Know Your Combat Robots! A Field Guide to Competition Weight Classes and Weapons

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Know Your Combat Robots! A Field Guide to Competition Weight Classes and Weapons

The combat robotics community spreads across the entire world, with major competitions happening on almost every continent. Combat robots are divided into different weight classes based on the robot’s overall weight, and within those classes an amazing number of weapon types are possible. In this guide we’ll introduce you to all of the most important weight classes and weapons.

THREE FLAVORS OF COMBAT

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In addition to weight, three different class modifiers are common in robot combat:

 Full Combat or Standard class is the
least restricted ruleset, allowing kinetic energy weapons, wedges, and a variety of control bots (lifters, etc.). Most events run
Full Combat classes.

• Sportsman classes add tip-speed limits to spinning weapons, drastically reducing their ability to do damage. They also require an active weapon on each robot, to stop passive wedges from dominating the competition. Sportsman classes are common at the 12lb and 30lb weight classes.

 Plastic classes, generally only found at the 1lb Antweight scale, require robots to be entirely fabricated from plastic parts (aside from electronics, motors, and fasteners). The Plastic Antweight is rapidly growing in popularity as an entry-level class due to the ease of 3D printing and the low damage rate, which make the robots extremely affordable compared to other weight classes. (Build your own on page 44.) You can learn more about combat rules online at sparc.tools.

ROBOT WEIGHT CLASSES

There have been many weight classes over
the last 30 years, from 25g to 350lbs, but not all
of them have active competitions today. Here’s your guide to currently active combat robot weight classes.

FAIRYWEIGHT (U.K. ANTWEIGHT) — 150g limit

A good class for beginners. Many free designs can be downloaded from Thingiverse and printed on a 3D printer; components are cheap, and the lack of devastating weapon power makes Fairyweight bots cheap to repair. Unfortunately, motors and electronics take up more of the overall weight of a Fairy, as it is hard to find super small components, making custom designs challenging. Although Fairyweights are usually made from 3D-printed plastic, the most competitive are packed with titanium, steel, and carbon fiber to maximize strength and performance.

ANTWEIGHT (U.S.) — 1lb limit

Antweights are the best place to start out. Designs that are mainly 3D printed or fabricated with cheap materials can be competitive. Weight is not as much of a limitation as it is in Fairies. Common robot construction methods, such as directly bolting weapons and wheels to motors, can be used successfully with little modification. This class is generally less destructive than the Beetleweight class, so your robot is less likely to get completely destroyed. And because the electronics take up less of the entire weight, robots that are homemade with cutting boards and duct tape can easily survive and even win Antweight events.

BEETLEWEIGHT — 3lbs (U.S.) / 1.5kg (U.K.)

By far the most competitive insect weight class. Brushless motors at this scale provide insane power-to-weight ratios, thanks to the huge market for micro drones which has driven the improved power density and affordability of small motors. This makes Beetleweight robots incredibly powerful and dangerous. While they can be made by hand and with 3D-printed parts, the most successful Beetleweights feature a multitude of custom machined parts in exotic materials. Solid engineering and design are
a must for this class, as robots regularly get ripped in half.

While you might expect bots in this class to be mostly metal, a surprising amount of engineering plastics are used, giving superior impact protection and strength while minimizing weight and cost. Additionally, 3D-printed materials such as nylon and TPU are heavily utilized, making fabrication much easier than with larger bots.

HOBBYWEIGHT — 12lbs

Hobbyweight bots are the smallest non-insect bot. This weight class allows for more creativity in robot design. The larger footprint provides space for more complex mechanisms, while the larger weight budget allows for more off-the-shelf components to be used, and the motors and electronics take up a smaller proportion, leaving more weight for frame and weapons. The weapons on these robots store incredible amounts of energy, so they must be designed more robustly than bots in smaller classes. Plastics are no longer the go-to for frame material; Hobbyweights start to use aluminum and steel frames in order to survive the devastating weapons that they face. While these bots are four times as heavy as Beetleweights, they’re nowhere near as power dense. No micro drone motors here — Hobbyweight bots work at a larger scale, with motors that are generally used in large RC cars and electric skateboards.

DOGEWEIGHT — 15lbs

Dogeweight robots are essentially overweight Hobbyweights. Most competitions that host larger bots host 12lb bots, but this 15lb class has found life in an educational setting. Leagues such as NRL and Xtreme BOTS host high school and college level competitions where engineering students can practically apply what they’re learning to fighting robots. As with Hobbyweights, these robots are incredibly dangerous and can do amazing amounts of damage.

FEATHERWEIGHT — 30lbs

This weight class is very popular for experienced builders, as a single builder with experience can design and fabricate a Featherweight. There are several competitions for these robots annually in the U.S. and worldwide. Featherweight robots are generally less weapon oriented than smaller robots and focus more on reliability and defensive armor. Good engineering becomes more important when fighting at the featherweight scale because parts need to be relatively lighter than smaller scale robots while providing the same performance.

LARGER WEIGHT CLASSES

Competitions for robots above the featherweight class are few and far between. Lightweights and Middleweights are practically extinct, and Heavyweight events have been limited to well-funded TV shows, or small sportsman events. The cost of building these robots grows exponentially between weight classes, so it takes a well-sponsored team with experience to compete at this scale. The best-known combat robot event is BattleBots, which is televised on Discovery and has near 1 million viewers each episode. Outside of BattleBots, various Robot Wars TV series from the U.K. are seen around the world, and several heavyweight competitions have been filmed in China including FMB (Fighting My Bots), Clash Bots, and KOB (King of Bots).

SPINNING WEAPON TYPES

Here’s how they work, and their advantages and disadvantages, especially in the smaller classes.

VERTICAL SPINNER

Vertical Spinner

These weapons store energy in a spinning mass that rotates in a vertical plane, generally sticking out the front of the robot. They spin so that the impact point of the weapon is moving upward, effectively delivering an “uppercut” to the opponent. Vertical spinners are very effective fighters, as the reaction force from the uppercut pushes the bot downward into the floor, keeping them planted.

The key to the vertical spinner is the ability to get under other robots. When an opponent rides up a vertical spinner’s wedge or forks, the spinning blade can impact the bottom edge of the opponent’s chassis, rather than grinding against a side wall. This results in significantly better energy transfer, meaning a harder hit and more damage.

Vertical spinners generally spin either a bar
or a disk weapon, and commonly switch between the two depending on the opponent they’re
facing. Disks are more popular because they store more energy for a given mass, but they’re vulnerable to horizontal spinners. Doubling the spinning speed of a weapon results in four times the kinetic energy, so vertical spinners will often deploy asymmetrical, single-tooth weapons in order to maximize their “bite.” This allows them to save weight while increasing the energy stored in the weapon.

One problem vertical spinners can suffer from is gyroscopic forces on the robot due to the high momentum of the weapon. Like a bike wheel, the spinning weapon acts as a gyroscope and resists the robot’s turning motion. Turn too fast, and one end of the robot “floats” off the ground. This effect can be used to self-right some robots but it can also flip a robot upside down. Because of this effect vertical spinners need to limit their turning speed, or turn in large arcs. These handicaps can give other robots an advantage.

DRUM SPINNERS

Drum Spinner

These weapons also spin vertically but they’re wider, with smaller diameters, and can spin at much higher speeds. Drum spinner robots are mostly driven with two-wheel drive and have smaller and more compact frames than any other weapon type, but their geometry is especially prone to float when turning.

Because the weapon on a drum spinner protrudes past the frame of the robot, their ground clearance is not as important as other vertical spinners. Most robots get away with a small piece of metal pointed toward the ground to stop other robot’s wedges from getting under, but even that’s not absolutely necessary to be competitive.

There are several popular ways to make the drum weapon. The first is to fabricate a single-piece drum out of a hardened steel. This is difficult without highly specialized tools. An easier way is to mount hardened steel “teeth” to a drum using bolts, but these weapons have to be manually balanced which takes considerable time. The easiest way is by layering hardened steel disks with spacers made from a softer material, such as aluminum. This simulates the large impact face of a unibody drum while also storing a large amount of energy in the rotating mass of the weapon.

BEATER BAR SPINNERS

Beater Bar

Beater bars are a weapon type that sits between verticals and drums. They span most of the width of the bot, like a drum, but they can store more energy with a larger diameter, like a vertical spinner. The general idea behind a beater bar is to maximize the moment of inertia (MOI) of the weapon while keeping the weight low. This is achieved by removing most of the material on the inside of the weapon, and keeping the outermost material, as mass further from the axis of rotation has more of an effect on MOI.

The most effective beater bars are solid steel, custom machined using CNC or wire EDM processes, but there are good low-cost alternatives. These budget weapons are cut out of a flat piece of aluminum in an outline of a rectangle, with a hollow interior; bolts are mounted to the top and bottom edges to act as a cheap hardened impacting surface. These can be cut with only a few operations and are nearly as effective as more expensive weapons at a fraction of the price.

HORIZONTAL SPINNERS

These weapons spin in the horizontal plane and are generally much larger in diameter than vertical spinners, but they spin at slower speeds. Most horizontal spinners can be split up into one of three categories:

Horizontal bar spinners or midcutters have their weapon mounted off the ground, between the bottom and top of their chassis. Horizontal spinners generally have bar weapons, as they are vulnerable to vertical spinners. Using a disk weapon this high off the ground will give vertical spinners great engagement into the weapon, making bending more likely. With a bar, there’s less surface for a vertical to hit, and the weapon can be made stronger, as it does not have to span as much area. Horizontal bar spinners are vulnerable to wedges, as their weapons struggle to get good bite on hardened surfaces and get deflected upward. These robots are also very prone to self-inflicted damage, as their weapon can be forced into a floor or wall, causing the robot to ricochet around the arena.

Undercutter weapons are mounted under the chassis, as close to the ground as possible. Undercutter robots attack low on their opponents with the goal of bypassing armor and directly hitting important components such as wheels. Because they spin so low to the ground, other robots have trouble getting under them without taking significant damage, and they’re less vulnerable to vertical spinners. Undercutters can use either disks or bars for weapons, but disks are more widely used as they store energy more efficiently.

Overhead bar spinners can spin weapons with much larger diameters than undercutters or midcutters, because they spin over the top of the entire robot chassis. These robots are very fun to design and build and can be unbelievably destructive due to their high energy storage, but they have several inherent flaws. They can’t drive while inverted, as the weapon lies between the wheels and the ground. Because they store more energy, they take the greatest strain on the chassis and weapon system whenever they land a hit. They’re also extremely vulnerable to vertical spinners as their weapons are mounted so high and cannot be supported on both top and bottom.

FULL BODY SPINNERS

Full body spinners have a spinning weapon that protrudes outside the robot’s drive chassis and surrounds the entire robot. They function as both weapon and armor, and usually weigh more than any other weapon type. Full body spinners are split up into three types:

Ring spinners spin a ring weapon around the robot’s frame. These rings are supported at the outside of the robot and do not cover the top. Therefore the robot can use wheels that are taller than the frame and stick out the top, which allows them to drive while inverted. While being invertible is a huge advantage, this weapon system is inherently fragile. Because it spins around the body of the robot (with several contact points to the frame) the weapon will fail with any deformation. This makes ring spinners one of the most complex weapon types to design successfully.

Shell spinners have a weapon that covers
their top and sides, protecting the entire robot. They can’t drive when inverted but have a much simpler weapon support compared to ring spinners. Shells generally have a protrusion on top that makes them unstable when inverted, allowing them to right themselves. These weapons are much more robust to damage, as they generally spin about a shaft in the center of the robot and deformation of the outer shell does not necessarily mean it will be disabled.

One problem that plagues both ring and shell spinners: because their weapons have large MOIs (due to the large radius and weight), they’re prone to losing drive traction when the weapon first starts spinning, which starts the chassis rotating in the opposite direction. This causes the driver to completely lose control of the bot.

Meltybrain spinners are a special type of robot that uses its entire body as a weapon and its drivetrain to power it. Meltybrain robots have very high-speed drivetrains that essentially spin them in place. They can then use sensors to modulate the speed of the motors to achieve translational movement. Because the full robot weight is in the weapon, these bots hit extremely hard. Their entire chassis must withstand thousands of RPMs, so they are generally very tough. While meltybrains are extremely damaging, they are very technically complex and require more computing than any other robot type.

NON-SPINNER WEAPONS

LIFTER ROBOTS

Lifter Bot

Robots that have a mechanism that does not store energy and is used to lift up and flip over other robots are called lifters. These weapons move slower than flippers (see below), and can be moved to any point along their travel.

Most insect-weight lifters use a high-power servomotor or gearmotor to power their lifting arms through a simple mechanism such as a lever or a four-bar linkage. Lifters are the safest bots that have active weapons, as they do not store any energy in their weapon system. (You can build one on page 44.) Some lifter robots also have integrated grabbing mechanisms that can hold their opponent as they lift them off the ground. Lifter robots can be very competitive in arenas that feature some sort of pit or “arena out” feature, as they can corral their opponent into the out-of-bounds area, winning the match. Lifters are strong against robots that are not invertible, as a single flip will render their opponent immobile, but they generally struggle to cause enough damage to score knockouts.

FLIPPER ROBOTS

Flipper Bot

Unlike lifter robots, flipper robots have a weapon that uses stored kinetic energy to lift or flip their opponents. Flippers feature complex mechanical systems that make reliability a challenge: springs or pneumatics for insect-weight flippers, spinning flywheels or hydraulics for the heavier classes. Some events ban pneumatic weapons due to safety concerns, which makes flipper robots even harder to successfully compete with. Flipper robots work best in an arena with an “arena out” feature but can be powerful enough to damage robots and achieve knockout victories in a standard arena.

OVERHEAD SAW ROBOTS

Overhead Saw

These robots have a spinning weapon at the end of an arm that can be independently articulated. Most have an actual saw blade that they use to cut through other robots, taking advantage of the lack of top armor. (Others have a weapon called a hammer saw that’s no different from a regular spinning weapon as it imparts kinetic energy through an impact.) Overhead saw bots usually have a very open front end with prongs protruding, as they need to control other robots and hold them still for the saw blade to be effective. These robots require a robust design and also a talented driver, as the articulated weapon is more demanding to operate and they usually need to pin their opponent in place to do meaningful damage.

HAMMER ROBOTS

Hammer Bot

Hammer robots and their cousins, axe robots, have a weapon that swings a mass over the top
of the robot in a limited arc to impart kinetic energy into their opponent. At the insect weight classes, these robots can be powered with either a high-speed servo or a motor. Like overhead saws, these robots take advantage of the light top armor on their opponents. There are two main problems with insect-weight hammer bots. First, their weapon must be triggered slightly before the opponent is in front of them because the hammer takes time to complete its swing. Insect bots move fast relative to their larger counterparts, which exacerbates this problem. Second, they struggle to store comparable energy to spinners as their weapon only has up to 180 degrees of motion to accelerate before impact.

CRUSHER ROBOTS

Crusher Bot

Crusher robots have an overhead arm that pierces through the top of their opponents, damaging the internals . At insect weight classes, crushers are designed to convert fast rotational movement from a motor into slow, powerful linear movement at the tip of their claw. The fatal flaw of crushers is that their actuators either are too slow to easily grab their opponents, or lack the force needed to pierce armor. Crusher robots are mechanically complex and require extremely precise driving.

WEDGE ROBOTS

Passive Wedge

Passive wedge bots don’t have an active weapon but they’re very durable, as they rely on pushing their opponent around until they break. These robots are the simplest of all — and very competitive! 

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Peter Garnache

Peter Garnache is a mechanical engineer, combat robot builder, and mentor to high school robotics teams.

View more articles by Peter Garnache
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