There are several approaches to creating haptic systems. Although they may look drastically different, they all have two important things in common -- software to determine the forces that result when a user's virtual identity interacts with an object and a device through which those forces can be applied to the user. The actual process used by the software to perform its calculations is called haptic rendering. A common rendering method uses polyhedral models to represent objects in the virtual world. These 3-D models can accurately portray a variety of shapes and can calculate touch data by evaluating how force lines interact with the various faces of the object. Such 3-D objects can be made to feel solid and can have surface texture.
The job of conveying haptic images to the user falls to the interface device. In many respects, the interface device is analogous to a mouse, except a mouse is a passive device that cannot communicate any synthesized haptic data to the user. Let's look at a few specific haptic systems to understand how these devices work.
- The PHANTOM® interface from SensAble Technologies was one of the first haptic systems to be sold commercially. Its success lies in its simplicity. Instead of trying to display information from many different points, this haptic device simulates touching at a single point of contact. It achieves this through a stylus which is connected to a lamp-like arm. Three small motors give force feedback to the user by exerting pressure on the stylus. So, a user can feel the elasticity of a virtual balloon or the solidity of a brick wall. He or she can also feel texture, temperature and weight. The stylus can be customized so that it closely resembles just about any object. For example, it can be fitted with a syringe attachment to simulate what it feels like to pierce skin and muscle when giving a shot.
- The CyberGrasp system, another commercially available haptic interface from Immersion Corporation, takes a different approach. This device fits over the user's entire hand like an exoskeleton and adds resistive force feedback to each finger. Five actuators produce the forces, which are transmitted along tendons that connect the fingertips to the exoskeleton. With the CyberGrasp system, users are able to feel the size and shape of virtual objects that only exist in a computer-generated world. To make sure a user's fingers don't penetrate or crush a virtual solid object, the actuators can be individually programmed to match the object's physical properties.
- Researchers at Carnegie Mellon University are experimenting with a haptic interface that does not rely on actuated linkage or cable devices. Instead, their interface uses a powerful electromagnet to levitate a handle that looks a bit like a joystick. The user manipulates the levitated tool handle to interact with computed environments. As she moves and rotates the handle, she can feel the motion, shape, resistance and surface texture of simulated objects. This is one of the big advantages of a levitation-based technology: It reduces friction and other interference so the user experiences less distraction and remains immersed in the virtual environment. It also allows constrained motion in six degrees of freedom (compared to the entry-level Phantom interface, which only allows for three active degrees of freedom). The one disadvantage of the magnetic levitation haptic interface is its footprint. An entire cabinet is required to house the maglev device, power supplies, amplifiers and control processors. The user handle protrudes from a bowl embedded in the cabinet top.
As you can imagine, systems like we've described here can be quite expensive. That means the applications of the technology are still limited to certain industries and specialized types of training. On the next page, we'll explore some of the applications of haptic technology.