01-01-2013, 01:49 PM
PC BASED DATA ACQUISITION SYSTEM
PC BASED DATA ACQUISITION SYSTEM.pdf (Size: 907.91 KB / Downloads: 108)
ABSTRACT
DAQ (Data AcQuisition) is simply the process of bringing a real-world signal, such as avoltage, into the computer,
for processing, analysis, storage or other data manipulation. A Physical phenomena represents the real-world
signal you are trying to measure. In order to optimize the characteristics of a system in terms of performance,
handling
capacity and cost, the relevant subsystem can be combined together. Analog data is generally acquired and
transformed into the digital form for the purpose of processing, transmission and display. Rapid advances in
Personal Computer (PC) hardware and software technologies have resulted in easy and efficient adoption of PCs
in various precise measurement and complex control applications. A PC based measurement or control application
requires conversion of real world analog signal into digital format and transfer of digitized data into the PC. A data
acquisition system that performs conversion of analog signal to digital data and the digital data to analog signal is
interfaced to a pc to implement the functions of a measurement and control instrumentation applications. In this
project we have used the electromagnetic sensor to acquire the data of a magnetic disk angular velocity, which we
have got in mili volts range. This has been further transformed approximately into the range of 5 volt by using an
operational amplifier of suitable gain (~30)and then rectified .We then transformed the analog voltage into digital
by using ADC 0804 and the processing part is done by using ATMEL89c51.In the second phase we have used the
data acquisition card PCL-208 and 207 to interface the amplified output to PC by the help TURBO C (C
compiler).
DATA ACQUISITION SYSTEM
DAQ (Data AcQuisition) is defined as the process of taking a real-world signal as input, such as avoltage or
current any electrical input, into the computer, for processing, analysis, storage or other data manipulation or
conditioning.A Physical phenomena represents the real-world signal we are trying to measure. Today, most
scientists and engineers are using personal computers with ISA, EISA, PCI or PCMCIA bus for data acquisition in
laboratory, research, test and measurement, and industrial automation applications. Many applications use
plug-in boards to acquire data and transfer it directly to computer memory. Others use DAQ hardware
remote from the PC that is coupled via parallel port, serial port, GPIB-Bus or Netoperate. Typically, DAQ plug-in
boards are general-purpose data acquisition device that are well suited for measuring voltage signals. However,
many real-world sensors and transducers output signals that must be conditioned before a DAQ
board or device can effectively and correctly acquire the signal. This front-end preprocessing, which is
generally referred to as signal conditioning, includes functions such as signal amplification, filtering,
electrical isolation,and multiplexing. Afterall, many transducers require excitation currents or voltages, bridge
completion, linearization, or high amplification for proper and accurate operation.
The PC as a platform for data acquisition
The field of data acquisition encompasses a very wide range of activities. At its simplest level, it involves reading
electrical signals into a computer from some form of sensor. These signals may represent the state of a physical
process, such as the position and orientation of machine tools, the temperature of a furnace or the size and shape of
a manufactured component. The acquired data may have to be stored, printed or displayed. Often the data have to
be analyzed or processed in some way in order to generate further signals for controlling external equipment or for
interfacing to other computers. This may involve manipulating only static readings, but it is also frequently
necessary to deal with time-varying signals as well. Some systems may involve data to be gathered slowly, over
time spans of many days or weeks. Other will necessitate short bursts of very high speed data acquisition –
perhaps at rates of up to several thousand readings per second. DAQ is used widely for laboratory automation,
industrial monitoring and control, as well as in a variety of other time-critical applications. The most central reason
for using the PC for data acquisition and control is that there is now a large and expanding pool of programmers,
engineers and scientists who are familiar with the PC.Indeed it is quite likely that many of these personnel will
have learnt how to program on an IBM PC or PC clone.
Input/output ports
In addition to its memory, the PC has another entirely separate address space. This is dedicated to transferring data
to or from marginal devices and is known as Input/Output space (or simply I/O space). Just as the PC’s memory
space is divided into separate byte locations, the I/O space consists of many byte-sized I/O ports. Each port is
addressable in much the same way as memory, although an additional control line is used within the PC to
differentiate between memory and I/O port accesses. I/O space consists of a contiguous series of I/O addresses.
Unlike memory space, the I/O address space is not segmented and cannot be paged. In fact, the processor
references I/O ports by means of a 16-bit address and this means that no more than 65 536 I/O ports can be
supported by the PC. In practice, this is added partial by the I/O address decoding scheme used on the PC and its
adaptor cards.The I/O ports provide a means of sending data to, and receiving data from, devices such as the video
adaptor, the disk subsystem, or analogue-to-digital transformers (ADCs) on plug-in data-acquisition cards.
Software can use the assembly language IN or OUT instructions,or their high level language counterparts, to
communicate with hardware devices via the I/O ports. These are discussed in more detail in Chapter 6, but for the
moment we will consider a simple example. Suppose that a plug-in 8-bit ADC card possesses control and data
registers that are each mapped to one of the PC’s I/O ports. The software starts the analogue-to-digital exchange
process by writing a bit pattern to the I/O port that maps to the ADC card’s control register. When the ADC has
finished the conversion it might set a bit (known as the End of Conversion, or EOC, bit) in another register to
indicate that digitized data is now available.In this way, the software is able to detect the EOC bit by reading the
corresponding I/O port.