Variable Stiffness Actuators Towards Passive Haptic Interfaces

Abstract

The main purpose of haptic devices is to enhance the interaction between the user and virtual or remote environments through the sense of 'touch', allowing them to perceive information more efficiently. Research has been oriented around the creation and the control of new haptic interfaces. Haptics nowadays are found in numerous applications, starting from joysticks in entertainment devices, to cellphones, medical applications and industrial applications where interaction between user and the virtual or remote environment is needed. The classifications of haptic interfaces go into two folds based on the nature of their actuation system; 'Active' or 'Passive'. Passive haptics is the category of haptic interfaces that incorporate passive actuation systems that either dissipate, redirect or store energy, (i.e. brakes, clutches, springs... etc.). The main advantage of using passive haptic devices lies in their intrinsic stability, which reflects their safety. Two main limitations hinder the capabilities of passive haptic devices. One lies in their inability to generate desired forces in any arbitrary direction and magnitude. The other one lies in their inability to fully simulate the interaction between the virtual tool and the deformable elastic body. The ideal solution for the latter limitation lies in incorporating variable stiffness mechanisms that are able to alter the stiffness of the haptic interface to equate (relate to) the level of stiffness of remote/virtual elastic body. This had become the main driver of this research. In this thesis, three novel passive variable stiffness joints were invented and their design, modeling and characterization for passive haptic interfaces are presented. The design criteria of these joints lie in their ability of moving freely at zero stiffness level, with several ranges of stiffness that can be achieved, taking account of the simplicity of the design, the energy consumption, and the speed of stiffness altering. The three novel achievements are called, Passive Variable Stiffness Joint (pVSJ), Passive Discrete Variable Stiffness Joint (pDVSJ, pDVSJ-II), and the Passive Binary-controlled Variable Stiffness Joint (BpVSJ). The main feature of the pVSJ is its capability to cover a wide-span of stiffness values (0 – 1000 N.m/rad) and the ability to perform infinite deflection at zero stiffness, benefiting from the concept of Variable Lever Mechanism (VLM) and the simple and cheap components. While the main features of the (pDVSJ, and pDVSJ-II), lie in their capability of instantaneous switching between the levels of stiffness at minimum energy consumption benefitting from the novel design of the Cord Grounding Units (CGUs) which are responsible for creating new grounding point altering the effective length of the involved elastic elements. Finally, in the BpVSJ, the concept of Series-Parallel Elastic Actuators (SPEA) had enabled achieving 2n levels of stiffness from (n) elastic elements with instantaneous stiffness level switching capability and low power consumption. In order to evaluate the capabilities of the proposed haptic interfaces to render the stiffness that could be recognized by human users, two series of psychophysical experiments were performed on human subjects. As the proposed devices are totally passive, only active-push cognitive tasks were performed. These tasks consisted of the Relative Cognitive Task (RCT) and Absolute Cognitive Task (ACT), based on literature. The Relative Cognitive Task tests the haptic interface performance by the user's recognition of different levels of the stimuli parameter (for our stiffness, its stiffness) being rendered by the device. The Absolute Cognitive Task tests the haptic interface performance by the user's recognition of different levels of the stimuli parameter and match it with a calibrated mimic device. The two tasks have been conducted on the pDVSJ-II & BpVSJ through 20 users (11 males and 9 females). The results show that the users have recognized the levels of stiffness rendered by pDVSJ-II with an average of 88% in the Relative Cognitive Task and 76% for the Absolute Cognitive Task, while for the BpVSJ, the results gave an average of 97% for the Relative Cognitive Task, and 83% for Absolute Cognitive Task. From these results it can be concluded that the devices are capable to render different levels of stiffness that is recognizable by human users. Furthermore, a qualitative experiment in a teleoperation scenario is presented as a case study to demonstrate the effectiveness of the proposed haptic interfaces and to show how a human can take advantage of stiffness rendering by the proposed device in applications e.g. remote palpation. Only the pDVSJ-II was subjected to the this experiments, and the results showed that the device was capable of successfully providing information about the stiffness of two different objects through the forces acting at the remote site, thus improving the overall telepresence in such applications. In total, this dissertation contributes to the passive haptics research by proposing new concepts of using variable stiffness joints to render force interaction levels in robotics teleoperation and by developing three novel variable-stiffness joints with modeling, design, and experimental validation. Indexing Terms: Variable Stiffness Actuators, Haptic Interfaces, Mechanical Design, Human-Robot Interaction, Psychophysical Experiments

Date of Award	May 2018
Original language	American English
Supervisor	Dongming Gan (Supervisor)