McGraw-Hill - Robot Mechanisms and Mechanical Devices Illustrated - 2003 Part 10 ppt

T he Romans extensively used two wheeled carts, pulled by horses. Pull on the right rein and the horse pulls the cart to the right, and vise versa. The two wheels on the cart were mounted on the same axle, but were attached in a way that each wheel could rotate at whatever speed was needed depending on whether the cart was going straight or around a corner. Carts got bigger and eventually had four wheels, two in front and two in back. It became apparent (though it is unclear if it was the Romans who figured this out) that this caused problems when trying to turn. One or the other set of wheels would skid. The simplest method for fixing this problem was to mount the front set of wheels on each end of an axle that could swivel in the middle (Figure 6-1). A tongue was attached to the axle and stuck out from the front of the vehicle, which in turn was attached to a horse. Pulling on the tongue aligned the front wheels with the turn. The back wheels followed. This method worked well and, indeed, still does for four wheeled horse drawn buggies and carriages. 189 Figure 6-1 Pivot mounted front wheels 190 Chapter 6 Steering History In the early 1800s, with the advent of steam engines (and, later, elec- tric motors, gas engines, and diesel engines) this steering method began to show its problems. Vehicles were hard to control at speeds much faster than a few meters per second. The axle and tongue took up a lot of room swinging back and forth under the front of the vehicle. An attempt around this problem was to make the axle long enough so that the front wheels didn’t hit the cart’s sides when turning, but it was not very con- venient having the front wheels wider than the rest of the vehicle. The first effective fix was to mount the two front wheels on a mecha- nism that allowed each wheel to swivel closer to its own center. This saved space and was easier to control and it appeared to work well. In 1816, George Lankensperger realized that when turning a corner with the wheels mounted using that geometry the inside wheel swept a differ- ent curve than the outside one, and that there needed to be some other mechanical linkage that would allow this variation in alignment. He teamed with Rudolph Ackerman, whose name is now synonymous with this type of steering geometry. Although Ackerman steering is used on almost every human controlled vehicle designed for use on roads, it is actually not well suited for high mobility vehicles controlled by comput- ers, but it feels right to a human and works very well at higher speeds. It turns out there are many other methods for turning corners, some intu- itive, some very complex and unintuitive. STEERING BASICS When a vehicle is going straight the wheels or tracks all point in the same direction and rotate at the same speed, but only if they are all the same diameter. Turning requires some change in this system. A two- wheeled bicycle (Figure 6-2) shows the most intuitive mechanism for performing this change. Turn the front wheel to a new heading and it rolls in that direction. The back wheel simply follows. Straighten out the front wheel, and the bicycle goes straight again. Close observation of a tricycle’s two rear wheels demonstrates another important fact when turning a corner: the wheel on the inside of the corner rotates slower than the outside wheel, since the inside wheel is going around a smaller circle in the same amount of time. This important detail, shown in Figure 6-3, occurs on all wheeled and tracked vehicles. If the vehicle’s wheels are inline, there must be some way to allow the wheels to point in different directions. If there are wheels on either side, they must be able to rotate at different speeds. Any deviation from this Chapter 6 Steering History 191 Figure 6-2 Bicycle steering Figure 6-3 Tricycle steering 192 Chapter 6 Steering History and some part of the drive train in contact with the ground will have to slide or skid. Driving straight in one direction requires at least one single direction actuator. A wind-up toy is a good demonstration of this ultra-simple drive system. Driving straight in both directions requires at least one bi- directional actuator or two single-direction actuators. One of those single direction actuators can power either a steering mechanism or a second drive motor. Add one more simple single-direction motor to the wind-up toy, and it can turn to go in any new direction. This shows that the least number of actuators required to travel in any direction is two, and both can be single-direction motors. In practice, this turns out to be quite limiting, at least partly because it is tricky to turn in place with only two single direction actuators, but mostly because there aren’t enough drive and steer options to pick from to get out of a tight spot. Let’s investigate the many varieties of steering commonly used in wheeled and tracked robots. The simplest statically stable vehicle has either three wheels or two tracks, and the simplest power system to drive and steer uses only two single-direction motors. It turns out that there are only two ways to steer these very simple vehicles: 1. Two single-direction motors powering a combined drive/steer wheel or combined drive/steer track with some other passive wheels or tracks 2. Two single-direction motors, each driving a track or wheel (the third wheel on the wheeled layout is a passive swivel caster) The simplest version of the first steering geometry is a single-wheel drive/steer module mounted on a robot with two fixed wheels. The com- mon tricycle uses this exact layout, but so do some automatic guided vehicles (AGVs) used in automated warehouses. Mobility is limited because there is only one wheel providing the motive force, while drag- ging two passive wheels. This layout works well for the AGV application because the warehouse’s floor is flat and clean and the aisles are designed for this type of vehicle. In an AGV, the drive/steer module usu- ally has a bi-directional steering motor to remove the need to turn the drive wheel past 180° but single direction steer motors are possible. There are many versions of AGVs—the most complicated types have four drive/steer modules. These vehicles can steer with, what effectively amounts to, any common steering geometry; translate in any direction without rotating (commonly called “crabbing”), pseudo-Ackerman steer, turn about any point, or rotate in place with no skidding. Wheel modules Chapter 6 Steering History 193 for AGVs are available independently, and come in several sizes ranging from about 30 cm tall to nearly a meter tall. The second two-single-direction motor steering layout has been suc- cessfully tried in research robots and toys, but it doesn’t provide enough options for a vehicle moving around in anything but benign environ- ments. It can be used on tracked vehicles, but without being able to drive the tracks backwards, the robot can not turn in place and must turn about one track. Figure 6-4 shows this limitation in turning. This may be acceptable for some applications, and the simplicity of single direction electronic motor-driver may make up for the loss of mobility. The biggest advantage of both of these drive/steering systems is extreme sim- plicity, something not to be taken lightly. The Next Step Up The next most effective steering method is to have one of the actuators bi-directional, and, better than that, to have both bi-directional. The Rug Warrior educational robot uses two bi-directional motors—one at each wheel. This steering geometry (Figure 6-5a, 6-5b) is called differential steering. Varying the relative speed, between the two wheels turns the robot. On some ultra-simple robots, like the Rug Warrior, the third wheel does not even swivel, it simply rolls passively on a fixed axle and skids when the robot makes a turn. Virtually all modern two-tracked Figure 6-4 Turning about one track 194 Chapter 6 Steering History vehicles use this method to steer, while older tracked vehicles would brake a track on one side, slowing down only that track, which turned the vehicle. As discussed in the chapter on wheeled vehicles, this is also the steer- ing method used on some four-wheel loaders like the well-known Bobcat. One motor drives the two wheels on one side of the vehicle, the other drives the two wheels on the other side. This steering method is so effective and robust that it is used on a large percentage of four-, six-, and even eight-wheeled robots, and nearly all modern tracked vehicles whether autonomous or not. This steering method produces a lot of skid- Figure 6-5a Differential steering Figure 6-5b Chapter 6 Steering History 195 ding of the wheels or tracks. This is where the name “skid steer” comes from. The fact that the wheels or tracks skid means this system is wasting energy wearing off the tires or track pads, and this makes skid steering an inefficient design. Placing the wheels close together or making the tracks shorter reduces this skidding at the cost of fore/aft stability. Six-wheeled skid-steering vehicles can place the center set of wheels slightly below the front and back set, reducing skidding at the cost of adding wobbling. Several all-terrain vehicle manufacturers have made six-wheeled vehi- cles with this very slight offset, and the concept can be applied to indoor hard-surface robots also. Eight-wheeled robots can benefit from lower- ing the center two sets of wheels, reducing wobbling somewhat. The single wheel drive/steer module discussed earlier and shown on a tricycle in Figure 6-6 can be applied to many layouts, and is, in general, an effective mechanism. One drawback is some inherent complexity with powering the wheel through the turning mechanism. This is usually accomplished by putting the drive motor, with a gearbox, inside the wheel. Using this layout, the power to the drive motor is only a couple wires and signal lines from whatever sensors are in the drive wheel. These wires must go through the steering mechanism, which is easier than passing power mechanically through this joint. In some motor-in- wheel layouts, particularly the syncro-drive discussed next, the steering Figure 6-6 Drive/steer module on tricycle 196 Chapter 6 Steering History mechanism must be able to rotate the drive wheel in either direction as much as is needed. This requires an electrical slip ring in the steering joint. Slip rings, also called rotary joints, are manufactured in both stan- dard sizes or custom layouts. One type of mechanical solution to the problem of powering the wheel in a drive/steer module has been done with great success on sev- eral sophisticated research robots and is commonly called a syncro- drive. A syncro-drive (Figure 6-7) normally uses three or four wheels. All are driven and steered in unison, synchronously. This allows fully holonomic steering (the ability to head in any direction without first requiring moving forward). As can be seen in the sketch, the drive motor is directly above the wheel. An axle goes down through the cen- ter of the steering shaft and is coupled to the wheel through a right angle gearbox. This layout is probably the best to use if relying heavily on dead reck- oning because it produces little rotational error. Although the dominant dead-reckoning error is usually produced by things in the environment, this system theoretically has the least internal error. The four-wheeled layout is not well suited for anything but flat terrain unless at least one wheel module is made vertically compliant. This is possible, but would produce the complicated mechanism shown in Figure 6-8. Figure 6-7 Synchronous drive Chapter 6 Steering History 197 All-terrain cycles (ATCs), when they were legal, ran power through a differential to the two rear wheels, and steered with the front wheel in a standard tricycle layout. ATCs clearly pointed out the big weakness of this layout, the tendency to fall diagonally to one side of the front wheel in a tight turn. Mobility was moderately good with a human driver, but was not inherently so. Quads are the answer to the stability problems of ATCs. Four wheels make them much more stable, and many are produced with four wheel drive, enhancing their mobility greatly although they cannot turn in place. They are, of course, designed to be controlled by humans, who can foresee obstacles and figure out how to maneuver around them. If a mobility system in their size range is needed, they may be a good place to start. They are mass-produced, their price is low, and they are a mature product. Quads are manufactured by a number of companies and are available in many size ranges offering many different mobility capabilities. As the number of wheels goes up, so does the variety of steering methods. Most are based on variations of the types already mentioned, but one is quite different. In Figure 4-30 (Chapter Four), the vehicle is divided into 2 sections connected by a vertical axis joint. This layout is common on large industrial front-end loaders and provides very good steering ability even though it cannot turn in place. The layout also Figure 6-8 Drive/steer module with vertical compliance 198 Chapter 6 Steering History forces the sections to be rather unusually shaped to allow for tighter turn- ing. Power is transferred to the wheels from a single motor and differen- tials in the industrial version, but mobility would be increased if each wheel had its own motor. [...]... used with six- or more-legged robots, but is not very common in robots because of the large numbers of joints and actuators Independent Leg Walking Virtually all other legged animals in nature that don’t use wave walking can control each leg independently Some animals are better than others, but the ability is there Figures 7-8 and 7-9 show four- and six-legged walkers with three rotary-actuated joints... eight-leg layout would have no less than 24 actuators The four- and six-legged versions Chapter 7 Walkers 209 Figure 7-8 Independent leg walker, four legs, twelve DOF Figure 7-9 Independent leg walker, six legs, eighteen DOF 210 Chapter 7 Walkers Figure 7-1 0 Extra wide feet provide two-legged stability theoretically have very high mobility Many research robots have been built that use four or six legs and. .. of the segments 206 Chapter 7 Walkers Figure 7-4 Two-DOF leg using rotary actuators Figure 7-5 Two-DOF leg using cable driven actuators Chapter 7 Walkers 207 Figure 7-6 Three-DOF leg using linear actuators Figure 7-7 Three-DOF leg using rotary actuators 208 Chapter 7 Walkers and the relative location of each actuator It is quite difficult to drive a two-DOF hip joint with cables, but it can be done... Chapter 7 Walkers Figure 7-1 2 Traversing/rotating frame eight-leg frame walker with single-DOF legs Figure 7-1 3 Eight-leg frame walker with two-DOF legs Chapter 7 Walkers like feet, making it quasi-statically stable Figure 7-1 2 shows an implementation of the traversing/rotating frame with simple one-DOF legs This layout has 10 DOF Figure 7-1 3 removes the rotating joint, which forces placing a second... Figure 7-3 shows a mechanism that keeps the second leg segment vertical as it is raised and lowered The actuator can be replaced with a passive link, making this a one-DOF leg whose second segment doesn’t swing out as much as the leg shown in Figure 7-2 Walkers 203 204 Chapter 7 Walkers Figure 7-1 One-DOF leg for frame walkers Figure 7-2 Two-DOF leg using linear actuators Chapter 7 Walkers 205 Figure 7-3 ... robot would lack the ability to stand level on uneven terrain Perhaps the best is a six-leg tripodgait frame walker with one linear DOF in each leg and two in the coupling, bringing the total DOF to eight Figure 7-1 4 shows just such a layout, perhaps the best walking layout to start with if designing a walking robot Figure 7-1 4 Six-legged tripod-frame walker with single-DOF legs 213 ... power to be useful iRobot’s Genghis robot used two hobby servos bolted together, acting as rotary actuators, to get a very effective twoaxis hip joint This robot, and several others like it, use simple straight legs These simple walker layouts are useful preliminary tools for those interested in studying six-legged walking robots To turn the two-DOF linear actuator layout into a three-DOF, a universal... foot can be placed anywhere it can reach and the robot will not fall over The wide feet must also prevent tipping over sideways and are so wide that they overlap each other and must be carefully shaped and controlled so they don’t step on each other Two-legged walking, with oversized and overlapping feet, is simply picking up the back foot, bringing it forward, and putting it down The hip joints require... example of a dynamically-stable walker in nature is, in fact, any two-legged animal They must get their feet in the right place when they want to stop walking to prevent tipping over Two-legged dinosaurs, humans, and birds are remarkably capable two legged walkers, but any child that has played Red-light/Green-light or Freeze Tag has figured out that it is quite difficult to stop mid-stride without falling... walking technique that must be mentioned, although it is simple in concept, for walking robots, it is less efficient than other methods The robot lifts its rear-most set of legs and swings them forward and sets them down, then the next set of legs is moved similarly When the front-most set of legs is moved, the whole robot chassis is moved forward relative to the legs The process can be smoothed out some . there. Figures 7-8 and 7-9 show four- and six-legged walkers with three rotary-actuated joints in each leg. An eight-leg layout would have no less than 24 actuators. The four- and six-legged versions. Walkers Figure 7-4 Two-DOF leg using rotary actuators Figure 7-5 Two-DOF leg using cable driven actuators Chapter 7 Walkers 207 Figure 7-6 Three-DOF leg using linear actuators Figure 7-7 Three-DOF leg. actuators bi-directional, and, better than that, to have both bi-directional. The Rug Warrior educational robot uses two bi-directional motors—one at each wheel. This steering geometry (Figure 6-5 a, 6-5 b)