Sensors

• 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

• 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

• TRANSDUCER– CONVERTS ENERGY FROM ONE FORM TO ANOTHERTEMPERATURE, STRAIN --------- ELECTRICAL ENERGYEXAMPLE- ACCELEROMETER GIVES OUTPUT VOLTAGE PROPORTIONAL TO THE

MECHANICAL MOTION OF THE OBJECT

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.

• 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

1. RANGE – DIFFERENCE BETWEEN MINIMUM AND MAXIMUM VALUES OF A QUANTITYEX. 10KN TO 100KN

2. SPAN – IT IS THE DIFFERENCE BETWEEN MAXIMUM AND MINIMUM VALUES OF THE QUANTITY TO BE MEASUREDEX. 100-10 = 90KN

3. ERROR – DEVIATION OF THE TRUE VALUE FROM THE MEASURED VALUEERROR = 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. 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 ORMINIMUM 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

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

• 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 TIME2. TIME CONSTANT- MEASURE OF INERTIA OF THE SENSOR3. RISE TIME – 10% - 95% OF STEADY STATE OUTPUT4. SETTLING TIME- TIME TAKEN FOR THE THE OUTPUT TO SETTLE TO WITHIN SOME PERCENTAGEEX. 2% OF STEADY STATE VALUE

THERMO METER

THERMISTOR

• THERMAL RESISTOR• NEGATIVE TEMP COEFFICIENT• 10 TIMES SENSITIVE THAN Pt100• RESISTANCE R OF THERMISTOR AT

TEMPERATURE T R= αeβT

α Temperature coefficient of materialΒ Thermistor constant

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

Types

• Large temperature coefficient of resistance.• Accurate in the range -100°c to 300°c.

+/- 0.1°c +/-0.005°c

Room temperature control

Thermostat

•"keeps heat the same“• Bimetallic strips

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.

RTD

RESISTANCE TEMPERATURE DETECTOR

TR60

BASIC CONCEPT

RESISTANCE OF ANY MATERIAL CHANGES WITH TEMPERATURE

Positive temperature coefficient

Thermocouple

DISPLACEMENT AND

POSITION SENSORS

DISPLACEMENT SENSORS- MEASURE THE AMOUNT BY WHICH AN OBJECT HAS BEEN MOVED

POSITION SENSOR- DETERMINE THE POSITION OF THE OBJECT IN RELATION TO SOMEREFERENCE POINT

TYPES OF DISPLACEMENT AND POSITION SENSORS

1. CONTACT SENSOR2. NON CONTACT SENSOR

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

FORMS OF CAPACITIVE SENSING ELEMENTS

LVDT (LINEAR VARIABLE DIFFERENTIAL TRANSFORMER)

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.

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

NUMBER OF PULSE PROPORTIONAL TO THE ANGLE THROUGH WHICH DISC IS ROTATED

WITH 60 SLOTS IN ONE REVOLUTION THE RESOLUTION IS 360/60 =6O

ABSOLUTE ENCODER

USED TO MEASURE ANGULAR DISPLACEMENT

GIVES OUTPUT IN THE FORM OF BINARY NUMBER OF SEVERAL DIGITS,EACHSUCH NUMBER REPRESENTING A PARTICULAR ANGULAR POSITION

TYPICAL ENCODER IS OF 10 TO 12 TRACKS10 TRACKS =210 =1024 POSITIONRESOLUTION = 360/1024 = 0.35O

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

STRAIN GUAGE

Bonded wire strain gauge