Paul E. Sandin - Robot Mechanisms and Mechanical Devices Illustrated (779750), страница 18
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This fast,short-stroke actuator is finding applications in industrial, office automation, and medical equipment as well as automotive applicationsThe PM armature has twice as many poles (magnetized sectors) as thestator. When the actuator is not energized, as shown in (a), the armaturepoles each share half of a stator pole, causing the shaft to seek and holdmid-stroke.When power is applied to the stator coil, as shown in (b), its associated poles are polarized north above the PM disk and south beneath it.The resulting flux interaction attracts half of the armature’s PM poleswhile repelling the other half. This causes the shaft to rotate in the direction shown.Figure 1-54 This bidirectionalrotary actuator has a permanentmagnet disk mounted on itsarmature that interacts with thesolenoid poles. When the solenoid is deenergized (a), the armature seeks and holds a neutralposition, but when the solenoid isenergized, the armature rotatesin the direction shown.
If theinput voltage is reversed, armature rotation is reversed (c).Chapter 1Motor and Motion Control SystemsWhen the stator voltage is reversed, its poles are reversed so that thenorth pole is above the PM disk and south pole is below it. Consequently,the opposite poles of the actuator armature are attracted and repelled,causing the armature to reverse its direction of rotation.According to the manufacturer, Ultimag rotary actuators are rated forspeeds over 100 Hz and peak torques over 100 oz-in. Typical actuatorsoffer a 45º stroke, but the design permits a maximum stroke of 160º.These actuators can be operated in an on/off mode or proportionally, andthey can be operated either open- or closed-loop.
Gears, belts, and pulleys can amplify the stroke, but this results in reducing actuator torque.ACTUATOR COUNTDuring the initial design phase of a robot project, it is tempting to addmore features and solve mobility or other problems by adding moredegrees of freedom (DOF) by adding actuators. This is not always thebest approach. The number of actuators in any mechanical device has adirect impact on debugging, reliability, and cost. This is especially truewith mobile robots, whose interactions between sensors and actuatorsmust be carefully integrated, first one set at a time, then in the wholerobot.
Adding more actuators extends this process considerably andincreases the chance that problems will be overlooked.DebuggingDebugging effort, the process of testing, discovering problems, andworking out fixes, is directly related to the number of actuators. Themore actuators there are, the more problems there are, and each has to bedebugged separately. Frequently the actuators have an affect on eachother or act together and this in itself adds to the debugging task.
This isgood reason to keep the number of actuators to a minimum.Debugging a robot happens in many stages, and is often an iterativeprocess. Each engineering discipline builds (or simulates), tests, anddebugs their own piece of the puzzle. The pieces are assembled intolarger blocks of the robot and tests and debugging are done on those subassemblies, which may be just breadboard electronics with some controlsoftware, or perhaps electronics controlling some test motors. The subassemblies are put together, tested, and debugged in the assembled robot.This is when the number of actuators has a large affect on debug complexity and time.
Each actuator must be controlled with some piece of6768Chapter 1Motor and Motion Control Systemselectronics, which is, in turn, controlled by the software, which takesinputs from the sensors to make its decisions. The relationship betweenthe sensors and actuators is much more complicated than just one sensorconnected through software to one actuator. The sensors work sometimes individually and sometimes as a group. The control software mustlook at the inputs from the all sensors, make intelligent decisions basedon that information, and then send commands to one, or many of theactuators. Bugs will be found at any point in this large number of combinations of sensors and actuators.Mechanical bugs, electronic bugs, software bugs, and bugs caused byinteractions between those engineering disciplines will appear and solutions must be found for them. Every actuator adds a whole group of relationships, and therefore the potential for a whole group of bugs.ReliabilityFor much the same reasons, reliability is also affected by actuatorcount.
There are simply more things that can go wrong, and they will.Every moving part has a limited lifetime, and every piece of the robothas a chance of being made incorrectly, assembled incorrectly, becoming loose from vibration, being damaged by something in the environment, etc.
A rule of thumb is that every part added potentially decreasesreliability.CostCost should also be figured in when working on the initial phases ofdesign, though for some applications cost is less important. Each actuator adds its own cost, its associated electronics, the parts that the actuatormoves or uses, and the cost of the added debug time. The designer ordesign team should seriously consider having a slightly less capable platform or manipulator and leave out one or two actuators, for a significantincrease in reliability, greatly reduced debug time, and reduced cost.Chapter 2Indirect PowerTransfer DevicesCopyright © 2003 by The McGraw-Hill Companies, Inc.
Click here for Terms of Use.This page intentionally left blank.As mentioned in Chapter One, electric motors suffer from a problemthat must be solved if they are to be used in robots. They turn toofast with too little torque to be very effective for many robot applications,and if slowed down to a useable speed by a motor speed controller theirefficiency drops, sometimes drastically. Stepper motors are the leastprone to this problem, but even they loose some system efficiency at verylow speeds.
Steppers are also less volumetrically efficient, they requirespecial drive electronics, and do not run as smoothly as simple permanent magnet (PMDC) motors. The solution to the torque problem is toattach the motor to some system that changes the high speed/low torqueon the motor output shaft into the low speed/high torque required formost applications in mobile robots.Fortunately, there are many mechanisms that perform this transformation of speed to torque.
Some attach directly to the motor and essentially make it a bigger and heavier but more effective motor. Othersrequire separate shafts and mounts between the motor and the outputshaft; and still others directly couple the motor to the output shaft, dealwith any misalignment, and exchange speed for torque all in one mechanism. Power transfer mechanisms are normally divided into five general categories:1.2.3.4.5.belts (flat, round, V-belts, timing)chain (roller, ladder, timing)plastic-and-cable chain (bead, ladder, pinned)friction drivesgears (spur, helical, bevel, worm, rack and pinion, and many others)Some of these, like V-belts and friction drives, can be used to providethe further benefit of mechanically varying the output speed. This abilityis not usually required on a mobile robot, indeed it can cause controlproblems in certain cases because the computer does not have direct control over the actual speed of the output shaft. Other power transferdevices like timing belts, plastic-and-cable chain, and all types of steelchain connect the input to the output mechanically by means of teeth just7172Chapter 2Indirect Power Transfer Deviceslike gears.
These devices could all be called synchronous because theykeep the input and output shafts in synch, but roller chain is usually leftout of this category because the rollers allow some relative motionbetween the chain and the sprocket. The term synchronous is usuallyapplied only to toothed belts which fit on their sprockets much tighterthan roller chain.For power transfer methods that require attaching one shaft to another,like motor-mounted gearboxes driving a separate output shaft, a methodto deal with misalignment and vibration should be incorporated.
This isdone with shaft couplers and flexible drives. In some cases where shockloads might be high, a method of protecting against overloading andbreaking the power transfer mechanism should be included. This is donewith torque limiters and clutches.Let’s take a look at each method. We’ll start with mechanisms thattransfer power between shafts that are not inline, then look at couplersand torque limiters. Each section has a short discussion on how well thatmethod applies to mobile robots.BELTSBelts are available in at least 4 major variations and many smaller variations. They can be used at power levels from fractional horsepower totens of horsepower.
They can be used in variable speed drives, remembering that this may cause control problems in an autonomous robot.They are durable, in most cases quiet, and handle some misalignment.The four variations are••••flat beltO-ring beltV-belttiming beltThere are many companies that make belts, many of which haveexcellent web sites on the world wide web. Their web sites contain anenormous amount of information about belts of all types.••••V-belt.comfennerprecision.combrecoflex.comgates.comChapter 2Indirect Power Transfer Devices• intechpower.com• mectrol.com• dodge-pt.comFlat BeltsFlat belts are an old design that has only limited use today.
The belt wasoriginally made flat primarily because the only available durable beltmaterial was leather. In the late 18th and early 19th centuries, it was usedextensively in just about every facility that required moving rotating powerfrom one place to another. There are examples running in museums andsome period villages, but for the most part flat belts are obsolete. Leatherflat belts suffered from relatively short life and moderate efficiency.Having said all that, they are still available for low power devices withthe belts now being made of more durable urethane rubber, sometimesreinforced with nylon, kevlar, or polyester tension members.
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