LOW LEVEL CURRENTS AND VOLTAGES MEASUREMENTS

Transcription

1 LOW LEVEL CURRENTS AND VOLTAGES MEASUREMENTS CĂTĂLIN VOINA 1, SIMONA MORARU 2, ANDREEA COSAC Key words: analogical data, acquisition. This paper presents one modern solution in order to read electrical quantities as analogical and digital signals. It can be used in the energetic applications as alternating solution for similar products made from various firms. Measurements instruments can be created and simulated using acquisition boards. Its main advantage, compared with a classical measurement instrument, is that it can be easily converted by soft. 1. INTRODUCTION With a competitive economics many firms want to increase work s productivity, to solve fast any faults that might appear. So products quality is a very important factor. In order to deal with this, firms use high and expensive technologies. A new, specialized technology has been developed, so it can solve any measurement, communication and information problems. SCADA systems ("Supervisory Control And Data Acquisition") were realized to solve these potential inconvenients. One of specialized soft that can monitorize and analyze data is LabVIEW System [1-3]. LabVIEW represents a graphical alternative to the conventional programming design for instrumentation. It is equipped with all necessary tools in order to test any measurement systems. LabVIEW is a graphical developed environment designed for create flexible and scalable tests, to measure and to control faster the applications, with a minimal price. The quickness of this program is high, due to the introduction of an intuitive graphical interface. LabVIEW uses a general graphical language for programming called G, containing wide libraries with proper functions. The LabVIEW programs are called virtual instruments and are made from two parts, distributed in two windows: front panel (containing the necessary elements for interactive operations and the display of the results) and block diagram (actually the source code, this one contains the corresponding instructions, constants, functions and pointers from the front panel). 1 - catalin_voina_g@yahoo.com, Polytechnic University Bucharest 1 - simona492273@yahoo.com, Polytechnic University Bucharest

2 Cătălin Voina, Simona Moraru, Andreea Cosac 2 Flowing data is determined in block diagram using links represented by lines between icons. We will present some analogical low level data acquisitions [4]. 2. THEORETICAL CONSIDERATIONS The rectifier has a converting function for the electrical energy, from AC into DC. Its working is depending on the load type, connected at its output. This dependence is shown in a very simple diagram, see figure 1. The diode should be an ideal one (u D =0 for working state; i D =0 for blocking state). Fig.1 Electrical diagram for one phase rectifier and the output voltage for a resistive load Sampling Theorem (Shannon, 1949) said that any continuous time signal, with a limited spectrum, can be represented without loosing information through a sample series of the original signal, or in other words, through a discrete signal. Johnson noise [5] is the voltage associated with the motion of electrons due to their thermal energy at temperatures above absolute zero (0K). All voltage sources have internal resistance, so all voltage sources develop Jhonson noise. It may be reduced by decreasing the temperature of the source resistance and by decreasing the bandwidth of the measurement. Magnetic fields generate spurious voltages in two circumstances: if the field is changing with time and if there is relative motion between the circuit and the field. Changing magnetic fields can be generated from the motion of a conductor in a magnetic field, from local AC currents caused by components in the test system or from the deliberate ramping of the magnetic field, such as for magneto resistance measurements. Even the earth s relatively weak magnetic field can generate nanovolts in dangling leads, so leads must be kept short and rigidly tied down. Noise and error voltages also arise from ground loops. We obtain such a loop when the source and measuring instruments are both connected to a common ground bus. The cure for ground loops is to ground all equipment at a single point. The easiest way of accomplishing this is to use isolated power sources and instruments, than to find a single, good earth-ground point for the entire system. To operate with discrete amplitude signals means a special attention. The result might be often a sum of quantification noises, with a statistical characterization factors and consequences.

3 3 LOW LEVEL CURRENTS AND VOLTAGES MEASUREMENTS The data acquisition systems principal components are sampled circuits, memorized ones and the analogical-numerical converters. The numerical and analogical signals caused by processing can be used to memorize and give back the information or to command the execution elements (motors, relay), which control the physical processes. 3. PROPER APPLICATION In our application we read, memorize and compute analogical and digital signals, particular currents and voltages, in order to analyse the behavior of certain system in stationery or permanent modes. Instrument measurement speed is important in many test situations. When specified, measurement speed is usually states as a specific number of readings per second for given instrument operating conditions. Certain factors such as integration period and the amount of filtering may affect overall instrument measurement speed. There is often a tradeoff between measurement speed and accuracy since changing these operating modes may alter resolution and accuracy. The acquisition board DAQ 6024E can operate with a maximum analogical scan rate of scans/second. DAQ 6024E board allows signal acquisition into ±10VDC limits. It s obviously that we need an intermediate electronic board to adapt the real acquired signals to the specified interval (±10VDC), with suitable scan factors. The board admits the independently scanning for each channel. The two application s windows are described in figure 2 and figure 5. In the program we used many specific functions: - every channel has its own configuration; - the operator can select one of the following operations: acquisition, visualization or saving the acquisitioned data in a specific file (figures 2, 3 and 4); - various possibilities for changing the parameters (scaling factors for each channel, delay factors on the OY axis for each channel, zoom, the used program s memory size, the scan rate, the channels number for reading and displaying, the cursor for reading the exact acquisitioned values); - operator can process through meaning or filtering the waveforms with another proper program (we use the arithmetical mean, with a setted number of points) is not this paper purpose.

4 Cătălin Voina, Simona Moraru, Andreea Cosac 4 Fig. 2. The Main Front Panel Fig. 3. The Front Panel - Acquisition

5 5 LOW LEVEL CURRENTS AND VOLTAGES MEASUREMENTS Fig. 4. The Front Panel - Visualization Fig. 5. Bloc Diagram Data can be displayed in two different ways: in real time (one second constantly updated) and historically (displaying the entire interval ordered by the user). Data reading is made as long as the program goes on. Data reading is permanently, but data recording has a controlled Start/Stop, given by the operator or presetted. This is a way to avoid a useless loading of memory or even an overcharge of hardware system. The diagrams allow to simultaneously displaying all channels for reading or only a few of them after selection. The utility of this program is the possibility to use it for tracing and visualization of electrical quantities, any deviation from the normal behavior is unliked and it must be eliminated without any delay. (Example: hydroelectric power stations, power stations the entire national circuit of electrical and terminal energy).

6 Cătălin Voina, Simona Moraru, Andreea Cosac 6 The possibility of changing the principal parameters, which interfere in the acquisition and in the recording, and also the filtering of the data are very important program performances. We can print all acquired diagrams. An example of acquisition is the waveform for the current and the voltage obtained at the output of one phase rectifier with a specific load: resistive load (R); inductive-resistive (RL); resistive load and a DC voltage supply (RE); resistiveinductive load and a DC voltage supply (RLE). Fig.6. The dropping voltage in case of resistive load for one phase rectifier. Fig.7. The current in case of resistive load for one phase rectifier. Fig.8. The dropping voltage in case of resistive-inductive load for one phase rectifier.

7 7 LOW LEVEL CURRENTS AND VOLTAGES MEASUREMENTS Fig. 9. The dropping voltage in case of resistive load and DC voltage supply for one phase rectifier. Fig. 10. The dropping voltage in case of RLE load for one phase rectifier. Fig. 11. The current in case of RLE load for one phase rectifier. 4. CONCLUSIONS The signal is a physical quantity or quality and it takes with him some specified information. For the numerical computation data are transformed first into analogical signals using transductors and than into numerical signals using data acquisition systems. The operation made from the numerical computing systems upon the resulted numerical signals from the acquisition can be: filtering, frequency domain representation, classification, and identification. It obtains computing numerical signals, which contained informations about physical

8 Cătălin Voina, Simona Moraru, Andreea Cosac 8 processes. These informations can be used for memorizing, communication or control. The numerical computing techniques are limitated from the maximum frequency for analogical input signals and also from numerical computed speed point of view. In an application these limitations are depending on the characteristics data acquisition system, on the work speed of the numerical computing systems and on the numerical computing algorithm s complexity. There are applications in which a real time data computing is demanded; it means that the computing algorithms are correlated with the data access speed. Because of the time axis discretization the analogical signals become discrete. The signal becomes discrete if we will also divide the OY axis. We are interesting in these requirements because the final purpose of this research paper is to simulate an industrial process, in the aim to know it better, to control and to predict it. The useful signal, representing the physical phenomenon or system s behavior, is mixed with perturbations, at acquisition and through the transmition channel. The discretization introduces a noise too. Noise is often a consideration when making virtually any type of electronic measurement, but noise problems can be particularly severe when making low level measurements. Thus, it is important that noise specifications and terms are well understood when evaluating the performance of an instrument. Significant errors may be introduced into low level measurements by offset voltage and noise sources that can normally be ignored when measuring higher signal levels. Noise sources include Johnson noise, magnetic fields and ground loops. An understanding of these noise sources and the methods available to minimize them is crucial to making meaningful low voltage measurements. The perturbations and noises are continuous time phenomena, like the useful signals. Between them is a subjective difference, the specialist s point of view. Because of the high mathematical level, it is hard to analyze and to separate them. The virtual instrumentation utilization advantages are decreasing the expenses with new instruments (the system acquisitioning price, the expenses with the development and the maintenance) and increasing performances (flexibility, reutilization, and reconfiguration). Low prices and high performances are the desired qualities customers expect from their delivers. REFERENCES 1. F. Cottet, O. Ciobanu, Bazele Programǎrii în LabVIEW, Ed. MatrixRom, Bucharest, V. Maier, C. Maier, LabVIEW în Calitatea Energiei Electrice, Ed. Albastrǎ, Cluj Napoca, LabVIEW User Manual, National Instruments, January, D. Stanomir, Semnale şi Sisteme Analogice, Ed. Politehnica, Bucharest, Low Level Measurements Handbook, Keithley, 2002.