Assistive Shopping Trolley Controller Design for the Elderly

Abstract

We are living in an aging society, which is putting an increasingly heavy strain on our healthcare system. As people age many become less mobile, leading to loneliness and a deteriorating health. Subsequently elderly often end up in nursing homes; an experience which is unpleasant as well as expensive. Assisting the elderly with robotic devices can help increase their mobility, therefore reducing healthcare costs and increasing elderly satisfaction. This thesis follows up on the master thesis of R. Berci-Hajnovics, where she sets out to design a novel concept for an assistive, motorized shopping trolley aimed at reducing the physical burden of carrying groceries over uneven terrain. In the current thesis, a control system for the proposed assistive shopping trolley is designed with the goal of attenuating disturbances like road inclination and added mass on the trolley’s dynamic behavior. Predictability of its behavior is considered as well in order to gain the confidence of the elderly user. An analysis of potentially suitable control types for implementation in the assistive shopping trolley has been performed, wherein their characteristics are compared on several different aspects relating to their predictability and disturbance attenuating behavior. Based on this analysis, a controller implementing a so called Disturbance Observer (DOB) is deemed best suited for the job. A DOB uses a dynamic model of the controlled plant to estimate the influence of present disturbances on the plant dynamics. This estimation can consequently be used to produce an opposing force, thereby canceling the effects of the disturbances. Furthermore, a DOB includes a filter which provides additional capabilities, such as robustness to uncertainty within the model and filtering of high-frequency noise in the data to further increase its performance. A DOB is most suitable for the current cause due to its overall adequate performance with respect to the imposed criteria, as well as its relatively high disturbance attenuation accuracy due to its ability to use the plant’s dynamics in making an estimation of the disturbances at play. Following the results of the analysis, this thesis proposes the implementation of a controller including a DOB for application in the assistive shopping trolley. In its design, a model of the trolley dynamics is being used, which provides a basis for the controller’s internal dynamic model. This model is furthermore used as a representation of the trolley plant in a stability analysis of the trolley-controller closed-loop behavior. The model is based on the dynamics of an inverted pendulum on wheels, with an additional constraint on the position of the trolley handle to represent the interaction with the human user. By neglecting minor, nonlinear dynamic effects such as variations in the trolley’s orientation and air drag, the general model is simplified to a linear equation. The controller’s internal model is constructed to represent the trolley’s nominal (desired) behavior, determined to be that of an empty trolley on a flat road, by substituting corresponding nominal values in the parameters of the general model. Subsequently, criteria for robust stability of the trolley-controller’s closed loop behavior are determined using a H_∞-approach; In this approach the system’s behavior is analyzed given the ‘worst-case’ uncertainties. Furthermore, the system’s performance with respect to disturbance estimation and noise filtering is analyzed using its loop transfer function. The insights gained from these analyses are used in the design of the controller and to make recommendations for future work. The obtained controller is implemented in the Robotic Operating System (ROS) framework; an open-source robotics platform which makes use of a decentralized system of processes, simplifying the creation and sharing of complicated software structures across a wide variety of applications. For this purpose, the controller is subdivided into multiple elements, each performing a specific function. These elements, called ‘nodes’, are submitted to a variety of unit tests to verify their proper implementation. The controller’s performance is put to the test in simulation using ROS’s accompanying simulation engine, ‘Gazebo’. The controlled trolley’s acceleration error is compared to that of a regular trolley in several scenario’s, where different types of disturbances are applied. The results show that the controller indeed reduces the effects of disturbances on the trolley’s behavior; the controller is able to recognize the trolley’s changing behavior due to an encountered disturbance and it correctly attenuates its effect by ordering the required compensatory motor torque. More research is required with respect to the effect of uncertainty within the model dynamics to ensure stability and controllability of the trolley system. Moreover, the closed-loop system response should be tested in the presence of various types of noise to determine how well the current results apply when the controller is used in a real-life scenario.