In one of my stories, I have a bipedal walker. Basically a copy of a Star Wars walker, but smaller and armed with M16s and grenade launchers. After a few minutes of research and using my common sense, I realized that such a walker would be impractical and nearly impossible. The main problem would be the CoG (Center of Gravity/Mass/Balance), which would probably be in the center of the 'head'. However, when taking a step, the narrow rectangle between the two legs where the CoG can be to avoid falling over becomes a small square over the foot. Here's an excerpt from the first time this walker was introduced to the story:

The warehouse doors were suddenly blown off, and inside was a huge bipedal mech. On top of the mech was (insert evil scientist name here), laughing that weird evil person laugh. "You fools! You have wandered right in front of my MMD, Mech of Mass Destruction! HAHAHAHAHA! Utilizing this stolen miniature nuclear generator prototype, I now have the unlimited power needed to power this mech!" He pushed a lever in front of him, and the mech fell over on its side, tearing open the side of the warehouse. "No! I told Gerald to fix that!" When the dust settled, evil scientist man had disappeared, and the nuclear generator prototype along with him.

What I think evil scientist man needs to do is either: make a way to change the CoG between steps, or have some really big feet pads. For the first option, my rather limited knowledge is imagining a giant servo on top that rotates a weight side to side each step, but this would make it much wider and it would be an obvious attack point. The second option would probably be easier, but would require smoother terrain to stay upright.

I want this solution to be 100% possible with today's technology, and preferably not something on the exterior of the mech.

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    $\begingroup$ So, the good news is that this problem shouldn't even exist. The bad news is that the reason why it shouldn't exist is because that large scale mechs will typically require way more power and weight in the legs than any other part of the mech because of the square-cube law, and the reason why its bad news is because such a design is horribly impractical because mechs don't gel well with hard science fiction $\endgroup$
    – Halfthawed
    Aug 6, 2020 at 18:31
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    $\begingroup$ Give your mech some birthing hips and an inner ear. Problem solved. $\endgroup$
    – DKNguyen
    Aug 6, 2020 at 18:58
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    $\begingroup$ This is basically a problem all existing bipedal robots have solved, and the higher your robot's center of gravity, the more time your robot has to solve it between steps. $\endgroup$
    – notovny
    Aug 6, 2020 at 19:10
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    $\begingroup$ maybe something like suction cup/plunger or finger like chicken feet? $\endgroup$
    – Li Jun
    Aug 7, 2020 at 2:54
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    $\begingroup$ > I want this solution to be 100% possible with today's technology, and preferably not something on the exterior of the mech. That solution already exists: bostondynamics.com/atlas $\endgroup$
    – alexgbelov
    Aug 7, 2020 at 13:52

7 Answers 7



If you don't want a machine to fall, you make it stand up by exerting some authority on it. Shouting is optional.

More specifically, we are talking about turning authority. This is a term that you will only find in three posts in aviation.se and in Kerbal Space Program discussions, but I like it. It refers to how much torque you can impose on a vessel to make it turn in some direction.

There are multiple ways to impose turning authority. On airplanes, you use rudders. But for land vessels, that is not enough. True authoritarianism requires reactionary devices. When you don't like a machine's attitude, you can use Reaction Control Systems:

A reaction control system (RCS) is a spacecraft system that uses thrusters to provide attitude control (...) An RCS is capable of providing small amounts of thrust in any desired direction or combination of directions. An RCS is also capable of providing torque to allow control of rotation (roll, pitch, and yaw).

Or Reaction Wheels.

A reaction wheel (RW) is a type of flywheel used primarily by spacecraft for three-axis attitude control, which does not require rockets or external applicators of torque. They provide a high pointing accuracy, and are particularly useful when the spacecraft must be rotated by very small amounts, such as keeping a telescope pointed at a star.

A reaction wheel is sometimes operated as (and referred to as) a momentum wheel, by operating it at a constant (or near-constant) rotation speed, in order to imbue a satellite with a large amount of stored angular momentum.

Reaction wheels can be really strong. They are used, for example, in cargo ships to keep them from doing rebellious things such as "rolling" (or whatever it is that teenagers call it nowadays):

Ship stabilizing gyroscopes are a technology developed in the 19th century and early 20th century and used to stabilize roll motions in ocean-going ships. Their function is similar to control moment gyroscopes or reaction wheels in spacecraft - they provide rotational stability via production of torque.

Here is a picture of a couple gyros on a ship. They weight 25 tonnes each and were installed on the USS Henderson:

Ship stabilizing gyroscopes
Source: https://en.wikipedia.org/wiki/USS_Henderson_(AP-1)

The egghead explanation goes thus, from the same link as above:

The ship gyroscopic stabilizer typically operates by constraining the gyroscope's roll axis and allowing it to "precess" either in the pitch or the yaw axes. Allowing it to precess as the ship rolls causes its spinning rotor to generate a counteracting roll stabilizing moment to that generated by the waves on the ship's hull.

I have tried the same principle in Kerbal Space Program to keep craft from rolling on steep surfaces. Turns out these things negate rotation so well, that ships can stand on one leg even if their center of mass is not supported by that leg. They can stand on very weird angles to the ground as if they were anchored or supported by invisible strings, so long as the gyros keep spinning.

Smooth criminality

Alternatively, each foot can anchor itself to the ground on every step. That way the vessel won't fall even if the center of mass is not over/between its feet.

For this, see US Patent #5,255,452, for:

A system for allowing a shoe wearer to lean forwardly beyond his center of gravity by virtue of wearing a specially designed pair of shoes which will engage with a hitch member movably projectable through a stage surface.

A system for allowing a shoe wearer to lean forwardly beyond his center of gravity by virtue of wearing a specially designed pair of shoes which will engage with a hitch member movably projectable through a stage surface.
Source: see link above

This patent was awarded to Michael Jackson (Oooo!), which used the concept to perform a dance move made famous through his Smooth Criminal music video (as if the Moonwalk alone wasn't cool enough!). Check the 7:03 mark.

The difference here is that your mech must be able to place the anchors as it walks. Michael used special preparations on stage so he could only anchor at specific places.

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    $\begingroup$ I think the spinning reaction wheels would be the way to go. Plus you could make them into the shape of arms on the outside of the mech and put giant fists on the end. People would call that mech "Punchy". A problem would be that if the fist hit something the mech itself would go flying as well, but that adds an element of intrigue to the fiction. $\endgroup$
    – Willk
    Aug 6, 2020 at 22:41
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    $\begingroup$ @Willk That would be an awesome way to take off the ground. $\endgroup$ Aug 6, 2020 at 23:19
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    $\begingroup$ Renan, you're my hero. I can't even imagine what karma in your life led you to know about that patent. $\endgroup$
    – JBH
    Aug 7, 2020 at 5:14
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    $\begingroup$ To reaffirm your answer: possibly the oldest possible "Western" mech franchise, BattleTech, uses exactly that. $\endgroup$
    – mikołak
    Aug 7, 2020 at 9:46
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    $\begingroup$ +1 gyro is awesome. And it leads to a cool failure mode as well. Those gyros have some serious stored energy that explodes in a rather lethally kinetic way. $\endgroup$
    – Yakk
    Aug 7, 2020 at 14:59

Or, it just does what humans and other bipeds do: it makes use of a combination of dynamic stability, making sure that weight is transferred from one foot to the other before it has time to fall, and flexibility in the hip joint to permit keeping the feet close to or directly under the center of gravity when planted and only moving outwards to swing around each other when moving.

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    $\begingroup$ The inverted pendulum problem is one of the problems humans solved before they even had a word for ‘problem’. $\endgroup$
    – Joe Bloggs
    Aug 6, 2020 at 18:38
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    $\begingroup$ Indeed, walking is nothing more than falling over forward and continually catching yourself. Most points in a stride are unstable, but if you can switch between unstable states the right way, you won't fall over. $\endgroup$ Aug 6, 2020 at 19:59
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    $\begingroup$ For some great examples, of dynamic stability in robots, check out the MIT Leg Lab. youtu.be/XFXj81mvInc $\endgroup$
    – Cecilia
    Aug 6, 2020 at 21:32
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    $\begingroup$ To see a human having to be less dynamically stable (and more statically stable), watch the walk by King George in Hamilton; the original crown and gear was so heavy, and the walk was so slow, that the actor had to walk as if on a tight-rope. $\endgroup$
    – Yakk
    Aug 7, 2020 at 14:56
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    $\begingroup$ You need to add: when walking humans deliberately "fall" forwards. They use the weight of the legs (+/-10kg) by throwing it forwards and then letting the body be "dragged" behind, basically falling onto the foot. This is why modern prostetics are so much harder to walk with: we make them lightweight. This shortens the step length people can make unless they start throwing their legs with more force. A bipedal mech would be able to use this as well. $\endgroup$
    – Demigan
    Aug 8, 2020 at 16:06

Easy but inelegant static stability

Your feet are C shaped, so they can be lifted over one another while keeping support below the centre of gravity

wind up toy robot

This model is exceptionally simple, and might be a good choice if you want your robot to advance slowly and inexorably, more like a tank than a human.

Hips and knees to move the weight from foot to foot

NAO robot

If you watch a NAO robot or Robonova moving e.g. https://www.youtube.com/watch?v=2STTNYNF4lk you'll see it moves its centre of gravity over one foot, lifts the other at a leisurely pace, puts it down, then moves its centre of gravity onto that and so on.

That means it's always in a 'dynamically stable' position, i.e. it can stop moving without falling over. The bigger your feet are, the easier.

You can watch videos like https://www.youtube.com/watch?v=UJxfQs0ajVk to see people building such robots and getting them to walk.

Of course, you can also find a lot of videos of these robots falling over, as while this model is simple to achieve on flat hard ground, it's not so hot on difficult terrain. A good choice if, for plot reasons, you want your robot to avoid stairs or be unable to travel through woodlands.

Dynamic balancing

Atlas robot

The fanciest movement, from robots like Boston Dynamics' Atlas robot https://www.youtube.com/watch?v=fRj34o4hN4I doesn't rely on either of those, which is why it looks a lot more human in its movement.

Humans don't need to touch the ground below their centre of gravity at all times - as is proven by the fact jumping is possible.

When you jump forward with both feet, you first give your body upwards momentum, then you can take both your feet off the ground before gravity takes over - so long as you move them somewhere appropriate to arrest your downward momentum at the end of the jump.

Robots designed to balance dynamically like this will be marked by smaller, lighter legs and feet and a higher centre of gravity - because light feet can move fast, meaning you need less air time to move your feet in a jump.

Walking and running are in a sense a variation of jumping: Dynamically unstable, if someone froze your legs in place mid-stride you would fall.

This is the best choice if you want terminator-style sprinting, jumping over things, and suchlike.

Just use a tank, missile or drone instead

Giant robots are very cool, but in a fight between a giant robot costing X weighing Y with a power source of Z horsepower, and a tank with the same cost, weight and power, my money would be on the tank every time.

So for the ultimate in realism, just put the guy in a tank instead.


how do living bipeds deal with it.

When you walk you put one foot in front of the other, you walk a tightrope without realizing it. you don't swing your feet straight forward and back but in a slight arc, swing out to get around the other foot then back before touching the ground. You also swing the rest of your body side to side to keep your center of gravity bobbing back and forth between sides. Your feet walk in a roughly straight line, your arms/hips swing side to side, and you fall forward catching yourself with each step. the only reason you don't notice is your necks swings your head side to side to keep it centered.

Dinosaurs and birds walk in a similar way but they move the center of gravity less but move the feet more, the feet often overlap in the stride thanks to wide toes that curl and uncurl during the stride. All this requires a lot of feedback sensors spread throughout the feet and legs of humans and dinosaurs.

Note that the star-wars walkers have "necks" the hips are not connected directly to the head. there is a joint in between that would allow for lateral motion without making the crew violently sea sick.


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Source 2


Step 1: Increase the size of the feet especially towards the CoG as much as fashionable (if aesthetics are not an issue, use U-shapes with openings facing each other and you did it). Have the feet somewhat flexible as needed helps with difficult terrain.

Step 2: Increase the mass of the feet relative to the mass of the head as much as possible. Ideally, the head is lighter than the atmosphere and keeps itself up and the feet each are heavier than anything else combined.

Step 3: Optimize the legs' movement. For a smooth and balanced gait, consider a female-like gait where the feet stay very close to the center at all time and the "hips" provide counter-movement. If you manage to give the Mech enough push with the hind leg to move the CoG well into the front leg's area of stability and move the hind leg over the front leg, you are about as stable as it gets with two legs and minimal joints. The legs would need to be pointed inwards while being placed as far out as possible.

Step 4: Admit that bipeds were a bad idea in the first place and just not worth it if you are not restricted by the number of limbs. We are about the only bipeds out there and even that only because we needed those limbs for utility. [Edit: Same goes for the few other bipedal animals that are not extinct yet. Birds use their front limbs for flying, a much more versatile way of movement, bats even use their front limbs for walking, grabbing and flying.] Evolution does not grow wheels or additional pairs of legs but Mechs can be designed in any desirable way. I imagine the mad scientist mastering his invention at the end of the story with dozens of joints, counterweights, and such - just in time to get bested by an "inferior" engineer who just builds a tank on tracks.

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    $\begingroup$ Humans are not the only bipeds out there. Birds and their ancestors, dinosaurs, are often bipedal. Then there are kangaroos, rodents, some lizards at high speeds, and other animals that are compensating for limb-loss. $\endgroup$
    – Cecilia
    Aug 6, 2020 at 21:28
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    $\begingroup$ "Increase the mass of the feet relative to the mass of the head as much as possible. Ideally, the head is lighter than the atmosphere and keeps itself up and the feet each are heavier than anything else combined." You have this the wrong way round. Try balancing a hammer on your hand. If you have the dense metal hammer-head on your fingers, you won't be able to balance the hammer as you walk. If you have the light handle on your fingers and the metal head in the air, with a little practice you'll be able t waltz around. A high CofG is good when your are moving. Think of an ostrich. $\endgroup$ Aug 6, 2020 at 22:55
  • $\begingroup$ @Cecilia Not strictly the only ones but the number of bipeds compared to non-bipeds is insignificant. We are the only ones that use this way of walking and for other kinds of bipedal animals, the same thing applies. They need their other limbs for something else. $\endgroup$
    – hajef
    Aug 7, 2020 at 13:37

Internal counterweight (Sumo wrestler effect)

Basically have a heavy pendulum inside the robot that swings from side to side. This can be likened to a Sumo wrestler swinging their arms, or to walking with a wide stance holding a bowling ball.

What is actually happening is:

  1. Initially the counterweight is centred
  2. The mech lifts right foot and starts tilting to the right
  3. This places a rightward force on the top of the counterweight
  4. Since the counterweight's CoG is much lower than that, it rotates clockwise more or less around its CoG irrespective of the mech body
  5. Once it is approaching 90° it places a large reaction force on the mech body to the left
  6. This slows the mech from moving further to left...
  7. ... giving time to plant the right foot back down
  8. Even better, now the counterweight is traveling to the right
  9. This momentum makes the next step of the opposite foot even easier (longer stride possible)

You can hide the counterweight inside the mech, or for comic effect it can dangle between the mech's legs...


The robot just needs to learn to walk on stilts


Los Chancaires (Béarnese French for "stilts") during Primtemps de l'arribèra on 2015. Photograph by Unuaiga, available on Wikimedia under the Creative Commons Attribution-Share Alike 4.0 International license.

People have been using stilts since times immemorial. We even have ancient Greek pictures (on pottery) showing people walking on stilts. If our puny biological brains can learn how to walk on stilts, then surely the lightning-fast electronic brain of a robot can learn too.

  • $\begingroup$ what's the point of this answer? $\endgroup$
    – carlo
    Aug 7, 2020 at 17:26
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    $\begingroup$ @carlo: The point is that humans can learn to walk on stilts, which elevate the center of gravity considerably, and have small footprints. If humans can do it, robots can do it too; it's not magical. $\endgroup$
    – AlexP
    Aug 7, 2020 at 17:40
  • $\begingroup$ But how combat effective are they? $\endgroup$
    – DKNguyen
    Aug 7, 2020 at 18:39
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    $\begingroup$ @DKNguyen: Not any more ineffective than a bipedal mecha. $\endgroup$
    – AlexP
    Aug 7, 2020 at 18:49
  • $\begingroup$ @AlexP Touche.. $\endgroup$
    – DKNguyen
    Aug 7, 2020 at 18:50

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