The use of Vehicle-Borne Improvised Explosive Devices (VBIED) has increased each year. Current anti-VBIED technology is not only expensive, but requires months or years of training by Explosive Ordinance Disposal (EOD) technicians to operate the equipment. The process of unloading the EOD robot, attaching the detonation wire to the robot, attaching the water charge to the EOD robot, driving the water charge to the VBIED, placing the charge under the vehicle, and then retrieving the EOD robot is a time consuming event. With a typical EOD robot costing $100k - $200k, there is a large financial risk to the EOD team if the robot is damaged or destroyed in the process. WM Robots PAWN was developed to offer the EOD technicians another option in reducing the time needed to neutralize the threat and cost of the operation.
Based on EOD Squad feedback, a need was identified for a low cost solution to complement the current procedures for VBIED neutralization. The feedback identified the major design criteria and represented some of the design challenges of PAWN:
- Low cost, expendable;
- Video for non-Line of Sight (nLoS) operation;
- 500 feet of tethered operation, including control, video transmission, and electronic detonation cable;
- Simple operation, minimal training to operate;
- Operation on semi-improved roads with normal debris;
- Deployment in third world countries.
The overall design philosophy was to minimize the Size, Weight and Power (SWaP) of the system. This was one of the major design challenges in development. Computer modeling of the chassis was utilized to simulate the stresses that would be encountered during operation, and allowed for the final design to be as minimal as possible. The requirement of being expendable dictated that the components used could not pose any additional hazards to the scene. By utilizing in-house rapid prototyping of components, Proof of Concept (PoC) and Prototype testing time was reduced, facilitating a reduced development schedule. The design teams approach was to define and design first the size, then the weight, and lastly the power subsystem.
Vehicles utilized in VBIED threats tend to be compact cars, with the IED being placed either in the trunk or backseat of the vehicle. The water charge used to neutralize the VBIED must be placed under the vehicle in these areas for maximum effectiveness. The average ground clearance of a compact car is 9" in the trunk section. In order to accommodate for the weight of the IED compressing the suspension of the vehicle, the maximum height was set to 7.5". This height allows the operator to place the water charge directly under the trunk or close to the backseat. This height requirement posed a challenge to the design team, such that the design of the chassis had to have minimal height, securely hold the water charge in place during operation and placement, and have adequate ground clearance for road debris. The chassis was designed using lightweight aluminum, reducing the frame weight, and providing the strength needed for the water charge weight.
To determine the final system weight, 3 areas needed consideration: The water charge system weight, the cable assembly, and the frame. A typical water charge contains 5 gallons of water and an explosive charge with a combined weight of approximately 40 lbs and fixes one part of the total system weight that could not be changed. The requirement of tethered operation, with an operational range of 500 feet, sets another part of the system weight. Design options for the cable assembly were completed and the resultant design of the cable assembly and spooling mechanism added another 16 lbs of weight to the system. The chassis design was finalized such that the final weight, with batteries and electronics, was 8lbs, setting the total system weight to 64 lbs.
With the size and weight parameters identified, the design team could then focus on the motor and power source subsystems. It was identified that a typical skid steering configuration would not be feasible in the design. Typical skid steer in a 4 wheel vehicle encompasses 2 drive wheels and 2 fixed front wheels. When turning a skid steer drive system, the drive wheels are driven in opposite directions, and the fixed front wheels are dragged across the surface, increasing the torque and battery requirements of that system. After evaluating several different steering configurations, the final design for the steering was a modified skid steering scheme that utilizes 2 drive motors and 2 caster front wheels instead of fixed wheels. During a turning maneuver, the caster wheels would rotate to the direction of the turn instead of dragging, reducing the motor torque and power source requirements by 25-40% depending on the terrain.
Design and evaluation of motors was another major design challenge for the team. A wheel diameter of 6" was chosen to give adequate ground clearance, while staying under the 7.5" height requirement. Design calculations showed that, based on terrain, weight, and wheel diameter requirements, that each motor would need a minimum of 21 lbf-in of torque. A suitable motor that meets the torque requirement and with a shaft RPM of 85 was identified. The speed was then calculated to be 1.5 mph, based on motor RPM and wheel diameter. At this speed the time needed to reach the target in a straight line, with an operational range of 500 ft, was calculated to be 3.75 minutes. The design operational time was increased to 15 minutes, to account for obstacle avoidance and nLoS (non-line-of-sight) operations.
The identified motor’s power requirements represented 90% of the total calculated power budget. In order to keep the total power budget to a minimum, the electronics and video camera were designed using low power components.
To address the power source options, the operating parameters needed to be carefully considered. The use of lithium based batteries was not an option, due to the fact that the sysrequirements. A drawback of alkaline batteries, while having a large mAh capacity, is the increased internal resistance over nickel-based or lithium-based batteries. This increased internal resistance limits the amount of power the batteries can supply with a large step increase in current. In order to design within this limitation the design team developed drive controls that ramped up the acceleration over time, reducing the step function of motor current.
Several PoCs (proof of concept) were constructed to evaluate the design parameters and initial calculations. The PoCs demonstrated that the initial design calculations for minimizing SWaP were correct, and that a final design that meets the end-user requirements was viable. Prototypes were then developed and tested on a variety of surface conditions and inclines. Testing showed that the caster steering operated nominally with small to medium debris on a roadway, including small potholes. Alkaline battery life testing under simulated real world scenarios resulted in a 12 - 15 minute runtime, depending on terrain, validating the power calculations. Testing was expanded to using NiMH batteries and resulted in a 15-17 minute runtime, on the same terrain that was used for the Alkaline battery testing. The use of NiMH batteries is an option for the operator, if the situation presents a need for a longer run time. The final design was then field tested and all design criteria were validated.
This article was written by Mark Giacobbe, R&D Manager, WM Robots (Colmar, PA). For more information, Click Here