Transcript The voltage and current induced in the second coil depend on the

Signal conditioning
Noisy
• Key Functions of Signal Conditioning:
Amplification
Filter
 Attenuation
 Isolation
 Linearization
ATTENUATION
• Attenuation is a general term that refers to any reduction in the
strength of a signal.
• occurs with any type of signal, whether digital or analog.
Voltage Divider
• Most data acquisition system inputs can measure voltages only within
a range of 5 to 10 V.
• Voltages higher than this must be attenuated.
• Simplest attenuation circuit
• Vout = Vin ( R2 / R1+R2)
• It is essential that any attenuator or voltage divider is driven from a low
impedance source
• the load (the impedance connected to the output) must be high compared
to the divider output impedance.
• It is generally considered that the signal source should have an impedance
of at most 1/10 that of the attenuator, and the load should have an
impedance (at least) 10 times the attenuator's output impedance.
There is a need to measure or monitor electrical signals
from multiple sensors.
So normally they are connected to a multiplexer
• However, for multiplexer inputs, the output impedance of a simple
voltage divider circuit is much too high
• For example, consider a 10:1 divider reading 50 V. If a 900 kΩ and a
100 kΩ resistor are chosen to provide a 1 MΩ load to the source, the
impedance seen by the analog multiplexer input is about 90 kΩ, still
too high for the multiplexed reading to be accurate.
• When the values are both downsized by a factor of 100 so the output
impedance is less than 1 kΩ, but the voltage measurement will be
affected as well.
• Hence, simple attenuation circuit is not practical with multiplexed
inputs.
• Buffered Voltage Divider
• The low impedance loading effect of
simple voltage dividers can be
overcome using unity-gain buffer
amplifiers on divider outputs.
• A dedicated unity-gain buffer has
high-input impedance in the MΩ
range and does not load down the
source, as does the network in the
previous example.
• Moreover, the buffers’ output
impedance is extremely low, which is
necessary for the multiplexed analog
input.
An op amp or a transistor serves as an
impedance matching buffer to prevent
the load from affecting the divider’s
output voltage.
ISOLATION
Isolated signal conditioning products protect and preserve valuable
measurements and control signals, as well as equipment, from the
dangerous and degrading effects of noise, transient power surges, internal
ground loops, and other hazards present in industrial environments.
Methods of Implementing Isolation
Isolation requires signals to be transmitted across an isolation barrier
without any direct electrical contact.
Light-emitting diodes (LEDs), capacitors, and inductors are three
commonly available components that allow electrical signal transmission
without any direct contact.
The principles on which these devices are based form the core of the
three most common technologies for isolation –
i. optical,
ii. capacitive
iii. inductive coupling.
• Optical Isolation
Optical isolation uses an LED along with a photodetector device to
transmit signals across an isolation barrier using light as the method of
data translation.
LEDs produce
light when a
voltage is
applied across
them
A photodetector
receives the light
transmitted by the LED
and converts it back to
the original signal.
ADVANTAGE:
• immunity to electrical and magnetic noise
DISADVANTAGE:
• transmission speed, which is restricted by the LED switching speed, high-power
dissipation, and LED wear.
• Capacitive Isolation
Capacitive isolation is based on an electric field that changes with the
level of charge on a capacitor plate. This charge is detected across an
isolation barrier and is proportional to the level of the measured signal.
ADVANTAGE:
• immunity to magnetic noise.
• support faster data transmission rates
DISADVANTAGE:
• capacitive coupling involves the use of electric fields for data transmission, it can
be susceptible to interference from external electric fields.
• Inductive Coupling Isolation
− current through a coil of wire produces a magnetic field.
− current can be induced in a second coil by placing it in close vicinity of the
changing magnetic field from the first coil.
− The voltage and current induced in the second coil depend on the rate of
current change through the first.
− This principle is called mutual induction and forms the basis of inductive
isolation.
Inductive isolation uses a pair of
coils separated by a layer of
insulation. Insulation prevents
any physical signal transmission
ADVANTAGE:
• support faster data transmission rates
DISADVANTAGE:
• susceptible to interference from external
magnetic fields.
Signals can be transmitted by
varying current flowing through one
of the coils, which causes a similar
current to be induced in the second
coil across the insulation barrier.
LINEARIZATION
Most sensor outputs are non-linear with respect to the applied stimulus. As a
result, their outputs must often be linearized in order to yield the correct
measurements.
For example:
Thermocouples, for example, have a nonlinear relationship from input
temperature to output voltage
Two methods of linearization (can be analog or digital)
i. Software linearization
ii. Hardware linearization
• Basically, it is a process of mapping/linearizing the output from the
sensors with the stimulus in order to achieve the correct
measurements
By taking
the slope
of these
data
Plot of voltage versus temperature
for three types of thermocouple
Plot of nominal Seebeck coefficient
versus temperature for three types of
thermocouple
• An ideal linear thermocouple would have a constant
Seebeck coefficient
• selecting a thermocouple for a particular temperature
range, we should choose one whose Seebeck coefficient
varies as little as possible over that range
For range between
250C to 500 C
For range between
400C to 750 C
Type S has wider
range of useful
temperature
MID TERM EXAM
• 30th March 2016
• 10 am, venue will be announced later
• Up until Lecture 5 including the handout notes