Paul E. Sandin - Robot Mechanisms and Mechanical Devices Illustrated (779750), страница 25
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This method works very well for robots that travel at slow speeds.In a case where the rotating shaft suddenly jams or becomes overloaded for some unexpected reason, the torque in the shaft could breakthe shaft, the gearbox, or some other part of the rotating system.Installing a device that brakes first, particularly one that isn’t damagedwhen it is overloaded, is sometimes required. This mechanical device iscalled a torque limiter.There are many ways to limit torque.
Magnets, rubber bands, frictionclutches, ball detents, and springs can all be used in one way or another,and all have certain advantages and disadvantages. It must be remembered that they all rely on giving off heat to absorb the energy of stopping the rotating part, usually the output shaft.
Figures 3-44 through 3-53show several torque limiters, which are good examples of the wide variety of methods available.TEN TORQUE-LIMITERSFigure 3-44 Permanent magnets transmit torque in accordance with their numbers andsize around the circumference ofthe clutch plate. Control of thedrive in place is limited to removing magnets to reduce the drive’storque capacity.122Chapter 3Direct Power Transfer DevicesFigure 3-45 Arms hold rollers inthe slots that are cut across thedisks mounted on the ends ofbutting shafts. Springs keep theroller in the slots, but excessivetorque forces them out.Figure 3-46 A cone clutch isformed by mating a taper on theshaft to a beveled central hole inthe gear.
Increasing compressionon the spring by tightening thenut increases the drive’s torquecapacity.Figure 3-47 A flexible beltwrapped around four pins transmits only the lightest loads. Theouter pins are smaller than theinner pins to ensure contact.Chapter 3Direct Power Transfer Devices123Figure 3-48 Springs inside theblock grip the shaft because theyare distorted when the gear ismounted to the box on the shaft.Figure 3-49 The ring resists thenatural tendency of the rollers tojump out of the grooves in thereduced end of one shaft. Theslotted end of the hollow shaftacts as a cage.Figure 3-50 Sliding wedgesclamp down on the flattened endof the shaft.
They spread apartwhen torque becomes excessive.The strength of the springs intension that hold the wedgestogether sets the torque limit.124Chapter 3Direct Power Transfer DevicesFigure 3-51 Friction disks arecompressed by an adjustablespring. Square disks lock into thesquare hole in the left shaft, andround disks lock onto the squarerod on the right shaft.Figure 3-52 Friction clutchtorque limiter. Adjustable springtension holds the two friction surfaces together to set the overloadlimit.
As soon as an overload isremoved, the clutch reengages. Adrawback to this design is that aslipping clutch can destroy itself ifit goes undetected.Figure 3-53 Mechanical keys. Aspring holds a ball in a dimple inthe opposite face of this torquelimiter until an overload forces itout. Once a slip begins, clutchface wear can be rapid.
Thus,this limiter is not recommendedfor machines where overload iscommon.Chapter 3Direct Power Transfer Devices125ONE TIME USE TORQUE LIMITINGIn some cases, the torque limit can be set very high, beyond the practical limit of a torque limiter, or the device that is being protected needsonly a one-time protection from damage. In this case, a device called ashear pin is used. In mobile robots, particularly in autonomous robots, itwill be found that a torque limiter is the better choice, even if a large oneis required to handle the torque. With careful control of motor power,both accelerating and braking, even torque limiters can be left out ofmost designs.Torque limiters should be considered as protective devices for motorsand gearboxes and are not designed to fail very often.
They don’t oftenturn up in the drive system of mobile robots, because the slow movingrobot rarely generates an overload condition. They do find a place inmanipulators to prevent damage to joints if the manipulator gets overloaded. If a torque limiter is used in the joint of a manipulator, the jointmust have a proprioceptive sensor that senses the angle or extension ofthe joint so that the microprocessor has that information after the jointhas slipped. Figure 3-54 shows a basic shear pin torque limiter.Figure 3-54 A shear pin is asimple and reliable torque limiter.However, after an overload,removing the sheared pin stubsand replacing them with a newpin can be time consuming. Besure that spare shear pins areavailable in a convenient location.This page intentionally left blank.Chapter 4Wheeled VehicleSuspensionsand DrivetrainsCopyright © 2003 by The McGraw-Hill Companies, Inc.
Click here for Terms of Use.This page intentionally left blank.Given the definition of robot in the introduction to this book, the mostvital mechanical part of a robot must be its mobility system, including the suspension and drivetrain, and/or legs and feet. The ability of thethese systems to effectively traverse what ever terrain is required is paramount to the success of the robot, but to my knowledge, there has neverbeen an apples to apples comparison of mobility systems.First, just what is a mobility system? A mobility system is all parts ofa vehicle, a land-based robot for the purposes of this book, that aid inlocomoting from one place to another.
This means all motors, gearboxes,suspension pieces, transmissions, wheels, tires, tracks, springs, legs, footpads, linkages, mechanisms for moving the center of gravity, mechanisms for changing the shape or geometry of the vehicle, mechanisms forchanging the shape or geometry of the drivetrain, mechanisms and linkages for steering, etc., are parts of mobility systems.The systems and mechanisms described in this book are divided intofour general categories: wheeled, tracked, walkers, and special cases.Each gets its own chapter, and following the chapter on special cases is aseparate chapter devoted to comparing the effectiveness of many of thesystems.There are some that are described in the text that are not discussed inChapter Nine.
These are mostly very interesting designs that are worthdescribing, but their mobility or some other trait precludes comparingthem to the other designs. Most of the systems discussed in ChapterEight fall into this category because they are designed to move throughvery specific environments and are not general enough to be comparable.Some wheeled designs are discussed simply because they are very simple even though their mobility is limited. This chapter deals withwheeled systems, everything from one-wheeled vehicles to eightwheeled vehicles.
It is divided into four sections: vehicles with one tothree wheels and four-wheeled diamond layouts, four- and five-wheeledlayouts, six-wheeled layouts, and eight-wheeled layouts.129130Chapter 4Wheeled Vehicle Suspensions and DrivetrainsWHEELED MOBILITY SYSTEMSBy far the most common form of vehicle layout is the four-wheeled,front-steer vehicle. It is a descendant of the horse-drawn wagon, but hasundergone some subtle and some major changes in the many decadessince a motor was added to replace the horses. The most importantchanges (other than the internal combustion engine) were to the suspension and steering systems. The steering was changed from a solid centerpivot axle to independently pivoting front wheels, which took up lessspace under the carriage.
Eventually the suspension was developed intothe nearly ubiquitous independently suspended wheels on all four corners of the vehicle.Although the details of the suspensions used today are widely varied,they all use some form of spring and shock combination to provide goodcontrol and a relatively comfortable ride to the driver. Most suspensionsare designed for high-speed control over mostly smooth surfaces, butmore importantly, they are designed to be controlled by a human. In spiteof their popularity and sometimes truly fantastic performance in racecarsand off-road vehicles, there are very few sprung suspension systems discussed in this book.
The exception is sprung bogies in some of thetracked vehicle layouts and a sprung fourth wheel in a couple four-wheeldesigns.WHY NOT SPRINGS?Springs are so common on people-controlled vehicles, why not includethem in the list of suspension systems being discussed?Springs do seem to be important to mobility, but what they are reallyaddressing is rider comfort and control in vehicles that travel more thanabout 8m/s. Below that speed, they are actually a hindrance to mobilitybecause they change the force each wheel exerts on the ground as bumpsare negotiated. A four-wheeled conventional independent suspensionvehicle appears to keep all wheels equally on the ground, but the wheelsthat are on the bumps, being lifted, are carrying more weight than theother wheels.
This reduces the traction of the lightly loaded wheels. Thebetter solution, at low speeds, is to allow some of the wheels to rise, relative to the chassis, over bumps without changing the weight distributionor changing it as little as possible. This is precisely what happens inrocker and rocker/bogie suspensions.Ground pressures across all vehicles range from twenty to eighty kilopascals (the average human foot exerts a pressure on the ground of aboutChapter 4Wheeled Vehicle Suspensions and Drivetrains35 kilo-pascals) for the majority of vehicles of all types. Everything fromthe largest military tank to the smallest motor cycle falls within thatrange, though some specialized vehicles designed for travel on loosepowder snow have pressures of as low as five kilo-pascals. This narrowrange of pressures is due to the relatively small range of densities andmaterials of which the ground is made.
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