JPH0321049B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (a) Field of Industrial Application This invention relates to an electro-pneumatic converter used for process control, etc., and particularly to an electro-pneumatic converter using a piezoelectric element in a nozzle flapper.

(b) Prior art As shown in Figure 1, in a conventional electro-pneumatic converter using a piezoelectric element, the input current Ii is connected to a Zener diode Z.
1 flows through the volume VR, and a voltage ei proportional to the input current Ii is obtained at the input resistor Ri of the volume VR, and this voltage ei and the voltage of the Zener diode Z2 are added by the resistors R2 and R3, and a micropower operational amplifier is generated. The output voltage Eo of the operational amplifier 1 is applied to the piezoelectric flapper 2. The piezoelectric element flapper 2 is constructed by sandwiching a metal plate 2a between piezoelectric elements 2b and 2c, and the output voltage Eo of the operational amplifier 1 described above is applied between the piezoelectric elements 2b and 2c and the metal plate 2a.
The piezoelectric element flapper 2 is then displaced according to the applied voltage.

On the other hand, in the pneumatic system, the blowhole of the nozzle 3 faces the piezoelectric element flapper 2 and receives supply pressure, and the back pressure is amplified by the pilot valve 4 and derived as an output pressure. In addition to a pneumatic conversion circuit consisting of a source 5 and a pressure sensor 6, the output pressure is converted into a voltage signal. Further, the voltage signal from the pressure sensor 6 is added to the bias voltage EB by the differential amplifier 7, and the result is added to the non-inverting input terminal of the operational amplifier 1. And operational amplifier 1
There is a difference between the input signal voltage and the voltage fed back from the differential amplifier 7, and when the input signal voltage is larger, the output voltage E ₀ is displaced so that the piezoelectric element flapper 2 approaches the nozzle 3. When the back surface of the nozzle 3 becomes larger accordingly, the voltage from the differential amplifier 7 also becomes larger and approaches the input signal voltage. When the input signal voltage and the voltage fed back from the differential amplifier 7 eventually become equal, the operational amplifier 1 outputs the voltage
Keep E ₀ as is. Therefore, the back surface of the nozzle 3 is also constant, and the output pressure is also constant. As the input signal voltage further increases, it becomes larger than the voltage fed back from the differential amplifier 7, so the operational amplifier 1 shifts the output voltage _E0 so that the piezoelectric element flapper 2 approaches the nozzle 3. The back pressure increases accordingly, and the voltage from the differential amplifier 7 also increases. When the signal input voltage and the voltage fed back from the differential amplifier 7 match, the operational amplifier 1 maintains its output voltage E ₀ at the value at that point, and the back pressure and output pressure of the nozzle 3 also change accordingly. It becomes constant. That is, this electro-pneumatic converter changes the output voltage E ₀ of the operational amplifier 1 so that the voltage fed back from the differential amplifier 7 approaches the input signal voltage, and so that both become equal. , holds the output voltage E ₀ and obtains an output air pressure proportional to the input signal voltage Ei and therefore to the input current Ii.

In the conventional electro-pneumatic converter described above, the control voltage Eo applied to the piezoelectric element flapper 2 has positive polarity, and as shown in FIG.
When changing within the range of 100% (characteristic Y1), the control voltage changes within the range of Eo1 to Eo2. Therefore, during operation, the piezoelectric element flapper 2 is always subjected to a considerable displacement in the same direction, even with an input of about 50%. As a result, the initial value of the mechanical strain of the piezoelectric element material changes, causing a phenomenon in which the operating point shifts. This phenomenon becomes more noticeable as the applied voltage increases and the time increases. Therefore, if the above-mentioned conventional electro-pneumatic converter is operated for a long period of time with a normal input signal applied, that is, with a considerable applied voltage, the piezoelectric element will change over time and mechanical initial conditions will change, e.g. As the flapper approaches the nozzle, the applied voltage Eo required to output the same output signal becomes smaller. Therefore, the operating point, which was originally Y1 shown in Figure 2 and could be controlled in the range of 0% to 10%, moves to Y2, for example.
The control voltage required to output a 0% signal is -
Eo'1, which deviates from the positive applied voltage range, and control from 0% to 100% is no longer possible. This makes it impossible to use the electro-pneumatic converter for a long period of time with peace of mind.

(c) Purpose In view of the above, the purpose of the present invention is to provide a stable electro-pneumatic converter in which the operating point does not change much due to changes in the initial value of mechanical strain of the piezoelectric element material.

(d) Configuration In order to achieve the above object, the electro-pneumatic converter of the present invention sets the control voltage applied to the piezoelectric element flapper to around 0V at the middle level of the input electric signal, and when the input electric signal is large or small. It is designed to be a positive electrode or a negative electrode depending on the situation. That is, the electro-pneumatic converter of the present invention includes a piezoelectric element that is displaced according to an applied voltage, a first DC voltage source that outputs a constant DC voltage and applies this DC voltage to the piezoelectric element, and an input electrical signal. and when the input electrical signal is at the middle level of the expected change range, outputs a DC voltage that has approximately the same value as the output DC voltage of the first DC voltage source, and converts this output DC voltage into the first DC voltage source. a second DC voltage source that applies to the piezoelectric element in a manner that cancels out the DC voltage from the DC voltage source; a nozzle that uses the displaced portion of the piezoelectric element as a flapper to receive supply pressure and derive back pressure; a pilot valve that converts back pressure into output pressure; a conversion circuit that converts the output pressure into an electrical signal; and applying the electrical signal converted by the conversion circuit to the input side of the second DC voltage source;
It is comprised of a feedback circuit that balances the input electrical signal.

(E) Examples The present invention will be explained in more detail below with reference to Examples.

FIG. 3 is a circuit diagram of an electro-pneumatic converter showing one embodiment of the present invention. In the figure, the same numbers and symbols as those of the conventional electro-pneumatic converter shown in FIG. 1 are given to circuits and components having the same functions, and detailed explanations thereof will be omitted.

Reference numeral 8 denotes a collector-tuned oscillation circuit (first oscillation circuit), which derives an oscillation output with a constant amplitude. 1
0 is a full-wave rectifier circuit (first rectifier circuit) that rectifies the oscillation signal from the oscillation circuit 8, and the DC voltage E1
is applied to the piezoelectric elements 2b and 2c.

Reference numeral 9 denotes a collector-tuned oscillation circuit (second oscillation circuit) which undergoes amplitude modulation by the output voltage Eo of the operational amplifier 1, and therefore derives an oscillation output with an amplitude corresponding to the input signal. 11 is an oscillation circuit 9
This is a full-wave rectifier circuit (second rectifier circuit) that rectifies the oscillation signal of the piezoelectric element flapper 2, and is configured to apply a DC voltage E2 to the metal plate 2a of the piezoelectric element flapper 2. Note that Ra and Rb are discharge resistances, and Cf and Rf are internal feedback impedance elements to increase the stability of the circuit.

In the electro-pneumatic converter of the above embodiment, if only the voltage circuit applied to the piezoelectric element flapper 2 is taken out and an equivalent circuit is shown, it becomes as shown in FIG. The output DC voltages E1 and E2 of the rectifier circuits 10 and 11 are both of positive polarity, but the DC voltage E1 is
In c, since the DC voltage E2 is applied to the metal plate 2a, both DC voltages E1 and E2 are applied to the piezoelectric element of the piezoelectric element flapper 2 in such a manner that they cancel each other out.
In other words, the voltage applied to the piezoelectric element flapper 2
E′o becomes E′o=E2−E1. Now here, DC voltage
If E2 can be changed from 0 to 2E1,
E′o will vary from −E1 to +E1, that is, from positive to negative polarity.

The DC voltage E2 is amplitude modulated by the output voltage Eo of the operational amplifier 1, and the output voltage Eo changes in response to the input current Ii.
For example, for an input current of 50%, the rectifier circuit 11
By selecting each circuit so that the output DC voltage E2 becomes E1, the input signal-control voltage characteristics of the electro-pneumatic converter of the above embodiment become as shown in FIG. As is clear from the figure, when the input signal is 50%, the control voltage E′o is 0V, and the control voltage changes positive and negative around this 0V according to changes in the input signal, so it is normal operation. In this state, only a small voltage is applied to the piezoelectric element flapper 2.

In addition, in this embodiment of the electro-pneumatic converter, the oscillation outputs of the oscillation circuits 8 and 9 are converted to the rectifier circuits 10 and 11, respectively.
Since the DC voltage obtained by rectification is applied to the piezoelectric element flapper 2, the oscillation circuits 8 and 9
By outputting an oscillation signal with a larger amplitude, it is possible to apply a high voltage to the piezoelectric element flapper 2, and there is a margin in setting the control voltage range. Even if the operating point fluctuates, the effect can be absorbed, and it can also follow changes in the operating voltage due to thermal strain and mechanical strain (posture error, vibration, changes over time) in the nozzle flapper system.

In the above embodiments, a collector-tuned oscillation circuit is used, but oscillation circuits of other oscillation types may be used.

(f) Effects According to the electro-pneumatic converter of the present invention, the voltage applied to the piezoelectric element flapper is not unidirectional, but has bipolar polarity, that is, bidirectional, depending on the input electrical signal. For example, when the input signal is at a medium level of about 50%, the applied voltage is
Since the voltage is 0V, the displacement under normal operating conditions is small, and therefore there is almost no initial value fluctuation of the piezoelectric element material, and the fluctuation of the operating point can be suppressed to a small level, so that it cannot be controlled. will be resolved.

[Brief explanation of the drawing]

Fig. 1 is a circuit diagram showing a conventional electro-pneumatic converter, Fig. 2 is a diagram showing input signal-control voltage characteristics to explain operating point fluctuations of the conventional electro-pneumatic converter, and Fig. 3
The figure is a circuit diagram of an electro-pneumatic converter showing one embodiment of the present invention, FIG. 4 is a diagram showing an equivalent circuit of the piezoelectric element flapper voltage application section of the electro-pneumatic converter, and FIG. 5 is the same embodiment. FIG. 3 is a diagram showing input signal-control voltage characteristics of an electro-pneumatic converter. 1; operational amplifier, 2; piezoelectric flapper, 3;
Nozzle, 4; Pilot valve, 5; Constant current source, 6;
Pressure sensor, 7; differential amplifier, 8, 9; oscillation circuit, 10, 11; rectifier circuit.

Claims (1)

[Scope of Claims] 1. A piezoelectric element that is displaced according to an applied voltage, a first DC voltage source that outputs a constant DC voltage and applies this DC voltage to the piezoelectric element, and a piezoelectric element that is displaced according to an input electric signal. When the input electric signal changes and the input electric signal is at a middle level of the planned change range, outputs a DC voltage that has approximately the same value as the output DC voltage of the first DC voltage source, and converts this output DC voltage into the first DC voltage. a second DC voltage source applied to the piezoelectric element in a manner to offset the DC voltage from the piezoelectric element; a nozzle that receives supply pressure and derives back pressure by using a displaced portion of the piezoelectric element as a flap; and a nozzle that receives supply pressure and derives back pressure; a pilot valve that converts the output pressure into an output pressure; a conversion circuit that converts the output pressure into an electrical signal; and the electrical signal converted by the conversion circuit is applied to the input side of the second DC voltage source, and the electrical signal is connected to the input electrical signal. An electro-pneumatic converter consisting of a balancing feedback circuit. 2. The first DC voltage source includes a first oscillation circuit that oscillates with a constant amplitude and a first rectifier circuit that rectifies the oscillation output of the first oscillation circuit, and the second DC voltage source Claim 1, characterized in that the device comprises a second oscillation circuit whose amplitude changes according to the input electric signal, and a second rectifier circuit which rectifies the oscillation output of the second oscillation circuit. Electro-pneumatic converter.