Data Acquisition System for a Vertical Axis Wind Turbine Prototype

Transcription

1 Data Acquisition System for a Vertical Axis Wind Turbine Prototype KOCH-CIOBOTARU C. Department of Automation and Applied Informatics Politehnica University from Timisoara Bvd. V. Parvan, Nr. 2, Timisoara, ROMANIA cosmin.koch@aut.upt.ro Abstract: - This paper illustrates the design of a prototype model of a Savonius vertical axis wind turbine (VAWT), including a few dynamic tests. This prototype is physical realised and equipped with a data acquisition system (DAQ). Further, a virtual instrument (VI) is implemented on a PC to collect data from the DAQ device through the USB port, to compute them, and to monitor the evolution on the PC. In the end are presented tests done on the system, on different wind speed regimes. The results obtained by the VI are discussed. Key-Words: - Vertical Axis Wind Turbine, Data Acquisition Systems, Microcontroller, USB Communication, LabView 1 Introduction The two major directions in the development of wind turbines, according to the position of the axis, are the Horizontal Axis Wind Turbines (HAWT) and the Vertical Axis Wind Turbines (VAWT)[1]. Both of them present advantages and disadvantages. The HAWT have a higher power coefficient, so they present a better efficiency being more attractive for large scale turbines and wind farms, on- and off-shore. The largest turbines, to date, are rated 5MW and are HAWT [2][3]. The VAWT are used especially in low power, until a few tens of kw, for insular, off-grid, applications or in the crowded areas, on top of buildings. VAWT have the tip speed ratio smaller then HAWT, run at lower speeds, and therefore are more silence. Another advantage is the high start-up torque, the VAWT being used in the past for numerous mechanical actions, like running water pumps [2]. For controlling the turbine during operation, to achieve an optimal regime, there are used transducers, data acquisition modules, digital signal processing units This paper illustrates the design and realization of a Savonius vertical axis wind turbine of low power, as a laboratory model. Numerous softwares were used to design the turbine, the data acquisition (DAQ) system that monitors the VAWT and the virtual instrument for data processing The paper is structured as follows: II the mechanical part of the wind turbine, III data acquisition system circuit, IV Virtual Instrument application and the communication between the PC and DAQ system through the USB port. 2 System description The first part of the system consists in a Savonius type VAWT prototype. The turbine captures the wind energy and converts it into mechanical energy, in the form of rotation motion around the vertical axis. This motion is transferred to a direct current generator through a transmission belt and a system of two wheels having the multiplication factor of 10. In this way, in the generator s windings, electric power is produced. The electric voltage at the generator end is read by a DAQ system controlled with a PIC18F4550. The DAQ system also reads the voltage of a thermic sensor. This sensor can be used to ensure the functionality of the turbine in normal safety parameters: if the temperature is too high, the system works in a danger zone, where the structural integrity is in danger, or even the destruction of the installation is possible. Fig. 1 Block diagram of the application 3 Mechanical part of the VAWT The chosen profile of the turbine is the Savonius vertical axis wind turbine concept which dates back tho the '30. The horizontal section resembles the letter S, ISSN: / ISSN: ISBN:

2 Fig. 2 Horizontal plan section of a Savonius VAWT turbine having a concave and a convex scoop. The working principle of this design may be seen in Fig. 2 [5]. Unlike the HAWT, where the rotational movement is due to the wing effect (the difference in the pressure on the two sides of the blade results in a lifting force, that moves the turbine around its central axis), in the case of Savonius design, the wind acts directly, with a drag force, as in the case of a sail [1]. For every type of wind turbine, the plot of the power coefficient against the tip-speed ratio is of great interest. The power coefficient (Cp) represents the percent of power collected from the wind; the theoretical maximum point is called Betz coefficient and has the value of The tip-speed ratio is defined as the product between the linear speed of the tip of the blade and the wind speed. For larger values of tip-speed ratio, the problem of noise emission is of serious concern [2]. In Fig. 3 are presented the efficiency curves of different types of wind turbine designs, by plotting the power coefficient against the tip-speed ratio of the turbine. The designed model has two levels. Each level consists of a classic two scoops Savonius design, joined together with a phase angle of 90 degrees between the horizontal axes. For the design of the turbine and simulation of aerodynamic tests, the software SolidWorks 2006 was used, with the modules CosmosWorks (see Fig. 4) for deformations study, and FluidWorks for aerodynamic study (see Fig. 6). Fig. 4 SolidWorks 2006 model of a Savonius VAWT The manufactured Savonius VAWT prototype (Fig. 5) has the following characteristics: height of the rotor : 50 cm diameter of the rotor : 45 cm For dynamic balance of the rotor at high speeds, the base of each level is a full disc, having the diameter with 5% larger than that of the two scoops. The air mass in motion with a certain speed the wind speed, at the contact with the two scoops profiles, the concave and the convex side, will create a pressure region on each. As it may be seen in Fig. 6, the pressure builds up on the side with the wind action. The difference of pressure between the two sides of a profile, acts as a force on the surface. The action of this force is at a certain distance of the rotation axis, and therefore, results a torque. At this level it is obvious the cause of the reduced efficiency: on each moment of the Savonius wind turbine function, there is a part of the turbine that acts in the opposite direction. Fig. 3 The power coefficient (Cp) of various types of wind turbines plotted against the tip-speed ratio Fig. 5 Savonius VAWT prototype ISSN: / ISSN: ISBN:

3 (a) (b) Fig. 6 FloWorks 2006 simulation of the wind action on the two sides of the Savonius blade The FloWorks module offers the fluid dynamic study on designed modules. The study from Fig. 6 was realized under the following simulation conditions: the air pressure: Pa the air temperature: 20 C the profiles are situated in a field where the wind speed has only an Ox component, of value 10m/s As it may be seen from Fig. 6, the difference between the two faces of the scoop, in the case when the wind acts from the concave side, is around 200 Pa. In the case when the wind acts from the convex side of the scoop, the pressure difference is only Pa. This difference gives the resulting force that moves the rotor. The rotation motion is sent from the turbine to a DC machine that is used in this case in its generator operation regime through a transmission belt and a couple of wheels, one on each end, having the ratio of Data Aquisition System For this circuit was used the microcontroller PIC18F4550, product of Microchip company. It provides analog inputs and analog to digital converters on 10 bits [4]. The advantage of using this particular controller is the fact that provides the hardware capability to use the Universal Serial Bus (USB) communication. The functionality of an USB device is structured on many levels, in a framework. Each level is associated to a specific function level of the device. The highest level, except the device, is the configuration. A device can have multiple configurations, for example for different power supplied applications. A configuration can have many interfaces. Under the interface is the endpoint. At this level the data is handled. There can be 16 bidirectional endpoints. The endpoint 0 is always for control and it must be free when the device is connected, for the configuration to take place. There are four types of transfers for USB: - Isochronous used for large packages of data, with on receiving time assurance. Used in cases where low losses are not critical, as in audio field - Bulk used for large packages, with integrity assurance; the receiving time is not controlled - Interrupt used for low size packages transmission. Both the receiving time and integrity is assured - Control used for device setup The devices use high-speed or low-speed communication. The first one supports all the above mentioned types of transfer, as for the low-speed communication, it support only the last two. SIE can be external supplied with 3.3 V, or by an internal circuit, case in which SIE is connected directly by USB cable to the other device and the conversion between the 5V to the 3.3V is made internal. The block diagram of the SIE is shown in Fig. 7. Fig. 7 General overview of the USB peripheral and its features ISSN: / ISSN: ISBN:

4 The controller has a dual port memory, which allows the access of the controller's central unit, as well as of the SIE module. PIC18F4550 has the capability to set descriptors for optimizing the applications through control registers. Those registers are: USB Control Register (UCON) USB Configuration Register (UCFG) USB Transfer Status Register (USTAT) USB Device Address Register (UADDR) Frame Number Registers (UFRMH: UFRML) Endpoint Enable Registers from 0 to 15 (UEPn) The circuit acquires two signals: from the generator and from a temperature sensor. It is necessary to calibrate the inputs for values between 0 and 5V. The electronic circuit design was realized in Pads The firmware for the microcontroller was implemented in MPLAB, in the C language. It has two major objectives Acquiring data from the sensors USB communication with the PC The logic flow structure of the firmware is presented below: Descriptors setup Set the analog port inputs; in the case of this application there will be used two inputs: 0 and 1 Set the input channel used by the analogic port; this function loads in the ADCON0 register the active port input address SIE verifies the USB port for commands; it is identified, analysed, and executed by the firmware as follows The input signals are subject to an analog to digital conversion; the voltage on the input is read as a 10 bits value Return the result of the analog to digital conversion under a short signed value (on 16 bits); the two registers that represent this value are ADRESH and ADRESL In the firmware, descriptors are configured for identifying, initializing, and setup of the communication between the host PC and de DAQ device. The following types are used. Device descriptors- they have a role in the identification of the device by the host PC Configuration descriptors- they define the number of interfaces, the current maximum value, Interface descriptors- very important for the setup of the communication type, of the input/output endpoint address and the size of the transit data 5 The virtual instrument implementation The data are processed with the help of a Virtual Instrument (VI) realized in LabView 6.1 software. For the communication between the two systems (PC and DAQ) NI-VISA is used. VISA is a high level API used for the bus communication on different instruments. It is an independent platform: the same API is used for communication between a device and LabView and for communication of a GPIB device and a Mac [6][7]. The USB is a message based communication bus. This means that the PC and the DAQ device communicate through commands and data send in text or binary form. Each USB device has its own command set. NI-VISA Read and NI-VISA Write can be used to send these commands to a device or to be able to read data from it. Information about the command set of the device is necessary in order to be able to communicate effectively; this set of commands is specified by the producer [8]. Beginning with the version 3.0, NI-VISA supports USB communication. There were implemented communications with two VISA classes: USB INSTR and USB RAW. The devices that use the USBTMC (USB Test and Measurement Class) protocol use the class USB INSTR. A special configuration for the communication with such a device is unnecessary. The USB RAW instruments are those instruments that do not respect the USBTMC protocol. This is the case of the presented DAQ device. The communication with a USB RAW device is more complicated because the device uses its own communication protocol. For an application to be interfaced with such a device, the producer has to provide all the necessary data. NI-VISA implement three types of data fluxes: control, bulk, and interrupt. When NI-VISA detects the USB device, it automatically scans for the lowest endpoint associated to each flux type. The DAQ device designed and used in this paper has a BULK type data flux. For using NI-VISA, the operation system must have a driver installed for the considered device. In Windows, this operation is done by installing an.inf file. The device driver, which consists in this.inf file, is realized using VISA Driver Development Wizard (DDW). ISSN: / ISSN: ISBN:

5 I II III IV a b c Fig. 9 VISA Driver Development Wizard panel By selecting the device and communication type and entering the Vendor ID and the Product ID (must be the same as the ones set in the device descriptors) DDW returns the necessary.inf file. Fig. 9 shows the DDW panel. The resulting.inf file is to be installed in Windows\Inf directory. At the first connection of the device, the device is installed using this file. The Virtual Instrument, in a repetitive structure, presents two sequences of device interrogation code. By two commands, A1xxx and A0xxx, the VI interrogates the device to return the read data from input 1 and input 0, which correspond to the generator voltage and temperature sensor. This response, sent on 10 bits, must be interpreted by the VI in order to reproduce the correct value; this is the reason why the VI designer has to know the exact structure of the response given by the DAQ device, in order to correctly interpret the received bits. The read data is represented on the front panel of the application with the help of control blocks. The voltage is represented in an interval of 60 seconds, for a better view of the phenomenon. Fig. 10 represents the control panel of the VI, the user interface of the device. In Fig. 11 is presented the block diagram of the application. For a better comprehension of the interface, a division in four quadrants was realised. In the first quadrant, are present the start and stop controls, which activate the necessary LabView commands to setup or to close the communication with the device. After the start control is activated, the VI interrogates the device descriptor and returns in a text box the manufacturer name. The installed driver for the operation system has also a name; this name, of the driver, is reproduced in the first textbox, in this case, USB_AAAA. Fig. 10 LabView 6.1 Virtual Instrument interface Fig. 11 LabView 6.1 Virtual Instrument block diagram In the quadrant II is represented the temperature value read by the thermic sensor. In quadrants III and IV is represented the voltage at the generator end. In III is presented the momentary voltage, and in IV is presented the voltage plot on a 60 seconds time interval. 5 Experimental results During tests, there were considered three wind regimes, depicted in Fig. 10 with a,b, and c: the rise of the wind speed is presented with a the wind speed increases, the turbine collects larger amount of energy which is passed to the generator; it can be seen in the figure the inertia of the turbine constant wind speed regime is presented with b it may be observed that the generator produces a constant amount of energy during this regime decrease of the wind speed is presented with c the turbine collects a smaller amount of energy and the generator is producing less voltage ISSN: / ISSN: ISBN:

6 6 Conclusions This paper uses numerous dedicated software for turbine design, mechanical stress and fluid dynamic simulation, developing the firmware, writing the firmware on the microcontroller, design of the electronic circuit, design of the virtual instrument. The developed acquisition device replaces the National Instrument s own device, hence reducing the cost of the application. This type of devices can use the LabView software for user interface and application monitoring. This particularly acquisition device is developed around a Savonius Vertical Axis Wind Turbine for monitoring the generated voltage. The DAQ device is supplied from the USB port of the PC and it doesn t need an external power source. This feature gives the device autonomy and simplicity. 7 Acknowledgment This work was partially supported by the strategic grant POSDRU 2009 project ID of the Ministry of Labour, Family and Social Protection, Romania, cofinanced by the European Social Fund Investing in People. 7 References [1] Gary L. Johnson, Wind Energy Systems, Electronic Edition 2006 [2] Mukund R. Patel, Wind and Solar Power Systems, CRC Press Florida, 1999 [3] European Wind Energy Association, Wind energy The facts 2009 [4] [5] [6] Gani, A.; Salami, M.J.E.; A LabVIEW based data acquisition system for vibration monitoring and analysis, Research and Development, SCOReD Student Conference on, 2002, pp [7] Swain, N.K.; Anderson, J.A.; Ajit Singh; Swain, M.; Fulton, M.; Garrett, J.; Tucker, O.; Remote data acquisition, control and analysis using LabVIEW front panel and real time engine, SoutheastCon, Proceedings. IEEE, 2003 [8] Chance Elliotta, Vipin Vijayakumara, Wesley Zinka and Richard Hansen, National Instruments LabVIEW: A Programming Environment for Laboratory Automation and Measurement, Journal of the Association for Laboratory Automation, Volume 12, Issue 1, February 2007, Pages ISSN: / ISSN: ISBN: