Electricity: battery or capacitor charging or discharging – Capacitor charging or discharging
Reexamination Certificate
1999-04-26
2001-05-22
Wong, Peter S. (Department: 2838)
Electricity: battery or capacitor charging or discharging
Capacitor charging or discharging
C310S317000
Reexamination Certificate
active
06236190
ABSTRACT:
BACKGROUND OF THE INVENTION
Field of the Invention
The invention relates to a method and device for driving a capacitive actuator, in particular a piezoelectric-operated fuel injection valve of an internal combustion engine.
Piezo actuators consist of a multiplicity of piezo-ceramic layers that form a so-called “stack.” The stack changes its dimensions when a voltage is applied, in particular, it changes its length s by a displacement ds, or it produces an electric voltage on mechanical stress or tension.
Various methods are known for driving piezo actuators, which behave electrically as capacitors. In those methods the voltage applied to the piezo actuator is monitored. In all prior art methods, the criterion used for terminating the charging is when a specific voltage across the piezo actuator is reached, unless an elaborate measurement of the relevant actuator displacement ds is provided. One example of this is a prior German patent application No. 1932872-1.
The electrical properties of a piezo stack of the foregoing type change with the temperature to which it is exposed. With increasing temperature, its capacitance increases, and the displacement increases as well. At temperatures, from about −40° C. to +150° C., which are meant to be taken into account for automotive applications, changes of up to a factor of 2 may in this case be observed.
If a piezo actuator is charged, at all operating points, for example to a constant voltage, which brings about the requisite displacement ds at low temperatures, then the displacement attained at high temperatures is considerably greater than necessary. In the context of fuel injection valves with constant fuel pressure, this leads to an excessive amount of fuel, or vice versa. Since the capacitance of the piezo stack is likewise greater at high temperatures, very much more charge and energy (E=½C·U
2
) than necessary is required.
U.S. Pat. No. 5,387,834 discloses a drive circuit for a piezoelectric element of a dot matrix printer, in which a temperature sensor senses the temperature of the piezoelectric element. The piezoelectric element is driven using charging times which are stored in a table as a function of the temperature.
U.S. Pat. No. 5,543,679 discloses a drive circuit for a piezoelectric element for driving a fuel valve. A presumably constant charge is drawn from a capacitor and supplied to the piezoelectric element via a transformer. None of the variations in the circuit due to temperature changes, humidity, component tolerances, aging, and the like are thereby taken into consideration.
Patent Abstracts of Japan, Vol. 018, No. 188 (E-1532), Mar. 31, 1994 & JP-A 05 344755 discloses a drive circuit for a piezoelectric element for driving a fuel valve. There, the piezoelectric element is charged via a first regulated voltage with a constant charge amount, and is entirely discharged to a regulated second negative voltage via a capacitance determination of the piezoelectric element of the voltage measured at discharge.
SUMMARY OF THE INVENTION
It is accordingly an object of the invention to provide a drive system for a capacitive actuator, which overcomes the above-mentioned disadvantages of the heretofore-known devices and methods of this general type without using a temperature sensor, such that a maximally constant displacement ds is achieved throughout the entire temperature range in which the actuator is operated. A further object of the invention is to provide a device for carrying out the method.
With the foregoing and other objects in view there is provided, in accordance with the invention, a method of driving a capacitive actuator, in particular a piezoelectrically operated fuel injection valve of an internal combustion engine. The method comprises the following steps:
predetermining an amount of energy required for a particular actuator displacement and defining a charging setpoint;
charging the actuator from a power source and multiplying a current flowing into the actuator and a voltage drop across the actuator with one another to form a product;
integrating the product with respect to time to form an integration value; and
terminating the charging step when the integration value reaches or exceeds the charging setpoint.
In an alternative mode of the invention, the method comprises the following steps:
predetermining an amount of energy required for a particular actuator displacement and defining a charging setpoint;
charging the actuator with a predetermined constant current;
integrating a voltage across the actuator with respect to time during the charging step to form an integration value; and
terminating the charging step when the integration value reaches or exceeds the charging setpoint.
With the above and other objects in view there is also provided, in accordance with the invention, a device for driving a capacitive actuator such as, in particular, a piezoelectrically operated fuel injection valve of an internal combustion engine. The device comprises:
a voltage source having a positive terminal and a negative pole;
a series circuit connected between the positive terminal and the negative terminal of the voltage source, the series circuit comprising a current-limiting element, an electronic switch, and an actuator to be driven;
a control circuit connected to the electronic switch, the control circuit having an input receiving a control signal defining a start, a duration, and an end of a drive process, an input receiving a signal representing a current supplied to the actuator and an input receiving a signal representing a voltage across the actuator, the control circuit turning the switch on in response to the control signal;
the control circuit including:
a multiplier element for multiplying the current with the voltage to form a product;
an integrator for integrating the product to form an integration value; and
a comparator for comparing the integration value with a predetermined setpoint; and
the control circuit turning the switch off when the integration value reaches or exceeds the predetermined setpoint.
In accordance with an added feature of the invention, the control signal has a switching edge, and the integrator is discharged by the switching edge of each control signal.
In an alternative embodiment of the invention, there is provided a device for performing the above-noted alternative mode of the novel method. The device comprises:
a voltage source having a positive terminal and a negative pole;
an electronic switch and an actuator to be driven connected in series between the positive terminal and the negative pole of the voltage source;
a constant-current source connected for charging the actuator;
a control circuit having an input for receiving a control signal for a start, a duration, and an end of a drive process, and an input receiving a signal representing a voltage across the actuator, the control circuit turning on the switch at a start of the control signal;
the control circuit including:
an integrator for integrating the signal representing the voltage across the actuator to form an integration value; and
a comparator for comparing the integration value with a predetermined setpoint; and
the control circuit turning the switch off when the integration value reaches or exceeds the predetermined setpoint.
Studies have shown that the energy fed to a capacitive actuator represents a very much more precise measure of the displacement ds than the applied voltage, and that charging with constant energy over the requisite temperature range leads to a substantially more constant displacement. The displacement varies approximately linearly with the applied voltage at a particular temperature. If the temperature changes, then the displacement for the same voltage changes. On the other hand, the displacement varies proportionately to the square of the applied energy (ds≈e
2
), but independently of the temperature.
In mass-produced piezo stacks, the thicknesses of the individual piezo layers are not exactly equal. It is possible, for example, to produce stacks having c
Freudenberg Hellmut
Gerken Hartmut
Hoffmann Christian
Greenberg Laurence A.
Lerner Herbert L.
Siemens Aktiengesellschaft
Stemer Werner H.
Tibbits Pia
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