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TACTILE SENSORS
LT A C THOMAS
SCOPE
• INTRODUCTION• RELEVANCE• TYPES• MARINE APPLICATIONS• CONCLUSION
INTRODUCTION
• DEFINITION : TACTILE SENSOR ARE DEVICES WHICH MEASURES THE PARAMETERS OF A CONTACT BETWEEN THE SENSOR AND AN OBJECT.
• THE HUMAN ELEMENT
RELEVANCE
• SINGLE POINT CONTACT• SENSITIVITY • SENSOR BAND WIDTH• LOW HYSTERESIS• ROBUST
TYPES OF TACTILE SENSORS
• OPTICAL• PIEZOELECTRIC• RESISTIVE• CAPACITIVE• MAGNETIC• STRAIN GAUGE
OPTICAL TACTILE SENSORS OPTICAL FIBRE
BASED SENSOR
• OPTICAL WAVE GUIDE
• SOFT RUBBER AT THE SURFACE
• A LIGHT SOURCE• OPTICAL DETECTOR
OPTICAL TACTILE SENSOR
• PHOTOELASTICITY• MODULATING THE INTENSITY OF LIGHT BY
MOVING AN OBSTRUCTION INTO THE LIGHT PATH
OPTICAL TACTILE SENSORS
• TYPES– INTRINSIC – EXTRINSIC
• ADVANTAGES– IMMUNITY TO EXTERNAL ELECTROMAGNETIC
INTERFERENCE– LOW WEIGHT AND VOLUME
PIEZOELECTRIC SENSORS
PRINCIPLE OF PIEZOELECTRICITY
• CHANGE IN THE PIEZOELECTRIC RESONANCE FREQUENCY OF THE MATERIAL WITH THE APPLIED STRESS.
• CHANGES IN ELECTRIC CHARGE
PIEZO ELECTRIC CRYSTAL
V
ELECTRIC PLATES
FORCE or VIBRATION
THE RESONANCE FREQUENCY OF THE RESONATING MATERIAL :
• T = THICKNESS OF THE PIEZOELECTRIC MATERIAL.
• F0 = NATURAL MECHANICAL RESONANCE FREQUENCY.
• K = STIFFNESS CONSTANT OF THE MATERIAL.
• P = DENSITY OF THE MATERIAL.
• F = APPLIED FORCE
• ∆f = CHANGE IN FREQUENCY
MATERIALS
POLYVINYLIDENE FLUORIDE (PVDF), QUARTZ, LEAD ZIRCONATE TITANAATE (PZT)
ADVANTAGE
• GETTING A CONTOUR OF THE OBJECT.• MEASUREMENT OF PRESSURE• SIGNAL REGARDING TOUCH AND NO-TOUCH OF
THE OBJECT
RESISTIVE TACTILE SENSORS
BASED ON THE CHANGE OF THE ELECTRICAL RESISTANCE BETWEEN A CONDUCTIVE POLYMER AND ATLEAST TWO ELECTRODE
WORKING PRINCIPLE
FIGURE 1
FIGURE 2
SALIENT FEATURES
• A SIMPLE SIGNAL CONDITIONING ELECTRONICS
• HYPEBOLIC STYLE CHARACTERISTIC BETWEEN THE LOAD APPLIED AND THE ELECTRICAL RESISTANCE
• ROBUST DUE TO SIMPLE CONSTRUCTION
CONSTRUCTION
FIGURE 1
FIGURE 2
SENSOR MATERIAL
• EVA FOAM• SILICON RUBBER• PTFE
EVA SILICON RUBBER
PTFE
Realizable measurement range
150 kPa 100 kPa 2.000 kPa
Temperature range
-70 to 60 °C -100 to 316 °C -260 to 300 °C
Ductility high high low
Polymer type thermoplastic thermoset thermoset
Processing sawing, (jet-) cutting, milling jet cutting
compression molding, injection molding
(jet-) cutting
CAPACITIVE TACTILE SENSORS
CAPACITIVE SENSORS UTILISE THE CHANGE OF CAPACITANCE BETWEEN TWO ELECTRODES COVERING A DEFORMABLE DIELECTRIC.
PRINCIPLE OF WORKING
The basic electrical property of capacitance – a measure of an object’s ability to store electrical charge –two electrodes with area A separated by an air gap d as shown. If the air gap decreases, the capacitance C goes up.
SENSOR TECHNOLOGY COMPARISON
MAGNETIC BASED SENSOR
• TWO APPROACHES(1) CHANGE OF FLUX DENSITY
-MEASURED USING MAGNETO RESISTIVE DEVICES
(2)CHANGE OF MAGNECTIC COUPLING BETWEEN WINDINGS-DEFORMATION OF MAGNECTO ELASTIC CORE
MAGNECTIC BASED SENSOR
• ADVANTAGES– HIGH SENSITIVITY– DYNAMIC RANGE– NO MECHANICAL HYSTERESIS– LINEAR RESPONSE– ROBUSTNESS
MARINE APPLICATIONS
• Local ice load on ships– test loads on hull in contact with ice– Tested in straight going and turning modes– Tested using two models– Interpreted for the vessel being built
MARINE APPLICATIONS
• Local ice loads on ships
– I-SCAN 210 tectile sensor sheets used
– Sensors located at four positions
– Measures pressure distribution
– Largest loading on bow shoulder
MARINE APPLICATIONS
• Shear and normal force measurements– Capacitive sensor is used– Phase and amplitude changes gives forces applied
• Finger tip sensing system in UW Gripper– Strain gauge sensor used– Measures force exerted by gripper