Robot Power Systems

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Power Systems

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. Caution

An obvious power source that we won't address here is address here is internal combustion. Internal combustion engines can be found on some field and combat robots, but given the danger of this fuel type for any active bot, batteries and other less volatile power sources are more commonly used.


Batteries

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!

Pressure Systemsfeedinco

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.


Solar Cells

For true robot autonomy (or any other type of power autonomy, for that matter), nothingbeats the big bright rays of our closest star. Solar-powered robots use what are calledphotovoltaic cells to covert the sun's radiation (photons) into electrical power (electrons). Although this is a miraculous feat, it's also an extremely inefficient one. Only a small percentage of the energy that reaches the solar cell is actually absorbed and converted into electrical current (the rest is lost as heat). To be able to use this current, it must be stored up and then accessed when there's enough of it to do anything useful.

Note:

In truth, hydraulic and pneumatic systems are actually hybrid systems, as they both use electricity (from batteries or AC) to power the pump(s) that circulate the liquid or air.


Some robots (like the Mars Sojourner) use batteries for storage, whereas other types of solar-powered robots, such as many robo-critters, use capacitors to store up their energy. We won't get into capacitors here (see the "Thumbnail Guide to Electronics" inChapter 6), but basically a capacitor is an electronic component that dams the flow of electrons. Eventually, that dam breaks and the stored electrons race ahead through the circuit. This storing and dumping of the electrons can be controlled further with electronics. Using a solar cell and capacitors (and other support electronics), one can make what's called a solarengine, a free, always-available (while the sun shines, anyway) power source. Of course, since the cells deliver slowly accumulating power, robots of this type live a manic-depressive existence of bursts of energy followed by languorous sunbathing. And while most robots have to be weight conscious ("Do these gearboxes make my hips look big?"), solar robots need to be obsessive about it.

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Other Power Sources

There are other, more exotic, power systems for robots being experimented with. One of them involves bots that actually eat! Dubbed gastrobots, these robots would have a gastric system that can break down sugars and starches in food and convert them into usable power. Imagine what a boon this would be for autonomous robotics. There's even a Gastrorobotics Institute at the University of Southern Florida (www.gastrobots.com). The Institute's director, Dr. Stuart Wilkinson, coined the term.

Another extremely promising new power technology, for robots and just about everything else, is the fuel cell. A cousin to the conventional battery, a fuel cell produces electricity through an electrochemical reaction. But unlike a battery, fuel cells use hydrogen (the most abundant element of Earth) and oxygen, and will continue to produce power as long as there's fuel available. Batteries need to be recharged. Besides not running down, fuel cells weigh a lot less than batteries (for comparable power delivered), are much more power efficient, and are, overall, better for the environment. There are problems with fuel cells. Hydrogen might be abundant, but getting it into a form that can be used by the cell takes processing, and a good hydrogen source such as methane, propane, or natural gas.

Tip:

For a user-friendly introduction to fuel cell technology, check out the article "How Fuel Cells Work," on the How Stuff Works Web site (www.howstuffworks.com/fuel-cell1.htm).

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