Many people in the field of experimental robots would not think of any way to make their robot move other than using tank-type treads. Others feel the same way about legs, whether two, four, or six. As mentioned earlier, many other means of lo-comotion and propulsion for robots are out there, including flying or swimming, but we’ll concentrate on wheels from this point on. Wheels are pretty much proven in all types of robot applications, from the smallest desktop Sumo machine to the largest mobile industrial robots. Even designers for NASA’s Mars-exploration ro-bots gave up on legs and other means of locomotion in favor of wheels.
Types of Steering
Wheels are generally categorized by steering method and mounting technique.
The two types of steering that are used with wheels are Ackerman steering and dif-ferential steering.
Ackerman Steering
Ackerman steering, also known as car-type steering, is familiar to all of us. Figure 3-3 illustrates several variations of Ackerman steering. Note that only a single motor source drives the wheels, and a separate motor controls the steering. This method uses two wheels in the front turning together to accomplish the turn. Sometimes a single wheel is used, as in some golf carts, or the rear wheels can turn, as in fork-lifts. A child pedaling a tricycle is powering the front wheel, but she is also using that same front wheel to control the direction of movement of the vehicle. This turning method has been used in robot applications, but it is not as popular as the differential drive method that we’ll discuss in a moment.
Ackerman steering is used in radio controlled (R/C) model race cars and in most children’s toys. It requires two sets of commands for control. Quite often, a model race car R/C system will have a small steering wheel on the hand-held trans-mitter to control the steering direction and another joy stick to control the speed, either forward or reverse. This type of steering has the capacity to be more precise than differential steering in following a specific path. It also works best for higher speeds, such as that of real cars of all types and model race cars. Its major disad-vantage is its inability to “turn on a dime,” or spin about its axis. This type of steering has a turning radius that can be onlysosmall; it’s limited by the front-rear wheel separation and angle that the front wheels can turn.
Differential Steering
Differential steering, sometimes called “tank-type” steering, is not to be confused with tank treads. The similarity is in the way an operator can separately control the speeds of the left and right wheels to cause a directional change in the motion of the robot. Figure 3-4 illustrates how controlling the speed and direction of both wheels with differential steering can result in all types of directional motion for the robot. Note that each of the two separately driven side wheels has its own motor, and no motor is required to turn any wheels to steer.
With differential steering, spinning on the robot’s axis is accomplished by mov-ing one wheel in one direction and the other in the opposite direction. A sharp turn is accomplished by stopping one wheel while moving the other forward or back-ward, and the result is a turn about the axis of the stopped wheel. Shallower turns are accomplished by moving one wheel at a slower speed than the other wheel,
FIGURE 3-3 Variations of Ackerman steering.
making the robot turn in the direction of the slower wheel. Variations in between can cause an infinite variety of turns. This type of control is most favored by re-mote-controlled robots on the battle floor and by promotional robots you might see in advertising. The wheels versus treads controversy has produced a design variation that does not use the free-moving caster illustrated in Figure 3-4, but in-stead uses a series of side-mounted wheels, similar to the idlers pressing downward on the inside of tank treads. See Figure 3-5. Some or all of the wheels on each side may be powered with a separate motor attached to each wheel, or with each set of wheels on either side interconnected by a single chain or belt drive, and a single motor per side. Yes, this method is not energy efficient for the same reason tank treads eat batteries—the front and rear wheels must skid in turns.
Chapter 13 shows you the construction techniques that were used to build the robotLive Wires.This four-wheeled combat robot was built on two cordless drill motors, one for each of its sides. For safety purposes, two drive sprockets on each drill motor were used with a separate chain going to each of the two racing go-kart wheels on either side of the motor. If one chain was broken,Live Wiresstill had mobility, and the differential steering capability was left mostly intact.
The multi-wheel platform does have an advantage: it can provide a lot of trac-tion with a low-profile robot fitted with small wheels. To achieve this tractrac-tion, however, the builder should independently spring each wheel a small amount to prevent high-centering, which can occur when the bottom of the robot gets caught on some obstruction, leaving the wheels lifted off the ground. For example, a four-wheel-drive vehicle can get high-centered after driving the front wheels over a large tree. If the vehicle gets stuck on the tree between the wheels, the wheels can’t get the traction needed to get off the tree.
FIGURE 3-4 Differential steering
High-centering is a greater problem with a typical two-side-wheel differential bot setup, where a front or rear caster is raised enough to bring the driving wheels off the floor. If all driven wheels are used to provide extra traction, accidentally raising one or more wheels reduces the available traction that a combat robot may need to defeat its opponent. When using casters in the front and rear of a differen-tially driven robot, you should have each of them spring-loaded to prevent the robot from rocking back and forth, but not too much so that the robot might be lifted off its drive wheels.
Wheel Configurations
Some of the several methods and configurations of wheel mounting are more ap-plicable to unique terrain conditions such as the “rocker bogie” system used on some of the Mars robot rovers developed at NASA’s Jet Propulsion Labs. The pre-decessors to the famous Sojourner robot that roved about Mars’s surface were named various forms of “Rocky,” after the wheel-mounting system used. This system employs two pairs of wheels mounted on swivel bars that can help the wheels conform to uneven surfaces.
In smaller robots, many experimenters mount the wheels directly to the output shaft of the gearmotor. This works fine for the light robots that are designed to follow lines on the floor or run mazes, but it doesn’t work well for larger machines, espe-cially combat robots that take a lot of abuse in their operation. The output shaft of most gearmotors may have a sintered bronze bushing on the output side, and many times such a shaft does not have any sort of bearing on the internal side of the gearcase. This type of shaft support is not made to take the side-bending mo-ment placed upon it by wheels and heavy loads.Bending moment is the name of the force that is trying to snap the shaft in two when one bearing is pressed
down-FIGURE 3-5 A robot design using a series of side-mounted wheels.
ward as the other bearing is forced upward. Bending moment forces on a robot’s wheel in combat are sometimes so severe that a gearmotor’s gearcase can be shat-tered, even if ball bearings are on both sides of the gearcase.
One unique configuration of wheel mounting can possibly save you if your ma-chine is ever flipped onto its back. Several robots have used identical sets of wheels on both the top and bottom, with mirror-image sets of top and bottom body shells; this allows the robot to continue its mobility while “upside-down.” The other, more popular, method is to add wheels of sufficient diameter to protrude equally above the top surface, thus allowing continued mobility while “up-side-down.” This system works well for the low-profile machines; but for larger machines, it obviously gets a bit more complicated because huge monster truck-style wheels might obstruct a robot’s mobility. For these types of bots, a top-flipping actuator can be used to right the robot after a flip.
Selecting Wheels for Your Combat Robot
Wheels are one of the most important considerations in the design of your robot.
They are your robot’s contact with the rest of the real world—namely, the battle area’s floor. They allow your robot to move, maneuver, and attack its opponent, as well as retreat from an unfavorable position. Knowing this, your opponent will do everything he can to remove your robot’s maneuvering ability, something you should also do to his robot at every opportune moment. So the words “sturdy,”
“tough,” “puncture-proof,” and “reliable” should all come to mind when you select wheels for your combat robot. And sometimes a wheel just looks too cool not to be used on the robot—take a look at Figure 3-6.
FIGURE 3-6 A 14-inch diameter,
flat proof, treaded wheel.(courtesy of National Power Chair, Inc.)
You must also remember that the floor in a combat robot arena is not exactly like Grandma’s living room floor. It includes some of the most destructive and de-vious hazards the contest producers can conjure up in their sadistic minds.
Metal-cutting saw blades, spikes, hammers and even water can all come together to ruin your robot’s day. You shouldn’t waste time worrying that another ma-chine or the hazards operator will attack your pride and joy in a contest.It will happen.Prepare for the worst. Have a wheel configuration and tire construction that will survive far more abuse than you can deliver in your garage tests, as you will be shocked at what a full-blown match can do to your machine.
You might be looking at a set of 20-inch bicycle tires for possible use in your ro-bot, thinking, “If a 150-pound bike rider can jump over curbs and logs for days on end, tires like these should survive a 3-minute robot battle.” If you watch a few ro-bot combats, though, you’ll see that wheel failure is not caused by downward force or even force from the front of the machine. What kills wheels is force from the side, hitting one side of the wheel, and bending or breaking the shaft or hub. A killer robot will “taco” a bike tire in seconds, or shred its spokes. Leave bike tires for benign robot designs.
Another favorite wheel of the beginning robot builder is the kind found on lawnmowers and other garden tools. Their ability to bounce over rough ground may seem to make them good potential robot wheels, but the same applies here as in bike tires. They cannot take side-bending forces. Most of the newer types use cheap plastic rims instead of metal. You find wheels and tires from so many sources—such as toys, disability equipment, hand-held golf carts, and barbe-cues—that we will not further elaborate. Consider the original intended use of the equipment and the expected loads the design team might have considered. Many companies have cut quality in areas to compete in the market pricewise. Look at all parts of the wheels you intend to use. Be cautious and use good sense here.
One of the best sources of tires and wheels for combat robots is from industrial applications. The hard rubber tires used in industrial parts carts made to handle thousands of pounds are among the best. Aerospace surplus yards generally have several varieties of these wheels, both mounted and unmounted. These wheels have stout rims and extremely tough tires. Some are non-rotating types and others are mounted in swivel assemblies as large casters. Most of these industrial wheels do not have any sort of tread, as they are used in passive applications that do not require traction.
Figure 3-7 shows a heavy-duty drive wheel.
One of the most popular wheel types used in combat robots are go-kart wheels, which come in a wide variety of rim and wheel types and shapes. They are readily available and easy to mount to a robot. Many top competitive ro-bots use go-kart wheels.
Tires
In addition to wheels, you need to carefully consider the rim and tire of your ro-bot’s assembly. The tire or rubber part of the wheel is probably the most critical consideration, because it is the most exposed part and takes the most abuse. It is the part that will encounter the kill saws at some point in aBattleBot competi-tion. Tire hazards wreck more robots than all the rest of the hazards combined.
Imagine what an opponent’s weapon or a kill saw can do to your intended wheels. How secure is the rubber mounted to the rim? Will the rubber stay on the rim if it’s partially shredded? How easily can the rubber be shredded? Are the tires pneumatic and can they be “popped?” If one or more wheels have a series of gashes in them, can you still maneuver your robot or allow it to escape your oppo-nent or the hazard to regroup? Can the tire be struck from the side and be knocked off? You must ask yourself these and many other questions before you select the tires used.
You may like a particular wheel/tire combination that you’ve located and want to make it a bit more resilient to the onslaught it will be facing on the battle floor.
You see a pneumatic that is the right size and has good traction, but you realize that it can easily be punctured and flattened, or it can be shredded by some weapon or hazard. In this case, consider filling the tire with a pliable rubber epoxy instead of air. The epoxy will bond to the inner part of the rim, and at the same
FIGURE 3-7 An 8-inch diameter
heavy-duty drive wheel.(courtesy of National Power Chair, Inc.)
time hold the inner part of the tire together, resulting in a puncture-proof combi-nation. Another option is to fill the tires with foam, which a lot of experienced ro-bot builders use to keep down the weight of their roro-bots.
Traction on the combat floor is important. Go-kart tires are made for extremely hard use, and their fairly soft surface has pretty good traction (see Figure 3-8).
Many of the pneumatic tires you might find in surplus houses or hardware stores have molded treads for traction purposes. The industrial cart tires mentioned earlier with the hard rubber tires are not pneumatic, but they can be modified with grooves, which some builders believe give traction to the wheels. Cutting with a knife or saw is not recommended, though, as any sharp cuts or gouges can easily propagate into a crack that can eventually sever the tire. Grinding the grooves is rec-ommended instead.
Mounting and Supporting the Wheels and Axles
The mounting of the wheel to the axle and other parts of the locomotive system is the next important consideration. Not only must the complete wheel assembly be securely attached to the axle, but the wheel should ideally be able to be rapidly moved if repairs and replacements are necessary between matches. An easily re-movable wheel can make the difference in winning or losing a competition. You can attach wheels to robot platforms in numerous ways. Attachment methods de-pend on the wheel configuration desired. A typical arrangement might be the one illustrated in Figure 3-9. Many robot designs involve some sort of metal box chas-sis with internal motors and associated equipment, and external wheels attached
FIGURE 3-8
Go-kart wheels give a robot the look of a racing car.
to axles protrude from the “box.” Fortunately for the combat robot designer, the terrain that the robot is to traverse is usually a flat floor with little deviations from level. A few bumps may result from joining floor surfaces, and some of the hazards present an uneven surface area in small spots. However, for the most part, the floor is flat in virtually all of the popular contests.
Such surfaces may not remain the case for future events, though, so a prospective designer may want to take into consideration possible variations in floor flatness.
Some present-day contests, such asRobotica, have ramps for the competing ro-bots to traverse, so builders must plan for a sudden change of the operating plane.
The robot may be high-centered as it starts up a ramp or reaches the top, so flexi-ble wheel mounting (where wheels can adapt to severely differing floor angles) is a must in these scenarios. Quite often, placing the driving wheels at the extreme ends can allow a robot to startupa ramp, but this same arrangement might not prevent high-centering as the robot reaches the top and teeters in that position. A series of driven middle wheels would give the robot the final push out of such a sit-uation, but many of the machines rely on inertia built up from speed to “dive”
over such obstacles.
Mounting Axles Using Various Types of Bearings
Certain styles of bearings seem to be a bit more popular than other types for robot use, especially in mounting axles for wheels. These are the pillow block and flange mount bearings. Some catalogs refer to pillow block bearings as those with a base mount, while other companies call pillow block bearings any configuration that has holes in a flange or base to bolt onto a surface.
FIGURE 3-9 A typical wheel
configuration arrangement where an axle is supported by two pillow block bearings. A sprocket is located between the pillow blocks, and the wheel is located to one side of the pillow blocks.
Throughout this text, we’ll refer to pillow block bearings as those with a rigid mount or base mount that supports the shaft in a position parallel to the surface on which the bearing is mounted. We’ll use the term “flange mount” bearings for those that have two or four holes, and mount the shaft perpendicular to and
Throughout this text, we’ll refer to pillow block bearings as those with a rigid mount or base mount that supports the shaft in a position parallel to the surface on which the bearing is mounted. We’ll use the term “flange mount” bearings for those that have two or four holes, and mount the shaft perpendicular to and