Journal of Sensors

Volume 2017, Article ID 1702671, 12 pages

https://doi.org/10.1155/2017/1702671

## Research on the Plasma Anemometer Based on AC Glow Discharge

Nanjing University of Aeronautics and Astronautics, Jiangsu Province Key Laboratory of Aerospace Power System, Key Laboratory of Aero-Engine Thermal Environment and Structure, Ministry of Industry and Information Technology, Nanjing 210016, China

Correspondence should be addressed to Bing Yu; nc.ude.aaun@302by

Received 6 December 2016; Revised 5 February 2017; Accepted 7 February 2017; Published 28 February 2017

Academic Editor: Paolo Bruschi

Copyright © 2017 Bing Yu et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

#### Abstract

A new plasma anemometer based on AC glow discharge is designed in this article. Firstly, theoretical analysis of plasma anemometer working principle is introduced to prove the feasibility of the experimental measurement method. Then the experiments are carried out to study the effects of different parameters on the static discharge characteristics of the plasma anemometer system, by which the system optimization methods are obtained. Finally, several groups of appropriate parameters are selected to build the plasma anemometer system based on resistance capacitance coupling negative feedback AC glow discharge, and different airflow speeds are applied to obtain the achievable velocity measurement range. The results show that there is a linear relationship between airflow velocity and discharge current in an allowable error range, which can be applied for airflow velocity measurement. Negative feedback coupling module, which is composed of the coupling resistance and the coupling capacitance, has good effects on improving the system stability. The measurement range of the airflow velocity is significantly increased when the electrode gap is 3 mm, coupling resistance is 470 Ω, and coupling capacitance is 220 pF.

#### 1. Introduction

Airflow velocity measurement technology is a long-studied subject, and it has been widely applied in many fields, such as aviation, spaceflight, meteorology, and military [1]. Due to the strict requirements of the traditional anemometers for measuring environment, these anemometers cannot measure the airflow velocity accurately and even cannot work in the harsh environment. Although the rotor or cup type mechanical anemometers can measure airflow from different directions, big size and poor accuracy limit their developments [2]. Pitot tube anemometers have better accuracy for measuring high airflow velocity and are not affected by air pollution; however large size and bad accuracy for low speed measurements are main defects [3]. The piezoelectric anemometers also can measure airflow from different directions; however, the anemometers are easily affected by temperature and even fail to work in high temperature environment [4]. The above anemometers all have some limitations, so new measurement technology which can measure airflow velocity in harsh environment and meet different measurement requirements is desired. And then a plasma airflow velocity measurement method based on gas discharge emerged.

The concept of anemometer based on gas discharge can be traced back to the last century; it was firstly presented through the relationship between voltage, current, and wind speed in gas discharge test by Lindvall [5]. A German research team inspired by the work of Lindvall applied gas discharge to measure turbulence and ultimately achieved success in 1941. In the following three years, relevant experimental data was summarized well by Fucks, and some airflow velocity measurement rules based on gas discharge were obtained [6]. A low noise DC glow discharge anemometer was designed successfully by Mettler in 1949, and it was applied to measure the 1.6 Ma supersonic flow field and obtained the good effect [7]. Corona discharge was employed to measure airflow velocity by Werner and Geronime, which made good achievement in 1953 [8]. With the rapid development of electronic technology and the researches on gas discharge, the plasma anemometer based on glow discharge designed by Vrebalovich was continually improved by Matlis and Corke after 2003. The frequency of the discharge power supply was increased, and the voltage of the discharge power supply was decreased, which greatly reduced the power required by the glow discharge and the damage to the plate. The biggest progress is mainly in two aspects: first, the measuring probe reaching *μ*m level through adopting MEMS (microelectromechanical system) manufacturing technology, which reduced the effect of the probe on the airflow field, and the sensor had better resolution; second, applying the constant current glow discharge system for error control and using the computer to collect discharge signal, process the data, and control current of the discharge circuit, which improved the measuring range and accuracy of system [9–16].

There are exactly many researches on discharge or plasma anemometers for decades; here, only three typical anemometers based on gas discharge are introduced as follows. In 1985, in order to observe stratospheric turbulence, [17] presented a glow-discharge ionic anemometer based on glow discharge. The anemometer has two mesh electrodes, and the third electrode is applied to collect the ions interacting with airflow. The ion collector of the anemometer is partitioned into four insulated segments. When glow discharge happens and there is no airflow, equal ion currents flow into the four segments. When the airflow appears, the ions will drift downstream and the partial currents flowing into the four segments will be different from each other. The glow discharge between mesh electrodes is stable under pressures of 10–30 Torr; therefore, the anemometer can only measure weak flows of low-pressure gases, which greatly limits its application. In 2011, [9] introduced a constant current plasma anemometer for measuring the airflow velocity from subsonic to hypersonic. By keeping the discharge current constant, the airflow velocity could be obtained by measuring discharge gap voltage. In order to keep the discharge current constant, an active closed-loop feedback controller has been added to the plasma anemometer system. Although the measuring method is advanced, the constant current control brings many difficulties and the cost improvements. In 2015, [1] presented a DC planar ionic anemometer for measuring the airflow velocity of boundary-layers near a surface; the anemometer includes two symmetrical cathodes and one anode with a sharp tip. When DC voltage is applied to electrodes, the ions will be attracted symmetrically to the two cathodes. If there is no airflow, the currents of two symmetrical cathodes are the same. When the airflow appears, the downstream electrode can receive more ions than the upstream electrode. Although the anemometer has high accuracy, the applied high DC voltage would cause great damage to the electrodes and is hard to generate stable glow discharge. Therefore, it cannot measure airflow for a long time and its measurement range of airflow velocity is limited.

The presented plasma anemometer has been improved greatly in many aspects; it can not only operate continually in standard atmospheric air but also greatly decrease the realization costs of the system. The working principle, design, parameters effects studies, and airflow measurement experiments of the presented plasma anemometer will be illustrated in the following sections.

#### 2. Theoretical Analysis of Plasma Anemometer Working Principle

The theoretical analysis of DC plasma anemometer was firstly proposed by Mettler in 1949 [7], which is also applicable to AC plasma anemometer. The plasma generator structure of the plasma anemometer is shown in Figure 1, where is the electrode gap and is the electrode diameter. Between two parallel electrodes, when the gas is punctured by high AC voltage, then a relatively steady plasma region is generated. If there is no airflow, the total number of electrons leaving from cm^{2}/s of cathode can be expressed aswhere is the constant number of electrons leaving from cm^{2}/s of cathode, which is caused by constant power voltage, is proportional constant depending on the energy of the positive ions, the field strength at the cathode, the cathode material, and the condition of cathode surface, and is the number of positive ions arriving at cm^{2}/s of cathode