Electrical generator or motor structure – Non-dynamoelectric – Piezoelectric elements and devices
Reexamination Certificate
1999-06-16
2001-07-31
Budd, Mark O. (Department: 2834)
Electrical generator or motor structure
Non-dynamoelectric
Piezoelectric elements and devices
Reexamination Certificate
active
06268681
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a piezoelectric transformer drive circuit and in particular to a drive method and circuit which is preferably used for driving of a piezoelectric transformer which is connected with a cold cathode tube having a high variation in impedance as a load.
BACKGROUND OF THE INVENTION
A prior art activating device in which the frequency of a voltage applied to a piezoelectric element is preset to a frequency which is higher than an intended drive frequency and is higher than the resonating frequency and is lower than the harmonics of the resonating frequency, and is gradually lowered is disclosed in, for example, Japanese patent Kokai publication JP-A-3-22873. This is a driving device of an ultrasonic motor for preventing the occurrence of uncomfortable noise on activation of the ultrasonic motor. This prior art device will be described with reference to FIG.
16
.
A plurality of electrostrictive elements of polarized piezoelectric material are disposed on the surface of an ultrasonic motor stator
34
. These plurality of electrostrictive elements are separated into two groups, section A and section B, which are disposed in such a manner that they are out of phase by 90 degrees to each other. The electrostrictive elements of sections A and B are provided with electrodes
47
a
and
47
b
, respectively. An electrode
47
c
is disposed on electrostrictive elements which are insulated from the electrodes
47
a
and
47
b
so as to output a detecting signal depending upon the oscillating condition of the electrostrictive elements.
When a voltage from a power source is applied to this circuit, a microcomputer
40
outputs data concerning an upper limit of the driving frequency to a VCO
38
via a D/A converter
39
. This causes a voltage of the upper limit frequency to be applied to the electrode
47
a
via a frequency divider
37
, amplifier
36
and a coil
35
. Simultaneously with this, a voltage having a frequency which is out of phase by 90 degrees from that applied to the electrode
47
a
is applied to the electrode
47
b.
The electrode
47
b
is driven by a frequency given by “a shift” register
44
having inputs connected to both sides of the frequency divider
37
, and an output connected to an amplifier
45
and further to a coil
46
.
However, since the upper limit frequency is far from the optimal drive frequency, the ultrasonic motor is not activated and no signal is obtained from the electrode
47
c.
Accordingly, a band pass filter (B.P.F.)
43
outputs a
0
level signal, which is fed to the microcomputer
40
via a rectifier
42
and an A/D converter
41
. The signal is compared with a predetermined reference value Ds in the microcomputer
40
for detecting that desired driving is not performed. In response to this result, the microcomputer
40
outputs data for presetting the drive frequency to a frequency which is lower by 0.1 KHz.
A series of above-mentioned operations is repeated until a signal which is input to the microcomputer from the electrode
47
c
via the band pass filter
43
, rectifier
42
and the A/D converter
41
becomes the predetermined reference value Ds, until then drive frequency is continued to lower.
The value of Ds is preset in such a manner that the signal input to the microcomputer
40
is higher than the reference value Ds when the drive frequency is in the vicinity of the resonating frequency after lowering of the drive frequency is continued. When the signal input to the microcomputer
40
is actually higher than the reference value Ds, the microcomputer
40
outputs data for increasing the drive frequency by 0.1 KHz. When the signal input to the microcomputer
40
is lower than the reference value Ds again, the microcomputer
40
outputs data for lowering the frequency correspondingly. Such a series of operations enables control of drive frequency so that the signal input to the microcomputer
40
converges to the vicinity of the reference value Ds.
Since the frequency of the voltage applied to the piezoelectric material is not identical with the resonating frequency on activation of the ultrasonic motor by the above-mentioned operation, uncomfortable noise does not occur.
SUMMARY OF THE DISCLOSURE
However, various problems have been encountered in the course of investigations toward the present invention. Namely, the above-mentioned drive circuit has a problem as follows:
As shown in
FIG. 16
, this drive circuit determines whether the drive frequency is elevated or lowered in the microcomputer
40
. This determination is made based upon the magnitude of the signal from the oscillating electrostrictive elements.
However, frequency control can not be performed based upon only a signal from an oscillating state of piezoelectric elements in case of, for example, a piezoelectric transformer inverter which is loaded with a cold cathode tube.
The reason will now be described.
The cold cathode tube has a negative impedance. The relation between the output voltage and the input current is shown in FIG.
12
. The output voltage and input current both increase in a range a in
FIG. 12
before starting of lighting. The input current increases as the output current decreases in a range b in
FIG. 12
after lighting. The output voltage exhibits almost no decrease although the input current increases in a range c in
FIG. 12
in which the input current increases.
A cold cathode tube in a back light for LCD (liquid crystal display) is generally used in the range c in
FIG. 12
in which the output voltage hardly decreases even if the input current increases.
The current is proportional to the brightness in a practical range of the cold cathode tube. It has been found from
FIG. 12
that the voltage hardly changes even if the current changes in the practical range of the cold cathode tube.
In the piezoelectric transformer, the signal which can be readily taken out from the oscillating piezoelectric transformer is an output voltage. From the foregoing, the piezoelectric transformer inverter can not be controlled at a high precision with a voltage which is a signal from oscillating state of the piezoelectric transformer.
Accordingly, a unique system for controlling a piezoelectric transformer inverter which is loaded with a cold cathode tube has been demanded.
Therefore, it is an object of the present invention to provide a method and circuit for driving a piezoelectric transformer which is capable of preventing the breaking down of the piezoelectric transformer caused by excessive oscillation on activation thereof and simultaneously of achieving high efficiency in the method and circuit for driving the piezoelectric transformer having a feature of high load dependency and which is loaded with a cold cathode tube having a high variation in impedance. Other objects and advantages of the present invention will become apparent from the entire disclosure including the following description.
According to an aspect of the present invention, the present invention is characterized in that it comprises control means to control the load power constant, wherein driving of said piezoelectric transformer is initiated at a frequency which is higher than the resonating frequency of the piezoelectric transformer on activation thereof, followed by gradually lowering the drive frequency, and drive frequency of the piezoelectric transformer is controlled so that it will not pass through the resonating frequency of said piezoelectric transformer on activation thereof.
In a second aspect of the present invention, the control means is characterized in that it controls the load current to a constant value at a frequency higher than the resonating frequency of the piezoelectric transformer.
Other aspects of the present invention will become apparent in the following description.
REFERENCES:
patent: 4459505 (1984-07-01), Lim
patent: 5329200 (1994-07-01), Zaitsu
patent: 5705879 (1998-01-01), Abe et al.
patent: 5731652 (1998-03-01), Shimada
patent: 5739622 (1998-04-01), Zaitsu
patent: 5796213 (1998-08-01), Kawasaki
patent: 589
Furuhashi Naoki
Yamaguchi Shuuji
Budd Mark O.
Laff, Whitesel & Saret, Ltd.
NEC Corporation
Whitesel J. Warren
LandOfFree
Method and circuit for driving piezoelectric transformer does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Method and circuit for driving piezoelectric transformer, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Method and circuit for driving piezoelectric transformer will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2566929