Melty Brain

Melty Brain Beetleweight

The Melty Brain robot is a modified Thwack Bot that allows 100% of the bots mass to spin, but unlike a traditional Thwack bot that spins in place, the Melty Brain robot adjusts the wheel speed on each rotation allowing it to effectively drive around the arena. This is also called translational drift or "tornado drive." Because the whole robot is the weapon, no weight goes into a separate weapon system – but a melty is complex to build and tune, and if the drive fails, the robot loses both its movement and its weapon.[1]

How it works

• The robot spins its whole body using its drive wheels.
• An accelerometer mounted away from the center measures the centrifugal force, which tells the controller how fast the robot is spinning.
• The controller uses that to keep track of where "forward" is during each rotation, and flashes an LED once per revolution at that point. To the driver, the flashes look like a steady dot showing the robot's heading.
• To move, the controller briefly changes the power to the wheels at the same point in every rotation. The robot drifts in that direction, while still spinning at full speed. The driver's stick controls which way it drifts, and turning left or right shifts the heading.[2]

1-wheeled robots: Although the design may seem counter-intuitive, there are advantages to using a single large motor to spin the robot. In addition to the increased power and durability of a single motor, tuning a single wheel to control the robots movement is generally easier.

2 wheeled robots: These melty brain robots have a more typical design with two wheels to balance the weight of the robot as it goes through it's rotation. Although these can be just as effective as 1-wheeled robots, the additional friction from the second wheel as well as weight-differences between the two wheels can make tuning the melty-brain more difficult. One advantage of a two-wheeled Meltybrain robot is that it can revert to (more usual) tank-drive steering, e.g. to escape from corners.

Mechanical Melty Brain: Although not a true Melty Brain, there have been examples where a small amount of weight is dedicated to a central 'directional' bot that maintains contact with the ground and uses a cam to adjust the wheel angle during the robots rotation. (Y-Pout)

Rich Olson over at Nothing Labs has an open source melty brain project and has successfully used his approach with a variety of weight-classes. http://www.nothinglabs.com/openmelt/

Hardware

The current version of Rich Olson's project, Open Melt 2 (published in December 2023), is a good starting point.[3] It runs on an Arduino-compatible ATmega32u4 board (such as an Arduino Micro) with a high-g accelerometer such as the H3LIS331DL (±400 g), and drives either MOSFETs for brushed motors or brushless ESCs. It uses a normal 3-channel radio with a working failsafe. The accelerometer has to be placed so it stays within its range at full speed, and the code includes safety features such as spinning down on signal loss. More advanced builds use faster microcontrollers, two accelerometers, and brushless ESCs that report motor speed back to the controller.[4]

Challenges

• Tuning: the controller needs calibrating so the LED heading stays steady. Open Melt 2 has a calibration mode for the accelerometer offset, the effective radius, and the LED position.
• Heading drift: an accelerometer alone can't hold an absolute heading, so the heading slowly drifts and the driver has to keep correcting it.[5]
• High g-forces at full speed put a lot of stress on the electronics, battery and wiring, which must be very well secured.
• Driving skill: steering a melty takes a lot of practice.

Notable meltybrains

• Blade Runner – an early meltybrain attempt in the original BattleBots era; the technology never worked as intended.
• Blade Runner – a middleweight built by Ilya Polyakov, the first robot to attempt meltybrain, in the original Comedy Central BattleBots; the technology never worked as planned.[6]
• Melty B – Rich Olson's antweight meltybrains, featured by Pololu in 2009.[7]
• Nuts 2 – the first meltybrain on Robot Wars (Series 10, 2017), which beat defending champion Carbide on the way to a joint 3rd place in the Grand Final.
• Project LiftOff – a 3 lb meltybrain that in 2021 became the first meltybrain to win an NHRL event, and finished 3rd at the 2022 NHRL World Championship.[8]

Rules

• Because a meltybrain's weapon is its drive, it can't use a normal weapon lock. NHRL requires meltybrains to use wheel locks or covers instead.[9]
• Some rule sets require a weapon that's independent of the drive. NHRL makes an exception for meltybrains that show controlled movement while spinning; BattleBots doesn't count translational drift as an active weapon.
• Check your event's rules before building one, and always follow the safety rules for spinning weapons (see Spinner Design and Safety).

See also

• Translational drift on Wikipedia
• Weapons – full-body spinners and thwackbots
• Spinner Design