WO1989006366A1

WO1989006366A1 - Process and device for verifying a capacitance

Info

Publication number: WO1989006366A1
Application number: PCT/DE1988/000643
Authority: WO
Inventors: Werner Nitschke; Wolfgang Drobny; Hugo Weller; Peter Taufer
Original assignee: Robert Bosch Gmbh
Priority date: 1987-12-30
Filing date: 1988-10-19
Publication date: 1989-07-13
Also published as: DE8717831U1; DE3744524A1

Abstract

In a process for verifying the capacitance of a capacitor (C40), the latter is discharged from a first potential (US1) to a predetermined second potential with a constant discharging current (IK). The discharging time (TE) is measured. The existing capacitance of the capacitor (C40) at a known charging resistance can be calculated from the measured values.

Description

Method and device for checking a capacity

State of the art

With safety-critical circuit arrangements, such as, for example, from "1141 Ingenieur de l'Automobile" (1982) No. 6, pages 69 to 77 known restraint systems is required that they can still perform their safety function for a predefinable minimum time after the supply voltage has been removed - for example, the vehicle battery has been torn off in the event of an accident. For this purpose, the circuit arrangement comprises an energy reserve, usually in the form of an electrolytic capacitor that can be charged by the vehicle battery. Since high-quality electrolytic capacitors are also subject to aging processes and / or their capacitance values change as a function of the ambient temperature, the electrolytic capacitors must be checked regularly in order to be able to guarantee a safe functioning of the circuit arrangement. From DE-AS 22 22 038 a test circuit for the triggering device of a safety device serving to protect the occupants of a vehicle during an accident is already known, in which a constant current source is provided for continuously checking the functionality of an ignition element provided in the triggering device, from which a constant current source is provided low current flows through the ignition element. The one on the Zündele The voltage drop is compared with the voltage of a constant current source. With this test circuit, however, it is not possible to check the energy reserve required for the safety device.

Advantages of the invention

The solution according to the invention with the characterizing features of claim 1 has the advantage that the operational safety of a safety device can be significantly increased, since the capacity of the capacitors provided as an energy reserve can be checked continuously.

Further claims provide useful facilities for carrying out the method according to claim 1.

In an advantageous embodiment of the invention, means for limiting the voltage at the energy reserve are specified.

drawing

Embodiments of the invention are shown in the drawing and explained in more detail in the following description. 1 shows the voltage profile across the capacitor as a function of time, FIG. 2 shows a simplified circuit diagram of a device for carrying out the method according to the invention, FIG. 3 shows a block diagram of a device for carrying out the method, FIG. 4 shows a flow chart to explain the method according to the invention and FIG further design of a device with voltage limitation. Description of the embodiment

According to the block diagram representation according to FIG. 3, a safety device 10 for vehicle occupants comprises an acceleration sensor 11, which converts an acceleration a into a voltage V. At the output of 11, therefore, either an acceleration-proportional signal is generated or a signal that only occurs when a definable acceleration threshold is exceeded. This signal is fed to an evaluation circuit 12, in which the signal emitted by the sensor 11 is examined to determine whether an accident has occurred and the safety device must therefore be activated. Restraining means 13 for the vehicle occupants, such as, for example, airbag and / or belt tensioners, are connected to the evaluation circuit 12 and can be actuated thereby. The device further comprises an energy reserve 14 which, in the event of an accident, ensures that the device 10 is still supplied with power even if, for example, the vehicle battery is torn off and is no longer connected to the vehicle electrical system. Electrolyte capacitors are expediently used as the energy reserve 14 since, despite a relatively large capacitance value, they have a comparatively small construction volume and are also inexpensive to manufacture. However, electrolytic capacitors are subject to aging processes, for example, which can adversely affect their capacitance value. The capacity value can also be temperature-dependent. For the operational safety of the device 10, it is therefore extremely important to regularly check the capacity value of the energy reserve 14. A suitable test circuit is represented in the block diagram according to FIG. 3 by the components 15, 16, 17 and 18, which will be explained in more detail below. First, however, the principle according to which the capacity of the energy reserve 14 is checked is explained using FIGS. 1 and 2. Figure 1 shows a diagram of the course of the voltage U on

Energy reserve 14 (capacitor C40) as a function of time. It is assumed that at the beginning of the test process, that is to say at time t ₁ , capacitor C40 is charged to voltage U _s1 . The

Voltage U on capacitor C40 is in accordance with the circuit arrangement in

Figure 2 determined by a voltage measuring device 21. The measured value is fed to a measuring circuit 15. Then the capacitor

C40 is discharged by a switching element S20 which can be actuated by the measuring circuit 15 via a discharge resistor R _E with a constant current I _K. The discharge current can be monitored by a measuring device 22 and fed to the measuring circuit 15. The duration of the unloading process is also recorded by a measuring device 23 and fed to the measuring circuit 15. The discharge of the capacitor C40 with the constant current I continues until a predeterminable voltage level U _{s2 is} reached, which _varies by the voltage value Δ U _E from

Voltage _value U _s1 differs. This second voltage _value U _s2 is reached after the discharge time t _E.

The relationship applies

(

After measuring the voltage value U _s1 and the discharge time t _E , which goes to the predeterminable value U _s2 when the voltage drops until the voltage drops, the current capacitance value of the capacitor C40 can thus be determined. In the diagram according to FIG. 1, the unloading process ends at time t ₂ . From this point in time, the capacitor C40 is recharged up to the voltage _setpoint U _s1 . The elements of the test circuit in the assembly 10 'are contained in the block diagram according to FIG. They include the measuring circuit 15 for measuring the voltage at the energy reserve 14 and the discharge time t _E , an evaluation circuit 16 which evaluates the measured values detected by 15 and controls the test sequence, a control circuit 17 which, initiated by the evaluation circuit 16, via the discharge circuit 18 causes the energy reserve 14 to be discharged for the purposes of testing and the measured value acquisition via the measuring circuit 15.

The individual steps of the test procedure are explained on the basis of the flow chart according to FIG. It is assumed that in step 100 the device is put into operation and in step 101 the energy reserve 14 or the capacitor C 40 are charged. The test process begins in step 102, in which the control circuit 16 (FIG. 3) first initiates a measurement and storage (step 103) of the voltage _value U _s1 . Subsequently, a discharge of 14, C40 with constant current I _{K is} initiated via the discharge circuit 18 (step 104). The discharge time t is measured (step 105). During the unloading process, the measuring circuit 15 continuously supplies the energy reserve 14

Voltage U _s measured (step 106) and checked whether the predetermined

Voltage level U _{s2 is} reached. As soon as this voltage level is reached, the discharge duration t _{E is} determined and stored in order to calculate the capacitance of the capacitor C40 according to the relationship given above.

In a particularly advantageous embodiment of the invention, all of the test steps described above are controlled by a computer; the test process can be carried out once each time the device is started up, i.e. when the vehicle is started or cyclically with a predefinable time sequence. As soon as it is determined during the test process that the Kapa is below a critical limit value and the functionality of the device is no longer guaranteed in an emergency, this can be expediently indicated by a warning signal. Furthermore, when equipping a vehicle with a plurality of safety devices, a priority can be set in such a way that if the capacity value of the energy reserve 14 is too low, only the safety devices of particularly vulnerable vehicle occupants can be activated.

In practical vehicle operation, there is a risk that the energy reserve 14 could be charged to a voltage value which is above the permissible peak voltage of the energy reserve 14 under adverse circumstances. This disadvantage can be avoided in a particularly simple manner by an advantageous development of the device according to the invention. For this purpose, the circuit arrangement is to be designed as shown in FIG. 5. A capacitor C40, to which a voltage divider R40, R41 is connected in parallel, serves as the energy reserve. The tap of the voltage divider R40, R41 leads to the measuring circuit 15, which measures the voltage at C40. The series circuit of the collector-emitter path of a first transistor T40 and a resistor R43 is connected in parallel with C40. The base connection of transistor T40 is connected to the junction of a resistor R42 and a Zener diode Z40, which are part of a series circuit, which also includes the discharge resistor R _E and the collector-emitter path of a second transistor T41. The base terminal of the transistor T41 is connected to the discharge circuit 18 and is triggered by this discharge circuit 18 to initiate a discharge process for test purposes. The connection point between the Zener diode Z40 and the discharge resistor R _E is led via a diode D40 to the connection 2, to which a stabilized supply voltage for charging the capacitor C40 can be placed. With uncontrolled unloading circuit 18 can the voltage U _s at C40 to a maximum value of

(2) U _s = U _{EB (T40)} + U _Z40 + U _FD40 + U _STAB

increase.

In this formula:

U _{EB (T40)} : base-emitter voltage of the transistor T ₄₀ ,

U _Z40 : Zener voltage of the Zener diode Z40,

U _FD40 : forward voltage of diode D40,

U _STAB : Value of the stabilized at connection 2

Supply voltage.

Advantageously, this circuit arrangement also ensures that the capacitor C40, which serves as an energy reserve, does not fall below a predeterminable minimum value if the discharge circuit is defective. This minimum value U _MIN is:

(3) U _MIN = U _{EB (T40)} + U _Z40 + I _Z40 R _E + U _{CE SAT (T41)}

The discharge circuit described above is also suitable for limiting the maximum voltage across the capacitor C40 when the discharge circuit is not activated and for limiting it to a minimum voltage if the discharge circuit is defective. This means two additional weighty safety functions that can be achieved without significant additional circuitry.

Claims

Expectations

1. Method and device for checking a capacity, in particular in the case of a safety device for vehicle occupants, characterized by the following features:

a) The voltage (U _s1 ) at the at least partially charged capacitance (14, C40) is measured at a time (t ₁ );

b) the capacity (14, C40) with a constant discharge current

((I _K ) discharge until a predeterminable voltage value (U _s2 ) is reached;

c) the discharge time (t _E ) is determined;

d) the value of the capacity (14, C40) is according to the formula

certainly. In this formula, U _{1 means} the voltage value at the capacitance (14, C40) measured at the start of the test process, Δ U _E the difference between the measured voltage values U _s1 and U _s2 , t _E the duration of the discharge process, R _E value of the discharge resistance and C40 Capacitance of the capacitor C40.

2. Device for performing the method according to claim 1, characterized by

- means for charging the capacity (C40),

Means for measuring the voltage across the capacitance (C40), means for discharging the capacitance (C40) with a constant discharge current (I _K ),

- Means for measuring the discharge time (t _E ) of the capacity

(C40),

- Means for calculating the capacity value of the capacity (C40).

3. Device according to claim 2, characterized in that the means for discharging the capacitance (C40) has a discharge resistor

(R _E ), which is connected in series with a switching element (S20) through which the discharge resistor (R _E ) can be connected in parallel with the capacitor (C40).

4. Device according to one of claims 2 and 3, characterized in that the switching element (S20) by a discharge circuit (18) controllable transistor (T41).

5. Device according to one of claims 2 to 4, characterized in that for charging the capacitor (C40), the series connection of a resistor (R42), a Zener diode (Z40) and a diode (D40) is provided, with between the connection point between the Zener diode (Z40) and diode (D40) and ground, the series circuit of the discharge resistor (R _E ) and the switching element (T41) are connected, and further parallel to the capacitor (C40) the series circuit consisting of a resistor (R43) and the base emitter path of a further transistor (T40) are connected, the base connection of the transistor (T40) being led at the connection point between the resistor (R42) and the Zener diode (C40).