Build Your Own Combat Robot phần 3 docx

UILDING a robot requires that you make many decisions—from the type of sensors you’ll use to the color you’ll paint it. Some of these decisions are trivial, while others will make or break your robot. One decision in the make-or-break category is motors—not just deciding which ones you’ll use, but determining how you’ll optimize their performance. Most robots use the same class of motor—the permanent magnet direct current (PMDC) motor. These commonly used motors are fairly low in cost and relatively easy to control. Other types of electric motors are available, such as series-wound field DC motors, stepper motors, and alternating current (AC) motors, but this book will discuss only PMDC -type motors. If you want to learn more about other types of motors, consult your local library or the Internet for that information. Some combat robots use internal combustion motors, but they are more com- monly used to power weapons than to drive the robots, largely because the inter- nal combustion engine rotates only in one direction. If you are using an internal combustion engine to drive the robot, your robot will require a transmission that can switch into reverse or use a hydraulic motor drive system. With electric mo - tors, however, the direction of the robot can be reversed without a transmission. Many combat robots combine the two, using electric motors for driving the robot system and internal combustion motors for driving the weapons. Another use for internal combustion engines is to drive a hydraulic pump that drives the robot and/or operates the weapons. Since most robots use PMDC motors, most of the discussion in this chapter will be focused on electric motors. At the end of this chapter is a short discussion of internal combustion engines. E lectric Motor Basics Because the robot’s speed, pushing capability, and power requirements are di - rectly related to the motor performance, one of the most important things to un - derstand as you design your new robot is how the motors will perform. In most robot designs, the motors place the greatest constraints on the design. 62 Direct current (DC) motors have two unique characteristics: the motor speed is proportional to the voltage applied to the motor, and the output torque (that is, the force producing rotation) from the motor is proportional to the amount of current the motor is drawing from the batteries. In other words, the more voltage you supply to the motor, the faster it will go; and the more torque you apply to the motor, the more current it will draw. Equations 1 and 2 show these simple relationships: The units of K v are RPM per volt and K t are oz in. per amp (or in lb. per amp). Torque is in oz in. and RPM is revolutions per minute. K v is known as the motor- speed constant, and K t is known as the motor-torque constant. These equations apply to the “ideal” motor. In reality, certain inefficiencies exist in all motors that alter these relationships. Equation 1 shows that the motor speed is not affected by the applied torque on the motor. But we all know through expe- rience that the motor speed is affected by the applied motor torque—that is, they slow down. All motors have a unique amount of internal resistance that results in a voltage loss inside the motor. Thus, the net voltage the motor sees from the bat- teries is proportionally reduced by the current flowing through the motor. Equation 3 shows the effective voltage that the motor actually uses. Equation 4 shows the effective motor speed. Where V in is the battery voltage in volts, I in is the current draw from the motor in amps, R is the internal resistance of the motor in ohms, and V motor is the effective mo - tor voltage in volts. It can easily be seen in Equation 4 that as the current increases (by increasing the applied torque), the net voltage decreases, thus decreasing the motor speed. But speed is still proportional to the applied voltage to the motor. With all motors, a minimum amount of energy is needed just to get the motor to start turning. This energy has to overcome several internal “frictional” losses. A minimum amount of current is required to start the motor turning. Once this threshold is reached, the motor starts spinning and it will rapidly jump up to the maximum speed based on the applied voltage. When nothing is attached to the output shaft, this condition is known as the no-load speed and this current is known as the no-load current. Equation 5 shows the actual torque as a function of the current draw, where I 0 is the no-load current in amps. Note that the motor de - livers no torque at the no-load condition. Another interesting thing to note here is Chapter 4: Motor Selection and Performance 63 4.1 4.2 4.3 rpm K V K (V I R)==− in in motor vv 4.4 64 Build Your Own Combat Robot that by looking at Equation 4, the voltage must also exceed the no-load current multiplied by the internal resistance for the motor to start turning. Some motors advertise their no-load speed and not their no-load current. If the motor’s specifications list the internal resistance of the motor, the no-load current can be determined from equation 4. With these equations, as well as the gear ratio, wheel size, and coefficient of friction between wheels and floor,you can determine how fast the robot will move and how much pushing force the robot will have. (How you actually determine this will be explained in Chapter 6.) If you want the robot to go faster, you can ei - ther run the motors at a higher voltage or choose a lower gear reduction in the drive system. Equation 5 is an important equation to know and understand, because it will have a direct effect on the type and size of the batteries that you will need. By rear - ranging this equation, the current draw requirements from your batteries can be determined. Equation 6 shows this new relationship. For any given torque or pushing force, the battery current requirements can be calculated. For worst-case situations, stalling the motors will draw the maximum current from the batteries. Equation 7 shows how to calculate the stall current, where I stall is the stall current in amps. The batteries should be sized to be able to de- liver this amount of current. Batteries that deliver less current will still work, but you won’t get the full performance potential of the motors. Some builders pur - posely undersize the battery to limit the current and help the motors and electron - ics survive, and others do this simply because they have run out of weight allowance. For some motors, the stall current can be several hundreds of amps. Another set of relationships that needs to be considered is the overall power being supplied by the batteries and generated by the motor. The input power, P in ,tothe motor is shown in equation 8. Note that it is highly dependent on the current draw from the motor. The output power, P out , is shown in mechanical form in equation 9 and in electrical from in equation 10. Motor efficiency is shown in equation 11. The standard unit of power is watts. 4.5 4.6 4.7 4.8 4.9 4.10 The output power is always less than the input power. The difference between the two is the amount of heat that will be generated due to electrical and frictional losses. It is best to design and operate your robot in the highest efficiency range to minimize the motor heating. If the motor is able to handle the heat build-up, it might be best to design the robot (or weapon) to be operated at a higher percent - age of the motor’s maximum power (to keep the motor as light as possible). For example, a motor that is used to recharge a spring-type weapon might be fine if operated at near-stall load for just a few seconds at a time. The maximum amount of heat is generated when the motor is stalled. A motor can tolerate this kind of heat for short periods of time only, and it will become permanently damaged if it’s stalled for too long a period of time. This heat is generated in the armature wind - ings and the brushes, components that are hard to cool by conduction. Figure 4-1 shows a typical motor performance chart. These charts are usually obtained from the motor manufacturer, or a similar chart can be created if you know the motor constants. The motor shown in Figure 4-1 is an 18-volt Johnson Electric motor model HC785LP-C07/8, which can be found in some cordless drills. The constants for this motor are shown in Table 4-1. This motor is dis- cussed here as an example motor to describe how all of the motor constants relate to each other and how they affect the motor performance. Figure 4-1 graphically displays how the motor speed decreases as the motor torque increases and how the motor current increases as the applied torque on the motor increases. For this particular motor, maximum efficiency is approximately Chapter 4: Motor Selection and Performance 65 4.11 FIGURE 4-1 Typical motor performance curves. 75 percent and it occurs when the motor is spinning at approximately 19,000 RPM. Maximum output power from this motor occurs when the motor speed decreases to about 50 percent of its maximum speed and the current is approximately 50 percent of the stall current. For all permanent magnet motors, maximum power occurs when 50 percent of the stall current is reached. Motor manufacturers recommend that motors be run at maximum efficiency; otherwise, motors will overheat faster. 66 Build Your Own Combat Robot I 0 1.934 amps R 0.174 ohms K v 1,234.6 rpm/volt K t 1.097 oz-in/amp TABLE 4-1 Motor Constants for Figure 4-1 n True Story: Grant Imahara and Deadblow Grant Imahara started his career in robotics as a kid by drawing pictures of robots from movies and television. Later, his designs evolved into LEGOs, and then cardboard and wood. “Only recently,” he laments, “have I had the tools and equipment to build them out of metal.” Though Grant got his start as part the Industrial Light and Magic team at Robot Wars in 1996 (he’s an animatronics engineer and model maker for George Lucas’ ILM special effects company), he is perhaps best known for his creation known as Deadblow. Deadblow is a robot with its share of stories. “The best match I ever fought was against Pressure Drop in season 1.0,” Grant recalls. “I had broken the end of my hammer off in a previous match against a robot named Alien Gladiator.” Grant had a spare arm, but, not really expecting to need it, he hadn’t fully prepared it to mate with the robot. Without the hammer head, he had no weapon, so a little quick construction work was called for. “‘No problem,’ I thought. I’ll just drive back to ILM and work on it at our shop. With three hours before the next match, I figured it would be a breeze.” Unfortunately, Grant soon uncovered a glitch. “We drove up to the shop and I started working on the hammer arm. I discovered to my horror that we were out of carbide mills, and I had to put two holes in case-hardened steel. After going through several high-speed steel bits and getting nowhere, I resorted to going through my co-worker’s desks, trying to find a carbide tool. Finally, I found a tiny 1/16-inch carbide bit. I took this bit and chucked it into a Dremel tool and painstakingly bored two 3/8-inch holes in the handle of my hammer by hand.” Chapter 4: Motor Selection and Performance 67 Determining the Motor Constants To use the equations, the motor constants, K v , K t , I 0 , and R must be known. The best way to determine the motor constants is to obtain them directly from the motor manufacturer. But since some of us get our motors from surplus stores or pull them out of some other motorized contraption, these constants are usually un- known. Fortunately, this is not a showstopper, because these values can be easily measured through a few experiments. You’ll need a voltmeter and a tachometer before you start. To determine the motor speed constant, K v , run the motor at a constant speed of a few thousand RPMs. Measure the voltage and the motor speed, and record these values. Repeat the test with the motor running a different speed, and record the second values. The motor speed constant is determined by dividing the measured difference in the motor speeds and the difference between the two measured voltages: All permanent magnet DC motors have this physical property, wherein the product of the motor speed constant and the motor torque constant is 1352. With this knowledge, the motor torque constant can be calculated by dividing the motor speed constant by 1352. The units for this constant is (RPM / Volts) × (oz in. / amps). Equation 13 shows this relationship. The next step is to measure the internal resistance. This cannot be done using only an ohmmeter—it must be calculated. Clamp the motor and output shaft so that they will not spin. (Remember that large motors can generate a lot of torque and draw a lot of current, so you need to make sure your clamps will be strong Grant Imahara and Deadblow (continued) With only an hour left and a 20-minute drive to get back to the competition, Grant still wasn’t overly concerned. “But then we hit Sunday evening traffic back into San Francisco. We were going to be late. Forty-five minutes later, I ran into Fort Mason with the new hammer in hand. And we threw it into the robot.” As the announcer called Team Deadblow to line up for the fight, they were still screwing the armor back onto the robot. “If you look carefully,” Grant says, “you can see that my normally put-together look had become severely disheveled. I was out of breath and about to pass out and the match hadn’t even started yet! I had a ‘go for broke’ attitude for that match, and the adrenaline was pumping. Deadblow went in and pummeled Pressure Drop with a record number of hits. By the end, I could barely feel my hands because they were tingling so much.” 4.12 4.13 enough to hold the output shaft still.) Apply a very low DC voltage to the motor—a much lower voltage than what the motor will be run at. If you do not have a variable regulated DC power supply, one or two D-cell alkaline batteries should work. Now measure both the voltage and current going through the motor at the same time. The best accuracy occurs when you are measuring several hundred milliamps to several amps. The internal resistance, R, can be calculated by divid - ing the measured voltage, V in , by the measured current, I in : It is best to take a few measurements and average the results. To determine the no-load current, run the motor at its nominal operating volt - age (remember to release the output shaft from the clamps, and have nothing else attached to the shaft). Then measure the current going to the motor. This is the no-load current. The ideal way to do this is to use a variable DC power supply. In - crease the voltage until the current remains relatively constant. At this point, you have the no-load current value. The no-load current value you use should be the actual value for the motor running at the voltage you intend to use in your robot. After conducting these experiments, you will now have all of the motor con- stant parameters to calculate how the motor will perform in your robot. Power and Heat When selecting a motor, you should first have a good idea of how much power that your robot will require. A motor’s power is rated in either watts or horse- power (746 watts equal 1 horsepower). Small fractional horsepower motors of the type that are usually found in many toys are fine for a line-following or a cat-annoying robot. But, if your plan is to dominate the heavyweight class at BattleBots, you will require heavyweight motors. This larger class of motors can be as much as 1,000 times more powerful than the smaller motors. A small toy motor might operate at 3 volts and draw at most 2 amps, for an input requirement of 6 watts (volts × amps = watts). If the motor is 50-percent efficient, it will produce 3 watts of power. At the other end of the spectrum are the robot combat class motors. One of these might operate at 24 or 48 volts and draw hundreds of amps, for a peak power output of perhaps 5 horsepower (3,700 watts) or more. Two of these motors can accelerate a 200-pound robot warrior to 15-plus mph in just a few feet, with tires screaming. One 1997 heavyweight (Kill-O-Amp) had motors that could extract 1,000 amps from its high-output batteries! The power that your robot will require is probably somewhere between these two extremes. Your bot’s power requirements are affected by factors like operating surface. For example, much more power is required to roll on sand than on a hard surface. Likewise, going uphill will increase your machine’s power needs. Soft tires that you might use for greater friction have more rolling resistance than hard tires, 68 Build Your Own Combat Robot 4.14 which will increase the power requirements. Do you have an efficient drive train, or are you using power-robbing worm gears? How fast do you want to go? An internal combustion engine produces its peak horsepower at about 90 per - cent of its maximum RPM, and peak torque is produced at about 50 percent of maximum RPM. The higher the RPM, the more energy it consumes. Compare this to the PMDC motor, which consumes the most energy and develops its peak torque at zero RPM. It consumes little energy at maximum RPM, and it produces its peak horsepower at 50 percent of its unloaded speed. At 50 percent of maximum speed, the PMDC motor will draw half of its maxi - mum stalled current, as seen earlier in Figure 4-1. Unfortunately, much of the cur - rent going into the motor at this high power level is turned into heat. Figure 4-2 shows how much heat is generated in the example motor used to create the statis - tics in Figure 4-1. It is obvious to see that the minimum amount of heating occurs when running the motor near its maximum speed and efficiency. It can also be seen in Figure 4-2 that as the motor torque increases, a near exponential increase in motor heat re - sults. Motors can tolerate this amount of heat only for short periods of time. Con- tinuously running a motor above the maximum power output level will seriously damage or destroy it, depending on how conservatively the manufacturer rated the motor. Many motors are rated to operate continuously at a certain voltage. You can increase the power of your motor by increasing the voltage. Figure 4-3 shows how a motor’s speed, torque, and current draw are affected by increasing the input voltage to the motor. In Figure 4-3, you can see that the motor speed is doubled Chapter 4: Motor Selection and Performance 69 FIGURE 4-2 Heat generated in an electric motor. and the maximum stall torque is doubled when the input voltage is doubled. Re- call from equation 4 that the motor’s speed is proportional to the applied voltage. In Figure 4-3, you will notice that the current draw line from the 18-volt and 36-volt cases are on top of one another. Remember that the current draw is only a function of the applied torque on the motors, and it is not related to the voltage. So for a fixed torque on the motor, the current draw will be the same regardless of the speed of the motor. Figure 4-4 shows how the output power from the motor is affected by doubling the applied voltage. You can see that increasing the voltage can significantly in - crease the output power of the motor. The maximum power at 36 volts is approxi - mately four times greater than the maximum power at 18 volts. The maximum power of this 18-volt motor is 448 watts, or 0.6 horsepower. By doubling the voltage, this motor has become a 2.5-horsepower brute! Not only does the power increase, so does the motor’s efficiency. The maximum efficiency of the motor at 18 volts is 74.5 percent, and at 36 volts the maximum efficiency is 81.6 percent—a 7 percent increase in efficiency just by doubling the voltage! A big factor in choosing a motor is the conditions under which it will operate. Will the motor run continuously, or will it have a short duty cycle? A motor can be pushed much harder if it is used for a short time and then allowed to cool. In fact, heat is probably the biggest enemy of the PMDC motor. By doubling the motor’s voltage, you can double the top speed of the robot, and you can even double the stall torque of the motor. But be forewarned: These im - provements do not come without a cost. Figure 4-5 shows the heat generated in the motors as the applied torque increases. Doubling the voltage, and therefore 70 Build Your Own Combat Robot FIGURE 4-3 Motor speed and torque changes by doubling the input voltage. the current, increases the heat by a factor of four! Stalling the motor will cause the motors to overheat and be seriously damaged in a short period of time. Nothing is free in the world of physics. Chapter 4: Motor Selection and Performance 71 FIGURE 4-4 Motor power changes by doubling the input voltage. FIGURE 4-5 Heat generated by doubling the applied voltage. [...]... battery performance characteristics used to create Figures 5-2, 5 -3, and 5-4 89 90 Build Your Own Combat Robot Battery Type Rated Capacity Multiply By 6-Minute Capacity 6-Minute Current 6-Minute Voltage NiCad 4.4Ahr 0.90 4.0Ahr 40 amps 10 .3 volts NiMH 6.5Ahr 0.92 6.0Ahr 60 amps 10 .3 volts SLA 12.0Ahr 0 .33 4.1Ahr 41 amps 11.5 volts SLA 17.5Ahr 0 .33 5.8Ahr 58 amps 11.5 volts TABLE 5-1 Battery Performance Characteristics... subjects presented in Chapters 3 through 7 relate to one another Now, this isn’t required—in fact, many robot builders simply pick a motor and build a robot around it If they’re lucky, everything works out just fine However, most robot builders learn the hard way, as things break because they inadvertently pushed components past their capabilities How you choose to build your robot is totally up to you... 73 74 Build Your Own Combat Robot Motor Sources You can acquire electric motors in two ways: you can purchase them from a motor manufacturer or retail store, or you can salvage them from other pieces of equipment Many robot builders use salvaged motors because they usually cost less than 20 percent of the original cost of buying a brand new motor Appendix B in this book lists sources for obtaining robot. .. gearbox, and clutch from a Bosch 18-volt cordless drill reconfigured into a robot gearbox to drive two sprockets FIGURE 4-8 Bosch 18-volt cordless drill motor converted into a robot drive motor 75 76 Build Your Own Combat Robot I nternal Combustion Engines Not all robots use electric motors to drive and power the weapons Some robots use internal combustion engines to perform this important task These... are much lower.) The key is to use the high-temperature insulation Current Minimum AWG 13 amps #20 18 amps #18 20 amps #16 28 amps #14 38 amps #12 53 amps #10 78 amps #8 105 amps #6 142 amps #4 196 amps #2 266 amps #0 TABLE 5 -3 American Wire Gauge Copper Wire Minimum Current Ratings n 91 92 Build Your Own Combat Robot You should use only multi-stranded wires—the more strands, the better Do not use solid... and to coil up 81 82 Build Your Own Combat Robot the wire so that it is easy to handle Temperature causes the resistance to change, so use the wire at room temperature and don’t use it so long that it heats up 1 Place the resistor in series with your robot s battery 2 Measure the voltage across the resistor (a 6.2-foot-long coil of #12 wire, or the high-wattage resistor) with the robot running in normal... large engines in model aircraft C onclusion The motors are the muscles of your robot By understanding how the motors work and how to push them to their limits, you will be able to determine the appropriate motors, the types of batteries, and the appropriate-sized electronic speed controllers for your robot When building your combat robot, the motors are usually the first major component that is selected... different battery types that can be used in combat robots Understanding how well the batteries perform is crucial to your ability to build a winning competition robot B attery Power Requirements The batteries’ primary purpose is to keep your robot powered during the competition These competitions can last up to 5 minutes, so the battery must supply all the power to the robot during that time Selecting an appropriately... (4.1Ahr = 0 .33 × 12Ahr) and will provide an average current of 41 amps (41 = 10 × 4.1Ahr) for the 6-minute duration Typical SLA batteries have a peak current delivery capacity of 10 times its 20-hour capacity In this example, the battery can supply a peak current of 120 amps (120 = 10 × 12Ahr) FIGURE 5-6 Various sealed lead acid batteries (courtesy of Hawker batteries) 93 94 Build Your Own Combat Robot Hawker... confidently run your robot throughout the entire match can be a significant competitive advantage The lightest battery will allow the robot to use the weight savings for other things, such as weapons and armor A properly selected battery will have enough capacity to supply full running current continuously to your robot s motors; and it will be able to supply the peak currents that will allow your robot s . note here is Chapter 4: Motor Selection and Performance 63 4.1 4.2 4 .3 rpm K V K (V I R)==− in in motor vv 4.4 64 Build Your Own Combat Robot that by looking at Equation 4, the voltage must also. at maximum efficiency; otherwise, motors will overheat faster. 66 Build Your Own Combat Robot I 0 1. 934 amps R 0.174 ohms K v 1, 234 .6 rpm/volt K t 1.097 oz-in/amp TABLE 4-1 Motor Constants for. uphill will increase your machine’s power needs. Soft tires that you might use for greater friction have more rolling resistance than hard tires, 68 Build Your Own Combat Robot 4.14 which will