Bi-manual Dexterous Manipulation for Maritime Explosive Ordnance Disposal
Since 2001, there has been a proliferation of robots within the U.S. military to assist with Explosive Ordnance Disposal (EOD) missions. Ground robotics systems are able to detect and dispose of Improvised Explosive Devices (IEDs) with the help of robotic manipulator arms.
These robotic arms are considered true “mobile manipulators”. Mobile manipulators are designed from the ground up to be power-efficient (running off of standard batteries), lightweight, and rugged enough for the rigors of mobile use. These manipulators are integrated into unmanned ground vehicles to enable operators to interact with their environment and neutralize threats. EOD mobile manipulators are specialized for outdoor environments, possess high strength-to-weight ratios, and are able to perform delicate and precise operations while operating in confined, concealed, and cluttered spaces. EOD mobile manipulators are also controlled using teleoperation because a human must be in the loop to deal with explosive threats.
Unfortunately, IED threats are not limited to ground operations. Waterborne IEDs can be hidden in our ports and waterways. The major challenge for manipulator arm manufacturers today is to come up with a marine variant that can effectively and efficiently perform EOD tasks underwater with low visibility, shifting currents, and corrosive environments.
RE2 Robotics is currently developing two types of underwater manipulator arms with the U.S. Navy’s Office of Naval Research. One is an electro-mechanical system and the other is an inflatable solution that utilizes soft robotics materials. In addition to developing these manipulators, RE2 is also creating novel human-robot interfaces to control high dexterity manipulation systems.
EOD operations within the Navy require Remotely Operated Vehicles (ROVs) that are small enough to maneuver in cluttered or confined spaces. These ROVs have limited space and power, but require dexterous manipulation capability. Hydraulic manipulation solutions, commonly used for underwater applications, are simply too large for these smaller ROVs. Although micro-hydraulics solutions are being researched, these systems are likely to be cost prohibitive in the near term since commercial-off-the-shelf parts are not yet available for hydraulics at that scale. As a result, adapting commonly used electromechanical mobile manipulation actuation for underwater use may be the most viable path for achieving dexterity and strength in a compact, lightweight form. RE2 is now adapting its ground-based electromechanical manipulation technology for use in a marine environment.
Through two distinct but complementary Small Business Innovation Research (SBIR) grants, one with ONR and one with the US Army, RE2 developed a highly dexterous manipulation system (HDMS). The Army system featured a total of 11 degrees of freedom (five degrees of freedom per arm plus a one degree of freedom torso). The Navy system’s dexterity mimicked a human being (7 degrees of freedom per arm), plus a torso tilt and yaw, totaling 16 degrees of freedom. In addition to its dexterity, HDMS has a 3:1 strength-toweight ratio. As an example, the Army HDMS weighed 35 pounds and was able to lift 110 pounds.
There were two major innovations related to joint modules which allowed RE2 to achieve this compact and lightweight, yet high-performance, electromechanical capability.
First, RE2 needed a compact, lightweight motor controller solution. Offthe- shelf motor controllers for DC motors were too large for HDMS. As an example, at the time of development, the smallest motor controller, with the attributes required for controlling HDMS, weighed 82 grams and had a volume of 48mm x 57mm x 38mm. At first glance, this may not appear to be significant. However, 82 grams multiplied by 16 degrees of freedom is approximately 2.9 pounds, which is greater than 8% of the Army’s HDMS weight budget. The heavier off-the-shelf controllers located throughout the arm would also require additional structure and actuator torque in order to maintain performance and house the bulkier off-the-shelf motor controllers, further increasing weight.
To overcome this, RE2 used a combination of off-the-shelf components (versus off-the-shelf controllers) and packaged them into a volume of 37mm x 52mm x 15mm that weighed only 17 grams. These resulting controllers significantly contributed to HDMS achieving such a high strength-to-weight ratio in a compact form.
Second, overheating is a major cause of DC motor failures. RE2 custom packaged off-the-shelf drive train components at each robotic arm joint to control the heat dissipation path. With optimized heat dissipation, smaller motors were used to achieve similar performance output as larger motors, serving as the main driver of the strength-to-weight ratio realized. By using off-the-shelf components for both the motor controllers and joint modules, both reliability and cost savings due to economies of scale were achieved.
Under an ONR research grant, RE2 is now adapting its innovative joint design for maritime use. Preliminary analysis indicates that minor adjustments are needed to support use undersea. Sealing designs will be modified and changes in materials will be required in places. The greatest benefit of the existing joint design is that it can be filled with oil without affecting the main components, allowing for a design where pressure can be compensated more easily than an air-filled solution. Pressure compensation is critical for use at various depths below sealevel. The oil-filled joint modules will also aid in the design of the marine variant of HDMS to be neutrally buoyant, a critical feature for minimizing the impact of HDMS on the ROV.
Under an ONR SBIR grant, RE2 is also developing an inflatable high-dexterity robotic manipulator. The inflatable manipulator is based on soft robotics technology. This arm is composed of many cells at each joint with pumps that allow the cell to be filled or emptied with water. As cells are filled, the distal end of the joint rotates “outward”, and when cells are emptied, the distal end of the joint rotates “inward.” If all cells of all joints are inflated, the arm will be fully extended, whereas if all cells of all joint are deflated, the arm will be fully retracted. The structure between the joints is also made of a soft material that can be filled with water to provide structure. The feasibility of using sea water to operate “structure” or “joint” cells is being explored. When emptied, the arm will stow into the ROV, reducing drag as the ROV swims through the water.
RE2 has also developed an innovative and proprietary joint that would be used at the wrist and shoulder of the arm. This joint uses water to produce a high strength-to-weight joint with continuous roll ability. Ground EOD operations rely on continuous wrist roll capability. Underwater EOD operations would also greatly benefit from this capability.
Control of Dexterous Manipulators
The Navy wants bi-manual dexterous manipulation for underwater EOD operations, whether it is an electro-mechanical or an inflatable solution in order to execute maritime IED operations more quickly and with less risk than current methods. These tools will harness the intellect and experience of the Navy diver but will increase the “length of their arms” when conducting operations.
Traditional control input devices, such as gaming joysticks (e.g. Xbox), are suboptimal for controlling dexterous robotic arms. Controlling a 7-degree-of-freedom arm with a gaming controller in order to “fly the end-effector” in Cartesian space is not very effective because the joint and link positions are indeterminate due to the redundant degrees of freedom, which could cause collisions with the arm. After a decade of researching various control input methods for dexterous robotic arms, RE2 has concluded that using an “imitative controller” is the most intuitive input method using technology available today.
An imitative controller is a scaled model of the arm(s) to be controlled. As the operator moves the scaled arm, the remotely located actual arm mimics the motions of the imitative controller. A gripper controller is mounted at the end of the imitative controller, allowing the operator to control the arm’s gripper too. When using the imitative controller, not only are operators controlling the end-effector, but they are also controlling all joint positions of the arm. This is important in EOD operations as you may need to control exactly where your elbow is, for example, allowing the operator to prevent all parts of the arm from colliding with obstacles in the environment.
When using only 2D visual feedback from cameras, the imitative controller also provides useful information to the operator – for example, the operator can determine if the two grippers of a bimanual dexterous manipulation system are touching or perpendicular to one another by simply looking at his or her own hands.
The greatest gauge of the imitative controller’s intuitiveness is training time. After a few minutes of training, only hours later, an unskilled operator can quickly become fairly proficient at controlling a 16-degree-of-freedom bimanual manipulation system.
In order to enable ROVs to truly perform EOD tasks underwater, new innovation is needed. RE2 is researching two viable solutions: electro-mechanical and inflatable. The results of these two research efforts will yield significant data to allow the Navy to assess the pros and cons of each approach so that the most viable solution can be integrated with the fleet of ROVs that will be performing these EOD missions. Regardless of the manipulation solution, an intuitive control device, such as the aforementioned imitative controller, will be critical to allow these dual-arm solutions to be effectively used.
This article was written by Jorgen Pedersen, President & CEO, RE2 Robotics (Pittsburgh, PA). For more information, Click Here .