Motion Control Resources
by Kristin Lewotsky, Contributing Editor
Motion Control & Motor Association Posted 09/17/2009
Today’s search and rescue robots put the focus on surveillance courtesy of precision motion from compact, high-efficiency servo motors.
Forget about anthropomorphic robots cradling human victims à la The Day the Earth Stood Still. In the latest search and rescue robots, size is the key - the smaller, the better. If an opening is large enough to admit a human rescuer, the reasoning goes, the first responders will worm their way in and do a more effective job of retrieval. The benefits of today’s ultracompact, high-power, unmanned ground vehicles (UGVs) lie in their ability to gather information, whether by slipping unnoticed among enemy combatants or driving through a gap in the rubble of a collapsed building to locate victims. “It’s about reconnaissance and the need for identifying an additional set of tactical responses," says Patrick McKinney, chief operating officer of Recon Robotics Inc. (Edina, Minnesota).
A typical search and rescue UGV starts with a rugged, low center-of-gravity chassis, typically - though not always - powered by servo motors. Some designs appear to be refugees from BattleBots, with tank-style treads and one or more articulated arms. Others are very different; the Recon Scout, for example, looks more than anything like a 10-pound dumbbell (see figure 1). At minimum, the units feature a camera, often with IR illumination and visible-light detectors with IR capabilities. Although tethered designs exist, units more often operate on battery power and wireless control, free of limiting and potentially encumbering wires. The wireless approach presents tradeoffs, however, as we will discuss.
The robot designers face conflicting demands at every turn. Traversing rubble and other rough terrain requires a significant amount of torque. The 4-in-tall wheels of the Scout, for example, need to be able to climb over 2-in-high obstacles. The easiest way to increase torque is to choose a bigger motor, but with the size constraints of the vehicles, that is not an option. The second easiest way is to add a gearhead. Unfortunately, that not only increases package size, it slows down the motion, which is an unacceptable tradeoff in a rescue situation. Finally, as if there weren’t enough barriers, battery-operated units must balance torque with current draw and power consumption to achieve the longest possible runtime.
Just like water flows downhill, so the issues faced by UGVs engineers get passed on to their component vendors. “Our challenge is to provide the speed and torque that they require within the constraints, if any, of their package size and weight” says Randy Cordes, President of Midwest Motion Products LLC (MMP) (Watertown, Minnesota), which supplies motion control components to manufacturers of UGVs for surveillance and ordnance disposal. “One of our approaches is to use a high-energy magnet such as neodymium iron boron in the motor. Sumarium cobalt or aluminum nickel cobalt (AlNiCo) are other options but neodymium iron boron is the magnet material of choice these days.”
This is the real world, of course, and nothing comes for free. The more exotic and powerful magnet materials involve a price premium. According to Cordes, a neodymium iron boron magnet can cost five to six times as much as a common ceramic magnet. It is important to note that magnet price is not directly proportional to motor price, however. “It's just the magnet that's five to six times more expensive," Cordes says. "The aggregate total for a motor with a neodymium iron boron magnet might be in the neighborhood of 30%-50% than a typical ceramic version.” The increased cost is partly tempered by the fact that UGVs typically incorporate only a handful of motors. In addition, the importance of performance and form factor in this application outweigh the cost concerns. Sometimes, there simply is no substitute.
At the same time, the overall system goals can’t be sacrificed for torque alone. “You can throw as much power at a given project as possible but then what happens to the life of your battery?" Cordes asks. "You consume so much power that the utility of the vehicle is lost because the batteries don't last long enough.” Minimizing size and weight of the unit are certainly factors but gearmotor efficiency is key (see figure 2).
“The higher the back-EMF constant of a given motor, the lower the amount of current required to produce a given value of torque, so that's something that comes into play when optimizing," he notes. "The winding characteristics play a very important role as well.” In addition to optimizing motor output, maximizing gearhead efficiency can also help extend battery life. This level of optimization can also provide ancillary benefits by better controlling the costs of the drive electronics, by virtue of lower powered output components.
Forward progress alone isn’t enough - the UGV needs to go where directed. The drive mechanisms of the individual wheels or drive treads need to be balanced so that the UGVs can be steered along a desired path. The Recon UGV design incorporates a closed-loop control architecture. A gyroscope provides feedback for the x-axis (yaw) and z-axis (pitch) and a multiaxis accelerometer provides feedback for the gyroscope orientation. Together, the loop allows the unit to compare motor position and directionality, allowing the robot to be steered as desired.
In addition to meeting the performance objectives, search and rescue UGVs need to be rugged enough to survive dusty, dirty, rubble-filled environments - and worse. To accelerate deployment, the Recon UGVs are intended to be thrown, rather than driven, into the area of interest. In a SWAT team scenario, for example, the Scout can be tossed in through a window or over a balcony. As a result, the engineering team had to design it to survive a minimum 30-foot drop onto solid concrete.
For starters, the crossbar that contains the camera and electronics features titanium housing for maximum protection (see figure 3). The wheels consist of a proprietary polyurethane blend that acts as a shock absorber. The challenge of the impact goes beyond just fracture or misalignment due to shock, however. When the robot lands, the kinetic energy of the fall is translated into an abrupt rotation of the wheels, which has the potential to strip the gears. To protect against this, the engineering team developed a patented mechanical clutch that spontaneously disengages when necessary. "It allows us to isolate the impact of the wheels from the pinion gear and motor," says McKinney. "That allows the wheel to float free quickly without causing problems for the motor." When the impact is over, the clutch re-engages and UGV is ready to roll, literally.
When the team adapted the original Scout design to disaster response, they had to develop a unit that could both traverse rough terrain and survive contamination: "Our original design was more focused on tactical law enforcement and military in a relatively clean urban environment," McKinney says. "The new unit involved a complete redesign of what we had before. It proved to be an interesting engineering challenge," he adds dryly. In addition to more muscle, the search and rescue design features spiked wheels that could double as lawn aerators (see figure 4). The projections allow the robot to climb over obstacles.
In addition to heat and dirt, UGVs can face serious electrical contamination. Older climate-control systems with high EMI levels can jam wireless communications. The problem is that rescue situations require deployment on a moment’s notice, into environments about which the responders have little knowledge. "We’re not always using these robots in bombed-out locations, we're using them in urban environments where the RF spectrum is packed," says McKinney. "RF topology in an operational environment is unknown and becomes a real issue for command and control." An obvious solution would be a radio with spread-spectrum and frequency-hopping capabilities, but such devices are far too large for the available footprint. "The smallest [spread spectrum] radio I've seen is a couple inches on a side and a quarter inch thick,” McKinney says. “It's just too big. Instead, we're back at the drawing board trying to figure out the best way to handle [electrical noise], to find a new way to shield our radios or use interesting filtering.”
The new breed of search and rescue UGVs may be small but they face tall orders. And no matter how low their profile, courtesy of smart engineering, they’re rising to the occasion.
Thanks go to Charlie Cayne of SuperDroidRobots for useful conversations.