Control moment gyroscope
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Control moment gyro(scope) is an attitude control device generally used in satellite attitude control systems. Commonly known as CMGs, they are used to control the orientation of spacecraft. A CMG consists of a spinning rotor and one or more motorized gimbals that tilt the rotor’s angular momentum. As the rotor tilts, the changing angular momentum causes a gyroscopic torque that rotates the spacecraft.
Fixed spinning rotors, known as reaction wheels, apply torque simply by changing the rotor spin speed, but CMGs are far more power efficient. For a few hundred watts and about 100 kg of mass, large CMGs have produced thousands of newton meters of torque. A reaction wheel of similar capability would require megawatts of power.
CMGs have been used for decades in large spacecraft, including Skylab and the International Space Station. In the years to come they will provide attitude control for several commercial earth-imaging satellites, such as Lockheed-Martin’s Ikonos, Ball Aerospace’s WorldView, and Astrium's Pleiades spacecraft.
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The most effective CMGs include only a single gimbal. When the gimbal of such a CMG rotates, the change in direction of the rotor's angular momentum represents a torque that reacts onto the body to which the CMG is mounted, e.g. a spacecraft. Except for effects due to the motion of the spacecraft, this torque is due to a constraint, so it does no mechanical work (i.e. requires no energy). Single-Gimbal CMGs exchange angular momentum in a way that requires very little power, with the result that they can apply very large torques for minimal electrical input.
These CMGs include two gimbals per rotor. As an actuator it is more versatile than a single-gimbal CMG because it is capable of pointing the rotor's momentum vector in any direction. However, the torque caused by one gimbal's motion often must be reacted by the other gimbal on its way to the spacecraft, requiring more power for a given torque than a single-gimbal CMG. If the goal is simply to store momentum in a mass-efficient way, as in the case of the International Space Station, dual-gimbal CMGs are a good design choice. Instead, if a spacecraft requires large output torque per available input power, single-gimbal CMGs are a better choice.
Most CMGs hold the rotor speed constant. Some academic research has focused on the possibility of spinning up and down the rotor as the CMG gimbals. These so-called variable-speed CMGs offer few practical advantages, mostly because the output torque from the rotor is likely orders of magnitude smaller than that caused by the gimbal motion. So, this effect adds nothing of practical value on the time scale of the motion typical of CMGs.
At least three CMGs are necessary for linear control of spacecraft attitude. However, no matter how many CMGs a spacecraft uses, gimbal motion can lead to relative orientations that produce no usable output torque along certain directions. These orientations are known as "singularities" and are related to the kinematics of robotic systems that encounter limits on the end-effector velocities due to certain joint alignments. Avoiding these singularities is naturally of great interest, and several techniques have been proposed. Two recent patents summarize some of these approaches: US Patent 7,014,150 & US Patent Application 20060027708
Honeywell Defense and Space Systems is the most prolific builder of CMGs. Their next-generation CMGs involve embedded control of multiple CMGs in a single momentum-control system and modular design, as described in Porter Davis's recent paper at the 2006 Guidance and Control Conference: 2006 AAS GNC Conference Program
- L3 Communications (the ISS CMGs, which include two gimbals per rotor) DGCMG 4800/250
- Astrium (a small single-gimbal CMG whose rotor is provided by Teldix): Astrium CMG
- Surrey Space Centre (a small CMG built for the Turkish BilSAT spacecraft): CMGs
- GDC Guidance Dynamics Corporation (non-flightworthy researach-grade variable-speed CMGs used in some flight simulators):
- Model 750 Educational Control Products (non-flightworthy classroom-demo multi-gimbal CMG)
CMG applications and fundamental research are undertaken at several institutions.
- Georgia Tech's Panagiotis Tsiotras has studied variable-speed CMGs in connection with flywheel energy storage and has built a spacecraft simulator based on them: faculty page
- Virginia Tech's Christopher Hall has built a spacecraft simulator as well: faculty page
- Texas A&M's John Junkins and Srinivas Vadali have written papers on VSCMGs for use in singularity avoidance: faculty page
- Cornell's Mason Peck is researching CMG-driven spaceborne robots CMG project page and has written about Honeywell's CMG-driven spacecraft simulator: faculty page
- Control Moment Gyros They also show a spacecraft simulator driven by CMGs:
- CMG Testbed Video