| Description |
Traditional haptic displays based on mechanical linkages and electric motors have a fundamental limitation in their ability to render the subtle forces and contact cues of low-friction and low-inertia environments, which are important for virtual training in areas such as microsurgery. An alternative to linkage-based haptic displays is Lorentz-force (Maglev) magnetic haptic displays, which can render high-fidelity forces with low friction, but with an increase in inertia and a reduction in workspace due to their construction. This dissertation describes the design and control of a new class of untethered magnetic haptic displays that are based on systems and techniques that have come from the literature on magnetic manipulation. It is known that in systems designed for magnetic manipulation that comprise stationary electromagnets, eight electromagnets are necessary to ensure that there are no singularities in the workspace. A variety of eight-electromagnet configurations have been proposed to date. The first contribution of this dissertation is a critical comparison of these proposed configurations. A new configuration is then proposed, which does not comprise the level of symmetry seen in prior systems, providing more access than prior systems for a hand-held stylus while still exhibiting an isotropic workspace with comparable performance. In magnetic-manipulation systems, the standard method for achieving some desired systemoutput (e.g., force and torque) is to forma matrix that maps the electromagnets' currents to the resulting output, and the pseudoinverse of that matrix is used to solve for the currents. In the second contribution of this dissertation, this standard methodology is revisited to account for the saturation limits of the amplifiers and power supply, as well as the temperature in the electromagnets, which all limit systemperformance in practice. It is shown that system performance can be improved by increasing both the achievable output magnitudes and the total operating time to achieve those outputs, and the risk of overheating can be eliminated. When a haptic device is manipulated, it must be localized with an update rate of approximately 1 kHz. Linkage-based haptic displays accomplish this using joint sensors (e.g., encoders), which are not available for untethered magnetic haptic displays. The third contribution of this dissertation is a high-speed tracking method that runs at haptic update rates. The method combines computer vision, dynamic region-of-interest cameras, Kalman filtering, and ArUco markers. The fourth contribution of this dissertation is the development of a new class of untethered magnetic haptic display, which combines the first three contributions with additional innovations in electromagnet design and thermal management. The device has a workspace and performance specifications that are targeted at simulation and training for eye surgery. The final contribution of this dissertation is the identification of a new haptic illusion in which underutilized torque actuation capacity in the haptic display can be used to create the illusion of harder virtual surfaces. This illusion is particularly applicable to untethered magnetic haptic displays, since magnetic torque tends to scale better with distance than does magnetic force. |
