Automatic measurement of air-pressure sensor based on two-pressure control instrument

DING Hong-ying(丁红英), ZHAO Zhan(赵 湛), XUAN Yun-dong(轩运动), FANG Zhen(方 震)

(1. The State Key Laboratory of Technology, Institute of Electronics, Beijing 100190, China;2. Graduate University, Chinese Academy of Sciences, Beijing 100190, China)

Automatic measurement of air-pressure sensor based on two-pressure control instrument

DING Hong-ying(丁红英)1,2, ZHAO Zhan(赵 湛)1, XUAN Yun-dong(轩运动)1,2, FANG Zhen(方 震)1

(1. The State Key Laboratory of Technology, Institute of Electronics, Beijing 100190, China;2. Graduate University, Chinese Academy of Sciences, Beijing 100190, China)

To measure the performance of high precision air-pressure sensors in below normal pressure, an automatic measurement instrument has been designed and implemented. It can simulate environment of low pressure from 300 hPa to 1 000 hPa with high accuracy by proportional-integral-derivative (PID) control quickly, and it can also generate various relative humidity by two-pressure control. The results show that this instrument can reach controlled pressure quickly. And it works well with the minimum average pressure difference, and the fluctuation is±0.02 hPa at 500 hPa. And it can keep in a stable status for a long time. It works well in performance testing of pressure sensors.The structure of the system is simple, takes small investment , and can be operated conveniently.

automatic measurement; two- pressure; pressure sensor; proportional integral derivative (PID) control

0 Introduction

Weather condition is so important that the parameters of air have been studied greatly. And low pressure experiment has been widely used in aviation, space flight, instrument and meter fields as well as information industry. It has been used to confirm the environment adaptability and experimental reliability of the product or components. The environmental conditions, especially in industries such as textile, medicine, metal manufacturing and so on, accurate measurement of relative humidity is essentially required. Thus, the calibration of hygrometers is of great importance in industries and research. The development of hygrometers need corresponding testing instrument. Two-pressure method[1]is wildly used in the design of humidity generator. Based on these ideas, the two-pressure control system is presented in this paper, which not only can implement continuous accuracy control of air pressure from vacuum to normal pressure, but also can generate various relative humidity. High accuracy pressure controllers are mainly designed based on field programmable gate array(FPGA) or programmable logic controller (PLC)[2]. And most of them use the fuzzy-PID(proportional integral derivative)[3]technology as the control strategy. Compared with them, this instrument composed by computer, its single chip is much simpler and costs low, and it can reach target pressure quickly. At the same time, it works well with the minimum average pressure difference, and can keep in a stable statue for a long time. It is hopeful to be used together with temperature and humidity in the environment simulation equipments in the near future.

1 System design

The system aims at controlling the air pressure to the desired point. The overall structure of the system is shown in Fig.1. The high pressure chamber is designed to saturate air by water in the chamber at high pressure. It provides saturated air to the testing chamber via electronic expansion valve, which can reduce air pressure to various low level pressures before it gets into the testing chamber. While air get into the testing chamber, the vacuum pump would let it out to reduce the pressure to dynamically control the air pressure of the testing chamber. The control of the digital mass flow controller (DMFC) has used the single loop feedback control mode[4]. The DMFC is controlled couple with pressure of the testing chamber. It is simple, taking little investment, and being operated conveniently. It can meet the common requirements of industrial production, and has been wildly used in the control of process. Once the single loop feedback control is installed, the quality of the control is relevant to the selected parameter of the controller.

Fig.1 Overall block diagram of measurement system

2 Hardware configuration and communication network of the system

As is shown in Fig.1, the system consists of high-pressure chamber, electronic expansion valve, testing chamber, vacuum pump, air-pressure sensors, DMFC, and so on.

The testing chamber is a vacuum chamber made of stainless steel. Pressure sensor 1 rangeing from 0 to 1.6 MPa has been set ahead of the electronic expansion valve. It is used to monitor the pressure of the high pressure chamber. And the air is saturated at this pressure. The pressure is very important at the generating of the relative humidity. A high accuracy gas plunger pressure sensor 2 made by Paroscientific has been set in the testing chamber. Its accuracy is 0.02%FS, while the tested pressure ranges 100 hPa to 1 500 hPa.

The communication network of the system is the key of totally automated control system. By using different communication agreements as shown in Fig.2, the communication between PC and other parts are realized. A secure and reliable network has been set up.

The system uses C8051F020 as its embedded controller. It also include A/D converter, I/O port and universal asynchronous receiver/transmitter(UART). It is the core of the controller, which works to calculate, transform and store the switch signals and analogy signals adopted by software module. The microprogrammed control unit (MCU) uses A/D converter to get data from pressure sensor 1. MCU simulates pulse-width modulation (PWM) by I/O module to control the electronic expansion valve controller. DMFC supports RS485 serial port communication. Electric cable and converter can be used together to connect DMFC with computer, and then DMFC then can be controlled directly by the computer via its own communication agreement. And also the computer can get data from the pressure sensor 2 via RS232 serial port directly.

Fig.2 Communication network of the measurement system

3 Software design

Visual Studio 2010 has been used as the developing environment of the software. The software of the system was developed by C# in the .NET framework, which is a very popular development platform for control system in recent years. It has been integrated with COM directly as Delphi. The software flow chart is shown in Fig.3.

In Fig.3, P1is the air pressure of the high pressure chamber, P2is the controlled pressure of the testing chamber and Ptis the target control pressure which has been set by computer. Interface has integrated a data gathering window, which can be used to gather and store data collected from serial ports. The front panel of the programme is designed as Fig.4.

In control system, PID technology has been used to control the open level of the DMFC, while the electronic expansion valve has been set in line with P1. PID control[5]is the most ancient, the most common and the strongest control method in automatic control. The principle sketch has been shown in Fig.5. With a PID controller, three variables have to be selected Kp, Kiand Kd. The digital control using the following relationships, where the variable e is the Error of pressure error, and U is the open level of DMFC. The error is computed after the pressure p2acquired per second.

(1)

Fig.3 Software flow chart of control system

Fig.4 Front panels interface

Fig.5 Principle sketch of PID control

4 Experimental results

The system has achieved the control of testing chamber pressure between 300 hPa-1 000 hPa, while the pressure of the high pressure chamber ranges from 1 024 hPa to 9 000 hPa. Figs.6 and 7 have shown the control of testing pressure. They are tested while the values of Kp, Kiand Kdare 2, 6 and 0, respectively. The set of the parameters is very important to the control of the pressure.

Fig.6 Whole control process of target pressure 1 000 hPa

Fig.7 Fuctuation of the pressure controlled at 500 hPa

Fig.6 shows the whole process of pressure control from vacuum to 1 000 hPa. The whole process lasts for only 8 min. Fig.7 has shows the fluctuation of the pressure controlled at 500 hPa, when the pressure is stable. The experimental results show that pressure can be controlled at 500 hPa with the fluctuation of 2 Pa. And it can be controlled steadily no more than 25 min. This system can be used to control pressure ranging from 300 hPa to 1 000 hPa accurately.

5 Conclusion

This instrument uses C8051F020 as the core of the single chip. Togethering with I/O ports, A/D converter and serial ports communication, the system realizes the control of pressure using PID technology. The stability and reliability of the system has been well reached. It can be used to measure air-pressure sensors automatically with little time cost.

The innovation of the system is as follows:

1) A two-pressure system with one loop feedback control has been used to accurately control pressure for calibrating the pressure sensor.

2) The two-pressure system is hopeful to be used in the testing of more parameters for integrated sensors, for example, humidity, temperature and so on.

[1] ZHANG Wen-dong，YIN Long-de，DING Yi-min, et al. Low dewpoint standard humidity generator using two-temperature and two-pressure method. Shanghai Measurement and Testing, 2002, 29(1): 24-26.

[2] YE Jian-xiong, ZHANG Fa-yun. Application of PLC in constant air pressure control. Proc. of the 2010 International Conference on Intelligent Computation Technology and Automation, IEEE Computer Society, WA,USA, 2010, 1: 771-773.

[3] HE Feng-you, BAO Wei-ning ZHANG Bing, et al. Research of an unattended intelligentized control system of air compressor for supplying constant-pressure air. In: Proceedings of the Second International Conference on Intelligent Computation Technology and Automation, 2009, 1: 838-841.

[4] XIE Jian-ying. Micro-computer control technology, Shanghai: Shanghai Jiaotong University, 1985.

date： 2012-08-29

National Basic Research Program of China (No.2011CB302104); Special Fund for Public Welfare (No.GYHY201004004)

ZHAO Zhan(zhaozhan@mail.ie.ac.cn)

CLD number： TP212.1 Document code： A

1674-8042(2013)01-0006-04

10.3969/j.issn.1674-8042.2013.01.002