JP2509705B2 - Control circuit of electro-pneumatic converter - Google Patents

Description

The present invention relates to a control circuit of an electropneumatic converter used for process control or the like, and particularly to a nozzle flapper mechanism using a piezoelectric element that is displaced according to an applied voltage. The present invention relates to a control circuit for an electropneumatic converter.

[Prior Art] As an electropneumatic converter of this type, there is an electropneumatic converter including a control unit 1, a nozzle flapper unit 2 and a pilot regulator unit 3 as shown in FIG. The nozzle flapper portion 2 is composed of an orifice 4, a nozzle 5, and a piezoelectric element flapper 6 that adjusts the opening portion facing the nozzle 5, and the piezoelectric element flapper 6 is a so-called piezo bimorph in which piezoelectric elements are bonded to both surfaces of a thin metal plate. The applied voltage causes displacement to manipulate the nozzle opening and control pilot pressure. In the pilot regulator section 3, a main valve 9 is operated via an exhaust valve 8 which is operated by the action of two diaphragms 7 having different pressure receiving areas. Then, the control circuit 10 compares the input signal with the feedback signal from the pressure sensor 11 to control the applied voltage to the piezoelectric element flapper 6, and when the input signal increases compared to the feedback signal, the control circuit 10 applies the voltage to the piezoelectric element flapper 6. Since the applied voltage increases, the tip of the piezoelectric element flapper 6 is displaced downward and tries to close the nozzle opening. Here, a part of the supply air flows into the pilot chamber 12 through the orifice 4, but when the nozzle opening is closed by the piezoelectric element flapper 6, the pressure in the pilot chamber 12 rises and the diaphragm 7 is exhausted. The main valve 9 is opened by pushing the valve 8 downward, and air is supplied from the input port 13 to the output port 14. When the pressure on the output port 14 side rises, the pressure between the two diaphragms 7 also rises, trying to push back the exhaust valve 8 upward, and at the same time the feedback signal from the pressure sensor 11 increases and becomes an input signal. Difference decreases, the displacement of the piezoelectric element flapper 6 decreases, the opening of the nozzle 5 opens, and the pilot pressure decreases, so that the exhaust valve 8 moves upward and the main valve 9 closes. When the output pressure is high with respect to the input signal, the diaphragm 7 is pushed further upward, the exhaust valve 8 is opened, and exhaust is performed from the exhaust port 15. With the above operation, an output corresponding to the input signal can be obtained.

Further, generally, a unidirectional (positive polarity) DC voltage is applied to the piezoelectric element to displace the piezoelectric element. However, if the piezoelectric element is controlled only by a unidirectional voltage, plastic deformation occurs in the direction in which the element itself attempts to control, and permanent strain occurs. This phenomenon becomes more remarkable as the applied voltage is higher and the voltage application time is longer. When the permanent distortion occurs, there is a disadvantage that the initial characteristics change and the nozzle opening cannot be adjusted accurately. In order to eliminate this inconvenience, Japanese Patent Laid-Open No. 59-138922 discloses that the voltage applied to the piezoelectric element flapper is not unidirectional but positive / negative bipolar, that is, bidirectional depending on an input electric signal. And a configuration in which the applied voltage is 0 V when the input electric signal is at a middle level of about 50% is disclosed.

[Problems to be Solved by the Invention] However, the one disclosed in the above publication cannot distinguish between the time when there is no input electric signal and the time when the input electric signal is about 50%. There is a problem that the air pressure cannot be reduced to zero. To solve this problem, JP-A-59-
In 141017, in addition to the configuration of Japanese Patent Laid-Open No. 59-138922, another DC voltage source is provided so that a negative DC voltage is forcibly applied to the piezoelectric element flapper when the input electric signal is interrupted. Although an empty converter is disclosed, there is a problem that the configuration becomes complicated.

Further, the electropneumatic converter using the piezoelectric element flapper 6 performs feedback control having the set pressure detecting means inside, and for example, the control circuit shown in FIG. 7 is adopted. In this control circuit, the pressure detection signal S1 of the pressure sensor 11 is input to the inverting input terminal of the signal comparator (opamp) 16, and the setting signal S is input to the non-inverting input terminal. Therefore, when the setting signal is input to the signal comparator 16 with no supply pressure or below the rated supply pressure, the signal comparator 16 always generates the maximum output and outputs the piezoelectric element flapper 6.
Also, the maximum voltage is applied to promote the plastic deformation of the piezoelectric element.

The present invention has been made in view of the above problems, and an object thereof is a stable electropneumatic device having a simple structure capable of preventing permanent strain of a piezoelectric element flapper due to plastic deformation of a piezoelectric element and having little change in initial characteristics. It is to provide a control circuit for the converter.

[Means for Solving the Problems] In order to achieve the above-mentioned object, in the present invention, in an electropneumatic converter using a piezoelectric element that is displaced according to an applied voltage to a nozzle flapper mechanism, the output pressure of the electropneumatic converter is A signal comparator for inputting a setting signal to be set and a pressure detection signal of a pressure sensor for detecting an output pressure and comparing both signals, and a unidirectional voltage is applied to the piezoelectric element based on the output signal of the signal comparator. With the piezoelectric element drive circuit section for outputting, when the pressure detection signal of the signal comparator is input to the inverting input terminal when there is no supply pressure to the electropneumatic converter or when the supply pressure is below the rated supply pressure, A voltage limiting circuit is provided for returning a part of the output voltage of the signal comparator to the inverting input terminal to which the pressure detection signal is input.

Further, according to the second aspect of the invention, in the electropneumatic converter using a piezoelectric element which is displaced in the nozzle flapper mechanism according to the applied voltage, a setting signal for setting the output pressure of the electropneumatic converter and the output pressure are detected. And a piezoelectric element drive circuit section for outputting a unidirectional voltage to the piezoelectric element based on the output signal of the signal comparator. In addition to providing the piezoelectric element drive circuit section, when the setting signal is not input to the signal comparator or when the value of the pressure detection signal is larger than the value of the setting signal, the piezoelectric element generates a voltage in the opposite direction to that during piezoelectric element drive control. A reverse voltage generating circuit is provided.

[Operation] In the control circuit of the electro-pneumatic converter of the present invention, the setting signal for setting the output pressure of the electro-pneumatic converter and the pressure detection signal of the pressure sensor for detecting the output pressure are input to the signal comparator, and the signal is inputted. A piezoelectric element drive circuit section that outputs a unidirectional voltage to the piezoelectric element is activated based on the output signal of the comparator. In the invention described in the first claim, when the electropneumatic converter has no supply pressure or less than the rated supply pressure, part of the output voltage of the signal comparator is caused by the action of the voltage limiting circuit. The pressure detection signal is returned to the inverting input terminal and the output voltage of the signal comparator does not reach the maximum output but becomes a value larger than the set signal voltage by a predetermined ratio.

According to the second aspect of the invention, the reverse voltage generating circuit of the piezoelectric element drive circuit section is in a state where the setting signal is not input to the signal comparator or when the value of the pressure detection signal is larger than the value of the setting signal. Is activated to apply a voltage in the opposite direction to that at the time of piezoelectric element drive control.

[First Embodiment] A first embodiment of the present invention will be described below with reference to FIGS. As shown in FIG. 1, the piezoelectric element drive circuit section 17 that operates based on the output signal of the signal comparator 16 and applies a unidirectional control voltage to the piezoelectric element flapper 6 has an NPN.
Transistor Tr1 and PNP transistor Tr2 have their emitters connected to each other and each base has a signal comparator 16
It is provided in a state of being connected to the output side of. The collector of the NPN transistor Tr1 is connected to the common of the DC power supply, and the collector of the PNP transistor Tr2 is connected to the negative (−) DC power supply and the negative (−) side of the piezoelectric element flapper 6. The emitters of both transistors Tr1 and Tr2 are connected to the positive (+) side of the piezoelectric element flapper 6.

The pressure detection signal S1 is applied to the inverting input terminal of the signal comparator 16,
The setting signal S is input to each non-inverting input terminal. The signal comparator 16 has a function of returning a part of the output voltage of the signal comparator 16 to the inverting input terminal when the electropneumatic converter has no supply pressure or is less than the rated supply pressure. However, during normal control, reverse input =
It is connected in parallel with a capacitor C for non-inverting input. The voltage limiting circuit 18 is composed of a PNP transistor Tr3 and a Zener diode Zd1, the emitter of the PNP transistor Tr3 is the anode of the Zener diode Zd1, the collector is the inverting input terminal of the signal comparator 16, and the base is the non-inverting of the signal comparator 16. Each is connected to the input terminal side.

Next, the operation of the control circuit configured as described above will be described. When the electropneumatic converter has a rated supply pressure, the setting signal S and the pressure detection signal S1 are input to the signal comparator 16,
The signal comparator 16 outputs a predetermined voltage based on both signals. Along with this, NPN transistor Tr1 is turned on,
The PNP transistor Tr2 is turned off. And the signal comparator 1
A high voltage proportional to the output voltage of 6 is applied to the piezoelectric element flapper 6 in its operating direction, and the piezoelectric element flapper 6 is drive-controlled. The relationship between the output voltage V IN of the signal comparator 16 and the piezoelectric element terminal voltage V OUT is as shown by the solid line in FIG.

When the electro-pneumatic converter has no supply pressure or does not reach the rated supply pressure, if the setting signal S is input to the signal comparator 16 when there is no voltage limiting circuit as in the conventional case, the signal comparator 16 The maximum output of the performance of the signal comparator 16 is always generated from 16 regardless of the magnitude of the setting signal S, and the maximum voltage is always applied between the terminals of the piezoelectric element flapper 6 as shown by the broken line in FIG. The plastic change of the piezoelectric element is promoted.

However, since the control circuit of this embodiment includes the voltage limiting circuit 18, the Zener voltage of the Zener diode Zd1 is input to the inverting input terminal of the signal comparator 16,
The maximum output voltage from 16 is the sum of the setting signal S and the Zener voltage. That is, when there is no supply pressure to the electropneumatic converter or when the supply pressure is below the rated supply pressure, the change in the voltage V OUT between the piezoelectric element terminals becomes as shown by the chain line in FIG. When there is pressure, the voltage V OUT between the piezoelectric elements when outputting the set pressure is suppressed to a value with a certain margin, and applied to the piezoelectric element flapper 6 as compared with the case where there is no voltage limiting circuit shown by the broken line. The applied voltage becomes smaller. Therefore, the voltage limiting circuit 1 having a simple structure of combining the transistor Tr3 and the Zener diode Zd1
By simply adding 8, unlike the prior art, the plastic change of the piezoelectric element is suppressed and the initial characteristics of the piezoelectric element flapper are maintained.

[Second Embodiment] Next, a second embodiment will be described with reference to FIGS. In the control circuit of this embodiment, in addition to the configuration of the above-described embodiment, when the setting signal S is not input to the signal comparator 16 or when the value of the pressure detection signal S1 is larger than the value of the setting signal S, the piezoelectric element flapper 6 is piezoelectric. The piezoelectric element drive circuit section 17 is provided with a reverse voltage generation circuit 19 that generates a voltage in the opposite direction to that during element drive control. That is, between the collector of the PNP transistor Tr2 and the common of the DC power supply, the reverse voltage generating circuit 19 in which the resistor R1 and the Zener diode Zd2 are connected in series is connected, and the cathode side of the Zener diode Zd2 is connected to the piezoelectric element flapper. 6 is connected to the negative (-) side.

Next, the operation of the control circuit configured as described above will be described. When the setting signal S is input to the signal comparator 16 and the pressure detection signal (S1) smaller than the setting signal S is input, the signal comparator 16 outputs a positive output voltage.
Along with this, the NPN transistor Tr1 is turned on and the PNP transistor Tr2 is turned off. Then, a high voltage proportional to the output voltage of the signal comparator 16 is applied to the piezoelectric element flapper 6 in its operating direction, and the piezoelectric element flapper 6 is drive-controlled.

On the other hand, when the setting signal S is not input to the signal comparator 16 or the pressure detection signal is larger than the setting signal S, the output voltage from the signal comparator 16 becomes zero. As a result, the NPN transistor Tr1 turns off and the PNP transistor Tr1
Tr2 turns ON. When the PNP transistor Tr2 is turned on, the positive (+) side of the piezoelectric element flapper 6 has a voltage of -V,
The negative (-) side of the piezoelectric element flapper 6 becomes a voltage obtained by adding -V and the Zener voltage of the Zener diode Zd2, and as a result, the piezoelectric element flapper 6 has a Zener diode Zd.
A constant voltage equal to the Zener voltage of 2 is applied in the opposite direction to its operating direction. That is, unlike the conventional case, when the piezoelectric element flapper 6 is not drive-controlled, a voltage is applied to the piezoelectric element which has a function of canceling a plastic change generated by a unidirectional voltage applied during drive control. Therefore, the plastic change of the piezoelectric element is suppressed and the initial characteristics of the piezoelectric element flapper 6 are maintained.

The present invention is not limited to the above-described embodiments, and for example, in the second embodiment, the voltage limiting circuit 18 may be omitted and only the reverse voltage generating circuit 19 may be provided. or,
Zener diode Zd2 as reverse voltage generator 19
2 as shown in FIG. 5 instead of the combination of the resistor and the resistor R1.
Alternatively, the resistors R1 and R2 may be used to generate a reverse directional voltage by resistance division.

[Effects of the Invention] As described in detail above, according to the invention described in the first claim, the electropneumatic converter has no supply pressure or is rated only by adding a voltage limiting circuit having a simple structure. If the setting signal is input when the supply pressure is not reached, the output of the signal comparator does not reach the maximum output voltage but becomes a value that is higher than the set signal voltage by a specified percentage, unlike the conventional case. The voltage applied to the piezoelectric element becomes small, the plastic change of the piezoelectric element is suppressed, and the initial characteristics are maintained. Especially when the set signal voltage is low, the effect becomes large.

Further, according to the invention described in the second aspect, by only adding the reverse voltage generating circuit having a simple structure, when the setting signal is not input to the electropneumatic converter or the pressure detection signal is generated. When the voltage is higher than the set signal, the reverse voltage generation circuit is activated to apply a voltage to the piezoelectric element in the opposite direction to that during normal drive control, which suppresses plastic deformation of the piezoelectric element and reduces the initial characteristics. Maintained.

[Brief description of drawings]

1 and 2 show a first embodiment embodying the present invention. FIG. 1 shows a circuit diagram, and FIG. 2 shows a relationship between an output voltage of a signal comparator and a voltage between piezoelectric element terminals. A diagram, FIGS. 3 and 4 show a second embodiment, FIG. 3 is a circuit diagram, and FIG. 4 is a diagram showing the relationship between the output voltage of the signal comparator and the voltage between the piezoelectric element terminals,
FIG. 5 is a diagram of a reverse voltage generating circuit of a modified example, FIG. 6 is a sectional view of an electropneumatic converter, and FIG. 7 is a conventional control circuit diagram. Piezoelectric element flapper 6, pressure sensor 11, signal comparator 16, piezoelectric element drive circuit unit 17, voltage limiting circuit 18, reverse direction voltage generating circuit 19, setting signal S, pressure detection signal S1, Zener diodes Zd1, Zd2, transistors Tr1, Tr2, Tr3.

Claims (2)

(57) [Claims] 1. An electropneumatic converter using a piezoelectric element that is displaced in accordance with an applied voltage to a nozzle flapper mechanism, a setting signal (S) for setting the output pressure of the electropneumatic converter, and a pressure sensor (for detecting the output pressure). 11) The pressure detection signal (S1) is input and a signal comparator (16) for comparing both signals is output, and a unidirectional voltage is output to the piezoelectric element based on the output signal of the signal comparator (16). A piezoelectric element drive circuit section (17) is provided, and the inverting input terminal to which the pressure detection signal (S1) of the signal comparator (16) is input is supplied to the electropneumatic converter in a state where there is no pressure or a rated supply. In the electro-pneumatic converter provided with a voltage limiting circuit (18) that returns part of the output voltage of the signal comparator (16) to the inverting input terminal to which the pressure detection signal (S1) is input when the pressure is below the pressure. Control circuit. 2. An electropneumatic converter using a piezoelectric element that is displaced in accordance with an applied voltage to a nozzle flapper mechanism, a setting signal (S) for setting the output pressure of the electropneumatic converter, and a pressure sensor (for detecting the output pressure). 11) The pressure detection signal (S1) is input and a signal comparator (16) for comparing both signals is output, and a unidirectional voltage is output to the piezoelectric element based on the output signal of the signal comparator (16). A piezoelectric element drive circuit section (17) for operating the piezoelectric element drive circuit section (17).
When the setting signal (S) is not input to 6) or when the value of the pressure detection signal (S1) is larger than the value of the setting signal (S), a reverse voltage is generated in the piezoelectric element in the opposite direction to the piezoelectric element drive control. Control circuit of an electropneumatic converter provided with a static voltage generating circuit (19).