Method and device for diagnosing a reservoir capacitor of a vehicle passenger protection system, and vehicle safety system incorporating such device

A device for diagnosing a reservoir capacitor of a vehicle safety system, the device comprising:

• a supply voltage generating circuit arranged to load the reservoir capacitor up to a given voltage value;
• a current sink arranged to selectively discharge the reservoir capacitor with a given fixed current of known value;
• and
• a control unit arranged to:
• obtain a first measure of a discharge rate of the reservoir capacitor when discharging the capacitor by a first undetermined current;
• and,
• after the reservoir capacitor has been reloaded up to the given voltage value by the voltage supply, obtain a second measure of the discharge rate of the reservoir capacitor when discharging said capacitor by the first current and by said given fixed current added to said first current by the current sink;
• determine the capacitance value of the reservoir capacitor from the difference between the first measure and the second measure, based on the known value of the given fixed current of the current sink;
• and
• compare the capacitance value of the reservoir capacitor to a threshold and, if the capacitance value is below said threshold, trigger an alarm signal indicating so.

A vehicle safety system comprising:

• a battery;
• an actuating device;
• a reservoir capacitor arranged to supply power to said actuating device in replacement of the battery when needed;
• and,
• a diagnosing device implemented to diagnose the reservoir capacitor, arranged to detect whether enough energy is available or not in said reservoir capacitor for actuating the actuating device, wherein said diagnosing device comprises
• a supply voltage generating circuit arranged to load the reservoir capacitor from a battery voltage supplied by the battery, up to a first given voltage value,
• a current sink arranged to selectively discharge the reservoir capacitor with a second fixed current of known value;
• a control unit arranged to:
• obtain a first measure of a discharge rate of the reservoir capacitor when discharging the reservoir capacitor by a first undetermined current,
• after the reservoir capacitor has been reloaded up to the first given voltage value by the supply voltage generating circuit, obtain a second measure of the discharge rate of the reservoir capacitor when discharging said capacitor by the first undetermined current and by the second fixed current added to said first undetermined current by the current sink,
• determine the capacitance value of the reservoir capacitor from the difference between the first measure and the second measure, based on the known value of the given fixed current of the current sink, and
• compare the capacitance value of the reservoir capacitor to a threshold and, if the capacitance value is below said threshold, trigger an alarm signal indicating so.