[Table of Content] [Appendices] [Abstract] Summary [Chapter 1] [Chapter 2] [Chapter 3] [Chapter 4] [Chapter 5] [Chapter 6]

Human Performance in Six Degree of Freedom Input Control

Shumin Zhai, Ph.D.


Progress in technology is continuously expanding the design space for 6 degree-of-freedom (6 DOF) human machine interfaces. It is both theoretically and practically necessary to understand how users' performance relates to various design dimensions for 6 DOF interfaces. This thesis investigates human performance in relation to the following critical issues: controller resistance in terms of isotonic, elastic, and isometric devices; transfer function in terms of zero order position control versus first order rate control; the size and the shape of the 6 DOF device that determine joints and muscle groups used, such as shoulder, elbow, wrist, and fingers; display methods to reveal users' input control action in relation to target objects in 3D, in particular stereopsis and partial occlusion through semi-transparency.

The methodology of this thesis is a pragmatic combination of analysis, literature review and experimentation. The cited design dimensions are analysed in terms of human proprioception and control feel (corresponding to device resistance), mental processing in forming control actions (corresponding to transfer function), motor and sensory cortex representation (corresponding to muscles and joints) and the nature of various visual depth cues (corresponding to display formats). The related literature in engineering psychology, human motor control, manual tracking and human computer interaction is also reviewed.

A series of five experiments are presented. Experiment 1 examined four 6 DOF manipulation techniques, based on a combination of isotonic versus isometric resistance modes and position control versus rate control transfer functions. A strong interaction was found between the resistance mode and the transfer function. In the position control mode, subjects had shorter mean completion times with the isotonic device than with the isometric device. In rate control mode, the relative advantage of isotonic and isometric device was reversed. This interaction pattern is the result of the compatibility between paired actions required in rate control and the self-centring effect of isometric devices.

Experiment 2 studied isometric versus elastic devices in rate control mode. Two factors, proprioception and compatibility, were identified as opposing properties in the design of elastic devices for rate control. As the stiffness of elasticity increases, the self-centring effect increases accordingly, hence enhancing the compatibility with rate control. On the other hand, stiff elastic controllers allow less movement, hence reducing proprioceptive feedback due to movement. In a 6 DOF docking task, Experiment 2 revealed that, if optimised between the two factors, the elastic device had performance advantages over the isometric device, but only at the early stage of learning. Experiment 3 pursued the same issue as in Experiment 2, but with a more demanding task: 6 DOF tracking. A more substantial difference was found between the two types of controllers but the general trend was the same as in Experiment 2: the elastic device was easier to learn than the isometric device, but the performance difference decreased as practice progressed. Consistent with many human motor control theories, the results of Experiment 2 and Experiment 3 imply that the basis of human motor skills shifts from closed-loop, feedback driven behaviour to open-loop, motor-program driven behaviours.

A detailed analysis of Experiment 3 results revealed interesting user strategies in 6 DOF tracking. In a rather consistent priority order, subjects tended to concentrate on fewer degrees of freedom at a time during early learning stages and progressed to cope with more degrees of freedom together during later learning stages. Between horizontal, vertical, and depth dimensions, the horizontal dimension appears to take attentional priority. Between translation aspects and rotation aspects, translation appears to take higher priority. It was found that after 40 minutes of practice more than 80% percent of the subjects were able to control all 6 DOF simultaneously.

The issue of which joints and muscle groups should be used for 6 DOF manipulation was studied in Experiment 4. Two isotonic position control techniques were tested in a 6 DOF docking task. One technique utilised the user's wrist, elbow and shoulder, while the other technique made use additionally of the user's fingers. The results showed that the participation of fingers significantly improved the task performance.

Experiment 5 investigated the issues of visual representation formats of users' input control actions in relation to the target object. In a 3D dynamic target acquisition task, it was found that both binocular disparity and partial occlusion through semi-transparency, a rather novel graphic technique, were beneficial. In particular, the use of semi-transparent surfaces appeared to enhance human performance in discrete tasks more than the classical stereoscopic viewing technique.

The experimental and analytical studies in this thesis significantly contribute to the understanding of human factors in 6 DOF manipulation. The highlights of the results can be summarised briefly as: (1) The physical properties of a 6 DOF input device should provide rich feedback so that the user can easily feel her control actions proprioceptively and thus learn the task quickly. (2) To the extent possible, fine small muscle groups and joints (fingers) should be included in the operation of input devices. (3) The transfer function used to interface a device with the computer should be compatible with the characteristics of the physical device. (4) The visual representation of the user's actions in relation to target object should be designed to allow immediate exteroceptive feedback and the application of semi-transparency serves this purpose well.