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Survey of Spaceborne Missions and Sensors
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Active sensor. A sensor having its own source of EMR (Electromagnetic Radiation); it
transmits a series of signals to the target and detects the echo. Information is obtained by
transmission and reception of radio waves (or light waves). A SAR instrument, a lidar, a
radar altimeter, a scatterometer and cloud profile radars --- are examples of active sensors.
Actuators. Refer to a class of onboard devices or techniques (manipulators) that are being
used in support of a great variety of functions. Some examples are:
• Actuators in the field of attitude control: reaction wheels, momentum wheels, magnetorquer coil/rods, permanent magnets, gravity---gradient boom, nutation damper, control
moment gyros, cold gas thrusters, solid thrusters, ion thrusters, mono--- or bi---propellant
engine, etc.
• Deployment and release mechanisms such as antenna and instrument deployment, positioners, aperture opening and closing devices
• Actuators are being used for autonomous sample acquisition, autonomous instrument
placement, robotic arms, rovers, etc.
• Starting with the 1990s, miniature actuators (demonstrators) are being introduced. Piezoelectric actuators 6395) (ultrasonic rotary motors) offer special characteristics that are of
interest for a number of applications, for instance: 6396) 6397)
--- Non---powered holding torque in the same range as the maximum driving torque (hard
brake without backlash)
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High positioning accuracy (micropositioning) in direct drive mode
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Feasibility of non---magnetic motor designs
The simplest actuators are direct piezoelectric actuators (DPA) which can produce displacements of the order of ten to one hundred microns and exhibit high stiffness. These actuators are robust when highly pre---stressed and may be used in active satellite structures
without the need for a locking mechanism during launch. --- The number of applications of
piezoelectric actuators in instruments flown in space is growing, in particular with regard to:
---
Precise pointing of optics (adaptive optics)
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Active damping of vibrations
• Small (mass, volume, power) CMGs (Control Moment Gyroscopes) are being
introduced at the start of the 21st century providing the dual function of actuator/sensor in
an attitude system of a spacecraft.
Adaptive optics. A technique which tries to compensate for the atmospheric degradation
(blurring) of the incoming signal of optical imaging systems. However, the compensations
achieved, regardless of method, are never “perfect.” The following approaches are in use:
6398) 6399)
• An on---line adaptive optics system (hardware solution). In this version, the adaptive
optics system is capable of compensating for the distortion of electromagnetic radiation as it
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6395) Note: The piezoelectric effect was discovered in 1881 when Pierre and Jacques Curie (Pierre Curie: 1859--- 1906;
Nobel prize in physics in 1903 together with Henri Becquerel) observed that quartz crystals generated an electric
field when stressed along a primary axis. The term piezoelectric derives from the Greek word “piezein,” meaning
to squeeze, and the electricity that results from pressure applied to the quartz crystal.
6396) R. Seiler, M.F. Six, M. Debarnot, R. Le Letty, F. Claeyssen, “The Ultrasonic Piezo Drive An Innovative Solution
for High--- Accuracy Positioning,” Proceedings of AIAA/USU Conference on Small Satellites, Logan, UT, Aug.
12--- 15, 2002, SSC02--- VIII--- 4
6397) Y. Bar--- Cohena, X. Baoa, W. Grandiab, “Rotary Ultrasonic Motors Actuated By Traveling Flexural Waves,” Proceedings of the Smart Structures and Materials Symposium, San Diego, CA, March 1--- 5, 1998, Paper 3329--- 82
6398) S. R. Restaino, D. M. Payne, “Adaptive Optics on a shoe string,” SPIE Vol. 3494, 1998, pp. 152--- 160
6399) D. Dayton, S. Browne, J. Gonglewski, “Control Loop Analysis for a Nematic Liquid Crystal Spatial Light Modulator Used in an Adaptive Optics System,” SPIE Vol. 3494, 1998, pp. 161--- 160--- 167