2012 9th International Conference on Electrical Engineering, Computing Science and Automatic Control Mexico City, Mexico. September 26-28, 2012
Instrumenting and Programming a Virtual Instrument for an Open Loop System to Control Gliding Centrifugal Thermal Plasma G. Jiménez-Aviles 1,2, R. Valdivia-Barrientos 1, J. O. Pacheco-Sotelo 1, J. Silva-Rosas 1,3, F. RamosFlores 1, M. Pacheco-Pacheco 1, C. Rivera-Rodríguez 1 1
Department of Environmental Sciences, ININ, Mexico, Mexico 2 Department of Chemistry, UAEMex, Mexico, Mexico 3 Department of Electronic Engineering, ITT, Mexico, Mexico Phone (722) 260-8001 E-mail:
[email protected]
Abstract –– This paper describes: 1) the implementation of one system for controlling and monitoring physical variables involved in starting and sustaining a three-phase centrifuged gliding-arc (TPCGA) discharge called gliding centrifugal thermal plasma (GCTP), and 2) the analysis of data, collected in spreadsheets by a virtual instrument (VI), to explain the behavior of plasma. The goal of this work was to program a VI through LabVIEW™ for establishing a central unit to control the performing of the GCTP, and for recording the measurements of the analog signals involved in the process. Data are used to support the analysis and diagnosis steps. In addition, it could also help to propose a closed loop control design, because of the identification of extreme values. Instrumentation also considers the installation of an AC voltage source, gas flow valves, data acquisition cards, and current, pressure, and temperature sensors. For security reasons, a remote interface version of the VI was uploaded to internet. It can be approached when toxic gases are treated.
maintained at atmospheric pressure are more desirable than low-pressure ones, because they do not require costly vacuum equipment. The properties of plasma leave open the possibility for treating pollutants, and so, it makes a new way to reduce the amount of wastes discarded to the environment. Since plasma was discovered, researches have tried to find applications to real world, and they have been successful. However, only a few research breakthroughs resulted into economically viable technologies for large-scale industrial applications [5]. Nowadays, plasma is a visionary way to fight against environmental pollution. Dielectric barrier discharges (DBDs) are being introduced to fuel reforming, surface treatment and synthesis of novel materials [6]. Thermal plasma can dissolve objects in contact with it owing to the use of its high temperatures, obtaining inert glasses or clean gasses [7]. Arc discharges are used to synthesize nanoparticles for sorption [8], and for chemical conversion of compounds. Recent applications have been focused on gasification of biomass, coal, and local wastes to produce synthetic gas (syngas), due to the increasing concern for energy problems [9].
Keywords –– Data acquisition, gliding centrifugal thermal plasma, LabVIEW™, plasma, virtual instrument.
I. INTRODUCTION 1 Gas is considered as an insulated medium. When the right amount of energy is applied to it, atoms or molecules lost electrons (or at least one), converting the atoms or molecules into positively charged ions, thus, making the gas electrically conductive. This is referred as gaseous plasma. It is estimated that plasma constitutes more than 99% of the visible universe [1]. Plasmas are complex because their controlled maintenance is a delicate work. The best way to generate them in a laboratory scale is employing electrical energy. They can evolve from a dark discharge to an electric arc, requiring different power levels, coming from tens of watts up to kilowatts [2], [3]; extending from room temperature to thousands of kelvins. During its evolution, plasma generates important active species (electrical and chemical) [4], e.g. ultraviolet or visible photons, ions, electrons, excited molecules or neutral particles. From a practical point of view, plasmas that can be generated and
The present text comes out from a research about a three-phase centrifuged gliding-arc (TPCGA) discharge called gliding centrifugal thermal plasma (GCTP), a selfoscillating discharge formed in three divergent electrodes and aimed to improve the environment conditions. The discharge emerges from the closest region between the electrodes, where the biggest electric potential difference is located. The discharge length grows until a critical value is attained, determined by the power supply capability. Beyond this point, the energy obtained by the supply can no longer sustain the equilibrium phase of the gliding arc, and the electrical conductivity in plasma channel decreases (meanwhile the arc length increases) until the discharge finally extinguishes. Then, a restrike appears, leading to a new periodic cycle [10], [11]. A mobile unit for treating wastes in situ was the study site. It involves many tools for the proper functioning of plasma (ignition, sustaining and turned off), namely reactor, voltage source, gasses lines, temperature sensor, cooler, pressure sensor and vacuum pump, gas analyzer, and gas washing system (Fig. 1). Some of the devices must be controlled and monitored simultaneously, so a reliable control system is essential to
This work was accomplished under the financial support of the Mexican Council for Science and Technology (CONACyT), the SENER-Hydrocarbons Fund, and the Mexican Institute for Nuclear Research (ININ). Grant: 127499. Projects: CO-048 and AM-111. IEEE Catalog Number: CFP12827-CDR ISBN: 978-1-4673-2168-6 978-1-4673-2169-3/12/$31.00 ©2012 IEEE
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2012 9th International Conference on Electrical Engineering, Computing Science and Automatic Control Mexico City, Mexico. September 26-28, 2012
the next paragraphs describe the configurations applied to the tools concerned in the GCTP process, applying the technical characteristics established in [13-15]. These parameters helped to make up the control unit represented in Fig. 2, where LabVIEW™ joins the hardware through DAQ devices, which receive conditioned signals from the instruments, and generate the necessary ones to control them. Instruments such as data acquisition cards (NI-9263 attached to NI-9162 carrier, and NI-6259), voltage driver (VS1MD230), gas and pressure controllers (gas valve MKS MFC MXXXB connected to MKS 247D, and MKS Pirani Series 317 connected to MKS HPS 937A), temperaturevoltage and pressure-voltage transducers (K thermocouple and Pirani sensor, respectively) are employed to control the variables and for sensing the analog signals provided during tests. Then, their magnitudes are displayed on a VI across an industrial touchscreen.
Fig. 1. Platform designed for treating wastes in situ, gasses in its first stage.
achieve the job required by the process, for instance, programmable logic controllers (PLCs), microcontrollers, or data acquisition (DAQ) cards. Combining DAQ cards with the Laboratory Virtual Instrument Engineering Workbench™ (LabVIEW™) is an efficient way of controlling plasma, because they offer a quick and easy form to program a virtual instrument (VI), through which the GCTP is sustained. The following sections describe the communications, operations and applications layers that constitute the final VI [12].
A. Power system The GCTP is electrically fed by a power system made up of one voltage driver supplied directly from a 220 average voltage (Vav) three phase line at 60 Hz. The inputs of the driver (R, S and T) have a constant 220 Vav and the outputs (U, V and W) can be controlled from 0 V to 220 Vav through varying the PWM frequency of its internal inverter. One 1:10 voltage transformer, in star-star topology, elevates the voltage at the outputs of the driver, and then, it supplies to three tungsten electrodes placed in a ceramic (see Fig. 3). When there is a gas flow, a discharge appears, gliding from the nearest region of the electrodes to the most divergent one. This movement is because of the gas flow and the electromagnetic forces created by the discharge.
II. TECHNICAL SETUP It is important to control and measure all variables in the system (flow rate, voltage, current, power, pressure and temperature), because the behavior of the plasma discharge depends on them. For example, it is so indispensable establishing equilibrium between gas flow and applied power. Too much gas can turn off the discharge, or a reduced pressure helps to ignite the plasma. In that order,
Fig. 2. Whole control unit scheme.
IEEE Catalog Number: CFP12827-CDR ISBN: 978-1-4673-2168-6 978-1-4673-2169-3/12/$31.00 ©2012 IEEE
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2012 9th International Conference on Electrical Engineering, Computing Science and Automatic Control Mexico City, Mexico. September 26-28, 2012
Fig. 3. Power and gas lines connections.
TABLE I RELATIONS FOR OBTAINING THE DISPLAYED PARAMETERS OF THE VOLTAGE DRIVER Displayed parameter
Voltaje Acometida Voltaje Primario Voltaje Secundario Corriente Acometida Corriente Inversor Corriente Primario % Corriente secundario
Where: = Gas Correction Factor for gas 𝑥. = (Standard Density of nitrogen) (Specific Heat of nitrogen). = Molecular Structure correction factor, 𝑚𝑠 where it is equal to: 1.030 for monoatomic gasses, 1 for diatomic gasses, 0.941 for triatomic gasses, and 0.880 for polyatomic gasses. = Standard Density of gas 𝑥, in 𝑔⁄𝑙 𝑑𝑥 [0℃ @ 760 𝑚𝑚 𝐻𝑔 ]. = Specific Heat of gas 𝑥, in 𝑐𝑎𝑙⁄𝑔℃. 𝐶𝑝𝑥 = Scaling Control Factor. 𝑆𝐶𝐹 = Gauge Factor. 𝐺𝐹 𝐺𝐶𝐹𝑥 0.3106
Description
Constant voltage equal to 220 Vav Proportional voltage to analog output signal from the NI-6259 card (AM, 0 V – 10 V). Considered equal to 0 V up to 220 Vav 1:10, relation of voltage for the transformer. Considered equal to 0 V up to 2200 Vav
Proportional current to the current through internal inverter. Considered equal up to 0 A to 96 A Value directly proportional to AM. Considered equal to 0 A up to 88 A
Corriente Inversor divided by 1.5 (Corriente Inversor/Corriente Primario)*100%. Considered equal to 0 % up to 150 % 10:1, relation of current for the transformer. Considered equal to 0 A up to 5.867 A
Equation (1) gives next results 𝐺𝐶𝐹𝐻𝑒 = 1.4434, 𝐺𝐶𝐹𝐴𝑟 = 1.4431, 𝐺𝐶𝐹𝐻 = 1.0105 and 𝐺𝐶𝐹𝐶𝑂2 = 0.7382. Depart from these values, equations (2) and (3) reveal the ones in Table II. Final gas installation seems like scheme represented in Fig. 3, gas lines are stainless steel tubing, the same material that builds the mixer.
Voltage is controlled on the touch panel by virtual switches and a slide. Switches send TTL signals to the VS1MD230 for turning on and off its outputs. The slide generates a voltage signal from the NI-6259 card to the keypad of the VS1MD230. Power values displayed on the interface are obtained according to the relationships deduced by [16], and the transformer ratio; no losses were taken into account in such magnitudes and it was considered the linearity between its input and output terminals. AM is the main variable that makes possible relations in Table I, which is a direct current voltage varying from 0 V – 10 V.
C. Transducers A K thermocouple (Chromium/Aluminum) was installed to sense temperature at second chamber of the reactor; it was packed in alumina. As temperature reference, a constant equal to 295.15 K (temperature measured in the laboratory) for cold junction compensation (CJC) was set into the program. Furthermore, pressure is gauged by a Pirani device, which has to be placed parallel to flow inside the reactor. Because this tool sends a logarithmic voltage, (4) is needed to correlate voltage and pressure quantities.
B. Flow system Instruments for gas flow were adjusted due to the fact that their default configuration represents a measurement for N2. This calibration was fulfilled thanks to (1), (2) and (3). Gasses used were He, Ar, H and CO2. 𝐺𝐶𝐹𝑥 =
(0.3106)(𝑚𝑠) (𝑑𝑥 )(𝐶𝑝𝑥 )
𝑆𝐶𝐹 = (𝐺𝐹)(𝐺𝐶𝐹𝑥 )
1 𝐸𝑞𝑢𝑖𝑣𝑎𝑙𝑒𝑛𝑡 𝑓𝑙𝑜𝑤 𝑟𝑎𝑡𝑒 𝑜𝑓 𝑛𝑖𝑡𝑟𝑜𝑔𝑒𝑛 = 𝐺𝐶𝐹𝑥 𝐹𝑙𝑜𝑤 𝑟𝑎𝑡𝑒 𝑜𝑓 𝑔𝑎𝑠 𝑥 IEEE Catalog Number: CFP12827-CDR ISBN: 978-1-4673-2168-6 978-1-4673-2169-3/12/$31.00 ©2012 IEEE
𝑃 = 10 �
𝑉 − 12� ∗ 133.322 0.6
(4)
Where 𝑃 is the pressure inside the reactor, in 𝑃𝑎, and 𝑉 is a proportional voltage to P, in 𝑉.
(1) (2)
D. Virtual Instrument
The VI was programmed according to the methodology and suggestions in the literature [17-19], and the G code was
(3)
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2012 9th International Conference on Electrical Engineering, Computing Science and Automatic Control Mexico City, Mexico. September 26-28, 2012
4. Radio buttons is a menu for surfing through panels. 5. When an instrument is turned on, LEDs turn on too. 6. Tiempo de salvado establishes the rate and the period while data is going to be saved, their default values are 0 s and 1 S/s, respectively. This area has switches for erasing saved data when the operator decides it. 7. WebCam makes possible the video connection when working with remote control panel. 8. Button ININ displays a description of the programmer. 9. Slide for controlling the flux allows opening the valve at specific flow. 10. MFC state enables flux. It works with inverse logic. 11. In the box Digite el caudal deseado, the operator types the gas flow quantity instead of using the slide. 12. Gauge corroborates the flux value sent by MKS 247D. 13. Control frequency slide of the voltage driver represents the proportional frequency at which the VS1MD230 is operated. The driver was programmed to be controlled from 0 Hz up to 60 Hz (the level is displayed in adjacent numerical indicator). It sends the control signal to the driver for enabling a voltage from 0 V up to 220 Vav at its outputs. 14. Arrancar sends a “high” signal to the VS1MD230 for allowing the use of 13. When it is switched, its tag changes to Parar. 15. Inhibir salidas suddenly inhibits the output voltage of the voltage driver. 16. Reset restores the VS1MD230 at its initial conditions. Control marked as 14 should be in Arrancar position before this button is pressed. 17. and 18. They corresponds to main voltages and currents involved in the process, respectively. (see Table I) 19. It displays the measurements for pressure inside the reactor and the temperature at the second chamber. The units can be changed from Torr to atm, Pa or bar; and from °C to °F or K degrees. 20. It is an auto-adjustable histogram that holds on all the pressure gages during testing.
TABLE II CONNFIGURATION OF THE MASS-FLOW CONTROLLERS Gas He Ar H CO2 a
𝑑𝑥 [𝑔⁄𝑙 ]a
0.1786 1.7820 0.0899 1.9640
𝐶𝑝𝑥 [𝑐𝑎𝑙⁄𝑔℃]a 1.2410 0.1244 3.4190 0.2016
N2 flow [slpm]
Equivalent flow
GF a
SCF
20 20 10 5
28.868 slpm 28.862 slpm 10.105 slpm 3.691 slpm
200 200 100 50
288.68 288.62 101.05 36.91
OBTAINED FROM [14]
predominant, avoiding express code (for more information, refer to [20]). Configuration for DAQ cards follows the structure Create>>Start>>Read/Write>>Stop>>Clean. The data are collected in spreadsheets, which ensure a subsequent analysis of the variables, namely voltage, current, temperature, pressure and flux; all of them are referenced to the variable time. Their respective primaryunits are: V, A, °C, Torr and slpm. However, they can be converted to SI, MKS or English units. By default, this subroutine saves 1 S/s, but rate and the total saving time can be modified to exercise requirements. III. GUI PERFORMANCE The GUI for the virtual instrument is shown in Fig. 4; a brief description of each item is given. (The numbers correspond to the ones in the image). 1. Registry access contains the parameters Operador and Clave for being typed at the beginning of the program; if they are wrong, the VI stops. 2. While the VI is running and Button ALTO is pushed, a warning message is displayed to inform that the routine is going to stop; if the operator agrees, VI and the devices are stopped. 3. Button Hold freezes the front panel.
Fig. 4. GUI of the ultimate VI.
IEEE Catalog Number: CFP12827-CDR ISBN: 978-1-4673-2168-6 978-1-4673-2169-3/12/$31.00 ©2012 IEEE
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2012 9th International Conference on Electrical Engineering, Computing Science and Automatic Control Mexico City, Mexico. September 26-28, 2012
21. It is an auto-adjustable histogram that holds on all the temperature changes during testing. 22. This table displays the data points saved on the exercise for the variables voltage, current, temperature, pressure and flow of He, Ar, H and CO2.
According to the saved spreadsheets, graphics shown in Fig. 5 were plotted. Moreover, it can be corroborated that plasma starts and sustains at atmospheric pressure (See Fig. 6). Consequently, the subsequent findings can be stated: i. The waveforms of pressure and of Ar flow are similar, therefore, the pressure is ideally going to be autocontrolled by the flux, but the intervention of the discharge must be considered.
The final VI was included into a web page for being used when toxic gasses are treated. The web page was uploaded to internet (server: 200.15.118.244), allowing manipulating the VI up to five users. Control is given to users by the server and then, it circles depending on the order of control requests. At any time, the server can take the control. While the approved clients are waiting for the control, they can see the front panel of the VI, as well as the plasma inside the reactor, or all what is happening in the platform. This is possible when a webcam is plugged in a USB port of the server.
ii. The current has a sinusoidal shape. Given that the device is used in processes where a purely inductive charge is attached to it, the measurement should be corroborated by another device, like a Hall Effect sensor on each line of the three phases. They must be electromagnetically isolated. iii. Temperature and voltage keep a constant average response from the beginning to the end of the tests. This is their desirable behavior.
IV. RESULTS AND DISCUSSION
Sometimes, the TPCGA is referred as hybrid plasma [21], because it includes thermal and non-thermal plasma (NTP) regions. Reactor is built by two chambers. The first one contains the electrodes and there, the highest temperature is reached in the nearest region of the electrodes [11]. The second chamber contains the thermocouple, which senses temperatures up to 425.15 K (it is a lower temperature compared to higher ones reached by thermal plasma [22]). It seems to be the correct temperature because of the nature of NTP described in [23].
The initial goals were achieved. Fueled by the systems listed before and the virtual instrument working as the central control unit, plasma discharges were successful. Experimental probes were executed with argon as plasmatic gas. Maximum power obtained from the circuit was 3 kW (measured from read signals through DAQ cards). Until here, some values and evolutions were obtained, and they could be taken in consideration for improving the virtual interface and the instrumentation in the near future. The prototype was tested with a synthetic sample of Ar and CO2, which was introduced to the reactor and the treated outlet gas was examined by a residual gas analyzer. Graphically, a decrease of CO2 was seen and increases of oxygen and CO were observed (this analysis will be part of another report). Probes with CO2 reinforce the proposal of using the design as a way to control gas pollutant emissions.
When process starts for the first time, plasma works with 3 slpm of Ar and the control frequency of the voltage driver set at 36 Hz. Plasma is stable when gas flow rises up to 6 slpm, however, it is more stable when argon reaches 10 slpm at the same frequency. From this point, voltage output of the driver can be controlled up to 220 Vav.
Fig. 6. Plasma reactor and performance of plasma (ignition and sustaining).
Fig. 5. Plot of variables involved in the process. ( “Argon” should be referenced to the scale at the right-hand of the image)
IEEE Catalog Number: CFP12827-CDR ISBN: 978-1-4673-2168-6 978-1-4673-2169-3/12/$31.00 ©2012 IEEE
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2012 9th International Conference on Electrical Engineering, Computing Science and Automatic Control Mexico City, Mexico. September 26-28, 2012
V. CONCLUSIONS [4]
It is important to know characteristics of plasma to quantify and analyze phenomena during discharges. A viable method to obtain it is via DAQ hardware and programming an interface in LabVIEW™. Hence, a brief description of the implementation and the programming of a GCTP controlling unit for treating wastes was described. Because of the fact that the project is a laboratory-prototype, the interface is an open loop controlled by an operator. In due course, the results will determine the parameters to suggest a totally-automated control system.
[5] [6] [7]
[8]
The VI is the basis for a closed loop system, where, at the beginning of the exercise, the precise levels for the variables will be set over specific time (ramps). Databases for atomic structure, 𝑑𝑥 and 𝐶𝑝𝑥 of many gases could be integrated to automatically obtain 𝐺𝐶𝐹𝑥 by applying (1), (2) and (3). Subsequent platform improvements could involve higher gas flux, treatment of powders, and liquid streams.
[9]
[10]
Some problems attached to the rise of plasma technology are: materials for long-term operation, energy supplies, reactor volume and geometry, and cost efficiency of plasma-based methods related to competing technologies. Besides, problems associated with the scale-up of laboratory reactors with few gas flow rates compared to industrial size units for flow rates of many cubic meters per second. The problems could be associated to the lack of research in topics such as the identification, quantification, and control of byproducts produced in the plasma chemical reactions, and the dependence of destruction efficiencies on pollutant concentration, and because of the fact that the plasma physics and plasma chemistry, involved in the destruction processes, are not well understood.
[11]
[12] [13] [14] [15] [16]
ACKNOWLEDGMENTS
[17]
The authors are very indebted to J. L. Portillo, M. Ibañez, N. Estrada, M. García, J. A. Salazar, G. Soria, M. Hidalgo and M. Durán, for their experimental support, as well to the CONACyT for the scholarship (412287).
[18] [19] [20]
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IEEE Catalog Number: CFP12827-CDR ISBN: 978-1-4673-2168-6 978-1-4673-2169-3/12/$31.00 ©2012 IEEE
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