High-Density Servos Make Possible Compact Recon Missions

A pair of high-torque servomotors was key in making it possible to field a miniature unmanned ground vehicle (UGV) agile enough to climb uneven terrain.

The dumbbell-shaped Recon Scout IR from ReconRobotics Inc., Edina, Minn., can unobtrusively slip into a collapsed building or near a hostage situation where a human can’t, all while operating untethered. It then beams back real-time images that let those in charge decide on a tactical response.

Each wheel has its own servomotor driven independently with the aid of a closed-loop control system that balances the drive signals. The motors are 8 and 10-mm coreless dc designs (about the diameter of a cigarette) known for high efficiency. This configuration was more practical, say Scout designers, than using a high-reduction-ratio gearhead to convert motor speed to torque. Such a scheme would occupy more than the available space and would only increase torque at the expense of reducing speed, unacceptable in a search and rescue robot.

Motor supplier MicroMo Electronics Inc., Clearwater, Fla., worked with designers of the robot to maximize power transfer while still maintaining a mean-time-to-failure that keeps robots in the field for years. MicroMo delivers an integrated package of motor, gearhead, and leads. In addition, its dedicated machine shop adds a pinion that ReconRobotics designed.

MicroMo was also able to upgrade parameters such as the brush material that let motors work reliably despite heavy use. In tests, the motors (which are more typically used in medical applications) have run continuously for well over 168 hr at a 50% load.

The motors work with a planetary gearbox providing a 64:1 reduction ratio to boost torque while taking up little space. A gyroscope provides Z-axis feedback (yaw) to ensure the wheels are driven evenly to keep the Scout moving in a straight line. The UGV steers to the desired path with help from an accelerometer which monitors x- and z-axis motion. A weighted tail stabilizes the rotational position of the crossbar holding the camera so the camera aperture remains in the correct spot for image capture.

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.

The Recon Scout IR is designed to be cast or dropped directly into surveillance locations. As a result, the ReconRobotics team had to build 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. Wheels use a proprietary polyethylene. They are designed, through geometry and material selection, to absorb much of the energy encountered in drops onto concrete without harming the electronics or mechanical systems inside the titanium shell.

When the device lands after a throw, the wheels hit the ground with enough force to potentially strip the gears. To protect against this, the team designed a patented mechanical clutch that disengages the gears on impact and reengages after the Scout has landed. With imager, illuminator, and wheels running constantly, the Scout has a per-charge runtime of 1 hr.

The original Scout was a surveillance robot targeted at military and tactical law-enforcement applications in relatively clean urban environments.

Another version targeted at search and rescue must operate amid rubble, dirt, and heat. The 4-in.-tall Scout must also be able to lift itself over 2-in.-high objects unaided. This version of the device uses a more-powerful 10-mm-diameter motor, slightly wider wheels, and fearsome inch-long spikes for traction. It weighs in at 1.2 lb.

The walkie-talkie style operator control unit for the Scout mainly consists of a single joystick and is designed to be easily run by operators with little training. 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 min, compared with 45 min for a competing device.