The Multi-segmented Magnetic Ro bot (MSMR) project addresses a capability gap in the intelligence, surveillance, and reconnaissance needs of the U.S. Navy visit, board, search, and seizure (VBSS); Navy SEALs; and Marine Force Reconnaissance teams. A segmented robotic platform with magnetic wheels and a minimal acoustic signature was developed that can navigate the hull, tanks, and passageways of a ship. The goal was to provide effective climbing and turning ability over and within a ferrous hull that typically features plumbing, protrusions, and indentations such as weld seams where hull plating meets. Such a robot will be able to climb the hull of a ship, provide covert perch-and-stare surveillance of the deck area, and wirelessly transmit audio/video before a search team boards. The technology is also promising for inspection of tanks, and dangerous or hard-to-reach passageways and voids in maritime vessels.
The MSMR robot system is composed of the robot modules, linkages, and magnetic wheels that provide attraction and traction with the surface being climbed. The robot modules contain the system electronics, motors, and batteries. The exterior of the robot module protects its contents from water, dust, dirt, and impact. The flexible linkages allow relative motion between robot modules so the system can turn, negotiate obstacles, and traverse around corners.
Robot Module Chassis
The chassis is the structural element of each robot module, enclosing the electronics, drive modules, and battery (see Figure 1). The first iteration of the robot module was developed to accommodate rapid changes in design by using modular mechanical interfaces for the linkage and drive module. O-rings and seals to protect internal components from the elements were not included in the first iteration; future iterations will include environmental sealing. A plastic prototyping machine was used to fabricate the robot module chassis with a polycarbonate ABS blend. Final versions of the robot module will most likely be machined from aluminum to increase strength and durability.
The magnetic wheel provides the attractive force between the MSMR and the surface it’s climbing. This feature allows the robot to traverse vertical and inverted surfaces, making it useful for exploring ships, shipping containers, and other ferrous environments. The primary design attributes that drive the effectiveness of a magnetic wheel are adhesion force, surface friction, acoustic signature, shock absorption, mass, cost, manufacturability, ease of assembly, and serviceability.
For a wheel to climb a surface effectively, it needs enough adhesion force to carry the weight of the MSMR so that the friction between the wheel and surface being climbed keeps the wheel from sliding. The acoustic signature of a wheel while climbing a surface is important for operations where stealth is required. Flexibility of the wheel provides survivability for the entire system under high-shock loading when the robot falls from a vertical surface. Minimizing the mass of the wheel and MSMR, in general, reduces the required magnetic forces for climbing, motor output torque, and electrical power.
The flux-plate wheel components consist of an elastomer wheel, two fluxplates, a flux-plate locator, a rigid hub, and an array of magnets oriented parallel to the central axis of the wheel. The magnets are positioned with all the north poles facing one side of the wheel, and the south poles facing the other. The elastomer wheel is made of 1-inch-thick neoprene. In addition to locating the other components of the wheel, it allows the entire assembly to flex during impact with the ground.
The outer surface of the wheel has a high coefficient of static friction, maximizing traction with hull surfaces. The rigid hub in the middle of the wheel translates torque from the output shaft of the drive motor to the wheel assembly to facilitate motion. The flux-plates direct the magnetic flux of the magnet array through the surface being climbed, providing adhesion. The flux-plate locators help keep the flux-plates centered on the elastomeric wheel. A prototype of the optimized design is 1.25" wide, 4" in diameter, and has a measured attraction force of 21 lbf.
The drive module provides the torque to rotate the wheels and move the robot. The main components are the motor, gearbox, output shaft, housing, motor shaft shock isolator, and bearings. The primary design considerations for the drive module were torque output, speed output, shock absorption, weight, robustness, and modularity. Required motor torque for a single motor output was 20 inch-pounds. The required speed output was 57 rpm. These values were used to select an optimal motor gearbox combination.
The development of the MMR was very dynamic, and experimentation and prototyping was used to try many different ideas. For this reason, modular designs were used for the drive modules, allowing them to be quickly moved from one robot chassis design to another. The entire drive module can be quickly detached from any chassis by removing four screws around the perimeter of the housing.
Most unmanned ground vehicles are subjected to large shock loads as the robot traverses over rough ground. Robots that climb or are thrown tend to experience even larger shock loads when they fall and hit the ground. Gearbox output shafts are often hardened and will easily break if a large radial load is experienced. For this reason, the drive module was designed to mitigate the effects of large shock loads when the robot falls onto the wheels. The output shaft is supported by two high-load bearings directly coupled to the drive module housing. If the robot falls and impacts a wheel, the radial loads are distributed through the output shaft to the drive module housing, and back to the robot chassis instead of to the gearbox output shaft. The motor shaft shock isolator allows relative motion between the output shaft and gearbox shaft during impacts that cause the output shaft to deflect, while allowing torque to be transmitted from shaft to shaft for vehicle motion.
The linkage, which makes the mechanical connection between the robot modules, must be flexible to allow the robot to turn and maintain wheel contact with the ferrous surface being climbed. It must also be able to transfer push (compressive) and pull (tension) forces between the robot modules so they can work in concert to overcome obstacles greater than the capability of any one robot module.
Drive modules with too many degrees of freedom (DoFs) may make control of the system overly complex and limit how force can be transmitted between robot modules. On the other hand, a linkage with insufficient DoFs will limit the maneuverability and obstacle traversing ability of the system. To better understand the linkage needs, a series of prototypes was designed, fabricated, and tested.
The elastomer bow linkage (EBL) was pursued because it appeared that it would provide both flexibility for maintaining wheel contact and the transmission of forces for push-pull benefits. At rest, the bow linkage is rainbow-shaped, allowing the robot to traverse external corners more easily. The design is extremely simple, robust, and easy to fabricate. Linkages from three different durometers of elastomer were fabricated and tested. Testing results showed the elastomer bow was effective at translating tension forces and providing flexibility to maintain wheel contact. However, during some cases when push force was needed, the elastomer linkage started to buckle and the rear wheels caught up to the front, creating a situation where the robot stuck to itself. For this reason, the hardest durometer linkage worked best but still had problems with buckling. Implementing a harder durometer elastomer would limit turning flexibility and would not allow for adequate pitch flexibility on a system with three or more robot modules (internal robot modules would lose surface contact while traversing internal corners).
It is conceivable the implementation of the coordinated control system may reduce the cases where the linkage is buckled. However, the sensors required to maintain information on relative position of robot modules are much more complex than linkages with discrete DOFs. It is expected that some type of vision or beacon system would be required to implement a coordinated controller for a system with this type of linkage.
A rigid-bow linkage was fabricated for evaluation on the two-segment prototype robot. The linkage looked exactly the same as the EBL, except that it was fabricated from a piece of plywood that made the entire robot structure rigid. The robot could traverse obstacles similar to those of the EBL as long as it was perpendicular to the obstacle. The rigid linkage did not perform well if the obstacles were approached at an angle, because not all four wheels could maintain contact with the surface being climbed. The rigid linkage also provided very poor turning capability.
A linkage design that includes a three-DOF ball joint that can lock the pitch-and-yaw DOFs when compressed was designed, prototyped, and tested. The idea is that for compressive force to be translated from one robot module to the next, the ball joint must be locked in the pitch DOF or the linkage will simply start to fold on itself. The ball-joint design allowed the wheels to maintain surface contact with pitch, roll, and yaw DOFs. The design also allowed pull forces through the ball joint. Inside the ball joint, a spring pushes the ball into the normal operating position, but when a compressive force between the robot modules starts to compress the spring, the ball slides further into its receptacle, and the pitch-and-yaw DOFs are constrained. As the compressive force diminishes, the spring returns to its original position and the joint regains its full range of motion.
The spring implemented on this design was too rigid and unable to be compressed by the robot modules. Additionally, the range of motion of the ball joint was not enough to allow the robot to traverse external corners. Conceivably, the lockable ball-joint design could be implemented on a bow-shaped linkage, allowing the traversal of external corners. A spring with a lower spring rate could also be integrated into the system to better evaluate the feasibility of this design.
For a robot with only two drive modules, a pitch DoF is not needed to maintain wheel contact when traversing an internal corner. For this reason, a single DoF bow linkage was developed. The first prototype simply included a hinge point in the middle of the bow linkage. The linkage proved effective for turning, but if the system was accidentally turned too much (which happened often with the manually controlled prototype), wheels from the adjacent robot modules could get stuck together.
To mitigate this failure mode, a second prototype was developed with a limited range of motion at the hinge point. Instead of using a traditional hinge, a leaf spring was designed as the pivot to try to bias the robot towards a position where the front and rear wheels were aligned. The design resembled a simplified saloon door concept. The wires connecting the front and rear robot module were run through the middle of the linkage and along the sides of the leaf spring. The pivot and range limiting portions of the design worked well, allowing effective turning without allowing wheel-to-wheel contact. The spring contribution of the design was ineffectual since the torque produced by the wheels could easily overpower the spring. For future designs, using a spring to provide a bias at a joint is ineffective for this application. The coordinated control system can implement the robot module relative position control more effectively and efficiently.
Two prototypes were developed. The first, a two-segment robot with RC components, was developed to quickly test different variations of subsystem components such as the drive module, magnetic wheels, and linkage. The second, a three-segment system with integrated processors, will be used to develop coordinated control software for the system (see Figure 2).
The RC prototype was tested with a variety of different linkage concepts. The yaw-bow linkage only provided a single DoF in the yaw axis, but proved the most effective of those tested with the RC controller. The system successfully climbed at 0.5 foot/second and negotiated internal corners, external corners, and obstacles as big as 3". It could also turn on vertical, horizontal, and inverted horizontal ferrous surfaces (see Figure 3). The linkage did not provide a roll DoF, resulting in the loss of wheel contact if an obstacle or angular transition was not traversed orthogonally to vehicle motion. Often, when wheel contact was lost, the robot would fall from the surface being climbed. It is expected that the addition of a roll DoF would improve the system’s mobility.
The prototype systems demonstrated that a multi-segmented magnetic robot with relative DoFs between modules can effectively climb and negotiate surfaces with discontinuities, obstacles, internal corners, and external corners. Additional development of the mechanical, electrical, and software systems is required before the system is ready for testing in operational scenarios. The technology is promising for use in maritime interdiction operations and vessel hull and tank inspections, with the potential to increase safety, effectiveness, and efficiency of personnel involved in maritime operations.
This article was written by Aaron Burmeister, Narek Pezeshkian, Kurt Talke, Abraham Hart, and Gary GilBreath of SSC Pacific, San Diego, CA. For more information, visit www.public.navy.mil/spawar/pacific .