Paul E. Sandin - Robot Mechanisms and Mechanical Devices Illustrated (779750), страница 26
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Vehicles with relatively lowground pressure will perform better on softer materials like loose sand,snow, and thick mud. Those with high pressures mostly perform betteron harder packed materials like packed snow, dirt, gravel, and commonroad surfaces. The best example of this fact are vehicles designed totravel on both hard roads and sand. The operator must stop and deflatethe tires, reducing ground pressure, as the vehicle is driven off a road andonto a stretch of sand. Several military vehicles like the WWII amphibious DUKS were designed so tire pressure could be adjusted from insidethe cab, without stopping.
This is now also possible on some modifiedHummers to extend their mobility, and might be a practical trick for awheeled robot that will be working on both hard and soft surfaces.This also points to the advantage of maintaining as even a groundpressure as possible on all tires, even when some of them may be liftedup onto a rock or fallen tree. Suspension systems that do this well willtheoretically work better on a wider range of ground materials.Suspension systems that can change their ground pressure in response tochanges in ground materials, either by tire inflation pressure, variablegeometry tires, or a method of changing the number of tires in contactwith the ground, will also theoretically work well on a wider range ofground materials.This chapter focuses on suspension systems that are designed to workon a wide range of ground materials, but it also covers many layouts thatare excellent for indoor or relatively benign outdoor environments.
Thelatter are shown because they are simple and easy to implement, allowing a basic mobile platform to be quickly built to ease the process of getting started building an autonomous robot. Vehicles intended for use inany arbitrary outdoor environment tend to be more complicated, butsome, with acceptably high mobility, are surprisingly simple.SHIFTING THE CENTER OF GRAVITYA trick that can be applied to mobile robots that extends the robot’smobility, independent of the mobility system, is to move the center ofgravity (cg) of the robot, thereby changing which wheels, tracks, or legsare carrying the most weight.
A discussion of this concept and some lay-131132Chapter 4Wheeled Vehicle Suspensions and Drivetrainsouts are included in this chapter, but the basic concept can be applied toalmost any mobile robot.Shifting the center of gravity can be accomplished by moving a dedicated weight, shifting the cargo, or reorienting the manipulator. Movingthe cg can allow the robot to move across wider gaps, climb steeperslopes, and get over or onto higher steps. If it is planned to move themanipulator, then the manipulator must make up a significant fraction ofthe total weight of the vehicle for the concept to work effectively. Whilemoving the cg seems very useful, all but the manipulator techniquerequire extra space in the robot for the weight and/or mechanism thatmoves the weight.The figures show the basic concept and several variations of cg shifting that might be tried if no other mobility system can be designed tonegotiate a required obstacle, or if the concept is being applied as a retrofit to extend an existing robot’s mobility.
Functionally, as a gap in the terrain approaches, the cg is shifted aft, allowing the mobility system’sfront ground contact point to reach across the gap without the robot tipping forward. When those parts reach the far side of the gap, the robot isdriven forward until it is almost across, then the cg is shifted forward,lifting the rear ground contact points off the ground. The vehicle is thendriven across the gap the rest of the way.For stair climbing or steep slopes, the cg is shifted forward so itremains over the center of area of the mobility system.
For climbing up asingle bump or step, it is shifted back just as the vehicle climbs onto thestep. This reduces the tendency of the robot to slam down on the frontparts of the mobility system. It must be noted that cg shifting can be controlled autonomously fairly easily if there is an inclinometer oraccelerometer onboard the robot that can give inclination.
The controlloop would be set to move the cg in relation to the fore and aft tilt of therobot. In fact, it might be possible to make the cg shifting system completely automatic and independent of all other systems on the robot, butno known example of this has been tested. Figures 4-1 and 4-2 show twobasic techniques for moving the cg.The various figures in this chapter show wheel layouts without showing drive mechanisms. The location of the drive motor(s) is left to thedesigner, but there are a few unusual techniques for connecting the drivemotor to the wheels that affect mobility that should be discussed. Someof the figures show the chassis located in line with the axles of thewheels, and some show it completely above the wheels, which increasesground clearance at the possible expense of increased complexity of thecoupling mechanism.
In many cases, the layouts that show the chassisdown low can be altered to have it up high, and vise-versa.Chapter 4Wheeled Vehicle Suspensions and Drivetrains133Figure 4-1 Method for shiftingthe center of gravity on a linearslideFigure 4-2 Shifting the cg on aswinging arm134Chapter 4Figure 4-3wheel hubWheeled Vehicle Suspensions and DrivetrainsGeared offsetFor the raised layouts, the drive axle is coupled to the wheel through achain, belt drive, or gearbox. The US Army’s High MobilityMultipurpose Wheeled Vehicle (HMMWV, HumVee, or Hummer), usesgeared offset hubs (Figure 4-3) resulting in a ground clearance of 16"with tires that are 37" in diameter.
This shows how effective the raisedchassis layout can be.WHEEL SIZEIn general, the larger the wheel, the larger the obstacle a given vehiclecan get over. In most simple suspension and drivetrain systems, a wheelwill be able to roll itself over a step-like bump that is about one-third thediameter of the wheel. In a well-designed four-wheel drive off-roadtruck, this can be increased a little, but the limit in most suspensions issomething less than half the diameter of the wheel.
There are waysaround this though. If a driven wheel is pushed against a wall that istaller than the wheel diameter with sufficient forward force relative to thevertical load on it, it will roll up the wall. This is the basis for the designof rocker bogie systems.Chapter 4Wheeled Vehicle Suspensions and DrivetrainsThree wheels are the minimum required for static stability, and threewheeled robots are very common.
They come in many varieties, fromvery simple two-actuator differential steer with fixed third wheel types,to relatively complex roller-walkers with wheels at the end of two oreven three DOF legs. Mobility and complexity are increased by addingeven more wheels. Let’s take a look at wheeled vehicles in rough orderof complexity.The most basic vehicle would have the least number of wheels.Believe it or not, it is possible to make a one-wheeled vehicle! This vehicle has limited mobility, but can get around relatively benign environments. Its wheel is actually a ball with an internal movable counterweight that, when not over the point of contact of the ball and theground, causes the ball to roll.
With some appropriate control on thecounterweight and how it is attached and moved within the ball, the vehicle can be steered around clumsily. Its step-climbing ability is limitedand depends on what the actual tire is made of, and the weight ratiobetween the tire and the counterweight.There are two obvious two wheeled layouts, wheels side by side, andwheels fore and aft. The common bicycle is perhaps one of the most recognized two-wheeled vehicles in the world.
For robots, though, it is quitedifficult to use because it is not inherently stable. The side by side layoutis also not inherently stable, but is easier to control, at low speeds, than abike. Dean Kamen developed the Segway two-wheeled balancing vehicle, proving it is possible, and is actually fairly mobile. It suffers fromFigure 4-4Bicycle135136Chapter 4Figure 4-5Wheeled Vehicle Suspensions and DrivetrainsTail draggerthe same limitation the single wheeled ball suffers from and cannot getover bumps much higher than one quarter a wheel height.The third, less obvious layout is to drag a passive leg or tail behind thevehicle. This tail counteracts the torque produced by the wheels, makesthe vehicle statically stable, and increases, somewhat, the height ofobstacle the robot can get over.
The tail dragger is ultra-simple to controlby independently varying the speed of the wheels. This serves to controlboth velocity and steering. The tail on robots using this layout must belight, strong, and just long enough to gain the mobility needed. Too longand it gets in the way when turning, too short and it doesn’t increasemobility much at all. It can be either slightly flexible, or completely stiff.The tail end slides both fore and aft and side to side, requiring it to be ofa shape that does not hang up on things.
A ball shape, or a shape verysimilar, made of a low friction material like Teflon or polyethylene, usually works out best.THREE-WHEELED LAYOUTSThe tail dragger demonstrates the simplest statically stable wheeledvehicle, but, unfortunately, it has limited mobility. Powering that thirdChapter 4Wheeled Vehicle Suspensions and Drivetrainscontact point improves mobility greatly.
Three wheels can be laid out inseveral ways. Five varieties are pictured in the following figures. Themost common and easiest to implement, but with, perhaps, the leastmobility of the five three-wheeled types, is represented by a child’s tricycle. On the kid-powered version, the front wheel provides both propulsion and steers. Robots destined to be used indoors, in a test lab or othercontrollable space, can use this simple layout with ease, but it hasextremely poor mobility. Just watch any child struggling to ride their tricycle on anything but a flat smooth road or sidewalk.
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