Servomotors Stabilize and Position Sensor Turrets on UAVs

There was a time when reconnaissance meant either multimillion-dollar satellites snapping a few shots per orbit or pilots risking their necks to fly over hostile territory. Today’s reconnaissance is dominated by unmanned aerial vehicles (UAVs). Not only are UAVs less expensive than manned vehicles by an order of magnitude, the approach presents no risk to operators who keep their feet firmly planted on the ground, frequently on the other side of the globe.

Imagery obtained from these flights is only useful if it’s crisp and clear. As anyone who has taken a picture from a moving vehicle can attest, the clarity of an image depends not only on camera focus, but on stability as well. Cameras mounted in aircraft or helicopters typically use gyrostabilized platforms to stay steady. However, those platforms are heavy and need plenty of power to operate. They do not adapt well to UAVs that weigh only 15 kg (33 lb) and operate continuously for 24 hr. Hood Technology Corp., Hood River, Oreg., tackled the task of creating just such a platform, creating the first 700-gm (1.5-lb) gyro-feedbackstabilized sensor turret for cameras in the ultralight class of UAVs. The forces needed to maintain turret stability come from a collection of lightweight, compact motors from MicroMo Electronics, Clearwater, Fla.

Hood Technology President Andreas von Flotow notes that larger, more-sophisticated UAVs can cost upwards of $10 million. In contrast, the ultralight class ranges from $100,000 to $200,000. “People have to make reservations days and weeks in advance for the big UAVs, while they can afford to distribute the ultralights widely so everyone can use them.”

Hood Tech currently uses four classes of imagers: a visible camera and a suite of three IR cameras that image over the shortwave, midwave, or longwave IR spectral bands, respectively. The cameras produce NTSC video with fields of view on the order of 10 to 30 ft for a 1,000 to 3,000 ft of altitude. Object-tracking feedback based on optical control loops lets the cameras lock onto objects. For example, once locked in, the system can automatically follow a truck driving along a road.

Under the hood
The Hood Tech turret keeps the 200-gm (0.44-lb) sensor payload and electronics steady and controllably aimed despite vibration, thermal loading, wind forces, and more. The 500-gm (1.1-lb) gyro-controlled gimbal mechanism uses a coarse/fine design, with the outer coarse stage powered by MicroMo motors. The inner vernier stage, which provides only a few degrees of motion, is controlled by Hood’s own direct-driven actuators and encoders.

The turret controls both coarse axes using microstepped stepmotors as rate actuators. Position feedback is handled by an absolute encoder. Stepmotors were chosen over torquers for ease of control in overcoming friction. The reasoning: When a stepmotor is given a command to step, it steps. When step commands are issued at a fixed rate, the motor responds with that velocity. Torquers, however, won’t move until the torque builds enough to overcome friction. Then the motor jumps to its new location.

The microstepped motors, however, are stepped at rates of over 10,000 steps/sec. At such rates, friction and stiction are almost irrelevant and motion becomes nearly smooth. The motors are teamed with dual-path spur gearboxes to produce a 200:1 reduction ratio. That yields an angular velocity of up to 90°/sec for the axis of rotation.

Backlash can introduce unacceptable error in a precision pointing application like aerial reconnaissance. To eliminate the issue, Hood used MicroMo’s zero-backlash gearboxes. The gearboxes incorporate two parallel gear trains that are wound elastically against each other. However, configuring the gearboxes for proper operation requires just the right amount of elastic tension. Too tight, and friction becomes so great as to make the gearbox useless. Too loose, and the risk of backlash remains. Hood had to develop the process in cooperation with MicroMo to set the elastic qualities of the gearbox just right.

Slip rings eliminate cable windup in the coarse stage, and hence there is no need for unwind maneuvers. While the visible-wavelength imagers come equipped with motorized focus and zoom, the IR turrets incorporate brushed-dc motors from MicroMo for their focus mechanism. Some of the IR imagers also have motorized zoom.

Meeting the stabilization and control specs would be difficult enough on the ground, but UAVs impose additional challenges. One obvious constraint is weight, but power is also an important issue. Minimizing power consumption for the turret prolongs battery life and lengthens vehicle range. A 200-W camera and turret demand would be as unwelcome on these small aircraft as a 5-kg (11-lb) payload. Through the use of high-efficiency motors, the stabilizing turret draws a mere 4 W on average. The cameras draw about 3 W, so the entire package demands only 7 W from the aircraft. The 7 W of power is practically nothing compared to the several hundred watts used by the UAV propulsion system.

Of course, even the least power-hungry unit isn’t much good if it only operates in the lab. The theaters of operation for UAVs present relentlessly hostile environments. Conditions range from the dust of Iraq or Afghanistan to the salt and humidity of naval operations at sea. But a far bigger problem is temperature.

Thermal overload affects the IR imagers in high elevations with hot locales like Afghanistan in the heat of summer. Heat from the overhead sun coupled with operations at altitudes up to 14,000 ft take their toll. The IR imagers need thermal stability to work properly. Thermal stabilization adds weight and power demands to the system, putting more pressure on the stabilization motors to deliver motion in a small, efficient, and economical package.

Of course, the challenges aren’t just restricted to the environment. The UAVs get catapulted into flight, a process that imposes 35 gs of force on the aircraft and turrets. Even more interesting is the landing — or rather, the capture. The planes are snared in flight when they fly past a vertical rope, catching one of their wing tips. While that method of landing would likely rip the wing off a jumbo jet, the advantages of scale simply means the UAV whirls around. Even so, the UAV sees 20 gs of centrifugal force as its forward motion is arrested. The turret is subjected to stringent shock and vibration testing before the product is shipped, and then undergoes at least two sessions of extreme shock every mission.

The UAVs are tough right down to the motors. “We have few, maybe even zero problems with motor failures,” says von Flotow, who notes the company is currently shipping about 1,000 units/year. “The most common way for our turrets to end their lives is not by being worn out. The airplanes don’t always make it home to the rope, and when they don’t, they often end up smacking into some hillside somewhere.”