Obviously, motors in a drive train are there to power something. That something is usually wheels, tank tracks, or legs. Let's take a peek at each of these.
The majority of robots use the good ol' wheel for their mobility. Wheels are readily available, relatively easy to put into service, and extremely reliable.
Many robots that use wheels use only two of them (or two drive wheels and two unpowered (idler) wheels, or two drive wheels and one idler wheel). The reason for this is simplicity of design. If you have four wheels-two drive wheels and two steering wheels-that's two extra wheels and a steering mechanism you have to deal with. Two drive wheels can do double-duty as steering wheels by simply turning the power of one wheel off (or running it in reverse) while the other wheel moves forward. This is often called skid steering or differential steering. The disadvantages of wheels are obvious: They can't climb over significant obstacles, or climb stairs (unless they're fancy wheels designed for this purpose, as shown in Figure 4.12).
Another advantage of the wheel is that it is a very mechanically efficient device to use. Tracks on a tank drive need to be bent and unbent around each wheel. Legs need back and forth motion, which is electrically "expensive" (you gain nothing motion-wise from moving a leg backward, other than to position it for the next forward step). With wheels, all the energy that's sent to them are used directly for vehicular motion.
The Mars rover Sojourner had a special kind of wheel suspension system called a rocker-bogey that allowed it to climb over rocks bigger than it could with conventional wheels. Photo used with permission of NASA/JPL/Cal Tech.
The cousin of the wheel is the tank track, two sets of wheels with rubber (or metal) belts connecting them on each side of the vehicle. Steering here works the same as on a two-drive-wheel robot: Stop or reverse one side, and the vehicle will slew toward that side. The advantage to this design is much greater traction. Tanks can climb up and down steep inclines that other vehicles can't handle. A major drawback to this type of locomotion is the mechanical complexity of building it. And, if you lose a tread, the whole side of your robot becomes "motion-liberated" (to steal a horrific technical euphemism). Skid steering also requires much more power on a track system. You're literally dragging around lots of rubber every time you make a turn.
I'll try to resist the temptation to quote from ZZ Top's song "Legs" here (oops, I guess I failed). Look around in nature. You don't see many animals with wheels. Wait, you don't see any animals with wheels. Evolution has decided that legs are the most versatile and hearty form of mobility. Eight out of ten roboticists agree, but making legs work on a robot can be difficult. They have to be powered in such a way that the legs on either side work in opposition to one another (one leg up while the other is down), and on bots with many legs, this can become very complicated. Then there's that drag called gravity to deal with. All of a sudden, you find yourself with numerous jointed leg segments to engineer, multiple servos to power and control, and some gnarly circuit designing or programming to turn all of this into a walking robot. Still, when it's done really well (like Honda's humanoids), a walking robot can go where other bots can't, like up and down stairs.
A robot isn't going anywhere without juice. Your body uses a complex process of breaking down food into chemicals that it then converts to energy for powering muscles, organs, and other systems. In robots, a number of different substances
are used to give a robot its get-up-and-go. The most common are electricity (from AC wall current, batteries, and solar power), air, and liquid. Let's take a look at each of these.
The majority of mobile robots consume battery power. These batteries process chemicals, either wet or dry, to create an electrical current that can be sent through wires to power actuators, sensors, and on-board controllers. Since batteries are such a common form of robot power, we should take a few minutes to detail some different battery types:
Alkaline-These are your run-of-the-mill home workhorse batteries, found in flashlights, smoke detectors, and toy firetrucks all over this great planet of ours. One drawback to the alkaline battery is that the voltage available decreases as the battery is used up (giving robots that "tired blood" feeling). They also don't perform well under pressure (when a high current draw is required). And, last but not least, regular alkaline batteries are not rechargeable. Since active robots consume the juice faster than an Irish pub on St. Patty's day, most serious robot builders use rechargeable batteries.
Nickel cadmium (NiCad)-The rechargeable NiCad (or NiCd) battery has been around since the 1950s. Although this battery has some advantages (it can deliver a very high discharge current, and it can be charged/discharged a lot), it's being phased out of the consumer market. That "cadmium" part of the name spells bad news for the environment (it's extremely toxic). Environmentally conscious robot builders have started to pass over this battery type as a power source.
Nickel metal hydride (NiMH)-In the past decade or so, this type of rechargeable battery has been gaining ground in the market. Batteries of this type don't require complete discharging before recharging (like NiCads), and they have a much higher energy density than NiCads. Unfortunately, they're a battery that will not be ignored. If left unused, they lose their charge faster than any other battery type (as much as 30% per month!).
Lithium ion (Li-Ion)-If you have a swanky new laptop computer, chances are, it sports a lithium ion battery. These batteries are very popular in consumer electronics, where size (as in: small) matters. A Li-Ion cell can deliver three times as much juice as a comparable NiCad or NiMH battery, in a much smaller package. Li-Ion batteries also have a very slow self-discharge rate. Unfortunately, this comes at a price: Li-Ion batteries are still very expensive.
Lead acid-Look under the hood of your car. See that battery in the corner? That's a lead acid battery. This type of battery is found in some field robots, but is not common in other robot circles. Unlike the "dry cell" batteries mentioned previously (where the chemicals are in a dry form), lead acid batteries have caustic, corrosive liquids inside of them. These batteries can leak if tilted or punctured.
Sealed lead acid (SLA)-A type of lead acid battery that's more common in robots is called a sealed lead acid battery. Here, the hazardous goodness of the lead acid is permanently sealed inside a rugged plastic case. SLAs are common in combat robotics because of their high-current capabilities, reasonable cost, and relative safety. Drawbacks to the SLA are size and weight. It's a big-boned fella that weighs more than any other common robot battery type. SLAs are also frequently referred to as gel-cell batteries.
Battery packs-This isn't really a specific battery type, but a way in which dry cell batteries are often grouped together. Frequently, on robots, you'll see an odd-shaped bumpy plastic brick. This is a battery pack. Inside is a cluster of rechargeable batteries (NiCad, NiMH, Li-Ion) all connected together and then shrink-wrapped inside a permanent plastic covering. These packs are convenient because separate battery holders don't need to be used, which can save precious space and weight, and all the batteries can be recharged at once.
There are many other types of batteries (carbon-zinc, lithium, polymer, silver), but we won't go into them here. I don't know about you, but I'm tired of talking about batteries!
Although battery power is extremely common in robots, it's not the only source in town. No amount of pouring battery juice into a hydraulic or pneumatic leg or arm is going to make it move. These actuator systems get their energy from liquid and compressed air. Again, like the cylinders themselves, the "circulatory systems" to power these two technologies are nearly identical. You have tanks that hold the liquid or air, strong flexible tubing to deliver the material, and a dizzying array of fittings, couplers, valves, pressure gauges, and other "plumbing" parts.