Paul E. Sandin - Robot Mechanisms and Mechanical Devices Illustrated (779750), страница 44
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The categories are proprioceptive and environmental. Proprioceptive things are part of the robot itself like theposition of the various segments of its manipulator, the temperature ofits motors or transistors, the current going to its motors, the position ofits wheels, etc. Environmental things are generally outside the robotlike nearby objects, ambient temperature, the slope of the surface therobot is driving on, bumps, or drop-offs, etc. This is an over-simplifiedexplanation because in several cases, the two categories overlap in oneway or another.
For instance, when the bumper bumps up against anobject, the object is in the environment (environmental sensing) but thebumper’s motion and location, relative to the robot, is detected by alimit switch mounted inside the robot’s body (proprioceptive sensing).In this book, anything that is detected by motion of the robot’s parts isconsidered proprioceptive, whether the thing being sensed is part of therobot or not.265266Chapter 11Proprioceptive and Environmental Sensing Mechanisms and DevicesThese two categories subject the switch to very different problems.Proprioceptive sensors usually live in a fairly controlled environment.The things around them and the things they sense are all containedinside the robot, making their shape unchanging, moving generally inthe same direction, and with the same forces.
This makes them easier toimplement than environmental sensors that must detect a whole rangeof objects coming from unpredictable directions with a wide range offorces. Environmental sensing switches, especially the mechanical type,are often very difficult to make effective and care must be taken in theirdesign and layout.Mechanical limit switches come in an almost infinite variety ofshapes, sizes, functions, current carrying capacity, and robustness.
Thischapter will focus on layouts and tripping mechanisms in addition to theswitches themselves. Some switch layouts have the lever, button,whisker, or slide directly moved by the thing being sensed. Others consist of several components which include one or more switches andsome device to trip them. In fact, several of the tripping devices shownin this chapter can also be used effectively with non-mechanicalswitches, like break-beam light sensors.
The following figures showseveral basic layouts. These can be varied in many ways to producewhat is needed for a specific application.The simplest form of mechanical limit switch is the button switch(Figure 11-1) It has a button protruding from one side that moves in andout. This opens and closes the electrical contacts inside the switch. Thebutton switch is slightly less robust than the other switch designsbecause the button must be treated with care or else it might be pushedtoo hard, breaking the internal components, or not quite inline with itsintended travel direction, breaking the button off. It is, theoretically, themost sensitive, since the button directly moves the contacts without anyother mechanism in the loop.
Some very precise button limit switchescan detect motions as small as 1mm.The lever switch is actually a derivative of the button switch and isthe most common form of limit switch. The lever comes in an almostlimitless variety of shapes and sizes. Long throw, short throw, with aroller on the end, with a high friction bumper on the end, singledirection, and bidirection are several of the common types. Figure11-2 shows the basic layout.
Install whatever lever is needed for theapplication.The whisker or wobble switch is shown separately in Figure 11-3even though it is really just another form of lever switch. The whiskerlooks and functions very much like the whiskers on a cat and, like a cat,the whisker directly senses things in the environment. This makes itChapter 11Figure 11-1 Button SwitchFigure 11-2 Lever SwitchProprioceptive and Environmental Sensing Mechanisms and Devices267268Chapter 11Proprioceptive and Environmental Sensing Mechanisms and DevicesFigure 11-3 Whisker SwitchFigure 11-4 Slide SwitchChapter 11Proprioceptive and Environmental Sensing Mechanisms and Devicesmore robust and easier to incorporate, but it is also much less precisesince the sensing arm is necessarily flexible.The whisker has the special property of detecting an object fromany direction, making it distinctly different from lever switches.
Sinceit bends out of the way of the sensed object, neither the object nor theswitch is damaged by impact. This trick can also be done with aroller-ended lever arm, but more care is needed when using a rigidarm than with the flexible whisker. Figure 11-3 shows a basic whiskerswitch.The last basic type of limit switch is the slide switch. This switch hasa different internal mechanism than the button switch and its variations,and is considered less reliable. It is also difficult to implement in a robotand is rarely seen. Figure 11-4 shows a slide switch.Magnetic limit switches come in several varieties and have the advantage of being sealed from contamination by dirt or water. The most common design has a sensitive magnet attached to a hinged contact so thatwhen a piece of ferrous metal (iron) is nearby on the correct side of theswitch, the magnet is drawn towards a mating contact, closing the electric circuit.
All of the mechanical limit switches discussed in the following sections can incorporate a magnetic limit switch with some simplemodification of the layouts. Just be sure that the thing being sensed isferrous metal and passes close enough to the switch to trip it. Besidesbeing environmentally sealed, these switches can also be designed tohave no direct contact, reducing wear.There are several ways to increase the area that is sensed by a mechanical limit switch. Figures 11-5 and 11-6 show basic layouts that can beexpanded on to add a large surface that moves, which the switch thensenses. There is also a form of mechanical switch whose area is inherently large. This type is called a membrane switch.
These switches usually are in the shape of a long rectangle, since the internal componentslend themselves to a strip shape. Membrane switches come with manydifferent contact surfaces, pressure ratings (how hard the surface has tobe pushed before the switch is tripped), and some are even flexible. Forsome situations, they are very effective.The huge variety of limit switches and the many ways they can beused to sense different things are shown on the following pages inFigures 11-5 and 11-6. Hopefully these pictures will spur the imagination to come up with even more clever ways mechanical limit switchescan be used in mobile robots.269270Chapter 11Proprioceptive and Environmental Sensing Mechanisms and DevicesINDUSTRIAL LIMIT SWITCHESActuatorsFigure 11-5a Mechanical, Geared, and Cam Limit SwitchesLinear Mechanical SwitchesFigure 11-5b Mechanical, Geared, andCam Limit SwitchesLatching Switchwith Contact ChamberGeared Rotary Limit SwitchesRotary-Cam Limit Switches272Chapter 11Proprioceptive and Environmental Sensing Mechanisms and DevicesFigure 11-6 Limit Switches in MachineryChapter 11Proprioceptive and Environmental Sensing Mechanisms and Devices273276Chapter 11Proprioceptive and Environmental Sensing Mechanisms and DevicesLAYOUTSWith the possible exception of the whisker switch, the limit switch typesdiscussed above almost always require some method of extending theirreach and/or protecting them and the object being sensed from damagingeach other.
There are many ways to do this. The next several figuresshow various basic layouts that have their own benefits and problems.In every sensor/actuator system, there is a time lag between when theswitch is tripped and when the actuator reacts. This time lag must betaken into account, especially if the switch or object could be damaged.Object, in this case, can mean something in the environment, or something attached to the robot that is designed to detect things in the environment. If the time lag between contact and reaction cannot be madeshort enough, the layout must provide some other means of preventingdisaster.
This is done by using one of three methods.Figure11-7 Direct sensing combined with direct hard stopChapter 11Proprioceptive and Environmental Sensing Mechanisms and Devices277Figure 11-8 Direct sensing withseparate hard stop• A hard stop that is strong enough to withstand the stopping force(and yet not damage the object) can be placed just after the trip pointof the switch.• The layout can allow the object to pass by the switch, tripping it butnot being physically stopped by anything.
The robot’s stoppingmechanism is then the main means of preventing harm.• The travel of the sensor’s lever or button, after the sensor has beentripped, can be made long enough to allow sufficient time for therobot to stop.Let’s take a look at each layout.Combination Trip (Sense) and Hard StopThis is probably the simplest layout to implement. The switch directlystops the sensed object (Figure 11-7), which means the switch must bestrong enough to withstand repeated impacts from the thing beingsensed. Alternatively, there is a separate hard stop that is in line with theswitch that absorbs the force of the impact after it has been tripped(Figure 11-8). Using a switch with a long throw eases implementation,and nearly any mechanical limit switch can be made to work with thislayout, though the button and lever designs are usually best.278Chapter 11Proprioceptive and Environmental Sensing Mechanisms and DevicesFigure 11-9 By-pass linearBy-Pass LayoutsThe by-pass layout shown in Figures 11-9 and 11-10 relieves the switchof taking any force, but, more importantly, is less sensitive to slight variations in the positions of the switch and the sensed object, especially if aswitch with a long throw is used.
Removing the hazard of impact andreducing sensitivity make this layout both more robust and less precise.With careful design, however, this layout is usually a better choice thanthe previous layout because it requires less precision in the relationshipbetween the hard stop and the switch’s lever or button. Remember thatthe object being sensed can be anything that is close to the robot, including the ground.This layout and its derivatives are the basis of virtually all mechanicaltimers.
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