Assessing and increasing Z-width of haptic displays with active electrical damping

The "Z-width" of a haptic display is the dynamic range of impedances that can be passively rendered. Haptic displays with larger Z-width generally render more realistic feeling virtual environments. The maximum passive impedance a haptic display can render is directly related to the quantity of physical damping in the mechanism, as previous research shows. To improve Z-width, I designed a novel linear current amplifier that incorporates an analog circuit to compute the back EMF of the motor, providing a direct velocity sensor in the motor amplifier without the use of an encoder. This analog velocity signal is fed back to the current amplifier providing active electrical damping for the haptic display. Active electrical damping is advantageous because it can be controlled dynamically at the signal level in the motor amplifier, circumventing the need to add mechanical components and complexity. I present the design and construction of a small 1 degree of freedom linear haptic display. It has very low apparent mass of approximately 6 grams, a 15 millimeter range of motion, position sensing resolution at the endpoint of 0.5 micrometers, and a continuous force output of 1.1 Newtons. The haptic display utilizes a low inertia brushless permanent magnet synchronous motor. I take advantage of the small angular range of motion by driving the haptic display using a single motor winding, avoiding the need for commutation. In order to assess the performance of the haptic display, I built an external "impedance probe" to measure the actual behavior of the haptic display. The impedance probe creates a disturbance force and directly measures the resulting velocity, providing a measure of the impedance of a haptic display. Instead of presenting Z-width by plotting virtual stiffness and damping, I develop a new method for illustrating Z-width. I show that plotting the envelope of the passive impedance range of a haptic display as a function of frequency provides additional information to complement traditional stiffness-damping plots. Using the impedance probe and Z-width plots combined with virtual stiffness-damping plots, I show that added active electrical damping improves the performance of the haptic display