Self-Righting

Sooner or later almost every robot ends up upside down, on its side or stuck against a wall. If it can't keep fighting, it loses. Self-righting is the ability to get back onto your wheels, and invertibility is the ability to keep driving upside down. Planning for one or the other is one of the most important parts of a robot's design.

Why it matters

Under the standard SPARC Match Rules, a robot that stops moving under control gets a 10-second count. To stop the count it has to show controlled translational movement – it must actually travel in a line, not just spin in place or twitch. A robot that's been flipped and can't drive is knocked out.[1]

Un-sticks don't always help either: under the standard "modified single un-stick" rule, a robot only gets an un-stick if its opponent didn't cause the problem. If you were flipped onto your back or pushed onto the wall, you're on your own.

There are two main approaches: make the robot work either way up, or give it a way to flip itself back over. Many robots combine both.[2]

Invertible designs

An invertible robot can drive normally upside down, usually because its wheels are taller than the chassis and stick out above and below it. Most radios can flip the controls with a switch so the robot drives the right way when inverted (see Radios/Mixing).

Advantages:

Nothing extra to build, power or break.
Getting flipped barely slows the robot down.

Disadvantages:

Exposed wheels are easier for opponents to hit.
Some weapons only work one way up. A vertical spinner, for example, hits downward when inverted, which is much less effective.
Driving upside down can be disorienting, so practice it.
Low, flat robots can be easy to get under.

Famous invertible robots include Tombstone, Minotaur, Tornado and Carbide.[3][4]

Self-righting mechanisms (srimechs)

A self-righting mechanism, or srimech, is anything the robot uses to push itself back over. The term was coined on the UK show Robot Wars.[5]

Using the weapon

Many robots right themselves with their main weapon, which saves weight and complexity:

Flippers and lifters push against the floor to roll the robot back over. A front- or rear-hinged flipper is one of the most reliable srimechs (see Flippers and Lifters).
Hammers and axes swing all the way over and push off the floor (see Hammers and Axes).
Vertical spinners and drums can use gyroscopic forces. Turning sharply with the weapon at speed lifts one side of the robot, and with practice the driver can tip it all the way over. BattleBots robots such as Minotaur, Copperhead and Yeti use this, and some drivers do it as a celebration ("gyro dancing").[6][7]

The downside is that if the weapon is damaged or (for pneumatics) runs out of gas, the robot can't self-right anymore.

Dedicated mechanisms

Some robots have a separate arm, bar or lid just for self-righting, for example an arm on the back of the robot driven by a servo or gearmotor. These work regardless of the weapon, but they add weight and are another part that can be damaged. For small robots with a servo arm, a servo of at least about 2.5 kg·cm torque is a good starting point.[8]

Shape-based (passive) self-righting

Some robots are shaped so they naturally roll back onto their wheels: a rounded top, a roll bar or a roll cage. Shell spinners often have a curved bar on top for this. Passive designs need room to roll, can fail when the robot is against a wall, and only work if the robot lands the "right" way – a robot flipped end-over-end may not roll back.[9]

Common ways to get stuck

On its side, balanced on one wheel or edge. Wide, low robots are less likely to end up here.
Against the arena wall, where a rolling or flipping srimech doesn't have room to work.
High-centered: resting on the chassis with the wheels not touching the floor, for example on an arena seam, a hazard or the opponent.
Srimech damaged, or out of gas or power.

Design tips

Decide early whether your robot will be invertible, self-righting, or both, and design the chassis around it.
Test it! Put your robot upside down and on each side, and make sure it can recover – including next to a wall.
Keep the center of gravity low and in the middle; a srimech has to lift the robot's weight.
For servo- or motor-driven srimechs, make sure the arm is strong enough to survive hits and that the mechanism can't jam.
Practice driving inverted and practice self-righting. Driver skill makes a big difference (see GettingStarted).

Self-Righting

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