CHAPTER 5
Data Acquisition with LabVIEW
5.1 Introduction 115
5.2 Computer-Based Data Acquisition 116
5.2.1 Acquisition of Data 116
5.3 National Instruments LabVIEW 117
5.3.1 Virtual Instruments 118
5.4 Introduction to Graphical Programming in LabVIEW 118
5.4.1 Elements of the Tools Palette 120
5.5 Logic Operations in LabVIEW 121
5.6 Loops in LabVIEW 123
5.7 Case Structure in LabVIEW 124
5.8 Data Acquisition Using LabVIEW 125
5.9 LabVIEW Function Generation 127
5.10 Summary 128
5.1 Introduction
This chapter is designed to introduce the reader to the concept of computer-based data acquisition and to LabVIEW, a software package developed by National Instruments. The main reason for focusing on LabVIEW is its prevalence in laboratory setting. To be sure there are other software tools that support laboratory data acquisition made by a range of vendors. These are reviewed briefly in the appendix due to their limited presence in the educational setting. We should also point out that Matlab and other software tools used to model and simulate dynamic systems are at times used in laboratory setting although their use is often limited to specialized applications such as real-time control. For this reason, these tools are not discussed in this chapter.
LabVIEW itself is as an extensive programming platform. It includes a multitude of functionalities ranging from basic algebraic operators to advanced signal processing components that can be integrated into rather sophisticated and complex programs. For pedagogical reasons we only
Measurement and Instrumentation: Theory and Application
# 2012 Elsevier Inc. All rights reserved. 115
introduce the main ideas from LabVIEW that are necessary for functioning in a typical
undergraduate engineering laboratory environment. Advanced programming skills can be
developed over time as the reader gains comfort with the basic functioning of LabVIEW and its external interfaces.
Specific topics discussed in this chapter and the associated learning objectives are as follows.
bull; Structure of personal computer (PC)-based data acquisition (DAQ) systems, the purpose of DAQ cards, and the role of LabVIEW in this context
bull; Development of simple virtual instruments (VIs) using basic functionalities of
LabVIEW, namely arithmetic and logic operations
bull; Construction of functionally enhanced VIs using LabVIEW program flow control operations, such as the while loop and the case structure
bull; Development of VIs that allow for interaction with external hardware as, for instance,
acquisition of external signals via DAQ card input channels and generation of functions using DAQ card output channels
These functionalities are essential to using LabVIEW in laboratory setting. Additional capabilities of LabVIEW are explored in the subsequent chapter on signal processing in LabVIEW.
5.2 Computer-Based Data Acquisition
In studying mechanical systems, it is often necessary to use electronic sensors to measure certain variables, such as temperature (using thermocouples or RTDs), pressure (using piezoelectric transducers), strain (using strain gauges), and so forth. Although it is possible to use oscilloscopes or multimeters to monitor these variables, it is often preferable to use a PC to view and record the data through the use of a DAQ card. One particular advantage of using computers in this respect is that data can be stored and converted to a format that can be used by spreadsheets (such as Microsoft Excel) or other software packages such as Matlab for more extensive analysis. Another advantage is that significant digital processing of data can be performed in real time via the same platform used to acquire the data. This can significantly improve the process of performing an experiment by making real-time data more useful for further processing.
5.2.1 Acquisition of Data
One important step in the data acquisition process is the conversion of analogue signals received from sensing instruments to digital representations that can be processed by the computer. Because data must be stored in the computerrsquo;s memory in the form of individual data points represented by binary numbers, incoming analogue data must be sampled at discrete time intervals and quantized to one of a set of predefined values. In most cases, this is
accomplished using a digital-to-analogue (D/A) conversion component on the DAQ card inside the PC or interconnected to it via a Universal Serial Bus (USB) port. Note that both options are used commonly. However, laptop computers and/or low-profile PCs generally require the use of USB-based DAQ devices.
5.3 National Instruments LabVIEW
LabVIEW is a software package that provides the functional tools and a user interface for
data acquisition. Figure 5.1 depicts a schematic of data flow in the data acquisition process. Note that the physical system may be a mechanical system, such as a beam subjected to stress, a chemical process such as a distillation column, a DC motor with both mechanical and electrical components, and so forth. The key issue here is that certain measurements are taken from the given physical system and are acquired and processed by the PC-based data acquisition system.
LabVIEW plays a pivotal role in the data acquisition process. Through the use of VIs, LabVIEW directs the real-time sampling of sensor data through the DAQ card (also known as the I/O card) and is capable of storing, processing, and displaying the collected data. In most cases, one or more sensors transmit analogue readings to the DAQ card in the computer. These analogue data are then converted to individual digital values by the DAQ card and are made available to LabVIEW, at which point they can be displayed to the user. Although LabVIEW is capable of some d
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