Data Accquisition
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theultrandomData Accquisition
Uploaded by
theultrandomData Acquisition
Acquisition
System Overview
• In the last few years, industrial PC I/O interface products have become
increasingly reliable, ccurate and affordable. PC-based data acquisition
and control systems are widely used in industrial and laboratory
applications like monitoring, control, data acquisition and automated
testing.
• Selecting and building a DA&C (Data Acquisition and Control) system
that actually does what you want it to do requires some knowledge of
electrical and computer engineering.
• • Transducers and actuators
• • Signal conditioning
• • Data acquisition and control hardware
• • Computer systems software
Data Acquisition System
Introduction I
A data acquisition system consists of
many components that are integrated
to:
• Sense physical variables (use of
transducers)
• Condition the electrical signal to make it
readable by an A/D board
Data Acquisition System
Introduction II
• Convert the signal into a digital format
acceptable by a computer
• Process, analyze, store, and display the
acquired data with the help of software
Data Acquisition System
Block Diagram
Transducers
Sense physical phenomena and
translate it into electric signal.
■ Temperature ■ Displacement
■ Pressure ■ Level
■ Light ■ Electric signals
■ Force ■ ON/OFF switch
Transducers and
Actuators
• 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.
Signal Conditioning
• 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.
• Examples of signal conditioning are signal scaling,
amplification, linearization, cold-junction
compensation, filtering, attenuation, excitation,
common-mode rejection, and so on.
Signal Conditioning
• 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.
Signal Conditioning
Electrical signals are conditioned so
they can be used by an analog input
board. The following features may be
available:
■ Amplification ■ Filtering
■ Isolation ■Linearization
Data Acquisition
• Data acquisition and control hardware
generally performs one or more of the
following functions:
• analog input,
• analog output,
• digital input,
• digital output and
• counter/timer functions.
Analog Inputs (A/D)
• 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.
Analog Inputs (A/D)
• The most significant criteria when selecting A/D
hardware are:
• 1. Number of input channels
• 2. Single-ended or differential input signals
• 3. Sampling rate (in samples per second)
• 4. Resolution (usually measured in bits of resolution)
• 5. Input range (specified in full-scale volts)
• 6. Noise and nonlinearity
Analog to Digital (A/D)
Converter
• Input signal ■ Resolution
■ Range
• Sampling rate
■ Gain
• Throughput
A/D Converter:
Input Signal
• Analog
✔ Signal is continuous
Example: strain gage. Most of transducers
produce analog signals
■ Digital
✔ Signal is either ON or OFF
Example: light switch.
A/D Converter:
Sampling Rate
Determines how often conversions take
place.
The higher the sampling rate, the better.
Analog 16
8
4 Samples/cycle
Samples/cycle
Samples/cycle
Input
A/D Converter:
Sampling Rate
• Aliasing.
✔ Acquired signal gets distorted if
sampling rate is too small.
A/D Converter:
Throughput
Effective rate of each individual channel is
inversely proportional to the number of
channels sampled.
Example:
• 100 KHz maximum.
• 16 channels.
100 KHz/16 = 6.25 KHz per channel.
A/D Converter:
Range
• Minimum and maximum voltage levels
that the A/D converter can quantize
■ Ranges are selectable (either
hardware or software) to
accurately measure the signal
A/D Converter:
Resolution
Analog Outputs (D/A)
• The opposite of analog to digital conversion is digital to
analog (D/A) conversion. This operation converts digital
information into analog voltage or current. D/A devices
allow the computer to control real-world events.
• Analog output signals may directly control process
equipment. The process can give feedback in the form of
analog input signals. This is referred to as a closed loop
control system with PID control.
• Analog outputs can also be used to generate waveforms. In
this case, the device behaves as a function generator.
Analog Outputs (D/A)
Data Acquisition Software
• It can be the most critical factor in obtaining
reliable, high performance operation.
• Transforms the PC and DAQ hardware into a
complete DAQ, analysis, and display system.
• Different alternatives:
• Programmable software.
• Data acquisition software packages.
Programmable Software
• Involves the use of a programming language,
such as:
• C++, visual C++
• BASIC, Visual Basic + Add-on tools (such as
VisuaLab with VTX)
• Fortran
• Pascal
✔ Advantage: flexibility
✔ Disadvantages: complexity and steep learning
curve
Data Acquisition Software
• Does not require programming.
• Enables developers to design the custom
instrument best suited to their
application.
Examples: TestPoint, SnapMaster,
LabView, DADISP, DASYLAB, etc.
Designing a DAS:
Factors to Consider
Is it a fixed or a mobile application?
Type of input/output signal: digital or analog?
Frequency of input signal ?
Resolution, range, and gain?
Continuous operation?
Compatibility between hardware and software. Are
the drivers available?
Overall price.