Data-acquisition-ppt-20020408490qwe5.pptx

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1. DATA ACQUISITION SYSTEM

2. 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.

3. 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

4. BASIC BLOCK DIAGRAM

5. 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.

6. 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 elements are required in the conditiong of data signal: Excitation System  Amplifier Sample & Hold Circuit Multiplexer

7. Amplification One 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. 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. Isolation Isolated 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 bot h the operators and expensive measurement equipment .

8. Filtering Filters 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. Linearization Linearization 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.

9. 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.

10. 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.

11. Sample & Hold Circuit Fig.:A DAS & conversion system Sample-and-hold (S/H) is an important analog building block with many applications, including analog-to-digital converters (ADCs) and switched-capacitor filters. The function of the S/H circuit is to sample an analog input signal and hold this value over a certain length of time for subsequent processing. A mux selects each signal is sequence. For that each signal is converted into a constant voltage over the gating time interval by means of S/H circuit.

12. Telemetry It is used to transmit measured quantity to a remote location for processing, recording & displaying. Following are the classes of Telemetry system.

13. Voltage Telemetry It use  primary sensing element which pproduces a proportional voltage signal. It must have high quality   circuits than current   telemetry   system and relatively high SNR. In the above fig.  as the pressure changes , the tip of the bordon tube changes its position which changes the sliding contact of  potentiometer,thereby changing the voltage proportionally. A voltage Telemetry System

14. Current Telemetry Basically it helps in connecting the transmission line from transmitter end to receiver end.  This type of telemetry employs a torque balance method in which the current in wire is used to apply a torque to balance a torque generated by the measurand. In this system, value of current is adjusted in an extended circuit to correspond a measured quantity and this value of current is  determined by an end device at a remote place.

15. Difference between Voltage & Current Telemetry    Voltage Telemetry    Current Telemetry Less immune to the effects of thermal and inductive voltage in the connecting loads. More immune to the effects of thermal and inductive voltage in the connecting loads. The system can't be used by adding output of several transmitters operating several receivers at a time. The system can be used by adding output of several transmitters operating several receivers at a time. Less susceptible for errors due to leakage currents to the ground More susceptible for errors due to leakage currents to the ground Cheaper More costly Develop less voltage Develop more voltage Speed of response isn't instantan. Speed of response is instantaneous

16. Position Telemetry Two Types : A) Wheatstone Bridge Position Telemetry B) Synchros Position Telemetry A) Wheatstone Bridge Position Telemetry There are two potentiometers. One at transmitting end and other at receiving end. Both are energized by a common supply. The slide contact at the transmitting end is positioned by Bourdon tube, when pressure is applied on the tube.If sliding contact at the receiving end is positioned, till the galvanometer indicates zero;  the position of the contact will assume the same position as the contact at the transmitter end. The receiving end sliding contact moves the pointer, which indicates the pressure to be measured

17. Automatic version of position telemetry The difference of the potential between points A and B is fed to chopper type amplifier. The output of chopper controls the movement of the shaded pole motor. The motor in turn move the point B in such that point A & point B are at equal potential.

18. Signal Recovery The signal at the receiving end are generally corrupted by noise. In situations where SNR is poor, it is necessary to employ a measurement technique to separate the noise from the signal. This is signal recovery. Four methods of detecting signals:     A) Signal Filtering       B) Signal Averaging     C) Signal Correlation C)Signal Coding

19. Radio Telemetry  In this method there is no physical link between transmitter and receiver. The link between transmission station (where the actual measurements are carried out) and the receiving station (where the measurable quantity is measured recorded and information used for control purposes) can be established only through radio links. At transmitter the signal is modulated. This can be amplitude (frequency) pulse modulation etc. The modulated signal is amplified and sent through the channels e.g. microwave channel. At the receiver the signal is deflected amplified and measured e.g. the receiver may have a frequency meter calibrated in units corresponding to the measured.

20. Data Processing, Display & Recording Generally it is necessary to process the received signals so as to modify the data before displayed or recorded or to analyse the data. Two forms of data modifications :      A)Correction to the received signal to compensate for scaling, non-          linearity,zero offset,temperature errors,etc.     B)Calculations such as addition, subtraction, multiplication or division          of two or more signals and their associated constants Often signals may be required to displayed or recorded. For this purpose indicating or recording instruments are required. Indication may be on a scale by a pointer or it may be a numeric display.

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