The preceding proposal suggests that improvement over the glove design for a 6 DOF isotonic device does not necessarily lie in moving operations from the large muscle groups to the smaller ones, but rather in using the small muscle groups in addition to the large ones. Motivated by this hypothesis, an isotonic position control alternative to the glove design, as shown in Figure 4.3, has been designed and implemented. This device has been dubbed the Fball, to reflect its nature of free-moving (isotonic) in 3D space ("flying"), as well as the intention of operation with arm and fingers.
The ball shape was chosen because a symmetrical ball shape can be easily grasped and manipulated by the fingers in all directions. The Fball is designed to be held and moved (rolled) by the fingers, wrist, elbow and shoulder, in postures that have been classified as "precision grasp", as opposed to "power grasp" (Cutkosky and Howe, 1990; MacKenzie and Iberall, 1994) . Precision grasping, while holding objects with the finger tips, puts emphasis on dexterity and sensitivity. In contrast, power grasping, while holding objects against the palm, puts emphasis on security and power. The Fball is also a versatile shape that can be engaged with different types of object shape in a manipulation task.
To take maximum advantage of finger operations, two additional features are desirable. One is to make the ball tetherless, so that the user can roll it between her fingers without interference. The second desirable feature is that the ball be made of an elastic, conductive material, so that the entire ball functions as a button that can be squeezed from any direction. Enabling technology for wireless design is not easily available. The Fball currently uses the Ascension Birdô mounted in the centre of an elastic ball 6 cm in diameter. The cord of the Bird is pointed away from the hand in the null position, so as to maximise the range of rotation without interference from the cord (Figure 4.3).
In the design of the glove (Figure 4.2), the clutch was an essential component. Since the glove requires rotation to be made with the wrist, the elbow and the shoulder, the range of rotation is limited in one movement. Whenever a limit is reached, the user needs the clutch to disengage the manipulated object and restore the hand to a more comfortable posture in order to recommence the manipulation. This is very similar to lifting a 2D mouse and starting from a new position on the mouse pad. We refer to this process as "re-clutching". It is also called "indexing" by some authors (Johnsen and Corliss, 1971) .
Since the Fball can be rotated up to 180 degrees in any direction, however, the clutch becomes unnecessary and has therefore been eliminated from the following experiment. The 3BallTM, manufactured by Polhemus, is a commercial product similar to the Fball design. The limitation of the 3Ball is its fixed button location. In order to access this button, users can not easily roll the ball between their fingers.
Other existing designs of freely moving 6 DOF devices, such as
the "Bat" (Ware, 1990) , the "Cricket" (DIDI, 1993) and the 3D mouse (Logitech, 1991) are similar to
the glove design in assigning wrist, elbow and shoulder muscle
groups for manipulating the six degrees of freedom, but not the
fingers. The "Bat" and "Cricket" are shown
in Figure 1.1 in Chapter 1.