Paul E. Sandin - Robot Mechanisms and Mechanical Devices Illustrated (779750), страница 45
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They are still found in dishwashers, washing machines, and anydevice where turning the knob results in an audible clicking sound as thearm or button on the switch jumps off the lobe of the cam. They can bestacked, as they are in appliances, to control many functions with a sin-Chapter 11Proprioceptive and Environmental Sensing Mechanisms and DevicesFigure11-10 Rotating camgle revolution of the timer. They can also be used as a very courseencoder to keep track of the revolutions or position of the shaft of amotor or the angle of a joint in a manipulator.Reversed BumpThe reversed bump layout shown in Figure 11-11 is a sensitive androbust layout.
The switch is held closed by the same springs that hold thebumper or sense lever in the correct position relative to the robot. Whenan object touches the bumper, it moves the sense arm away from theswitch, releasing and tripping it. A high quality switch is tripped veryearly in the travel of the sensing arm, and as far as the switch is concerned, there is no theoretical limitation on how far the bumper travelsafter the switch has been tripped. For this reason, it is an effective layoutfor sensing bumps.279280Chapter 11Proprioceptive and Environmental Sensing Mechanisms and DevicesFigure 11-11 Reversed bumpBUMPER GEOMETRIES AND SUSPENSIONSThe robot designer will find that no matter how many long and shortrange noncontact sensors are placed on the robot, at some point, thosesensors will fail and the robot will bump into something.
The robot musthave a sensor to detect collisions. This sensor may be considered redundant, but it is very important. It is a last line of defense against crashinginto things.The sensor must be designed to trip quickly upon contacting something so that the robot’s braking mechanism can have the maximum timeto react to prevent or reduce damage. To be perfectly safe, this sensormust be able to detect contact with an object at any point on the outersurface of the robot that might bump into something.
This can be donewith a bumper around the front and sides of the robot, if the robot onlygoes forward. Robots that travel in both directions must have sensorsaround the entire outer surface. It is important that the bumper be largeChapter 11Proprioceptive and Environmental Sensing Mechanisms and Devicesenough so that it contacts the object before any other part of the robotdoes, otherwise the robot may not know it has hit something. Some robotdesigns attempt to get around this by using a measure of the currentgoing to the drive wheels to judge if an object has been hit, but thismethod is not as reliable.A bumper, though seemingly simple, is a difficult sensor to implementeffectively on almost any robot.
It is another case in which the shape ofthe robot is important as it directly affects the sensor’s design and location. The bumper is so tricky to make effective as to be nearly impossibleon some larger robots. Unfortunately, the larger the robot, the moreimportant it is to be able to detect contact with things in the environment,since the large robot is more likely to cause damage to itself or the thingsit collides with.
In spite of this, most large teleoperated robots have nocollision detection system at all and rely on the driver to keep from hitting things. Even large autonomous robots (robots around the size ofR2D2) are often built with no, or, at most, very small bumpers.Simplifying any part of the robot’s shape, or its behaviors, that cansimplify the design of the bumper is well worth the effort. Making theshape simple, like a rectangle or, better yet, a circle, makes the bumpersimpler. Having the robot designed so that it never has to back up meansthe bumper only has to protect the front and possibly the sides of therobot.
Having the robot travel slowly, or slowing down when other sensors indicate many obstacles nearby means the bumper doesn’t have torespond as fast or absorb as much energy when an object is hit. All thesethings can be vital to the successful design of an effective bumper.There are several basic bumper designs that can be used as startingpoints in the design process. The goal of detecting contact on all outersurfaces of the robot can be achieved with either a single large bumper,or several smaller ones, each of which with its own sensor.
These smallerpieces have the added benefit that the robot’s brain can get some idea ofwhere the body is hit, which can then be useful in determining the bestdirection to take to get away from the object. This can be done with a single piece bumper, but with less sensitivity.A clever design that absolutely guarantees the bumper will completelycover the entire outer surface of the robot is to float the entire shell of therobot and make it the bumper, mounted using one of the techniquesdescribed later. Place limit switches under it to detect motion in anydirection of this all-in-one bumper/shell. This concept works well forsmall robots whose shells are light enough not to cause damage to themselves but may be difficult to implement on larger robots.Not only is it helpful to know the location of the bump, it is even better to be able to detect bumps from any direction, including from above281282Chapter 11Proprioceptive and Environmental Sensing Mechanisms and Devicesand below.
This is due to the possibility that the robot might try to driveunder something that is not quite high enough, or try to drive up ontosomething and get the bottom edge of the bumper stuck, before it tripsthe sensor. Both of these cases are potential showstoppers if the robot hasno idea it has hit something.
This is where a bumper compliant and sensitive to bumps coming from any direction is very helpful. If there is achance the robot will be operating in an environment where this problemwill arise, this additional degree of freedom, with sensing, makes thebumper’s suspension system more complex but vital.
Let’s start by looking at the simplest case, the one-dimension sensing bumper.SIMPLE BUMPER SUSPENSION DEVICESThe one-dimension (1D) bumper only detects bumps that hit the bumperrelatively straight on, from one direction. Although this may seem toolimiting, it can be made to work well if there are several smallerbumpers, each with their own 1D sensor. Together they can sense a largearea of bumps from many directions. There are also layouts that are basically 1D in design, but, by being compliant, can be made to sense bumpsfrom arbitrary directions.Since straight-on or nearly straight-on bumps are the most commonand produce the largest forces, it is better to use a design that allows thebumper to have the longest travel in that direction. Bumps can bedetected around the sides of the robot without as much motion from thebumper.
This is why a compliant 1D bumper suspension can be used for2D detection. There are many ways to attach bumpers that are basically1D bumpers, but that can also function as 2D bumpers.Some of these methods, or variations of them, can be used as is, withno additional devices required. Usually, though, a secondary device mustbe incorporated into the layout to positively locate the nominal positionof the bumper.
This facilitates repeatable sensing by the limit switch. Thespring-centered plate layout is shown in Figure 11-14. The moving plateis so loosely positioned it requires a vibration damper or it will wobbleconstantly.The V-groove centering block shown in Figure 11-12, is a basicmethod of realigning the bumper after encountering a bump, but thereare several others that work nearly as well. The V-groove layout is essentially two reversed bump limit switch layouts at 90° to each other. It istherefore effective for bumpers designed to detect bumps from straight ornearly straight on.Chapter 11Proprioceptive and Environmental Sensing Mechanisms and Devices283Figure 11-12 Tension springV-groove layoutThree Link PlanarA very useful and multipurpose mechanical linkage is something called a four-bar link.
It consistsof four links attached in the shape of a quadrilateral. By varying the lengths of the links, manymotions can be generated between the links. A3D version of this can be built by attaching twoplanes (plates) together with three links so thatthe plates are held parallel, but can move relativeto each other.This could be called a five-bar link, since thereare now essentially five bars, but the term fivebar link refers to a different mechanism entirely.A better name might be 3D four-bar, or perhapsthree link planar.
Figure 11-13 shows the basicidea. If the base is attached to the chassis, and thetop plate is attached to the bumper orbumper/shell, a robust layout results. This systemis under-constrained, though, and requires someFigure 11-13 Three link planar284Chapter 11Proprioceptive and Environmental Sensing Mechanisms and DevicesFigure 11-14 Tension springstar layoutother components to keep it centered, like the V-groove device discussedpreviously, and some sort of spring to hold the top plate in the groove.Tension Spring StarA simple to understand spring-centering layout uses three tensionsprings in a star layout (Figure 11-14). The outer ends of the springs areattached to the chassis and the inner three ends all attach to a plate orother point on the frame that supports the bumper.
This layout is easy toadjust and very robust. It can be used for robot bumpers that must detectbumps from all directions, provided there is an array of sensors aroundthe inner edge of the bumper, setup as a switch-as-hard-stop layout.This layout requires a damper between the chassis and plate to reducewobbling.Torsion Swing ArmThe torsion or trailing arm car suspension system (Figure 11-15) firstappeared in the early 1930s and was used for more than 25 years on theChapter 11Proprioceptive and Environmental Sensing Mechanisms and Devices285Figure 11-15 Torsion swing armVW Beetle. It is similar in complexity to the sideways leaf spring shownin the next section, but is somewhat more difficult to understand becauseit uses the less common property of twisting a rod to produce a spring.The mechanism consists of a simple bar with trailing links at each end.The center of the beam is attached to the chassis, and each end of thetrailing links supports the bumper.
If the beam is properly sized and sufficiently flexible, it can act as both support and spring with proper passive suspension points.Horizontal Loose Footed Leaf SpringAnother suspension system, used since the days of horse drawn buggies,that can be applied to robot bumper suspensions is a leaf spring turned onit side.
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