User Stories 

“[These] modern engineering modeling tools allow engineers to create realistic simulations of controlled mechanical systems. This approach frees engineers from the timeconsuming and errorprone process of manually writing their own equations of motion.”
Carl Baker, PNNL
Richland, Washington–based Pacific Northwest National Laboratory (PNNL) was contracted by the U.S. Department of Energy to develop a robotic arm to break up sediment in nuclear waste. The waste was stored in tanks 40 feet underground. Because access was limited to a 42inch hole in the top of the tank, the movable end of the robotic arm had to be long and thin, with relatively flexible links and low natural frequencies. This design made the arm prone to vibration and oscillation.PNNL engineers settled on a design concept that included a longreach manipulator (LRM), used for coarse positioning, with a lighterduty shortreach manipulator (SRM) mounted on its tip. With sensors and control computers in place to provide feedback control for oscillation damping, the basic idea was to alter the intended command trajectory of the short arm to take advantage of the arm’s own inertial forces. The movement would cancel oscillations while still meeting the normal requirements of stability, robustness, and reliability.
This highly nonlinear system would be difficult to model with conventional design tools.
PNNL engineers built a prototype of the proposed design in half scale. The test bed was a steel beam 15 feet long, one foot high, and 0.75 inches thick. The beam was fixed at one end and free to move in a horizontal plane at the other. The free end was supported by an air bearing that provided a lowfriction interface with the floor while restricting any torsion about the longitudinal axis of the beam. A sixdegreesoffreedom hydraulic manipulator was mounted on the beam at the freefloating end to represent the SRM.
While the physical prototype could be used to evaluate many control strategies, engineers could spend years in this physical testing environment in a search for the optimal approach.
PNNL engineers chose MATLAB^{®} and Simulink^{®} to evaluate alternative control strategies. Computer simulation allowed them to consider many operational parameters, such as the position, velocity, and acceleration of every point at each time step in the simulation. That data was then used in analysis routines to calculate optimal control equations.
PNNL engineers used DADS (Dynamic Analysis and Design System) 3D mechanical system simulation software from Computer Aided Design Software, Inc., to model the mechanics. This software, which interfaces with MATLAB and Simulink, allowed engineers to quickly model, simulate, and animate the basic test bed system. The model included open and closedloop control systems and realistic hydraulic valve and actuator properties. It also incorporated finite element results that accurately modeled the flexible link.
The tools had met the biggest simulation challenge—accurately reflecting physical reality. Once linearized, the model was used to develop the constantcoefficient differential equations that drive the controller design models (CDMs), the mathematical models that compute the actuator responses needed to dampen vibrations. The step functions developed from the simulation output were applied to the azimuth angle of the SRM, and the LRM tip response signals were measured.
PNNL engineers analyzed those responses to produce optimized linear transfer functions based on the step response of the robotic system. The analysis was performed under a variety of loading conditions to obtain a family of transfer functions.
Develop control strategies to compensate for oscillation and vibration in a flexible, 30footlong robotic arm built to handle nuclear waste
Use MATLAB and Simulink to create a comprehensive, dynamic model of the arm and evaluate control options