Customized Servo Motors Position Gyro-Stabilized Sensor Platform
A Dedicated Machine Shop and Design Capabilities Allow MICROMO to Tailor Motors for Land and Air
The best camera and sensor systems in the world are nothing if they’re not placed on a stable platform that can point them in the desired direction. No company understands that better than surveillance specialist Gyrocam Systems, LLC (Sarasota, FL), which has developed a gyro-stabilized, servo-motor-controlled platform capable of handling a variety of sensor payloads, including visible and infrared cameras, inertial navigation systems, and more. Initially, they served the aerial camera market; for example, news and law-enforcement helicopters. More recently, they've shifted to ground-based, mast-mounted sensors for improvised explosive device (IED) detection systems in theaters like Iraq and Afghanistan.
Servo-Motor Positioning
The platform consists of a sensor payload and two positioning subsystems: the inner positioning axes and outer positioning axes.
A set of covers rides on the outer axes and protects the interior of the gimbal. The inner positioner carries the payload and features a pair of fine-motion axes, one devoted to elevation (tilt) and one devoted to azimuth (pan). The inner axes and the gimbal exterior covers are in turn carried by the outer coarse motion-control system, which features its own elevation and azimuth axes.
“The inner payload can’t move very far,” says Matt Kornblum, senior engineer at Gyrocam. “It’s limited to about 3 or 4 deg in travel around either the elevation or the azimuth axis but it has a very, very high frequency response. Outside of that we have the coarse axis; we have about 40 ft-lbs of torque that we can apply to it [the payload, inner axes and the covers].”
Enter MICROMO, which produces the two 35-mm-diameter coarse-positioning gearmotors that deliver roughly 10 ft-lbs to a 4:1 miter-and-bevel reduction gear set that changes the direction of rotation by 90 deg. “They’re doing all the heavy lifting in terms of moving the payload, inner axis, and the covers around,” says Kornblum. “It gives a fairly strong outer system that can accelerate and move rapidly, on the order of 72 deg/s. At the same time, we’re also stabilized to the sub-pixel level even for a 0.8 deg horizontal field of view.”
The mast-mounted platform delivers this kind of performance while traveling over potholes and ruts on mine-resistant, armor-protected (MRAP) vehicles. “They ride these things over the worst roads in the world and they get bounced around and beat up every day,” says Kornblum, “and every day the cameras go out and work.”
Customized Components
The coarse-positioning motor consists of a 12-V armature design that can be operated at higher speeds when necessary. “You have to run the armature very fast to get enough power out [on the order to 100 to 130 W],” says Kornblum. “We run it up to 24 V to get to our top speeds but we don't stay there for very long. Most of our work, aerial and mast-mounted, is done down at sub-degree-per-minute speeds. For us, it's more about being able to precisely control position.”
Accurate positioning can be challenging, especially in such harsh conditions. In aerial applications, for example, the gimbals operate in the airstream beneath the belly of a helicopter or plane, which can fly at 250 knots or more. To deliver, the motors need to provide a lot of torque in a small package. The team at MICROMO’s dedicated machine shop worked with Gyrocam to customize the motor to their needs by changing the shaft design to accommodate torque.
“Primarily this thing uses a miter and bevel gear set to redirect the force of the motor through a 4:1 gear reduction and we also alter the direction of rotation by 90 deg, so there’s a lot of axial loading on the shaft that you wouldn't necessarily see in a lot of applications,” says Kornblum. “There's a lot of side loading on the gear train. MICROMO made sure that they had one of their good support bearings and a thick shaft to support that. They did a very nice job for us.”
The team also tailored other aspects of the gearmotor. “When we first started working with MICROMO, I think we might have shipped one camera system a month,” Kornblum says. “All the motors we used went through their job shop and they improved the bearings, they put a larger shaft on them, worked on the keyway, things like that. From that perspective, MICROMO certainly helped us out because they were willing to make some of these types of changes that frankly some other vendors would be less inclined to do at the kinds of quantities we were looking at.” The results wound up benefiting both companies. “Now,” he adds, “we're typically shipping two systems a day.”
Retrofitting Gearboxes
When Gyrocam’s defense business took off, the company sought to repurpose inventory acquired for the aerial platforms for use in the mast-mounted surface systems. The problem was that the gearmotors for the aerial platforms featured a 246:1 reduction ratio. That was sufficient for the slow scans and high torques typically required for airborne applications, but ground-level scans take place more quickly and with less wind loading, which means they require a different reduction ratio.
Because the existing gearmotors were customized with their gearheads and motors sealed together, simply breaking them loose and swapping them for different gearheads could have resulted in motor damage. Instead, MICROMO’s machine shop broke the Loctite adhesive bond that held on the front deck of each gearbox and pulled out the components of the planetary gearhead, repopulating them with new gearing to achieve a reduction ratio of 159:1 for the complete assembly. The shop then machined fixturing to reinforce the Loctite seals of the front decks, enhancing the robustness of the design.
Gyrocam worked directly with MICROMO, even bringing in their prototype for discussions. The results included improvements in the keyway to minimize slop and nonlinearities in the coupling of motor to gearhead, and on mounting and lubrication methods to improve lifetime in harsh conditions.
In addition to the coarse-positioning gearmotors, MICROMO motors are used in some of the outsourced subsystems in the Gyrocam sensor platforms, Kornblum says. For his part, he’s quite happy with MICROMO’s products and performance. “They've never, to my knowledge, held us up from a shipment,” he says. “They consistently deliver what they say they're going to deliver when they say they're going to do it.”
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Application Case Study: Servo Motors Position Gyro Sensor Platform
