M C Good

Dynamic Models for Control System Design of Integrated Robot and Drive Systems

The design of high performance motion controls for industrial robots is based on accurate models for the robot arm and drive systems. This paper presents analytical models and experimental data to show that interactions between electromechanical drives coupled with compliant linkages to arm link drive points are of fundamental importance to robot control system design. Flexibility in harmonic drives produces resonances in the 5 Hz to 8 Hz range. Flexibility in the robot linkages and joints connecting essentially rigid arm members produces higher frequency modes at 14 Hz and 40 Hz. The nonlinear characteristics of the drive system are modeled, and compared to experimental data. The models presented have been validated over the frequency range 0 to 50 Hz. The paper concludes with a brief discussion of the influence of model characteristics on motion control design.

Dynamic effects in visual closed-loop systems

In this paper we argue that the focus of much of the visual servoing literature has been on the kinematics of visual control and has ignored a number of fundamental and significant dynamic control issues. To this end the concept of visual dynamic control is introduced, which is concerned with dynamic effects due to the manipulator and machine vision sensor which limit the performance. These must be explicitly addressed in order to achieve high-performance control. The paper uses simulation and experiment to investigate the feedback control issues such as the choice of compensator, and the use of axis position, velocity or torque controlled inner-loops within the visual servo system. The limitations of visual feedback control lead to the investigation of target velocity feedforward control, which combined with axis velocity control, is shown to result in robust and high performance tracking.

Re-definition of the robot motion control problem: Effects of plant dynamics, drive system constraints, and user requirements

The objective of this paper is a redefinition of the robot control problem, based on realistic (1) models for the industrial robot as a controlled plant, (2) end effector trajectories consistent with manufacturing applications, and (3) the need for end-effector sensing to compensate for uncertainties inherent to most robotic manufacturing applications. Based on extensive analytical and experimental studies, realistic robot dynamic models are presented that have been validated over the frequency range 0 to 50 Hz. These models exhibit a strong influence of drive system flexibility, producing lightly damped poles in the neighborhood of 8 Hz, 14 Hz, and 40 Hz, all unmodeled by the conventional rigid body multiple link robot dynamic approach. The models presented also quantify the significance of nonlinearities in the drive system, in addition to those well-known in the linkage itself. Realistic simulations of robot dynamics and motion controls demonstrate that existing controls coupled with effective path planning produce dynamic path errors that are acceptable for most manufacturing applications. Major benefits are projected, with examples cited, for use of end-effector sensors for position, force, and process control that compensate for uncertainties encountered on the factory floor.

The form drag of two-dimensional bluff-plates immersed in turbulent boundary layers

Measurements of the distributions of pressure on a bluff flat plate (fence) have been correlated with the characteristics of the smooth-wall boundary layer in which it is immersed. For zero pressure-gradient flows, correlations are obtained for the variation of form drag with plate height h which are analogous in form to the ‘law of the wall’ and the ‘velocity-defect law’ for the boundary-layer velocity profile. The data for adverse pressure-gradient flows is suggestive of a ‘law of the wake’ type correlation. Pressures on the upstream face of the bluff-plate are determined by a wall-similarity law, even for h /δ > 1, and are independent of the pressure-gradient history of the flow; the separation induced upstream is apparently of the Stratford-Townsend type. The effects of the history of the boundary layer are manifested only in the flow in the rear separation bubble, and then only for h /δ > ½. The base pressure is also sensitive to free-stream pressure gradients downstream of the bluff-plate. The relative extent of upstream influence of the bluff-plate on the boundary layer is found to increase rapidly as h /δ decreases. One set of measurements of the mean flow field is also presented.

Redefinition of the robot motion-control problem

The objective of this paper is a redefinition of the robot control problem, based on (1) realistic models for the industrial robot as a controlled plant, (2) end-effector trajectories consistent with manufacturing applications, and (3) the need for end-effector sensing to compensate for uncertainties inherent to most robotic manufacturing applications. Based on extensive analytical and experimental studies, robot dynamic models are presented that have been validated over the frequency range 0 to 50 Hz. These models exhibit a strong influence of drive system flexibility, producing lightly damped poles in the neighborhood of 8 Hz, 14 Hz, and 40 Hz, all unmodeled by the conventional rigid-body multiple-link robot dynamic approach. The models presented also quantify the significance of non-linearities in the drive system, in addition to those well known in the linkage itself. Simulations of robot dynamics and motion controls demonstrate that existing controls coupled with effective path planning produce dynamic path errors that are acceptable for most manufacturing applications. Major benefits are projected, with examples cited, for use of end-effector sensors for position, force, and process control.

Control of tool/workpiece contact force with application to robotic deburring

The design and implementation of a microprocessor-based system to control the interaction forces between a five-axis articulated robot and a workpiece is described. The control system worked in parallel with a robot controller by calculating position corrections that allowed forces to be controlled in the desired manner. These corrections were successfully interfaced to the controllers position control loop on an individual-axis level. Stable force-control algorithms were designed in spite of limitations imposed by flexibility in the robot drive train. For multi-degree-of-freedom force control, it is shown that each axis can be considered autonomous, obviating the need for a multivariable approach. Force control was implemented in both edge following and deburring experiments. In edge following, the commanded normal force ranged from 1 to 15 N, while the root mean square (rms) force errors remained constant. Errors increased from 0.5 to 1.5 N rms as tangential speed was increased from 1 to 9 cm s-1. The performance of the force control system during deburring operations was characterized across the full force and speed range of the cutting tools used. The smoothness of cut was shown to be consistent with manual deburring operations in terms of optimal feed and metal removal rates.

Dynamic effects in high-performance visual servoing

The authors discuss three dominant dynamic effects that are manifested in the use of robot end-mounted cameras for high-performance tracking and positioning tasks. They are time delay (or latency in the sensing and control blocks), time-varying loop gain due to perspective, and vibrations due to structural and drive compliance. Analysis of the dynamic is presented and compared with experimental results.<<ETX>>

Control of an Electromechanical Brake for Automotive Brake-By-Wire Systems with an Adapted Motion Control Architecture

A disk brake clamp force controller for electromechanical brakes (EMB) in automotive brake-by-wire systems may be obtained from a standard motion control architecture with cascaded position, speed and current control loops by replacing the outer position control loop with a force control loop. When implemented with proportional, integral and differential (PID) controllers this architecture generally performs well for standard motion control problems, but the EMB control problem is differentiated by a large operating range in which non-linear load disturbances such as friction become significant at high clamp forces of up to 30kN. This paper investigates the feasibility of a cascaded PI control architecture for an EMB with the intention of establishing a baseline standard against which the performance of future control schemes may be compared. Simulation results are presented based on an accepted EMB model.

Electromechanical Brake Modeling and Control: From PI to MPC

The electromechanical brake (EMB) force control problem has been approached in prior work using cascaded proportional-integral (PI) control with embedded feedback loops to regulate clamp force, motor velocity, and motor current/torque. However, this is shown to provide limited performance for an EMB when faced with the challenges of actuator saturation, load-dependent friction, and nonlinear stiffness. There is a significant margin for improvement, and a modified control architecture is proposed using techniques of gain scheduling, friction compensation, and feedback linearization. A further improvement is then achieved by incorporating a model predictive control that better utilizes the available motor torque. Simulation and experimental results are presented to demonstrate the improvement in performance.

Model Predictive Control for Reference Tracking on an Industrial Machine Tool Servo Drive

The benefits of model predictive control (MPC) have been well established; however its application to reference tracking on digital servo drives (DSDs), which typically have very fast update rates, is limited by the computational power of present-day processors. This paper presents a novel MPC formulation, which provides a mechanism to trade-off online computation effort with tracking performance, while maintaining stability. This is achieved by introducing a trajectory horizon, which is distinct from the prediction and control horizons typically encountered in MPC formulations. It is shown that increasing the trajectory horizon inherently leads to improved tracking; however larger horizon lengths also have the unwanted effect of increasing online computation. The proposed MPC formulation is compatible with recently developed explicit MPC solutions, and hence the burden of online optimization is avoided. The new approach is successfully implemented on an industrial machine tool DSD, and in terms of tracking accuracy, is shown to outperform the incumbent approach of cascaded PID control.