Force Sensing Technologies

7/30/2019 Force Sensing Technologies

1/49


2/49


3/49

Contents

Abstract iii

List of Figures v

List of Tables vi

1 Introduction 11.1 Goal . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.2 Structure of the Report . . . . . . . . . . . . . . . . . . . . . . . . . 1

2 Tactile Sensing Technologies 32.1 Mechanical Tactile Sensors . . . . . . . . . . . . . . . . . . . . . . . 3

2.1.1 Whiskers and Antennae . . . . . . . . . . . . . . . . . . . . . 32.1.2 Mechanical Displacement . . . . . . . . . . . . . . . . . . . . 42.1.3 Pneumatic Touch Sensor and Foil Switches . . . . . . . . . . 42.1.4 Digital Tactile Sensor Array . . . . . . . . . . . . . . . . . . . 5

2.2 Capacitive Force Sensors . . . . . . . . . . . . . . . . . . . . . . . . . 52.3 Strain Gauges . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 72.3.1 Metal Strain Gauges . . . . . . . . . . . . . . . . . . . . . . . 72.3.2 Semiconductor Strain Gauges . . . . . . . . . . . . . . . . . . 8

2.4 Piezoresistive Force Sensors . . . . . . . . . . . . . . . . . . . . . . . 82.4.1 Conductive Elastomers . . . . . . . . . . . . . . . . . . . . . . 82.4.2 Carbon Felt and Carbon Fibers . . . . . . . . . . . . . . . . . 9

2.5 Piezoelectric Force Sensors . . . . . . . . . . . . . . . . . . . . . . . . 102.6 Pyroelectric Force Sensors . . . . . . . . . . . . . . . . . . . . . . . . 102.7 Optical Force Sensors . . . . . . . . . . . . . . . . . . . . . . . . . . 10

2.7.1 Frustrated Internal Reflection . . . . . . . . . . . . . . . . . . 102.7.2 Opto-Mechanical . . . . . . . . . . . . . . . . . . . . . . . . . 112.7.3 Fiber-Optic . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

2.7.4 Photoelasticity . . . . . . . . . . . . . . . . . . . . . . . . . . 142.7.5 Tracking of Optical Markers . . . . . . . . . . . . . . . . . . . 15

2.8 Magnetic Force Sensors . . . . . . . . . . . . . . . . . . . . . . . . . 162.8.1 Hall Effect and Magnetoresistance . . . . . . . . . . . . . . . 162.8.2 Magnetoelastic . . . . . . . . . . . . . . . . . . . . . . . . . . 17

2.9 Ultrasonic Force Sensors . . . . . . . . . . . . . . . . . . . . . . . . . 182.10 Electrochemical Force Sensors . . . . . . . . . . . . . . . . . . . . . . 18

3 Evaluation of Force Sensing Technologies 21

4 Additional Sensor Features 254.1 Multi-Axes Sensors . . . . . . . . . . . . . . . . . . . . . . . . . . . . 254.2 Slip Detection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 254.3 Measuring Curvature . . . . . . . . . . . . . . . . . . . . . . . . . . . 25

i


4/49

5 Force Sensors on the Market 275.1 Load Cells and Single Force Sensing Elements . . . . . . . . . . . . . 275.2 Description of the Parameters . . . . . . . . . . . . . . . . . . . . . . 28

5.3 Sensors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29

6 Applications of Force Sensors 356.1 Surgical Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . 356.2 Rehabilitation and service Robotics . . . . . . . . . . . . . . . . . . . 356.3 Agriculture and Food Processing . . . . . . . . . . . . . . . . . . . . 366.4 Specific Applications of force sensors . . . . . . . . . . . . . . . . . . 36

7 Conclusions 37

Bibliography 39

ii


5/49

Abstract

This report contains an overview over the fundamental force sensing technologieswith examples of manufactured sensors. These technologies are evaluated in terms oftheir applicability for docking maneuvers with unmanned helicopters. Force sensorsthat are available on the market are listed with their specifications and evaluated

likewise. Furthermore, some additional features of certain sensing technologies aregiven and the field of applications of force sensors is described briefly. In a conclu-sion, the suited technologies and sensors are summarized and a recommendation fora force sensor to use on a helicopter is given.

iii


6/49

iv


7/49

List of Figures

2.1 Whisker contact sensor by Clements and Rahn [15] . . . . . . . . . . 32.2 Mechanical transducer with a linear potentiometer [2] . . . . . . . . 42.3 Cross-section view of the pneumatic touch sensor [1] . . . . . . . . . 52.4 Digital tactile Array sensor [1] . . . . . . . . . . . . . . . . . . . . . 6

2.5 Capacitive touch sensor [1] . . . . . . . . . . . . . . . . . . . . . . . 72.6 Typical metallic strain gauge pattern [36] . . . . . . . . . . . . . . . 72.7 Piezoresistance using a separator [1] . . . . . . . . . . . . . . . . . . 92.8 Carbon felt tactile sensor [1] . . . . . . . . . . . . . . . . . . . . . . . 92.9 Tactile sensor based on frustrated internal reflection [1] . . . . . . . 112.10 Detecting shear forces with a microlever [1] . . . . . . . . . . . . . . 112.11 An opto-mechanical array touch sensor [1] . . . . . . . . . . . . . . . 122.12 Optical fiber sensor based on varying coupling between crossed fibers

[1] . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 122.13 Light radiation due to microbending [1] . . . . . . . . . . . . . . . . 132.14 Tactile sensor based on a deformable elastic reflective surface and

fiber-optic technology [1] . . . . . . . . . . . . . . . . . . . . . . . . . 142.15 Measuring stresses using photoelasticity [1] . . . . . . . . . . . . . . 152.16 Photoelastic sensor [1] . . . . . . . . . . . . . . . . . . . . . . . . . . 152.17 Hall effect [35] . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 162.18 Magnetoresistive sensor using current-carrying wires [1] . . . . . . . 172.19 Magnetoresistive sensor using magnetic dipoles [1] . . . . . . . . . . 172.20 Changes in flux distribution caused by applied force [1] . . . . . . . . 182.21 Tactile sensor using ultrasonic pulses to detect elastic skin thickness

[1] . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 182.22 Schematic diagram of the streaming potential sensor [1] . . . . . . . 19

5.1 ATI Nano17 6-axis force and torque transducer . . . . . . . . . . . . 275.2 A Honeywell FSS1500NSB force sensor . . . . . . . . . . . . . . . . . 285.3 FlexiForce foil sensor . . . . . . . . . . . . . . . . . . . . . . . . . . . 28

v


8/49

List of Tables

3.1 General advantages and disadvantages of different sensor technologies 223.2 General advantages and disadvantages of different sensor technologies

(continued) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 233.3 Sensor technologies categorized with respect to the desired sensor

properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24

5.1 Different force sensing elements with their specifications . . . . . . . 305.2 Different force sensing elements with their specifications (continued) 315.3 Some examples of special force sensors that are available on the market 32

vi


9/49

Chapter 1

Introduction

1.1 GoalThe goal of this report is to collect information about existing technologies andproducts for force sensing devices. The existing technologies and the availablesensors on the market should be described and evaluated. The evaluation refers tothe suitability of these technologies and particular sensors for docking maneuvreswith an unmanned rotorcraft. Preferred attributes of the force sensor are low weight,small size, robust especially in the sense of shock resistance, low price and a simpleconstruction in case it has to be customized.

1.2 Structure of the Report

This report is organized as follows: Chapter 2 introduces the different force sen-sor technologies. It explains all the fundamental transduction methods for forcesensing. In Chapter 3, this technologies are evaluated and their advantages anddisadvantages are registered. Additional features of sensors that some technologiesprovide besides sensing of normal forces are explained in Chapter 4. Common forcesensors that are available on the market are presented in Chapter 5 together withtheir properties as far as known. Chapter 6 presents the main fields where forcesensors are applied as well as some specific applications of commercial force sensors.In the last Chapter, conclusions, based on the evaluation of sensor technologies andsensors on the market, are drawn regarding docking maneuvers with unmannedrotorcrafts.

1


10/49

Chapter 1. Introduction 2


11/49

Chapter 2

Tactile Sensing Technologies

This chapter gives an overview and a description of the function principle of theknown force sensor technologies. A concrete example with a figure for illustrationis given to the described technologies. The technologies presented here are for themost part fundamental transduction methods presented by Russell [1] and Nicholls[2] as well as from chapter 19 of the Handbook of Robotics [7]. The books of Russelland Nicholls were published in the year 1990 and 1992 respectively. But accordingto newer literature by Lee and Nicholls [6], most of the possible forms of physicaltransduction methods have now been explored and there seems little scope for newfundamental transducers. So the fundamental transduction methods are still thesame. The current research offerings are mainly concerned with novel packagings,better designs, improved engineering and more complete analysis.

2.1 Mechanical Tactile Sensors

2.1.1 Whiskers and Antennae

Whisker or antenna sensors are in essence a hybrid of proprioceptive and tactileinformation. The main components of the earliest sensors of this type are a baseangle sensor and a tip contact sensor to explore the environment. This simpleassembly provides information whether contact occurs and if so, also the contactlocation. For many animals, whiskers or antennae provide an extremely accuratecombination of contact sensing and proprioceptive information. Examples of suchsensors are reported by Kaneko [14] or Clements and Rahn [15].

Figure 2.1: Whisker contact sensor by Clements and Rahn [15]

3


12/49

Chapter 2. Tactile Sensing Technologies 4

2.1.2 Mechanical Displacement

This kind of force sensors rely on a mechanical displacement caused by an appliedforce. The simplest example is a spring loaded switch giving on-off contact readings.A linear potentiometer provides a graded scale of deflection, and the output canbe considered in terms of either force or linear displacement (see Figure 2.2). Acommon example of mechanical displacement in simple touch sensors is the move-ment of a linear probe. For example, Presern et al. [16] designed a three degrees offreedom probe for arc welding applications where it is intended for seam tracking.

Figure 2.2: Mechanical transducer with a linear potentiometer [2]

2.1.3 Pneumatic Touch Sensor and Foil Switches

A switch is one of the simplest touch-actuated devices for detecting the presenceof an object. A very simple and small form of a switch are foil layers as they

are used in cheap calculator keyboards. These switches use two foil layers with aconductor and one layer to separate them. If enough force is applied the two exter-nal layers are pressed together and contact between the two conductors is produced.

Pneumatic touch sensors are more complex than foil layers but work in a simi-lar way. Usually, pneumatic touch sensors are built by a shallow spherical domemade of thin sheet metal. When a critical value of force is applied the dome col-lapses with a click, and later returns to its original shape as the force is removed.As the dome collapses, electrical contact with an electrode on the inside of the domeis produced. The sensor sensitivity varies with different geometries and materialproperties of the dome as well as with its pressure inside. Figure 2.3 shows a cross-section view through part of a pneumatic sensor array by Garrison and Wang [11].The sensitivity of this sensor can be varied by changing the pressurized fluid insidethe domes. The single sensing elements are spaced 2.54mm apart.


13/49

5 2.2. Capacitive Force Sensors

Figure 2.3: Cross-section view of the pneumatic touch sensor [1]

2.1.4 Digital Tactile Sensor Array

A simple switch indicates only whether or not the applied force exceeds a set thresh-old. With a closely grouped array of switches, each having a different threshold,the magnitude of an applied force can be estimated. For example, this can be im-plemented with a conductive, elastic material pressed against a V-shaped notch asillustrated in Figure 2.4. More pressure is required to force the elastomer into thenotch as the width of the notch narrows. Commonly, a row of aluminium pads de-posited along the bottom of the notch forms one electrode and a sheet of conductiveelastomer makes the other electrode. Like this a small array of switches, with eachswitch in the array having a different pressure threshold, can be produced. Linear,

logarithmic, or exponential response to pressure can be obtained by varying theshape of the notch. An advantage of this sensor design is that no analog-to-digitalconverter is necessary. A disadvantage of this sensor is that the fragile silicon chipis close to the point of contact with external objects. A tactile sensor array basedon this idea has been constructed by Raibert [12] where a sheet of elastic materialis pressed against a round hole.

2.2 Capacitive Force Sensors

Over small distances capacitance can be used to measure the separation betweentwo conductive plates. In principle, capacitance can be used to measure both shearand normal forces. Shear forces can alter the area of overlap between two platesand normal forces can affect the plate separation. However, it is difficult to separatethe two effects when trying to measure both at the same time. An example of ancapacitive sensor array constructed by Siegel et al. [13] is shown in Figure 2.5. Thissensor uses a set of row and column electrodes which are spatially separated by adielectric. A multiplexing scheme allows the capacitance at the cross point of anyrow and column electrode to be measured and hence the deflection at that pointis determined. Capacitive sensor arrays can be molded and provide very accuratemeasurements of skin deflection. The properties of the sensor, in terms of hysteresis,creep, memory, non-linearity, etc., are governed by the elastic material between thecapacitor plates.


14/49


Figure 2.4: Digital tactile Array sensor [1]


15/49

7 2.3. Strain Gauges

Figure 2.5: Capacitive touch sensor [1]

2.3 Strain Gauges

Strain gauges are a common tool to measure forces and are often applied in com-mercial force sensors. So even if semiconductor strain gauges belong to the categoryof piezoresistive force sensors they are described in this separate section. For mea-surements of small strain, semiconductor strain gauges are often preferred over foilgauges. Still both types of strain gauges are presented and their differences andadvantages are explained.

2.3.1 Metal Strain Gauges

A strain gauge consists of an insulating flexible backing which supports a metallicfoil pattern. A typical pattern is shown in Figure 2.6 which measures strain inlongitudinal direction. The strain gauge is attached to the object by a suitableadhesive. When the conductive metallic foil is stretched within the limits of itselasticity, it will become narrower and longer, changes that increase its electricalresistance. The pattern with several paths in parallel increases the effect of a changein the total resistance. From the electrical resistance of the strain gauge, the amountof applied stress may be inferred typically using a Wheatstone bridge. By using thematerial properties of the object which the gauge is attached to, also the appliedforce can be calculated.

Figure 2.6: Typical metallic strain gauge pattern [36]


16/49


2.3.2 Semiconductor Strain Gauges

Additionally to the change of resistance due to the geometrical change of the con-

ductor in metal strain gauges, the resistance of piezoresistive strain gauges alsochange because of the new mechanical state of stress. This new state of stresschanges the relative resistance of the piezoresistive material. So the change of thetotal resistance consists of a geometric part and a material specific part. The totalresistance with respect to the strain is given by

R() =()l()

A()(2.1)

with

() = 0(1 + ) (2.2)

l() = l0(1 + ) (2.3)

A() = A0(1 )2 (2.4)

This can be transformed into

R

R0= (1 + 2+ ) = K (2.5)

Where R is the change in resistance, R0 is the resistance of the unstressed gauge, is the Poissons ratio of the conductor, is a factor of the change of the rela-tive resistance due to strain and is the strain. The factor (1 + 2+ ) is calledthe K-factor and is a measure of the sensitivity of the strain gauge. For metallicfoil gauges, the K-factor is around 2.1 and is dominated by the deformation. For

semiconductor strain gauges the change of the total resistance is based up to 98%on the change of the specific resistance. Semiconductor strain gauges can reachK-factors of up to 150 which allows stiffer deformable elements of the sensor. Thislarge sensitivity is the major advantage of semiconductor strain gauges. In addi-tion, semiconductor techniques allow much smaller strain gauges than metal straingauges. The major disadvantage of semiconductor strain gauges is that they aremore sensitive to temperature changes and they are not as robust as metal straingauges. Erler [9] discusses the properties of semiconductor strain gauges in detailin Chapter 6 of his book.

2.4 Piezoresistive Force Sensors

2.4.1 Conductive Elastomers

Conductive elastomers are insulating natural or silicone-based rubbers made con-ductive by adding particles of conducting or semiconducting materials such as silveror carbon. Most of these forms of conductive rubber show little change in bulk re-sistance as they are compressed. However, area of contact and hence inverse contactresistance can be made to vary with applied force. An example of a sensor usingconductive elastomers described by Hillis [17] is shown in Figure 2.7. The elastomerand the contact pad are separated by a woven mesh of a nylon stocking which givesno contact, hence infinite resistance, for zero normal force. At a certain thresholdforce the conductive elastomer makes contact with the electrode. Additional forceincreases the area of contact and thus reduces the contact resistance. The sensor ofHillis has an array of 256 tactile sensor elements in the area of 1cm2 and a sensingrange of 1-100g.


17/49

9 2.4. Piezoresistive Force Sensors

Figure 2.7: Piezoresistance using a separator [1]

2.4.2 Carbon Felt and Carbon Fibers

Larcombe [28] has described piezoresistive sensors constructed by sandwiching car-bon felt and carbon fibers between metal electrodes as shown in Figure 2.8. Asthe load increases, the carbon fibers are compacted together, making more electri-cal contacts and reducing the felt resistance. At loads in excess of 5 kg the areaof contact between touching fibers starts to increase and this leads to a furtherreduction in resistance. Carbon fiber and carbon felt sensors are rugged and canbe shaped. They withstand very high temperatures and considerable overloads. Adisadvantage of this sensor is a great deal of electrical noise with a load of less than10 g. However, these sensors are very robust and well suited for sensing in veryinhospitable environments.

Figure 2.8: Carbon felt tactile sensor [1]


18/49


2.5 Piezoelectric Force Sensors

The piezoelectric effect gets crystals of quartz to produce an electrical voltage when

pressure is applied to the crystal. The piezoelectric effect only occurs in crystalswhich do not have a center of symmetry. Depending on the design of the sensor, dif-ferent modes to load the piezoelectric element can be used: longitudinal, transversaland shear. The voltage generated across a sensing element is proportionally relatedto the applied pressure. If a load is maintained, then the sensor output decays tozero. Therefore, these sensors are most suited for sensing dynamic forces. One ofthe most common material used for piezoelectric sensors is a polymer known aspolyvinylidene fluoride (PVF2 or PVDF). PVF2 has good mechanical properties, isa durable material and shows one of the largest piezoelectric effect. Its flexibility,sensitivity, and large electrical output offer many advantages for touch sensors inparticular. Piezoelectric materials are also pyroelectric as described in the nextsection. A problem with materials that are both piezoelectric and pyroelectric is

separating the two effects, thus protection from thermal variations may be necessaryif pressure variations are important. Dario and Buttazzo [19] have developed a skin-like sensor based on PVF2 film. This sensor contains two force-sensing layers andhas the additional capability of sensing thermal properties. The sensing elementsare arranged in a hexagon at 5mm spacing. A similar sensor built by Dario et al.[20] has a dynamic range of 4000:1, where load over 0 .01 40N were established.

2.6 Pyroelectric Force Sensors

Correspondingly to the piezoelectric effect, the pyroelectric effect is the generationof a voltage when the sensing element is heated or cooled. As piezoelectric sensors,pyroelectric sensors are inherently dynamic which means that the sensor output

decays to zero if the temperature is constant. The above presented PVF2 materialis also highly pyroelectric. A pyroelectric sensor consists of a pyroelectric elementand a heat source. The heat source causes the device to heat up. When an objecttouches the sensor surface, heat flows from the sensor into the object ore vice versa ifthe object is hotter than the sensor. The resulting temperature change can then bemeasured. The pyroelectric effect is not well suited to measure scaled forces but hasto be taken into account when building a piezoelectric sensor (Dario and Buttazzo[19]). In return, a pyroelectric sensor is well suited to detect slip on a surface. Ifthe sensor has no slip, the surface temperature at the point of contact becomesthe same as the sensor temperature. As soon as the sensor moves, a temperaturechange and therefore slip can be detected.

2.7 Optical Force Sensors

2.7.1 Frustrated Internal Reflection

A sheet of clear plastic can act as a light guide. Light introduced at one edge willpropagate across the sheet by total internal reflection and emerge at the oppositeedge. Total internal reflection occurs when light is propagating in the denser oftwo media and strikes the interface at an angle larger than a particular criticalangle with respect to the normal to the interface. When the surface of the lightguide comes into contact with an external object then at that point total internalreflection is frustrated and light emerges from the opposite side of the light guide.In practice , a reflective rubber is placed on the light guide to protect it and toexclude external light as it is shown in Figure 2.9. If the rubber sheet is moldedwith a textured surface then an output proportional to the area of contact, and


19/49

11 2.7. Optical Force Sensors

hence applied force, can be obtained (Tanie et al. [22]). The light which emergesfrom the back of the light guide is detected either by an array of photodiodes, solidstate optical sensors, or transported away from the sensor by optical fibers. This

kind of sensor can also be made sensitive to shear forces in the reflective rubbermaterial as the microlever proposed by Dario et al. [21] that is illustrated in Figure2.10.

Figure 2.9: Tactile sensor based on frustrated internal reflection [1]

Figure 2.10: Detecting shear forces with a microlever [1]

2.7.2 Opto-Mechanical

The Lord Corporation produced an opto-mechanical touch sensor as shown in Figure2.11. It contains a rubber skin with an array of mushroom-shaped projectionsmolded into its surface (Rebman and Morris [23]). The head of the mushroomconcentrates the normal force and the stalk acts as an optical shutter to modulatelight transmission between a light-emitting diode and a photo detector dependingupon normal force. Because the deflecting element is made of rubber, the sensorresponse will be subject to the usual problems of creep, hysteresis, memory, andtemperature variation. The Lord LTS-210 for example consists of an array of opto-mechanical force sensors on a 1.8mm spacing. The Construction of this sensor isquite labor intensive.


20/49


Figure 2.11: An opto-mechanical array touch sensor [1]

2.7.3 Fiber-Optic

Light Coupling Between Adjacent Fibers

Light propagates along an optical fiber with very little lost due to radiation; how-ever, if the surface of the fiber is roughened then at that point light can leave andenter the fiber. If two optical fibers pass close to each other and both have a rough-ened surface then light can pass between the fibers. The sensor design shown inFigure 2.12 uses D-section cords made of rubber as a deformable member and thelight coupling between crossed plastic optical fibers to measure the resulting deflec-tion. A 4x4 array with 1cm spacing has been reported by Schoenwald et al. [24].

Loads applied normal to the sensor surface compress the D-section elastomer cordsmoving the fibers closer together and thus increases the light coupling. This sensoris flexible and can conform to complex curved surfaces. It has the advantage of noiseimmunity associated with optical transducers and the imperfections introduced bythe use of elastomer materials as the deformable member.

Figure 2.12: Optical fiber sensor based on varying coupling between crossed fibers[1]

Bending Losses in Optical Fibers

Light propagates through an optical fiber by repeated internal reflection from thecladding interface. For total internal reflection to occur, light must strike thissurface at an angle greater than a critical angle with respect to the normal to thesurface. If the fiber is subjected to a significant amount of bending over a lengthcomparable to the distance between successive internal reflections then the angle of


21/49


incidence can be reduced sufficiently for the light to leave the core. This effect isillustrated in Figure 2.13. Under these conditions of microbending, the amount oflight transmitted by an optical fiber is greatly reduced. This effect has been put

to use in microbend touch sensors. An experimental 2x2 robot sensor has beenreported by Winger and Lee [25]. This sensor is capable of detegting a 5g variationin applied load in its linear region which ranges between 125g and 225g per sensorelement. Hysteresis proved to be a large problem and this was thought to be causedby the cladding material.

Figure 2.13: Light radiation due to microbending [1]

Optical Skin Thickness Sensor

Figure 2.14 shows a cross-section of a sensor that determines the thickness of atransparent, deformable elastomer layer by measuring the intensity of light reflectedback from the far side of the layer. Light is introduced into the sensor via anoptical fiber. A widening cone of light propagates out through a layer of transparentelastomer and is reflected by an outer skin of white elastomer. The reflected light isreceived by a second fiber which is viewed by a computer vision system to measurethe reflected light. When an external force compresses the transparent elastomerthis shortens the distance travelled by the light cone, limiting the light dispersionand thus reducing the light gathered by the receiving fiber. A sensor described bySchneiter and Sheridan [26] contains 2100 sensitive points per square inch (6.45cm2)and exhibits a dynamic range of only 18:1. This sensor is very labor intensive sinceeach optical fiber was positioned by hand and it is rather frail since the clear rubbermaterial was found to fatigue after only a few hundred operating cycles.


22/49


Figure 2.14: Tactile sensor based on a deformable elastic reflective surface andfiber-optic technology [1]

2.7.4 Photoelasticity

Photoelasticity can be used to measure stresses in a sample of optically activematerial. Consider the experimental set-up shwon in Figure 2.15.

(a) Light radiated from a light source contains many waves of differing polarizationand amplitude. Both polarization and amplitude vary with time.

(b) Polarizer 1 only allows through components of each wave having a particularplane of polarization, and blocks those components which are at right angles.

(c) Upon entereing the birefringent material light is split into two components,polarized at right angles. These components are aligned with the planes of

maximum and minimum stress in the material.(d) If the two waves emerge from the birefingent material with the same relative

phase that they had when they entered then the original wave is reconstructed.

(e) Polarizer 2 is rotated 90 with respect to polarizer 1 and therefore blocks trans-mission of the reconstructed wave.

If the two waves do not emerge with the same relative phase that they had whenthey entered the birefringent material then an elliptically polarized wave results.Part of this elliptically polarized wave is passed by polarizer 2. Dark areas, orfringes, where light is not transmitted through the system, are the result of twoeffects:

1. Isoclinics: Areas where light from polarizer 1 is in line with one of theprincipal stress axes. In this case the light is not split into two componentsand therefore emerges unaltered. This effect provides information about theorientation of principal stresses in the birefringent material.

2. Isochromatics: Result when light is split by the birefringent material butemerges with the same phase that it entered. This will be true if the principalstresses in the material are identical or differ by an amount which producesan integral number of phase rotations.

An example of such a sensor was developed by Cameron et al. [27] and is shownin Figure 2.16. It measures the forces applied to a sheet of birefringent material.The sheet of birefringent material is illuminated by circularly polarized light tovisualize isochromatic fringes. A CCD camera records the resulting stress pattern.


23/49


The camera image can be analysed to determine the distribution of stresses withinthe birefringent material.

Figure 2.15: Measuring stresses using photoelasticity [1]

Figure 2.16: Photoelastic sensor [1]

2.7.5 Tracking of Optical Markers

This principle is based on the idea of using deformable tactile sensors. It combinesoptical tracking with models of the sensors skin to predict the sensor skin defor-mation. The sensor skin consists of markers on the inside which are tracked by atiny CCD camera. The measured data of the camera is then combined with themechanical model of the skin to derive its deformation and the applied force on theskin. Ferrier and Brocket [29] implemented a tactile sensor with a CCD camerafocused on a 7x7 array of dots that are marked on the inside of a gel-filled siliconefingertip membrane. An algorithm is then used to construct a 13x13 grid over thearray of dots. Another interesting tactile sensor uses vision to track an array ofspherical markers embedded in a transparent elastomer to infer the stress state ofthe skin material due to applied force. This sensor is commercialized under thetradename GelForce by the Tachi Lab [37].


24/49


2.8 Magnetic Force Sensors

2.8.1 Hall Effect and Magnetoresistance

The Hall effect is closely related to the motor effect observed as a force on a current-carrying conductor in a magnetic field. In Figure 2.17, if charge carriers (1) flowthrough a conductive material (2) and a magnetic flux (4) is established, then theyexperience a force orthogonal to their flow direction and the magnetic field direction.This deflection of the charge carriers is then producing a resulting Hall potentialin direction of the deflection which can be measured. Due to this deflection ofthe charge carriers, they take a longer path to travel the length of the conductive

material. Effectively the deflected particles have a lower mobility and this showsas an increased electrical resistance. This effect is known as magnetoresistance.Both the Hall effect and magnetoresistance can be used to measure magnetic fieldintensity. Note that a Hall effect sensor is only sensitive to magnetic fields in onedirection while the magnetoresistive effect can be used to detect magnetic fieldhaving any orientation within a plane normal to the current flow. A schematicexample of a magnetoresistive sensor is shown in Figure 2.18. The magnetic fieldis provided by current-carrying wires within an elastomer. When the elastomeris compressed, a change of the magnetic field in the magnetoresistive element canbe measured. Hackwood et al. [30] developed a magnetoresistive sensor that canmeasure normal forces, tangential forces and torques. In this sensor design, a sheetof silicone elastomer contains an array of embedded magnetic dipoles (see Figure

2.19). Beneath each dipole four Permalloy magnetoresistive sensors are mountedon a rigid substrate. Magnetic field strength at each of the four magnetoresistivesensors is used to determine both position and orientation of the magnetic dipole.

Figure 2.17: Hall effect [35]


25/49

17 2.8. Magnetic Force Sensors

Figure 2.18: Magnetoresistive sensor using current-carrying wires [1]

Figure 2.19: Magnetoresistive sensor using magnetic dipoles [1]

2.8.2 Magnetoelastic

Magnetoelastic sensors are made from a magnetostrictive material. These materialschange their magnetic permeability when they are deformed. The sensor elementshown in Figure 2.20 contains two windings arranged at right angles. In the un-stressed condition the magnetostrictive material is isotropic and hence there is noflux coupling between the two windings and no output voltage at the secondarywinding. When a force is applied to the sensor the permeability of the magnetostric-tive material decreases in the vertical and increases in the horizontal direction. Thischanges the magnetic field of the primary winding in such a way that flux links thesecondary winding and an output voltage is produced in the secondary winding. Asensor array of 16x16 magnetoelastic sensor elements with 2.5mm spacing has beenreported by Luo et al. [31]. The sensor array was covered by a sheet of elastomer toprovide protection and an improved gripping surface. Good sensitivity and linearity,and low hysteresis are claimed for the sensor but the sensors and their associatedcircuits are relatively complicated.


26/49


Figure 2.20: Changes in flux distribution caused by applied force [1]

2.9 Ultrasonic Force Sensors

Ultrasound has been in use for a long time to measure the thickness of objects. Wecan measure the time taken for an ultrasonic pulse to travel through the material,reflect off the back surface and return. If the speed of propagation of the ultrasonicwave in the material is known then the material thickness can be calculated. Thisprinciple can be used to measure the thickness of a flexible elastomer layer at manyclosely spaced points. Like this the deformation of the elastomer can be measured asit is shown in Figure 2.21 and the pressure applied on its surface can be calculated.Grahn and Astle [32] claim a dynamic range of 2000:1 and a spatial resolutionof 0.5mm for their ultrasonic force sensor. If the sensor is immersed in a liquidor touches an object with a similar acoustic impedance to the skin material, it is

possible that the ultrasonic pulse propagates past the surface of the skin materialwithout being reflected. So attention has to be paid on the material properties ofthe sensor surface and the objects that should be touched.

Figure 2.21: Tactile sensor using ultrasonic pulses to detect elastic skin thickness[1]

2.10 Electrochemical Force Sensors

Chemical-impregnated gels have been formulated to make them sensitive to defor-mation. The sensor shown in Figure 2.22 by De Rossi et al. [33] uses a gel thatcontains an immobile negative charge which is balanced by a mobile positive charge.An ionized gel disk 1cm in diameter and 0.40.5mm thick is made from polyacrylic


27/49

19 2.10. Electrochemical Force Sensors

acid and polycinylic acid. When pressure is applied to the gel, positively chargedliquid is forced out of the gel and an inhomogeneity of charge is formed which con-stitutes the streaming potential. This potential is picked up by two thin electrodes.

The sensor can detect low-frequency deformations but has no steady state response.

Figure 2.22: Schematic diagram of the streaming potential sensor [1]


28/49



29/49

Chapter 3

Evaluation of Force Sensing

Technologies

First, a few terms which are needed for the evaluation of the force sensing technolo-gies are explained.

1. Dynamic Range: The ratio of largest to smallest detectable force.

2. Spatial Resolution: The space that one single sensing element takes. Sofor an Array sensor the spatial resolution gives the amount of single sensingelements that can be placed in a given length or area.

3. Inherently Dynamic: Sensor output decays to zero for constant load.

A very general evaluation of the different sensor technologies is given in Table 3.1and Table 3.2. Due to the fact that most of the technologies are only applied inresearch it makes no sense to give specific characteristics of the built sensors sincethey vary very much between different sensor designs within the same technology.Some specific examples are given in Chapter 2 to give an impression on the limita-tions of the corresponding technology.Table 3.3 categorizes the technologies with respect to the desired sensor propertiesintroduced in Section 1.1. In this table ++ denotes very good or well suited and denotes bad or not at all suited.

21


30/49

Chapter 3. Evaluation of Force Sensing Technologies 22

Table 3.1: General advantages and disadvantages of different sensor technologies


31/49

23

Table 3.2: General advantages and disadvantages of different sensor technologies(continued)


32/49

Chapter 3. Evaluation of Force Sensing Technologies 24

Table 3.3: Sensor technologies categorized with respect to the desired sensor prop-erties


33/49

Chapter 4

Additional Sensor Features

4.1 Multi-Axes Sensors

Load cells (see Section 5.1) usually measures forces and torques in multiple axis.But also some of the presented fundamental force sensing technologies are able tomeasure more than just normal force in one direction.A sensor based on frustrated internal reflections for example have been made sensi-tive to shear forces by Dario et al. [21]. This sensor is described in Section 2.7.1 andillustrated in Figure 2.10. With piezoelectric sensors (see Section 2.5), dependingon the design of the sensor, different modes to load the piezoelectric element can beused: longitudinal, transversal and shear. A further example is the sensor describedin Section 2.8.1 by Hackwood et al. [30] is illustrated in Figure 2.19. This hall effectsensor can measure normal and shear forces as well as torques.

4.2 Slip Detection

Several force sensing technologies can also be used to detect slip. Pyroelectricsensors (see Section 2.6) are well suited to detect slip by measuring a change inthe temperature as soon as the sensor starts to move along a touched object. Alsocarbon felt sensors (see Section 2.4.2) have been used to detect slip by measuring thenoise due to the friction at the contact point. A capacitive sensor which is capableof detecting slip is described by Luo [34]. This sensor uses the change in capacitancecaused by relative contact movement between sensor and object. The contactingsensor surface comprises a set of parallel rollers. Each roller is a half cylinder of

conductive material, and a half cylinder of nonconductive material and therefore actas a variable capacitor. Other approaches have been realized by detecting the soundgenerated by movement against the sensor surface with piezocrystal microphones.Of course there are many other approaches to detect slip, which are not discussedhere.

4.3 Measuring Curvature

Provancher and Cutkosky [10] have built a sensor that can measure the curvatureof a touched object. The sensor consists of an array of strain gauges embedded ina compliant membrane. The curvature is estimated with a least squares procedurewith the measurements of all the strain gauges. The resulting sensor is inexpensiveand robust and can be used for object handling and exploration. The curvature

25


34/49

Chapter 4. Additional Sensor Features 26

measurements provide information for manipulation planning and control and pro-vide an estimate of the local object geometry.


35/49

Chapter 5

Force Sensors on the Market

5.1 Load Cells and Single Force Sensing Elements

There are two different main fields of force sensors on the market. One are so calledLoad cells (see Figure 5.1) and the other are small single sensing elements. Thesesensing elements are based on piezoresistive transduction and can again be split upin two categories, namely sensors that use some sort of steel ball to concentratethe force to a silicon sensing element (see Figure 5.2) and sensors that consist ofpiezoresistive foil layers (see Figure 5.3). Load cells are a mechanical arrangementwhere the applied forces and torques are measured by semiconductor strain gauges(see Section 2.3.2). Usually 6-axis transducers can be found on the market. Mostly

these load cells are designed for large industrial applications and can measure largeforces up to hundreds of kilo Newton. Since they are rather large, heavy andexpensive they are not considered in this report which focuses on applications on asmall helicopter. The smallest available 6-axis transducer is the ATI Nano17 shownin Figure 5.1. Its properties and performances can be found in Table 5.3. With itssize of 17mm in diameter and 15mm height and a weight of 9g it would also besuitable for applications on a unmanned helicopter. Nonetheless such a transducerwill not be first choice since it is very expensive and force and torque measurementsin all axis together is hardly required for docking maneuvers with a helicopter.

Figure 5.1: ATI Nano17 6-axis force and torque transducer

27


36/49

Chapter 5. Force Sensors on the Market 28

Figure 5.2: A Honeywell FSS1500NSB force sensor

Figure 5.3: FlexiForce foil sensor

5.2 Description of the Parameters

In this section the parameters which are used to evaluate the sensors found on themarket are explained in case their meaning or definition is not clear. In Tables 5.1and 5.2 the unit [%Span] can be found for different parameters. This denotes therelative error compared to the whole span of the sensor.

1. Range: The range of forces which the sensor is dimensioned for. In Tables5.1 and 5.2 the forces are given in [N] but one can often find the unit [kg] or[kgf] which means the force corresponding to the declared mass in a 9.81m/s2

gravitational field. For array sensors the range is given in [Pa].

2. Dynamic Range: The ratio of largest to smallest detectable force.


37/49

29 5.3. Sensors

3. Over Force: The maximum force which may safely be applied to the sensorfor it to remain in specification once force is returned to the operating forcerange.

4. Sensitivity: The ratio of output signal change to the corresponding inputforce change. Sensitivity is determined by computing the ratio of Span to thespecified operating force range.

5. Span: The algebraic difference between output signal measured at the upperand lower limits of the operating force range. Also known as full scale outputor simply span. For some sensors only the output resistance difference isdeclared.

6. Linearity Error: The maximum deviation of the true response to the bestfit straight line (BFSL).

7. Repeatability: The maximum difference between output readings when thesame force is applied consecutively, under the same operating conditions, withforce approaching from the same direction within the operating force range.

8. Temperature: The temperature range over which the product will producean output proportional to force but may not remain within the specified per-formance limits. This is the operating temperature range, the storage tem-perature range may be different.

9. Mechanical Hysteresis: The maximum difference between output readingswhen the same force is applied consecutively, under the same operating con-ditions, with force approaching from opposite directions within the operatingforce range.

10. Elements: The number of simple sensing elements contained in an arraysensor.

11. Resolution: The distance between to simple sensing elements in an arraysensor.

5.3 Sensors

This Section presents the sensors that were found on the market at different dis-tributors. In Table 5.1 and Table 5.2 standard single element force sensors arepresented. Table 5.3 presents some special sensors such as sensor arrays and a 6-axis transducer. The introduced properties of each sensor are gathered from datasheets that were found in the internet. Since the quality and contained informationof the data sheets varies very much not every property could be evaluated for eachof the presented sensors.


38/49


Table 5.1: Different force sensing elements with their specifications


39/49

31 5.3. Sensors

Table 5.2: Different force sensing elements with their specifications (continued)


40/49


Table 5.3: Some examples of special force sensors that are available on the market


41/49

33 5.3. Sensors

We can now generally evaluate the three different sensor categories on the marketin terms of the desired properties described in Section 1.1.

1. Load Cells: As mentioned before, load cells are constructed for large forcesand therefore are also very robust. But they have some crucial disadvantagesfor applications on a small helicopter. They are very expensive, rather largeand heavy.

2. Silicon-sensing Elements: These elements are small, light and not tooexpensive. But they have one major disadvantage, which is their robustness.Their sensing range goes only up to around 15N and their over force is onlyaround 45Nwhich can already be reached when the helicopter bounces againsta wall.

3. Foil Sensors: These sensors are very light, small (especially thin) and verycheap. Sensors for forces up to 445N are available which is entirely sufficient

for applications on a small helicopter. So generally these sensors fit the desiredproperties very well.


42/49



43/49

Chapter 6

Applications of Force Sensors

During the 1980s, most authors predicted that the major application area for tactilesensing would be industrial automation tasks such as robotic assembly. However, thedemand for such sensors proved to be low. There are still very few fully developedapplications of tactile sensing but this chapter considers current trends. We see threemain fields where tactile sensing is likely to play a key role. These are: medicalprocedures, especially surgery; rehabilitation and service robotics; and agricultureand food processing. In addition, some examples of applications using commercialforce sensors are given.

6.1 Surgical Applications

Surgery is perhaps the most exciting and rapidly developing area where tactile sens-

ing is actually of central importance. Minimally Invasive Surgery (MIS) is still ayoung method but is now routinely used as the preferred choice for many opera-tions. However, despite its advantages, MIS severely reduces the surgeons sensoryperception during manipulation. Surgery is essentially a visual and tactile expe-rience and any limitations on the surgeons sensory abilities are most undesirable.For example, in laparoscopy long slender tools are inserted through small punctureopenings in the abdominal wall and the surgeon uses a range of tip mounted in-struments guided by video feedback images. As the instruments are rigid rods andeffectively have fixed pivots at the entry points, the available degrees of freedomare restricted. This is one of the main difficulties together with lack of depth from2D vision and the almost complete lack of sense of touch. The reason that tactilesensing is so important in surgery is that soft tissue can only be properly examinedand identified by assessing its softness, viscosity and elasticity properties. So it isclear that tactile sensing is greatly needed in this area.

6.2 Rehabilitation and service Robotics

A major concern for the next century is the enormous numerical increase in theelderly population that will generate great economic pressures. This demographicchange is well accepted and many governments have initiated programmes of re-search in health care, hospital services and social support. It is clear that therewill be greatly increased demand on these services and researchers are looking formethods of support and assistance that do not involve central services but can bedistributed as aids within the home and community.Despite the contrast with industrial robotics, there are some emerging interests inhuman-robot cooperation within manufacturing. As the next stage in automation,

35


44/49

Chapter 6. Applications of Force Sensors 36

Toyota envisage workers and robot machines coexisting in a safe partner relation-ship.As yet there is little in this area that is new specifically concerning tactile sensing.

However, as for medicine, we can see many opportunities where sense of touch willbe a real need.

6.3 Agriculture and Food Processing

The field of agriculture and food production is now well automated but, like ser-vice robotics, also does not feature many new technological advantages in tactilesensing. Uses of tactile sensing are often mundane, but the importance of this areais its potential for imminent development. Unlike manufacturing automation, theprocessing of natural produce usually involves high numbers of human operators.This is because of the problems of handling soft, delicate and highly variable itemsby machine have not been solved at low enough cost. Recently, there has beenincreased interest in the prospect of reducing human involvement in order to re-duce hygiene risks, eliminate human errors and use efficient but more hazardousenvironments.

6.4 Specific Applications of force sensors

Most of the research of tactile sensing in the field of robotics is made for producingrobotic hands. It deals with the problem of gripping objects and therefore measur-ing contact forces and contact locations. Usually an array of force sensing elementsis placed into the skin of the robotic hand to be able to measure contact over thewhole surface of the hand. The goal of most of the technologies presented in Chap-

ter 2 is to build a robotic hand covered by an artificial skin.

Further applications are in human and robot interaction such as haptic devicesused for rehabilitation. Another example are touch screens which are very popularnowadays and are mostly based on a resistive detection of the contact location.Some also use capacitive techniques to detect contact on the surface.The Tachi Lab [37] developed some very interesting applications of force sensing.Their field of research is virtual reality. They have several projects dealing withhaptic interaction between humans and a virtual reality such as the GravityGrab-ber. They also did research on force sensing technologies and developed for exampleGelForce, a force sensor which uses tracking of optical markers (see Section 2.7.5)to measure the distribution of both the magnitude and direction of force.

The most frequently applied force sensors are strain gauges. They are cheap andcan be easily used in a custom application. A lot of larger commercial force sensorsare using strain gauges to measure the applied forces. Load cells (see Section 4.1)for instance use several strain gauges to measure forces and torques in multipledirections. Another example of a sensor using strain gauges is explained in Section4.3.Foil sensors such as produced by FlexiForce are also commonly used in commercialapplications. One example are PlayStation controllers which have buttons that donot only have an on-off function but also a scaled input according to the appliedforce on the button.


45/49

Chapter 7

Conclusions

Over the past four decades, tactile sensing has developed into a sophisticated tech-nology. There has been a long-standing and widely held expectation that tactilesensors would have a major impact on industrial robotics and automation. How-ever, this promise has not been realized, and few, if any, tactile sensors can befound in factory based applications [4]. The main fields where tactile sensing islikely to play a key role are medical procedures, especially surgery; rehabilitationand service robotics; and agriculture and food processing. After all the productionof new designs and configurations of sensors continues apace. However, most ofthe possible forms of physical transduction methods have now been explored andthere seems little scope for new fundamental transducers. The basic transductionmethods are described by Russell [1] and Nicholls [2] and were already developed in1990. The current research offerings are mainly concerned with novel packagings,better designs, improved engineering and more complete analysis.

The technologies that satisfy the requirements of a small, light, robust, simpleand cheap force sensing element best, are mechanical displacement sensors, straingauges and more advanced, a carbon felt and carbon fibers sensor. A carbon feltsensor may be considered for sensing in inhospitable environments and if large forcesor shocks on the sensor are possible. The problem is that no commercial sensor us-ing this technology is available and the development would be very time-consumingand expensive. Therefore it makes more sense to use strain gauges which are themost commercialised sensors that can measure forces. They are cheap compared toother force sensors and can be attached to a simple customized deformable element,such as a small metallic arc, at the end of the helicopter. Finally the simplest con-siderable solution is a self-built mechanical displacement sensor maybe combined

with some kind of damping element to prevent the helicopter to bounce back from atouched object. Such a sensor could be developed very cheaply and can be adaptedquite easily to a given task.

The commercial sensors presented in Chapter 5 are nearly all based on piezore-sistive technologies. Sensors like those produced by Honeywell will most likely notbe suitable for the purpose of docking maneuvers with a helicopter because the max-imum force of 40Nwhich these sensors withstand and the actual range of 015N isbarely enough. Foil sensors such as FlexiForce have a much larger range of 0440Nand are therefore more appropriate. A disadvantage for both of these sensor typescan be that the movable part of the sensors can only perform small deflections. Forinstance if the helicopter is flying towards a wall, no damping of the impact can berealized just by the sensor, so one have to consider to build an assembly of such asensor with some damping element.

37


46/49

Chapter 7. Conclusions 38

After all, the two force sensing methods that fit best for a helicopter are on the onehand mechanical displacement and on the other hand a customized assembly witha foil sensor. Both fit the desired properties introduced in Section 1.1 very well and

are easy to build by yourself. Therefore, these two should be considered first andtested if their measuring performance is suitable.


47/49

Bibliography

[1] R.-A. Russel: Robot Tactile Sensing, Prentice Hall, New York; Sydney, 1990

[2] H.-R. Nicholls: Advanced Tactile Sensing for Robotics, World Scientific Pub-lishing Co., Inc., 1992

[3] H.-R. Nicholls, M.-H. Lee: A Survey of Robot Tactile Sensing Technology,The International Journal of Robotics Research, Vol. 8, No. 3, 1989, pp. 3-30

[4] M.-H. Lee: Tactile Sensing: New Directions, New Challenges, The Interna-tional Journal of Robotics Research, Vol. 19, No. 7, 2000, pp. 636-643

[5] R.-S. Fearing: Tactile Sensing Mechanisms, The International Journal ofRobotics Research, Vol. 9, No. 3, 1990, pp. 3-23

[6] M.-H. Lee, H.-R. Nicholls: Tactile sensing for mechatronicsA state of theart survey, Mechatronics, Vol. 9, No. 1, 1999, pp. 1-31

[7] M.-R. Cutkosky, R.-D. Howe, W.-R. Provancher: Handbook ofRobotics, Springer, 1990, Chap. 19

[8] J. Rausch: Engineering Haptic Devices, Springer, 2009, Chap. 10

[9] W. Erler: Elektrisches Messen nichtelektrischer Grossen mit Halbleiter-widerstanden, Verlag Technik, Berlin, 1973

[10] W.-R. Provancher, M.-R. Cutkosky: Sensing Local Geometry for Dex-terous Manipulation, ISER, 2002, pp. 507-516

[11] R.-L. Garrison, S.-S.-m. Wang: Pneumatic touch Sensor, IBM TechnicalDisclosure Bulletin, Vol. 16, No. 6, 1973, pp. 2037-2040

[12] M.-H. Raibert: An All Digital VLSI Tactile Array Sensor, Proceedings of theIEEE International Conference on Robotics, 1984, pp. 314-319

[13] D.-M. Siegel, et al.: A Capacitive Based Tactile Sensor, SPIE Conference onIntelligent Robots and Computer Vision, Cambridge, Massachusetts, 1985, pp.153-161

[14] M. Kaneko, N. Ueno, T. Tsuji: Active antenna-basic considerations onthe working principle, Proceedings of the IEEE International Conference onIntelligent Robotic Systems, Vol. 3, 1985, pp. 1744-1750

[15] T.-N. Clements, C.-D. Rahn: Three-dimensional contact imaging with anactuated whisker, IEEE International Conference on Intelligent Robotic Sys-tems, 2005, pp. 598-603

[16] S. Presern, F. Dacar, M. Spegel: Design of three active degrees of freedomtactile sensor for industrial arc welding robots, Proceedings of the 4th BritishRobot Association conference, Brighton, UK, 1981, pp. 207-213

39


48/49

Bibliography 40

[17] W.-D. Hillis: Active Touch sensing, MIT AI Memo 629, April 1981

[18] M.-H.-E. Larcombe: Carbon Fibre Tactile Sensors, Proceedings of the First

International Conference on Robot Vision and Sensory Controls, Bedford, UK,1981, pp. 273-276

[19] P. Dario, G. Buttazzo: An Anthropomorphic Robot Finger for Investigat-ing Artificial Tactile Perception, International Journal of Robotics Research,Vol. 6, No. 3, 1987, pp. 25-48

[20] P. Dario, D. De Rossi, C. Domenici, R. Francesconi: Ferroelectric poly-mer tactile sensors with anthropomorphic features, Proceedings of the IEEEInternational Conference on Robotics, Atlanta, GA, 1984, pp. 332-340

[21] P. Dario, et al.: Advanced Rehabilitative Robots, Proceedings of the Inter-national Symposium and Exposition on Robots, Sydney, Australia, 1988, pp.

687-703

[22] K. Tanie, et al.: A High-Resolution Tactile Sensor, Robot Sensors, Vol. 2,1986, pp. 189-198

[23] J. Rebman, K.-A. Morris: A tactile Sensor with Electro-optical Transduc-tion, Robot Sensors, Vol. 2, 1986, pp. 145-155

[24] J.-S. Schoenwald, et al.: A Novel Fiber Optic Tactile Array Sensor, Pro-ceedings of the IEEE International Conference on Robotics and Automation,Raleigh, North Carolina, 1987, pp. 1792-1797

[25] J. Winger, K.-M. Lee: Experimental Investigation of a Tactile Sensor Based

on Bending Losses in Fiber Optics, Proceedings of the IEEE International Con-ference on Robotics and Automation, Raleigh, North Carolina, 1988, pp. 754-759

[26] J.-L. Schneiter, T.-B. Sheridan: An Optical Tactile Sensor for Manipu-lators, Robotics and Computer-integrated Manufacturing, Vol. 1, No. 1, 1984,pp. 65-71

[27] A. Cameron: Touch and Motion, Proceedings of the IEEE International Con-ference on Robotics and Automation, Philadelphia, 1988, pp. 1062-1067

[28] M.-H.-E. Larcombe: Carbon Fibre Tactile Sensors, Proceedings of the FirstInternational Conference on Robot Vision and Sensory Controls, Bedford, UK,

1981, pp. 273-276

[29] N.-J. Ferrier, R.-W. Brockett: Reconstructing the shape of a deformablemembrane from image data, The International Journal of Robotics Research,Vol. 19, No. 9, 2000, pp. 795-816

[30] S. Hackwood: A Torque-Sensitive Tactile Array for Robotics, The Interna-tional Journal of Robotics Research, Vol. 2, No. 2, 1983, pp. 46-50

[31] R.-C. Luo, et al.: An Imaging Tactile Sensor with Magnetoresistive Transduc-tion, Proceedings of the First International Conference on Intelligent Sensorsand Computer Vision, Cambridge, Massachusetts, 1983

[32] A.-R. Grahn, L. Astle: Robotic Ultrasonic Force Sensor Arrays, Proceed-ings of the Robots 8 Conference, Detroit, Michigan, 1984, pp. 21.1-21.18


49/49

41 Bibliography

[33] D. De Rossi, A. Nannini, C. Domenici: Artificial Sensing Skin MimickingMechanoelectrical Conversion Properties of Human Dermis, IEEE Transac-tions on Biomedical Engineering, Vol. 35, No. 2, 1988, pp. 83-92

[34] R.-C. Luo: Design and implementation of hand-based tactile sensors for in-dustrial robots., Proceedings of the International Conference on Robotics andFactories of the Future, 1984, pp. 423-432

[35] Wikimedia: Hall Effect, http://commons.wikimedia.org/wiki/File:Hall_effect_A.png

[36] Wikimedia: Strain Gauge Pattern, http://upload.wikimedia.org/wikipedia/commons/thumb/5/53/Strain_gauge.svg/424px-Strain_

gauge.svg.png

[37] Tachi Lab: http://tachilab.org/
http://commons.wikimedia.org/wiki/File:Hall_effect_A.pnghttp://commons.wikimedia.org/wiki/File:Hall_effect_A.pnghttp://upload.wikimedia.org/wikipedia/commons/thumb/5/53/Strain_gauge.svg/424px-Strain_gauge.svg.pnghttp://upload.wikimedia.org/wikipedia/commons/thumb/5/53/Strain_gauge.svg/424px-Strain_gauge.svg.pnghttp://upload.wikimedia.org/wikipedia/commons/thumb/5/53/Strain_gauge.svg/424px-Strain_gauge.svg.pnghttp://tachilab.org/http://tachilab.org/http://upload.wikimedia.org/wikipedia/commons/thumb/5/53/Strain_gauge.svg/424px-Strain_gauge.svg.pnghttp://upload.wikimedia.org/wikipedia/commons/thumb/5/53/Strain_gauge.svg/424px-Strain_gauge.svg.pnghttp://upload.wikimedia.org/wikipedia/commons/thumb/5/53/Strain_gauge.svg/424px-Strain_gauge.svg.pnghttp://commons.wikimedia.org/wiki/File:Hall_effect_A.pnghttp://commons.wikimedia.org/wiki/File:Hall_effect_A.png

Force Sensing Technologies

Documents

Transcript of Force Sensing Technologies