Handheld haptic feedback for grasping in virtual reality
Keywords: Mechanical Engineering.

For more realistic and useful interaction in Virtual Reality (VR), haptic feedback with natural gestures is necessary because it plays a large role in conjunction with visual feedback when interacting with objects. While recent advances in technology have made commercial Head-Mounted Displays (HMDs) available to consumers at a low cost, the user interfaces are still limited to conventional controllers and existing styles of gestural input. It is desirable to allow users to interact as realistically in VR as they interact with objects in the real world. To develop a haptic user interface for VR with HMDs, wearability and mobility are extremely important. Traditional kinesthetic haptic interfaces are fixed externally in a room and thus have a limited workspace. Handheld or wearable haptic interfaces can address this fundamental challenge of limited workspace. However, achieving rich haptic sensations with ungrounded haptic devices is especially challenging due to the following two reasons. First, the selection of an actuator is limited to lightweight and low power actuators due to the constraints of the mobile form factor. Second, it is hard to create kinesthetic force feedback external from a human body because the devices are grounded to the user's body, not to the environment. This thesis proposes two strategies for addressing these challenges. First, the thesis introduces novel force feedback mechanisms using clutches and brakes to make ungrounded haptic devices lightweight, compact, safe, power-efficient, and low cost. Second, it shows it is desirable to simulate kinesthetic haptic sensations through vibrotactile feedback because vibration can be easily implemented with small and light voice coil actuators. In this thesis, I propose five force feedback methods based on these two strategies. The first half of this thesis introduces novel force feedback mechanisms making handheld or wearable haptic devices compact, safe, power-efficient, and low cost. First, the thesis introduces a unidirectional brake mechanism to generate a rigid stiffness with a lightweight form factor. The self energizing effect of the unidirectional brake enables a high dynamic range of force with low power consumption and does not require a force sensor to release. After evaluating the brake's performance, the brake mechanism is integrated into a wearable haptic interface, named Wolverine. Second, after addressing the strengths and weaknesses of the unidirectional brake, the thesis introduces a clutch-equipped hybrid actuation system to utilize compact geared motors. Geared motors create high force in a compact form factor but also involve large inertia, damping, and friction. The proposed system is composed of a geared motor and two unidirectional clutches in series. By selectively engaging the end-effector to the geared motor or the handle grip, the system improves the performance of force feedback in terms of rendering free space and solid objects. Third, a soft and high force density linear brake utilizing layer jamming is introduced. It functions as a compact brake module by dissipating mechanical energy through coulomb friction. The braking force is controllable by changing the vacuum pressure inside a flexible and extensible enclosure. The thesis introduces a dynamic model to calculate the tension force of the brake and experimentally verifies the model. The second half of the thesis explores effects of vibrotactile feedback on human perception to improve perceived kinesthetic haptic feedback of handheld or wearable haptic devices. First, the thesis investigates the effect of transient vibration for augmenting perceived softness in the context of VR interaction with haptic proxy objects and brake-based haptic devices. Two studies show that active transient vibration added to rigid kinesthetic force feedback changes users' softness perception. By changing the frequency of the transient vibration to be lower or higher than the natural frequency occurring from the actual impact in the mechanism, the transient vibration can simulate softer or harder objects. Second, the thesis investigates the effect of asymmetric vibration to create an illusory kinesthetic force for simulating the sensation of weight. Two user studies show that users feel illusory weight sensations with a maximum force of 0.2N. The amount of the weight sensation is controllable by changing the amplitude of the asymmetric vibration. Two voice coil actuators creating asymmetric vibration are integrated into a wearable haptic device for grasping, named Grabity.