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Unique Ring Motor for DARPA Unmanned Vehicles
Motor shape suits special needs of ducted-fan and screw propulsions
Al Presher -- Design News, June 27, 2005
A high-efficiency brushless ring motor based on patented electromotive coil technology is reducing energy consumption and increasing operational time for DARPA Unmanned Vehicles. For ground and underwater vehicles, the ring motor can eliminate transmissions, allow larger payloads, and provide an ability to move faster, farther, or longer between battery charges.
Ring motor technology
The brushless ring motor (TG8250) developed by ThinGap Corp. (www.thingap.com) under DARPA contract is in development for a variety of air (UAV), ground (UGV), and underwater (UUV) applications. The motor delivers 1.5 hp continuous per pound, provides a form-fitting platform that the company claims is quieter than previous designs, conserves battery power, and provides a gyroscopic effect that reduces vibration and stabilizes the UAV.
"The motor looks like a steel ring with an outside diameter of 8.25 inches, coupled to an iron, inner ring, with an inside diameter of 7.5 inches and height of 1.4 inches," says Greg Graham, chief technology officer for ThinGap. "A copper centered ring forms the stator that sits inside the iron rings."
"In fact, it doesn't look like a motor with its large open inner diameter surrounded by the 0.374-inch thick ring that makes the motor," Graham says. "The shape of the ring motor is part of what makes it a solution for ducted-fan, direct wheel, and screw drive propulsion."
In ducted-fan applications, Graham says the blades fit within the inside diameter of the ring motor and the outer ring becomes the rotating member over which nacelles or composite structures can shield the mechanics of the outer ring. With its large diameter ring, the motor creates its own gyroscopic effect stabilizing the craft similar to the rotors of a helicopter in flight. The stator and rotor's magnetic fields create a fixed path at the centerline of rotation.
Offsetting the rotor magnetic field from its path within the stator magnetic field creates a magnetic force that resists the offset. Graham says this increases stability, allowing UAV's to fly during rough weather conditions and to dampen vibration and improve imaging capabilities.
Unique technology
Ring motor with rendering of fan blades developed by ThinGap for use in unmanned vehicles.
The ring motor is fundamentally different from a conventional dc brushless motor in two areas: the coil and rotating parts of the motor. The ring motor coil (stator) replaces wire windings with precision-machined copper sheets formed into a circular coil, allowing a higher copper-packing density than copper wire. The coil assembly is a free-standing coil structure without supporting laminations.
By winding the coils in parallel or in series, the stator assembly can be operated at 50V to 280V or at three different torque constants. At 72V, the motor operates at 75 percent efficiency, for motor and controller with all 4× coils in series, and provides shaft output power of 1,200W. With quadruple redundancy, if one coil fails, the motor continues to operate. And since iron is not used in the coil, Graham says the magnetic field does not affect inertia, reduces rotational losses (eddy currents in the laminations), and allows the non-operating coil to freewheel like auto-rotating helicopter blades.
Different applications
Under the latest DARPA contract, a larger motor is being designed to develop 6.5 hp constant output from 500 rpm to 5,600 rpm to match the requirements of a UGV. Graham says the speed torque curve of the motor suggests that optimized output will reach 40 hp or 30 kW output. For UGV applications, the motor will deliver enough peak and high torque at low speed to eliminate gearboxes and drive trains in smaller vehicles although larger vehicles will still use gearboxes.
Graham claims that by eliminating the magnetic detent, the motors will deliver very smooth and cogging-free rotation, even at slow speeds. The ring motor is also envisioned to mount directly to the wheel, improving control and maneuverability.
For more information, contact ThinGap Motor Technologies: http://rbi.ims.ca/4393-512
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