Control of Vacuum Pressure between Negative and Positive Pressure - Electronic Flow Control Valve Application

Continuous Control of Vacuum Pressure between Low and High Pressure (Negative and Positive Pressure)

Abstract: For some vacuum pressure applications, it is necessary to reach one-way or alternating continuous precision control between low pressure and high pressure (or negative pressure to positive pressure). This article focuses on the application of electronic flow control valve to reach alternating positive and negative pressure control, different configuration and technical parameters of regulating valve are introduced in detail.

1. Introduction

In some vacuum pressure applications, the air pressure is often required to change in one direction or alternately between low pressure and high pressure (negative pressure to positive pressure), and the entire change process requires precise control. Typical applications in this regard are:

(1) Calibration device of pressure sensor: For some pressure sensors which measurement range covers negative pressure to positive pressure, their calibration requires a corresponding calibration chamber, and the calibration chamber needs to simulate the corresponding vacuum pressure environment from negative pressure to positive pressure . And in the calibration process, it is necessary to set up multiple calibration points in the range of low pressure to high pressure, and continuously control and measure from high to low (or from low to high), and perform calibration.

(2) Human lung organ performance research device: Controlling the simulated breathing process through positive and negative pressure changes to study the dynamic characteristics of lung organs, thereby guiding and improving ventilators and related instruments.

(3) Atmospheric pressure environment simulation device: In various aviation aircraft, motor vehicles and electrical instruments and other industries, it is necessary to conduct assessment tests in an atmospheric pressure simulation environment, and the corresponding atmospheric pressure simulation chamber also needs continuous positive and negative pressure ranges of control. Sometimes even requires rapid changes between positive and negative pressure to simulate the dynamic characteristics of aircraft's rapid changes in altitude.

(4) Positive and negative pressure conversion of isolation rooms in hospitals: Many hospitals’ operating rooms are mostly positive pressure rooms. With the emergence of covid-19, the positive pressure room needs to be transformed into a negative pressure room. It is even required that the positive pressure room can be used as required to switch between pressure and negative pressure.

(5) Flash evaporation process: The function of flash evaporation process makes the liquid superheat and volatilizes into steam quickly in the environment of rapid change of positive and negative pressure to play a rapid drying effect, and at the same time, it can be used to increase the penetration of liquid to solid.

(6) Soft pneumatic actuators for manipulators: Most soft pneumatic actuators used to generate bending actuation utilize positive or negative pressure, and the bending forces of the positive and negative pressure actuators are combined into a single actuation structure, and generate a large blocking force and still be able to produce a large bending deformation, which improves the effective technical means for the soft robot gripper in applications requiring delicate touch.

This article will propose corresponding solutions for the above-mentioned applications that need to reach one-way or alternating continuous precision control between low pressure and high pressure (or negative pressure to positive pressure). For different vacuum pressure ranges, different control valve configurations and technical parameters are introduced in detail

2. Technical solutions

The basic principle of continuous control of positive and negative pressure intervals is shown in Figure 1. Its purpose is to precisely control the continuous monotonic change (or reciprocating change) of air pressure in vacuum pressure vessel from low pressure to high pressure (or from high pressure to low pressure). The following is the specific content of the control principle:

(1) The control principle is based on the dynamic balance method of inlet and outlet of vacuum pressure vessel, which is a typical closed-loop control loop.

The PID controller collects the pressure sensor signal and compares it with the set value and adjusts the opening of intake electronic flow control valves and exhaust electronic flow control valves, and finally makes the sensor measured value relative to set value to reach accurate vacuum pressure control.

(2) In order to cover the entire vacuum pressure range from low pressure to high pressure, at least two vacuum pressure sensors are configured to be responsible for negative pressure and positive pressure respectively. The PID controller is dual-channel simultaneous control to correspond to the control of low pressure and high pressure ranges, and the PID controller can automatically switch sensors according to different vacuum pressure ranges.

(3) The control loop is equipped with a vacuum pump (negative pressure source) and a high pressure gas source (positive pressure source) to provide sufficient low pressure and high pressure capabilities.

(4) When the control changes from low pressure to high pressure, the opening of intake electronic flow control valve (intake flow) at the beginning is much smaller than the opening of the exhaust electronic flow control valve (extraction flow). Different balance states are used to reach different vacuum pressure control, and finally the opening of intake electronic flow control valve is gradually much larger than that of exhaust control valve, thereby reaching continuous and precise control of a series of set points in the range of low pressure to high pressure. For the change control from high pressure to low pressure, the above process is reversed.

3. The specific configuration of the program

The technical solution proposed in this article includes two parts to cover the following two different vacuum pressure ranges.

(1) The absolute pressure is up to 7bar to the lowest 0.01mbar (1Pa).

The structure of the control system in this vacuum pressure range is shown in Figure 2.

An electronic flow control valve is used for precise control of high vacuum, and the positive pressure withstand pressure of electric needle valve is only 7 bar, so it is determined that high-pressure control range of control system of this configuration does not exceed 7bar.

In addition, for applications that require rapid alternating control of positive and negative pressures, the intake and exhaust electronic flow control valve valves are required to have a high response speed. At this time, an electronic flow control valve with a faster response speed is required.

(2) The absolute pressure is up to 15bar to the lowest 15mbar (1.5kPa).

In order to meet the needs of higher pressure, it is necessary to solve the high pressure bottleneck in the solution in Figure 2. Therefore, the electronic flow control valve with high pressure and pressure difference in Figure 2 is replaced with a vacuum type air-controlled back pressure valve, which can greatly expand the high pressure range, but with corresponding sacrifices in the low pressure range. The control system structure of this high-voltage type is shown in Figure 3.

The valve is driven by an air-operated pilot valve. Due to the use of an air-controlled vacuum type back pressure valve, the high pressure control range can be increased to 15bar, but the corresponding negative pressure has also been increased to 15mbar. If desired, the upper high pressure limit can be raised further, but the lower low pressure limit can also be raised.

In this pilot valve-driven back pressure valve control method shown in Figure 3, in addition to shifting the entire control range to the high pressure side, it also has two characteristics. First is the vacuum pressure control of the larger container. Second is the fast response speed of back pressure valve, which can meet the fast control of the positive and negative pressure reciprocating alternating.

4. Conclusion

Through the above technical solution, the continuous control and the reciprocating alternating control of vacuum pressure in the range of positive and negative pressure can be completely realized, and high control precision and speed can be reached.

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