László Bencsik

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.

Stability case study of the ACROBOTER underactuated service robot

The dynamics of classical robotic systems are usually described by ordinary differential equations via selecting a minimum set of independent generalized coordinates. However, different parameterizations and the use of a nonminimum set of (dependent) generalized coordinates can be advantageous in such cases when the modeled device contains closed kinematic loops and/or it has a complex structure. On one hand, the use of dependent coordinates, like natural coordinates, leads to a different mathematical representation where the equations of motion are given in the form of differential algebraic equations. On the other hand, the control design of underactuated robots usually relies on partial feedback linearization based techniques which are exclusively developed for systems modeled by independent coordinates. In this paper, we propose a different control algorithm formulated by using dependent coordinates. The applied computed torque controller is realized via introducing actuator constraints that complement the kinematic constraints which are used to describe the dynamics of the investigated service robotic system in relatively simple and compact form. The proposed controller is applied to the computed torque control of the planar model of the ACROBOTER service robot. The stability analysis of the digitally controlled underactuated service robot is provided as a real parameter case study for selecting the optimal control gains.

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

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.

Reduction of the effect of actuator saturation with periodic servo-constraints

Saturation is an undesired event in trajectory tracking control of mechanical systems. When the actuators of a robotic device saturate, the solution of the inverse dynamics problem cannot fully be realized, which results in deviations from the desired trajectory and loss of performance. It is generally hard to consider the limited actuator torques and the corresponding nonlinear effects in the control design. The most common way to handle the problem is recalculating the control forces and trying to adjust the desired trajectory such that saturation will not happen. In contrast we propose a switched control approach, where, upon saturation, different sets of inputs are varied periodically to keep the reference point of the robot on the desired trajectory. For this, the desired motion is formulated by means of servo-constraints, and the periodic switching of these constraints is adjusted according to the variation of a new, manipulability type performance measure. It is demonstrated that the proposed controller can effectively reduce the trajectory following error due to actuator saturation. A typical robotic benchmark example is provided to show the application of the method, and to compare it with other approaches taken from the literature.

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.