data-acquisition-system

2. 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.  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

5.  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.  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. 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.
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 both the operators and expensive measurement equipment.
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.

8.  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.

9.  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.

10. 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.

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

12. • 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

13. • 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.

14. 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

15. 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

16. • 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.

17.  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

18. • 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.

19.  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.