Simplifying the Complex in High-Risk Duty Robots

In a world in which an abandoned suitcase, a box, or even a pile of rubble can conceal an improvised explosive device (IED), bomb-disposal robots have become essential. Many units in the field bristle with cameras, feedback devices and processors. Such sophisticated robots offer a significant amount of capability and autonomy, but their increasing complexity also drives up weight, size and cost. As a result, organizations often have to share one system across several units.

To address this issue, Mechanical Mobile Platforms (MMPs), a family of simplified, man-portable robots from The Machine Lab Inc. has been developed to perform a range of surveillance and bomb disposal tasks at a price point low enough for wide deployment. For the cost of a high-end system, which can exceed several hundred thousand dollars, users can purchase nine or 10 MMPs. Powered by compact, economical, high-torque motors from the Faulhaber group's value-added reseller Micromo, The Machine Lab's robots are making a difference around the world.

Kick-Starting the Design Process

The Machine Lab engineering team started out with one overarching principle: keep it simple. This required an immediate trade-off. Instead of a robot that can perform 95 percent of all possible tasks needed, an MMP might only handle 60 percent of the tasks. The bottom line, as all engineers know, is that a solution doesn't need to be perfect, it just needs to be adequate to the application at hand.

So, rather than focusing on building a single product, The Machine Lab took the big picture approach and developed a modular design platform that can be applied to a wide range of implementations. The basic line includes a quartet of direct-drive MMPs ranging from a four-wheel model that can carry a 4 lb payload to a six-wheel model that can handle up to 30 lb. For loose terrain, the company also offers two track-driven versions: a four-wheeled version and a six-wheeled version. Here, too, the focus on simplicity came into play. Instead of driving with all wheels, each of the track-driven MMPs sports two passive wheels, providing savings in cost and weight without compromising performance.

The team avoided onboard processors and feedback devices, opting instead for direct user control via wirelessly linked joysticks or switches. The operator determines the path of the robot based on images transmitted by the unit's wide-angle IR camera. The mobile platforms provide a zero-degree turn radius and linear speeds as high as 6.0 ft/s. They range in size from 5 lb and 11.5 x 11.2 inch to 40 lb and 29 x 19 inch, although custom versions can be larger, such as the 1,500-lb unit built for clearing vegetation in mine fields.

Although cost control is important, a low price point would be meaningless if a unit couldn't perform or failed after a few weeks in the field. To obviate this issue, the foundation of each MMP consists of a rugged aluminum chassis mounted on nylon wheels with rubber tires. Steel gears and IP-rated motors help ensure survival.

"The robots we send to Afghanistan have to be pretty much IP62 weatherproof," says company president Zack Bieber. "They need to be able to be washed down and they need to be able to resist the very fine, abrasive dust that you find out there."

Gripper Design Strategy

Perhaps the most critical aspect of a bomb-disposal robot is the robot arm. Here, the engineering team faced a number of challenges, particularly in the gripper and wrist-rotation elements. To maximize the amount of payload a robot could handle while ensuring stability, they had to minimize the weight of the arm. That meant working with the lightest possible components. At the same time, those components had to power the arm so that it could grip a wide variety of materials and not only lift but rotate payloads as high as 40 lb.

One obvious way to get the torque required was to gear down a high-speed motor, but that approach adds size and weight, as well as complexity and cost. In keeping with the company's philosophy of simplicity, The Machine Lab team decided to skip additional gearing in the wrist rotator and look for a motor that provided enough muscle to do the job on its own.

Size presented another constraint. The design required a motor that would supply the required torque and still fit inside the 30-mm-diameter tube of the robot arm.

"It was a very tight physical dimensional requirement," says Bieber. "We picked the Micromo motor because of the power-to-weight ratio. It was really the only one we could find that would fit our application."

The actual gripper design was a little trickier. The device needs to operate slowly to provide operators with the precision movement they need to handle explosive devices. At the same time, the grip needs to be fail-safe - once an arm picks up an IED, it cannot drop it. To handle these requirements, the design incorporates a gearmotor fitted with a planetary gearhead that provides a 246:1 reduction ratio. An additional worm-drive gearbox resists act driving, providing an additional 15:1 reduction ratio for a total of 3,690:1. Although using multiple sets of gearing can increase torque, in practice, stacking up gear reductions can actually rob the system of power. Using a motor with a high-efficiency planetary gearhead provided the performance the design required.

Efficiency was another important design constraint. Running out of power in a cell phone is merely annoying. Running out of power during the disposal of an IED can cost lives. At every step the team focused on specifying components for the most efficient possible design.

With the high-efficiency design used, the units can deliver as much as an hour-and-a-half of constant operation on a single charge from a 25V lithium ferrous phosphate battery pack that weighs just 1.5 lb.

Above all, the most challenging aspect of the gripper design was duty cycle, according to Bieber. The motors for the wrist and gripper are specified to run 24 hours a day, but their actual duty cycle is closer to 5 or 10 percent of that. During that time, however, they must routinely perform to their limits and sometimes beyond.

"The types of tasks these robots have to do are all over the place, and it's all dependent on the operator," he says. "So, if we say this can only lift and rotate 5 lb, we know that the user may well end up trying to grab a 20 lb object with it."

To eliminate overdriving or stalling the motors in those situations, the design includes adjustable slip clutches on both the gripper and wrist rotator.

So far, attention to detail has paid off. "We've got over 200 robots out in Afghanistan and surrounding countries, and probably another 50 to 80 out in law enforcement here in the U.S.," says Bieber. The company also recently secured another U.S. Navy contract for 250 more robots.

Kristen Lewotsky is a technical writer for Micromo. For more information, go to www.micromo.com.