Investment Fosters Future Motor & Control Technology

Electric motors and motor controllers play an important role in the future of space exploration, military operations, and environmental conservation. Consequently, federal agencies continue to invest in these technologies by funding projects that improve their performance and efficiency. This work will not only shape the future of these specific application areas, but will also have significant crossover into the commercial marketplace.

Figure 1. NASA’s four-wheeled, autonomous mobile robot, Thibodeaux, will be used in future lunar and Mars missions.

Roughly the size of an all-terrain vehicle, Thibodeaux is designed to pre-scout areas for astronaut missions, and to carry heavier payloads for construction and in situ science tasks. The robot is directed by astronaut voice commands, and can be driven remotely using wireless communications and onboard video cameras. Four onboard, sealed lead-acid batteries provide power.

Currently reaching speeds of only 3 miles per hour (mph), one 1.8-kW electric motor (the patented 2x motor from WaveCrest Laboratories of Dulles, Virginia) is being outfitted into each wheel to enable speeds up to 30 mph. Originally designed for electric scooters, cargo scooters, and motorcycles with top speeds up to 37 mph, each motor will be current limited to produce 180 Newtonmeters (Nm) of peak torque for the NASA project.

The permanent magnet, DC brushless motor (see Figure 2) has an inverted architecture - meaning the rotor surrounds and rotates around the centermounted stator. The stator consists of a series of independently controlled electromagnets driven by a proprietary power electronics module. Conventional steel laminations are used as the stator core material. The rotor has rare earth-based permanent magnets and housings that are arranged in a proprietary design. Connected to the power electronics, a digital signal processor activates the electromagnets by analyzing motor position, desired torque, and the electrical characteristics of the energy management system powering the motor. Patented adaptive algorithms adjust the current and excitation sequence of each electrical phase.

Figure 2. In-wheel motors with an inverted architecture and integrated motor control are being outfitted into each of Thibodeaux's four wheels to enable speeds up to 30 mph.

"One of the key things about our technology is the patented software used in the integrated motor and control. It allows the motor to reconfigure itself within nanoseconds," said Tim Hassett, vice president and general manger of motors and operations for WaveCrest Laboratories. "The software senses load such that it reconfigures the motor and allows it to run at an optimal performance while properly dissipating heat." Integrating the motor and control also eliminates electric and magnetic field (EMF) issues and shortens cable lengths to reduce line chatter.

Additional customization is necessary for the NASA project mainly because Thibodeaux will be an all-wheel drive robot requiring communication among four-wheels, whereas the scooter only had a single wheel on the back drive. While specifics cannot be provided due to the nature of the work, software will be altered so that it works with the robot's existing vehicle control and some of the components on the power board will be changed to better suit the application. According to Hassett, alterations to the housing or anything mechanical will not be necessary.

Military Operations
The U.S. military invests heavily in the development of technology aimed at reducing the number of causalities on the battlefield. Consequently, unmanned ground, air, and underwater vehicles receive ample attention and funding. In 2004, for instance, the Defense Advanced Research Projects Agency (DARPA) kicked-off its annual Grand Challenge in response to a Congressional and U.S. Department of Defense (DoD) mandate. The Grand Challenge aims to accelerate the research and development of autonomous ground vehicles (AGVs). More discrete projects have been taking place all over the country with assistance from programs like the DARPA Small Business Innovation Research (SBIR) program for years.

Figure 3. Cross-section of the mating rotor ring assembly, consisting of a U-shaped iron channel with a thin wall and magnets mounted to the inside of the outer steel ring.

Under a DARPA contract ThinGap Corporation of Ventura, California, has developed an 8.3-inch diameter brushless ring motor (the TG8250 motor) based on their patented electromotive coil design that replaces an iron core and wire windings with a free-standing, precision-machined copper sheet coil, see Figure 3. A version of this motor powers a ducted fan that has already successfully lifted an unmanned air vehicle (UAV) in initial tests. The unique feature of this lightweight thin ring motor (7.5" ID) is that the propeller is mounted inside of it.

Based on a Phase II SBIR contract granted by DARPA in October 2004, the company will be taking the technology a step further, adapting it into a larger, 14-inch model (see Figure 4) for use as a potential electric-drive in a six-wheel, unmanned ground vehicle (UGV). The UGV could be used for a variety of missions, including hauling equipment, detecting land mines, and land assault. Application requirements include high torque at start up - 1,200 ft-lbs at 28 rpm for 500 seconds (8.33 minutes) and high speed at 260 rpm with efficient heat dissipation. The application requires 6.5 hp constant power output throughout the power curve.

According to Greg Graham, Thin- Gap's vice president and chief technology officer, a key difference between the original motor and the UGV motor will be the capability to integrate a gearbox inside the ring motor, resulting in a very thin and lean assembly. "Gearboxes help electric motors perform in vehicles. It traditionally requires a lot more motor, if you will, when you have to direct drive," said Graham. "Here we can have the entire motor assembly built into this one plane with a large, 12-inch through hole for all of the drive mechanics such as gear reduction, ball bearings, brakes, and everything else. Conventional motors usually have too much material within the motor to do this."

Figure 4. Ring motor with rendering of fan blades developed by ThinGap for use in unmanned vehicles.

Graham also noted that the unique freestanding coil allows the magnet assembly to be more efficient: Eliminating laminations gets rid of the iron losses typical of conventional brushless motors. The only remaining parasitic losses are the eddy current losses (AC losses), which are minimized by the coil design. An optimal thermal path dissipates the remaining I2R heat more efficiently, allowing the motor to perform at high power output levels.

Conservation
Growing concerns regarding the price of fossil fuels and the environmental impact of using them to generate power have prompted many motor and control manufacturers to look at new ways of gaining efficiency. "If you can enhance performance and get more power out of the motor without added cost, it will have a big impact," said Dr. George Holling, chief engineer at Raser Technologies in Provo, Utah, a motor and controller technology licensing company. "If you can improve energy consumption of motors by just a few percent, the overall savings to the economy and total energy consumption are tremendous numbers."

More stringent performance requirements have prompted a continuous trend toward variable speed drive, intensifying the relationship between controller and motor. "Our Symetron™ technology changes how you design and control the motor, so both the motor and controller are improved," said David West, Raser Technologies' vice president of marketing. (Symetron collectively refers to patent-pending pieces of technology that are selectively applied based on the motor or controller type.) According to West, the technology's other advantage in the marketplace is that it can be manufactured in existing facilities with existing technologies.

With an Energy Conversion Science grant from the State Technologies Advancement Collaborative (STAC) of the U.S. Department of Energy (DOE), the company is now mid-way through a project aimed at creating a universal motor controller. The company expects to meet the specifications while achieving the targeted efficiency improvement of 2 percent. "The FLEXMOD™ controller applies our Symetron technology and packages it into a universal controller that can drive a lot of different motors. That's the key to creating economy of scale," explained West. "We are hoping that the availability of a universal, highefficiency controller will let prices come down so that a lot more applications will be controller driven."

Targeting electric-powered transportation vehicles (e.g., electric buses, industrial trucks, and hybrid electric vehicles) for the DOE project, additional applications range from industrial pumps and motors to washing machines and air conditioners.

For more information on the companies and agencies covered in this article, visit:
Defense Advanced Research Projects Agency (DARPA) www.darpa.mil
National Aeronautics & Space Administration (NASA) www.nasa.gov
Raser Technologies, Inc. www.rasertech.com
ThinGap Corporation www.thingap.com
U.S. Department of Energy (DOE) www.energy.gov
WaveCrest Laboratiories, LLC www.wavecrestlabs.com