Gyrostabilized turrets for unmanned aerial vehicles
Micro servo motors stabilize and position sensor turrets on ultralight unmanned aerial vehicles
No so long ago, reconnaissance meant either multimillion-dollar satellites snapping a few shots per orbit or pilots risking their necks to fly over hostile territory. Today, recon is dominated by unmanned aerial vehicles (UAVs). This offers a less expensive approach that presents no risk to the operators, who are not only on the ground but frequently on the other side of the globe.
These systems require not only focus but also stabilization. That’s where Hood Technology Corp. comes in. Eight years ago, Hood developed the first 700-g gyro-feedback-stabilized sensor turret that enabled a new class of ultralight UAVs weighing 15 kg and offering 24 hrs of endurance. Core to the success of these ultralight systems is a collection of lightweight, compact motors from MICROMO.
“We’re trying to squeeze as much as we can into this class of UAV, keep them small and light and affordable,” says Hood Tech president Andreas von Flotow, noting that larger, more sophisticated UAVs can run around $10 million, compared to $100,000 to $200,000 for the ultralights. “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 1000 ft to 3000 ft altitude. Object-tracking feedback based on optical control loops allows the cameras to lock onto objects, for example to automatically follow a truck driving along a road.
Under the hood
The Hood Tech turret keeps the 200 g sensor payload and electronics steady and controllably aimed in the face of vibration, thermal loading, wind forces, and other factors. The 500-g gyro-controlled gimbal mechanism consists of a coarse/fine design, with the outer coarse stage powered by MICROMO motors. The inner vernier fine stage, which can provide a few degrees of motion, is controlled by Hood’s own direct-driven actuators and encoders.
Using micro-stepping stepper motors as rate actuators, the turret controls both coarse axes and closes the loop with an absolute encoder. Why not just use a torque? Simple, says von Flotow. “With a stepper motor, when you command a step you get one, when you command a step rate, you get that velocity. When you command a torquer, you don't get any motion until the torque you command overcomes the friction and then it
jumps, so you have to do clever friction compensation. Most people think of steppers as doing a discrete number of steps but we step these things at tens of thousands of times per second.” At such rates, friction and stiction are almost irrelevant and the motion becomes nearly smooth. “It’s basically then an angular velocity actuator,” von Flotow says. 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 deg/s for the axis of rotation.
In a precision pointing application like aerial reconnaissance, backlash can introduce unacceptable error. To eliminate the issue, Hood turned to MICROMO’s zero-backlash gearboxes. The components essentially incorporate two parallel gear trains that are wound elastically against each other. “When you use such a gearbox, it's always a compromise between winding it too tight, generating so much friction that usable actuation torque is reduced to being useless, or winding it not quite tight enough, to where you still have a little bit of backlash,” says von Flotow. “We had to work with MICROMO to get that right. We had to develop the process together.”
The coarse stage features a large articulation, endless pan with slip rings to eliminate cable windup, and therefore the need for unwind maneuvers. While the visible-wavelength imagers come equipped with motorized focus and zoom, the IR turrets also incorporate brushed DC motors from MICROMO for their focus mechanism. Some of the IR imagers also incorporate motorized zoom.
Dust in the wind The stabilization and control specs would be difficult to meet 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, lengthening the vehicle’s range. The cameras draw about 3 W, and the team worked to restrict the platform requirements to the minimum, in part by using high-efficiency motors. A typical stabilizing/pointing turret draws about 4W, so that the entire package demands only 7 W from the aircraft. Since the propulsion system itself demands only a few hundred watts, 7 W is a good number for camera-system powera 200-W camera payload would be about as unwelcome on these small aircraft as a 5-kg payload.
Of course, even the lowest-power unit isn’t much good if it only operates in the lab. The theater of operations for UAV presents a relentlessly hostile environment. “They can get unbelievably dusty over there in Iraq or Afghanistan,” says von Flotow. “You should see some of the stuff that comes back.” It’s not just dust, either—the U.S. Navy operates a number of the ultralight UAVs, and there the issue is less dust than the salt and humidity of sea conditions. Corrosion damage is rare, though, von Flotow says. A far bigger problem is temperature.
“We have to watch for effects of thermal overload, first detectable in its effects upon the imagers,” he notes. “This has been an issue especially in high elevation, hot situations like Afghanistan in the heat of the summer.” The aircraft are often operated at 14,000 ft density altitude. The IR imagers require thermal stabilization, which adds weight and power demands to the system, putting more pressure on the stabilization motors to deliver motion in a small, efficient, economical package.
Additional challenges exist as well.. The UAVs get catapulted into flight, a process that imposes 35 Gs of force on the aircraft and turrets. Even more interesting is the landingor rather, the capture. The planes are snared in flight when they fly by a vertical rope, catching one of their wing tips. It’s a method that would likely rip the wing right off a jumbo jet, but due to the advantages of scale, it simply whirls the UAV around—while subjecting it to 20 Gs of force. The turret must undergo stringent shock and vibration testing before the product is shipped, then extreme shock every mission once deployed. “We’ve had no G issues,” says von Flotow. “We’ve made them tough enough to survive.”
Indeed, the UAVs are tough right down to the motors. “We have very few, maybe even zero problems with motor failures,” says von Flotow, who notes the company is currently shipping about 1000 units per year. “The most common way for our turrets to end their lives is not being worn out. The airplanes don’t always make it home to the rope, and when they don’t, they end up smacking into some hillside somewhere.”
FIGURES Figure 1 Ultralight UAVs are launched by catapult, suffering 35 Gs of force. Photo credit: Insitu Inc.
Figure 2 The sensor turret (left) on the ultralight UAVs weighs 700 g. Photo credit: Insitu Inc.
Figure 3 For capture, the UAV snags a wingtip on a vertical rope, for a rapid deceleration that exerts 20 Gs of applied force. Photo credit: Insitu Inc.
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Application Note: Turrets for UAV Systems
