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4 Haptic Interfaces
Pages 161-187

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From page 161...
... Such haptic interactions may or may not be accompanied by the stimulation of other sensory modalities, such as vision and audition. Computer keyboards, mice, and trackballs constitute relatively simple haptic interfaces.
From page 162...
... It is quite likely that much greater immersion in a NIE can be achieved by the synchronous operation of even a simple haptic interface with a visual and auditory display, than by large improvements in, say, the fidelity of the visual display alone. Real environments or VEs in which one is deprived of the touch and feel of objects seem impoverished, seriously handicap human interaction capabilities, and, at worst, can be disorienting.
From page 163...
... The mechanical loading on the skin, the transmission of the mechanical signals through the skin, and their transduction by the cutaneous mechanoreceptors are all strongly dependent on the mechanical properties of the skin and subcutaneous tissues (Phillips and Johnson, 1981b; Srinivasan, 1989; Srinivasan and Dandekar, 1992~. Tactual sensory information from the hand in contact with an object can be divided into two classes: (1)
From page 164...
... In experiments involving lifting of objects held in a pinch grasp, it has been shown that motor actions such as increasing grip force are initiated as rapidly as within 70 ms after an object begins to slip relative to the finger pad, and that the sensory signals from the cutaneous afferents are critical for task performance (Iohansson and Westling, 1984; lohansson and Cole, 1992~. Clearly, the mechanical properties of the skin and subcutaneous tissues, the rich sensory information provided by a wide variety of sensors that monitor the tasks continuously, and the coupling of this information with the actions of the motor system are responsible for the human abilities of grasping and manipulation.
From page 165...
... In contrast, for the detection of surface texture or slip, tactile information is fundamental, whereas kinesthetic information is supplementary (Srinivasan et al., 1990~. Here, the sensing of spatiotemporal force diskibution within the contact region provides the basis for inferences concerning the contact conditions and object properties.
From page 166...
... The source of kinesthetic information that enables us to know the relative positions of limb segments or joint angles is still controversial (Clark and Horch, 1986~. Initially, it was proposed that the receptors in the joints were the source (Skoglund, 1956; Mountcastle and Powell, 1959~.
From page 167...
... Overall contact force is probably the single most important variable that determines both the neural signals in the sensory system as well as the control of contact conditions through motor action. It appears that the IND for contact force is 5-15 percent of the reference force value over a wide range of conditions involving substantial variation in force magnitude, muscle system, and experimental method, provided that the kinesthetic sense is involved in the discrimination task (Iones, 1989; Pang et al., 1991; Tan et al., 19921.
From page 168...
... However, when tactile information was blocked using local anesthesia, this ability deteriorated significantly because the subjects could not sense contact conditions such as the occurrence of slip and hence did not apply appropriate compensating grasp forces. Thus, good performance in tasks involving contact requires the sensing of appropriate forces as well as using them to control contact conditions.
From page 169...
... have revealed another class of slowly adapting affexents that are primarily sensitive to skin stretch and are associated with Ruffini endings. The response properties, such as thresholds and bandwidths of each of the receptor types obtained through neurophysiological experiments, give some of the design specifications for tactile display part of haptic interfaces.
From page 170...
... When the surface is rigid, kinesthetic information is necessary for discrimination, and the discriminability is much poorer than that for objects with deformable surfaces. For deformable objects with rigid surfaces held in a pinch grasp, the IND for compliance is about 5-15 percent when the displacement range is fixed, increases to 22 percent when it is roved (varied randomly)
From page 171...
... Spatial resolution on the finger pad, as measured by the localization threshold of a point stimulus is about 0.15 mm, whereas the two point limen is about 1 mm. Human detection thresholds for features on a smooth glass plate are a 2 ~ high single dot and a 0.06 ,u high grating.
From page 172...
... . Classification of Haptic Interfaces A primary classification of haptic interactions with real environments or VEs that affects interface design can be summarized as follows: (1)
From page 173...
... If these forces are self-equilibrating, as in simulating the contact forces that occur when we squeeze an object, then the interface need not be mechanically grounded. However, if the forces are unbalanced, as in pressing a virtual object with a single finger pad, the equilibrium of the interface requires that it be attached somewhere.
From page 174...
... The consequences of using a body-based display to simulate contact forces that really stem from ground-based sources are not known and warrant investigation. A further example of improperly grounded displays occurs with most tactile stimulators.
From page 175...
... (3) Tactile displays · shape changers - shape memory actuators pneumatic actuators microelectromechanical actuators · vibrotactile · electrotactile Joysticks are probably the oldest of these technologies and were originally conceived to control aircraft.
From page 176...
... PERForce is a 6-DOF hand controller that delivers high performance (Cybernet Systems, 1992~. The PHANToM, built in the MIT Artificial Intelligence Laboratory, is a multilink, low-inertia device that can convey the feel of virtual objects (Massie and Salisbury, 1994~.
From page 177...
... Rigid link exoskeletons that provide force reflection in addition to joint angle sensing have also been designed and built. Shimoga (1992)
From page 178...
... , the software necessary to generate factual images can be classified into three major groups: haptic interaction software, physical models of virtual objects and environments, and software for rendering factual images. Haptic interaction software mainly consists of reading the state of the haptic interface device.
From page 179...
... Because force displays are prone to mechanical instabilities and human users are sensitive to even low disturbances unrelated to the task, real-time control of the interface devices needs to be of high quality. In the robotics and teleoperation literature (Chapter 9)
From page 180...
... Substantial theoretical research is currently being pursued in the areas of multivariable control and advanced nonlinear techniques, such as adaptive and robust control. Summary of Current Technology and Future Possibilities Computer keyboards, mice, and trackballs are the simplest haptic interfaces and are being widely used to interact with computers.
From page 181...
... To determine the nature of these approximations or, in other words, to find out what we can get away with in creating synthetic haptic experiences, quantitative human studies are essential. Basic understanding of the biomechanical, sensorimotor, and cognitive abilities of the human haptic system is critical for proper design specification of the hardware and software of haptic interfaces.
From page 182...
... Human Haptics As mentioned above, the biomechanical, sensorimotor, and cognitive abilities of humans set the design specifications for devices. Therefore, multidisciplinary studies involving biomechanical and psychophysical experiments together with computational models for both are needed in order to have a solid scientific basis for device design.
From page 183...
... (5) Theoretical developments concerning informationfitow: Theoretical developments concerning the task-specific flow of sensory information and control of motor action are needed to generate testable hypotheses on our haptic interactions with both real environments and VEs.
From page 184...
... To fully match human haptic sensory capabilities, the tactile or force displays should have a bandwidth of about 1 kHz, whereas the signals representing the human motor action need to have a bandwidth of only 10 Hz. In order to prevent false cues to the user, vibrations that are not part of the intended display should have amplitudes less than human detection threshold, which is about 25 m at 0.4 to 3 Hz, 3 m at 30 Hz, 0.3 m at 250 Hz, and is higher for higher frequencies.
From page 185...
... It has been estimated that real-time mechanical interactions with typical finite element models need computational speeds on the order of Gflops (Hunter et al., 1990~. Similar to graphics engines used commonly with visual displays, special computational hardware specifically designed to accelerate the computations needed for haptic displays will become necessary in the near future.
From page 186...
... to fool the human user into believing a less than perfect multimodal display. Techniques such as filtering the user's normal tremor or the use of sensory substitution within a modality (e.g., the use of tactile display to convey kinesthetic information)
From page 187...
... An example of the former is the investigation of the consequences of using an ungrounded display to simulate contact forces that really stem from grounded sources. In the latter question, the word change is to be interpreted in a broad sense and includes modifications of the interface hardware, object models, interaction software, and addition/subtraction of visual or auditory modalities.


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