Sensors in Mechatronics

1. Sensors

2. • IN MECHATRONICS SYSTEM WE NEED TO
MEASURE THE FOLLOWING PHYSICAL
QUANTITIES
– DISPLACEMENT
– TEMPERATURE
– PRESSURE
– STRESS
SENSORS AND TRANSDUCERS ARE THE KEY
ELEMENT USED FOR THE MEASUREMENT OF THE
PHYSICAL QUANTITIES

3. • SENSORS
– ELEMENT IN A MEASUREMENT SYSTEM THAT ACQUIRES A PHYSICAL
PARAMETER AND CHANGES INTO A SIGNAL(ALSO CAN BE DEFINED AS
PART OF A TRANSDUCER WHICH SENSES OR RESPOND TO A PHYSICAL
QUANTITY OR MEASURAND
SENSOR NORMALLY SENSES THE FOLLOWING PHYSICAL
QUANTITIES
- POSITION
- FORCES
- DISTANCE
- STRAIN
- VIBRATION
- TEMPERATURE
- ACCELERATION ETC.
EXAMPLE OF SENSOR– A THERMOCOUPLE SENSES THE CHANGE IN
TEMPERATURE

4. • TRANSDUCER
– CONVERTS ENERGY FROM ONE FORM TO ANOTHER
TEMPERATURE, STRAIN --------- ELECTRICAL ENERGY
EXAMPLE- ACCELEROMETER GIVES OUTPUT VOLTAGE PROPORTIONAL TO THE
MECHANICAL MOTION OF THE OBJECT

5. Active transducers generate electric
current or voltage directly in response to
environmental stimulation
Passive transducers produce a change in
some passive electrical quantity, such as
capacitance, resistance, or inductance, as
a result of stimulation. These usually
require additional electrical energy for
excitation.

6. • CHARACTERISTIC PARAMETERS USED IN TRANSDUCERS AND SENSORS
• STATIC CHARACTERISTIC (MEASURE UNVARYING PROCESS CONDITION)
• 1. RANGE
• 2. SPAN
• 3. ERROR
• 4. ACCURACY
• 5. PRECISION
• 6. SENSITIVITY
• 7. HYSTERISIS
• 8.REPEATABILITY
• 9. REPRODUCIBILITY
• 10. READABILITY
• 11. RESOLUTION
• 12. DEAD ZONE
• 13. DEAD TIME
• 14. BACKLASH
• 15. BIAS
• 16. TOLERANCE
• 17. DRIFT
• 18. UNCERTAINITY

7. 1. RANGE – DIFFERENCE BETWEEN MINIMUM AND MAXIMUM VALUES OF A QUANTITY
EX. 10KN TO 100KN
2. SPAN – IT IS THE DIFFERENCE BETWEEN MAXIMUM AND MINIMUM VALUES OF THE
QUANTITY TO BE MEASURED
EX. 100-10 = 90KN
3. ERROR – DEVIATION OF THE TRUE VALUE FROM THE MEASURED VALUE
ERROR = MEASURED VALUE – TRUE VALUE
4. ACCURACY – IT REPRESENTS HOW CLOSELY THE MEASURED VALUE AGREES WITH THE
TRUE VALUE
5. PRECISION – IT REFERS TO REPEATABILITY OR CONSISTENCY OF MEASUREMENTS WHEN
THE MEASUREMENTS ARE CARRIED OUT UNDER IDENTICAL CONDITIONS AT SHORT
INTERVAL OF TIME
6. SENSITIVITY – IT IS THE RATIO OF THE MAGNITUDE OF THE OUTPUT SIGNAL TO THE
MAGNITUDE OF THE INPUT SIGNAL
SENSITIVITY = OUTPUT/INPUT
7. HYSTERISIS – DIFFERENCE IN THE OUTPUT FOR A GIVEN INPUT WHEN THIS VALUE IS
APPROACHED FROM THE OPPOSITE DIRECTION

8. 8. REPEATABILITY – CLOSENESS OF AGREEMENT AMOUNG NUMBER OF CONSECUTIVE
MEASUREMENTS OF THE OUTPUT FOR THE SAME VALUE OF INPUT UNDER THE SAME
OPERATING CONDITIONS
9. REPRODICIBILITY - CLOSENESS OF AGREEMENT AMOUNG NUMBER OF CONSECUTIVE
MEASUREMENTS OF THE OUTPUT FOR THE SAME VALUE OF INPUT UNDER THE SAME
OPERATING CONDITIONS OVER A PERIOD OF TIME
10. READABILITY – CLOSENESS WITH WHICH THE SCALE OF AN ANALOGUE INSTRUMENT
CAN BE READ
11. RESOLUTION – SMALLEST CHANGE IN A MEASURED VARIABLE TO WHICH AN
INSTRUMENT WILL RESPOND OR
MINIMUM VALUE OF THE INPUT SIGNAL REQUIRED TO CAUSE AN APPRECIABLE
CHANGE OR AN INCREMENT IN THE OUTPUT
12. DEAD ZONE – TIME TAKEN BY AN INSTRUMENT TO BEGIN ITS RESPONSE
13. BACKLASH –IT IS THE LOST MOTION OR FREE PLAY OF THE MECHANICAL ELEMENTS
SUCH AS GEARS, LINKAGES ETC
14. BIAS – THE CONSTANT ERROR THAT EXISTS OVER THE FULL RANGE OF MEASUREMENT
OF AN INSTRUMENT

9. 15. TOLERANCE – MAXIMUM ALLOWABLE ERROR IN THE MEASUREMENT
16. DRIFT – THE VARIATION OF CHANGE IN OUTPUT FOR A GIVEN INPUT OVER A PERIOD
OF TIME
17. UNCERTAINITY – DOUBT ABOUT THE UNCERATAINITY OFTHE EXACTNESS OF
MEASURENT RESULTS
18. ZERO DRIFT – CHANGES THAT OCCUR IN THE OUTPUT WHEN THERE IS ZERO INPUT

10. • DYNAMIC CHARACTERISTICS
– INPUT VARIES WITH TIME
• STEPPED INPUT – INPUT SUDDENLY CHANGED FROM ZERO TO A CONSTANT VALUE
• RAMP INPUT – INPUT IS CHANGED AT A STEADY RATE
• SINOSOIDAL INPUT- INPUT OF SPECIFIED FREQUENCY
DYNAMIC RESPONSE IS THE BEHAVIOUR OF AN INSRUMENT UNDER TIME-VARYING
INPUT-OUTPUT CONDITIONS
1. RESPONSE TIME
2. TIME CONSTANT- MEASURE OF
INERTIA OF THE SENSOR
3. RISE TIME – 10% - 95% OF STEADY
STATE OUTPUT
4. SETTLING TIME- TIME TAKEN FOR THE
THE OUTPUT TO SETTLE TO WITHIN
SOME PERCENTAGE
EX. 2% OF STEADY STATE VALUE

11. THERMO METER

14. THERMISTOR
• THERMAL RESISTOR
• NEGATIVE TEMP COEFFICIENT
• 10 TIMES SENSITIVE THAN Pt100
• RESISTANCE R OF THERMISTOR AT
TEMPERATURE T
R= αeβT
α Temperature coefficient of material
Β Thermistor constant

15. Materials used
• Manufactured from oxides of metals like
manganese, nickel, cobalt, copper, iron, zinc,
aluminium, titanium, magnesium, uranium.
• Mixed in appropriate proportion pressed and
sintered
• Electrical leads are embedded before
sintering.
• Resistance of thermistor at room temperature
is 25°c

16. Types

17. • Large temperature coefficient of resistance.
• Accurate in the range -100°c to 300°c.
+/- 0.1°c +/-0.005°c

18. Room temperature
control

19. Thermostat
•"keeps heat the same“
• Bimetallic strips

20. Advantages
• robust & simple
• fully mechanical devices no need of power
source.
Disadvantages
• not very accurate
• not suitable for measuring lower temperature
as the metals and metallic alloys show nearly
same expansion or contraction in lower range
of temperature.

21. RTD
RESISTANCE TEMPERATURE
DETECTOR

22. TR60

23. BASIC CONCEPT
RESISTANCE OF ANY MATERIAL CHANGES WITH
TEMPERATURE
Positive temperature coefficient

25. Thermocouple

28. DISPLACEMENT
AND
POSITION SENSORS

29. DISPLACEMENT SENSORS- MEASURE THE AMOUNT BY WHICH AN OBJECT HAS BEEN
MOVED
POSITION SENSOR- DETERMINE THE POSITION OF THE OBJECT IN RELATION TO SOME
REFERENCE POINT
TYPES OF DISPLACEMENT AND POSITION SENSORS
1. CONTACT SENSOR
2. NON CONTACT SENSOR

30. POTENTIOMETER SENSOR
A POTENTIOMETER CONSIST OF A RESISTANCE ELEMENT WITH A SLIDING CONTACT
WHICH CAN BE MOVED ALONG THE LENGTH OF THE ELEMENT . SUCH ELEMENT
CAN BE USED FOR LINEAR OR ROTARY DISPLACEMENTS, THE DISPLACEMENT
BEING CONVERTED INTO A POTENTIAL DIFFERENCE

33. FORMS OF CAPACITIVE SENSING ELEMENTS

34. LVDT (LINEAR VARIABLE DIFFERENTIAL TRANSFORMER)

35. The linear variable differential transformer (LVDT) is a type of electrical transformer used for
measuring linear displacement. The transformer has three solenoidal coils placed end-to-end
around a tube. The center coil is the primary, and the two outer coils are the secondaries. A
cylindrical ferromagnetic core, attached to the object whose position is to be measured, slides
along the axis of the tube.
An alternating current is driven through the primary, causing a voltage to be induced in each
secondary proportional to its mutual inductance with the primary. The frequency is usually in the
range 1 to 10 kHz.
As the core moves, these mutual inductances change, causing the voltages induced in the
secondaries to change. The coils are connected in reverse series, so that the output voltage is the
difference (hence "differential") between the two secondary voltages. When the core is in its
central position, equidistant between the two secondaries, equal but opposite voltages are
induced in these two coils, so the output voltage is zero.
When the core is displaced in one direction, the voltage in one coil increases as the other
decreases, causing the output voltage to increase from zero to a maximum. This voltage is in
phase with the primary voltage. When the core moves in the other direction, the output voltage
also increases from zero to a maximum, but its phase is opposite to that of the primary. The
magnitude of the output voltage is proportional to the distance moved by the core (up to its limit
of travel), which is why the device is described as "linear". The phase of the voltage indicates the
direction of the displacement.
Because the sliding core does not touch the inside of the tube, it can move without friction,
making the LVDT a highly reliable device. The absence of any sliding or rotating contacts allows the
LVDT to be completely sealed against the environment.
LVDTs are commonly used for position feedback in servomechanisms, and for automated
measurement in machine tools and many other industrial and scientific applications.

36. OPTICAL ENCODER
AN ENCODER IS A DEVICE THAT PROVIDES A DIGITAL OUTPUT AS A RESULT
OF LINEAR OR ANGULAR DISPLACEMENT
BASICALLY THEY ARE TWO TYPES
1. INCREMENTAL ENCODER
2. ABSOLUTE ENCODER
INCREMETAL ENCODER DETECTS CHANGES IN ROTATION FROM SOME DATUM
POSITION
ABSOLUTE ENCODER GIVE THE ACTUAL ANGULAR POSITION

37. NUMBER OF PULSE PROPORTIONAL TO THE ANGLE THROUGH
WHICH DISC IS ROTATED
WITH 60 SLOTS IN ONE REVOLUTION THE RESOLUTION IS 360/60 =6O

38. ABSOLUTE ENCODER
USED TO MEASURE ANGULAR DISPLACEMENT
GIVES OUTPUT IN THE FORM OF BINARY NUMBER OF SEVERAL DIGITS,EACH
SUCH NUMBER REPRESENTING A PARTICULAR ANGULAR POSITION
TYPICAL ENCODER IS OF 10 TO 12 TRACKS
10 TRACKS =210 =1024 POSITION
RESOLUTION = 360/1024 = 0.35O

39. INORDER TO PREVENT MISALIGNMENT DUE TO TWO OR MORE BIT CHANGE,
SINGLE BIT CHANGE IS USED

40. STRAIN GUAGE
Bonded wire strain gauge