Introduction to Haptics

Haptics refers to the modality of touch and associated sensory feedback. Researchers working in the area are concerned with the development , testing, and refinement of tactile and force feedback devices and supporting software that permit users to sense (" feel ") and manipulate three-dimensional virtual objects with respect to such features as shape, weight, surface textures, and temperature. In addition to basic psychophysical research on human haptics, and issues in machine haptics such as collision detection, force feedback, and haptic data compression, work is being done in application areas such as surgical simulation, medical training, scientific visualization, and assistive technology for the blind and visually impaired. How can a device emulate the sense of touch? Let us consider one of the devices from SensAble Technologies. The 3 DOF (degrees-of-freedom) PHANToM is a small robot arm with three revolute joints, each connected to a computer-controlled electric DC motor. The tip of the device is attached to a stylus that is held by the user. By sending appropriate volt-2 Introduction to Haptics Chapter 1 ages to the motors, it is possible to exert up to 1.5 pounds of force at the tip of the stylus, in any direction. The basic principle behind haptic rendering is simple: Every millisecond or so, the computer that controls the PHANToM reads the joint encoders to determine the precise position of the stylus. It then compares this position to those of the virtual objects the user is trying to touch. If the user is away from all the virtual objects, a zero voltage is sent to the motors and the user is free to move the stylus (as if exploring empty space). However, if the system detects a collision between the stylus and one of the virtual objects, it drives the motors so as to exert on the user's hand (through the stylus) a force along the exterior normal to the surface being penetrated. In practice, the user is prevented from penetrating the virtual object just as if the stylus collided with a real object that transmits a reaction to the user's hand. Different haptic devices—such as Immersion Corporation's CyberGrasp—operate under the same principle but with different mechanical actuation systems for force generation. Although the basic principles behind haptics are simple, there are significant technical challenges, such as the construction of the physical devices (cf. Chapter 4), real-time collision detection (cf. Chapters 2 and 5), simulation …