Build a Robot II
This is a material that is made by suspending carbon fiber sheets in an epoxy. The resulting material can be formed, similar to fiberglass, into whatever shape you want. Layers of the material, called plies, can be built up to create an incredibly strong, lightweight robot body part. The robot hobbyist can make carbon composite parts, but it's icky work (again, like fiberglass). Also, when made by an amateur, it can be prone to cracks between the layers of the carbon material (a process called delamination), which will greatly weaken the structure (did we mention fiberglass?).
Printed Circuit Board (PCB)
Sometimes, the body you need is no body at all, or at least not a structure separate from other components. In making miniature robots, especially robo-critters, builders sometimes cut down on precious weight (most critical in solar-powered robots) by using the other components (motors, capacitors, solar panels) as the robot's body. As the printed circuit board (the plastic or fiberglass board that all of the electronics are soldered to) is often the largest part of the robot, it gets pressed into service as the body (see Figure 4.9).
The ScoutWalker, a robo-critter from Solarbotics.com, which uses its printed circuit board as its body. Photo used with permission of Solarbotics.com.
The LEGO Group is building a bridge to the future, one little plastic brick at a time, with its amazing MINDSTORMS Robotics Invention System. This building set uses the same blocks that you made chunky-lookin' cars and rocket ships out of when you were a kid, but LEGO has added electrified bricks (with power connectors, sensors, and lights) and a "computer brick" (called the RCX) into which you can load control programs. This system is not just kid's stuff, either. MINDSTORMS can be found in active use in grade schools, college computer classes, and robotics labs throughout the world. It's a great way to create quick experimental robots that can be easily altered. Some builders of small robots even use them for permanent robot bodies by gluing the bricks together to create the structural components they need.
The preceding examples are only some of the more popular structural materials. There are also robot body parts made from magnesium, PVC pipe, tin, iron, brass, and even foam board. Whatever the materials, the perpetual struggle is between strength and weight. Depending on the environment the robot will inhabit, there's also the durability factor. And then there's the money. Cost always crashes the party eventually to wake you from your fever dream of building the ultimate all magnesium/ titanium/carbon composite super-bot.
Your own skeleton is covered by musculature, a system of long, stretchy fibers that contract when stimulated, delivering power for moving your body. On a robot, such systems are called actuators (from actuate-to put into action). These are the motors, gears, hydraulic or pneumatic cylinders, or other active components that work to put your robot into motion. Let's look briefly at the most common types of actuators.
Motors and Gears
The most tried and true means of delivering power to move robot parts is direct current (DC) motors and sets of gears. Sometimes, each joint of a manipulator arm (or other moving appendage) will have a dedicated motor and set of gears. Other times, sets of gears will be designed to transfer the power of one motor to a number of places where motion is needed. One popular type of motor is called a servo motor. It has a DC motor and a set of gears (called a gearbox) inside a housing that protects the whole assembly. Inside the housing is also usually a board of controller electronics that allows for precise control of the motion of the motor's axle. (We'll be discussing servo motors, gearboxes, and controllers in other sections of this book.)
Even more important than moving power around, gears are used to convert the high-speed, low-torque output from a DC motor to the slow and strong motion you need to move arms, legs, and other robot body parts. Gears are the most compact way of upping the torque/lower revolutions per minute (RPM) of a motor.
A form of robotic muscle frequently found in heavy-duty industrial and field robotics is the hydraulic cylinder. Hydraulics simply means the use of fluid to power something. In common practice, this usually takes the form of a cylinder that has a piston in it. The piston moves, and delivers power, when a pressured liquid (usually an oil-based fluid) is pumped into a chamber within the cylinder. Hydraulic systems can deliver an impressive amount of power (hence their widespread industrial use), but they are also heavy, temperamental, and extremely messy when tubes, fittings, and cylinders leak (which they are prone to doing eventually). Robots and liquids don't get along very well.
A type of "pressure system" like hydraulics, pneumatics does with air what hydraulics does with liquid. The techniques and hardware are very similar between the two systems. Obviously, one big advantage of pneumatics is the lack of yucky fluids that can ruin the rest of your robot. The downside is noise. Pneumatics hiss and purr as they work, and depending on the application, that sound pollution can be a problem. This type of actuation can also be temperamental, and it is very difficult to get consistently smooth movement because of the compressibility of air.
Shape memory alloy (SMA), also known as muscle wire, is a fascinating material that holds great promise for certain types of robotic (and other) applications. Muscle wire has the unique capability to be formed into a shape, and then when a small electrical current is applied, it will return to its original unstretched shape. This process can be harnessed to deliver motorless power (see Figure 4.10).
A robotic arm kit from The Robot Store (www.robotstore.com) designed to demonstrate the capabilities of shape memory alloy. Photo by JFM Digital, used with permission of Robot Store.com.
The most common form of SMA is made of a combination of nickel and titanium and is called nitinol (pronounced "night in all"). Unfortunately, the high cost of the materials has limited SMA's widespread use, but as the price of titanium continues to drop, this will become less of an issue.
One drawback to SMA is that it uses the heat (resistance) from an electrical current to change its shape. Converting all of that useful DC electricity into heat to get a contraction wastes a lot of power, which is something that's always at a premium for robot builders.
The part of a robot's actuation system that allows it to move through the world is called the drive train. This includes the motors, gears, drive belts, axles, wheels, legs, or tracks, and any other components directly related to robotic locomotion. The job of the robot's drive train is loosely analogous to that of our own legs and feet.
The heart of the drive train is the motor. There are basically three types of motors commonly found in robots; each of them is covered in the following sections.
If you've ever cannibalized a remote-controlled vehicle or other motorized toy (and who hasn't?), you're familiar with this type of motor. It uses direct current (either from batteries or from AC wall current converted within the device to DC) to power the motor and spin the drive shaft. DC motors come in all sizes, from tiny pager motors (used to vibrate the case of the pager to alert you that your mother-in-law "needs to talk"), to big beefy jobs that can power the largest of robots. DC motors are great for use in robots, but they spin too quickly to send their power directly to wheels or legs. They generate the most power of the motors used in robots, but the high rotation speeds can make them difficult to take advantage of on their own. This means that they require the addition of mechanical systems (such as gears, belts, sprockets, and chains) to slow down their speed and increase their torque.
A servo motor is an ingenious little machine. It usually consists of a DC motor, a gearbox, and a motor controller inside a rectangular plastic housing (see Figure 4.11).
The beauty of the servo is that it has its gearbox already built in (so you don't have to go loony trying to wrestle with gear reduction formulas), and its built-in circuitry allows for precise control over the motion the servo delivers. When you buy the hobby type of servo motor, it is typically limited to 130 to 180 degrees of back and forth movement (what's required for the remote-controlled vehicles, where servos are commonly found). Robot builders often change this limited rotation to continuous rotation with a little "hardware hacking." We'll detail how this is done in Projects 1 and 2. Although the hobby servo is the most common type, there are larger high-performance servo motors that can deliver serious motive power.
The Hitec Servo Motor. The types of servo motors used in small-to medium-size robots are the same types found in remote control (R/C) vehicles (planes, boats, and cars).
A DC motor uses magnets and windings of wire to generate rotation through electromagnetic induction. A stepper motor operates on the same basic principles, except its innards are arranged differently. In exact opposition to a regular DC motor, the stepper has its magnets on the rotating shaft (the rotor) and its wire windings on the inside motor wall. This arrangement allows the rotor to be moved from one discrete coil winding (called a stator) to the next. The action of moving from one stator, or "step," to another gives the motor its name. The stepper motor design allows for more precise speed control than with conventional DC motors or servo motors. This control also often means that gears don't have to be used to slow down the motor's rotation. Unfortunately, steppers take more power to operate, need special electronics to power, and don't deliver as much work in return as the other motor types.
The Absolute Beginner's Guide to Building Robots by Gareth Branwyn, ISBN-10: 0-78972971-7 is available from InformIT. Permission to reprint granted by Pearson Publshing.