11-Apr-17

Unit-IV
6ME3A: Mechatronics
Contents
Data Acquisition & Data Acquisition and related Instrumentation:
Related Instrumentation Introduction to Data Acquisition Measurement Techniques: Sensors and
Transducers, Quantizing theory, Analog to Digital Conversion, Digital to Analog
Tarun Kr. Aseri (D/A) conversation, Signal Conditioning.
Asst. Prof.
Mechanical Engineering Real time Instrumentation:
Govt. Engineering College Ajmer
Barliya Chouraha, NH-8, Computer-Based Instrumentation Systems,
Ajmer-305001, India Software Design and Development, Data Recording and Logging.
Email: tarunaseri@gmail.com

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Data Acquisition (DAS or DAQ) Test section to DAS to Logger

• The purpose of a data acquisition system is to capture and analyze
some sort of physical phenomenon from the real world.
• Light, temperature, pressure, and torque are a few of the many
different types of signals that can interface to a data acquisition
system.
• A data acquisition system may also produce electrical signals
simultaneously. These signals can either intelligently control
mechanical systems or provide a stimulus so that the data acquisition
system can measure the response.
• A data acquisition system provides a way to empirically test designs,
theories, and real world systems for validation or research.

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DAS: Block Diagram DAS: Block Diagram

Physical Sensors Signal A/D
Computer
System Transducer Conditioning Converter

• Sensing
• Electrical Signal Conditioning
• Multiplexing, Sample and Hold
• A/D conversion
• Interfacing with computer
• Storage, processing and display in the computer with software

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The signal conditioner accepts the electrical output of the transducer and Data Acquisition
transmits the signal to the comparator in a form compatible with the
reference input. The functions of the signal conditioner include:
• Since data acquisition devices acquire an electric signal, a transducer or a
sensor must convert some physical phenomenon into an electrical signal.
 Amplification/attenuation (scaling)
 Isolation • In case of thermocouple, as the temperature increases, the voltage produced
 Sampling by the thermocouple increases.
 Noise elimination
 Linearization • A software program can then convert the voltage reading back into a
 Span and reference shifting temperature for analysis, presentation, and data logging.
 Mathematical manipulation (e.g., differentiation, division, integration, multiplication, root finding, squaring, subtraction,
or summation) • Many sensors produce currents instead of voltages.
 Signal conversion (e.g., DC–AC, AC–DC, frequency–voltage, voltage–frequency, digital–analog, analog–digital, etc.)
 Buffering • A current is often advantageous because the signal will not be corrupted by
 Digitizing small amounts of resistance in the wires connecting the transducer to the data
 Filtering
 Impedance matching
acquisition device.
 Wave shaping • A disadvantage of current-producing transducers is that most data acquisition
 Phase shifting
devices measure voltage, not current.

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Data Acquisition Data Acquisition

• The heart of a data acquisition device is a digital-to-analog converter (DAC), • The data that is transferred from the ADC and to the DAC travels to the computer over
an analog-to-digital converter (ADC), or some combination of the two. a bus.
• An ADC has a finite list of values which represents voltages. • A bus is a group of electrical conductors that transfer information inside a computer.
• The purpose of the ADC is to select a value from this list, which is closest to • Some common examples of a bus are PCI (Peripheral Component Interconnect) and
an actual voltage at a specified time. The value is then transferred in binary USB (Universal Serial Bus).
format to a computer. • The bus can carry both control information and binary measurement data to and from
• Alternatively, a DAC can produce an analog voltage from a list of binary measurement hardware.
values. • One of the most important considerations in selecting a bus is bus transfer rate, usually
• The voltage generated by a basic DAC stays the same until it receives another expressed in megabytes per second (Mbytes/s).
value from the computer. • Data acquisition devices often have on-board memory to serve as a holding place for
data when the bus is not available.
• In order to acquire and produce analog waveforms, the DAC and ADC must
activate at precise intervals. • In very fast data acquisition routines, the memory can hold all the data, and at the end
of the acquisition, all the data can be transferred to the computer for processing.
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Data Acquisition Data Acquisition

• When data is acquired at high speeds on multiple channels, it is often • The difference between an actual analog voltage and the closest
important to understand the phase relationship from one signal to the next. voltage from the list of binary values is called the quantization error.
• If the signals are generated or acquired on multiple data acquisition devices, • In a perfect digitizing measurement system free of noise, the
there are a number of ways to synchronize the systems and preserve relative quantization error would solely explain any difference between the
phase relationships. actual voltage and the measured voltage.
• One way is to share the ADC and DAC clock between the data acquisition • No measurement hardware and no environment, however, are perfect.
devices.
• The real-time system integration bus (RTSI) is a bus that can connect multiple
• The accuracy of an instrument describes the amount of uncertainty
devices together to share timing circuitry among multiple devices. when considering quantization error, unavoidable system noise, and
hardware imperfections.
• Phase-lock looping (PLL) is a more sophisticated synchronization method.
• Accuracy is sometimes confused with precision.
• A reference signal is supplied to all the data acquisition devices, and the
internal clocks stay in phase with the reference signal.

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Data Acquisition Data Acquisition

• The accuracy of a data acquisition system can change with temperature, • Data acquisition software supplied by manufacturer or programmed by
time, and usage. end user in suitable platforms using variety of programming
• Data acquisition hardware can store on-board correction constants for languages.
offset and gain errors. • The application is then ready for an end user to easily control and
• An offset error is a constant difference between the measured and acquire data from the hardware—a custom instrument built
actual voltage, regardless of the voltage level. specifically for the user’s needs.
• A gain error increases linearly as the measured voltage increases.
• Some data acquisition hardware also include an accurate voltage source
on-board that can be periodically used as a reference to correct the gain
and offset error parameters.

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Error Signal
• An automatic control system is said to be error actuated because the
Measurement Techniques: forward path components (comparator, controller, actuator, and plant
or process) respond to the error signal.
Sensors & Transducers • The error signal is developed by comparing the measured value of the
controlled output to some reference input, and so the accuracy
and precision of the controlled output are largely dependent on the
accuracy and precision with which the controlled output is measured.
• It follows then that measurement of the controlled output,
accomplished by a system component called the transducer, is
arguably the single most important function in an automatic control
system.

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Automatic Control System Transducer and Sensor

• A transducer senses the magnitude or intensity of the
controlled output and produces a proportional signal in an
energy form suitable for transmission along the feedback path
to the comparator.

• The element of the transducer which senses the controlled

output is called the sensor; the remaining elements of a
transducer serve to convert the sensor output to the energy
form required by the feedback path.

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Motion and Force Transducers Displacement (Position) Transducers

• Force is closely associated with motion, because motion is the result of • Displacement transducers may be considered according to application
unbalanced forces, and so force transducers are discussed concurrently. as gross (large) displacement transducers or sensitive (small)
displacement transducers.
• Rectilinear/Linear Motion
• The demarcation between gross and sensitive displacement is
• Straight line motion within a stationary frame of reference somewhat arbitrary, but may be conveniently taken as approximately 1
• Angular/Rotation/Rotary Motion mm for rectilinear displacement and approximately 10′ arc (1/6°) for
• Circular Motion about a Fixed Axis angular displacement.
• The predominant types of gross displacement transducers are:
• Potentiometers
• Variable differential transformers (VDT)
• Synchros
• Resolvers
• Position encoders

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Potentiometer Variable differential transformers (VDT)

• Potentiometer-based transducers are simple to implement and require
the least signal conditioning, but potentiometers are subject to wear • VDTs are not as subject to wear as potentiometers, but the
due to sliding contact between the wiper and the resistance element maximum length of the stroke is small, approximately 25 cm or
and may produce noise due to wiper bounce. less for a linear VDT (LVDT) and approximately 60 degree or
less for a rotary VDT (RVDT).
• Potentiometers are available with strokes ranging from less than 1 cm
to more than 50 cm (rectilinear) and from a few degrees to more than • VDTs require extensive signal conditioning in the form of
50 turns (rotary). phase-sensitive demodulation of the AC signal; however, the
availability of dedicated VDT demodulators in integrated circuit
(IC) packages mitigates this disadvantage of the VDT.

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Synchros: a transformer Resolvers

• Due to inherent physical properties and mechanical & electrical designs,
synchros make possible the accurate transmission and reproduction to a • Resolvers are simpler and less expensive than synchros, and they
remote location of any data or information which can be converted to have an advantage over RVDTs in their ability to measure angular
angular rotation. displacement throughout 360oC of rotation.
• Synchros are rather complex and expensive three-phase AC machines, • Dedicated ICs are available for signal conditioning and for
which are constructed to be precise and rugged. conversion of resolver output to digital format.
• Synchros are capable of measuring angular differences in the positions
(up to ±180oC) of two continuously rotating shafts.
• In addition, synchros may function simultaneously as reference input,
output measurement device, feedback path, and comparator.

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Position encoders Velocity Transducers

• Signal conditioning techniques make it possible to derive all motion
• Position encoders are highly adaptable to digital control schemes
measurements—displacement, velocity, or acceleration—from a
because they eliminate the requirement for digital-to-analog
measurement of any one of the three.
conversion (DAC) of the feedback signal.
• The analog transducers frequently used are:
• The code tracks are read by track sensors, usually wipers or
• Magnet-and-coil velocity transducers
electro-optical devices (typically infrared or laser).
• Tachometer generators
• Position encoders are available for both rectilinear and rotary • Counter-type velocity transducers
applications, but are probably more commonly found as shaft
encoders in rotary applications.

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Velocity Transducers: Magnet-and-coil velocity transducers Velocity Transducers: Tachometer generator

• The operation of magnet-and-coil velocity transducers is based on • A tachometer generator is, as the name implies, a small
Faraday’s law of induction. AC or DC generator whose output voltage is directly
• For a solenoidal coil with a high length-to-diameter ratio made with proportional to the angular velocity of its rotor, which
closely spaced turns of fine wire, the voltage induced into the coil is is driven by the controlled output shaft.
proportional to the velocity of the magnet. • Tachometer generators are available for shaft speeds of
• Magnet-and-coil velocity transducers are available with strokes 5000 rpm, or greater, but the output may be nonlinear
ranging from less than 10 mm to approximately 0.5 m. and there may be an unacceptable output voltage ripple
at low speeds.
• AC tachometer generators are less expensive and easier
to maintain than DC tachometer generators, but DC
tachometer generators are directly compatible with
analog controllers and the polarity of the output is a
direct indication of the direction of rotation.
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Velocity Transducers: Counter-type velocity transducers Acceleration Transducers

• Counter-type velocity transducers operate on the principle of counting • As with velocity measurements, it
electrical pulses for a fixed amount of time, then converting the count is sometimes preferable to
per unit time to velocity. measure acceleration directly, Rectilinear Acceleration Transducers

rather than derive acceleration

• Counter-type velocity transducers rely on the use of a proximity sensor from a displacement or velocity
(pickup) or an incremental encoder: measurement.
• Electro-optic • The majority of acceleration
• Variable reluctance transducers may be categorized as
• Hall effect seismic accelerometers because Rotary Accelerometer
• Inductance the measurement of acceleration Force balance
is based on measuring the accelerometer
• Capacitance
displacement of a mass called the
seismic element.
Piezoelectric
Accelerometer.

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Force Transducers Process Transducers

• Force measurements are usually • This type of transducers are being used in measuring and controlling
based on a measurement of the the process variables most frequently encountered in industrial
motion, which results from the processes, namely,
applied force. • Fluid pressure
• If the applied force results in Spring Scale • Fluid flow
gross motion of the controlled Gyroscope • Liquid level
output, and the mass of the • Temperature
output element is known, then
any appropriate accelerometer Load Cell
attached to the controlled output
produces an output proportional
to the applied force (F = Ma).

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Analog vs. Digital signals Analog Signal & Digital Signal

• An Analog signal is any continuous signal for which the time varying • Analog and digital signals are used to transmit
feature (variable) of the signal is a representation of some other time information, usually through electric signals.
varying quantity, i.e., analogous to another time varying signal. It • In both these technologies, the information, such
differs from a digital signal in terms of small fluctuations in the signal as any audio or video, is transformed into electric
which are meaningful. signals.
• A digital signal uses discrete (discontinuous) values. By contrast, non- • The difference between analog and digital
digital (or analog) systems use a continuous range of values to technologies is that in analog technology,
represent information. Although digital representations are discrete, information is translated into electric pulses of
the information represented can be either discrete, such as numbers or varying amplitude.
letters, or continuous, such as sounds, images, and other measurements • In digital technology, translation of information
of continuous systems. is into binary format (zero or one) where each bit
is representative of two distinct amplitudes.

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Analog Digital
Properties of Digital vs Analog signals
Signal Analog signal is a continuous Digital signals are discrete time signals
Digital information has certain properties that distinguish it from analog communication signal which represents physical generated by digital modulation.
methods. These include
measurements.
• Synchronization – digital communication uses specific synchronization sequences for Waves Denoted by sine waves Denoted by square waves
determining synchronization.
Representation Uses continuous range of values Uses discrete or discontinuous values to
• Language – digital communications requires a language which should be possessed by
both sender and receiver and should specify meaning of symbol sequences. to represent information represent information
• Errors – disturbances in analog communication causes errors in actual intended Example Human voice in air, analog Computers, CDs, DVDs, and other digital
communication but disturbances in digital communication does not cause errors enabling electronic devices. electronic devices.
error free communication. Errors should be able to substitute, insert or delete symbols to Technology Analog technology records Samples analog waveforms into a limited set
be expressed. waveforms as they are. of numbers and records them.
• Copying – analog communication copies are quality wise not as good as their originals Data transmissions Subjected to deterioration by Can be noise-immune without deterioration
while due to error free digital communication, copies can be made indefinitely.
noise during transmission and during transmission and write/read cycle.
• Granularity – for a continuously variable analog value to be represented in digital form write/read cycle.
there occur quantization error which is difference in actual analog value and digital
representation and this property of digital communication is known as granularity. Response to Noise More likely to get affected Less affected since noise response are
reducing accuracy analog in nature
Flexibility Analog hardware is not flexible. Digital hardware is flexible in
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Analog Digital
Uses Can be used in analog devices only. Best suited for Computing and digital
Best suited for audio and video electronics.
transmission.
Applications Thermometer PCs, PDAs
Bandwidth Analog signal processing can be done There is no guarantee that digital signal
in real time and consumes less processing can be done in real time and Thank You
bandwidth. consumes more bandwidth to carry out
the same information. For
Memory Stored in the form of wave signal Stored in the form of binary bit
Your Attention
Power Analog instrument draws large power Digital instrument draw only negligible
power
Cost Low cost and portable Cost is high and not easily portable
Impedance Low High order of 100 megaohm
Errors Analog instruments usually have a Digital instruments are free from
scale which is cramped at lower end observational errors like parallax and
and give considerable observational approximation errors.
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