APPLICATION OF VI :
Today conservation of energy is of utmost importance in every aspect. The proper monitoring of power and maintaining the rated power factor can obtain the efficient usage of power in any industry. But the major problem encountered in every instrument is the error. So instruments of very high accuracy and precision are needed. We have developed a Virtual Instrument capable of measuring rms and peak values of current and voltage, frequency, active power, reactive power, apparent power and power factor using the software Lab VIEW 6i (a product of National Instruments, USA). The VI meter is successfully tested in our laboratory.
For the measurement of instantaneous power, the first step is to monitor the instantaneous values of the voltage and current from the circuit. These values of voltage and current are to be step down since the rated voltage for the interfacing kit is +10V or -10V. A standard 1ohms(wire-wound resistor) is connected in series with the circuit and the voltage across the resistor (equal to current) is given to the PCI. The interfacing kit used is PCI-6O24E. The voltage and current (voltage) signals are given at 2 pins of the kit. The signals are sampled at a rate of 10,000 scans/sec and 1000 samples are taken and are stored in the form of a matrix. This operation is performed by using a analog acquire input function
The two signals are sampled consequently and so there will be a time delay between the scan of one signal to the other.So the exact values of voltage signals corresponding to the current signals can be calculated knowing the delay 'd'.
t = (1/ w) x inv.sin[V / Vm]
v = Vm sin w( t+d )
CALIBRATION OF DELAY
DETERMINATION OF POWER AND POWER FACTOR
From the instantaneous values of current and voltage obatained as a result of the above operations ,the value of instataneous power 'p' can be found as,
p = ( v ) x ( i )
The relation between the instantaneous power and Vrms , Irms is given by,
p = ( v ) x ( i )
v = Vm sin wt
i = Im sin (wt- f )
therefore, p = ( Vm sin wt ) x ( Im sin (wt- f ))
p = ( Vm ) x ( Im/2 ) x (cos f - cos (2q - f)
The average power P is given by,
P = ( Vm ) x (Im) / ( 4p ) x INTEGRAL [(cos f - cos (2q - f )).dq ,0 , 2 p ]
P = ( Vm ) x ( Im/2 ) x (cos f )
P = ( Vrms ) x ( I rms ) x (cos f)
The average power can also be calculated by finding the average of the instantaneous power determined from v and i values .
P = S [ ( v ) x ( i ) ] / number of samples.
The rms values of V and I are found by using the DC RMS function present in the WAVEFORM MEASUREMENTS toolbox of LabVIEW 6i. The waveforms of voltage and current acquired using the ANALOG ACQUIRE function are given as inputs to the DC RMS function.
Now the powerfactor can be easily determined by ,
cos f = P / ( ( Vrms ) x ( Irms ) )
The reactive power can be determined by knowing the values of Vrms , Irms and the phaseangle.
f = inv. cos ( P / ( ( Vrms ) x ( Irms ) )
Reactive power = ( Vrms ) x ( Irms ) x (sin f )
The frequency of the input signal is found by using the SEARCH FREQUENCY and the MEASURE FREQUENCY fuctions.The peak values of V and I are found by using the peak detectors created in the LabVIEW environment.
The above figure is the VI diagram of the virtual meter developed using LabVIEW 6i.
EXPERIMENTAL RESULTS :
The values of the parameters obtained by the proposed meter and the conventional type meters are tabulated as follows.
DEVELOPED VI SOFTWARE DIAGRAM :
ADVANTAGES OF THE DEVELOPED VI METER:
The accuracy and precision of the developed virtual meter are very high compared to the conventional meters
The resolution is also greatly increased.
The errors due to the temperature change are eliminated.
The same meter can be used for the wide range of power and powerfactor.
The calibration of the meter is not affected by the variation in voltage and waveform.
All the discussed errors exhibited by the conventional meters are eliminated in the VI meter.
The measurement of energy can be done easily from the outputs of the meter.
Our works in labview
The labview is a development environment based on graphical programming. It is integrated fully for communication with hardware like GPIB. It contains front panel for user interface and block diagram for flowchart.
It has many applications as said earlier. As our part we have verified Ohm's law, Kirchoff's laws, Maximum power transfer theorem, Thevenin's theorem and Norton's theorem. We have interfaced with the external circuit and experimentally verified it. It explains the modes of input, Resolution, Range, Gain, code width, sampling rate and averaging and thus it serves as a basic learning tool for beginners.
Labview for windows DAQ VIs access the national instruments standard NI-DAQ for windows 32-bit dynamic link library (DLL). The nidaq32.dll file interfaces with the windows Registry to obtain the configuration parameters defined by Measurement and Automation Explorer.
Measurement and Automation explorer:
It is the unit where the channels of DAQ are configured. The measurement type, channel name, units, range, scale, scale type, channel number, input mode are given.
Consider the Kirchoff’s voltage law verification:
A circuit with dc voltage source is connected in series with two 10 Kohm resistances and the supply is switched ON. The three voltages from the circuit are taken and given to channel 1, 2 and 3 respectively to the interfacing device BNC 2120. In the program, voltages are acquired using the AI ONE PT vi through PCI DAQ card.
The acquired voltages are given to the Basic Dc rms sub vi to get the rms value of the acquired voltages. The rms voltages found are displayed in the digital indicators. The three rms voltages are added as per the kirchoff's voltage law. Program is such that if the voltage sum is zero then the square LED will glow. In our experiment, we got the voltage sum as zero and thus the square LED glowed. Thus the kirchoff's voltage law is verified.
Noise Reduction in virtual Instrumentation:
A high frequency noise is added with a sine wave signal using insert array function. The noise added waveform is given to a waveform graph and Butterworth low pass filter. The sampling frequency, higher cutoff frequency and order number are configured for the Butterworth filter. The frequency of the input is found with the Buneman Frequency Estimator.vi. Ten times the frequency is given as the sampling rate to the filter. The higher sample rate is for better accuracy. The output of the Butterworth filter is given to the waveform graph. It is observed that the noise is largely removed. The same will be true for the external noises
Advantages of VI :
Once people used typewriters, which were then replaced by the word processors that offered new levels of productivity and flexibility. Much as the same way, VIs will replace the physical instruments.
The picture on the top is that of a real instrument and the one on the bottom is that of a virtual one. Today's computer's graphic capabilities even make VIs look like real instruments.
Virtual instruments are better connected to PC technologies widely used in industries. With VI, users can move applications seamlessly between several bus architectures, such as PC Card, plug-in DA hardware, and VXI. This portability offers the flexibility to take advantage of improved bus standards as they arise.
VIs are fully customizable. You can select the range, accuracy, amount of information provided by them. You can even save the data provided by it in a computer and send it over the internet. They can be made to sense any physical quantity offering any range with the use of corresponding sensors. A vi can be made to control any other instruments including another vi.
Virtual instruments are less expensive and maintenance free. They work very fast, handle repetitive tasks, process data, store results and also generate reports. Reuse of code is also possible. Thus the same code can be used for testing similar instruments which saves time and money. Doing things in less time would definitely mean being able to do more. With less time, money and human labor for the same product, the productivity increases rapidly.
Lowers cost for customers
Physical instruments are not suited for extreme weathers, but VIs can work in any hazardous environments. When a new product is tested using expensive test equipments, it naturally raises the overhead cost. Employing VI there, reduces the testing cost. They also increase the test safety. They do not get damaged and thus no need of replacements. This lessens the price of the final product for customers.
The software used for VI is user friendly. In LabVIEW, new users can step through the dialogue boxes and quickly build a fully functioning DA application. LabVIEW will help define signal types, connections, and transducer equations before building the system. Thus, the learning curve is shortened significantly. More experienced developers can use the DAQ Wizards to prototype a system.
Applications: Most VI system’s hardware block diagrams are almost the same.
Most VI system contains a microprocessor, a data-acquisition unit or system, an I/O port, a display or a way of reporting the results and an analysis engine. Since most of the hardware is the same, only the front-end of the device needs to be changed to suit the equipment's purpose. With a personal computer, a software could do the analysis after acquiring the needed data. Thus, the software could be written as such to specifically answer what is called for by the analysis. The hardware could be changed according to the design of the experiment, whether it will receive data from an apparatus or control it. Instead of using separate hardware front-ends, software could be written for measuring different quantities. Thus with sensors and appropriate software, any physical quantity can be acquired and measured in VI.
Some other applications of VI are data analysis, systems control, process automation, testing and calibration of instruments, telecommunications, semiconductor manufacturing, automotive testing, robotics, automation, embedded systems, etc.
The future of virtual instrumentation is promising. As such companies as Intel and Microsoft continue to usher in new technologies for advanced productivity and connectivity; virtual instrumentation’s benefits will increase. Improvements in PC technology and VI hardware and software will make new applications possible. Companies like National Instruments are promoting VI to make it reach everyone. Quite sooner the traditional instrumentation will be completely moved inside the computer.