Introduction to Robotics Course

1. INTRODUCTION
TO ROBOTICS
LECTURE: CONTACT AND NON
CONTACT SENSORS
DR. HEMA C.R.

2. ROAD MAP
• POSITION SENSORS
• CONTACT TYPE
• POTENTIOMETERS
• LVDT
• NON-CONTACT TYPE
• PROXIMITY SENSORS
• ENCODERS
• VELOCITY SENSORS
• SPEED SENSORS
• ACCELEROMETERS
• RANGE FINDERS

3. CONTACT AND NON-CONTACT
SENSORS
Contact sensors detect the change in position, acceleration,
force, torque etc. at the end effecter, whereas
Non-contact sensors detect presence, distance, features of work
piece etc. contact sensing may be of various types such as, touch
and force sensing, proximity or displacement sensing, slip
sensing.
 Potentiometer
 LVDT
 Proximity sensors
 Encoders
NON CONTACT TYPE
CONTACT TYPE

4. POSITION SENSOR
Position sensors are employed to determine the
position of an object in relation to some reference
point. Position Sensors can detect the movement of an
object in a straight line using Linear Sensors or by its
angular movement using Rotational Sensors .

5. POTENTIOMETERS
Potentiometers are simplest type of position sensors. They can
be used for linear as well as angular position measurement. They
are the resistive type of transducers and the output voltage is
proportional to the displacement and is given by:
xi is the input displacement,
xt is the total displacement and
E is the supply voltage
These sensors are primarily used in the control systems with a
feedback loop to ensure that the moving member or component
reaches its commanded position.

6.  During the sensing operation, a voltage E is applied across the
resistive element.
 A voltage divider circuit is formed when slider comes into contact
with the wire.
 The output voltage (eo) is measured.
 The output voltage is proportional to the displacement of the
slider over the wire.
 Then the output parameter displacement is calibrated against the
output voltage eo

7. LINEAR VARIABLE DIFFERENTIAL
TRANSFORMER
 The term LVDT stands for the linear variable differential
transformer .
 It is the most widely used inductive transducer that covert the
linear motion into the electrical signals.

8. CONSTRUCTION
 The transformer consists of a primary winding P and two secondary
winding S1 and S2 wound on a cylindrical former (which is hollow in
nature and will contain core).
 Both the secondary windings have equal number of turns and are
identically placed on the either side of primary winding
 The primary winding is connected to an AC source which produces a
flux in the air gap and voltages are induced in secondary windings.
 A movable soft iron core is placed inside the former and
displacement to be measured is connected to the iron core.

9.  The iron core is generally of high permeability which helps in
reducing harmonics and high sensitivity of LVDT.
 The LVDT is placed inside a stainless steel housing
because it will provide electrostatic and electromagnetic
shielding.
 Both the secondary windings are connected in such a way that
resulted output is the difference of the voltages of two
windings.

10. WORKING
 CASE I - When the core is at null position (for no displacement) then the
flux linking with both the secondary windings is equal so the induced
emf is equal in both the windings. So for no displacement the value of
output Vout is zero as V1 and V2 both are equal. So it shows that no
displacement took place.
 CASE II - When the core is moved to upward of null position (For
displacement to the upward of reference point) In the this case the flux
linking with secondary winding S1 is more as compared to flux linking
with S2. Due to this V1 will be more as that of V2. Due to this output
voltage Vout is positive.
 CASE III When the core is moved to downward of Null position (for
displacement to the downward of reference point). In this case
magnitude of V2 will be more as that of V1. Due to this output Vout is
negative

11. PROXIMITY SENSOR
 Proximity sensors do not actually measure displacement
or angular rotation they are mainly used to detect the
presence of an object in front of them or within a close
proximity, hence their name “proximity sensor”.

12. INDUCTIVE PROXIMITY
SENSOR
 Inductive proximity switches are basically used for detection of
metallic objects.
 It comprises of a coil, an oscillator, a detector and a triggering circuit.
 An alternating current is supplied to the coil which generates a
magnetic field.
 When, a metal object comes closer to the end of the coil, inductance
of the coil changes.
 This is continuously monitored
by a circuit which triggers a
switch when a preset value of
inductance change is occurred.

13. INDUCTIVE PROXIMITY
SENSOR
 Inductive proximity sensors allow for the detection of metallic
objects in front of the sensor head without any physical contact
of the object itself being detected.
 This makes them ideal for use in dirty or wet environments. The
“sensing” range of proximity sensors is very small, typically
0.1mm to 12mm.
 Application
Industrial automation: counting of products during
production or transfer
Security: detection of metal objects, arms, land mines

14. ENCODERS
 Encoders are another type of position non contact
sensor where optical devices are used for converting the
angular position of a rotating shaft into a digital data
code.
 They convert mechanical movement
into an electrical signal
(preferably digital).
 All optical encoders work on the
same basic principle.

15. Light from an LED or infra-red light source is
passed through a rotating high-resolution
encoded disk that contains the required code
patterns, either binary, grey code or BCD (Binary
coded Decimal)
 Photo detectors scan the disk as it rotates and an electronic
circuit processes the information into a digital form as a
stream of binary output pulses that are fed to counters or
controllers which determine the actual angular position of the
shaft.

16. OPTICAL ENCODERS
 Optical encoders provide digital output as a result of
linear/angular displacement.
 These are widely used in the Servo motors to
measure the rotation of shafts. Figure shows the
construction of an optical encoder. It comprises of a
disc with three concentric tracks of equally spaced
holes.
 Three light sensors are employed to detect the
light passing through the holes.
 These sensors produce electric pulses which give the
angular displacement of the mechanical element e.g.
shaft on which the Optical encoder is mounted.
Light emitters
Shaft rotates

17.  The inner track has just one hole which is used locate the ‘home’
position of the disc.
 The holes on the middle track offset from the holes of the outer track
by one-half of the width of the hole.
 This arrangement provides the direction of rotation
 When the disc rotates in clockwise direction, the pulses in the outer
track lead those in the inner; in counter clockwise direction they lag
behind.
 The resolution can be determined by the number of holes on disc.
With 100 holes in one revolution, the resolution would be, 360⁰/100 =
3.6⁰

18. VELOCITY SENSORS
 Tachogenerator works on the principle of variable
reluctance.
 It consists of an assembly of a toothed wheel and a
magnetic circuit as shown in figure.
 Toothed wheel is mounted on the shaft or the element of
which angular motion is to be measured.
 Magnetic circuit comprising of a coil wound on a
ferromagnetic material core.
 As the wheel rotates, the air gap between wheel tooth and
magnetic core changes which results in cyclic change in
flux linked with the coil.

19.  The alternating emf generated is the measure of
angular motion. A pulse shaping signal conditioner is
used to transform the output into a number of pulses
which can be counted by a counter.

20. SPEED SENSORS
 The simplest way for speed measurement of a rotating body is to
mount a tachogenerator on the shaft and measure the voltage
generated by it that is proportional to the speed. However this is a
contact type measurement.
 A tachogenerator, or tachometer generator, is an electromechanical
device used to accurately measure the speed of engines and
motors.
 A tachogenerator converts mechanical energy into electrical
energy.

21.  There are other methods also for noncontact type measurements. An
opaque disc transparent windows at regular interval is mounted on the
shaft whose speed is to be measured.
 A LED source is aligned on one side of the disc in such a way that its light
can pass through the transparent windows of the disc.
 As the disc rotates the light will alternately passed through the transparent
windows and blocked by the opaque sections.
 A photo detector fixed on the other side of the disc detects the variation of
light and the output of the detector after signal conditioning would be a
square wave (as shown) whose frequency is decided by the speed and the
number of holes (transparent windows) on the disc.

22. ACCELEROMETER
 What does an accelerometer measure?
 Acceleration ? not really true.
 An accelerometer actually measures normal force or restoring force which we
equate to acceleration using the formula, F=ma.
 Accelerometer Fundamentals
 At the core all accelerometers contain five essential elements:
A mass that is able to move around, called a proof mass
A spring suspension to hold the proof mass
Some damping either due to designed damper or due to air resistance
A displacement sensor to measure the proof mass movement
A frame to connect the accelerometer to the thing you want to
measure

23.  The basic principle of operation is that the as the frame of the
accelerometer moves, the inertia of the proof mass wants to keep the
proof mass stationary.
 The inertial forces on the proof mass are equalized by the tension in the
springs attaching the proof mass to the frame, so that the proof mass
doesn't just crash into the frame.
 The support beams act as a spring, and the fluid (usually air) trapped
inside the cylinder acts as a damper, resulting in a second order lumped
physical system.
 A sensor measures the displacement between the proof mass and the
frame which is calibrated and converted to a measurement
of acceleration in units of gravities (g) or units
of m/s2. One g unit = 9.81 m/s2.
 Consistent with Newton's second law of motion
(F = ma ), as an acceleration is applied to the device, a
force develops which displaces the mass.

24.  When a spring mass damper system is subjected to acceleration,
the mass is displaced, and this displacement of the mass is
proportional to the acceleration.
 Hence a measure of displacement of the mass becomes a measure
of acceleration (rate of change of velocity).
 a = acceleration m/s2, v= velocity m/s x = displacement m

25. What physical quantities are accelerometers good at measuring?
Acceleration
Vibrations
Tilt angle relative to gravity
Centripetal force and acceleration
Collisions
Fast movements
What physical quantities are accelerometers bad at measuring?
Velocity
Position
Projectile motion

26. COMMON TYPES OF
ACCELEROMETERS
Capacitive -Metal beam or micro machined feature produces
capacitance; change in capacitance related to acceleration
Piezoelectric -Piezoelectric crystal mounted to mass –voltage
output converted to acceleration
Piezo resistive -Beam or micromachined feature whose
resistance changes with acceleration
HallEffect -Motion converted to electrical signal by sensing of
changing magnetic fields
Magneto resistive -Material resistivity changes in presence of
magnetic field

27. RANGE FINDERS
 Range finders are sensors that are used to measure distance
 In robotics they are used to measure distance between robots
and objects
 In robotics two type of Range finders are used
Laser Ranger Finder
Ultrasonic Range Finders
 Laser range finders calculate distances from objects by utilising a
laser beam. Using this principle, they can also detect any
obstacles that may lie in their path.

28. LASER RANGE FINDER
LASER LIGHT BASICS
Propagates dominantly in one direction in the form of a beam
Beam can maintain integrity for very long distances
Lasers are ideal to measure distances
Different methods used to measure different
lengths
i.e. Triangulation and Time of Flight

29. PROPERTIES
 Laser Light is Monochromatic it contains one specific wavelength
of photons
 Coherent : All the photons have wave fronts that are in unison
 Directional : Propagates generally in one direction in a
concentrated beam
 Atoms are excited and release photons that travel down the tube
reflecting off mirrors and exciting more atoms until they leave the
tube through a lens in a concentrated laser beam.

30.  Time of Flight
 Measures the time the laser take to travel the distance and back
 Short pulse fired out and time measured for a reflected portion to
come back
 Longer time farther away
 temporal accuracy must be very high e.g.
1ns for a spatial accuracy of 15cm
 Triangulation
 Have sensors offset to measure angle of reflectance
 Based on geometry, distance can be determined

31. IMPLEMENTATION
 Laser is installed to additional sensing circuitry
 A photo sensor receives the reflected signal
 Triggers timer stopping circuitry or angle valve input
 Data from the time of flight or the angle of impact is used to determine the distance to
target
 In time of flight method , due to the high speed of light, this technique is not appropriate
for high precision sub-milli meter measurements, where triangulation and other
techniques are often used.
CHALLENGES ISSUES WITH LASERS
 Tend to be expensive
 Dependent on line-of-sight (wall vs. chair)
 Can involve complicated circuitry that might not
be necessary
 Laser pointing fluctuations
 Laser noise issues

32. ULTRASONIC RANGE SENSORS
 Ultrasonic sensors (also known as transceivers when
they both send and receive) work on a principle
similar to radar or sonar which evaluate attributes of a
target by interpreting the echoes from radio or sound
waves respectively.
 An ultrasonic transducer is a device that
converts energy into ultrasound, or sound waves
above the normal range of human hearing.

33.  Basic principle of operation:
 Emit a quick burst of ultrasound (50kHz), (human hearing: 20Hz to 20kHz)
 Measure the elapsed time until the receiver indicates that an echo is
detected.
 Determine how far away the nearest object is from the sensor
Bat, dolphin,

34.  Ultrasonic sensors generate high frequency sound waves
and evaluate the echo which is received back by the
sensor.
 Sensors calculate the time interval between sending the
signal and receiving the echo to determine the distance
to an object.