DATA ACQUISITION SYSTEM

BYAyantika Mukherjee

EIE,3rd YearRoll No:09187005016

WHAT IS DATA ACQUISITION?

Data acquisition is the process of sampling signals that measure real world physical conditions and converting the resulting samples into digital numeric values that can be manipulated by a computer. Data acquisition systems (abbreviated with the acronym DAQ) typically convert analog waveforms into digital values for processing.

STEPS REQUIRED FOR DAQ

Sense physical variables (use of transducers)

Condition the electrical signal to make it readable by an A/D board

Convert the signal into a digital format acceptable by a computer

Process, analyze, store, and display the acquired data with the help of software

BASIC BLOCK DIAGRAM

FUNCTIONS OF THE BLOCKS Physical system: This term refers to the all physical phenomena that are to be

measured. Temperature, pressure, light, force, displacement, and level all these can be included as the physical quantity that we measure directly with the help of sensors and transducers.

Transducer and actuators: This is the section which converts sensed physical quantity to its

equivalent electrical signal. A transducer converts temperature, pressure, level, length, position, etc. into voltage, current, frequency, pulses or other signals.

An actuator is a device that activates process control equipment

by using pneumatic, hydraulic or electrical power. For example, a valve actuator opens and closes a valve to control fluid rate.

FUNCTION OF THE BLOCKS Signal conditioning: Electrical signals are conditioned so they can be

used by an analog input board. Signal conditioning circuits improve the quality of signals generated by transducers before they are converted into digital signals by the PC's data-acquisition hardware. The following features may be available:› Amplification› Isolation› Filtering› Linearization

AmplificationOne of the most common signal conditioning functions is amplification. For maximum resolution, the voltage range of the input signals should be approximately equal to the maximum input range of the A/D converter. Amplification expands the range of the transducer signals so that they match the input range of the A/D converter. For example, a x10 amplifier maps transducer signals which range from 0 to 1 V into the range 0 to 10 V before they go into the A/D converter. Amplifiers increase voltage level to better match the analog-to-digital converter (ADC) range, thus increasing the measurement resolution and sensitivity. In addition, using external signal conditioners located closer to the signal source, or transducer, improves the measurement signal-to-noise ratio by magnifying the voltage level before it is affected by environmental noise.IsolationIsolated signal conditioning devices pass the signal from its source to the measurement device without a physical connection by using transformer, optical, or capacitive coupling techniques. In addition to breaking ground loops, isolation blocks high-voltage surges and rejects high common-mode voltage and thus protects both the operators and expensive measurement equipment.FilteringFilters reject unwanted noise within a certain frequency range. Oftentimes, low pass filters are used to block out high-frequency noise in electrical measurements, such as 60 Hz power. Another common use for filtering is to prevent aliasing from high-frequency signals. This can be done by using an antialiasing filter to attenuate signals above the Nyquist frequency.LinearizationLinearization is necessary when sensors produce voltage signals that are not linearly related to the physical measurement. Linearization is the process of interpreting the signal from the sensor and can be done either with signal conditioning or through software. Thermocouples are the classic example of a sensor that requires linearization.

FUNCTION OF THE BLOCKS

Analog to digital converter: Analog to digital (A/D) conversion changes

analog voltage or current levels into digital information. The conversion is necessary to enable the computer to process or store the signals.

The resolution of the converter indicates the number of discrete values it can produce over the range of analog values. The values are usually stored electronically in binary form, so the resolution is usually expressed in bits. In consequence, the number of discrete values available, or "levels", is a power of two. For example, an ADC with a resolution of 8 bits can encode an analog input to one in 256 different levels, since 28 = 256. The values can represent the ranges from 0 to 255 (i.e. unsigned integer) or from −128 to 127 (i.e. signed integer), depending on the application.Resolution can also be defined electrically, and expressed in volts. The minimum change in voltage required to guarantee a change in the output code level is called the least significant bit (LSB) voltage.

Resolution

Sampling

In signal processing, sampling is the reduction of a continuous signal to a discrete signal. A common example is the conversion of a sound wave (a continuous signal) to a sequence of samples (a discrete-time signal).A sample refers to a value or set of values at a point in time and/or space. A sampler is a subsystem or operation that extracts samples from a continuous signal. A theoretical ideal sampler produces samples equivalent to the instantaneous value of the continuous signal at the desired points. The analog signal is continuous in time and it is necessary to convert this to a flow of digital values. It is therefore required to define the rate at which new digital values are sampled from the analog signal. The rate of new values is called the sampling rate or sampling frequency of the converter.A continuously varying band limited signal can be sampled (that is the signal values at intervals of time T, the sampling time, are measured and stored) and then the original signal can be exactly reproduced from the discrete-time values by an interpolation formula. The accuracy is limited by quantization error. However, this faithful reproduction is only possible if the sampling rate is higher than twice the highest frequency of the signal.

AliasingIf the digital values produced by the ADC are, at some later stage in the system, converted back to analog values by a digital to analog converter or DAC, it is desirable that the output of the DAC be a faithful representation of the original signal. If the input signal is changing much faster than the sample rate, then this will not be the case, and spurious signals called aliases will be produced at the output of the DAC. The frequency of the aliased signal is the difference between the signal frequency and the sampling rate. For example, a 2 kHz sine wave being sampled at 1.5 kHz would be reconstructed as a 500 Hz sine wave. This problem is called aliasing.To avoid aliasing, the input to an ADC must be low-pass filtered to remove frequencies above half the sampling rate. This filter is called an anti aliasing filter, and is essential for a practical ADC system that is applied to analog signals with higher frequency content.

FUNCTION OF THE BLOCKS

Computer:

This is the final or ultimate section that actually produces the output in digital format. In A/D converter section the signal already gets transformed into digitized form that can be understood by the computer. Now we can access the output through some hardware and soft wares.

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 motherboard. 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 permissible to block it with simple polling loops. For example: Waiting for a trigger, starting the ADC, looking up the time, waiting for the ADC to finish, move value to RAM, switch multiplexer, get TTL input, let DAC proceed with voltage ramp. Many times reconfigurable logic is used to achieve high speed for specific tasks and digital signal processors are used after the data has been acquired to obtain some results. 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. It generates a complete DAQ, analysis, and display system. Data acquisition applications are controlled by software programs developed using various general purpose programming languages such as BASIC,C, Fortran, Java, Lisp, Pascal. Specialized software tools used for building large-scale data acquisition systems include EPICS. Graphical programming environments include ladder logic, Visual C++, Visual Basic, and LabVIEW. It does not require programming.