Narsil

This page explains the design and development of my 3lb combat robot Narsil. This robot must be able to operate for 3-4 minutes, weight 3lbs and disable/destroy other robots through kinetic energy transfers.

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Mechanical Design

Drivetrain

The first 2 versions of Narsil used 2 brushed drive motors each producing 70W. I switched to brushless outrunners in version 3 to save weight and have greater drive power. Version 3 uses two 1806 brushless outrunners each capeable of producing 100W. The smaller form factor of these motors allowed for the center of mass to be moved closer to the wheel and weight could be distributed elsewhere on the robot. The weapon motor was tucked further into the chassis, almost between the wheels. One disadvantage of the brushless motors is that the can of the outrunner spins meaning it needs to be shielded from the wiring of the robot. Wide foam wheels provide great grip on plywood and prevents the motor getting destroyed when directly hit. This also reduced the material and weight of the 3D printed Chassis.

Weapon System

I have a 0.5lb weapon that is driven by a single 3530 brushless outrunner that can produce 500W. This motor is 1,400KV and at maximum throttle it reaches 25520 rpm. This motor is geared 1-1 to the weapon which is not ideal for battery life but allows for the weapon to store more rotational kinetic energy. The moment of inertia of the weapon is 6.138E-4 kg*m^2. This means if the weapon was able to reach maximum RPM it would store 2,192 Joules of energy. The muzzle energy produced by an AK47 is 1,990 Joules.

Render Of mk4 Weapon Assembly

Fusion 360 Shape Optimization

Shape Optimization

While designing a new top plate, I wanted to optimize the design for the load experienced during a hit. I created a rectangle in CAD with the correct hole spacing for the robot mounting and the weapon axle. I applied a 2,000lb bearing load in several directions, one if shown to the right. I chose a 2,000lb load since this was the maximum dynamic radial load for my bearings. The simulated load is far greater than the measured forces are closer to 200lb The mounting points were fixed in space for this simulation. This showed my the load path and thickness of material required to handle a hit that would break my weapon bearings. This showed that top plate I had designed had a large amount of unnecessary material. Versions 3 and above use 500lb loads as the max conditions for optimizing top plate geometry. I performed simulations with different loading directions to gather greater information to optimize the design.

Weapon Motor Graph

I generated a Speed Vs Torque graph for an un-sensored 3530 propdrive V2 brushless outrunner. From this graph and the corresponding equations I was able to determine the current drawn while spinning up and the maximum angular acceleration of the weapon. I use this as a design tool to verify that Moment of inertia changes to the weapon will not result in unacceptable spinup times. Ignoring free run current or additional resistance during a match the weapon will be able to spinup 17 times before the battery is depleted.

Custom Designed Wheels

The goal of the custom wheels was to provide equivalent or better traction that off the shelf replacements, while reducing the overall width of the wheel. This reduction in width allowed for the chassis to decrease in weight.

2 Shot Wheel Mold

I designed a 2 shot expansion mold to produce a set of wheels with each mix. A 2 part polyurethane foam was chosen as the wheel material. This was chosen due to the low density and the ability to tear without delaminating the tire from the hub. A Nylon 6/6 hub is machined and inserted into the mold. The foam is then molded onto the hub.

Finished Wheels

A finished set of production wheels are ready to be put on the robot. Since the mold is able to maintain pressure while the foam is expanding, fine details can be molded into the wheels.

Wheels on Robot

Wheels are assembled onto the mk4 version of the robot for combat testing. From testing in competition the wheels seem to perform as intended. The foam tears on a hit but does not loose the whole tread. When the nylon hubs are impacted they shatter, preventing shock loads from transferring load to motor gearboxes.

Bill Of Materials

Narsil V2 Weight Breakdown

Old Versions

Narsil 1.0

The first version of Narsil suffered from time constraints and optimistic estimations of the failure modes of the assembly. The robot suffered from poor traction, this made precise maneuvers and pushing other robots nearly impossible. The weapon assembly was missing key spacers between the races of the bearings. This prevented me from pre-loading the weapon assembly. This meant that and forces perpendicular to the weapon would result in damage to the weapon bearings. I was pleased with the first competition weapon power and was surprised with how much energy it was able to deliver in each hit. It went 1:2 at the Norwalk Havoc event I attended with version 1.0.

Narsil 2.0

The second version of my beetle Narsil took great advantage of the experience gained from the first competition with Narsil 1.0. The only major component change was the wheels. I changed from a homemade foam wheel and hub to an off the shelf wheel from BotKits. Wheel guards were added to prevent the wheels from being easily removed by other robots. The addition of wheel guards and heavier wheels resulted in weight reductions else where in the robot. Although the chassis occupies greater volume it is 50g lighter than the previous version, weight was also removed from the top weapon assembly. I was worried that this would compromise the strength of the assembly when encountering a vertical load. I simulated the top aluminium plate in CAD with various load cases and determined that removing the material covering the weapon motor would not compromise the structural integrity. Narsil 2.0 went 7:0 at its second event, winning the entire event.

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