Servo Motors Power Unmanned Ground Vehicle
Miniature unmanned ground vehicles reliably perform reconnaissance in the harshest environments, with the help of compact, high-torque servo motors.
Imagine the scene: A trio of terrorists are holed up in a hotel room, holding a group of terrified staff hostage. Outside, the SWAT team gathers. One of the members pulls a pin from an object and tosses it onto the room’s balcony, where it rolls to the sliding glass door. Moments later, chaos erupts. Commandos burst into the room through a side door, surprising the terrorists and freeing their captives. No, the object on the balcony wasn’t a grenade, it was a miniature unmanned ground vehicle (UGV), and the images it transmits have the power to save lives.
When it comes to search and rescue robots, the philosophy today is the smaller, the better. It's unlikely that even the most sophisticated robot can do a more effective job than a person at pulling victims from rubble or picking up a downed soldier. A UGV like the Recon Scout IR from ReconRobotics Inc. (Edina, MN) can easily and unobtrusively slip into a collapsed building or a hostage situation where a person can’t, however, and send back real-time images that allow responders to identify a tactical response. Although it looks for all the world like a small dumbbell, the Scout can climb uneven terrain, navigate a path as commanded, and send back a steady stream of images, all while operating untethered (see figure 1). Using sophisticated design, robust materials, and compact, high-torque servomotors from MICROMO to build the device, ReconRobotics has positioned itself as a major player in the search and rescue robot market.
Tiny But Mighty
The UGV may be small but designing it was a tall order. The engineering team faced conflicting performance demands at every turn: The unit had to be powerful but small, high torque but with long lifetime on a single charge. It took tradeoffs, optimizing components, and prioritizing specifications, but the resultant robot can do nearly everything well.
The primary order of business was motive power—a search and rescue UGV isn’t much good if it can’t reach the target area. Traveling over rubble or uneven ground requires an appreciable amount of torque. Normally, an engineer would just choose a motor large enough to provide the power required, but the Scout’s compact form factor presented limitations. The second obvious approach, adding a high-reduction-ratio gearhead to convert motor speed to torque wasn’t an option—not only would it increase size beyond the available space, but it would only increase torque at the expense of reducing speed, which is unacceptable in a search and rescue robot.
What the project required was high-power-density motors the diameter of a cigarette. MICROMO provided the answer with 8- and 10-mm DC motors. “They had the best available solution in the size range we were looking for,” says Patrick McKinney, chief operating officer of ReconRobotics. “They do small really well.” The motors were more typically used for medical applications, which brought the added benefit of a dependable solution.
"They have to operate in an environment where lives depend on their function,” he adds. “We've had these motors running continuously in tests for well over 168 hours at a 50% load. They’re very impressive.” An additional planetary gearbox provides a 64:1 reduction ratio to raise the torque even further with a modest size increase.
The crossbar of the UGV contains a visible imager with IR sensitivity, an IR illuminator, the motors, drives, and controller, as well as the radio transceiver that provides wireless operation (see figure 2). Each of the Recon Scout’s two wheels is driven independently, with the aid of a closed-loop control system that balances the drive signals. A gyroscope provides the z-axis feedback (yaw) to ensure that the wheels are driven evenly so the Scout can move in a straight line. To allow the UGV to be steered on the desired path, an accelerometer monitors x- and z-axis motion. A weighted stabilizing tail stabilizes the rotational position of the crossbar so that the camera aperture remains in the correct spot for image capture.
Forget about driving to the region of interest; the Recon Scout IR is designed to be cast or dropped directly into the surveillance location. As a result, the ReconRobotics team had to design the unit to survive an impact on concrete from 30 vertical feet. They began with a chassis of aircraft-grade aluminum, which also helps dissipate heat. A titanium housing on the crossbar protects the contents. The wheels consist of proprietary polyethylene that doubles as a shock absorber.
When the device lands after a throw, the wheels hit the ground with enough force to potentially strip the gears. To protect against this possibility, the team designed a patented mechanical clutch that disengages the gears on impact and re-engages after the Scout has landed. With imager, illuminator, and wheels running constantly, the Scout has a per-charge runtime of 1 hr, thanks to the high-efficiency operation of MICROMO’s motors.
For efficient, economical production, the ReconRobotics philosophy is to outsource whenever possible and save the inhouse engineering efforts for value-added aspects like controls. To assist, MICROMO delivers an integrated package of motor, gearhead, and leads. In addition, their dedicated machine shop [link to machine shop page] adds a pinion designed by ReconRobotics. “What we're buying from MICROMO is a solution that allows us to drop it into our robot, plug it into the boards and be up and running very quickly,” says McKinney. “MICROMO has done an excellent job for us over the years. They bend over backward to help us out.”
The walkie-talkie style operator control unit for the Scout features a single joy stick and can be operated one-handed. McKinney points to the intuitive nature of the device operation as a big part of its value proposition. “Law enforcement agencies are under severe budget constraints. Training is a luxury. I think that as you look at reconnaissance and rescue robots going forward, intuitiveness and ease of use is really going to drive their acceptance in the marketplace.”
In tests with the New York Port Authority, a response team was able to deploy the Scout, capture images, and rescue a downed victim in 12 minutes, compared to 45 minutes for a competing device. ReconRobotics isn’t done yet, though. They’ve got big plans for the future and MICROMO will be there with them. “We've staked out this miniature unmanned ground vehicle market and will be looking to MICROMO for solutions to some ideas we have in the future,” McKinney says. “We can’t speak to those yet but we're certainly looking forward to it.”