Perceptual Decomposition of Virtual Haptic Surfaces

couples the human operator to a source of mechanical power—either electromagnetic, electrohydraulic, or electropneumatic actuators. The computation engine governs the behavior of the actuators and linkage as a function of kinematic and force measurements from interface transducers, according to algorithms and equations that describe the VE models to be simulated. A force reflecting interface thus stimulates the human limb and muscle sense of physical dynamics, and, in most cases, either by default or by design, the tactile sensors in the skin. A distinguishing feature of the haptic channel—unlike VE systems which display visual or aural information and require a physical (typically manual) response—is that the same body part and interface hardware is used to transfer information back and forth between the human and the VE simulation. Consequently, haptic information transfer (i.e. , what the human operator “feels”) is affected not only by the processing capacity of the computation engine and the comprehensiveness of the VE model, but by the controlled dynamics of the interface linkage and of the human limb itself. In this paper, the analysis and construction of virtual haptic surfaces are considered from a perceptual point of view rather than from the dynamics and controls approach of prior work. We developed a perceptual decomposition of surface contact sensation by examining three qualities associated with the different stages of interaction with a haptic wall simulation. These qualities are the crispness of initial contact, the hardness of surface rigidity, and the cleanness of final release from the virtual wall’s surface. These qualities, plus an overall rating of wall quality, were employed consistently by seven subjects to evaluate a set of six simple haptic wall simulations. Three of the wall models consisted of single linear springs; the remainder, single viscous dampers. Highest rankings of subjective hardness were associated with the spring models; damper models received the highest crispness rankings. Subjects favored the simple spring models as having, overall, the more wall-like perceptual character. Because of the instantaneous transitions required when moving from one set of mechanical dynamics in free space to another set when in contact with a surface, and because of the inherent physical properties of the coupled interface and human limb system, high fidelity haptic simulation of surface contact presents a demanding technical challenge in the design of force reflecting VEs. Simple virtual haptic surfaces have been demonstrated in a variety of implementations [1], [7], [9], [12], [13], [18]. Acknowledged shortcomings of haptic wall simulations include: high frequency vibration [15]; low frequency instability [6]; excessive compliance [9], [13], [14], [18]; and stickiness [2]. Jex [8], in reporting on informal industry “rules of thumb” for high-performance aircraft simulators, suggested that the ability to produce convincing walls that are not spongy and do not creep is one of four simple simulations that together demonstrate the capacity of a haptic interface to produce any general simulation.