Jason J. Gorman

Dynamic analysis of the cable array robotic crane

Offshore loading and unloading of cargo vessels and on board cargo relocation during conditions of Sea State 3 or greater have been found to be difficult with existing crane technology due to oscillation of the payload. A new type of crane which uses four actuated cables to control the motion of the payload is presented. The closed chain configuration will intuitively provide more stability with respect to the motion of the sea compared to existing cranes. The kinematics and dynamics are derived using cable coordinates. Since there are four cables and three degrees of freedom, the system is redundant. This problem is solved by applying a geometric constraint to the equations of motion such that the reduced number of equations equals the degrees of freedom. The force distribution method is applied using linear programming to solve for the required cable tensions. Simulation results showing cable tensions and cable lengths during a typical crane operation are presented.

Analysis and design of parallel mechanisms with flexure joints

Flexure joints are frequently used in precision-motion stages and microrobotic mechanisms due to their monolithic construction. The joint compliance, however, can affect the static and dynamic performance of the overall mechanism. In this paper, we consider the analysis and design of general platform-type parallel mechanisms containing flexure joints. Based on static performance measures such as task-space stiffness and manipulability, and constraints such as joint stress, mechanism size, and workspace volume, we pose the design problem as a multiobjective optimization. We first calculate the Pareto frontier, which can then be used to select the desired design parameters based on secondary criteria, such as performance sensitivity and dynamic characteristics. To facilitate design iteration, we apply the pseudo rigid-body approach with a lumped approximation of the flexure joints. A planar mechanism is used to illustrate the analysis and design techniques.

Optimal force distribution applied to a robotic crane with flexible cables

A multiple cable robotic crane designed to provide improved cargo handling is investigated. The equations of motion are derived for the cargo and flexible cables using Lagranges equations and the assumed modes method. The resulting equations are kinematically redundant due to fewer degrees of freedom of the cargo than the number of cables. A nonlinear transformation is used to reduce the number of variables. An optimal force distribution method is then applied to the equations to solve for a set of necessary cable tensions which will cause the system to track a desired trajectory. These tensions are tested on the dynamic model using computer simulation. The results are compared against desired cable lengths and results gained in previous research using a rigid cable model.

Large Stroke Electrostatic Comb-Drive Actuators Enabled by a Novel Flexure Mechanism

This paper presents in-plane electrostatic comb-drive actuators with stroke as large as 245 μm that is achieved by employing a novel clamped paired double parallelogram (C-DP-DP) flexure mechanism. The C-DP-DP flexure mechanism design offers high bearing direction stiffness Kx while maintaining low motion direction stiffness Ky over a large range of motion direction displacement. The resulting high (Kx/Ky) ratio mitigates the onset of sideways snap-in instability, thereby offering significantly greater actuation stroke compared with existing designs. Further improvement is achieved by reinforcing the individual beams in this flexure mechanism. While the traditional paired double parallelogram (DP-DP) flexure design with comb gap G = 3 μm and flexure beam length L1 = 1 mm results in a 50- μm stroke before snap-in, the reinforced C-DP-DP design with the same comb gap and flexure beam length achieves a stroke of 141 μm. Furthermore, this C-DP-DP flexure design provides a 215- μm stroke with G = 4 μm and a 245-μm stroke with G = 6 μm. The presented work includes closed-form stiffness expressions for the reinforced C-DP-DP flexure, a design procedure for selecting dimensions of the overall comb-drive actuator, microfabrication of some representative actuators, and experimental measurements demonstrating the large stroke.

Automated Multiprobe Microassembly Using Vision Feedback

This paper describes the algorithm development and experimental results of a vision-guided multiprobe microassembly system. The key focus is to develop the capabilities required for the construction of 3-D structures using only planar microfabricated parts. Instead of using grippers, multiple sharp-tipped probes are coordinated to manipulate parts by using vision feedback. This novel probe-based approach offers both stable part grasping and dexterous part manipulation. The light weight of the part and relatively slow motion means that only kinematics-based control is required. However, probe motions need to be carefully coordinated to ensure reliable and repeatable part grasping and manipulation. Machine vision with multiple cameras is used to guide the motion. No contact force sensor is used; instead, vision sensing of the probe bending is used for the grasp force control. By combining preplanned manipulation sequences and vision-based manipulation, repeatable spatial (in contrast with planar) manipulation and insertion of a submillimeter part have been demonstrated with an experimental testbed consisting of two actuated probes, a passive probe, an actuated die stage, and two cameras for vision feedback.

Force Control of Linear Motor Stages for Microassembly

The microassembly of microelectromechanical systems from various micro-components requires the development of many new robotic capabilities. One of these capabilities is force control for handling micro-scale components with force control resolution on the order of micronewtons. In this paper, the force control of linear motor stages is discussed with application to the microassembly of MEMS. Linear motor stages provide an attractive solution for microassembly robots because they have a large working volume and can achieve high-precision positioning. However, the nonlinear friction and force ripple effects inherent in linear stages provide an obstacle to the required level of force control. A model of a single motor stage has been developed including dynamic friction effects. Based on this model, a robust nonlinear force controller has been designed to meet the microassembly requirements. The controller has been tested in simulation to demonstrate its effectiveness.Copyright

Design and Modeling of Thermally Actuated MEMS Nanopositioners

Several micro-scale nanopositioning mechanisms, or MEMS nanopositioners, have been developed for application in nanotechnology and optical sensors. In this paper, the design and modeling of these devices is presented along with initial experimental results. The MEMS nanopositioner is comprised of a parallel bi-lever flexure mechanism and a bent-beam thermal actuator. The flexure mechanism is designed to amplify and guide the motion of the actuator with high precision, while the thermal actuator provides the necessary force and displacement. The relationship between the applied voltage and resulting displacement for this mechanism has been calibrated using a scanning electron microscope and a simple image processing technique. A finite difference thermal model along with a FEA representation of the flexure mechanism and actuator is used to estimate the motion range of the device. Results from this method are compared with experimental calibrations, showing that the model provides a sufficient approach to predict the mechanism’s static performance. Finally, an open-loop controller based on calibration data was used to demonstrate the nanopositioning capabilities of these devices. The motion repeatability was found to be less than +/- 7 nm and step sizes well below 50 nm are possible, indicating suitable performance for many nanopositioning applications.

Control of MEMS Nanopositioners With Nano-Scale Resolution

Several approaches for the precision control of micro-scale positioning mechanisms, or MEMS nanopositioners, are presented along with initial experimental results which demonstrate nano-scale positioning resolution. The MEMS nanopositioners discussed in this paper are novel precision mechanisms comprised of a bent-beam thermal actuator and a flexure mechanism for each degree of freedom (DOF). These mechanisms can be used for a host of ultra-precision positioning applications, including nanomanipulation, scanning probe microscopy, high-density data storage and beam steering arrays. Concentrating on a 1 DOF MEMS nanopositioner, empirical static and dynamic models have been derived using characterization data obtained from experiments with optical and laser probe microscopes. Based on these models, three control approaches have been developed: 1) a quasi-static nonlinear open-loop controller, 2) a nonlinear forward compensator, and 3) a nonlinear PI controller. Simulation and initial experimental results are presented, and the benefits of each of these approaches are discussed.Copyright

Multiloop control of a nanopositioning mechanism for ultraprecision beam steering

Beam steering accuracy is critical to the successful operation of optical communications systems, especially those which take place over extreme length scales, such as for an interstellar spacecraft. In this paper, a novel beam steering mechanism and several control system approaches for ultra-precision beam steering are discussed. The beam steering mechanism is a nanopositioning device which utilizes a parallel cantilever configuration and a piezoelectric actuator to obtain extremely high positioning accuracy with minimal parasitic errors. A robust motion controller is presented for this mechanism which is designed to compensate for modeling uncertainty. This controller is intended for use with feedback from the nanopositioner’s built-in capacitance probe. Due to the need to track the trajectory of the steered beam, two additional control approaches are presented which combine the robust motion controller with additional feedback for the actual beam displacement. These multi-loop control approaches provide a level of robustness to thermal effects and vibrations which could not be obtained from a single sensor and feedback loop. Simulation results are provided for each of the control designs.

Significantly improved trapping lifetime of nanoparticles in an optical trap using feedback control.

We demonstrate an increase in trapping lifetime for optically trapped nanoparticles by more than an order of magnitude using feedback control, with no corresponding increase in beam power. Langevin dynamics simulations were used to design the control law, and this technique was then demonstrated experimentally using 100 nm gold particles and 350 nm silica particles. No particle escapes were detected with the controller on, leading to lower limits on the increase in lifetime for 100 nm gold particles of 26 times (at constant average beam power) and 22 times for 350 nm silica particles (with average beam power reduced by one-third). The approach described here can be combined with other techniques, such as counter propagating beams or higher-order optical modes, to trap the smallest nanoparticles and can be used to reduce optical heating of particles that are susceptible to photodamage, such as biological systems.