Max Donath

Dynamic Feedback Linearization for Electrohydraulically Actuated Control Systems

Using the dynamic inversion principal, a globally linearizing feedback control law is developed for an electrohydraulic servo system. The proposed control law is implemented on a rotational joint driven by a linear actuator. The results from experiments indicate that better uniformity of response is achieved across a wider range of operating conditions than would otherwise be possible. Improved symmetry is obtained for the extension and retraction phases of motion for an asymmetric actuator under various loading conditions and actuator positions. As a result of the improvements in linearity, significantly better performance is achieved when using linear controllers. To incorporate the effects of parametric uncertainties on the feedback linearization, a state space linear fractional representation of the parametrically uncertain linearized system is also developed. This uncertainty model is specifically suited for the design of robust control systems using the μ-synthesis and H∞ based approach.

Animal behavior as a paradigm for developing robot autonomy

We have been examining naturally occurring examples of autonomous systems in order to identify characteristics that might provide insight into our research on autonomy. In the first part of this paper, we review relevant research which has occurred in the area of animal behavior. Based on certain observations, we have proposed a number of primitive reflexive behaviors which are then used to develop several useful emergent behaviors. These emergent behaviors were demonstrated on a simulated mobile robot and then successfully implemented on Scarecrow, an actual robot. Scarecrow allows us to demonstrate that behavioral control strategies do indeed provide us with a powerful strategy for robust operation in dynamically changing unstructured environments in which one cannot impose unrealistic expectations on the performance of the machine or its sensors. A consequence of this is, that given the unpredictability of human actions, such behavioral control strategies may facilitate the safe interaction of man and machine.

The Minnesota Scanner: a prototype sensor for three-dimensional tracking of moving body segments

An advanced method for tracking the three-dimensional motion of bodies is presented. This system, which is presently being used for human motion tracking, has the potential for dynamically characterizing robot motion and also provides a means for facilitating robot endpoint control. Three rotating planes of laser light, fixed and moving photovoltaic diode targets, and a pipelined architecture composed of analog and digital electronics are used to locate multiple targets whose number is only limited by available computer memory. Data collection rates are a function of the laser scan rotation speed and are selectable up to 480 Hz. The tested performance on a preliminary prototype designed for 0.1-in. accuracy at a 480-Hz data rate includes a resolution of 0.8 mm, a repeatability of +or-0.636 mm, and an absolute accuracy of +or-2.0 mm within an eight cubic meter volume with all results applicable at the 95% level of confidence along each coordinate direction. The system can be used to reduce XYZ target position data to body angular orientation which, for this first prototype, ranges in accuracy from +or-0.5 degrees to +or-1 degrees . Moving targets can be tracked at speeds exceeding 1 m/s with signal integrity tested but not limited to 25-Hz motions. >

Coulomb friction joint and drive effects in robot mechanisms

Robot design and evaluation requires an extensive analysis of the nature and the quantitative interrelations of the dynamic factors that govern the operation of a manipulator. Coulomb friction is an inherent phenomenon in devices with moving parts. In multi-degree of freedom mechanical devices, Coulomb friction causes highly nonlinear coupling of the corresponding differential equations of motion. Furthermore, the presence of Coulomb friction is responsible for motion inaccuracies. This paper addresses the fundamental issues associated with Coulomb friction and investigates their consequences on design and performance considerations. Moreover, it develops the reasoning that permits the incorporation of Coulomb friction into the study of the dynamic response of a robot. Within this framework, a recursive approach that reveals the motion progress of a manipulator may be developed for digital simulation-oriented applications, implementation of the properties and principles discussed therein, may help uncover critical aspects of a manipulators functionality and suggest ways for design improvements.

Robust control of flexible manipulators via μ-synthesis

Abstract An experimental flexible arm serves as testbed to investigate the efficacy of the μ -synthesis design technique in the control of flexible manipulators. A linearized model of the testbed is derived for control design. Discrepancies and errors between the linearized model and the physical system are accounted for in the control design via uncertainty models. These uncertainties include: unmodeled high-frequency dynamics, errors in natural frequencies and damping levels and actuator and sensor errors. Colocated and noncolocated controllers are designed using μ -synthesis. It is observed, theoretically and experimentally, that the μ -synthesis design technique is a viable control tool for tip tracking with flexible manipulators.

Evaluation of in-vehicle GPS-based lane position sensing for preventing road departure

We present a systematic method for quantifying the dynamic performance of differential GPS, and in particular the Novatel RT-20 DGPS unit, for determining a vehicles lateral position in a highway lane. Image processing is used. Novatels RT-20 Double Differencing Carrier Phase Measurement System is specified to achieve real-time positioning performance of better then 20 cm nominal accuracy. This paper documents the results from a series of dynamic tests carried out on the RT-20 to verify its actual accuracy while on a moving vehicle. The approach adopted here incorporates synchronized data acquisition using two separate computer systems, and experimental verification of the computational latency of the RT-20. The image processing scheme used for this analysis achieved high accuracy by taking advantage of subpixel resolution in the image processing algorithm. Our results indicate that the RT-20 system exhibited a mean error of 2.03 cm in the lateral direction, and 3.16 cm in the longitudinal direction (note that both lateral and longitudinal are with respect to the moving truck) while moving at speeds ranging from 15 mph through 40 mph. The corresponding error standard deviations were 1.98 cm and 34.87 cm respectively. Our main interest is in the lateral positioning performance of the RT-20, which turns out to be very good. Furthermore, we believe that the longitudinal error standard deviation exhibited by the RT-20 can be reduced further by using an algorithm that eliminates the outlier data points.

Dynamic model of a one-link robot manipulator with both structural and joint flexibility

Equations were developed which describe the motion of a robot manipulator which incorporates both structural and joint flexibility. The emphasis was on the dynamic model of a one-link flexible manipulator to show how critical it is to incorporate joint flexibility and to demonstrate the significance of cross coupling among state variables for small deflection. A set of decoupled dynamic equations was obtained based on the assumed-mode method and on orthogonality relations. By comparison to a model which incorporates cross coupling, it was shown that it is sufficiently accurate to use the decoupled dynamic equations for small joint and structural deflections. Simulations of varying joint stiffness characteristics indicates that there is no rigid mode for the flexible joint system. As such, both structural and joint flexibility must be considered in the analysis and control of such systems.<<ETX>>

A fundamental investigation of tilt control systems for narrow commuter vehicles

One way of addressing traffic congestion is by efficiently utilizing the existing highway infrastructure. Narrow tilting vehicles that need a reduced width lane can be part of the solution if they can be designed to be safe, stable, and easy to operate. In this paper, a control system that stabilizes the tilt mode of such a vehicle without affecting the handling of the vehicle is proposed. This control system is a combination of two different types of control schemes known as steering tilt control (STC) and direct tilt control (DTC) systems. First, different existing variations of both STC and DTC systems are considered and their shortcomings analysed. Modified control schemes are then suggested to overcome the deficiencies. Then a new method of integrating these two control schemes that guarantees smooth switchover between the controllers as a function of vehicle velocity is proposed. The performance of the proposed STC, DTC, and integrated systems is evaluated by carrying out simulations for different operating conditions and some experimental work. The design of a second-generation narrow tilting vehicle on which the developed control system has been implemented is presented.

Road bank angle considerations in modeling and tilt stability controller design for narrow commuter vehicles

Narrow tilting vehicles that occupy a half width lane can play a big role in addressing traffic congestion problems by effectively doubling the capacity of existing highway lanes. In designing the tilt stability control system of such a vehicle consideration of the road bank angle is crucial since it plays a big role in minimizing the torque requirement of the tilt actuation system. In this paper the dynamic model of a tilting vehicle that takes into account the road bank angle is first developed. Then the design of a direct tilt control scheme that stabilizes the tilt mode of such a vehicle is outlined. Since the controller designed makes use of current road bank angle data, a method is outlined on how to determine the road bank angle from accelerometer readings. Finally simulation results are presented and discussion of the results is given

Optimizing the location of assembly tasks in a manipulator's workspace

Off-line programming of robotic workcells has become an invaluable tool for the workcell designer, however, current robot programming systems lack the capability to automatically determine the locations for performing assembly operations within the manipulators workspace. A gradient function of manipulability in Cartesian space was developed based on an explicit determination of the manipulability function and the gradient of the manipulability function in joint space. A modified method of the steepest descent optimization procedure was then used as the basis for an algorithm that automatically locates an assembly task away from singularities within a manipulators workspace. This algorithm can readily be extended to any nonredundant manipulator.