Warren P. Seering

Understanding bandwidth limitations in robot force control

This paper provides an analytical overview of the dynamics involved in force control. Models are developed which demonstrate, for the one-axis explicit force control case, the effects on system closed-loop bandwidth of a) robot system dynamics that are not usually considered in the controller design; b) drive-train and task nonlinearities; and c) actuator and controller dynamics. The merits and limitations of conventional solutions are weighed, and some new solutions are proposed. Conclusions are drawn which give insights into the relative importance of the effects discussed.

Time-Optimal Negative Input Shapers

Input shaping reduces residual vibration in computer controlled machines by convolving a sequence of impulses with a desired system command. The resulting shaped input is then used to drive the system. The impulse sequence has traditionally contained only positively valued impulses. However, when the impulses are allowed to have negative amplitudes, the rise time can be improved. Unfortunately, excitation of unmodeled high modes and overcurrenting of the actuators may accompany the improved rise time. Solutions to the problem of high-mode excitation and overcurrenting are presented. Furthermore, a simple look-up method is presented that facilitates the design of negative input shapers. The performance of negative shapers is evaluated experimentally on two systems; one driven by a piezo actuator and the other equipped with DC motors.

On dynamic models of robot force control

A series of lumped-parameter models is developed in an effort to predict the dynamics of simple force-controlled robot systems. The models include some effects of robot structural dynamics, sensor compliance, and workpiece dynamics. A qualitative analysis suggests that the robot dynamics contribute to force-controlled instability.

Three dynamic problems in robot force control

Three dynamic problems which arise in robot systems are discussed: rigid-body bandwidth; dynamically noncolocated flexible modes; and dynamically colocated flexible modes. These effects combine to set the closed-loop bandwidth achievable in the individual joint control loops. Simple models of robot systems are presented to illustrate these three dynamic effects. Some laboratory data are then presented and analyzed in these terms. Finally, the implications for robot system design is discussed in the hope that these issues will be considered in the development of the next generation of robot systems and machine tools.<<ETX>>

Using input command pre-shaping to suppress multiple mode vibration

Spacecraft, space-borne robotic systems, and manufacturing equipment often utilize lightweight materials and configurations that give rise to vibration problems. Prior research has led to the development of input command preshapers that can significantly reduce residual vibration. These shapers exhibit marked insensitivity to errors in natural frequency estimates and can be combined to minimize vibration at more than one frequency. A method for the development of multiple mode input shapers which are simpler to implement than previous designs and produce smaller system response delays is presented. The new technique involves the solution of a group of simultaneous nonlinear impulse constraint equations. The resulting shapers were tested on a model of MACE, an MIT/NASA experimental flexible structure.<<ETX>>

The function of the primary ligaments of the knee in varus-valgus and axial rotation

Abstract Four in vitro human knee specimens have been loaded, two with varus-valgus femoral rotations and two with internal and external axial tibial rotations. Each specimen has been tested in full extension and in 30° of flexion. All orthogonal components of applied force and moment required to cause the rotations were measured as were all resultant orthogonal components of load on the femur. Curves fit to the data were studied to establish the portions of the applied load transmitted by each ligament.

Slewing Flexible Spacecraft with Deflection-Limiting Input Shaping

A control scheme is described for slewing e exible spacecraft with both suppression of dee ection during the slew and elimination of residual oscillations. The method minimizes the maneuver time subject to constraints on residual vibration magnitude, sensitivity to modeling errors, rest-to-rest slew distance, and the transient dee ection amplitude. Furthermore, a solution is sought that provides inherent fuel efe ciency. The feasibility of the approach is demonstrated with linear and nonlinear computer simulations.

A zero-placement technique for designing shaped inputs to suppress multiple-mode vibration

The performance of many flexible systems can be improved by employing command shaping techniques to reduce machine vibration. The input shaping technique, in particular, has proven to be highly effective for a wide class of systems. Due to the mathematical complexity of higher-order problems, multiple-mode input shapers can be tricky to derive and difficult to understand. This paper proposes a new approach for deriving input shapers using zero-placement in the discrete domain. The flexibility and simplicity inherent in this formulation provide many options for maximizing shaper performance.

A nonlinear model of a harmonic drive gear transmission

Harmonic drives can exhibit very nonlinear dynamic behavior. In order to capture this behavior, not only must dynamic models include accurate representations of transmission friction, compliance, and kinematic error, but also important features of harmonic-drive gear-tooth geometry and interaction must be understood. In this investigation, experimental observations were used to guide the development of a model to describe harmonic-drive operation. Unlike less detailed representations, this model was able to replicate many of the features observed in actual harmonic-drive dynamic response. Unfortunately, since model parameters can only be derived from careful analysis of experimental dynamic response, it seems unlikely that any comparably sufficient representation can be constructed with parameter values obtained from catalogs or simple experimental observations.

The function of the primary ligaments of the knee in anterior-posterior and medial-lateral motions

Abstract The functions of the primary ligaments of the human knee have been investigated during anterior, posterior, medial and lateral tibial displacements. The ligaments of in vitro knee specimens have been loaded to large load levels and all three resultant orthogonal components of force and of moment acting on the femur as a result of the applied tibial displacements have been measured. Least squares cubic spline curves fit to the data have been analyzed to determine what percentage of a given applied load is transmitted by each ligament. These curves can also provide data for a computer model of the human leg.