Title: Analog Sensors for Motion Measurement
1Analog Sensors for Motion Measurement
- Presented By
- Pratyush Singh
- Naga. C. Pemmaraju
- Sandeep Panjala
- Sreenivas
- Prasanth Bandi
- March 20, 2006.
2CONTENT
- Introduction
- Motion Transducers
- Variable Inductance Transducers
- Permanent Magnet Transducers
- Eddy Current Transducers
- Variable Capacitance Transducers
- Piezoelectric Transducers
- Other types of Transducers
- Design Criterion for Control System
- Conclusion
3INTRODUCTION
- Measurement of plant outputs and feedback
signals is very important - Sub measurement system are used for this purpose
and are made up of transducers and sensors and
they are used in feedback systems - TRANSDUCER Transducer is a device, that converts
one type of energy to another. - Transducers are used for compensation in
different plants and systems. - Help in reducing the sensitivity of a system to
parameter change.
4Motion transducers
- Motion here means the four kinematics variables
- Displacement
- Velocity
- Acceleration
- Jerk
- Note That each variable is a time derivation of
the preceding one. - Motion measurement is extremely important for
systems or plant proper functioning - The basic working principle of motion transducer
have been discussed with illustration examples.
5Basic Inductive Displacement Sensor.
- Uses the simple principle of mutual inductance.
- AC input is applied to coil A output is
measured across B. - Movement of the ferromagnetic core results in
inductance from A to B
6Linear Variable Differential Transformer.
- Used for measurements in the range of mm or cm.
- Supply is provided to the middle coil and output
is taken across the other two coils. - Differential amplifier is used to measure the
differential output from the windings.
7Moire Fringe Detector
- It converts Light energy to electricity to
measure displacement. - Uses two slides with identical patterns and a
photocell for the measurement. - Displacement is measured as the amount of light
passing through the slides changes
8Working principle of Accelerometers
- Based on the Principle of F Ma.
- Basic concept of an accelerometer has been shown
in the diagram. - Acceleration is measured by calculating the
amount of force applied to the restoration
spring.
9Magnetic Drag For Car Speedometers
- Most commonly used technique for speedometers in
cars. - Based on the principal of magnetic inductance.
- A magnet is connected to the moving shaft and it
induces a torque in the stationary disc. - This torque is measured to compute the speed of
the cars.
10VARIABLE-INDUCTANCE TRANSDUCERS
- Motion transducers that employ the principle of
electromagnetic induction are termed
variable-inductance transducers. - BASIC PRINCIPLE When the flux linkage through
an electrical conductor changes, a voltage is
induced in the conductor. This in turn ,generates
a magnetic field that opposes the primary field.
If the change in flux linkage is brought about by
a relative motion ,the mechanical energy is
converted in to electrical energy. - DIFFERENT TYPES
- 1.Mutual-induction transducers
- 2.self-induction transducers
- 3.permanent-magnet transducers
11Mutual-induction transducers and Differential
Transformers.
- The basic arrangement of a Mutual-induction
transducer constitutes two coils, the primary and
secondary winding. one of the coils carries an AC
excitation that induces a steady AC voltage in
the other coil - In Mutual-induction transducers, a change in the
flux linkage is effected by one of the two common
techniques. one technique is to move an object
made of ferromagnetic material within the flux
path. - The other common way to change the flux linkage
is to move one coil with respect to the other.
12LINEAR VARIABLE DIFFERENTIAL TRANSFORMER(LVDT)
- AN LVDT transducer comprises a coil former on to
which three coils are wound. - The primary coil is excited with an AC current,
the secondary coils are wound such that when a
ferrite core is in the central linear position,
an equal voltage is induced in to each coil. - The secondary are connected in opposite so that
in the central position the outputs of the
secondary cancels each other out.
13LVDT contd
- The excitation is applied to the primary winding
and the armature assists the induction of current
in to secondary coils. -
- When the armature is in the central position
there is an equal voltage induced in to both
secondary coils. The sum of secondary outputs
cancels each other out resulting in a zero
output. - As the armature moves in to sec.1,the result is
that sum of sec1 and sec2 favors sec1. - As the armature moves in to sec2,the sum favors
sec2. - The out put is an AC waveform .
-
14Contd
- Signal conditioning associated with differential
transformers includes filtering and amplification
15POTIENTIOMETERS
- The potentiometer is a displacement transducer.
- The active transducer consists of resistive
material whose resistance is proportional to its
length. - A fixed voltage is applied across the coil using
an external voltage supply. - The output signal is the DC voltage between the
movable contact sliding on the terminal of the
coil. - Slider displacement x is proportional to the
output voltage
16Self Induction Transducers
- Based on the principle of self induction.
- Only a single coil is employed.
- Self Induction transducers are usually
variable-reluctance devices. - This can be used as a displacement sensor.
17Self Induction Transducers Contd..
- Self
Induction proximity sensor
18Permanent Magnet Transducers
- A permanent magnet is used to generate a uniform
and steady magnetic field. - Permanent magnet transducers are used in
measuring speed. - Two types of speed are measured.
- Rectilinear speed
- Rectilinear velocity transducer is used to
measure rectilinear speed. - Angular speed.
- DC tachometer-generator and AC
tachometer-generator are used in measuring
angular speed.
19Permanent Magnet Transducers Contd..
- Rectilinear velocity transducer
20Permanent Magnet Transducers Contd..
21Permanent Magnet Transducers Contd..
- AC Permanent magnet Tachometers
- Contains a permanent magnet rotor and 2 sets of
stator windings. - One set of stator windings is energized using AC
reference voltage and the other set of windings
is used to measure output. - When
- Rotor is stationary gt output is a constant
voltage. - Rotor is moving at finite speed gt induced
voltage proportional to speed is generated at
secondary winding.
22Permanent Magnet Transducers Contd..
- AC Induction Tachometers
- Similar in construction to two-phase induction
motors. - Stator arrangement is similar to AC permanent
magnet tachometer and rotor windings are shorted
and not energized by any external source. - Voltage is induced in rotor windings which has 2
components, one due to the transformer action of
AC supply and the other due to the speed of
rotation of rotor. - Voltage induced in the output stator windings is
due to both the primary stator winding and rotor
winding.
23Eddy Current Transducers
- Eddy current
- The current generated when a conducting medium
is subjected to fluctuating magnetic field. - Eddy currents are circular induced currents
generated by an AC current in the nearby coil. - Eddy currents generate their own magnetic fields.
- This principle is used in Eddy current proximity
sensor.
24Eddy Current Proximity Sensor
25Eddy Current Proximity Sensor Contd..
- Schematic diagram of eddy current proximity sensor
26Variable Capacitance Transducer
- VARIABLE CAPACITOR
- Capacitance may be changed either mechanically
or electronically. - Capacitance C KA/x.
- The variable capacitance used to convert a
physical phenomena into electrical signals is
called a Variable Capacitance Transducer. - Example capacitor microphone.
27Variable Capacitance Transducer Contd..
- Capacitance transducers are commonly used to
measure small transverse displacements, large
rotations, fluid levels and angular velocities. - Capacitive rotation sensor
- The angular displacement of one of the plates
causes a change in A.
28Variable Capacitance Transducer Contd..
- Capacitive Displacement Sensor In this a
transverse displacement of one of the plates
changes x.
29Variable Capacitance Transducer Contd..
- Capacitive Liquid level sensor
- A change in k is produced as the fluid level
between the capacitor plates changes. - The advantage of capacitance transducer is
negligible loading effects. - The capacitance bridge compensates the errors.
30Piezoelectric Transducers
- Piezoelectricity is the electricity produced by
applying pressure. - Piezoelectric materials Barium titanate, single
crystal quartz. - Piezoelectric Effect
- When mechanical stress or strain is applied to
the piezoelectric material, generates an electric
charge and associated potential difference. - The direct application of piezoelectric effect is
used in pressure and strain measuring devices
31Piezoelectric Sensor
- It may be represented as a charge source with a
series capacitive impedance Z. - Z 1/jwc
- Piezoelectric sensors have a limitation on the
useful lower frequency.
32Piezoelectric accelerometer
- Accelerometers are acceleration measuring
devices. - The piezoelectric accelerometer is a
piezoelectric motion transducer. - It is based on d Alemberts principle which
states that - If a force of magnitude Ma were applied to the
accelerating mass in the direction opposing the
acceleration, then the system could be analyzed
using static equilibrium considerations. - Advantage Light weight and High frequency
response.
33Piezoelectric accelerometer Contd
- Piezoelectric velocity transducer
- It uses piezoelectric accelerometer and an
integrating amplifier along with impedance
matching amplifier. - Piezoelectric displacement transducer
- It is obtained by using a double integration of
piezoelectric accelerometer.
34OTHERS TYPES OF SENSOR
- Fiber optic sensors and lasers
- Fiber optic Position sensor
- Fiber Optic Gyroscope
- Ultrasonic Sensors
- Gyroscopic sensors
35Fiber optic sensors and lasers
- The Characteristic component in a fiber optic
sensor is a bundle of glass fiber (Typically a
few hundred ) that can carry light. - Basically Two types fiber optic sensors
- Indirect type the optical fiber acts only as
the medium in which sensed light is transmitted
. In this type the sensing element itself does
not consists of optical fibers. Ex Fiber Optic
motion sensor and Tactile sensor . - Direct type The optical fiber bundle acts as
sensing element. When the conditions of the
sensed medium change, the light propagation
properties of the optical fiber change, providing
a measurement of the change in conditions. Ex
Fiber optic gyroscope and fiber optic hydrophones
36Fiber optic Position sensor
- The Optical fiber bundle is divided into two
groups. - Transmitting fiber
- Receiving fiber
- Light from the light source is transmitted along
the first bundle of fibers to the target object
whose position is being measured. Light reflected
onto the receiving fibers by the surface of the
target object is carried to a photo detector. The
intensity of the light received by the photo
detector will depend on position x of the target
object.
37Fiber Optic Gyroscope
- This is an angular speed sensor that uses fiber
optics. - Two loops of optical fibers wrapped around a
cylinder are used in this sensor. - One loop carries a monochromatic light (or laser)
beam in the clockwise direction, Other loop
carries a beam from the same source in counter
clockwise direction. Since the laser beam
traveling in the direction of rotation of the
cylinder has a higher frequency than that of the
other beam, the difference in the frequencies of
the two lasers beams received at a common
location will measure the angular speed of the
cylinder. Angular displacement can be measured
with the same sensor simply by counting the
number of cycles and clocking fractions of the
cycles. Acceleration can be determined by
digitally determining the rate of change of speed.
38Ultrasonic Sensors
- In distance measurement using ultrasonic, aburst
of ultrasound is projected at the target object,
and the time taken for the echo to be recived is
clocked. A signal processor computes the position
of the target object ,possibly compensating for
environmental conditions . alternatively velocity
of the target object can also be measured, using
the Doppler effect, by measuring the change in
frequency between the transmitted wave and the
received wave.
39DESIGN CRITERION FOR CONTROL SYSTEM
- Accuracy is affected by parameter changes in the
control system components and by the influence of
external disturbances - Consider general feedback control system to
parameter changes and to external disturbances. - GP(s) Transfer function of the plant ( Of the
system to be controlled) - G c(s) Transfer function of the controller (
Including Compensators) - H(s) Transfer function of the output feedback
system ( Including the measurement system) - u System input command
- ud External disturbance input
- y system output
40Contd..
- After analyzing the feedback back control
system we can stipulate the fallowing design
criterion for the system. - Make the measurement system (H) very accurate and
stable - Increase the loop gain to reduce the sensitivity
of the control system to changes in the plant and
controller . - Increase the gain of Gc H to reduce the
influence of external disturbances
41Conclusion
- Analog Transducers play a very important part in
insuring proper functioning of the systems. - They are simple, User friendly and reliable.
- But with increase in complexity and need for
accuracy in modern day plants use of analog
transducers is very limited. Newly developed
Digital and Optical Transducers are more apt for
use in these plants.