Ambrus Zelei

Motion Control of an Under-Actuated Service Robot Using Natural Coordinates

The recent work presents the motion control of a pendulum like under—actuated service robot AC ROBOT ER. This robot is designed to be applied in indoor environments, where it can perform pick and place tasks autonomously and/or with close cooperation with humans. It can also serve as a platform that carries other service robots with lower mobility. The cable suspended robot has a complex structure and its dynamics is difficult to model using conventional robotic approaches. Instead, in this paper natural (Cartesian) coordinates are used to describe the configuration of the robot, while its dynamics is modeled as a set of differential algebraic equations. The method of computed torque control with a PD controller is applied to the investigated under—actuated system. The inverse dynamics solution is obtained via direct discretization of the DAE system. Results for a real parameter case study are presented by numerical simulations.

Servo-constraint based computed torque control of underactuated mechanical systems

This paper aims to generalize the computed torque control method for underactuated systems which are modeled by a non-minimum set of generalized coordinates subjected to geometric constraints. The control task of the underactuated robot is defined in the form of servo constraint equations that have the same number as the number of independent control inputs. A PD controller is synthesized based on projecting the equations of motion into the nullspace of the distribution matrix of the actuator forces/torques. The results are demonstrated by numerical simulation and experiments conducted on a two degrees-of-freedom device.Copyright

Estimation of human reaction time delay during balancing on balance board

Human balancing on a balance board is modelled as a delayed proportional-derivative control mechanism with unknown feedback delay. The mechanical model implies that there exists a critical delay, for which no control gain parameters can stabilize the system. This theoretical critical delay is determined by numerical analysis for different geometries of the balance board. Then the results are compared to real balancing trials on balance boards with the same geometries. Comparison of the unsuccessful balancing trials to the theoretical critical delay suggests that the feedback delay of human balancing task is between 20ms and 110ms.

Stabilization of Internal Dynamics of Underactuated Systems by Periodic Servo-Constraints

The model-based motion control of underactuated, multiple degree-of-freedom, complex multibody systems is in focus. Underactuated mechanical systems possess less number of independent control inputs than degrees-of-freedom. The main difficulty in their control is caused by the dynamics of the uncontrolled part of the system. The complexity of multibody systems makes the dynamical and control formulation difficult. The direct application of traditional control techniques available in the literature can lead to unstable dynamic behavior in many cases. In order to avoid instability, these general methods are usually adapted for specific problems in an intuitive way. Here, we present a direct, more algorithmic approach, and propose the use of periodic servo-constraints to overcome stability problems and enhance the dynamic behavior. An exact, stability analysis-based method is also proposed for tuning the control parameters. A stability analysis procedure is developed which is directly applicable for investigating the dynamics of mechanical systems described by dependent coordinates and mathematically formulated as a set of algebraic differential equations.

Handling Actuator Saturation as Underactuation: Case Study With Acroboter Service Robot

Model based control methods, like inverse dynamics control and computed torque control encounter difficulties if actuator saturation occurs. However, saturation is a common phenomenon in robotics leading to significant non-linearity in system behavior. In this study, the saturation of the actuator torques is considered as a temporary reduction of the number of independent control inputs. The reduction of the number of actuators leads to an underactuated control problem which typically involves the handling of differential algebraic equation systems. The saturated system may become especially complex when intricate combinations of the actuator saturations appear. A servo-constraint based inverse dynamics control method for underactuated multibody systems is applied for the treatment of actuator torque saturation. In case of human-friendly robots, the problem of saturation cannot be avoided on the level of trajectory planning because unexpected human perturbations may take place, which result such abrupt changes of the desired trajectory that lead to saturation at some actuators. A case study for the service robot Acroboter shows the applicability of the proposed approach.

COMPUTED TORQUE CONTROL METHOD FOR UNDER-ACTUATED MANIPULATOR

The paper presents the dynamic analysis of a crane-like manipulator system equipped with complementary cables and ducted fan actuators. The investigated under-actuated mechanical system is described by a system of differential- algebraic equations. The position/orientation control problem is investigated with respect to the trajectory generation and the fine positioning of the payload. The closed form results include the desired actuator forces as well as the nominal load angle corresponding to the desired motion of the payload. Considering a PD controller, numerical simulation results and also experiments demonstrate the applicability of the concept of using complementary actuators for controlling the swinging motion of the payload.

Four-Bar Mechanism Substitution for Balance Board Experiments: A Parametric Study

Our research aims the study of balancing on a rolling balance board with respect to dynamic properties such as stability and stabilizability. The goal is to identify the parameter regions where human subjects are able to keep themselves stable in the upright position for at least 60 s. The radius of the balance board and the height of the foot platform are adjusted for each individual test, which is a time demanding process. We give a preliminary design of a substituting four-bar mechanism in order to speed up the balance board experiments and to extend the limits of the parameter study. The mechanism is tunable quickly in order to imitate the motion of the balance board with different radii and platform heights; whilst the agreement of the kinematic behaviour is almost perfect for tilt angles within the region of \(\pm 30 ^{\circ }\). The dynamic behaviour of the mechanism and the balance board are compared based on theoretically derived stability diagrams associated with the underlying mechanical models. The balancing process is modelled by a proportional-derivative delayed feedback controller in order to account with the reaction time delay of the subject. We show that the stable parameter regions of the balance board and the mechanism are in good agreement, therefore the mechanism can be used as a substituting device for balance board.

Development of the Acroboter Service Robot Platform

The domestic robot platform Acroboter exploits a novel concept of ceiling based locomotion. The robot platform is designed to perform pick and place tasks as well as carry other service robots with lower mobility. The crane-like Acroboter platform extends the workspace of these robots to the whole cubic volume of the indoor environment by utilizing the almost obstacle free ceiling. We summarize the evolution of the structure of the robot, the dynamic modelling concept and the control strategy which are the results of concurrent engineering.

Redundancy Resolution of the Underactuated Manipulator

The domestic robot platform ACROBOTER exploits a novel concept of ceiling based locomotion. A climber unit moves on the almost obstacle free ceiling, while carries a swinging unit with a system of suspending and orienting cables. The objective of the robot is the fine positioning of the swinging unit that accomplishes path following or pick and place tasks. Its motion is controlled by ducted fan actuators additionally to the variable length suspending cables. The complexity of the mechanical structure induces the use of natural coordinates for the kinematical description. An algorithm is proposed to control this underactuated and also redundant manipulator, which can be characterized as a control-constraint based computed torque control strategy.

The ACROBOTER platform - Part 1: Conceptual design and dynamics modeling aspects

This paper presents the conceptual design and the dynamics modeling aspects of a pendulum-like under–actuated service robot platform ACROBOTER. The robot is designed to operate in indoor environments and perform pick and place tasks as well as carry other service robots with lower mobility. The ACROBOTER platform extends the workspace of these robots to the whole cubic volume of the indoor environment by utilizing the ceiling for planar movements. The cable suspended platform has a complex structure the dynamics of which is difficult to be modeled by using conventional robotic approaches. Instead, in this paper natural (Cartesian) coordinates are proposed to describe the configuration of the robot which leads to a dynamical model in the form of differential algebraic equations. The evolution of the ACROBOTER concepts is described in detail with a particular attention on the under–actuation and redundancy of the system. The influence of these properties and the applied differential algebraic model on the controller design is discussed.