ANALOG SENSORS FOR MOTION MEASUREMENT

1
ANALOG SENSORS FOR MOTION MEASUREMENT

• Presented By
• Vikram Maduri
• Avinash Reddy Yerva

2
TOPICS

• Introduction
• Motion transducers
• Potentiometers
• Variable inductance transducers
• Permanent magnet transducers
• Eddy current transducers
• Variable capacitance transducers
• Piezoelectric transducers
• Design criterion for control systems

3
INTRODUCTION

• Measurement of plant outputs and feedback signals
  are very important.
• The measurement subsystem in a control system
  contains sensors and transducers that detect
  measurands and convert them into acceptable
  signals- typically voltages
• Sensor A device for measuring some quantity. The
  sensor usually converts the measurement space to
  an electrical signal.
• Transducer It 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.
• Several analog sensor-transducer devices are
  commonly used in control system instrumentation
• We will deal with several analog motion
  transducers.

4
MOTION TRANSDUCERS

• Motion here means the four kinematics variables
  
• Displacement
• Velocity
• Acceleration
• Jerk
• Each variable is a time derivation of the
  preceding one.
• Motion measurement is extremely important for
  systems or plant proper functioning.

5
Examples

• Proximity sensors (to measure displacement) and
  accelerometers are the two most common types of
  measuring devices used in machine protection
  systems for condition monitoring, fault
  detection, control of large machinery.
• Strain gages are used in measuring strains, they
  can be adopted to measure displacement by using
  auxiliary sensor element i.e. cantilever
  (spring), Resolver provides angular displacement.
• Pulse generating transducers like digital
  tachometers can serve as both displacement and
  velocity transducer depending on absolute number
  of pulses generated is counted or the pulse rate
  is measured.

6
Motion Transducers

• Motion transducers will be limited mainly to
  following types of devices
• Potentiometers
• Variable inductance transducers
• Eddy current transducers
• Variable capacitance transducers
• Piezoelectric transducers

7
Potentiometers

• Uniform coil of wire or a film of high resistive
  material carbon, platinum or conductive plastic
• Resistance is proportional to its length.

Resistive

Element

Wiper

Arm

v




ref

v
x
ref

v
(Supply)

Load


o

(Measurand)
v

Z


(Measurement)


Impedance

o


i


i

-
-
No Current

Nonzero

Current

FIG 1
FIG 2

• Slider displacement x is proportional to the
  output voltage

8

• This relationship is valid only if the output
  terminals are open circuit (no current) as shown
  in FIG 1.
• The output voltage drops when a load with a
  finite impedance is connected which is shown in
  FIG 2.
• Because of this loading effect the linear
  relationship will no longer be valid
• Loading can also affect the supply (reference)
  voltage
• To minimize loading effects
• Use a stabilized power supply with low output
  impedance
• Signal conditioning circuitry with high input
  impedance
• An element with high resistance will have reduced
  power dissipation and less thermal effects
• However, increased resistance increases the
  output impedance resulting in loading
  non-linearity error
• FIG 3 shows the linear motion of the translatory
  potentiometer whereas the FIG 4 shows the angular
  motion of Rotatory potentiometer.

9
Translatory and Rotatory potentiometers
Resistive Element
(Measurand)
vo (output)
Wiper
Wiper
vo (output)
x (Measurand)
vref
FIG 3
Translatory for Rectilinear Motions
vref
FIG 4
Rotatory for Angular Motions
10
VARIABLE-INDUCTANCE TRANSDUCERS

• These motion transducers employ the principle of
  electromagnetic induction
• Types of variable inductance transducers include
• Mutual induction transducers
• Self-induction transducers
• Permanent magnet transducers
• An AC excitation in the primary winding induces
  an AC voltage in the secondary winding as
  indicated in FIG 5.
• The amplitude of the induced voltage depends on
  the flux linkage between the two coils
• In mutual induction transducers change in the
  flux is effected by either
• Moving a ferromagnetic material on the flux path
  LVDT, RVDT, mutual induction proximity probe
• Moving one coil with respect to the other
  resolver, synchro-transformer

FIG 5
11
MUTUAL INDUCTANCE TRANSDUCER(Linear variable
differential transformer (LVDT))

• An LVDT transducer shown in FIG 6 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.

FIG 6
12
LINEAR VARIABLE DIFFERENTIAL TRANSFORMER

• 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 sec1,the result is
  that sum of sec1 and sec2 favors sec1.
• As the armature moves in to sec2,the sum favors
  sec2.
• The output is an AC waveform
• which is indicated in FIG 7

FIG 7
13
LINEAR VARIABLE DIFFERENTIAL TRANSFORMER

• Signal conditioning associated with differential
  transformers includes rectification and
  demodulation.
• FIG 8 shows Rectification.
• FIG 9 shows Demodulation.

FIG 8
FIG 9
14
Self Induction Transducers

• Based on the principle of self induction.
• Only a single coil is employed as shown in FIG
  10.
• Self Induction transducers are usually
  variable-reluctance devices.
• This can be used as a displacement sensor

FIG 10
15
Permanent 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 shown in FIG 11
  is used to measure rectilinear speed
• Angular speed.
• DC tachometer-generator in FIG 12 and AC
  tachometer-generator are used in measuring
  angular speed.

16

• Rectilinear velocity transducer

DC Tachometer-generator
FIG 12
FIG 11
17
Eddy Current Transducers

• Principle of Eddy current
• An eddy current is caused by a moving magnetic
  field intersecting a conductor or vice-versa.
• The relative motion causes a circulating flow of
  electrons, or current, within the conductor.
• These circulating eddies of current create
  electromagnets with magnetic fields that oppose
  the change in the external magnetic field.
• The stronger the magnetic field, or greater the
  electrical conductivity of the conductor, the
  greater the currents developed and the greater
  the opposing force.
• This principle is used in eddy current proximity
  sensor
• FIG 13 illustrates concept of Eddy current

FIG 13
18
Eddy current proximity sensor

• The Eddy Current Transducer uses the effect of
  eddy (circular) currents to sense the proximity
  of non-magnetic but conductive materials.
• A typical eddy current transducer contains two
  coils an active coil (main coil) and a balance
  coil as shown in FIG 14.
• The active coil senses the presence of a nearby
  conductive object, and balance coil is used to
  balance the output bridge circuit and for
  temperature compensation.

FIG 14
19
Schematic diagram of eddy current proximity sensor

• FIG 15
• Active coil and compensating coil forms arms of
  inductance bridge.
• When a measurand brought to near to active coil,
  due to eddy current which produces eddy current
  magnetic field that opposes active coil field
  causes change in inductance and thus creates
  imbalance in inductance bridge.
• This change is noted in calibrated unit.

20
Variable capacitance transducers

• A variable capacitor is a capacitor whose
  capacitance may be intentionally and repeatedly
  changed mechanically or electronically.
• Capacitance of two plate capacitor is given by
• CKA / x
• A change in anyone of K,A,x may be used in
  sensing process.
• Variable capacitance is used to convert physical
  phenomena into electrical signals
• Types of capacitor sensors
• capacitive rotation sensor
• capacitive displacement sensor
• capacitive liquid level sensor

21
Variable capacitance transducers

• Capacitive rotation sensor
• Angular displacement of one of the plates
  causes the a change in A (area of plate) which is
  shown in FIG 16.

FIG 16
22
Variable capacitance transducers

• Capacitive Displacement sensor
• Transverse displacement of one of the plates
  changes x (distance between plates) as shown in
  FIG 17.

FIG 17
23
Variable capacitance transducers

• Capacitive liquid level sensor
• A change in K (depends on dielectric
  properties of medium between two plates) is
  produced as the fluid level between the capacitor
  plate changes
• The advantage of capacitance transducer is
  negligible loading effects.
• FIG 18 shows change in K value used to measure
  the displacement

FIG 18
24
Piezoelectric transducers

• 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

25
The Piezoelectric Effect
Crystal material at rest No forces applied (as
shown in FIG 19), so net current flow is 0
Crystal
- - -
Current Meter 0
Charges cancel each other, so no current flow
- - -
FIG 19
26
The Piezoelectric Effect
Crystal material with forces applied in
direction of arrows (FIG 20).
Crystal
- - -
Current Meter deflects in direction
Force

Due to properties of symmetry, charges are net
on one side net - on the opposite side
crystal gets thinner and longer
FIG 20
27
The Piezoelectric Effect
Changing the direction of the applied
force..(FIG 21)
Crystal

Current Meter deflects in - direction
Force
- - - - -
. Changes the direction of current flow, and
the crystal gets shorter and fatter.
FIG 21
28
Piezoelectric accelerometer

• Piezoelectric velocity transducer
• It uses piezoelectric accelerometer and an
  integrating amplifier along with impedance
  matching amplifier which is show in FIG 22.
• Piezoelectric displacement transducer
• It is obtained by using a double integration of
  piezoelectric accelerometer.

FIG 22
29
Piezoelectric Sensor

• It may be represented as a charge source with a
  series capacitive impedance Z as shown in FIG 23
• Z 1/jwc
• Piezoelectric sensors have a limitation on the
  useful lower frequency.

Fig 23
30
Piezoelectric 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.

FIG 24
31
DESIGN 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 as shown in FIG
  25 system to parameter changes and to external
  disturbances.
• GP(s) Transfer function of the plant ( Of the
  system to be controlled)
• Gc(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

FIG 25
32

• After analyzing the feedback back control system
  we can stipulate the following 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 GcH to reduce the influence
  of external disturbances

33
Conclusion

• 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.

34
References

• Sensors and Actuators by C W Desilva.
• http//en.wikipedia.org/wiki/Sensor
• http//en.wikipedia.org/wiki/Eddy_current
• http//www.infoplease.com/ce6/sci/A0839004.html
• http//www.encyclopedia.com/doc/1E1-piezoele.html

35
Discussions

• Discuss briefly Eddy current Proximity sensor.
• How does linear displacement is measured using
  potentiometer ?
• Explain how different motions are measured using
  Variable capacitance transducers ?

36
Questions and comments
37

• Thanks to one and everyone