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