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Data acquistion is a great demand in industry and consumer applications. Data acquisition involves gathering signals form measurement sources and digitizing the signal for storage, analysis and presentation on a personal computer (PC). The purpose of data acquisition is to measure an electrical or physical phenomenon such as voltage, current, temperature, pressure, or sound. PC-based data acquisition uses a combination of modular hardware, application software, and a computer to take measurements. Data acquisition systems incorporate signals, sensors, actuators, signal conditioning, data acquisition devices, and application software.
Multichannel data acquisition (DAQ) is needed in many real-time applications for the purpose of surveillance, monitoring, and/or control. These applications include wideband communications, command communication and control, space exploration, medical diagnosis, etc. In our proposed system multichannel DAQ is implemented using virtual instrumentation software LABVIEW. If the signals are simultaneously acquired, simultaneous acquisition of additional data can be used to obtain additional information within the same acquisition time  However, existing computer based multi-channel DAQ systems are rugged, expensive and require expensive instruments. To reduce cost and power consumption of existing computer based DAQ an embedded system based DAQ was designed. An embedded system is a special-purpose computer system which is a combination of hardware and software designed to perform one or a few dedicated functions, often with real-time computing constraints.
Embedded systems when employed into the data acquisition environments can achieve low cost, low power consumption and portability but this also has few disadvantages [2, 3]. An embedded system has however it is not reconfigurable due to its fixed hardware architecture , . Fixed architecture embedded microcontroller DAQ systems have many disadvantages. It is not easy for an user to replace the design as per requirements as in case of embedded based DAQ system. If an existing DAQ system uses one processor with 16 bits, for example, there are times when it is needed to enhance the performance by using two processors with 32 bits each. In such cases it will be required to replace the whole embedded microcontroller DAQ system with a new one.
Further, the size of the cache of a microcontroller is a design parameter that cannot be reconfigured after fabrication. In addition, inserting floating point operations, or changing the ALU functionality is not possible. Also, in fixed architecture DAQs, robustness, and fault tolerance are achieved by redundancy. This in turn increases the size, power consumption and the cost of the system. The proposed system uses the facilities of Data acquisition system using virtual instruments which eliminates the use of physical instruments.
Multichannel DAQ is performed to acquire the channels simultaneously without interference. The importance of multichannel DAQ is observed in many fields . The purpose of acquisition of many channels has much impact in biomedical and space applications .
Signal ParametersAnalog inputs
Fig. 1 Block Diagram of DAQ using VI
Figure 1 shows the block diagram of DAQ implemented in Labview using virtual instruments. The analog channels are acquired in virtual instruments. Each channel is analysed with signal parameters like amplitude, frequency and multiplexed. The multiplexed signals are stored and displayed in multichannel. Thus multichannel is acquired in a single waveform and alternatively the signal parameters can be varied. This method has a greater advantage that a controller can easily monitor and control the channels simultaneously.
The objective was to design the DAQ System to target the recent needs in the industries and make it compatible with the new trends in the technology and to reduce the cost constraints. The utilization of the facilities, flexibilities and the available recourses to match the requirements is targeted in the design. This project is designed with keeping in mind the recent trends of applications and its requirements along with the cost constraints. There are certain other goals which are targeted through this design which are as included like industrial application, affordable to small scale Industries, system can take place of systems based on monitoring temperature, pressure, humidity, level, sound, heat, pH voltage.
DATA ACQUISITION SYSTEM
Generally DAQ system always has a controlling unit or the processing unit. Function of this unit is to acquire the data and convert it to the usable format. The virtual instruments is capable of taking data and responsible for giving output in designed range of analog input i.e. within reference voltage limits. Signal conditioning of the sensor signals are to be carried out before it could be connected to the system. Computer manipulates the data as well as stores it in a file, thus it also does function of data logging . The system as a whole classified in two primary design modules as, temperature acquisition and other physical data acquisition like pressure, sound, pH, level etc with GUI (Graphical User Interface) based software for display of acquired data by software tool LabVIEW.
The rapid advancement and adoption of computers in the last two decades has given a great improvement in instrumentation test and measurement. Continuous reduction of personal computers and availability of low cost high performance software packages has boosted the systems for Automatic Test Equipment (ATE) based on programmable instrumentation. GPIB (General Purpose Interface Bus) based programmable instrumentation has gained tremendous spread in the last decade for designing ATE system with the concept of â€œvirtual instrumentationâ€Â. Virtual instruments replace part of signal acquisition, processing and display, in traditional instruments, by using personal computer. By graphical programming, the computer monitor can be turned into the front panel of the traditional instruments and, in fact, with enhanced features. Plug-in data acquisition boards acts as interface between computer and outside world, it functions as a device that digitizes incoming analog signals. .
General VI is defined as the combination of hardware and software with industry-standard computer technologies to create user-defined instrumentation. In this type of test instrumentation that is basically software reliant and primarily dependent on a computer to control test hardware and equipment, analyze, and present test results. The power of VI application software lies in the fact that it empowers the user to include test equipments as objects in their programs.
Virtual instrumentation uses highly productive software, modular I/O and commercial platforms. National Instruments LabVIEW, a premier virtual instrumentation graphical development environment, uses symbolic or graphical representations to speed up development. The software symbolically represents functions . Another virtual instrumentation component is modular I/O designed to be rapidly combined in any order or quantity to ensure that virtual instrumentation can both monitor and control any development aspect.
The analog inputs are acquired initially using virtual instruments. In LabVIEW many virtual instruments like switch, knob, meter etc are implemented in order to avoid using traditional instruments in data acquisition.. All the channel are grouped in a single block called cluster.
Fig. 2 Cluster Block Diagram view
Figure 2 shows the structure of a cluster in block diagram. The above structure shows the implementation of all data types like integer, Boolean etc., in a single cluster. It combines different data types within a single structure. It reduces the connection terminals to sub VI.
Fig. 3 Cluster Front Panel view
Figure 3 shows the structure of cluster in front panel. The main cluster operations bundle, unbundled, bundle by name and unbundle by name. The following are the cluster operations.
Extract individual data elements from a cluster.
Add individual data elements to a cluster
Break a cluster out into its individual data elements.
Proposed system uses temperature acquisition as a separate module. Here the temperature is acquired using virtual instruments and it is measured. Acquisition of temperature is useful in many applications like weather forecasting, space applications and medical applications.
Fig. 4 Temperature Acquisition
Figure 3 shows the temperature acquisition using virtual instruments. Here the temperature is acquired with the help of meter. Initially the random data is recorded using meter and the data is varied. The recorded data is displayed in thermometer. The user can easily monitor the various changes occuring in the thermometer and displayed in the waveform. Using the display, the user can detect the errors and control the variations.
Multiplexing plays an important role in multichannel DAQ. Here it groups the signal of varying amplitude and frequency and displayed in the waveform. Multiplexing is the process of scanning through number of input channels and sampling in each rotation . Multiplexing allows single ADC to do the work of several channels. Rather than dedicating a ADC to each channel, a single convertor can be used with the help of multiplexing. This can save power, as ADC uses significant amount of power than switches does.
Unbundle operation performs demultiplexing of virtual channels. Hence it is demultiplexed using unbundling function. This splits the various signal parameters and given as input to function generator.
COMPARISON TABLE AMONG EXISTING DAQ SYSTEMS
Computer based DAQ
Microcontroller based DAQ
FPGA based DAQ
High cost due to fixed architecture
No power consumption
Applicable in real-time
Portability High performance
Full capabilities of FPGA, Single multiplexed ADC,
Graphical way to acquire channels. Easy measurement task.
In Table I it is shown that the proposed system is more advantageous than existing system interms of cost and hardware complexity. The performance is higher in virtual channels than in physical channels. Instead of using physical instruments, many virtual instruments can
be used which enables easier control and measurement task.
RESULTS AND DISCUSSION
The multichannel DAQ system have shown that many channels can be controlled simultaneously as per user requirements. The proposed system uses virtual instruments to control and monitor the channels with low cost.
Labview simulation results can be viewed in front panel. Here the eight channel acquisition is split up into four waveforms in which each contains two channel waveform.
Fig. 5 Two Channel Waveform
Figure 5 shows the two channel waveform which shows the acquisition of data using two virtual channels. The amplitude level can be varied according to the incoming data. The waveform chart shows the variation in incoming data and it can be easily controlled by the user.
Fig. 6 Multichannel DAQ System
Figure 6 shows the entire simulation results of proposed system. It has four waveform charts in which each contain two channel waveforms. The signal parameters like amplitude, frequency and signal type is given as input to the basic function generator. The function generator changes the output according to the input. The multichannel waveform is applied for various input. The variance in signal parameters depends on signal input. Based on the amplitude and frequency value the multichannel waveform is obtained. This approach is useful in medical diagnosis. To acquire and study ECG signals and to study and human generated signals which are of different amplitude and frequency this approach can be implemented.
Each channel acquisition consists of various parameters and adjusted to user requirements. Temperature acquisition is done unique using virutal instruments. The temperature variance can be viewed clearly in front panel.
The conceptual design of multichannel DAQ using virtual instruments has been provided. Using virtual instrumentation technique eight channel data acquisition is obtained. The channels are multiplexed in a multichannel waveform and number of channels can be displayed in a single waveform. Multichannel DAQ is universal and it is applied in various areas like wideband communication, radar application, medical and environmental applications. The main functions of the DAQ part have been designed, built. The stand-alone DAQ has good performance. The device cost is only a fraction of existing multichannel DAQs. The implementation of DAQ in Soc FPGA is the subject of future paper which implements the advantage of using reconfigurable hardware.