Robotic manipulators mounted on spacecraft experience a number of kinematic, dynamic, and control problems because the motion of the spacecraft is affected by the robot motion. In this paper, the general three dimensional equations of motion are derived for an n link manipulator mounted on a non-fixed base object. Instead of performing a single inverse kinematic calculation at the beginning of a movement to determine the required joint setpoints, multiple inverse kinematic updates are done throughout a movement. The updating sequence is determined by an optimal inverse kinematic updating algorithm. This motion control algorithm is based on experimental simulation results performed in Matlab and a set of performance indices that are used as guidelines. Simple PD joint controllers and a special joint trajectory generator are used for servoing the manipulator joints for a planar robot application. The derived motion control techniques incorporate the base motion without base motion control.

In this paper we present an experimental comparison between different decentralized controllers for industrial robot manipulators. In spite of the fact that robot manipulators are highly nonlinear and coupled multivariable systems, decentralized controllers have been widely adopted in industrial environments for trajectory tracking, because of their simplicity and their fault tolerance feature. Different kinds of controllers have been investigated: the typical PID-based controller, a nonlinear three-term controller devised by Tarokh, and a variable structure control law, a discontinuous integral control, both in the continuous and in the discretised version. Allowances are made for the tuning of the parameters in order to obtain low tracking errors. Results show that better performances are obtained by controllers other than PID, especially by the discontinuous integral control, so that they appear to be particularly appropriate for the use in industrial robots.

For securing and increasing the quality of various material processing technologies a flexible sensor data integration is indispensable. Hence, a powerful and configurable sensor-robot-system was designed and implemented, integrating sensor data into industrial robot controls within the interpolation or the position control clock rate. Different controllers were designed and the system responses of the control loops were simulated and optimized by an efficient control design tool. Experimental results on the achievable path accuracy are presented. Finally, some application for the developed sensor control loops are introduced.

In this paper, a study of a quadrupedal walking robot capable of walking on the irregular ground is presented. The robot has specially-designed feet with toes making soft and safe landings on uneven surfaces. Using the toe angle and landing force sensors integrated within the feet, its gait control algorithm can adapt the foot trajectory to the ground profile, while the body weight is evenly distributed on the supporting legs. After the landing of a foot, the roll and pitch angles of the body are measured and controlled to keep the body attitude parallel to the plane of support. The contents of this study are the design concept and working principle of the foot, including the gait control algorithm for walk on the irregular ground. Experimental results are reported.

The force distribution problem in multilegged vehicle is a constrained, optimization problem. The solution to the problem is the setpoints of the leg contact forces for a particular system task. In this paper, the efficient Compact QP method which takes into account both the linear and quadratic objective functions is adopted to resolve this constrained, optimization problem. Various objective functions such as minimum force, load balance, safety margin on friction constraints can be considered by the Compact QP method. This method can also be applied to smooth discontinuities in commanded forces by manipulating the homogeneous solution and including smoothing periods when the leg phase alternates between support and transfer. This smoothing scheme does not require force sensors. Multiple goals which consider several alternative objective functions can also be achieved by the Compact QP method.

In co-operation with an Austrian small sized company, the Institute for Handling Devices and Robotics at the Vienna University of Technology started a project with the goal to reduce the time necessary for planning, programming and set-up of robot equipped manufacturing cells. In this contribution a modular, object-oriented control system for robotized cells is presented. This system ‘C_CTRL' is responsible for all necessary tasks, like sequencing, supervising, controlling of basic functions, error handling, and recording of statistic data. The control system is ‘self-generating' during program start-up – using hardware information from a simple ASCII configuration file. Regardless which and how many components used in a particular cell, there is no additional programming effort for generation of the entire control software.

The goal of this paper is to present a new method to control pushing operations with several fingers. We take into account some dynamical aspects that have not yet been investigated in pushing studies such as the object's center of mass acceleration correction and optimal force distribution. To do this, we use a general method for multi-chain mechanisms based on a dynamical control with finger coordination. We first detail the problems encountered in this type of manipulation and the way to take these points into account with specific constraints definition for the support contact. According to the parameters of the model such as optimization criterion and object characteristics, we present simulation results of object pushing with two fingers for a rotational and translational motion.

The constraints on the physical limit should be considered in a kinematic redundancy resolution problem of a robot. This paper proposes a new optimization scheme to resolve kinematic redundancy of the robot while considering physical constraints. In the proposed scheme, quadratic inequality constraints are used in place of linear inequality constraints, thus a quadratically constrained optimization technique is applied to resolve the redundancy. It is shown that the use of quadratic inequality constraints considerably reduces the number of constraints. Therefore, the proposed method reduces the problem size considerably and makes the problem simple resulting in computational efficiency. A numerical example of a 4-link planar redundant robot is included to demonstrate the efficiency of the proposed optimization technique. In this example, simulation results using the proposed method and another well-known method are compared and discussed.

The paper considers the influence of external forces on the behaviour of a redundant manipulator. It is assumed that the forces can act anywhere on the body of the manipulator. First, the equivalent generalized forces in the task space and the null space are defined and several special manipulator configurations regarding the equivalent forces and torques are identified. Next, two measures for the quantification of the influence of external forces on the task space are proposed. These measures are then used in the control algorithm to minimize the influence of external forces on the task space position accuracy. The control is based on the redundancy resolution at the acceleration level and the gradient projection technique. Improvement of the position accuracy is illustrated using the simulation of a four link planar manipulator.

Adaptive gait planning is an important aspect in the development of control systems for multi-legged robots traversing on rough terrain. The problem of adaptive gait generation can be viewed as one of finding a sequence of suitable foothold on rough terrain so that legged systems maintain static stability and motion continuity. Due to the limit of static stability, deadlock situation may occur in the process of searching for a suitable foothold, if terrain contains a large number of forbidden zones. In this paper, an improved method for adaptive gait planning is presented by active compensation of stability margin, through center of gravity (CG) adjustment in the longitudinal axis and/or body translation in the lateral direction. An algorithm for the proposed method is developed and embedded in a computer program. Simulation results show that the method provides legged machines with a much larger terrain adaptivity and better deadlock-avoidance ability.

Singularity of a robot manipulator is one of the obstacles that influences its capabilities. This paper discusses constrained and allowable rotational motion resulting from lost translational freedom when the robot is singular. A convenient method and simple and clear expression to determine the allowable rotational axes and the subspace that they form, under Jacobian singularity, is analyzed and presented. Different configurations, reciprocal screws, and subspaces of allowable-rotational-axes are derived in a case study involving a classic robot. The result is useful in applications involving robot path planning in task space as it extends the usable workspace of rotational axes.

This paper addresses the swing-free transport of simply suspended objects which cannot be grasped by robot manipulators, and therefore, must be carried by a hook or a similar device attached to the manipulator endpoint. Two methods are presented to stop the suspended object in a swing-free state at the end of a move/gross motion; (1) limiting transportation time, thus stopping the manipulator at the instant when the object completes one or more full cycle(s), and (2) adjusting traveling time of each section of a three-piece continuous trajectory provided that a given transportation time is unchanged. A hydraulically actuated robot manipulator carrying a compound pendulum was employed as an experimental system to test the methods. Simulation and experimental results are presented to demonstrate the feasibility of both methods. It is concluded that while limiting transportation time is not a preferred way to eliminate swing at the end of the move as it depends on the period of oscillation of the suspended object, the latter is a more practical and applicable method and is valid for moves of any length. The results reveal that by properly planning the acceleration of the transporting device, a robot manipulator or a similar device such as a crane, a swing-free stop is obtainable. The proposed approaches are simple and easy to implement.

To increase operator safety in robotic working cells, a capacitive sensor for the detection of humans has been developed. The sensor consists of two electrically conducting planes, insulated electrically from the ambient. When the dielectric or conductive conditions in the space between the planes are altered, e.g. a person enters, the sensor will indicate the presence of the person. The strength of the electric field caused by the sensor is kept low by lock-in amplifier technique. In the article the basic detection principle, the sensor electronics and the sensor signal handling are described and some practical experiments are presented.

The peg-in-hole insertion process is the most common task in the robotic/automatic assembly industry. However, the inaccuracy of the assembly robot in positioning the inserted peg into the hole due to the uncertainty of the assembly environment, represents an obstacle in this area of automation. To overcome this problem, complex and expensive equipment has been designed. The objective of this paper is to provide an in-depth understanding of the chamferless peg-hole assembly, by showing the geometrical, dynamical conditions for a successful assembly operation. Also, the results of this analysis have been simulated to demonstrate the chamferless peg-hole insertion process.

The paper deals with the kinematic redundancy control of a 3DOF linear hydraulic manipulator moving in the vertical plane. The analysis is carried out in actuator coordinates so as to make the results usable in control schemes with actuator position feedback. The idea is to use the initial manipulator configuration as an optimization parameter in order to: (I) further minimize the actuator velocities obtained by a pseudoinverse solution, (II) simultaneously avoid actuator limits without recourse to a gradient projection approach. An improved pseudoinverse redundancy solution is thus obtained and implemented in a simple, non-iterative algorithm suitable for real-time applications. Simulations of a typical task with the proposed method show that minimizing the actuator velocity norm yields better results than minimizing the manipulator kinetic energy.

ELEVENTH INTERNATIONAL CONGRESS OF CYBERNETICS AND SYSTEMS

Dedicated to the memory of the late

Professor Frank H. George

Brunel University, Uxbridge (West London),

Middlesex, United Kingdom, 23–27 August 1999

This work focuses on the dynamic modeling of a flexible robotic manipulator with two flexible links and two revolute joints, which rotates in the horizontal plane. The dynamic equations are derived using the Newton-Euler formulation and the finite element method, based on elementary beam theory. Computer simulation results are presented to illustrate this study. The dynamic model becomes necessary for use in future design and control applications.

In this paper, a mathematical representation of constrained robot systems in the form of a differential-algebraic equation model is first considered. This model in modified further to include the joint flexibility between the linkages of the robot, and the actuator dynamics. The objective is to design a feedback control law for the system so that the position output variables (typically the end-effector position) and the force output variables (typically the contact force between the robot's end-effector and the contact surface) of the robot follows the desired position and the desired force trajectories, respectively, despite the presence of joint flexibility and actuator dynamics. A systematic procedure is developed for designing a feedback control law which ensures that the position variables track the desired position trajectories exponentially, and the force variables track the desired force trajectories exponentially. Since the development of the control law is based on the model of a constrained robot system which includes the effects of actuator dynamics and joint flexibility, it is possible to achieve better tracking performance using the force/position control law developed in this paper in cases where such effects are significant.

A small-scale flexure-based gripper was designed for manipulation tasks requiring precision position and force control. The gripper is actuated by a piezoelectric ceramic stack actuator and utilizes strain gages to measure both the gripping force and displacement. The position and force bandwidths were designed for ten Hertz and one hundred Hertz, respectively, in order to afford human-based teleoperative transparency. The gripper serves effectively as a one degree-of-freedom investigation of compliant mechanism design for position and force controlled micromanipulation. Data is presented that characterizes the microgripper performance under both pure position and pure force control, followed by a discussion of the attributes and limitations of flexure-based design.

A free gait algorithm is proposed utilizing a new method of gait generation called primary/secondary gait. The primary gait is a fixed sequence of leg transfers with modified leg-end kinematic limits according to the obstacle presence, while the secondary gait is a flexible gait which is generated to adjust the leg-end position. The primary gait is generated considering the following four constraints: stability constraint, kinematic constraint, sequential constraint and neighboring constraints. Primary gait parameters are modified by the influence of the obstacle. Normally, the machine tends to move with the primary gait. When the primary gait cannot move the vehicle, the secondary gait is adopted to serve as a complement of the primary gait. With the proposed primary/secondary gait, it is expected to improve the efficiency of free gait generation while maintaining the mobility of the vehicle. Simulation results are given to demonstrate the efficiency of the proposed methodology.