Editor's Note: This excerpt series is designed not to be a step-by-step instruction set for building this robot. It is designed to whet your appetite enough to buy the book and follow the steps to success!
Robots: the Exploded View
Okay, so let's unbolt some bots, spread their parts all over our virtual workbench, and talk about components. Figure 4.6 provides a basic rundown of robot anatomy, based on three common types of robots.
Regardless of robot type, most robots have similar subsystems, and those
systems are not entirely unlike those found in our own bodies.
Armed with this information, you should be able to look at nearly any robot and identify many of its subsystems. You'll be surprised how much this will increase your confidence in understanding how robots work. You'll be able to look at a bot and say: "Oh, cool. They used hydraulics for the arm actuators, heavy-duty servos for the end effectors, and stepper motors for the drive train. And look, there's the PCB for the motor controller." Come with us now, and you too can learn how to talk bot just like the pros!
Everybody needs a body, a skeletal structure upon which to attach all of the other stuff. In a human, this is obviously made of bone. In robots, it's made of just about everything but bone. Robot builders are constantly trying out new materials. Which materials are used has a lot to do with the robot's application. Obviously, strength is almost always important, but as we'll see throughout this book, there is a constant conflict in robot building between strength/durability versus power requirements.
Most robots run on battery power and battery life is still preciously short. The stronger and heartier the building materials, the more the robot weighs. The more weight, the more battery power required to move it. More batteries mean more weight, which requires more power to move, and so the vicious cycle goes. Recent materials innovations, such as titanium and carbon composites, are shifting this equation somewhat, but such materials are still very expensive. Let's run down some of the commonly used structural materials and their trade-offs:
As you might imagine from the diversity of robot definitions in Chapter 1, whether most embedded robots are actually robots or not is a matter of considerable debate. Many say, if it doesn't actuate (move) anything, it isn't a robot. But isn't a "smart house" that turns on the outside lights when you call it from your car, or shows you the security image on your TV when the doorbell rings, "actuating" the switches to these systems? But, the argument goes, because a computer is nothing more than a massive collection of tiny switches, it would have to be a robot, too. The debate rages on...
Hey, don't laugh. Wood can be a perfectly reasonable building material for the right kind of robot. The disk-type of developmental robot is sometimes made of wood, and I've seen fairly substantial robot arms (experimental, not industrial) made out of it. In my son's Tech Ed class, he made an extremely cool hydraulic arm out of nothing more than 3/4-inch plywood, wooden pegs, plastic tubing, and medical syringes. Pushing and pulling the water pressurized inside of the syringes and plastic tubing powers the components of the arm (see Figure 4.7). Obviously, problems with wood include its relatively low strength-to-weight ratio and its low durability (at least under many active conditions). It's also, well, wood-not the most twenty-first century material from which to build a robot.
My son Blake's hydraulic arm made from 3/4- inch plywood, plastic tubing, syringes, and hardware. Just add water!
Steel is a common building material because of its considerable strength and durability. Unfortunately, that strength comes at a high weight cost. Using thin sheet steel and then bending it for greater structural integrity sometimes overcomes this cost. To do this, specialized tools are required that make steel an undesirable material for robot builders on a budget. Some robot kits (such as the Parallax Boe-Bot, which we'll discuss in Chapter 6, "Acquiring Mad Robot Skills") use a stamped steel frame. Stainless steel is sometimes used when a great degree of precision is required in the robot's components. The use of stainless is usually kept to a minimum because of its high cost and weight. If you have the equipment, steel can be welded relatively easily and takes to physical fasteners (nuts, bolts, screws) admirably well.
Aluminum is probably the most commonly used material in medium to large-size robots. It is extremely strong for its light weight and can be worked easily with common shop tools. Square-tube, C-channel (three-sided), or L-shaped (two-sided) extruded aluminum stock (where the molten metal has been pushed through a mold like a Play-Doh machine) can add a lot of extra strength to the material disproportional to the increased weight (see Figure 4.8).
Evolution Robots (www.evolution.com) uses extruded aluminum Tinkertoy-like "Xbeams" in its robots. It also sells the beams and connectors separately.
There are a number of plastics that are used in robot construction. Probably the most common is acrylic resin (also known by the trade name Plexiglass). Plexi is very strong (though a bit heavy because of its high density), fairly easy to work with, and can be tapped (fitted with threads) to accept screws directly. It can also form a very strong bond with itself using special solvent cement.
Another commonly used plastic is polycarbonate resin (known by the trade name Lexan). You might also know it as bulletproof "glass." This material, as you might expect from something that thwarts bullets, is extremely strong and durable. Lexan is frequently found on combat robots (Battlebots, Robot Wars, and so forth) for this reason. One advantage to using these plastics in robot bodies is that they can be molded and shaped by applying heat.
A third type of plastic material gaining popularity among builders of small robots is expanded foam PVC (also known by the brand name Sintra). This fascinating material weighs about half as much as a comparable amount of acrylic, but is nearly as strong.
Titanium is the Cadillac of robotic building materials. It is extremely strong and surprisingly lightweight. It's also surprisingly expensive. Luckily, the widespread demand for it, for use in everything from Apple PowerBooks to golf clubs to combat robots, is helping to bring down the price. Having a glittering chunk of this amazing alloy as my right hip, I can attest to its remarkable properties.