Encyclopedia of Computer Science and Technology

data acquisition 129For example, an abstract stack data type includes astructure for storing data (such as a list or array) and aset of operations, such as “pushing” an integer onto thestack and “popping” (removing) an integer from the stack.(For the process of combining data and operations into asingle entity, see encapsulation.) Abstract data types canbe implemented directly in object-oriented programminglanguages (see class, c++, Java, and Smalltalk).One advantage of using abstract data types is that itseparates a structure and functionality from its implementation.In designing the abstract stack type, for example,one can focus on what a stack does and its essential functions.One avoids becoming immediately bogged downwith details, such as what sorts of data items can be placedon the stack, or exactly what mechanism will be used tokeep track of the number of items currently stored. Thisapproach also avoids “featuritis,” the tendency to see howmany possible functions or features one can add to thestack object. For example, while it might be useful to give astack the ability to print out a list of its items, it is probablybetter to wait until one needs such a capability than to burdenthe basic stack idea with extra baggage that may makeit more cumbersome or less efficient.An abstract data type or its embodiment, a class, is notused directly by the program. Rather, it is used to create anentity (object) that is a particular instance of the abstractdata type (for example, an actual stack that will be usedto manipulate data). The data stored inside the object isnot accessed directly, but through functions that the objectreceives from the abstract data type (such as the push andpop operations for a stack). (For more information abouthow such objects are used, see class.)Because the abstract data type is not directly used bythe program, the implementation of how the data is storedor manipulated can be changed without affecting programsthat use objects of that type. This information hiding isone of the chief benefits of object-oriented programming.Another advantage is inheritance, the ability to derive morespecialized versions of the abstract data type or class. Thus,one can create a derived stack class that includes the printingfunction mentioned earlier.the ASCII character code for the key pressed. Moving themouse sends a stream of signals that are proportional to therotation of the ball which in turn is calibrated into a seriesof coordinates and ultimately to a position on the screenwhere the cursor is to be moved. Digital cameras and scannersconvert the varying light levels of what they “see” intoa digital image.Besides the devices that are familiar to most computerusers, there are many specialized data acquisition devices(DAQs). Indeed, most instruments used in science andengineering to measure physical characteristics are nowdesigned to convert their readings into digital form. (Sometimesthe instrument includes a processor that provides arepresentation of the data, such as a waveform or graph. Inother cases, the data is sent to a computer for processingand display.)Components of a Data Acquisition SystemThe data acquisition system begins with a transducer,which is a device that converts a physical phenomenon(such as heat) into a proportional electrical signal. Transducersinclude devices such as thermistors, thermocouples,and pressure or strain gauges. The output of the transduceris then fed into a signal conditioning circuit. The purposeof signal conditioning is to make sure the signal fits intothe range needed by the data processing device. Thus theFurther ReadingCarrano, Frank M. Data Abstraction and Problem Solving with C++:Walls and Mirrors. 4th ed. Reading, Mass.: Addison-Wesley,2004.“Introduction to Data Abstraction.” MIT Press. Available online.URL: http://mitpress.mit.edu/sicp/full-text/sicp/book/node27.html. Accessed July 3, 2007.Koffman, Elliot B., and Paul A. T. Wolfgang. Objects, Abstraction,Data Structures and Design Using Java Version 5.0. New York:Wiley, 2004.data acquisitionThere are a variety of ways in which data (facts or measurementsabout the world) can be turned into a digitalrepresentation suitable for manipulation by a computer. Forexample, pressing a key on the keyboard sends a signal thatis stored in a memory buffer using a value that representsData acquisition is the process of gathering real-time data fromscientific instruments and making it available in digital form. Sensorsignals are “conditioned” by filtering extraneous values, andare then sampled and digitized. Software can now provide elaborategraphic displays as well as alert scientists to unusual readings.