McGraw-Hill - The Robot Builder''''s Bonanza Part 8 docx

motor, of course). Read more about servo motors in Chapter 20, “Working with Servo Motors.” OMNIDIRECTIONAL To have the highest tech of all robots, you may want omnidirectional drive. It uses steer- able drive wheels, usually at least three, as shown in Fig. 16.13. The wheels are operated by two motors: one for locomotion and one for steering. In the usual arrangement, the drive/steering wheels are “ganged” together using gears, rollers, chains, or pulleys. Omnidirectional robots exhibit excellent maneuverability and steering accuracy, but they are technically more difficult to construct. Calculating the Speed of Robot Travel The speed of the drive motors is one of two elements that determines the travel speed of your robot. The other is the diameter of the wheels. For most applications, the speed of the drive motors should be under 130 rpm (under load). With wheels of average size, the resul- tant travel speed will be approximately four feet per second. That’s actually pretty fast. A better travel speed is one to two feet per second (approximately 65 rpm), which requires smaller diameter wheels, a slower motor, or both. How do you calculate the travel speed of your robot? Follow these steps: 1. Divide the rpm speed of the motor by 60. The result is the revolutions of the motor per second (rps). A 100-rpm motor runs at 1.66 rps. 2. Multiply the diameter of the drive wheel by pi, or approximately 3.14. This yields the cir- cumference of the wheel. A 7-inch wheel has a circumference of about 21.98 inches. 3. Multiply the speed of the motor (in rps) by the circumference of the wheel. The result is the number of linear inches covered by the wheel in one second. With a 100-rpm motor and 7-inch wheel, the robot will travel at a top speed of 35.168 inches per second, or just under three feet. That’s about two miles per hour! You can read- ily see that you can slow down a robot by decreasing the size of the wheel. By reducing the wheel to 5 inches instead of 8, the same 100-rpm motor will propel the robot at about 25 inches per second. By reducing the motor speed to, say, 75 rpm, the travel speed falls even more, to 19.625 inches per second. Now that’s more reasonable. CALCULATING THE SPEED OF ROBOT TRAVEL 231 Steering wheel Drive wheels FIGURE 16.12 In tricycle steer- ing, one drive motor powers the robot; a single wheel in front steers the robot. Be wary of short wheelbases as this can introduce tipping when the robot turns. Ch16_McComb 8/18/00 2:14 PM Page 231 Bear in mind that the actual travel speed once the robot is all put together may be lower than this. The heavier the robot, the larger the load on the motors, so the slower they will turn. Round Robots or Square? Robots can’t locomote where they can’t fit. Obviously, a robot that’s too large to fit through doorways and halls will have a hard time of it. In addition, the overall shape of a robot will also dictate how maneuverable it is, especially indoors. If you want to navigate your robot in tight areas, you should consider its basic shape: round or square. ■ A round robot is generally able to pass through smaller openings, no matter what its ori- entation when going through the opening (see Fig. 16.14). To make a round robot, you must either buy or make a rounded base or frame. Whether you’re working with metal, steel, or wood, a round base or frame is not as easy to construct as a square one. ■ A square robot must orient itself so that it passes through openings straight ahead rather than at an angle. Square-shaped robot bases and frames are easier to construct than round ones. While you’re deciding whether to build a round- or square-shaped robot, consider that a circle of a given diameter has less surface area than a square of the same width. For example, a 10-inch circle has a surface area of about 78 square inches. Moreover, because the surface of the base is circular, less of it will be useful for your robot (unless your print- ed circuit boards are also circular). Conversely, a 10-inch-by-10-inch square robot has a surface area of 100 inches. Such a robot could be reduced to about 8.5 inches square, and it would have about the same surface area as a 10-inch round robot, and its surface area would be generally more usable. 232 ROBOT LOCOMOTION PRINCIPLES Steering and drive wheels FIGURE 16.13 An omnidirectional robot uses the same wheels for drive and steering. Ch16_McComb 8/18/00 2:14 PM Page 232 ROUND ROBOTS OR SQUARE? 233 Path of robot Path of robot Path of robot Square robot of same dimensions as circular robot won't fit through opening Square robot of slightly smaller dimensions as circular robot fits through opening 10" 8.5" 10" FIGURE 16.14 A round robot versus a square robot. All things being equal, a round robot is better able to navigate through small openings. However, rounded robots also have less usable surface area, so a square-shaped robot can be made smaller and still support the same onboard “real estate.” Ch16_McComb 8/18/00 2:14 PM Page 233 From Here To learn more about… Read Selecting wood, plastic, or metal to Chapter 8–10 construct your robot Choosing a battery for your robot Chapter 15, “All about Batteries and Robot Power Supplies” Selecting motors Chapter 17, “Choosing the Right Motor for the Job” Building a walking robot Chapter 22, “Build a Heavy-duty, Six-legged Walking Robot” Constructing a treaded robot Chapter 23, “Advanced Locomotion Systems” Controlling the speed of a two- motor-driven robot Chapter 38, “Navigating through Space” 234 ROBOT LOCOMOTION PRINCIPLES Ch16_McComb 8/18/00 2:14 PM Page 234 Motors are the muscles of robots. Attach a motor to a set of wheels and your robot can scoot around the floor. Attach a motor to a lever, and the shoulder joint for your robot can move up and down. Attach a motor to a roller, and the head of your robot can turn back and forth, scanning its environment. There are many kinds of motors; however, only a select few are truly suitable for homebrew robotics. In this chapter, we’ll examine the various types of motors and how they are used. AC or DC? Direct current—DC—dominates the field of robotics, either mobile or stationary. DC is used as the main power source for operating the onboard electronics, for opening and closing solenoids, and, yes, for running motors. Few robots use motors designed to operate from AC, even those automatons used in factories. Such robots convert the AC power to DC, then distribute the DC to various sub- systems of the machine. DC motors may be the motors of choice, but that doesn’t mean you should use just any DC motor in your robot designs. When looking for suitable motors, be sure the ones you buy are reversible. Few robotic applications call for just unidirectional (one-direction) motors. You must be able to operate the motor in one direction, stop it, and change its direction. DC motors are inherently bidirectional, but some design limitations may prevent reversibility. 17 CHOOSING THE RIGHT MOTOR FOR THE JOB 235 Ch17_McComb 8/29/00 8:36 AM Page 235 Copyright 2001 The McGraw-Hill Companies, Inc. Click Here for Terms of Use. The most important factor is the commutator brushes. If the brushes are slanted, the motor probably can’t be reversed. In addition, the internal wiring of some DC motors pre- vents them from going in any but one direction. Spotting the unusual wiring scheme by just looking at the exterior or the motor is difficult, at best, even for a seasoned motor user. The best and easiest test is to try the motor with a suitable battery or DC power supply. Apply the power leads from the motor to the terminals of the battery or supply. Note the direction of rotation of the motor shaft. Now, reverse the power leads from the motor. The motor shaft should rotate in reverse. Continuous or Stepping DC motors can be either continuous or stepping. Here is the difference: with a continuous motor, like the ones in Fig. 17.1, the application of power causes the shaft to rotate con- tinually. The shaft stops only when the power is removed or if the motor is stalled because it can no longer drive the load attached to it. With stepping motors, shown in Fig. 17.2, the application of power causes the shaft to rotate a few degrees, then stop. Continuous rotation of the shaft requires that the power be pulsed to the motor. As with continuous DC motors, there are subtypes of stepping motors. Permanent magnet steppers are the ones you’re likely to encounter, and they are also the easiest to use. The design differences between continuous and stepping DC motors need to be addressed in detail. Chapter 18, “Working with DC Motors,” focuses entirely on continu- 236 CHOOSING THE RIGHT MOTOR FOR THE JOB FIGURE 17.1 An assortment of DC motors. Ch17_McComb 8/29/00 8:36 AM Page 236 ous motors. Chapter 19, “Working with Stepper Motors,” focuses entirely on the stepping variety. Although these two chapters focus on the main drive motors of your robot, you can apply the information to motors used for other purposes as well. Servo Motors A special “subset” of continuous motors is the servo motor, which in typical cases com- bines a continuous DC motor with a “feedback loop” to ensure the accurate positioning of the motor. A common form of servo motor is the kind used in model and hobby radio-con- trolled (R/C) cars and planes. R/C servos are in plentiful supply, and their cost is reasonable (about $10–12 for basic units). Though R/C servos are continuous DC motors at heart, we will devote a separate chapter just to them. See Chapter 20, “Working with Servo Motors,” for more information on using R/C servo motors not only to drive your robot creations across the floor but to operate robot legs, arms, hands, heads, and just about any other appendage. Other Motor Types There are many other types of motors, some of which may be useful in your hobby robot, some of which will not. DC, stepper, and servo motors are the most common, but you may also see references to some of the following: OTHER MOTOR TYPES 237 FIGURE 17.2 An assortment of stepper motors. Ch17_McComb 8/29/00 8:36 AM Page 237 ■ Brushless DC. This is a kind of DC motor that has no brushes. It is controlled elec- tronically. Brushless DC motors are commonly used in fans inside computers and for motors in VCRs and videodisc players. ■ Switched reluctance. This is a DC motor without permanent magnets. ■ Synchronous. Also known as brushless AC, this motor operates synchronously with the phase of the power supply current. These motors function much like stepper motors, which will be discussed in Chapter 19. ■ Synchro. These motors are considered distinct from the synchronous variety, described above. Synchro motors are commonly designed to be used in pairs, where a “master” motor electrically controls a “slave” motor. Rotation of the master causes an equal amount of rotation in the slave. ■ AC induction. This is the ordinary AC motor used in fans, kitchen mixers, and many other applications. ■ Sel-Syn. This is a brand name, often used to refer to synchronous AC motors. Note that AC motors aren’t always operated at 50/60 Hz, which is common for house- hold current. Motors for 400-Hz operation, for example, are common in surplus stores and are used for both aircraft and industrial applications. Motor Specifications Motors come with extensive specifications. The meaning and purpose of some of the specifi- cations are obvious; others aren’t. Let’s take a look at the primary specifications of motors— voltage, current draw, speed, and torque—and see how they relate to your robot designs. OPERATING VOLTAGE All motors are rated by their operating voltage. With small DC “hobby” motors, the rating is actually a range, usually 1.5 to 6 volts. Some high-quality DC motors are designed for a specific voltage, such as 12 or 24 volts. The kinds of motors of most interest to robot builders are the low-voltage variety—those that operate at 1.5 to 12 volts. Most motors can be operated satisfactorily at voltages higher or lower than those spec- ified. A 12-volt motor is likely to run at 8 volts, but it may not be as powerful as it could be, and it will run slower (an exception to this is stepper motors; see Chapter 19, “Working with Stepper Motors,” for details). You’ll find that most motors will refuse to run, or will not run well, at voltages under 50 percent of the specified rating. Similarly, a 12-volt motor is likely to run at 16 volts. As you may expect, the speed of the shaft rotation increases, and the motor will exhibit greater power. I do not recommend that you run a motor continuously at more than 30 or 40 percent its rated voltage, however. The windings may overheat, which may cause permanent damage. Motors designed for high-speed operation may turn faster than their ball-bearing construction allows. If you don’t know the voltage rating of a motor, you can take a guess at it by trying var- ious voltages and seeing which one provides the greatest power with the least amount of heat dissipated through the windings (and felt on the outside of the case). You can also lis- ten to the motor. It should not seem as if it is straining under the stress of high speeds. 238 CHOOSING THE RIGHT MOTOR FOR THE JOB Ch17_McComb 8/29/00 8:36 AM Page 238 CURRENT DRAW Current draw is the amount of current, in milliamps or amps, that the motor requires from the power supply. Current draw is more important when the specification describes motor loading, that is, when the motor is turning something or doing some work. The current draw of a free-running (no-load) motor can be quite low. But have that same motor spin a wheel, which in turn moves a robot across the floor, and the current draw jumps 300, 500, even 1000 percent. With most permanent magnet motors (the most popular kind), current draw increases with load. You can see this visually in Fig. 17.3. The more the motor has to work to turn the shaft, the more current is required. The load used by the manufacturer when testing the motor isn’t standardized, so in your application the current draw may be more or less than that specified. A point is reached when the motor does all the work it can do, and no more current will flow through it. The shaft stops rotating; the motor has “stalled.” Some motors, but not many, are rated (by the manufacturer) by the amount of current they draw when stalled. This is considered the worse-case condition. The motor will never draw more than this cur- rent unless it is shorted out, so if the system is designed to handle the stall current it can handle anything. Motors rated by their stall current will be labeled as such. Motors designed for the military, available through surplus stores, are typically rated by their stall current. When providing motors for your robots, you should always know the approx- imate current draw under load. Most volt-ohm meters can test current. Some special-pur- pose amp meters are made just for the job. Be aware that some volt-ohm meters can’t handle the kind of current pulled through a motor. Most digital meters (discussed more completely in Chapter 3, “Tools and Supplies”) can’t deal with more than 200 to 400 milliamps of current. Even small hobby motors can draw in excess of this. Be sure your meter can accommodate current up to 5 or 10 amps. If your meter cannot register this high without popping fuses or burning up, insert a 1- to 10-ohm power resistor (10 to 20 watts) between one of the motor terminals and the positive supply rail, as shown in Fig. 17.4. With the meter set on DC voltage, measure the voltage developed across the resistor. A bit of Ohm’s law, I ϭ E/R (I is current, E is voltage, R is resistance) reveals the cur- rent draw through the motor. For example, if the resistance is 10 ohms and the voltage is 2.86 volts, the current draw is 286 mA. You can watch the voltage go up (and therefore the current too) by loading the shaft of the motor. MOTOR SPECIFICATIONS 239 6 5 4 3 2 1 0 Current (amps) Load (lb-ft) 0 1 2 3 4 5 6 7 8 9 Increasing load FIGURE 17.3 The current draw of a motor increases in proportion to the load on the motor shaft. Ch17_McComb 8/29/00 8:36 AM Page 239 SPEED The rotational speed of a motor is given in revolutions per minute (rpm). Most continuous DC motors have a normal operating speed of 4000 to 7000 rpm. However, some special- purpose motors, such as those used in tape recorders and computer disk drives, operate as slow as 2000 to 3000 rpm. For just about all robotic applications, these speeds are much too high. You must reduce the speed to no more than 150 rpm (even less for motors dri- ving arms and grippers) by using a gear train. You can obtain some reduction by using elec- tronic control, as described in Part 5 of this book, “Computers and Electronic Control.” However, such control is designed to make fine-tuned speed adjustments, not reduce the rotation of the motor from 5000 rpm to 50 rpm. See the later sections of this chapter for more details on gear trains and how they are used. Note that the speed of stepping motors is not rated in rpm but in steps (or pulses) per second. The speed of a stepper motor is a function of the number of steps that are required to make one full revolution plus the number of steps applied to the motor each second. As a comparison, the majority of light- and medium-duty stepper motors operate at the equivalent of 100 to 140 rpm. See Chapter 19, “Working with Stepper Motors,” for more information. TORQUE Torque is the force the motor exerts upon its load. The higher the torque, the larger the load can be and the faster the motor will spin under that load. Reduce the torque, and the motor 240 CHOOSING THE RIGHT MOTOR FOR THE JOB Motor 1Ω 10 watt Resistor Power Meter (set to read voltage) FIGURE 17.4 How to test the current draw of a motor by measuring the voltage developed across an in-line resistor. The actual value of the resistor can vary, but it should be under about 20 ohms. Be sure the resistor is a high-wattage type. Ch17_McComb 8/29/00 8:36 AM Page 240 [...]... external control Use the wiring diagram in Fig 18. 2 to prepare yourself for these later chapters Controlling the direction of the motor is only a little more difficult This requires a double-pole, double-throw (DPDT) relay, wired in series after the on/off relay just described (see Fig 18. 3; refer to the parts list in Table 18. 2) With the contacts in the relay in one Ch 18_ McComb 8/ 18/ 00 2:11 PM Page 256... REDUCTION 12-tooth driver (1,000 rpm) 245 12-tooth pinion fixed on 60-tooth gear 60-tooth gear 4 8- tooth driven gear (50 rpm) FIGURE 17 .8 True gear reduction is achieved by ganging gears on the same shaft of the motor Note as well that the running and stall torque of the motor will be greatly increased Make sure that the torque specification on the motor is for the output of the gearbox, not the motor... ounces, then the motor is said to have a torque of two ounce-inches, or oz-in (Some people reverse the “ounce” and “inches” and come up with “inch-ounces.”) The unit of length for the lever usually depends on the unit of measurement given for the weight When the weight is in grams, the lever is in centimeters (gm-cm) When the weight is in ounces, as already seen, the lever used is in inches (oz-in) Finally,... read more about this particular design in Chapter 22, “Build a Heavy-duty, Six-legged Walking Robot. ” Another example is shown in Fig 17.14 Here, the motor has mounting holes on the end by the shaft, but these holes are in the wrong position for the design of the robot Two commonly available flat corner irons were used to mount the motor This is just one approach; a number of other mounting schemes... each of the drive motors The wiring diagram for these robot motors is duplicated in Fig 18. 1 for your convenience The DPDT switches used here have a center-off position When they are in the center position, the motors receive no power so the robot does not move You can use the direction control switch for experimenting, but you’ll soon want to graduate to more automatic control of your robot There are... more commonly, a spring-loaded scale (as shown in the figure) Turn the motor on and it turns the lever The amount of weight it lifts is the torque of the motor There is more to motor testing than this, of course, but it’ll do for the moment Now for the ratings game Remember the length of the lever? That length is used in the torque specification If the lever is one inch long, and the weight successfully... series (such as the IRF-520, IRF-530, etc.), from International Rectifier, one of the world’s leading manufacturers of power MOSFET components These N-channel MOSFETs come in a T 0-2 20-style transistor case and can control several amps of current (when on a suitable heat sink) A basic, semi-useful circuit that uses MOSFETs is shown in Fig 18. 8 (see the parts list in Table 18. 5) Note the similarity between... flexible; that is, they give if the two shafts aren’t perfectly aligned These are the best, considering the not-too-close tolerances inherent in home-built robots Some couplers are available that accept two shafts of different sizes Finally, there are dozens of other methods for attaching wheels, gears, and other objects to motor shafts Several of these will be detailed in context in the chapters to come... Motor FIGURE 17.5 The torque of a motor is measured by attaching a weight or scale to the end of a lever and mounting the lever of the motor shaft Ch17_McComb 8/ 29/00 8: 36 AM Page 242 242 CHOOSING THE RIGHT MOTOR FOR THE JOB STALL OR RUNNING TORQUE Most motors are rated by their running torque, or the force they exert as long as the shaft continues to rotate For robotic applications, it’s the most important... reduction systems, the output shaft is opposite the input shaft (but usually off center) With other boxes, the output and input are on the same side of the box When the shafts are at 90 degrees from one another, the reduction box is said to be a “rightangle drive.” If you have the option of choosing, select the kind of gear reduction that best suits the design of your robot I have found that the “shafts on . when the robot turns. Ch16_McComb 8/ 18/ 00 2:14 PM Page 231 Bear in mind that the actual travel speed once the robot is all put together may be lower than this. The heavier the robot, the larger the. the force the motor exerts upon its load. The higher the torque, the larger the load can be and the faster the motor will spin under that load. Reduce the torque, and the motor 240 CHOOSING THE. speed of the motor (in rps) by the circumference of the wheel. The result is the number of linear inches covered by the wheel in one second. With a 100-rpm motor and 7-inch wheel, the robot will