James K. Roberge

Classical dual-inverted-pendulum control

A cart with two independent inverted pendula, called a dual-inverted-pendulum system, is analyzed and compared to the single-inverted-pendulum system using classical linear methods. Using only the angles of the pendula and the position of the cart, a classical controller is designed that stabilizes the pendula in the inverted position with the cart at the center of the track. Simulations of the transient response to initial conditions are presented. Intuitive reasoning and an insightful approach to the control design are major emphases of this effort.

Spatial tracking using an electro-optic nutator and a single-mode optical fiber

Recently, the potential advantages of using single-mode optical fibers within free-space laser communication systems has been recognized. Of particular importance within an optical receiver is the spatial tracking system, which must couple the received signal into a single-mode optical fiber in the presence of environmental disturbances. Because optical beamwidths are small (microrad) and typical disturbances can be large (mrad), this requirement makes the design of the spatial tracking subsystem an important and difficult part of an optical receiver. Previous systems have utilized mechanical techniques for nutating the tip of the receiving fiber to derive tracking information. A new technique, based on using a fixed fiber and an electro-optic beam deflector, is proposed. Design considerations and experimental results are presented.

Design considerations and experimental results for direct-detection spatial tracking systems

The effect of signal and background shot noise, as well as device noise, on the performance of a direct-detection spatial tracking system is investigated for arbitrary detector arrays, assuming linear loop operation. The performance of quadrant detectors as a function of background radiation and detector radius is then analyzed and compared to some performance bounds. Tracking systems based on avalanche photodetectors (APD) and PIN detectors also are compared. The effects of non-focal-plane processing, focus error, and pupil walk on tracking performance are investigated. Experimental results of a 2.4 kHz two-axis tracking loop operating at low signal power using an APD quadrant detector are presented.

Space Radiation Damage Measurements in the Earth Synchronous Orbit

For direct measurement of the integrated radiation dose experienced in Earth synchronous orbit, p-i-n diodes were flown as radiation dosimeters on LES-6. The diode, which has a lifetime of 10-4 seconds in the intrinsic region, was originally developed as a neutron dosimeter, but can detect 1-MeV electron fluences as low as 1013 e·cm-2. Observations over three years in orbit are presented.

A combined display for computer venerated data and scanned photographic images

Project Intrex (INformation TRansfer EXperiments) is a program of research and experiments intended to provide a foundation for the design of future information-transfer systems. The library of the future is conceived as a computer-based communications network, but at this time we do not know enough details about such a network to design it. Lacking are the necessary experimental facts, especially in the area of users interaction with such a system. To discover these facts, we want to conduct experiments not only in the laboratory, but above all, in the real-life environment of a useful operating library.

A High-Accuracy Shunt Switch for D/A Conversion

A current-summing digital-to-analog converter can be constructed by combining a switched, binary-weighted resistor network with an operational amplifier. Contributions to total system errors from reference inaccuracies, resistor ratio tolerances, and operational amplifier errors can be held to less than one part in 105 and are normally far outweighed by error contributions from the switches employed. This correspondence describes a technique which reduces the switch error contributions to an insignificant level.

Low Power Fast Pulse Circuit Techniques in the MIT Gamma-Ray Telescope

Usually, circuits for handling fast pulses are designed at low impedance levels and hence operate with relatively high power consumption. In the electronics instrumentation of the MIT Gamma-Ray Telescope, the analog portion of the electronics, which must respond to input pulses as short as 3 ns and which contains about 430 transistors, operates with a power requirement of 350 mw. The entire electronics package of 8000 components (1300 transistors) has a total power consumption of only 500 mw. The initial section of this paper will briefly describe the salient points in the design and operation of the Gamma-Ray Telescope. The subsequent portions will then discuss the signal processing involved in the analog section of the instrument and some design problems peculiar to the conflicting requirements of high speed input pulses and low power circuits.