This paper presents the self-calibration of a novel biologically inspired 7 DOF cable-driven robotic arm. Similar to the human arm, the proposed robotic arm consists of three sequentially connected modules, i.e., a 3 DOF shoulder module, a 1 DOF elbow module, and a 3 DOF wrist module. Due to factors like manufacturing defects, assembly misalignments, compliance, and wear of connecting mechanisms, errors in the geometric model parameters always exist. Hence, the identification of such errors is critical for path planning and motion control tasks. Self-calibration models of the various modules in the robotic arm are formulated based on the differential change in the cable end-point distances. Due to the linear nature of these self-calibration models, an iterative least-squares algorithm is employed to identify the errors in the geometric model parameters. The calibration does not require any external pose measurement devices, because it utilizes the cable length data obtained from the redundant actuation scheme of the cable-driven arm. Computer simulations and experimental studies were carried out on both the 3 DOF and 1 DOF modules, to verify the robustness and effectiveness of the proposed self-calibration algorithm. From the experimental studies, errors in the geometric model parameters were precisely recovered after a minimum number of pose measurements.

Self-Calibration of a Biologically Inspired 7 DOF Cable-Driven Robotic Arm

In this paper, we present the dynamics, control, and preliminary experiments on a wearable upper arm exoskeleton intended for human users with four degrees-of-freedom (dof), driven by six cables. The control of this cable-driven exoskeleton is complicated because the cables can transmit forces to the arm only under tension. The standard PD controllers or Computed torque controllers perform only moderately since the cables need to be in tension. Future efforts will seek to refine these control strategies and their implementations to improve functionality of a human user.

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Dynamics and control of a 4-dof wearable cable-driven upper arm exoskeleton

In this paper, a technique to generate the wrench-closure workspace for general case completely restrained cable driven parallel mechanisms is proposed. Existing methods can be classified as either numerically or analytically based approaches. Numerical techniques exhaustively sample the task space, which can be inaccurate due to discretisation and is computationally expensive. In comparison, analytical formulations have higher accuracy, but often provides only qualitative workspace information. The proposed hybrid approach combines the high accuracy of the analytical approach and the algorithmic versatility of the numerical approach. Additionally, this is achieved with significantly lower computational costs compared to numerical methods. It is shown that the wrench-closure workspace can be reduced to a set of univariate polynomial inequalities with respect to a single variable of the end-effector motion. In this form, the workspace can then be efficiently determined and quantitatively evaluated. The proposed technique is described for a 3 degrees of freedom (3-DOF) and a 6-DOF cable driven parallel manipulator. A detailed example in workspace determination using the proposed approach and comparison against the conventional numerical approach is presented. © 2011 American Society of Mechanical Engineers.

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Wrench-Closure Workspace Generation for Cable Driven Parallel Manipulators Using a Hybrid Analytical-Numerical Approach

Theoretical and experimental determination of capstan drive slip error

A method of verifying force-closure condition for general cable manipulators with seven cables

A solution-in-nature approach has been adopted in the design of a 7-DOF humanoid robotic arm. Similar to a human arm, the proposed robotic arm consists of three sequentially connected modules, i.e., a 3-DOF shoulder module, a 1-DOF elbow module, and a 3-DOF wrist module. To mimic the driving scheme of the human arm, all of the three arm modules are driven by cables which are functionally equivalent to human muscles (can pull but cannot push). Such a robotic arm has the advantages of light weight, high dexterity, and large reachable workspace. Hence, it can be an ideal versatile arm for various service robots to perform human-like operations. Two critical design analysis issues, i.e., displacement analysis and cable-tension analysis are addressed in more details. The closed form solution is derived for the forward displacement analysis, while the inverse displacement solution is obtained through an optimization technique. An effective cable-tension analysis algorithm is proposed in light of the analysis method for force-closure grasps.

Cong Pham

Papers

Kinematic design of a 7-DOF cable-driven humanoid arm: a solution-in-nature approach