Przemysław Dąbek

Requirements for Tire Models of the Lightweight Wheeled Mobile Robots

Tire models for vehicle dynamics studies have been developed for many years to suit the needs of automobiles and the automotive industry. Recently, the growing use of advanced simulation techniques in design of wheeled mobile robots calls for analysis of the possibility to use the existing automotive tire models in the wheeled mobile robots dynamics studies. This analysis is especially important in the case of the skid-steered lightweight mobile robots, which are very common type of design, but exhibit many differences in the tire–ground system as compared to a typical car. In the present work the differences between lightweight wheeled robots and automobiles are examined in the following areas: tires, environment, maneuvers, ways of control, and vehicle systems. The influence of the found differences on the tire–ground system is examined in detail. Finally, the requirements for the tire models of the lightweight wheeled mobile robots are formulated with emphasis on the requirements different than those for tire models of the automobiles.

Determination of Motion Parameters with Inertial Measurement Units – Part 1: Mathematical Formulation of the Algorithm

The paper tackles the problem of determination of motion parameters of a wheeled mobile robot using the inertial measurement method. By the motion parameters one means: positions, linear velocities and accelerations of characteristic points of the robot, as well as Euler angles and angular velocity and acceleration of a robot body. Existing methods of determination of robot motion parameters, including the inertial method, the satellite navigation method and hybrid methods, are briefly discussed. The method of determination of motion parameters of a wheeled mobile robot with Inertial Measurement Units is described in details. It involves measurement of three components of acceleration of a selected point on the robot using a three-axial accelerometer and three components of angular velocity of the robot body using a three-axial gyroscope. Desired motion parameters are obtained as a result of differentiation, integration and other mathematical transformations. It was assumed that most of the analyzed motion parameters are calculated both in the coordinate system associated with the robot (moving) and in the reference coordinate system (stationary). The presented method is simple, but enables measurement of wide range of 3D motions, and as such it can be used as a benchmark for advanced algorithms of determination of motion parameters. In the Part 2 of this article, the proposed measurement method is verified in empirical experiments with a wheeled mobile robot using the Inertial Measurement Unit based on low-cost MEMS sensors.

Determination of Motion Parameters with Inertial Measurement Units – Part 2: Algorithm Verification with a Four-Wheeled Mobile Robot and Low-Cost MEMS Sensors

The paper is concerned with the problem of determination of motion parameters of a wheeled mobile robot using the inertial measurement method. The algorithm proposed in Part 1 of the article is verified in empirical experiments with a four-wheeled mobile robot PIAP SCOUT. Main design features of the robot are presented. The measurement and control system is described in details. The measurement system is based on a low-cost MEMS Inertial Measurement Unit. Selected results of empirical experiments are shown and thoroughly discussed. Performance of the algorithm with the low-cost sensors is evaluated. It is concluded that the presented simple method enables determination of unknown motion parameters, especially in applications where only short duration of experiments is required. Quality of the obtained results, however, shows scope for improvement. The weakest point of the measurement system are unreliable changes of the Euler angles obtained from the low-cost MEMS gyroscopes.

Trajectory Tracking Control of a Four-Wheeled Mobile Robot with Yaw Rate Linear Controller

The paper concerns the problem of trajectory tracking control of a four-wheeled PIAP SCOUT mobile robot with non-steered wheels. For this kind of wheeled robots, it is impossible to find kinematic relationship between robot’s body motion and motion of driven wheels, because of inherent sliding of wheels on the ground during turning. This is an important problem from the point of view of control of the robot. The approach followed in the present work relies on introducing a simple linear controller with feedback of actual yaw rate of robot’s body. The yaw velocity is measured by inexpensive MEMS gyroscope. Experiments were conducted on two kinds of floor typical for office buildings: PVC flooring and carpet flooring. Measurements of motion parameters were possible with INS technique. It was found that the proposed yaw rate controller significantly reduces the angular error of path tracking for 90 degrees turn maneuver.

Comparison of Configurations of Inertial Measurement Units for Determination of Motion Parameters of Mobile Robots – Part 1: Theoretical Considerations

The paper is concerned with the problem of determination of motion parameters of a mobile robot using Inertial Measurement Units (IMUs) in three different configurations. The practical goal of this research is investigation of possibilities of improvement of quality of data obtained from a 3D scanning head thanks to the application of information about motion parameters of the robot in a cost effective way. PIAP GRANITE four-wheeled skid-steered mobile robot was used as a mobile platform for the investigations. The following configurations of IMUs are considered: 1st – comprising one 3-axis accelerometer and one 3-axis gyroscope located in central part of the robot, 2nd – consisting of four identical 3-axis gyroscopes located on the robot to reduce the effect of drift of measurement of the angular velocities, 3rd – containing four 3-axis accelerometers deployed in different locations on the robot to realize so-called Gyroscope-Free Inertial Measurement Unit. The three configurations of IMUs are discussed and formulas for obtaining the motion parameters are given. In Part 2 of this article, the results of experimental research involving the three above mentioned configurations of IMUs are presented and discussed.

Comparison of Configurations of Inertial Measurement Units for Determination of Motion Parameters of Mobile Robots – Part 2: Experimental Investigations

The paper is concerned with the problem of determination of motion parameters of a mobile robot using Inertial Measurement Units (IMUs) in three different configurations. The practical goal of this research is investigation of possibilities of improvement of quality of data obtained from a 3D scanning head thanks to the application of information about motion parameters of the robot in a cost effective way. PIAP GRANITE four-wheeled skid-steered mobile robot was used as a mobile platform for the investigations. In Part 1 of the paper, the three configurations of IMUs are discussed and formulas for obtaining the motion parameters are given. In Part 2, methods discussed in Part 1 are compared experimentally. Main metrological properties of the IMUs used during experiments are provided. The precisions of measurements for particular configurations of IMUs were estimated using the introduced quality indexes. Main findings are stated in the conclusion.

Tire Models for Studies of Wheeled Mobile Robot Dynamics on Rigid Grounds – A Quantitative Analysis for Longitudinal Motion

The work is concerned with quantitative analysis of tire models from the point of view of their use in studies of dynamics of lightweight wheeled mobile robots. The analyses are carried out for the four-wheeled skid-steered mobile robot dedicated for investigations of robot kinematics and dynamics. Robot kinematics and definitions of parameters describing wheel slips are discussed. Two tire models selected for the quantitative analysis are presented. The model of robot dynamics used during studies which allows simulation of longitudinal motion enhanced with drive unit model is described. Within work, simulation investigations of the full vehicle with successively connected different tire models are conducted. Results of simulations are benchmarked against data obtained during experiments with the real mobile robot.

Comparative Analysis of Posture Controllers for Tracking Control of a Four-Wheeled Skid-Steered Mobile Robot – Part 2. Dynamics Model of the Robot and Simulation Research of Posture Controllers

The paper is the second part of the work concerned with the problem of trajectory tracking control of a four-wheeled PIAP GRANITE mobile robot. The first part of the work was devoted to theoretical considerations. Research object and its kinematics were described. Robot motion control system structure comprising posture controller and drive unit controller was presented. Various solutions for posture controller were discussed and their modifications proposed. A methodology of posture controller tuning was introduced in which controller parameters for particular solutions are determined from conditions for maximum velocities of robot motion and maximum posture errors. In the present work dynamics model of the robot is described. It takes into account tire-ground contact conditions and wheel slips. The tire-ground contact conditions are characterized by coefficients of friction and rolling resistance. A simple form of the tire model, which includes only the most important effects of tire-ground interaction, is used. The robot dynamics model also contains the electromechanical model of a servomotor drive unit. The developed model of robot dynamics is used in the simulation studies in which the effectiveness of particular solutions of posture controller is benchmarked. Evaluation of the analyzed solutions is carried out using the introduced quality indexes.

Comparative Analysis of Posture Controllers for Tracking Control of a Four-Wheeled Skid-Steered Mobile Robot – Part 1. Theoretical Considerations

The paper is concerned with the problem of trajectory tracking control of a four-wheeled PIAP GRANITE mobile robot. All wheels of the robot are non-steered and the servomotors are used for driving the robot. Kinematic structure of the robot and its kinematics are described. Structure of robot motion control system containing posture controller and drive controller is presented. Various solutions of the posture controller which allow realization of tracking control are discussed. Because of limitations of the analyzed solutions their modifications are proposed. Methodology of posture controller tuning is proposed. According to it the controller parameters for particular solutions are determined from conditions for maximum velocities of robot motion and maximum posture errors.

Motion Stabilization System of a Four-Wheeled Mobile Robot for Teleoperation Mode: Experimental Investigations in Indoor Environment

The paper is concerned with the problem of straight-line motion stabilization of a wheeled mobile robot with non-steered wheels. This system is dedicated for teleoperation mode of the robot and aims at improvement of control experience of a human operator. The structure of the robot motion stabilization system based on a PD regulator with feedback from actual linear velocity and yaw rate of a robot body is proposed. The motion stabilization system was implemented using a low-cost MEMS Inertial Measurement Unit and the PIAP SCOUT four-wheeled robot with non-steered wheels. In the present work the motion stabilization system is implemented in a form reduced to yaw angle stabilization only, with stabilization of linear velocity not taken into account. Reliable functioning of the linear velocity stabilization (e.g. wheel slip reduction) based on measurement signals from the low-cost IMU requires additional research. Functioning of the proposed robot motion stabilization system was verified in experimental research. Experiments were conducted for various robot velocities in indoor environment on a horizontal and even ground. Results of investigations without and with the motion stabilization system were compared. A significant improvement in accuracy of realization of desired motion was observed in the case when the motion stabilization system was active.