We don't really see too many walking robots around us, but everyone knows how they should look like. After all, we've seen a lot of them in the movies, comic books, and computer games. There is a slight problem with that. You see, those robots were conceived by artists, not engineers, and their only goal is to evoke a certain set of associations, such as swiftness or clumsiness, slowness or speed, friendliness or danger, sturdiness or fragility, cuteness or awesomeness, fantasy or realism, etc. Most of them couldn't be built with today's technology, and some of them with any technology, assuming today's understanding of physics is not completely wrong. Even if they could be built, they would be hopelessly impractical and inefficient.

There is nothing wrong with that, of course. Those robots do the job they were created for. They are an element of plot. Only rarely the designer is so incompetent, as to completely destroy the experience, as in the Interstellar movie. Unless, just like me, you want to build a walking robot, and you unconsciously try to make it look like those fantasy robots, because that's the only robot you've seen.

I want to list here the most common differences between those "artistic" robots and the robots that would actually make sense in the Real World™.

The Design Process

Robo Cat

I think the most common approach to designing a cool robot for a comic, movie or game is to just take an existing animal, paint it in a metallic color and add some gears, radiators or antennas here and there. Or take a human actor, make some parts of her body transparent with computer special effects, and add blinking lights. This doesn't work on several levels.

First of all, if you didn't pick a crab or insect, your animal has most likely an internal skeleton, and your robot has a metal shell, which pretty much amounts to exoskeleton. The two approaches have radically different strong points and require radically different mechanical design. Unless, of course, your robo-animal has both internal skeleton and external armor, which is just wastefully stupid.

Second, muscles have completely different characteristics than servomotors or hydraulics, which most likely move your robot. Not only the power density is completely different, but you also attach them in different places and they have best efficiency in different situations.

Lastly, the constructions that nature arrived at are not very optimal from the point of view of technology. Nature has completely different constraints -- the animal must be able to build itself in the womb, then grow, heal itself, find food and digest it, reproduce, etc. The animals also had to evolve from existing earlier animals, and at each step they had to survive -- so they couldn't suddenly just sprout an extra set of limbs, rearrange its organs, etc. The nature also works with tissues, which are limited in many ways. For instance, you can't have wheels or other rotating parts, but it's easy to have soft or flexible elements.

Robo Car

Second approach is to take an existing machine or vehicle, and, um, transform it into a robot, usually by making it sprout hands and legs (and a head). Of course the transformation is purely visual. This is an easy solution for artists, because the machine already looks mechanical, so they don't need to think about all those parts they are drawing and what sense they make. Different elements of the machine get transformed into parts of the robot based exclusively on their visual appearance. And so, anything round may become a joint, anything oblong can be part of a hand or leg, and anything that has two holes in it can be the head. Any notion of structural consistency, fitting the actuators in there or powering them is completely ignored. You basically get a junk sculpture.

Note that this mistake is often made in a smaller degree when designing parts of the robot. The artists will often look for photos of machinery, and incorporate patterns they saw in them in their robot designs. And since it's all purely visual, it will rarely make any sense.

Robo Tank

There is a whole category of military mecha, called "smart tanks", which is basically a tank on legs. Sometimes spider-like, sometimes more humanoid or mammal-like, they are supposed to replace the modern tanks giving more mobility, speed, all-terrain capability and awesomeness. In an attempt to make them look realistic, they often use elements borrowed from modern-day combat vehicles and hardware, and they are painted brown (brown is more realistic). That's where their realism ends, though.

You see, the armored vehicles have this particular shape and armor to make them fast and well protected while they travel on wheels or tracks. Just replacing the tracks with spider legs is not going to do much good. First of all, the legs are incredibly vulnerable. Any hit in there is going to immobilize the vehicle, and immobilized tank on the battlefield quickly turns into a pile of scrap metal, no matter how much armor it has on all its other parts. And no, you can't really properly armor thin, long legs protruding from the tank in all directions. There are more problems. The tanks have this particular flat shape to make their profile as low as possible, and all surfaces angled, so that they are harder to hit directly. Taking that and putting it several meters up in the air on legs kinda cancels the whole design, and makes it useless. Lastly, there is the problem of the crew having to survive large accelerations while the vehicle runs in leaps and bounds across the battlefield to avoid being hit. The stresses are much worse, because the mechas are so large and heavy.

If we will have walking robots in the army, they won't look like huge spiders with tanks for bodies. They will probably be small, lightly armored and most likely remote controlled or autonomous.

Robo Washer

We already have robots in our homes, in a sense. Take the washing machine -- it automates a large part of the process of doing the laundry. But whether it's a washing machine, roomba, electric shaver, sewing machine or food processor, one thing among them is common -- they do the equivalent of common human jobs, but they do them completely differently than humans. The washing machine looks and works completely different than a washerwoman. The sewing machine uses two strings, not one, and makes completely different stitches. Optimal ways to do things with a machine may be completely different from the optimal ways to do them by hand.

The same applies to the many menial jobs done by robots in science fiction. Robot-guards patrolling around, instead of a fixed sensor network and an alarm system. Robot-repairers, instead of replaceable modules. Robot-window-washers, instead of a washing system built into them. You know how a reasonable robot-soldier looks like? It's oblong, with fins at one end, has explosives inside and you shoot it from a ship or plane at the target.

Robo Robot

Of course the best approach to the problem is to actually sit down with the engineers, think about what the robot is supposed to do in the fictional world (not just as a plot device) and design a machine for doing that. Then build a prototype and test it, working around any technology differences between the fantasy world and reality. Rinse and repeat until satisfied. Of course that's horribly costly and inevitably produces very dull designs...

Interestingly, some of the robots from Star Wars were designed that way. You get much better grounding in reality when you have to build actual physical props, and not rely on computer graphics.

Individual Aspects

Degrees of Freedom

Simplifying a little, the number of degrees of freedom is basically the number of ways the robot can bend its limbs. Each degree of freedom means another actuator and another complication in the control code, so you usually want as few degrees of freedom as possible. On the other hand, you want to have enough of them to have the robot move properly and be able to reach into all the necessary places.

For instance, for a properly walking robot you will need at least three degrees of freedom per leg. One or two more if you need to control the angles at which the feet touch the ground. A robot with fewer degrees of freedom will have its legs slip when walking or when turning, damaging the legs and the ground -- a little bit how a tank damages the ground when it turns on the tracks. Its movements will also be much more imprecise because of that.

On the other hand, especially when you are mimicking an animal, you might be tempted to give the robot much more degrees of freedom than needed. Animals have an insane number of degrees of freedom in their bodies, 80 and more depending how you count (Did you know that each individual hair in the fur has a muscle that raises it? That means each hair adds a degree of freedom...). That's because evolution is wasteful in the short term, and it likes to keep options for further evolution. So unless you have exceptionally light and strong actuators, infinite power supply and control programs orders of magnitude better than what we have today, you want to limit the ways in which your robots can move.

Stride Length

Believe it or not, but the shape of your robot's legs has a direct impact on how fast and over what terrain it can move. The longer the steps it can make, the faster it will be, assuming a constant rate of steps. On the other hand, the higher it raises its legs, the rougher terrain it can negotiate. However, since raising the leg also takes time, you don't want to do it unnecessarily, so it's best to have appropriate sensors for that. Now, the longer the leg, the heavier it is and the longer is the lever on which the whole robot has to be carried -- so the stronger actuators are required. It means that longer legs tend to move slower...

All in all, it's an exercise in trade-offs. And to decide on the trade-offs, you have to know what is actually needed -- how fast and over what terrain your robot is going to move. But there are also combinations that are to be avoided, because they are ridiculous. If your robot has one segment of its legs very long, but other segments very short, it gets the penalty for long legs, but no bonus for stride length -- because it can still only make very small movements with that leg in straight lines. Same goes for the allowed angle of movement -- if it's very constrained, it doesn't make much sense.

Leg Strength

How strong should each segment of the leg be? Obviously, it needs to be able to at least support its part of the weight of the robot, and whatever additional cargo it carries. Plus it needs to withstand, and possibly even absorb, the shocks from the legs hitting the ground or starting/stopping suddenly. That means that the legs would probably have to be made from materials as strong and thick as the main body, or better. And they would need to be pretty much the same thickness throughout, no silly large bulky feet at the ends of scrawny thin legs or the opposite — insect legs ending with a spike.

How about the thickness of the joints? Obviously this depends on what kind of actuators you have, how strong they need to be, and where you want to put them for each of the joint, but generally they would be about the same size as the rest of the leg, perhaps a little bit smaller, if made of solid material.

Head and Senses

For animals, eyes are very complex, very delicate and very precious parts. Large animals only have one pair of them, so they need to be carefully placed to make the best use of them. They also need to be protected both passively (eyebrows) and actively (blinking).

Not so much for robots. Cameras are pretty cheap to make, and other kinds of visual sensors (reflective or even time-of-flight) are even cheaper. Lidars are a bit obnoxious, with their need to rotate and to be unobscured, but I can imagine that technology improving rapidly as it gets more common. Unless you require a high-quality high-precision or otherwise specialized image data, there is no reason for your robot to even have a head. You can just put the cameras all over it, especially on the hands.

There is also no reason, unless it is being tele-operated by a human, for a robot to have a normal human-like RGB vision. Sensors sensitive to a wider spectrum of light are actually cheaper (no need for filters) and more sensitive. And if it's an expensive robot, why not give it full-spectrum, light-field cameras that are also sensitive to polarization? One look at a surface would tell it what it is made of! It's similar with hearing. No need to have a pair of ears on the sides of the head. An array of directional microphones, together with vibration sensors on the feet and fingers would work so much better. There are also active sensors: sonar, radar, lidar, metal detector, etc. And the senses we don't have or have in a very poor way: electric, magnetic, radiation, chemicals, temperature, atmospheric pressure, etc.

Of course, a robot will only have the senses that it actually needs to do its work. An exploration or combat robot would probably have every single sensor they can cram in it, and more, but a more specialized one would only have the necessary subset.

While the sensors themselves are relatively cheap, both in terms of monetary cost and design trade-offs, the bandwidth and computing power necessary to process them are not. There will probably be significant progress in this area by the time we actually have usable walking robots, but I don't expect miracles. So while the robot will probably have "eyes" all over its body, it will still probably mostly focus on a few of them at a time.

Size

Movie robots basically have four sizes: pet, man, car, giant.

It doesn't make much sense to have a robot smaller than, say, a cat in a movie, unless you are doing some macro-photography shots, because it simply wouldn't be visible on the screen. At this size it will also often need to fly, jump or climb vertical surfaces — so that it can navigate the same environment as humans do.

Man-sized robots vary from a child to an overgrown thug, but are generally designed to act the roles that a human would do if only the settings was less high-tech.

Car-sized mecha are perceived more like vehicles, whether autonomous or manned, and will therefore be mostly used for transportation, exploration and maybe combat.

Finally Japanese giant robots are a category of their own.

Real world is not nearly as well classified. We have (or can conceive of) robots as small as a few molecules, through insect and mouse sizes, to cat, dog, and child. Man-sized robots are already difficult, but exist, despite their poor usefulness, mostly as research or entertainment pieces. There are a few walking vehicles out there, mostly as extreme terrain solutions. No practical giant robots in the foreseeable future, apart from some publicity stunts.

As a rule of thumb, you want the robot to be as small as possible, while still containing all the components it needs, and having enough reach to freely move in the environment for which it is designed. Anything larger, and you start paying for this because of the cube law. Anything smaller will have problems executing its tasks.

For interaction with humans, you probably need the cat size to be noticed at all, and the child size to actually have face-to-face interactions. For merely using human equipment, probably something the size of an orangutan is sufficient.

Weight

Ideally, you also want your robots to weight as little as possible. When walking, you have to spend energy on carrying all of that weight. So no thick metal armor, even for the military robots — you want to use modern, strong and light materials. The bulk of the robot will probably be its power source, then the actuators, the skeleton, and all the rest. 

Since biological muscles have such a great energy density compared to the actuators we have, a robot as strong (or stronger) as a human will probably be considerably heavier than an average man, even without any armor — weighting easily 2x or 3x times more. An armored robot could be even heavier. On the other hand, we can better distribute the strength in our designs, so the robots probably won't need to be as strong as humans to perform the same tasks — and conversely, not as heavy.

Shape

Unless it's an entertainment piece, or a teleoperated servo, there is no reason why such a robot should have exactly two legs, two arms and be human-shaped. Even for operating in human environments and using human tools, something like the robo-simian might make more sense.

We have two long legs, because that turns out to be the most efficient energetically, and we evolved to pursue our prey until it dies of exhaustion. However, if you need a robot that can move around with maximum energetic efficiency, you simply give it wheels. Or a hybrid leg/wheel solution, if it also needs to be all-terrain. So two legs don't make much sense when four give much better stability and versatility. Six legs might make sense in some use cases, both for increased stability and speed, and for redundancy. More than six probably doesn't improve things much.

The simplest robots won't need hands at all. They might have a gripper attached directly to their body. More complex robots would probably have one or two universal arms, and possibly additional specialized arms with tools and sensors. This is generally highly dependent on the robot's purpose.

As we already saw before, no head is necessary, as well as no tail. Specialized arms can fill those roles better.

Clothes

Intuitively, why should a robot need clothes? In the movies robots are naked to show off their mechanical nature. In reality, some forms of clothing is actually useful for machines.

Military robots will need armor. We already noted that metal armor is probably too heavy for them — they will probably be armored with something similar to the bulletproof jacket.

Even robots that don't need armor will need some environmental protection. They don't need to keep themselves warm, probably, but they need to have their joints protected from dust and sand, their sensors protected from scratches and their inside protected from water. Textiles are lighter and easier to maintain than a rigid shell.

The utility robots will also probably need to be marked in some ways, so that we know which is which.