Abstract

In practice, models almost always have misfit. The misfit of a structural equation model (SEM) can affect the estimation of model parameters in terms of point estimates, standard errors (SEs), and confidence intervals (CIs). In this article, the performance of different methods are compared including: the delta method, the nonparametric bootstrap (NP-B) method, and the semi-parametric bootstrap (SP-B) method in estimating standardized model parameters under the influence of model misfit. Two methods are used to construct the CI of the bootstrap method, the standard percentile method and the bias-corrected and accelerated (BCa) method. A simulation study is conducted using an SEM model with different amounts of model misfit and various sample sizes. The results show that if the model is correctly specified, all methods give correct point estimates, SE estimates, and CI coverage rates. However, as the amount of model misfit increases, the estimates based on NP-B remain accurate and consistent, whose based on the delta method and SP-B deteriorate

Keywords: misspecification, standard error, SEM, standardized parameters

The properties of actuation is absolutely the basic and foundation among all factors that determine the overall performance of a system, and this has been true for all living creatures, and other machines as well. Ever since the first industry robot being invented half a century ago, the robotics field have been developed rapidly, and all sorts of robots are playing more and more important roles in all human production and daily life activities.

The main requirements for robots have been focused on rigidity, high torque and precision motion until recently, as traditionally the major application of robots are along factory production lines, with their working environment well known and controlled, performing jobs that are highly repetitive. To ensure the consistency of production, rigid robots that provides precise motion with high payload capacity are preferred. Thus, position controlled high torque servos have been dominating the industry. These actuators are also combined with force/torque sensors to enable force control capability on robotic systems, but due to various limitations of such setup, it is difficult to realize dynamic motions where impulsive loading is frequently recurrent. During the recent decade, research efforts in the robotics field have been made in the development of Series Elastic Actuators (SEA), seeking for alternative actuation solutions to meet the requirements for dynamic robotic systems. Although there are a hand full of successful cases, the complicity and difficulty down the route is quite obvious. Besides, the respond bandwidth of any SEA is strictly limited by the property of its elastic component, resulting in customization that is very application specific.

The recent rapid development of Proprioceptive Actuators, also know as Quasi-Direct Drive Actuators (or QDD for abbreviation), provides a simple actuation solution that not only features torque control capability, but also provides a very wide response bandwidth as well as inherent compliance. Thus its application on various robotic systems is definitely intriguing and worth of exploration.

The work presented in this dissertation involves the development and discussion of the application of Proprioceptive Actuators on several different dynamic robotic systems, including a three-finger robotic hand called DAnTE, a non-anthropomorphic bipedal robot called NABi-2 and a quadruped robot with very unique kinematics configuration, called ALPHRED-2. Underactuated anthropomorphic fingers are developed and applied on DAnTE, and a potential energy flow theory is developed to assist the development of the mechanical intelligence in these underactuated fingers to realize gesture dexterity. Along with the application of Proprioceptive Actuators on legged robots, an Impact Transfer Factor theory is proposed to quantify the response of a robotic limb to a impact in terms of impact sensing from a whole-system point of view. As outlook of the presented work, an upgraded version of DAnTE as well as an open source kid-size humanoid robot platform are introduced. Both systems are driven by well-packaged Proprioceptive Actuator modules and are expected to possess dexterous force control and high dynamic performance capabilities, and both of them are designed to serve as reliable robot platforms to carry further research in related topics.