Chapter 5.1 Sensors: Touch, Force, and Torque Richard M. Crowder University of Southampton, Southampton, England 1.1 INTRODUCTION The objective of any robotic sensing system is to acquire knowledge and resolve uncertainty about the robot's environment, including its relationship with the workpiece. Prior to discussing the require- ments and operation of speci®c sensors, the broad objectives of sensing need to be considered. The con- trol of a manipulator or industrial robot is based on the correct interpretation of sensory information. This information can be obtained either internally to the robot (for example, joint positions and motor torques) or externally using a wide range of sensors. The sensory information can be obtained from both vision and nonvision sensors. A vision system allows the position and orientation of the workpiece to be acquired; however, its performance is dependent on lighting, perspective distortion, and the background. A touch, force, or torque sensor will provide information regarding the contact between the sensor and workpiece, and is normally localized in nature. It is recognized that these sensors will not only complement vision sensing, but offer a powerful sensing capability in their own right. Vision may guide the robot arm through many manufacturing operations, but it is the sense of touch that will allow the robot to perform delicate manipulations and assembly tasks. 1.2 TOUCH AND TACTILE SENSING Touch and tactile sensors are devices which measure the parameters of a contact between the sensor and an object. This interaction obtained is con®ned to a small de®ned region. This contrasts with a force and torque sensor, which measures the total forces being applied to an object. In the consideration of tactile and touch sensing, the following de®nitions are commonly used: Touch sensing. This is the detection and measure- ment of a contact force at a de®ned point. A touch sensor can also be restricted to binary information, namely, touch and no touch. Tactile sensing. This is the detection and measure- ment of the spatial distribution of forces perpen- dicular to a predetermined sensory area, and the subsequent interpretation of the spatial informa- tion. A tactile sensing array can be considered to be a coordinated group of touch sensors. Slip. This is the measurement and detection of the movement of an object relative to the sensor. This can be achieved either by a specially designed slip sensor or by the interpretation of the data from a touch sensor or a tactile array. Tactile sensors can be used to sense a diverse range of stimuli, from detecting the presence or absence of a grasped object to a complete tactile image. A tactile 377 Copyright © 2000 Marcel Dekker, Inc. sensor consists of an array of touch-sensitive sites; the sites may be capable of measuring more than one prop- erty. The contact forces measured by a sensor are able to convey a large amount of information about the state of a grip. Texture, slip, impact, and other contact conditions generate force and position signatures that can be used to identify the state of a manipulation. This information can be determined by examination of the frequency domain, and is fully discussed in the literature [1]. As there is no comprehensive theory available that de®nes the sensing requirements for a robotic system, much of the knowledge is drawn from investigation of human sensing, and the analysis of grasping and manipulation. Study of the human sense of touch suggests that creating a gripper incorporating tactile sensing requires a wide range of sensors to fully deter- mine the state of a grip. The detailed speci®cation of a touch sensor will be a function of the actual task it is required to perform. Currently, no general speci®- cation of a touch or tactile sensor exists. Reference 2, though dated, can be used as an excellent basis for de®ning the desirable characteristics of a touch or tactile sensor suitable for the majority of industrial applications: A touch sensor should ideally be a single-point con- tact, though the sensory area can be any size. In practice, an area of 1±2 mm 2 is considered a satis- factory compromise between the dif®culty of fab- ricating a subminiature sensing element and the coarseness of a large sensing element. The sensitivity of the touch sensor is dependent on a number of variables determined by the sensor's basic physical characteristic. In addition the sen- sitivity may also be the application, in particular any physical barrier between the sensor and the object. A sensitivity within the range 0.4 to 10 N, together with an allowance for accidental mechanical overload, is considered satisfactory for most industrial applications. The minimum sensor bandwidth should be 100 Hz. The sensor's characteristics must be stable and repeatable with low hysteresis. A linear response is not absolutely necessary, as information-pro- cessing techniques can be used to compensate for any moderate nonlinearities. As the touch sensor will be used in an industrial application, it will need to be robust and pro- tected from environmental damage. If a tactile array is being considered, the majority of applications can be undertaken by an array 10± 20 sensors square, with a spatial resolution of 1±2 mm. In a dexterous end effector, the forces and relative motions between the grasped object and the ®ngers need to be controlled. This can be achieved by using a set of sensors capable of determining in real time the magnitude, location, and orientation of the forces at the contact point. This problem has been approached by using miniature force and torque sensors inside the ®ngertips, to provide a robot with an equivalent to the kinesthetic sense found in humans. The integration of skinlike and kinesthetic-like sensing will result in robots being equipped with arti®cial haptic perceptions [3]. The study of human touch and the use of perceived information indicates that other variables, such as hardness and thermal properties, can also be mea- sured, and this allows greater ¯exibility in an auto- mated process. Human touch is of considerable complexity, with sensors that respond to a range of stimuli including temperature, pain, acceleration, velo- city, and intensity. The human touch sensors in the skin may have many purposes, but are predominantly protective to prevent self-in¯icted damage to the body. The human touch sense is obtained by a combination of four sensors: a transient load detector, a continuous force sensor, a position transducer to give propriocep- tive data, and an overload sensor (i.e., pain) reacting both to force and other external environmental condi- tions. This combination of sensors is very sensitive, e.g., a ®ne surface texture can be detected, but there is poor spatial resolution; the dif®culty in reading Braille is readily apparent. Humans are very good at learning about an unknown object from touch. The information from the sensors is brought together through the nervous system to give us the sense of feel. It should be noted that the sensory information is processed and interpreted both locally (peripheral nervous system) and centrally (spinal cord and the brain). 1.2.1 Touch Sensor Technology Many physical principles have been exploited in the development of tactile sensors. As the technologies involved are very diverse, this chapter can only con- sider the generalities of the technology involved. In most cases, the developments in tactile sensing technol- ogies are application driven. It should be recognized that the operation of a touch or tactile sensor is very dependent on the material of the object being gripped. 378 Crowder Copyright © 2000 Marcel Dekker, Inc. The sensors discussed in this chapter are capable of working with rigid objects. However, if nonrigid mate- rial is being handled, problems may arise. Work has shown that conventional sensors can be modi®ed to operate with nonrigid materials [4]. 1.2.1.1 Mechanically Based Sensors The simplest form of touch sensor is one where the applied force is applied to a conventional mechanical microswitch to form a binary touch sensor. The force required to operate the switch will be determined by its actuating characteristics and any external constraints. Other approaches are based on a mechanical move- ment activating a secondary device, such as a potenti- ometer or displacement transducer. 1.2.1.2 Resistive-Based Sensors The use of compliant materials that have a de®ned force-resistance characteristics have received consider- able attention in touch and tactile sensor research [5]. The basic principle of this type of sensor is the mea- surement of the resistance of a conductive elastomer or foam between two points. The majority of the sensors use an elastomer that consists of a carbon-doped rub- ber. The resistance of the elastomer changes with the application of force, resulting from the deformation of the elastomer altering the particle density (Fig. 1). If the resistance measurement is taken between opposing surfaces of the elastomer, the upper contacts have to be made using a ¯exible printed circuit to allow move- ment under the applied force. Measurement from one side can easily be achieved by using a dot-and-ring arrangementonthesubstrate(Fig.2).Resistivesensors have also been developed using elastomer cords laid in a grid pattern, with the resistance measurements being taken at the points of intersection. Arrays with 256 elements have been constructed [6]. This type of sensor easily allows the construction of a tactile image of good resolution. The conductive elastomer or foam-based sensor, while relatively simple, does suffer from a number of signi®cant disadvantages: An elastomer has a long nonlinear time constant. In addition the time constant of the elastomer, when force is applied, is different from the time con- Sensors 379 Figure 1 Resistive sensor based on a conductive foam or elastomer. (a) Principle of operation. (b) Normalized resistance against applied force. Copyright © 2000 Marcel Dekker, Inc. stant when the applied force is removed. The force±resistance characteristics of elastomer- based sensors are highly nonlinear, requiring the use of signal-processing algorithms. Due to the cyclic application of forces experienced by a tactile sensor, the resistive medium within the elastomer will migrate over a period of time. Additionally, the elastomer will become perma- nently fatigued, leading to permanent deforma- tion of the sensor. This will give the sensor a poor long-term stability and will require replacement after an extended period of use. Even with the electrical and mechanical disadvan- tages of conductive elastomers and foams, the majority of industrial analog touch or tactile sensors have been based on the principle of resistive sensing. This is due to the simplicity of their design and interface to the robotic system. 1.2.1.3 Force-Sensing Resistor A force-sensing resistor is a piezoresistive conductive polymer, which changes resistance in a predictable manner following application of force to its surface. It is normally supplied as a polymer sheet which has had the sensing ®lm applied by screen printing. The sensing ®lm consists of both electrically conducting and nonconducting particles suspended in a matrix. The particle sizes are of the order of fractions of microns, and are formulated to reduce the temperature dependence, improve mechanical properties and increase surface durability. Applying a force to the surface of a sensing ®lm causes particles to touch the conducting electrodes, changing the resistance of the ®lm. As with all resistive-based sensors the force-sen- sitive resistor requires a relatively simple interface and can operate satisfactorily in moderately hostile envir- onments. 1.2.1.4 Capacitive-Based Sensors The capacitance between two parallel plates is given by C "A d 1 where A is the plate area, d the distance between the plates, and " the permittivity of the dielectric medium. A capacitive touch sensor relies on the applied force either changing the distance between the plates or the effective surface area of the capacitor. In Fig. 3a, the two conductive plates of the sensor are separated by a dielectric medium, which is also used as the elastomer to give the sensor its force-to-capacitance characteristics. 380 Crowder Figure 2 A resistive tactile sensor based on a dot-and-ring approach. (a) (b) Figure 3 (a) Parallel plate capacitive sensor. (b) Principal components of a coaxial force sensor. Copyright © 2000 Marcel Dekker, Inc. To maximize the change in capacitance as force is applied, it is preferable to use a high-permittivity dielectricinacoaxialcapacitordesign.Figure3b shows the cross-section of the capacitive touch trans- ducer in which the movement of one set of the capaci- tor's plates is used to resolve the displacement and hence applied force. The use of a highly dielectric poly- mer such as polyvinylidene ¯uoride maximizes the change in capacitance. From an application viewpoint, the coaxial design is better as its capacitance will give a greater increase for an applied force than the parallel plate design. In both types of sensors, as the size is reduced to increase the spatial resolution, the sensor's absolute capacitance will decrease. With the limitations imposed by the sensitivity of the measurement tech- niques, and the increasing domination of stray capa- citance, there is an effective limit on the resolution of a capacitive array. To measure the change in capacitance, a number of techniques can be, the most popular is based on the use of a precision current source. The charging character- istic of the capacitive sensor is given by I Cdv dt "A d dV dt 2 hence, the voltage across the sensor over a period of time is de®ned as dV Idtd "A 3 As the current source, I, and sampling period, dt, are de®ned, the capacitance and hence the applied force can be determined [7]. A second approach is to use the sensor as part of a tuned or LC circuit, and mea- sure the frequency response. Signi®cant problems with capacitive sensors can be caused if they are in close proximity with the end effector's or robot's earthed metal structures, as this leads to stray capacitance. This can be minimized by good circuit layout and mechanical design of the touch sensor. It is possible to fabricate a parallel plate capacitor on a single silicon slice [8]. This can give a very compact sensing device; this approach is discussed in Sec. 1.2.1.10. 1.2.1.5 Magnetic-Based Sensor There are two approaches to the design of touch or tactile sensors based on magnetic transduction. Firstly, the movement of a small magnet by an applied force will cause the ¯ux density at the point of mea- surement to change. The ¯ux measurement can be made by either a Hall effect or a magnetoresistive device. Second, the core of the transformer or inductor can be manufactured from a magnetoelastic material that will deform under pressure and cause the magnetic coupling between transformer windings, or a coil's inductance, to change. A magnetoresistive or magne- toelastic material is a material whose magnetic charac- teristics are modi®ed when the material is subjected to changes in externally applied physical forces [9]. The magnetorestrictive or magnetoelastic sensor has a number of advantages that include high sensitivity and dynamic range, no measurable mechanical hyster- esis, a linear response, and physical robustness. If a very small permanent magnet is held above the detection device by a compliant medium, the change in ¯ux caused by the magnet's movement due to an applied force can be detected and measured. The ®eld intensity follows an inverse relationship, leading to a nonlinear response, which can be easily linearized by processing. A one-dimensional sensor with a row of 20 Hall-effect devices placed opposite a magnet has been constructed [10]. A tactile sensor using magneto- elastic material has been developed [11], where the material was bonded to a substrate, and then used as a core for an inductor. As the core is stressed, the material's susceptibility changes; this is measured as a change in the coil's inductance. 1.2.1.6 Optical Sensors The rapid expansion of optical technology in recent years has led to the development of a wide range of tactile sensors. The operating principles of optical- based sensors are well known and fall into two classes: Intrinsic, in which the optical phase, intensity, or polarization of transmitted light are modulated without interrupting the optical path Extrinsic, where the physical stimulus interacts with the light external to the primary light path. Intrinsic and extrinsic optical sensors can be used for touch, torque, and force sensing. For industrial appli- cations, the most suitable will be that which requires the least optical processing. For example, the detection of phase shift, using interferometry, is not considered a practical option for robotic touch and force sensors. For robotic touch and force-sensing applications, the extrinsic sensor based on intensity measurement is the most widely used due to its simplicity of construction and the subsequent information processing. The poten- tial bene®ts of using optical sensors can be summarized as follows: Immunity to external electromagnetic interference, which is widespread in robotic applications. Sensors 381 Copyright © 2000 Marcel Dekker, Inc. Intrinsically safe. The use of optical ®ber allows the sensor to be located some distance from the optical source and receiver. Low weight and volume. Touch and tactile optical sensors have been developed using a range of optical technologies: Modulating the intensity of light by moving an obstruction into the light path. The force sensitiv- ity is determined by a spring or elastomer. To prevent crosstalk from external sources, the sen- sor can be constructed around a deformable tube, resulting in a highly compact sensor (Fig. 4a). A design approach for a re¯ective touch sensor is shown in Fig. 4b, where the distance between the re¯ector and the plane of source and the detector is the variable. The intensity of the received light is a function of distance, and hence the applied force. The U-shaped spring was manufactured from spring steel, leading to a compact overall design. This sensor has been successfully used in an anthropomorphic end effector [12]. A re¯ective sensor can be con- structed with source±receiver ®ber pairs embedded in a solid elastomer structure. As shown in Fig. 4c, above the ®ber is a layer of clear elastomer topped with a re¯ective silicone rubber layer. The amount of light re¯ected to the receiver is determined by an applied force that changes the thickness of the clear elastomer. For satisfactory operation the clear elastomer must have a lower compliance than the re¯ective layer. By the use of a number of transmitter±recei- ver pairs arranged in a grid, the tactile image of the contact can be determined [13]. 382 Crowder (a) (b) (c) Figure 4 (a) Optical touch sensor based on obstructing the light path by a deformable tube. (b) Optical re¯ective touch sensor. (c) Optical re¯ective sensor based on two types of elastomer. Copyright © 2000 Marcel Dekker, Inc. Photoelasticity is the phenomenon where stress or strain causes birefringence in optically transpar- ent materials. Light is passed through the photo- elastic medium. As the medium is stressed, it effectively rotates the plane of polarization, and hence the intensity of the light at the detector changes as a function of the applied force [14]. A suitable sensor is discussed in Section 1.2.2.2. A change in optical density occurs at a boundary, and determines if total internal re¯ection may occur. As shown in Fig. 5, an elastomer mem- brane is separated by air from a rigid translucent medium that is side illuminated. If the elastomer is not in contact with the surface, total internal re¯ection will occur and nothing will be visible to the detector. However, as the membrane touches the top surface of the lower medium, the bound- ary conditions will change, thus preventing total internal re¯ection, and the light will be scattered. Hence an image will be seen by the detector. The generated image is highly suitable for analysis by a vision system [15]. 1.2.1.7 Optical-Fiber-Based Sensors In the previous section, optical ®bers were used solely for the transmission of light to and from the sensor; however, tactile sensors can be constructed from the ®ber itself. A number of tactile sensors have been developed using this approach. In the majority of cases either the sensor structure was too big to be attached to the ®ngers of a robotic hand or the opera- tion was too complex for use in the industrial environ- ment. A suitable design can be based on internal-state microbending of optical ®bers. Microbending is the process of light attenuation in the core of ®ber where a mechanical bend or perturbation (of the order of few microns) is applied to the outer surface of the ®ber. The degree of attenuation depends on the ®ber para- meters as well as radius of curvature and spatial wave- length of the bend. Research has demonstrated the feasibility of effecting microbending on an optical ®ber by the application of a force to a second ortho- gonal optical ®ber [16]. One sensor design comprises four layers of ®bers, each layer overlapping orthogon- ally to form a rectangular grid pattern. The two active layers are sandwiched between two corrugation layers, where the ®bers in adjcent layers are slightly staggered from each other for better microbending effect. When the force is applied to a ®ber intersection, microbe- nding appears in the stressed ®bers, attenuating the transmitted light. The change in the light intensity pro- vides the tactile information. 1.2.1.8 Piezoelectric Sensors Although quartz and some ceramics have piezoelectric properties, polymeric materials that exhibit piezoelec- tric properties are suitable for use as touch or tactile sensors; polymers such as polyvinylidene ¯uoride Sensors 383 Figure 5 Optical boundary sensor. Copyright © 2000 Marcel Dekker, Inc. (PVDF) are normally used [17]. Polyvinylidene ¯uor- ide is not piezoelectric in its raw state, but can be made piezoelectric by heating the PVDF within an electric ®eld. Polyvinylidene ¯uoride is supplied as sheets between 5 mm and 2 mm thick, and has good mechan- ical properties. A thin layer of metalization is applied to both sides of the sheet to collect the charge and permit electrical connections to be made. In addition it can be molded, hence PVDF has number of attrac- tions when considering tactile sensor material as an arti®cial skin. As a sensing element the PVDF ®lm acts as a capa- citor on which charge is produced in proportion to the applied stress. The charge developed can be expressed in terms of the applied stress, r 1 ; 2 ; 3 T , the piezoelectric constant, d d 1 ; d 2 ; d 3 T , and the surface area, giving q A Á r 4 The piezoelectric touch transducer is most often used in conjunction with a charge ampli®er; this results in an output voltage that is proportional to the applied stress. Using a high-impedance ®eld-effect transistor (FET) input ampli®er (Fig. 6), the ampli®er's output voltage is given by v dq dt R f AR f d Á dr dt 5 which can be calibrated to give a force measurement. The piezoelectric sensors are essentially dynamic, and are not capable of detecting static forces. In prac- tice their use is restricted to specialist applications such as slip and texture detection. The use of PVDF in piezoelectric sensors causes dif®culty in scanning an array of sensing elements, as PVDF exhibits pyroelec- tric effects. Therefore some applications require a reference sensor of unstressed PVDF to allow the separation of the piezoelectric effect from the pyroelec- tric signal. 1.2.1.9 Strain Gages in Tactile Sensors A strain gage, when attached to a surface, will detect the change in length of the material as it is subjected to external forces. The strain gage is manufactured from either resistive elements (foil, wire, or resistive ink) or from semiconducting material. A typical resistive gage consists of a resistive grid bonded to an epoxy backing ®lm. If the strain gage is prestressed prior to the appli- cation of the backing medium, it is possible to measure both tensile and compressive stresses. The semicon- ducting strain gage is fabricated from a suitably doped piece of silicone; in this case the mechanism used for the resistance change is the piezoresistive effect [18]. When applied to robotic touch applications, the strain gage is normally used in two con®gurations: as a load cell, where the stress is measured directly at the point of contact, or with the strain gage positioned within the structure of the end effector. 1.2.1.10 Silicon-Based Sensors Technologies for micromachining sensors are currently being developed worldwide. The developments can be directly linked to the advanced processing capabilities of the integrated circuit industry, which has developed fabrication techniques that allow the interfacing of the nonelectronic environment to be integrated through microelectromechanical systems [19]. Though not as dimensionally rigorous as the more mature silicon pla- nar technology, micromachining is inherently more complex as it involves the manufacture of a three- dimensional object. Therefore the fabrication relies on additive layer techniques to produce the mechanical structure. 384 Crowder (a) (b) Figure 6 PVDF touch sensor. (a) De®nition used in the polarization of PVDF ®lm. (b) Equivalent circuit of a sensor. Copyright © 2000 Marcel Dekker, Inc. The excellent characteristics of silicon, which have made micromachined sensors possible, are well known [20], and include a tensile strength comparable to steel, elasticity to breaking point, very little mechanical hys- teresis in devices made from a single crystal, and a low thermal coef®cient of expansion. To date it is apparent that microengineering has been applied most successfully to sensors. Some sensor applications take advantage of the device-to-device or batch-to-batch repeatability of wafer-scale processing to remove expensive calibration procedures. Current applications are restricted largely to pressure and acceleration sensors, though these in principle can be used as force sensors. As the structure is very delicate, there are still problems in developing a suitable tactile sensor for industrial applications [21]. 1.2.1.11 Smart Sensors The most sign®icant problem with the sensor systems discussed so far is that of signal processing. Researchers are therefore looking to develop a com- plete sensing system rather than individual sensors, together with individual interfaces and interconnec- tions. This allows the signal processing to be brought as close as possible to the sensor itself or integrated with the sensor. Such sensors are generally termed smart sensors. It is the advances in silicon fabrication techniques which have enabled the recent develop- ments in smart sensors. There is no single de®nition of what a smart sensor should be capable of doing, mainly because interest in smart sensors is relatively new. However, there is a strong feeling that the mini- mum requirements are that the sensing system should be capable of self-diagnostics, calibration, and testing. As silicon can be machined to form moving parts such as diaphragms and beams, a tactile sensor can, in prin- ciple, be fabricated on single piece of silicon. Very little commercial success has been obtained so far, largely due to the problems encountered in transferring the technology involved from the research laboratory to industry. In all tactile sensors there is a major problem of information processing, and interconnection. As an n Ân array has 2n connections and individual wires, any reduction in interconnection requirements is welcomed for ease of construction and increased reliability. A number of researchers have been addressing the pro- blem of integrating a tactile sensor with integral signal processing. In this design the sensor's conductive elas- tomer sheet was placed over a substrate. The signi®- cant feature of this design is that the substrate incorporates VLSI circuitry so that each sensing ele- ment not only measures its data but processes it as well. Each site performs the measurements and proces- sing operations in parallel. The main dif®culty with this approach was the poor discrimination, and sus- ceptibility to physical damage. However, the VLSI approach was demonstrated to be viable, and alle- viated the problems of wiring up each site and proces- sing the data serially. 1.2.1.12 Multistimuli Touch Sensors It has been assumed that all the touch sensors dis- cussed in this section respond only to a force stimu- lus. However, in practice most respond to other external stimuli, in particular, temperature. If PVDF has to be used as a force sensor in an environment with a widely varying ambient temperature, there may be a requirement for a piece of unstressed PVDF to act as a temperature reference. It is possible for a sensor to respond both to force and temperature changes. This has a particular use for object recogni- tion between materials that have different thermal conductivity, e.g., between a metal and a polymer [22]. If the complexity of the interpretation of data from PVDF is unsuitable for an application, touch sensors incorporating a resistive elastomer for force, and thermistors for temperature measurement can be constructed. By the use of two or more force-sensitive layers on the sensor, which have different character- istics (e.g., resistive elastomer and PVDF), it is possi- ble to simulate the epidermal and dermal layers of human skin. 1.2.2 Slip Sensors Slip may be regarded as the relative movement of one object's surface over another when in contact. The relative movement ranges from simple translational motion to a combination of translational and rota- tional motions. When handling an object, the detec- tion of slip becomes necessary so as to prevent the object being dropped due to the application of a low grip force. In an assembly operation, it is possible to test the occurrence of slip to indicate some predeter- mined contact forces between the object and the assembled part. For the majority of applications some qualitative information on object slip may be suf®cient, and can be detected using a number of different approaches. Sensors 385 Copyright © 2000 Marcel Dekker, Inc. 1.2.2.1 Interpretation of Tactile-Array Information The output of a tactile-sensing array is the spatial dis- tribution of the forces over the measurement area. If the object is stationary, the tactile image will also remain stationary. However, if the pattern moves with time, the object can be considered to be moving; this can be detected by processing the sensor's data. 1.2.2.2 Slip Sensing Based on Touch-Sensing Information Most point-contact touch sensors are incapable of dis- crimination between relative movement and force. However, as the surfaces of the tactile sensor and the object are not microscopically smooth, the movement of an object across the sensor will cause a high-fre- quency, low-amplitude vibration to be set up, which can be detected and interpreted as movement across the sensor. This has been achieved by touch sensors based on the photoelastic effect [23] and piezoelectric [24] sensors. In a photoelastic material the plane of polarization is a function of the material stress. Figure7ashowsasensor,developedatthe University of Southampton, to detect slip. The sensor uses the property of photoelastic material, where the plane of the material's polarization is rotated as the material is stressed. In the sensor, light is ®rst passed through a polarizing ®lm (polarizer), the material, then a second polarizing ®lm (analyzer). As the stress applied to the material changes, the amount of received light varies. Typical results are shown in Fig. 7b; the changes in stress are caused by vibrations due to the photoelastic material slipping±sticking as the object moves relative to the sensor. The sensitivity of the sensor can be increased by arti®cially roughening the surface area of the sensor. In addition to slip detection, the infor- mation from the sensor can be used to determine infor- mation about the surface roughness of the gripped object by measurement of the vibration characteristics. 1.2.2.3 Sensors to Speci®cally Detect Slip It is possible to develop sensors that will respond only to relative movement. They are normally based on the principle of transduction discussed for touch sensors, but the sensors' stimulus comes from the relative movement of an area of the gripper. Several methods to detect slip have been reported. One sensor requries a sapphire needle protruding from a sensor surface to touch the slipping object; this gen- erates vibrations which in turn stimulate a piezoelectric crystal. The disadvantage of this approach is that it picks up external vibrations from the gripper and robot mechanics, and the needle frequently wears out. The improved version of this sensor uses a steel ball at the end of the probe, with the piezoelectric crystal replaced by a permanent magnet and a coil enclosed in a damping medium. To avoid the problem of interference signals from external vibrations, a range of interrupt-type slip sensors have been designed. In one design, a rubber roller has a permanent magnet passing over a magnetic head which generates a vol- tage when slip occurs. In a similar design the roller has a number of slits which interrupt an optical path; this allows an indication of slip to be obtained. Though these sensors give a very good indication of the speed and direction of slip there are disadvantages with poor slip resolution and the possibility of jamming of the roller. 1.2.3 Summary This section has discussed the technology available for touch, tactile, and slip sensors. In the interpreta- tion of a sensor's information, consideration should be taken of its design and use. One aspect that is often overlooked is the mechanical ®ltering of the sensory information caused by the sensor's protective covering material. Work has shown [25] that a cover of as little as 0.2 mm thick will degrade a sensor that is required to have a spatial resolution of less than 1mm.AsshowninFig.8,apointcontactisdiffused so that a number of sensors are stimulated. The degree of ®ltering is a function of the covering mate- rial, its thickness, and its physical properties, and requires the use of ®nite-element tecniques to be analyzed fully. For any application the characteristics of a number ofsensorsmayneedtobecompared.Table1presents a summary of the major advantages and disadvan- tages, allowing this comparison to be made between the transduction techniques. 1.3 FORCE AND TORQUE MEASUREMENT As noted earlier, touch sensors operate at the point of contact. If, however, it is required to measure the glo- bal forces and torques being exerted on an object by a robotic system, a multiaxis force measurement system is needed. If an object ®xed in space is considered and 386 Crowder Copyright © 2000 Marcel Dekker, Inc. [...]... called the iris The retina, as shown in Fig 2, consists of about 1 25 million light-sensitive receptors that, because of the many-to-one connections, have some processing capabilities These receptors consist of color-sensitive ``cones'' and brightness-sensitive ``rods.'' The central part of the retina is called the fovea It contains a dense cluster of between six and seven million cones that are sensitive... is already generating millions of dollars per year in thousands of successful applications Machine vision is becoming established as a useful tool for industrial automation, where the goal of 100% inspection of manufactured parts during production is becoming a reality The purpose of this chapter is to present an overview of the fundamentals of machine vision A review of human vision is presented ®rst... can be stored in computer memory A common set of gray values might range from 0 to 255 so that the value may be stored in an 8-bit byte Usually 0 corresponds to dark and 255 to white The effects of gray-level quantization are shown in Fig 6, which shows the same image displayed at 1; 2; 4 and 8 bits per pixel The 1-bit, or binary, image shows only two shades of gray, black for 0 and white for 1 The binary... Figure 4 Black-and-white image function Each element of the image f x; y may be called a picture element or pixel The value of the function f x; y is its gray-level value, and the points where it is de®ned are called its domain, window or mask The computer image is always quantized in both spatial and gray-scale coordinates The effects of spatial resolution are shown in Fig 5 The gray-level function... training data This type of learning is frequently referred to as self-organization A particular class of unsupervised learning rule which has been extremely in¯uential is Hebbian learning [12] The Hebb rule acts to strengthen often-used pathways in a network, and was used by Hebb to account for some of the phenomena of classical conditioning Primarily some type of regularity of data can be learned by... functions, in response to the advancement of semiconductor technology, such as progress in system-on-chip con®gurations and wafer-scale integration It may also be possible to realize one-chip intelligent processors for high-level processing, and to combine these with one-chip rather low-level image processors to achieve intelligent processing, such as knowledgebased or model-based processing Based on these... determine the location of the part and then the orientation so that the gripper can be properly aligned to grip the part Machine Vision Fundamentals 2 .5. 3 .5 Bin Picking The most common form of part representation is a bin of parts that have no order While a conveyor belt insures a rough form of organization in a two-dimensional plane, a bin is a three-dimensional assortment of parts oriented randomly... point of light we observe is actually the far-®eld Fraunhoffer diffraction pattern or Fourier transform of the image of the star The twinkling is due to the motion of our eyes The moon image looks quite different, since we are close enough to view the near-®eld or Fresnel diffraction pattern While the most common transform is the Fourier transform, there are also several closely related trans- Figure... astronauts took a photograph of their damaged spacecraft, but it was out of focus Image processing methods allowed such an out -of- focus picture to be put back into focus and clari®ed 2.4.3 dx dy Histograms The simplest types of image operations are point operations, which are performed identically on each point in an image One of the most useful point operations is based on the histogram of an image Figure 11... in Fig 25 The result of this operation is shown in Fig 26 2.4.4.2 Three DimensionalÐStereo The two-dimensional digital images can be thought of as having gray levels that are a function of two spatial variables The most straightforward generalization to three dimensions would have us deal with images having gray levels that are a function of three spatial vari- Figure 23 The horizontal edges of the . (of the order of few microns) is applied to the outer surface of the ®ber. The degree of attenuation depends on the ®ber para- meters as well as radius of curvature and spatial wave- length of. group calledtheiris.Theretina,asshowninFig.2,consists of about 1 25 million light-sensitive receptors that, because of the many-to-one connections, have some processing capabilities. These receptors consist of color-sensitive ``cones''. majority of the sensors use an elastomer that consists of a carbon-doped rub- ber. The resistance of the elastomer changes with the application of force, resulting from the deformation of the elastomer