulse of the diagnostic command, the number of pulses of the diagnostic command and the inductive value of the coil (7) are optimized taking into account one or more of the following constraints: - minimize the charging of the capacitor (6) during each pulse of the diagnostic command; - minimize the voltage rise time on the load (3) during the activation command of the switch (5); - minimize the number of pulses of the diagnostic command.

6. Electrical system (1) according to any one of the preceding claims, characterized in that the diagnostic control comprises between 5 and 15 pulses.

7. Electrical system (1) according to any one of the preceding claims, characterized in that the diagnostic control comprises 10 pulses.

8. Electrical system (1) according to any one of the preceding claims, characterized in that the switch (5) is a high-side switch and the load (3) is mounted between the output of the switch (5) and a voltage reference low point, or the switch (5) is a low-side switch and the load (3) is mounted between the output of the switch (5) and a voltage reference high point.

9. Electrical system (1) according to any one of the preceding claims, characterized in that the inductance of the coil (7) is between 1 and 50mH.

10. Motor vehicle, characterized in that it comprises an electrical system (1) according to any one of the preceding claims.

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