Electrical generator or motor structure – Non-dynamoelectric – Piezoelectric elements and devices
Reexamination Certificate
2002-10-08
2004-11-30
Dougherty, Thomas M. (Department: 2834)
Electrical generator or motor structure
Non-dynamoelectric
Piezoelectric elements and devices
Reexamination Certificate
active
06825591
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to a method for controlling a piezoelectric drive and a piezoelectric drive suited for the implementation of the method.
2. Description of the Prior Art
Piezoelectric drives can find an application in various fields. An application are proportioning devices, particularly for small volumes proportioned in the nanolitre to millilitre ranges.
WO 99/10099 discloses various microproportioning systems having an open-jet proportioner and/or a micro-diaphragm pump. Microdiaphragm pumps and open-jet proportioners have a displacement chamber and a deformable displacement wall or diaphragm associated therewith which can be driven by a piezoelectric actuator. The displacement chamber is connected to a reservoir for the supply of liquid and feeds a proportioning port which can be formed as a nozzle. In a microdiaphragm pump, the liquid flows off or drops out of a proportioning port. In an open jet proportioner, in contrast, the liquid is expelled from the proportioning port in an open jet. From this document, it has been known already to install a fluid module from a reservoir and a displacement chamber in a reception duct of a proportioning device which has an actuator associated with the reception duct which acts on the diaphragm of the module.
WO 99/37400 has made known another microproportioning device which has a proportioning module which is exchangeably arranged in a casing. The proportioning module comprises a proportioning chip with a displacer diaphragm and a pressurized chamber which is connected to a nozzle and a medium reservoir. Also here, the actuator of the microproportioning device and the displacer of the proportioning module are not fixedly connected to each other. Rather, the actuator is merely brought into contact with the displacer. The actuator is a piezoelectric stack actuator (“piezostack”) the force of which is transmitted to a pressure point of the proportioning chamber via a rotatably supported lever actuator. The proportioning volume is dependent on the voltage applied to the piezostack. In another embodiment, a control signal U (t) of a low edge steepness is provided to the actuating device at an interval called a suction phase, which causes a slow actuation of the diaphragm out of the initial position. In a proportioning phase, the control signal is brought down to a low level at a very short interval to expel the liquid through the nozzle in an open jet.
The voltage control which has been known before has the disadvantage that the accuracy in deflecting the piezostack does not always satisfy the requirements.
Accordingly, it is the object of the invention to provide a method for controlling a piezoelectric drive at an improved accuracy. In addition, the aim is to provide a suitable piezoelectric drive for implementing the method.
SUMMARY OF THE INVENTION
The object is attained by a method for controlling a piezoelectric drive in which:
a required voltage to be applied to a piezoactuator for a desired deflection of the piezoactuator is determined by means of the deflection characteristic curve of the piezoactuator,
the piezoactuator is supplied with a current by means of a current source,
the voltage is measured on the piezoactuator while the piezoactuator is being supplied with the current from the current source,
the voltage measured is compared to the voltage required, and
the supply of the piezoactuator with the current from the current source is turned off once the required voltage is reached.
A piezoelectric drive which is suitable for implementing the method has a piezoactuator,
a device for determining a required voltage to be applied to the piezoactuator for a desired deflection of the piezoactuator,
a current source connected to the piezoactuator for supplying the piezoactuator with a current,
a voltage measuring device connected to the piezoactuator for measuring the voltage on the piezoactuator,
a device connected to the voltage measuring device and the device for determining a required voltage to compare the voltage measured and the voltage required, and
an electric control device connected to the device for comparing and the current source to turn off the source of the piezoactuator with the current of the current source voltage once the required voltage is reached.
Analyses made on the voltage control, which has been known before, in conjunction with the present invention have revealed that a predetermined voltage will adjust itself only gradually on the piezostack because the latter electrically corresponds to a capacitor so that the voltage will rise according to an exponential function. However, since the deflection of the piezostack is proportional to the voltage the deflection desired will be reached only after a certain time lag. Moreover, the exponential variation of deflection is accompanied by a steadily varying dynamics. The additional undesirable side effect which presents itself in voltage control is a dielectric hysteresis, which makes it very difficult to ascertain the relationship between the voltage applied, dynamics, and deflection. What adds to such effect in the aforementioned embodiment that uses the control signal exhibiting control edges is the change in time of the triggering signal
Instead of working with a voltage source, the inventive method and the piezoelectric drive use a current source. The current source features a large internal resistance so as to supply a substantially constant current. The internal resistance of an ideal current source tends to approach infinity. Charging or discharging the piezoactuator with an impressed constant current achieves a linear rise or drop of the voltage on the piezoactuator and, as a result, the gradient or dynamics of this rise or drop is dependent on the driving current strength. Studies made in conjunction with the invention have also shown that this way of triggering avoids or minimizes a hysteresis of the path.
The easiest way of forcing the piezoactuator to effect a defined deflection by means of current control would consist in using a timed current source. The accuracy required for the current source and its timing could be ensured at a relatively low expenditure. However, the voltage arising during the time of current application and, hence, the deflection of the piezoactuator are also a function of piezostack capacitance. More detailed studies have proved that it does not react like an ideal capacitor, but that capacitance varies with the deflection of the piezoactuator with no linear correlation existing between the capacitance and deflection. Furthermore, dependencies of capacitance on the temperature, ageing, and the mechanical bias on the piezoactuator would result in inaccuracies relating to voltage and, hence, to deflection.
Therefore, the invention uses a current source feeding the piezoactuator and measures the voltage which actually is applied to the piezoactuator while the current is being fed. The voltage measured is compared to the voltage required to achieve the desired deflection which can be determined by means of the deflection characteristic curve of the piezoactuator, i.e. the dependence of the piezoactuator deflection on the voltage applied. Once the voltage measured matches the voltage required the piezoactuator is disconnected from the supply and, hence, the desired deflection is exactly achieved. This is the way the piezoactuator may both be charged and discharged to expand or contract the piezoactuator, i.e. to deflect it in various directions.
In this mode of operation, a non-constant capacitance of the piezoactuator still has only a small influence on the dynamics of deflection, but no longer on deflection itself. The accuracy of deflection is primarily governed by the accuracy at which the voltage is measured on the piezoactuator. Such accuracy may be ensured by a calibrated voltage measuring device. In contrast, the current source may basically be operated in an unbalanced state, nor needs the capacitance of the piezoactuator be taken into account in any way. There is
Baumgartner Matthias
Jansen Dirk
Dougherty Thomas M.
Eppendorf AG
Sidley Austin Brown & Wood LLP
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