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Stryker's Optical Viewer Goes Servo
The military’s increasing use of servo direct-drive motors in battlefield systems such as the Stryker armored vehicle show just how rugged these motion systems can be
Joe Ogando -- Design News, January 7, 2008
When it comes to servo technology, the U.S. military tends to lead — and follow.
As John Riley, a senior applications engineer for Galil Motion Control points out, the military still lags behind the industrial world in the adoption of servos, at least in battlefield equipment. “In general, the military is understandably conservative,” Riley says.
That conservative streak really is understandable because soldiers’ lives are on the line. And many applications for battlefield equipment do not need the precision required by high-end industrial motion systems, such as those used in semiconductor manufacturing. “Military applications can be precise,” Riley says, “but it’s not like they have to place a wafer board within tens of nanometers.”
But when the military does use servos, it leads in making them robust. Riley says the military tends to require temperature ranges, shock and vibration loads and packaging that far exceed those found in industrial applications.
For a good example of what servos can do on military machines, consider the U.S. Army Stryker, a family of eight-wheeled, all-wheel-drive, armored combat vehicles currently in use in Iraq. Among the Stryker’s advanced features is a custom optical viewing system that gives the commander a 360-degree view of the battlefield — without sticking his head out of the vehicle.
Developed by PVP Advanced Electro-Optical Systems and called the Commander’s Pan Viewer (CPV), this high-tech periscope makes use of inertial sensors in two axes and a gyro mechanism to give the commander a stable line of sight even as the vehicle rides over rough terrain or experiences the recoil from its weapons’ systems.
According to Geoff Miller, an engineer and PVP’s military program manager, the CPV offers an absolute pointing accuracy “within a few arcseconds.” It also stabilizes the viewer to less than 100 microradians across a wide envelope of operating conditions — as defined by vehicle speed, temperature conditions, vibration and shock loads that can reach hundreds of g’s, solar exposure and more. And it has been designed for a long, low-maintenance life under those tough conditions. “Keep in mind that the systems we do for the military have to last 20 years or more,” Miller says.
PVP has years of experience building related optical systems and sensor platforms for military and homeland security applications, and in the past the company relied solely on stepper motors to drive any gyro-based motion systems. “We know steppers very well, so they’re definitely the conservative way to go,” Miller says. Yet for the CPV project, PVP’s engineers went with a closed-loop servo system for the first time after assuring themselves it could hold up to the military’s demanding environmental and life cycle specs. They ended up building the CPV’s motion system around direct-drive frameless motors from several vendors and a DMC-1425 two-axis controller from Galil. This controller had to be physically customized with regard to the layout of the board and the VME backplane connections. Galil also helped PVP with custom firmware changes to accommodate a non-standard communications protocol used throughout the Stryker and to work with the signals from the systems’ Netzer electronic encoder.
Miller says the motion control system works in two complementary modes: A position mode responds to the commander’s requests for specific elevation and azimuth angles, while a rate mode closes the loop on position to provide the stabilization function. The system has to work quickly to make the necessary corrections given the fleeting nature of the terrain- and weapon-induced disturbances, so it has an update time of 2 kHz.
The direct-drive technology played a role in the system’s performance, too. Not only does it save space and increase reliability by eliminating the need for powertrain components but also contributed to a more responsive system by getting rid of the hysteresis associated with those powertrain components. “You end up with a faster-acting system,” Miller says.
Despite PVP’s stepper experience, the reasons for going servo in this application were compelling. For one thing, the system’s update time would have been a problem for stepper systems, according to John Hayes, a Galil senior applications engineer who worked with PVP to customize the DMC-1425 controller for the Stryker application. “One of the key goals was speed of response. Even a microstepper would have been too slow and had too much jitter for this application,” he says. Miller likewise cites the performance boost offered by servos. “The accuracy requirements on this project required servos,” Miller says.
Even if the requirements didn’t rule out steppers, though, PVP has another reason for what has turned out to be a continuing shift toward servos. Miller says the same Galil servo platform it used for the CPV has found a home in other new product development efforts. “It’s become a valuable tool for us to cut development times and costs,” he says.
For example, after its success with the Stryker viewing system, PVP developed a new surveillance tool for homeland security applications. This Night Hawk system is a pan-and-tilt gimbal capable of positioning optical and other types of sensors within a few arcseconds. “With the Night Hawk, we were able to use the same servo system as the CPV. We were even able to re-use about 95 percent of the firmware,” Miller says.
Other forthcoming products likewise take advantage of PVP’s servo platform. “We’re at the point now where our new products will be servo unless a customer specifically requests stepper,” Miller says.
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