Structural principle of piezoelectric valves and their application in intelligent valve positioners

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The pneumatic components of an intelligent valve positioner generally consist of an electro-pneumatic converter and a pneumatic amplifier. Electro-pneumatic converters primarily use two types of technologies: piezoelectric valve technology based on the piezoelectric effect of crystals with asymmetric structures, which typically receives digital signals (electrical pulses) to generate two-stage pneumatic outputs ； I/P converter technology based on electromagnetic principles and pneumatic nozzle/valve mechanisms typically accepts analog electrical signals to generate continuous pneumatic output. The intelligent valve positioners of Siemens’ SIPARTPS SP2 in Germany and NelesMetso’s ND9000 in the United States use electro-pneumatic converters with a piezoelectric valve structure ； Valve positioners such as Fisher’s DVC6000 in the United States, SAM-SON373X in Germany, and ABB’s SVP3000 in Japan use electro-pneumatic converters with a nozzle-and-damper structure. The electro-pneumatic converters of these two structures represent the mainstream of intelligent positioners in the international market today. A piezoelectric valve is a new type of control valve that utilizes the piezoelectric effect of piezoelectric materials to operate the valve; it boasts advantages such as high precision, fast response, low power consumption, long service life, and a compact design. This text introduces the structure and operating principle of several commonly used piezoelectric valves, as well as the pneumatic structure of intelligent valve positioners equipped with piezoelectric valves. Using Siemens’ SIPART SP2 intelligent valve positioner as an example, it explains the working principle of intelligent valve positioners that employ piezoelectric valve technology. 1. Structure and working principle of piezoelectric valves: yunrun.com.cn/tech/2210.html Piezoelectric valves operate on the principle of the inverse piezoelectric effect. They feature energy efficiency and low power consumption (with a drive current of only 10 MA), precision and miniaturization, fast response times, and good durability. They also facilitate the full digitalization of valve positioners. At present, the piezoelectric valve components used in the pneumatic parts of intelligent valve positioners are mostly produced by the German company Hoerbiger. The P9 series of piezoelectric valve elements and the P20 series of piezoelectric valve assemblies are commonly used; the piezoelectric valve components in Siemens’ PS2 model are also custom-made from Hoerbiger. Taking the piezoelectric valve of Holbig Company as an example, its structure and working principle are introduced. ①Direct-acting piezoelectric valve: The principle of a piezoelectric valve is to utilize the deformation of piezoelectric materials under the influence of an electric field in order to open and close the air inlet of the pneumatic valve. The structure of the micro direct-acting directional valve is shown in Figure 1: http://yunrun.com.cn/upload/201810/16/201810160037223794.png Figure 1 shows the structure and working principle of the direct-acting piezoelectric valve. The curved component located in the middle of the piezoelectric valve is a piezoelectric sheet made of piezoelectric material. The structure of the piezoelectric element consists of a piezoelectric crystal formed by bonding two extremely thin elastic metal sheets together; electrodes are created on the two working surfaces of this piezoelectric element through vacuum coating. The deformation of the piezoelectric element under the influence of an electric field is utilized to achieve two-position switching in micro air circuits. When the piezoelectric crystal is not powered, the inlet port 1 for compressed air is closed, the outlet port 2 is connected to the atmosphere-port 3, and the output air pressure is at atmospheric pressure, which is equivalent to the valve being closed ； When the piezoelectric crystal is powered, the upper layer of the crystal contracts while the lower layer expands; the mechanical deformation resulting from this upward movement is several dozen micrometers. When the air hole 3 is closed, compressed air flows from hole 1 to hole 2, generating an air pressure signal, which is equivalent to a valve being opened. The bending degree of the piezoelectric sheet is related to the input voltage; the response time is less than 2 ms, and the hysteresis voltage for the operation of the two-position switch is approximately 4 V. Piezoelectric valves can also be made as proportional-output types, but due to the large hysteresis between their upward and downward operations (with a difference in operating voltage of about 2V), they are rarely used in the pneumatic components of intelligent valve positioners. ②Power-amplifying piezoelectric valves: Direct-acting piezoelectric valves enable the conversion of electrical signals into pneumatic signals. However, in valve positioners, it is necessary to amplify the pneumatic signals in order to drive the actuator of the control valve. To address this issue of pneumatic signal amplification, manufacturers have developed power-amplifying piezoelectric valves. The internal structure and working principle of the P20 series piezoelectric valves are shown in Figure 2: http://yunrun.com.cn/upload/201810/16/201810160050443785.png Figure 2 illustrates the structure and working principle of the direct-acting piezoelectric valve. The P20 consists of a P9 direct-acting piezoelectric valve element, a pneumatic amplifier (composed of an air chamber, diaphragm, and valve core), a micro-reductor, and a filter. Its air circuit structure is similar to that of a 2/3-way solenoid valve, featuring an air supply port, an air inlet (connected to the actuator), and an exhaust port (connected to the atmosphere). The operating voltage is 24VDC, the response time is less than 20ms, the gas supply pressure ranges from 120 to 800 kPa, and the maximum gas flow rate is 7.8 Nm3/h. In Figure 2, the left side shows the state of the gas circuit when it is powered off, while the right side shows its state when it is powered on. When the piezoelectric valve is powered, the piezoelectric element moves to connect the air source, which has been depressurized and filtered, to the output port of the direct-acting piezoelectric valve. The air signal acts on the diaphragm of the pneumatic amplifier, causing the main valve to open; this allows the air signal to pass through the valve core of the main valve and reach the inlet 2, from where it is then sent to the actuator to drive the valve into action, while simultaneously closing the exhaust port. When the piezoelectric valve loses power, the piezoelectric element acts to seal the air inlet port; at the same time, the output air port and the exhaust port become connected. The gas in the air chamber of the pneumatic amplifier is released through the exhaust port. The diaphragm of the pneumatic amplifier returns to its original position under the action of the main valve spring, causing the valve core of the main valve to close. As a result, the inlet and exhaust ports of P20 become connected, and the gas inside the actuator is expelled through the exhaust port. The valve moves in the opposite direction due to the loss of pressure, driven by the spring. A power-amplifying piezoelectric valve is structurally equivalent to a combination of a direct-acting piezoelectric valve and a pneumatic signal amplifier. It enables the conversion of electrical signals into pneumatic signals, while also amplifying the power of the pneumatic signals. This facilitates the design and manufacture of intelligent valve positioners; as a result, power-amplifying piezoelectric valves are widely used in the pneumatic control systems of intelligent valve positioners. 2. Intelligent valve positioner with piezoelectric valve: ① An intelligent positioner that uses a piezoelectric valve in its gas circuit structure typically consists of two power-amplifying piezoelectric valve components (PV1, PV2) and two check valves (RV1, RV2), as shown in Figure 3. http://yunrun.com.cn/upload/201810/16/201810160102257019.png Figure 3: Pneumatic circuit structure of the intelligent valve positioner with a piezoelectric valve. The pneumatic components can be in three different pneumatic circuit states: a) State 1: PV1 is powered on, PV2 is powered on, RV1 is open, and RV2 is closed ； The positioner outputs a pneumatic signal to the diaphragm chamber of the pneumatic actuator of the control valve, causing the valve stem of the control valve to move in one direction. b、State 2: PV1 powered off, PV2 powered on, RV1 closed, RV2 closed ； The air circuit of the positioner is sealed, preventing air pressure from reaching the pneumatic actuator; thus, the air circuits of the entire positioner and the control valve remain in their original state, with the valve position of the control valve remaining unchanged. c、State 3: PV1 powered off, PV2 powered off, RV1 closed, RV2 open ； The air circuit of the locator is in exhaust mode; the diaphragm chamber of the pneumatic actuator is connected to the atmosphere through the air circuit of the piezoelectric valve, thereby expelling the gas from within the actuator’s diaphragm chamber. The valve stem of the control valve moves in the opposite direction (compared to State 1). The intelligent valve positioner achieves regulation of the control valve’s air circuit through the above three air circuit states, thereby controlling the operation of the control valve. ②Working principle of intelligent valve positioners: Compared with traditional positioners, intelligent valve positioners have essentially the same control principle; both adjust the output pressure signal after comparing the input signal with the position feedback. However, intelligent positioners and conventional positioners differ significantly in terms of their actuating elements, that is, their working mechanisms: intelligent valve positioners use a microprocessor as their core and employ new types of electro-pneumatic conversion devices. The Siemens SIPARTPS2 series of intelligent electrical valve positioners are based on microprocessors; they use new piezoelectric valves to replace the nozzle and baffle pressure-regulation systems found in traditional positioners, thereby enabling precise control of the output pressure and ensuring accurate positioning of the valve. The structure of the SIPART PS2 series positioners mainly includes components such as a piezoelectric valve unit, a piezoelectric drive circuit unit, a microprocessor unit, an LCD operation panel, a valve position sensor, and input circuits. Working principle of SIPART PS2: The valve stem position sensor detects the actual opening degree of the valve, converts it into a digital signal through A/D conversion, and compares this digital signal with the digital value set externally (inputted) by the positioner in the CPU. The difference between the two values is calculated; if this difference exceeds the positioning accuracy, the CPU outputs a control signal to activate the two piezoelectric valves. When the external set signal is greater than the valve position feedback, the piezoelectric valve V1 opens, the output air pressure P1 increases, and the increase in pressure within the actuator’s diaphragm chamber leads to an increased valve opening degree, thereby reducing the difference between the valve position feedback signal and the input signal ； When the external set signal is lower than the valve position feedback signal, the piezoelectric valve V2 opens, allowing air to be exhausted through the exhaust port and thereby reducing the pressure P1 of the output air source. The decrease in pressure within the actuator’s diaphragm chamber results in a reduced valve opening, which in turn reduces the discrepancy between the two signals. Features of the SIPART PS2 intelligent locator: Output pressure regulation is achieved using PID pulse width modulation (PWM) technology, ensuring speed and accuracy ； A new type of piezoelectric valve device is used to ensure high control precision ； The air consumption of the locator is extremely low. Intelligent positioners that utilize piezoelectric valve technology offer advantages such as high control precision, fast adjustment speeds, and low gas consumption; they represent the future direction of development for intelligent positioners. As time goes by and technology advances, the use of piezoelectric valves will become even more widespread.