# What good reason for a full metal humanoid robot with powerful hydraulic legs need to sit on chair?

Set in near future, the human and robot coexist together amicably and conflict is rare.

They are designed to resemble us as close as possible for our sake, so we feel at ease and less paranoid by their presence.

Even their behavior and speech pattern feels personal and genuine especially those micro facial expressions that doesn't create the uncanny valley effect commonly found in dolls, but one thing stands out like a sore thumb. They appears to sit on chairs designed for human!

They are equipped with powerful hydraulic legs and sitting on a chair would stress the actuators for 2 reasons, first their body is very heavy and can easily break the chair with their full weight so they need to engage the actuators to support most of their weight and number 2 is the center of gravity is much higher for them because of the heavy battery pack located inside their chest and balancing it put more stress on the actuators.

So despite all the trouble why do these robots still want to sit on a chair when clearly they know such action is very inefficient?

• i believe you answered your own question already: so we feel at ease and less paranoid by their presence. a roughly human-shaped being that never sits down would feel very alien to us. also: in such a mixed society, i believe furniture would be designed with robot weight in mind. Dec 1 '21 at 7:41
• They will need to sit at least when at dentist's - you don't want the robotic dentist to climb on an even higher implement for the procedure. As such, they may get a habit of doing it in more than one occasion. Dec 1 '21 at 7:52
• @Adrian Colomitchi robots going to dentists for a procedure? I don't think I've seen that yet :) Dec 1 '21 at 18:43
• @Alexander they do have a bluetooth, don't they? Now and then, it'll need to be at least cleaned. Dec 1 '21 at 19:43
• Just a tiny comment on chairs: If loaded only vertically even the most fragile looking chair will hold a very high load. We recently had a ... eh ... super sized friend over (like literally 2.20 m and carrying around some extra weight), but the wooden chairs that rock around when I (1.86m, ~95kg) sit on them held him easily. I guess he adapted to being careful not to put too much strain on a chair ... and I guess a robot could do so easily.
– fho
Dec 2 '21 at 10:44

## The world is still designed for humans

In any social engagement it is important to know when to sit or stand. Depending on the social situation sitting, standing or the refusal of sitting or standing can mean a lot in social standing, ease of a person, threat and more. It is important to keep to these social rules if they want to fit in.

More important is that the world is designed for humans. Many things are more easily accessible by sitting. Desks, computer terminals, dining tables or even just filling a car or a public transport vehicle. All can be used or used more to the full extent. Saves a ton in (design) costs if you don't need to adapt part of the world to robots with special computer terminals and cars.

You ask: "So despite all the trouble why do these robots still want to sit on a chair when clearly they know such action is very inefficient?"

Humanoid robots are inefficient by definition. Adding an inefficient method would be expected – to make it fit in – rather than frowned upon.

• Plus it would be physically uncomfortable for sitting humans to interact with a standing humanoid robot for extended periods of time, like a meeting or a meal. Dec 1 '21 at 10:52
• +1 for mentioning vehicles, but worth adding that the same laws of physics apply for vehicles that robots ride in as for people. Much more air resistance in a car or small plane (and water resistance for small submersibles) that has to accommodate standing robots compared to seated bipeds. (The same or better cross section could result from a prone position, but that doesn't work as well for humans, so in order to use the same vehicles without unnecessary air resistance, the robots need to sit down.) Dec 2 '21 at 5:49

## Bipedal locomotion is inherently unstable.

We never actually stand still. Try it!
You'll see that you wobble and waver constantly, with micro-adjustments to your posture and center of mass all the time to avoid falling over.

Standing completely still is not a natural state of things for a human. You can do it with some effort, but it's not comfortable for most of us.

Likewise, if your robots have similar proportioned body-parts to humans, your robots must also constantly adjust themselves to stay upright.

Even for a robot, sitting in a chair is more energy-efficient than standing.

• Humans constantly need to re-balance because it's not really possible to keep your muscles perfectly still. But that should be no trouble at all for a hydraulic, mechanical robot. Once balanced, it'll stay there forever - a coat rack does not need to expend any effort to keep from falling over. These robots would need to be specifically designed to knock themselves off-balance to give themselves a human-like wobble. Dec 1 '21 at 15:34
• @NuclearHoagie a coat rack does not need to expend any effort to keep from falling over because most coat racks have 3 to 4 legs which is inherently stable. A biped that is unstable gains efficiency while walking or moving at the expense of stability when standing still (or vice versa), hydraulics or not. Dec 1 '21 at 17:27
• not to mention coat racks aren't particularly good at walking. Dec 1 '21 at 17:38
• I don't think this is a good answer to the question. The OP stated that the robots cannot sit on chairs for the purpose of support; they are too heavy and the chair would break. Instead they go through great effort to enter a state visually similar to sitting, even though it actually requires an even more difficult balancing act. An answer claiming that sitting is beneficial to balance should address that premise in some way. Dec 1 '21 at 17:48
• @YoungJohn A object with two legs is stable so long as its feet aren't very small. Heck, a cylinder standing on its end effectively balances on one foot. Once the center of mass is positioned above the area of support, it will not spontaneously fall over. Bipedal walking is inherently unstable, bipedal standing is not - a well-balanced mannequin can stand just fine despite having only two feet and no muscles. Dec 1 '21 at 17:48

The robots compete with each other to see who can be the most human-like.

Sitting down was a piece of the repertoire from long ago, when the competition among robots first started. Robots now not only sit down, but tip chairs back, cross legs over, bounce nervously and other pieces from the human repertoire their keen observations have picked up.

A robot that comes up with a new human behavior to mimic will get appreciative nods from the other robots who notice. They wrinkle their noses, seem to doze off, push hair out of their eyes. Certain behaviors are trendy - perhaps long hair and a head toss to throw it over a shoulder. And this summer all the robots have their hair long and are tossing their heads!

Occasionally a robot will pick up and copy some behavior he saw someone do that is not normal human behavior - a person squirming uncomfortably from ill fitting underclothes or a painful hip. Particularly edgy emulators will cherish such one-off behaviors. More conservative robots will stick to common behaviors.

• I really like this one. Dec 2 '21 at 5:13

Frame Challenge: They would not break a normal chair by placing their whole weight on it.

### How heavy could these robots actually be?

Normally if an OP says "a robot is heavy enough to break a chair", I would just take this at face value; however, the OP specifically gave enough details to prove that his robots should not be heavy enough to do this.

the center of gravity is much higher for them because of the heavy battery pack located inside their chest

Since you mention that robots are top heavy because of the weight of their batteries, we can assume that a robot has a density of somewhat less than that of a battery. Older styles of robotics were very heavy because of their use of lead-based batteries; however, lead based batteries are no longer used in robotics. Instead, we now use lithium ion batteries which hold more power while being about 3 times less dense. A standard automotive lithium ion battery has a density of about 0.92g/cm^3 which is actually slightly less than the density of the human body; so, for a chest full of batteries to make a robot top heavy, it would have to actually be LIGHTER than humans, not heavier.

Even if we assume the use of older style lead batteries, your robot would still have to be significantly less dense than its batteries to make it top heavy. If the android is the size of an average human, then that means it weighs in at somewhere less than 200kg to be able to meet the description given by the OP.

### How heavy should they be?

As it turns out, the batteries are not actually the heavy part of a robot anymore. To figure out what an actual android should weigh, the best thing we can do is look at actual human sized robots being built today for public consumption, and see what they weigh. The two best examples you will find are probably the Promo-bot (about 152 cm tall and 100 kg) and the Kiki (about 160 cm tall, 80 kg). These are both a bit on the short end; so, if we assume your androids are made a bit taller to equal a more average Western human's height of about 172cm we'd get something in the 107-145kg range. So, even in the real world with our relatively privative technology, human-like robots only have a density of about 30-80% greater than humans. These robots also both drive on wheels instead of walking and have slightly over exaugurated body volumes; so, with legs and actual human proportions, they should actually have very close to human weights.

Even if you assume a higher density metallic framed robot designed for heavy labor, it will still be much less dense than you may assume. Significant amounts of the robot's internal space needs to be dedicated to air gaps to give room for things to move around, hydraulic fluids (1.0-0.8 g/cm^3), assorted electronic components (0.6-0.4 g/cm^3), the afore-mentioned batteries (0.92g/cm^3), etc... So even if you assume the structure of the android is made of a denser structural metal like steel (as opposed to something more practical like aluminum, titanium, fiber composites, or polymers), the steel bits are still unlikely to account for more than 20-30% of its actual volume... so even in this case, it is still very unlikely for a human sized, heavy labor robot to exceed 200kg.

### Would this break a human chair?

The average human chair is rated for about 115-365kg; so in theory, a human sized robot could exceed the recommended weight capacity of a poorly made human chair but chairs are not just designed to survive the weight of a human, they are designed to survive the impact of a human sitting on a chair. Since humans can be quite clumsy, we often inflict an impact force much greater than our resting force on the objects we interact with. So the weight ratings on chairs are normally 1/3 the load weight that the chairs actually break under during testing to account for how humans are expected to interact with it. This means that as long as you are careful about how you set your load on a human chair, it is actually designed to have a holding capacity of about 345-1095kg. If we assume that androids are even just a little less clumsy than humans, then thier extra weight could be easily offset by how carefully they sit allowing any android to sit down safely on practically any human chair.

### So why then would Androids Sit?

Apart from the obvious points already made about the social advantages of sitting, even when there are no humans around to perform for, they would would still benefit from sitting down to relax. It takes less energy to stay upright in a sitting position than a standing one because minor changes in your environment are less likely to knock you over. Sitting vs standing may not seem to conserve any power if your robot can just lock into a standing position, but in a standing position, it will need to continue to maintain full awareness of its body and surroundings to make sure nothing changes that would cause it to fall over and risk being damaged. The safer nature of siting means it can reduce power to its sensory and processing systems by reducing the cycle rates of how often it processes its environment. This also extends how long it takes for its electronic components to burn out if they are not always running at the higher capacity.

It may also slightly reduce stress on the robot's hydraulics causing those to fail slower too. Even though your hydraulic pumps may lock into a position, the outward pressure of a hydraulic system on its container is higher when it is under load increasing the risk of it forming a leak.

• This also leads to a fun pastime for teens - Android tipping, which involves sneaking up on an android who has powered down some of its systems upon sitting and pushing it over before it can power them back up and react. Dec 1 '21 at 23:33
• But would Android tipping work on an iPhone......? :) Dec 2 '21 at 6:10
• @Stilez Yes... but you are more likely to face criminal charges for destruction of property since iPhones cost twice as much and don't survive 3ft falls nearly as well. Dec 2 '21 at 17:46

Apparently continually standing when there is a seat available in a conversation with a sitting person is not considered socially ok, or is seen as aggressive or makes people nervous or isn't polite... if personal experience is anything to go by. They already alter themselves for social reasons, might as well suffer a bit for social reasons as we all do anyway.

I wouldn't put it past them to go the extra mile to keep up appearances and remain polite with humans.

Inductive charging

Are your robots powered by a nuclear rtg and run forever? Or do they have a battery pack that needs to recharge?

If you own a business that owns robots, or have your own robot maid at home, you'll probably have purchased said robot(s) with a charging dock - which just so happens to be in the shape of a rather ergonomic chair.

No need for messy wires or tedious battery changeouts, all new robots come with built-in inductive charging!

Sociology.

The group psychology of amicability is pretty simple, even if we're unaware of it. Standing in the presence of other people who have to sit is an intimidating act, if not outright hostile. The mafia don, the kings of old, if you were in their presence and someone was standing, it would be them and not you. Even in the fictional portrayals that you see on tv today where it's the opposite... the petitioners enter the court with the big guy sitting on throne, think about the details that go into that.

As they approach they must bow or kneel (no longer stand). And there are big beefy guys with spears/swords/guns standing between them and the king, ready to put an end to any nonsense that might occur. (Guards and servants being the exception to the "no one stands" rule, and their loyalty is heavily policed while they are made to know their "place".)

If Mr. Robot is going out to a work lunch with his colleagues, worse than him not eating with the rest is his tendency to stand there, all stiff like the T-800 endoskeleton from The Terminator. Ready to pounce and murder the savior of humanity or not. Boardroom meetings. Casual visits. None of these things work if they insist on standing. Perhaps it'd be no big deal if they were little tin cans like R2-D2, but if they are proper (humanoid) androids, then they just have to sit.

This is so basic though, that I would hesitate to bother to explain it at all. No one reading your story or playing your rpg will seek to poke holes in this, though if they do now you can wallop them with a good one.

# Don't tell the humans, but it's a toilet!

Hydraulic humaniform robots need state-of-the-art lubrication. Molecularly engineered, tested durable <insert motor oil ad here> ... but nonetheless, degradable lubricating oil. They take an occasional social drink to refresh themselves. And every day or so, when no one is looking, they use a fine gauge prehensile needle mounted in their ... posterior port ... to penetrate the fabric of a neglected chair they find some pretense to sit upon. The needle threads its way around the undercarriage and gives each joint and wheel some expired and degraded robot lubricating oil that is nonetheless quite suitable to keep a human-grade chair spinning and wheeling across the floor smoothly enough to remain the office toddler's favorite toy.

The humans still think the robots on the night shift must have a can of WD40 stashed away somewhere.

• Meh, large Diesel engines never change their oil, they simply do hydroscopic testing at intervals, looking for coolant or hydraulic oil (transmission fluid) contamination, bearing wear, and the like. So, no oil changes unless a problem is detected. Each bearing is potted differently so the spectral analysis will say which bearing is starting to fail. Dec 3 '21 at 20:32

## For Safety

Standing is relatively unstable for a humanoid biped. Especially when such a biped is top-heavy. These robots have vital, heavy parts (batteries) located in their chests. Should the robot fall over from a standing position this can damage the battery and effectively destroy the robot.

When not actively moving, the robots are eager to stabilize themselves either by leaning on walls, sitting, or whenever possible lying down. Every opportunity to lower their center of gravity is a chance to reduce the risk of damaging their vitals should they fall over or be knocked over. It is worth the extra energy cost to reduce the risk of irreparable damage.

Perhaps when they sit, a third stabilizing leg can be extended from their spinal column to reduce any weight placed on a chair and reduce the need for keeping actuators actively balancing them in a sitting position. This would give them a strong preference for chairs that have no back or that have an appropriate hole in the back.

Lying down would be preferred when stationary for long periods of time, but is avoided otherwise because it requires the most floor space, and takes the most effort for getting back into a standing position.

Ah, you must be talking about the A301-B model house droid. I was on the design team and I get this question a lot. When Nano Robotics wanted to break into the domestic market, we were tasked with converting our widely popular Mudrertron 9000 killbot into a more family-friendly model. We didn't have time to completely re-engineer the internal model so we had to get creative.

At first, we simply re-trained the internal reality model to do nothing at all when it had the urge to kill. However, that resulted in long periods where the robot would become unresponsive any time you tried to talk to it and the marketing guys didn't like that. It was actually my colleague Markus who came up with the idea to have them sit. That way when you talk to it it seems like it is really listening!

So, that's your answer. Any time you see your A301-B sitting down it's because it actually wants to kill you but can't!

• lol +1 but did you folks try to reset the training completely ? Wipe it out ? This unresponsiveness normally occurs when you feed its neural net too much inconsistent information. Maybe the neural net tries to keep the old battle knowledge in, resulting in permanent confusion ! Dec 3 '21 at 9:58

# Chairs are docking stations that connect into mains power and high-speed network links

Another answer already mentions that they might double as chargers and maintenance stations, but they can serve another purpose too. For the really computationally demanding tasks that make your overclocked brain take up 10x the energy, or that demand really high capacity network flow, you want to be wired into the grid. In real life, high-performance server equipment never uses WiFi or battery power, and instead always uses wired power and network connectivity.

It's worth considering when the robots frequently sit down. Sometimes it's for committees (social and political emulation is a major power hog), sometimes it's for social interaction with humans over food and drink (again, social and emotional emulation at that level can be demanding), sometimes it's just for sitting down in small group to hash out a difficult problem (a lot of required computations for simulating possible solutions, network connectivity needs to be very fast to cross-reference and retrieve external data). Each of these are so demanding that it would make sense that chairs are designed with these facilities in mind.

The same reason that humans sit on chairs, to relax

I've been working with robots for a couple of years now and there's a big hidden issue that most people aren't aware of when it comes to actuator design. This mostly applies to electric actuators, but there are some similarities with hydraulic ones. If you read the MIT Mini Cheetah actuator paper you'll see that low gear reduction and a big motor is preferred compared to the high gear reduction and strong (but slow) motors that most people expect in robots. This style of actuator transmits the forces from the ground back into the motor very efficiently allowing the motor to act as a virtual spring that's computer controlled. This also means that the motors need to be constantly consuming power to maintain balance, just like human muscles. It then makes a lot of sense for the robots to sit down so they only have to balance the upper half whilst sitting on chairs. Or almost no balance and no power required if they're also resting their arms and back comfortably on the chair.

This answer somewhat ignores the premise of your question, but I believe hydraulic actuators that will be used in robots will require the same backdriving allowance. This means there needs to be a path for the hydraulic fluid to flow backwards with an active controller that maintains balance by turning the piston into a virtual spring, but using power in the process. The alternative is the robot locks it's legs into position which will require no extra power, but that makes the legs perfectly stiff and unable to balance. Stiff legs are also incredibly prone to damage in case of falls; you really want everything to be floppy to absorb as much energy before breaking.

The other premise that robots need to partially use their legs to avoid putting all the weight on the chair is also questionable. If you insist on doing that then the robots will be sitting quite menacingly on chairs, leaning forward and with the legs firmly placed below their center of gravity. They'll look like their about to pounce on you from whatever chair their sitting on; just try sitting on a chair with only half your weight and see how you look. The balancing physics of a humanoid robot and a human will be very similar even if the center of gravity is a bit off. I'd suggest you make your robots no more than twice heavier than a human (you probably want the arms and legs made of light carbon fiber anyway). In that case most chairs will be fine under a robot, but it would be a nice detail of the story that all your robots need to carefully inspect the structural soundness of their chairs before sitting down.