Design and control of a robotic cable-suspended camera system for operation in 3-D industrial environment

Thesis (S.B.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 2008.Includes bibliographical references (leaves 52-54).Cable-suspended robots offer many advantages over conventional serial manipulators. The main benefit of cable robots is their large workspace size, which makes them well suited for broadcasting, transporting/loading, and construction applications. Since cables can only pull and not push the end-effector however, designing and controlling cable robots becomes more challenging. This thesis describes the design of a three-cable underconstrained robot which was built and then tested using a velocity feedback loop with a built-in PI controller. The endeffector of the robot consists of a camcorder mounted on a platform. The objective of the robot is to manipulate the camcorder in 3-D space with minimal tracking error. The dynamic equations of the system are derived along with the kinematic relationships and a closed-loop controller is designed. The controller is tested by prescribing a trajectory to the end-effector. Simulink derives the motor velocities given the desired Cartesian positions of the end-effector and simultaneously controls all three motors. The results of the experiment show that the error in the trajectory, which is on the order of about seven centimeters in the x -y plane, is small compared to the size of the robot's workspace. However, depending on the required precision, improvements may have to be made to the robot to reduce error. Future research ideas are presented to expand the scope of the robot.by Vladimir Gordievsky.S.B

Natural oscillations of underactuated cable-driven parallel robots

Underactuated Cable-Driven Parallel Robots (CDPR) employ a number of cables smaller than the degrees of freedom (DoFs) of the end-effector (EE) that they control. As a consequence, the EE is underconstrained and preserves some freedoms even when all actuators are locked, which may lead to undesirable oscillations. This paper proposes a methodology for the computation of the EE natural oscillation frequencies, whose knowledge has proven to be convenient for control purposes. This procedure, based on the linearization of the system internal dynamics about equilibrium con_gurations, can be applied to a generic robot suspended by any number of cables comprised between 2 and 5. The kinematics, dynamics, stability and stiffness of the robot free motion are investigated in detail. The validity of the proposed method is demonstrated by experiments on 6-DoF prototypes actuated by 2, 3, and 4 cables. Additionally, in order to highlight the interest in a robotic context, this modelling strategy is applied to the trajectory planning of a 6-DoF 4-cable CDPR by means of a frequency-based method (multi-mode input shaping), and the latter is experimentally compared with traditional non-frequency-based motion planners

Air vehicle simulator: an application for a cable array robot

The development of autonomous air vehicles can be an expensive research pursuit. To alleviate some of the financial burden of this process, we have constructed a system consisting of four winches each attached to a central pod (the simulated air vehicle) via cables - a cable-array robot. The system is capable of precisely controlling the three dimensional position of the pod allowing effective testing of sensing and control strategies before experimentation on a free-flying vehicle. In this paper, we present a brief overview of the system and provide a practical control strategy for such a system. ©2005 IEEE

Real-time motion planning based vibration control of a macro-micro parallel manipulator system for super antenna

A macro-micro manipulator (M3) system, composed of a rigid parallel manipulator serially mounted on a flexible cable suspended parallel manipulator, is used to precisely position the feed source of a super antenna. In order to reduce the impact of mechanical vibrations of the macro manipulator and achieve accurate positioning and orientating of the micro manipulator, a real-time motion planning based vibration control strategy is presented. This strategy comprises: (1) To determine the optimal position and orientation of the cable driven parallel manipulator, the real-time optimization is conducted according to the principle of uniform tension in the six driving cables; (2) Synchronized points and the “judge and wait” technique ensure the continuity and synchrony of the trajectory tracking of the two parallel manipulators; (3) The preadjustment of the micro parallel manipulator minimizes the drastic dynamical coupling as a result of its high-speed manipulation. Experimental results of the field model validate the high precision of the M3 system for super antenna when tracking a circular arc trajectory

Disturbance Robustness Measures and Wrench-Feasible Workspace Generation Techniques for Cable-Driven Robots

Cable robots are a type of robotic manipulator that has recently attracted interest for large workspace manipulation tasks. Cable robots are relatively simple in form, with multiple cables attached to a mobile platform or end-effector. The end-effector is manipulated by motors that can extend or retract the cables. Cable robots have many desirable characteristics, including low inertial properties, high payload-to-weight ratios, potentially vast workspaces, transportability, ease of disassembly/reassembly, reconfigurability and economical construction and maintenance. However, relatively few analytical tools are available for analyzing and designing these manipulators. This thesis focuses on expanding the existing theoretical framework for the design and analysis of cable robots in two areas: disturbance robustness and workspace generation. Underconstrained cable robots cannot resist arbitrary external disturbances acting on the end-effector. Thus a disturbance robustness measure for general underconstrained single-body and multi-body cable robots is presented. This measure captures the robustness of the manipulator to both static and impulsive disturbances. Additionally, a wrench-based method of analyzing cable robots has been developed and is used to formulate a method of generating the Wrench-Feasible Workspace of cable robots. This workspace consists of the set of all poses of the manipulator where a specified set of wrenches (force/moment combinations) can be exerted. For many applications the Wrench-Feasible Workspace constitutes the set of all usable poses. The concepts of robustness and workspace generation are then combined to introduce a new workspace: the Specified Robustness Workspace. This workspace consists of the set of all poses of the manipulator that meet or exceed a specified robustness value.Ph.D.Committee Chair: Dr. Imme Ebert-Uphoff; Committee Member: Dr. Harvey Lipkin; Committee Member: Dr. Jarek Rossignac; Committee Member: Dr. Magnus Egerstedt; Committee Member: Dr. William Singhos

Low Mobility Cable Robot with Application to Robotic Warehousing

Cable-based robots consist of a rigid mobile platform connected via flexible links (cables, wires, tendons) to a surrounding static platform. The use of cables simplifies the mechanical structure and reduces the inertia, allowing the mobile platform to reach high motion acceleration in large workspaces. These attributes give, in principle, an advantage over conventional robots used for industrial applications, such as the pick and place of objects inside factories or similar exterior large workspaces. However, unique cable properties involve new theoretical and technical challenges: all cables must be in tension to avoid collapse of the mobile platform. In addition, positive tensions applied to cables may affect the overall stiffness, that is, cable stretch might result in unacceptable oscillations of the mobile platform. Fully constrained cable-based robots can be distinguished from other types of cable-based robots because the motion and force generation of the mobile platform is accomplished by controlling both the cable lengths and the positive cable tensions. Fully constrained cable-based robots depend on actuator redundancy, that is, the addition of one or more actuated cables than end-effector degrees of freedom. Redundancy has proved to be beneficial to expand the workspace, remove some types of singularities, increase the overall stiffness, and support high payloads in several proposed cable-based robot designs. Nevertheless, this strategy demands the development of efficient controller designs for real-time applications. This research deals with the design and control of a fully constrained cable-based parallel manipulator for large-scale high-speed warehousing applications. To accomplish the design of the robot, a well-ordered procedure to analyze the cable tensions, stiffness and workspace will be presented to obtain an optimum structure. Then, the control problem will be investigated to deal with the redundancy solution and all-positive cable tension condition. The proposed control method will be evaluated through simulation and experimentation in a prototype manufactured for testing

Direct kinematics of CDPR with extra cable orientation sensors: the 2 and 3 cables case with perfect measurement and sagging cables

International audienceDirect kinematics (DK) of cable-driven parallel robots (CDPR) based only on cable lengths measurements is a complex issue even with ideal cables and consequently even harder for more realistic cable models such as sagging cable. A natural way to simplify the DK solving is to add sensors. We consider here sensors that give a partial or complete measurement of the cable direction at the anchor points and/or measure the orientation of the platform of CDPR with 2 or 3 cables and we assume that the measurements are exact. We provide a solving procedure and maximal number of DK solutions for an extensive combination of sensors for CDPR with sagging cables. We show that at least two measurements are necessary for the planar 2 cables case while six are necessary for the spatial 3 cables case. For spatial CDPR with n cables we prove that at least 2n additional sensors will be required to get a closed-form solution of the DK

Automatic Self-Calibration of Suspended Under-Actuated Cable-Driven Parallel Robot using Incremental Measurements

International audienceThis paper focuses on the problem of the initial-pose estimation by means of proprioceptive sensors (self-calibration) of suspended under-actuated Cable-Driven Parallel Robots (CDPRs). For this class of manipulators, the initial-pose estimation cannot be carried out by means of forward kinematics only, but mechanical equilibrium conditions must be considered as well. In addition , forward kinematics solution is based on cable-length measurements, but if the robot is equipped with incremental sensors cables' initial values are unknown. In this paper, the self-calibration problem is formulated as a non-linear least square optimization problem (NLLS), based on the direct geometrico-static problem, where only incremental measurements on cable lengths and on swivel pulley angles are required. In addition, a data acquisition algorithm and an initial value selection procedure for the NLLS are proposed, aiming at automatizing the self-calibration procedure. Simulations and experimental results on a 3-cable 6-degree-of-freedom robot are provided so as to prove the effectiveness of the proposed methodology

On the robustness of cable configurations of suspended cable-driven parallel robots

International audienceCable-driven parallel robot (CDPR) are parallel robots that use coilable cables as legs. We are interested here in suspended CDPR for which there is no cable that exert a downward force on the platform. If we assume that the cables are mass-less and not elastic it has been shown that at a given pose whatever is the number m > 6 of cables there will always be at most 6 cables under tension simultaneously. A cable configuration (CC) at a given pose is the set of cables number that are under tension and usually there are several possible CC for the same pose. These CC are not equivalent in terms of cable tensions, sensitivity to measurement errors and therefore it make sense from a control viewpoint to enforce the " best " CC to obtain the optimal robot configuration, which can be done by controlling the length of the cables that are not members of the CC so that we are sure that they are slack. Hence we are interested in ranking the different CC in term of ro-bustness. We propose several ranking indices for a CC and algorithms to calculate these indices at a pose, on a tra-jectory or when the robot moves on a surface and we show examples for a CDPR with 8 cables