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Chapter: Measurements and Instrumentation - Transducers and Data Acquisition Systems

Data Acquisition Systems and its Elements

Definition, Diagram, Fundamental elements of data acquisition system, Explanation, Source, Signals, DAQ hardware, DAQ software, Advantages, Disadvantages, Applications.

DATA ACQUISITION SYSTEMS

 

Definition

 

Data acquisition is the process of real world physical conditions and conversion of the resulting samples into digital numeric values that can be manipulated by a computer. Data acquisition and data acquisition systems (abbreviated with the acronym DAS) typically involves the conversion of analog waveforms into digital values for processing.

 

The components of data acquisition systems include:

 

i) Sensors that convert physical parameters to electrical signals.

 

ii)Signal conditioning circuitry to convert sensor signals into a form that can be converted to digital values.

 

iii) Analog-to-digital converters, which convert conditioned sensor signals to digital values.

 

Diagram

 


Fundamental elements of data acquisition system

Explanation

 

Data acquisition is the process of extracting, transforming, and transporting data from the source systems and external data sources to the data processing system to be displayed, analyzed, and stored.

 

 A data acquisition system (DAQ) typically consist of transducers for asserting and measuring electrical signals, signal conditioning logic to perform amplification, isolation, and filtering, and other hardware for receiving analog signals and providing them to a processing system, such as a personal computer.

 

 Data acquisition systems are used to perform a variety of functions, including laboratory research, process monitoring and control, data logging, analytical chemistry, tests and analysis of physical phenomena, and control of mechanical or electrical machinery.

 

 Data recorders are used in a wide variety of applications for imprinting various types of forms, and documents.

 

 Data collection systems or data loggers generally include memory chips or strip charts for electronic recording, probes or sensors which measure product environmental parameters and are connected to the data logger.

 

 Hand-held portable data collection systems permit in field data collection for up-to-date information processing.

 

Source

 

Data acquisition begins with the physical phenomenon or physical property to be measured.

 

 Examples of this include temperature, light intensity, gas pressure, fluid flow, and force. Regardless of the type of physical property to be measured, the physical state that is to be measured must first be transformed into a unified form that can be sampled by a data acquisition system.

 

The task of performing such transformations falls on devices called sensors.

 

A sensor, which is a type of transducer, is a device that converts a physical property into a corresponding electrical signal (e.g., a voltage or current) or, in many cases, into a corresponding electrical characteristic (e.g., resistance or capacitance) that can easily be converted to electrical signal.

 

 The ability of a data acquisition system to measure differing properties depends on having sensors that are suited to detect the various properties to be measured. There are specific sensors for many different applications.

 

 DAQ systems also employ various signal conditioning techniques to adequately modify various different electrical signals into voltage that can then be digitized using an Analog-to-digital converter (ADC).

 

Signals

Signals may be digital (also called logic signals sometimes) or analog depending on the transducer used. Signal conditioning may be necessary if the signal from the transducer is not suitable for the DAQ hardware being used.

 

The signal may need to be amplified, filtered or demodulated.

 

Various other examples of signal conditioning might be bridge completion, providing current or voltage excitation to the sensor, isolation, and linearization. For transmission purposes, single ended analog signals, which are more susceptible to noise can be converted to differential signals. Once digitized, the signal can be encoded to reduce and correct transmission errors.

 

DAQ hardware

DAQ hardware is what usually interfaces between the signal and a PC. It could be in the form of modules that can be connected to the computer's ports (parallel, serial, USB, etc.) or cards connected to slots (S-100 bus, Apple Bus, ISA, MCA, PCI, PCI-E, etc.) in the mother board.

 

 Usually the space on the back of a PCI card is too small for all the connections needed, so an external breakout box is required. The cable between this box and the PC can be expensive due to the many wires, and the required shielding

 

 DAQ cards often contain multiple components (multiplexer, ADC, DAC, TTL-IO, high speed timers, RAM). These are accessible via a bus by a microcontroller, which can run small programs.

 

 A controller is more flexible than a hard wired logic, yet cheaper than a CPU so that it is alright to block it with simple polling loops.

 

 The fixed connection with the PC allows for comfortable compilation and debugging. Using an external housing a modular design with slots in a bus can grow with the needs of the user.

 

 Not all DAQ hardware has to run permanently connected to a PC, for example intelligent stand-alone loggers and oscilloscopes, which can be operated from a PC, yet they can operate completely independent of the PC.

 

DAQ software

 

 DAQ software is needed in order for the DAQ hardware to work with a PC. The device driver performs low-level register writes and reads on the hardware, while exposing a standard API for developing user applications.

A standard API such as COMEDI allows the same user applications to run on

 

 different operating systems, e.g. a user application that runs on Windows will also run on Linux and BSD.

 

Advantages

 

Reduced data redundancy

 

Reduced updating errors and increased consistency

 

Greater data integrity and independence from applications programs

 

Improved data access to users through use of host and query languages

 

Improved data security

 

Reduced data entry, storage, and retrieval costs

 

Facilitated development of new applications program

 

Disadvantages

 

Database systems are complex, difficult, and time-consuming to design Substantial hardware and software start-up costs

 

Damage to database affects virtually all applications programs

 

Extensive conversion costs in moving form a file-based system to a database system Initial training required for all programmers and users

 

 

Applications

 

Temperature measurement

 

Recommended application software packages and necessary toolkit

 

Prewritten Lab VIEW example code, available for download

 

Sensor recommendations

 

Video tutorials for hardware setup and software programming

Study Material, Lecturing Notes, Assignment, Reference, Wiki description explanation, brief detail
Measurements and Instrumentation - Transducers and Data Acquisition Systems : Data Acquisition Systems and its Elements |


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