Push-Pull Pair of Single Action Mechanisms to Preload Strut and Node Joints in the Robotic Assembly of Structures in Space

When strut and node components are used for truss construction or repair, an assembly problem occurs if a strut must fit between nodes whose separation distance is either more or less than the design specification. In such circumstances two actions would permit continued assembly: 1. Change strut length 2. Move the nodes. Variable length struts fit between nodes and (or) move them. They are preloaded at the joint against a reference length in an attempt to maintain the desired dimension. As a by-product they either pull the nodes together or push them apart. Most cannot do both, and are therefore characterized as “single action”; those that can do both are “double action”. Double action mechanisms are currently being used for robotic truss assembly because they solve the above stated crucial problem in both directions. Single action mechanisms tend to exhibit superior performance in all other categories. They benefit from the attributes that accompany their simplicity. The titled concept combines the major advantage of double action with the simplicity of single action. This is demonstrated with an example.

Predicting Force Redistributions on Walking Robots for Reliable Locomotion: Modeling and Experiments

A large number of walking robots walk with a statically-stable gait. A statically-stable walker has at least three feet that are in ground contact at any time. If there are more than three feet in ground contact, the normal (vertical) forces exerted by the ground on the feet of the walker are indeterminate, unless they are measured. Some walking robots may walk with more than three legs in ground contact in order to achieve greater stability. To ensure this stability it is desirable to predict how vertical forces passively redistribute underneath the feet during walker motions. Predictions of future foot forces can be used as a basis for accepting or rejecting any planned walker motion. Two methods — the least-squares method and the compliance method — for predicting this redistribution of forces in the face of static indeterminacy are presented in this work. Both methods are computationally efficient, and give reasonably accurate predictions, as verified by experiments on a walking robot.

A Sensitivity Analysis Based Method for Robot Calibration

A new robot kinematic calibration procedure is presented. The parameters of the kinematic model are estimated through a relationship established between the deviations in the joint variables and the deviations in the model parameters. Thus, the new method can be classified as an inverse calibration procedure. Using suitable sensitivity analysis methods, the matrix of the partial derivatives of joint variables with respect to robot parameters is calculated without having explicit expressions of joint variables as a function of task space coordinates (closed inverse kinematic solution). This matrix provides the relationship between the changes in the joint variables and the changes in the parameter values required for the calibration. Two deterministic sensitivity analysis methods are applied, namely the Direct Sensitivity Approach and the Adjoint Sensitivity Method. The new calibration procedure was successfully tested by the simulated calibrations of a two degree of freedom revolute-joint planar manipulator.

Walking Machine Design Based on Certain Aspects of Insect Leg Design

A walking machine design originating from observations of insects is presented. The primary concept derived from insects is a leg used to apply force to the body without applying significant moments about the point of body attachment. This is accomplished with legs which have kinematic equivalents to ball-and-socket joints at body attachment and ground contact, with joints in the middle which only change distance between body and ground. Standing and walking with 6 legs of this design requires careful attention to static equilibrium equations but does not necessitate a control system which actively distributes forces to the legs. This paper considers necessary observational data, assumptions on which control is based, mathematical development for control and problems such as foot slip.

Optimization of the Robot Calibration Process

Most industrial robots cannot be off-line programmed to carry out a task accurately, unless their kinematic model is suitably corrected through a calibration procedure. However, proper calibration is an expensive and time-consuming procedure due to the highly accurate measurement equipment required and due to the significant amount of data that must be collected. To improve the efficiency of robot calibration, an optimization procedure is proposed in this paper. The objective of minimizing the cost of the calibration is combined with the objective of minimizing the residual error after calibration in one multiple-objective optimization. Prediction of the residual error for a given calibration process presents the main difficulty for implementing the optimization. It is proposed that the residual error is expressed as a polynomial function. This function is obtained as a result of fitting a response surface to either experimental or simulated sample estimates of the residual error. The optimization problem is then solved by identifying a reduced set of possible solutions, thus greatly simplifying the decision maker’s choice of an effective calibration procedure. An application example of this method is also included.

Path Generation for Two Cooperative Puma Robots

This paper describes the development of generating collision-free paths for a pair of cooperative PUMA robots as their end effectors grasp a workpiece in an obstacle-strewn environment. After the initial and goal positions of the wrist center are specified, a collision-free path for this pair of manipulators to move the workpiece safely to the final destination is generated. The algorithm is demonstrated via computer graphics animation on a Silicon Graphics IRIS 4D/70GT workstation.

Optimal Synthesis and Analysis of a New Leg Mechanism: The Planetary Gear Leg

The leg mechanism of a walking machine has a strong influence on the performance of the machine. A successful leg mechanism should be energy efficient, compact in size, strong and simple. In order to achieve good energy efficiency, a walking machine leg should be able to generate an exact or approximate straight line at the foot with only one driving actuator. This paper deals with the synthesis and analysis of a new leg mechanism — the planetary gear leg mechanism. Four types of planetary gear legs are studied. By the SUMT optimization method, a 20 inch tall leg is able to generate an approximate straight line trajectory with a maximum deviation of 0.12805 inches in a 20 inch stroke. The direct and inverse kinematics and velocities of the legs are analyzed. Also, the distribution of actuator force/moment during walking are studied. The results show that this leg design has great potential to be used as a practical walking machine leg.

Extrinsic Calibration of Portable Manipulators

Portable manipulators are installed for operation and then removed upon completion of their task. Typical applications of portable manipulators include the inspection of nuclear reactors, inspection and repair of nuclear steam generators and asbestos removal in buildings. In such operations, it is difficult to precisely position the manipulator at a fixed location within its workplace, yet this is critical for accurate tool positioning. It can be possible, however, to position the tool tip at several points in the environment using video feedback and manual operator control of the manipulator. This provides sufficient information to determine the position and orientation of the manipulator base frame with respect to the environment, hereafter referred to as extrinsic calibration. Following extrinsic calibration, subsequent moves of the manipulator can be automated. This paper describes a closed-form method for performing extrinsic calibration by contacting the tool to a total of six places on three orthogonal plane surfaces of reference.

Optimal Trajectory Resolution for the Coordination of Two Robots in Contact Operations

A new method is presented for the optimal coordination of a two-robot system performing contact operations. One of the robots carries a tool and performs the specific contact operation on a workpiece which is grasped and maneuvered by the second robot. The two robots move simultaneously relative to each other so that the tool maintains contact with the workpiece while moving along its prescribed trajectory at a constant speed. This prescribed trajectory, which is specified with respect to the workpiece frame, is thus resolved into a pair of conjugate trajectories, one for each robot, and specified in the world coordinate frame. This resolution process does not yield a unique solution, i.e. there exist an infinity of conjugate-trajectory pairs corresponding to a given tool trajectory. This paper presents a technique for resolving the original tool trajectory, where the robots’ conjugate trajectories are parameterized using polynomial functions. A method is then developed for selecting the optimal pair of conjugate trajectories on the basis of minimizing a given choice of cost function. This optimization is further enhanced by coupling it to a procedure for selecting the optimal layout of the robots within the workcell, resulting in the best possible solutions. Numerical simulation results support the validity of the proposed technique.

Optimal Design of a Six-Bar Linkage for an Anthropomorphic Three-Jointed Finger Mechanism

We treat the design of a three-jointed, anthropomorphic, finger mechanism for prostheses and robotic end-effectors. Based on the study of configurations for the human finger, we propose a six-bar linkage with one degree of freedom for the finger mechanism. A model of the fingertip displacement of the mechanism is derived by a vector analysis approach. We study the effects of joint friction on the transmission efficiency. By measuring the joint positions of a human finger, we develop a mathematical model of the pinching and holding configurations for the human finger. Optimal parameters for the finger mechanism are obtained by nonlinear programming based on motion posture, locus, transmission efficiency, and weight subject to geometric and bionic constraints. Simulations indicate that the mechanism is useful in a variety of prosthetic and robotic devices.