Jeffrey S. Schoenwald

Integration of multiple sensors to provide flexible control strategies

The design and operation of an experimental robot workstation that adaptively responds to a disordered or changing environment are presented. The adaptive control is provided by the use of several sensor subsystems in a distributed architecture workcell. Included in the workstation are a commercial 6-axis robot, a microprocessor-based vision subsystem, an acoustic-ranging sensor subsystem, and a force-torque sensing subsystem; each subsystem is microprocessor based, and supervisory control is provided by a workcell host computer. Five demonstrations of adaptive control are described, including real-time path modification by single sensors and integrated use of multiple sensors.

A novel fiber optic tactile array sensor

A new robotic tactile sensor based on fiber optics is described. The sensor array is based on a matrix of optical fibers in perpendicular rows and columns separated by an elastomeric pad. The elastomer provides compliant response to normal force applied to the structure, and the magnitude of the force is measured by a change in transmitted light coupling across a gap between the fibers. Location of the force applied to the pad is obtained by the matrix addressing of rows of fibers individually excited by light emitting diodes (LEDs) and orthogonally arranged columns of signal pickup fibers conducting the sensed light to photodetectors. The structure can be made thin enough to wrap conformally over a curved surface. The sensor array lends itself to sensing and control applications in robotic end effectors and as a tactile imaging platform for part identification and location to complement or replace optical vision systems.

Improved robot trajectory from acoustic range servo control

A technique is described for improving the accuracy of the path followed by a robot arm in precision tracking of an object surface contour. Acoustic pulse echo data from a sensor mounted on an end effector was processed to compute distance to an object surface several inches from the sensor. Real-time path modification was then based on this computed distance. An eddy current proximity sensor positioned approximately 30 mils from the surface was used to monitor the acoustic sensor and robot performance. Both sensors produced identical results in tracking variations in distance from the surface with the robot in open loop mode for a linear path over a precision ground surface for which the robot was taught the two endpoints of the trajectory. Variation from the linear path was as large as 10 mils. With the acoustic range sensor providing input for a control algorithm to maintain a fixed offset as the robot traversed the surface, path deviation was reduced to +/- 3 mils using single measurements in the real-time mode. This is attributable to the inherent noise level of the sensor system. Averaging 10 measurements reduced the standard deviation in position measurement and control to +/- 1.2 mils.

Acoustic Range Sensing Servo Control: Improved Robot Positioning and Trajectory

Absrracr-A technique is described for improving the accuracy of the path followed by a robot arm in precision tracking of an object’s surface contour. Acoustic pulse-echo range data from a sensor mounted on a robot end effector was used to enable real time path modification, correcting for errors in the robot’s internal sensors, calibration, and servo-control. An eddy current proximity sensor positioned approximately 30 mil from the surface monitored the acoustic sensor and robot performance. In open-loop mode, both sensors produced identical results in tracking variations in distance from the surface. Variation from a linear path was as large as 10 mil. The acoustic range sensor enabled a servo control algorithm to reduce path deviation lo +3 mil using single measurements in real time. This variation is attributable to the inherent noise level of the sensor system and ambient fluctuations in sound velocity. Averaging ten measurements reduced the standard deviation in position measurement and control to +_l2 mil. The acoustic ranging system has also proved capable of correcting position error induced by a dynamic change in mass loading at the end effector.

Design of a Laser RF Modulation Phase Sensitive Scheme for Sensing and Decoding Multimodal Vibration in Large Composite Space Structures

Current designs for large light-weight space structures prompt the need for enhanced damping of the structural system to improve system performance and/or simplify the controls designs. Enhancement of passive damping with local active damping is very desirable. Development of embedded sensors, actuators, and signal processing systems would provide local control superior to passive damping techniques.

A Novel Fiber Optic Tactile Array Sensor

A new robotic tactile sensor based on fiber optics is described. The sensor array is based on a matrix of optical fibers in perpendicular rows and columns separated by an elastomeric pad. The elastomer provides compliant response to normal force applied to the structure, and the magnitude of the force is measured by a change in transmitted light coupling across a gap between the fibers. Spatial structure of the force applied to the pad is obtained by the matrix addressing of rows of fibers individually excited by light emitting diodes (LEDs) and orthogonally arranged columns of signal pickup fibers conducting the sensed light to photodetectors. The structure can be made thin enough to wrap conformally over a curved surface. The sensor array lends itself to sensing and control applications in robotic end effectors and as a tactile imaging platform for part identification and orientation to complement or replace optical vision systems.

Amplitude-modulated laser-driven fiber-optic RF interferometric strain sensor

A new fiber optic sensor for use in flexible structures is reported. The sensor is based on an extension of Rogowskis design, in which the laser-driven optical beam in the fiber is modulated at radio frequency and strain is detected by a shift in the phase delay as the fiber dimension is strained with the structure. This sensor -- FORISS -- differs in that it consists of an embedded closed loop of fiber coupled to a laser/detector/fiber optic delay line circuit through a 2 X 2 coupler. The closed loop has the characteristics of a Fabry-Perot cavity operating at radio frequency wavelengths within the fiber. A parametric model of the sensor that enables both physical characterization of prototype sensors and insights which guide design optimization of the sensor is described. Experiments were performed on fiber- embedded composite specimens tension-stressed to failure at 26 kpsi and 7400 microstrains. The sensor survived to the point of coupon failure. The data indicates that the sensor possesses the properties predicted by the theory.

Embedded Sensors and Actuators for Lightweight Structures

Much effort has been invested in developing control methodologies that modify the joint torque profiles in a lightweight, high speed robot manipulator in order to suppress vibration in the flexible links and improve end-point positional accuracy [1-7]. Similar concerns arise regarding the interaction between structures and control methodologies for large space structures [8]. These techniques rely on a model of the flexural behavior of the link(s). Some of them operate in real-time [1,2], while others are computed prior to motion and may depend on inverse dynamics [3–6]. The principal drawback to all of these is that while the vibration modes are properties of the flexible links, the attempted solutions rely on actuation at the joint motors. This condition of non-collocation of the vibrational coordinates and the actuators’ degrees-of-freedom has hampered the satisfactory identification of a real-time controller. Furthermore, techniques for sensing have also relied on location of sensors remote from the actual vibrational coordinates [1–6,8].

Overview of fly-by-light research and development at Rockwell International

An overview is presented of fiber optic smart structure research at Rockwell for Fly-by-Light technology. The methods emphasize the use of radio frequency (rf) amplitude modulation of the optical intensity and detection of phase and amplitude of the rf signal transmitted through various optical fiber systems. Strain is transferred from metallic or composite structures to the embedded optical fibers.

UHF Modulation and Fourier Transform Differential Time Domain Techniques for Measuring Strain Via Fiber Optics

The design and manufacture of future space transportation and delivery systems will be strongly driven by safety, cost, maintenance and reliability considerations. Advanced composite structural components are likely to be a key element in realizing these system objectives. Composites have the additional potential of enabling the embedment of sensors for system health monitoring, which supports requirements for low cost safety, maintenance and repair diagnosis.