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In this report, a method for exploiting the redundancy of a hand-arm mechanical system for manipulation tasks is illustrated. The basic idea is to try to exploit the different intrinsic capabilities of the arm and hand subsystems. The Jacobian transpose technique is at the core of the method: different behaviors of the two subsystems are obtained by means of constraints in Null(J) generated by non-orthogonal projectors. Comments about the computation of the constraints are reported in the memo, as well as a description of some preliminary experiments on a robotic system at the A.I. Lab., M.I.T.

This thesis describes the open-loop behavior of an ionic polymer metal composite (IPMC) strip as a novel actuator, the empirical force and position models, the control system and the improved dynamic characteristics with the feedback control implemented. Ionic polymer metal composite is a novel polymer in the class of electroactive polymers. IPMC consists of a base polymer coated with electrodes made up of highly conducting pure metals such as gold. The actuation behavior of IPMC can be attributed to the bending of an IPMC strip upon application of voltage across its thickness. The main reasons for the bending are ion migration on the application of voltage and swelling and contraction caused by water content. An experimental setup to study the open-loop force and tip displacement of an IPMC strip in a cantilever configuration was developed, and real time controllers were implemented. In open loop, the force response of the IPMC strip of dimensions 25 mm x 3.9 mm x 0.16 mm to a 1.2-V step input is studied. The open-loop rise time was 0.08 s and the percent overshoot was 131.62 %, while the settling time was about 10 s. Based on this open-loop step response using a least-square curve-fitting methodology, a fourth-order empirical transfer function from the voltage input to the force output was derived. The tip displacement response of an IPMC strip of dimensions 23 mm x 3.96 mm x 0.16 mm to a 1.2-V step input was also studied. The step response exhibited a 205.34 % overshoot with a rise time of 0.08 s, and the settling time was 27 s. A fourth-order empirical transfer function from the step input to the tip displacement as output was also derived. Based on the derived transfer functions lead-lag feedback controllers were designed for precision control of both force and displacement. The control objectives were to decrease the settling time and the percent overshoot, and achieve reference input tracking. After implementing the controllers, the percent overshoot decreased to 30% while the settling time was reduced to 1.5 s in case of force control. With position control, the settling time was reduced to 1 s while the percent overshoot decreased to 20%. Precision micro-scale force and position-control capabilities of the IPMC were also demonstrated. A 4 ?N force resolution was achieved, with a force noise of 0.904-?N rms. The position resolution was 20 ?m with a position noise of 7.6-?m rms.

This thesis describes the open-loop behavior of an ionic polymer metal composite (IPMC) strip as a novel actuator, the empirical force and position models, the control system and the improved dynamic characteristics with the feedback control implemented. Ionic polymer metal composite is a novel polymer in the class of electroactive polymers. IPMC consists of a base polymer coated with electrodes made up of highly conducting pure metals such as gold. The actuation behavior of IPMC can be attributed to the bending of an IPMC strip upon application of voltage across its thickness. The main reasons for the bending are ion migration on the application of voltage and swelling and contraction caused by water content. An experimental setup to study the open-loop force and tip displacement of an IPMC strip in a cantilever configuration was developed, and real time controllers were implemented. In open loop, the force response of the IPMC strip of dimensions 25 mm x 3.9 mm x 0.16 mm to a 1.2-V step input is studied. The open-loop rise time was 0.08 s and the percent overshoot was 131.62 %, while the settling time was about 10 s. Based on this open-loop step response using a least-square curve-fitting methodology, a fourth-order empirical transfer function from the voltage input to the force output was derived. The tip displacement response of an IPMC strip of dimensions 23 mm x 3.96 mm x 0.16 mm to a 1.2-V step input was also studied. The step response exhibited a 205.34 % overshoot with a rise time of 0.08 s, and the settling time was 27 s. A fourth-order empirical transfer function from the step input to the tip displacement as output was also derived. Based on the derived transfer functions lead-lag feedback controllers were designed for precision control of both force and displacement. The control objectives were to decrease the settling time and the percent overshoot, and achieve reference input tracking. After implementing the controllers, the percent overshoot decreased to 30% while the settling time was reduced to 1.5 s in case of force control. With position control, the settling time was reduced to 1 s while the percent overshoot decreased to 20%. Precision micro-scale force and position-control capabilities of the IPMC were also demonstrated. A 4 ?N force resolution was achieved, with a force noise of 0.904-?N rms. The position resolution was 20 ?m with a position noise of 7.6-?m rms.

Control of the contact force between a robot manipulator and a workpiece is critical for successful execution of tasks where the robot’s end effector must perform a contact operation along the contour of a workpiece. Representative tasks include polishing, grinding and deburring. Considerable research has been conducted on force control with electric robots. By contrast, little research has been conducted on force control with pneumatic robots. The later has the potential to be considerably cheaper. However, the compressible nature of air as the working fluid and relatively high friction means pneumatic robots are more difficult to control. The subject of this thesis is the design and testing of a controller that regulates the normal contact force and tangential velocity of the end effector of a pneumatic gantry robot while tracking the contour of a planar workpiece. Both experimental and simulation results are presented. A PI Force Velocity (FV) controller for contour tracking was designed and tested experimentally. Three different workpiece edge geometries were studied: straight, inclined and curved. The tracking performance with the PI FV controller was comparable to the performance reported by other researchers with a similar controller implemented with an electric robot. This result confirms the potential of pneumatically actuated robots in force control applications. A system model was developed and validated in order to investigate the parameters that affect performance. A good match between experiment and simulation was achieved when the friction of the z-axis cylinder was modeled with a Displacement Dependent Friction Model (DDFM) instead of a Velocity Dependent Friction Model (VDFM). Subsequently, a DDFM based friction compensator was designed and tested. However, it was found that performance could not be improved even with perfect friction compensation, due to the effects of system lag. Two Neural Network (NN) compensators were designed to compensate for both the lag and friction in the system. Simulation results for straight and curved edges were used to examine the effectiveness of the NN compensators. The performance of the PI FV controller was found to improve significantly when a NN compensator was added. This result confirms the value of NN’s in control compensation for tracking applications with pneumatic actuation. / Thesis (Ph.D, Mechanical and Materials Engineering) -- Queen's University, 2008-09-16 12:29:44.679

This thesis presents the design and control of a cable-actuated mobile snake robot. The goal of this research is to reduce the size of snake robots and improve their locomotive efficiency by simultaneously actuating groups of links to fit optimized curvature profiles. The basic functional unit of the snake is a four-link, single degree of freedom module that bends using an antagonistic cable-routing scheme. Elastic elements in series with the cables and the coupled nature of the mechanism allow each module to detect and automatically respond to obstacles. The mechanical and electrical designs of the bending module are presented, with emphasis on the cable-routing scheme, key optimizations, and the use of series elastic actuation. An approximate expression for the propulsive force generated by a snake as a function of its articulation (i.e. the number of links it contains divided by its body length) is derived and a closed-form approximation for the optimal phase offset between joints to maximize the speed of a snake is obtained by simplifying a previous result. A simplified model of serpentine locomotion that considers the forces acting on a single link as it traverses a sinusoid is presented and compared to a detailed multibody dynamic model. Control strategies for snake robots with coupled joints are developed, along with a feedback linearization of the joint dynamics. Experimental studies of force control, locomotion, and adaptation to obstacles using a fully integrated prototype are presented and compared with simulated results. / MS / This thesis presents the development of a cable-driven snake robot, with the goal of decreasing the size and mass of these devices and increasing their efficiency. Snake robots have potential applications in exploration and manipulation in cluttered or confined environments. The cable transmission system presented in this thesis allows for multiple links in a snake robot to be actuated simultaneously, allowing for increased articulation in a robot of fixed size and mass. Serpentine locomotion, in which a sinusoidal wave is propagated down the robot’s length, is a silent and energy-efficient mode of transportation, widely employed in the animal kingdom. Snake robots achieve serpentine locomotion by driving their joints sinusoidally, with adjacent joints moving asynchronously, with the time lag between joints set by the value of a phase offset. An expression for the optimal phase offset to maximize forward velocity is derived by simplifying a previous result from the literature. An approximation of the dynamics of serpentine locomotion for a snake traveling at constant velocity is then derived, and this model is used to obtain an approximate limiting expression for the propulsive force generated per link as a function of the number of links in the snake. Methods to control a snake composed of coupled linkages are explored and the mechatronic design of a fully integrated prototype is presented. Experiments on force control, locomotion and turning, and detection and interaction with obstacles using the prototype are then described.

Multiple robot manipulators cooperating in a common manipulation task can accomplish complex tasks that a single manipulator would be unable to complete. To achieve physical cooperation with multiple manipulators working on a common object, interaction forces need to be controlled throughout the motion. The aim of this research is to develop an inverse dynamics model-based cooperative force and position control scheme for multiple robot manipulators. An extended definition of motion is proposed to include force demands based on a constrained Lagrangian dynamics and Lagrangian multipliers formulation. This allows the direct calculation of the inverse dynamics with both motion and force demands. A feedforward controller based on the proposed method is built to realise the cooperative control of two robots sharing a common load, with both motion and force demands. Furthermore, this thesis develops a method to design an optimal excitation trajectory for robot dynamic parameter estimation utilising the Schroeder Phased Harmonic Sequence. This method yields more precise and accurate inverse dynamics models, which result in better control. The proposed controller is then tested in an experimental set-up consisting of two robot manipulators and a common load. Results show that in general the proposed controller performs noticeably better position and force tracking, especially for higher speed motions, when compared to traditional hybrid position/force controllers.

For robotic manipulation tasks in uncertain environments, research typically revolves around developing the best possible software control strategy. However, the passive dynamics of the mechanical system, including inertia, stiffness, damping and torque limits, often impose performance limitations that cannot be overcome with software control. Discussions about the passive dynamics are often imprecise, lacking comprehensive details about the physical limitations. In the first half of this paper, we develop relationships between an actuator's passive dynamics and the resulting performance, to better understanding how to tune the passive dynamics. We characterize constant-contact physical interaction tasks into two different tasks that can be roughly approximated as force control and position control and calculate the required input to produce a desired output. These exact solutions provide a basis for understanding how the parameters of the mechanical system affect the overall system's bandwidth limit without limitations of a specific control algorithm. We then present our experimental results compared to the analytical prediction for each task using a bench top actuator. Our analytical and experimental results show what, until now, has only been intuitively understood: soft systems are better at force control, stiff systems are better at position control, and there is no way to optimize an actuator for both tasks. / Graduation date: 2012

For precise robot control, endpoint compliance strategies utilize feedback from a force sensor located near the tool/workpiece interface. Such endpoint force control systems have been observed in the laboratory to be limited to unsatisfactory closed-loop performance. This paper discusses the particular dynamic properties of robot systems which can lead to instability and limit performance. A series of lumped-parameter models is developed in an effort to predict the closed-loop dynamics of a force-controlled single axis arm. The models include some effects of robot structural dynamics, sensor compliance, and workpiece dynamics. The qualitative analysis shows that the robot dynamics contribute to force-controlled instability. Recommendations are made for models to be used in control system design.

Dynamic models have been developed in an attempt to match the response of a robot arm. The experimental data show rigid-body and five resonant modes. The frequency response and pole-zero arrays for various models of structural flexibility are compared with the data to evaluate the characteristics of the models, and to provide insight into the nature of the flexibility in the robot. Certain models are better able to depict transmission flexibility while others describe types of structural flexibility.