Electrical generator or motor structure – Non-dynamoelectric – Piezoelectric elements and devices
Reexamination Certificate
2000-01-24
2002-01-22
Ramirez, Nestor (Department: 2834)
Electrical generator or motor structure
Non-dynamoelectric
Piezoelectric elements and devices
C310S316010
Reexamination Certificate
active
06340858
ABSTRACT:
FIELD OF THE INVENTION
The invention concerns a method for calibrating a piezoelectric actuating drive which includes an electronic circuit for energizing a stack of piezoelectric elements which perform a displacement according to the piezoelectric effect in response to an energizing signal.
BACKGROUND INFORMATION
A piezoelectric actuating drive is known from German Patent Publication DE-AS 43 03 125 comprising control electronics and a piezoelectric actuator. The known drive includes a circuit arrangement which improves the correlation of the piezoelectric positioning movement or displacement with the electric voltage of an energizing signal applied to a piezoelectric member. The conventional correlation wants to make sure that particularly influences on the piezoelectric displacement due to hysteresis are reduced. For this purpose a second actuating drive is provided, the output signal of which is linearly linked or coordinated with the positioning movement or displacement of the first actuating drive.
The precise stroke resolution or transformation (stroke as a function of signal size) of stroke or displacement causing piezoelectric actuating drives is impaired if environmental operating conditions change for such drives. Such changes may involve the temperature, the mechanical load, and/or instability effects of the piezoelectric ceramic member due to aging. This applies to controlled open loop systems and regulated closed loop systems with a feedback of a displacement representing signal. A precise functioning of the actuating drives is of considerable technical and economic relevance, for example in connection with piezoelectrically controlled valves of high precision, in particular for fuel injection valves in automotive engineering.
OBJECTS OF THE INVENTION
In view of the foregoing it is the aim of the invention to achieve the following objects singly or in combination:
to provide a piezoelectric actuating drive having a positioning movement or displacement which does not depend on environmental operating conditions and not on piezoelectric instabilities without requiring an auxiliary drive;
to calibrate the displacement of a stack of piezoelectric elements caused by an expansion or contraction of the stack, so that the displacement corresponds accurately to the energizing control signal applied to the stack; and
to base such calibration on a response characteristic of the piezoelectric stack such as the electric capacitance or the electric charge impulse or a similar characteristic that is measurable when the displacement encounters a stop.
SUMMARY OF THE INVENTION
According to the invention the aforementioned objects have been achieved by a process for self-calibration of a piezoelectric actuating drive, advantageously using the piezoelectric effect. This calibration can be performed as required or as an automatic cyclic operation. Without requiring any additional components, the method according to the invention can be carried out with a conventional actuating drive comprising an electronic control circuit, an actuator and at least one preferably two limit stops.
The use of the piezoelectric effect according to the invention is based on the coupling of mechanical and electric properties by such an effect. A change of mechanical marginal or limiting conditions has effects that can be evaluated. One extreme condition is, for example, a locking of the piezoelectric actuator when it strikes a rigid limit stop. Another extreme condition that can be evaluated is an unhindered longitudinal displacement of the actuator. These conditions have an electric effect in such a way that, in the case of locking, the electric capacitance C
B
of the piezostack of the actuator is less than the electric capacitance C
F
in the case in which the piezostack can expand or contract unhindered. For the aforementioned condition the relation C
B
=C
F
(1−k
2
) is valid whereby k is a material parameter, also called coupling coefficient typically having a value of 0.65. Between these two extreme conditions the electric capacitance of the piezostack continually changes according to the degree of motion inhibition exerted on the piezostack.
The calibration method of the invention comprises the following steps:
a) positioning a piezoelectric actuator so that a piezoelectric displacement caused by an expansion or contraction of the piezoelectric actuator encounters at least one stop,
b) applying an energizing control signal to said piezoelectric actuator or a stack of piezoelectric elements forming part of the actuator, for causing piezoelectric displacement,
c) measuring a characteristic of said piezoelectric actuator and measuring at least one point of time (t
1
, t
2
) when said displacement is stopped by said at least one stop at one end of said displacement,
(d) evaluating said energizing control signal and said characteristic of said piezoelectric actuator for determining a correlation between said energizing control signal and respective displaced positions of said piezoelectric actuator, and
(e) using said correlation for said calibrating or as a calibration.
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“Calibration and characterization of piezoelectric elements as used in scanning tunneling microscopy”, by van de Leemput et al.; Rev. Sci. Instrum. 62(4), Apr., 1991, American Institute of Physics, pp. 989 to 992.
“Subnanometer behavior of a capacitive feedback, piezoelectric displacement actuator”, by Harb et al., Rev. Sci. Instrum. 63(2), Feb., 1992, American Institute of Physics, pp. 1680 to 1689.
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Tamer et al. Feedback Control of Piezoelectric Tube Scanners. Proceedings of the 33rd Conference on Decision and Control. Dec. 1994.
Daimler-Chrysler AG
Medley Peter
Ramirez Nestor
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