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`Introduction
`
`
`
`In the previous chapters, we discussed the two most common propulsion systems
`wheels and legs, and looked into the many details regarding possible implementg ;
`tion schemes.Actually we didn’t exhaust all the possibilities, so we are going to .
`describe a few more robotic vehicles suited to very special tasks.
`What mainly affects the mobility of a vehicle is the nature of the terrain that
`it has to move over. The scale of the robot, also, has a strong influence on the size
`of the obstacles it can overcome: A pebble two inches high is nothing for a wheel I
`20 inches in diameter, but it’s insurmountable for a differential drive with wheels
`
`I
`
`of only 3.5 inches (the largest contained in the MINDSTORMS kit). Scaling
`your robot up, however, is not always a practical option. In the specific case of
`LEGO, you’re limited to the size of the available parts and their mechanical prop-
`erties, and just like with real—life robots, they, too, often face constraints when it
`comes to weight and size.
`The two robots of this chapter, a SI-IRIMP rover and a skier, are completely
`different in nature, but share the fact that they are designed for special surfaces:
`rough terrain for the first model, and snow for the second.
`
`.Cre.ati.n.g._.Yo.u.rQwn MS HR.l.lViP...
`
`H
`
`. H
`
`The original SHRIMP is a high—mobility wheeled rover designed by the
`Autonomous Systems Lab based in Lausanne, Switzerland. It features six wheels
`
`powered by independent motors: one front wheel mounted on an articulated
`fork; one rear wheel directly connected to the body; four wheels mounted on
`two lateral swinging bogies. (A bogie is a wheeled assembly that pivots one or
`more axles.) It performs amazingly well on many surfaces and against many kinds
`of obstacles. It’s able to overcome obstacles as high as its wheels, even if they take
`the form ofa stairway
`
`During summer 2000, we built our first LEGO version of SHRIMP, which
`had capabilities very similar to that of the original robot that inspired its design.
`The version described here is our first attempt at a turning SHRIMP; the first
`one, like the original, was not designed to turn.
`Unfortunately, this project requires a lot of extra parts: seven motors, six gear—
`boxes, ten universal joints, two polarity switches... not to mention a couple
`dozen of 1 X 16 beams. Later on in the chapter, we will give you some sugges—
`tions to help reduce the requirements, but the project remains rather demanding.
`
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`ow it worl<:s.
`
`Let’s start by looking at the SHRIMP in ac
`tion to understand h
`’\While the first wheel climbs the obstacle (Figu
`re 16.1), the wheel a
`Vertical, attached to the body
`ssenibly remains
`with two parallel pairs of
`beams. This parallelogram
`g6OI11€t1"JV1-S
`the key to all of t
`he SHRIMP abilitieszllh
`e beams that connect the
`wheel assembly to
`the body convert the push from the
`other five wheels into Ver-
`tical lifi, while the
`front Wheel itself foll
`ows the shape of the obstacle.
`SHRlMP Front Wheel Climbs a Step
`
`Figure 16.1 The
`
`
`
`\X/hen the first whe
`el is up, it’s the bogies” turn to climb (Figure l6.2).They
`rely on the same principle:The bogie is
`a parallelograin attached to the body in
`the midpoints ofits horizontal sides.\X/h
`en the bogie approaches the obstacle,
`those horizontal beams act like lev
`ers, with their fiilcrunis on the second wheel
`ofthe bogie. The load is
`applied in the I‘]}ldpQl}:rt,Oftll€.,l€L‘
`,,Wl1t‘.E‘l h=.1s—t~o litt“oul’y h
`ers; thus the first
`a
`alfloftheweiglit applied to the bogie.
`In Figure l6.3, the bogies are over
`the step. and pull up the rear
`SHRIMP has
`an incredible ability to adapt to Very complex terr
`rations. Some otaits wheels may descend While others clirnb. Nevertl
`ain configu—
`A l§Ody remains stable (Figure 16.4).
`ieless, the
`To turn properly, that is, wit
`h no sliidding, the SHRHVIP
`mum of four wheels. Do you 1'
`should rotate a min-
`enieniber the rule?
`through the axles ofall tl
`If we extend imaginary lines
`ie Wheels, 1‘/my mus! H/Ct“
`perfect, but V
`I at £7 514/lg/K’ pa//1!. Tliis would be
`ery coniplex to build and
`control.
`
`wheel.
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`Figure 16.2 The First Wheels of the Bogies Climb the Step
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`Figure 16.4 The SHRii\/iP Traversing a Rough Terrain
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`
`
`In our turning Sl-lRll\/IP, we adopted a simplified scheme: the front and rear
`wheels turn, while the bogie wheels behave like a skid—steer. In other words, the
`inner ones stop while the outer ones continue to run.This is an approximate
`solution that introduces some slippage, but that in practice works Very well on
`most terrains.
`
`Though we reduced the complexity, there are still too many motors to con-
`trol for a single RCX:
`
`% The front and rear wheels form a group. Their motors need to always be
`powered when the robot is in motion.
`
`5 A motor turns the front and rear wheels.
`
`—E~
`
`a——The wheels of the left bogie*always"run except when the robot turns left.
`
`E The wheels of the right bogie always run except when the robot
`turns right.
`
`We really wanted to avoid 21 second RCX, niostly so as not to add further
`i Weight to the robot.After long experimenting, and many useful tips from the
`LUGNET friends, we came out with a solution that saves not only the fourth
`in motor port, but the third one alsol
`Our design requires two polarity switches, here used as simple on/off
`e SWitches_The idea is that the steering system controls those switches too, and
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`when the front and rear Wheels turn, the inner bogie stops as a result. Figure 1653
`should help to explain the concept.
`T
`
`Figure 16.5 The SHRIMP Steering Control System
`
`
`
`Two rubber bands keep the polarity switches gently pulled back in the on
`position.The pivoting axle of the rear wheel mounts a traverse axle, then, when
`turned, pushes the inner switch to the off position (the left switch controls the
`left bogie).
`In the same picture, notice the touch sensor that detects the neutral position
`of the steering system, the only sensor used on this robot.
`A single motor operates the entire steering system. its placed in the bottom
`part of the body, and connects to the main steering axle through a pulley—belt—
`worm—24t geartrain (see Figure 16.6).
`The main steer axle is a longjoined axle that turns both the front and rear
`wheels.The rear steer assembly is rigidly attached to the body, while the front
`one forms one side of the parallelogram described previously. For this reason the
`steer axle requires two universal joints positioned precisely where the swinging
`beams connect to the body and to the steer assembly (see Figure 16.7)-
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`Unconventional Vehicles ° Chapter 16
`Figure 16.6 Bottom View: The SHRIMP Steering Motor
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`The long steer axle ends with bevel gear pairs on both sides, which transfer
`
`motion to the pivoting axles (see Figure 16.8).
`
`Figure i6.8 The SHRIMP Rear Wheel, Rear View
`
`The bogies mount four identical wheel groups, where a motor powers the
`wheel through ajoined axle and a gearbox (see Figure 16.9).
`In our first SHRIMP, the front and rear wheels were identical to the side
`
`ones, while in this version, the motor has been moved down to make the
`
`assembly more compact and leave the space above for the steering mechanism
`(Figure 16.’lO).
`
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`

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`Figure 15.9 C!ose—Up of a SHRIMP Side Wheei
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`The RCX stays on the top,just behind the point where the bogies connect;
`to the body (see Figure 16.11).
`
`Figure ‘£6.11 The SHRIMP Top View
`
`
`
`Building a SHRIMP
`
`If you want to create your own SHRIMP, but don’t have all the parts We use 4,
`the LEGO inventory offers many possible substitutes:
`
`Gearboxes A gearbox is a convenient way to match a worm gear to at
`24t. But as you’Ve seen in this book, there are many other assembly solu—
`tions, they’re just a bit Iess compact.
`
`E Universal joints Those that power the wheels are easily avoidable with a
`different construction.‘For example, our setup for the front wheel doesn’t
`use them, and you can replicate this for all the Wheels. In Figure 16.12, we
`show a Wheel with no universal joints and no gearbox.
`
`E Polarity switches You can use the free port of the RCX to control
`one bogie, and connect the other to the same port that drives the front
`and rear wheels. No polarity switches are needed for this configuration,
`but your SHRIMP would only turn in one direction.
`
`3 Motors A nonsteering SHRIMP also saves one 1notor.We didn’t try,
`but We are sure it’s possible to use a single motor instead of two in each
`bogie. In theory, with a lot of gearing, you can power your SHRIIVIP
`
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`with a single central motor: transport motion to the bogies through their
`supporting axles, and to the front and rear wheels with a system similar
`to the one We used for the steering setup. Such a SHRIMP will suffer
`from a lack of power and therefore climbing ability, due to the reduced
`number of motors and increased fiiction.
`
`Figure 16.12 A Wheel Assembly
`
`Creating a Skier
`V What we find most interesting in this project is notjust the fact that this “skibot”
`robot can be used in the snow, but that without propulsion it descends snowy
`is Slopes like a true skier (Well, alniostl). It uses a technique known as sizowploiiliirg,
`ii due to the ‘\/—shape of the skis, often used by human skiers. In snowplowing of
`V @116 human Variety, the skier angles his toes inward in order to put the tips of his
`Skis together and simultaneously dig the inside edges of the skis into the snow. To
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`reduce speed, the skier pushes out the tails of‘ his skis, incre
`asing their angle to
`make a Wider V; to increase speed, th.e skier dr
`aws the tails nearer. niaking the
`V narrower.
`
`Our robotic skier is based on tl
`1e same principle. lt mounts onto a P1
`iii‘ of
`skis, and while descending a slope, it varies the angle of the sl<
`is to increase or
`decrease the resistance and maintain a roughly constant spee
`d. It uses only one
`motor and one sensor:The motor is on the back
`and operates the legs, niaking
`them more or less convergent, t}
`ius keeping the speed in the d
`esired range; the
`sensor is a rotation, attached to a wheel
`at the end of the left ski pole, and serves
`to measure the speed.
`Another interesting feature of this robot is that its geometry and the position
`of its center of gravity niake it alw
`ays point toward the direction of the lH11I{-—
`
`" happens
`automatically because the motion along the longitudinal axis of the robot is tl
`one that oders the lower resistance.
`A general View ofour skier c
`
`an be seen in Figure 16.l3.
`
`16
`
`Figure £6.13 The Skier
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`Unconventional Vehicles ° Chapter 16
`To build the skis you need some extra beams and, more important, many tiles
`(see Figure l6.l4).We used 36 2
`2 and two 1 X 4 tiles (available as spare parts at
`the LEGO Online Shop). Ifyou are open to employing non—LEGO parts, you can
`build the skis from other materials, like strips ofplastic available in hobby shops.
`Figure “£5.14 Side View of the Skier
`
`323
`
`
`
`The legs are not vertical, but rather
`are inclined outward.This is very impor-
`tant. For a human skier, it’s wliat keeps the skis resting on their inner edges and
`producing the necessary resistance to gravity when in the convergent (snowplow)
`position (see Figure 16.15).
`‘We acliieved this effect by using some hinges and forming the legs from the
`diagonal ofa perfect right triangle. it‘ you doift have hinges, other possible .solu—
`tions exist, like the one shown in Figure l6.l6.
`V Ej‘1cli lcgis rigidly attached to €L'4-‘lflf’gC31'.F.ln.l1&" pictures don’t show them, but
`we used the extra crossed holes of the ge;1rs to place pin—aXle connectors into.
`The two gears meet a wor 1 gear in the 1}‘1iClLll.€ of the assenibl
`y, which receive
`motion from the motor
`an d controls the convergence Of. the legs (Figure l().l7).
`Looking at the bottom, you notice a longitudinal beam that locks the struc-
`axle and beam that serve as boundaries to the movement
`tures, and a CI_'?lHSV’€I‘S€
`Ofthe legs (Figure l(i. l8).There are no limit switches. If the RCX tries to close
`:— Or open the legs niore than whats allowed, the belt will slip on the pulley.
`
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`Figure 16.15 Front View of the Skier
`
`[Figure 16.16 [Leg Geometry
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`Figure 16.17 Rear View of the Skier
`
`. 2;»
`
`Figure 16.18 Bottom View of the Skier
`
`
`
`
`j V/hat’s peculiar about this robot is that it has bezuiis with all the possible ori-
`nl3tiOns.The skis are studs down, the legs studs front, the body partly studs front,
`E‘ (A Frc :3./) L‘
`‘ght, and partly studs up (see Figure 16.19).
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`Figure ‘£6.19 Skier Top View (RCX Removed)
`
`There’s not much to say about the ski poles. The right one is just deco zxtive.
`placed there for the sake ofsy1nn1etry.Tl1e left one, meanwhile, incorporates a
`rotation sensor that’s directly connected to a wheel (see Figure 1620).
`
`Figure 16.20 Detail of the Left Ski Pole
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`Prograniniing this robot is so simple that the topic deserves only a few words.
`Inside the main program cycle, test the increment in the rotation sensor counts: If
`this falls in the range that represents the desired speed you chose, switch the
`motor off; if it’s above the range, start the m
`otor in the direction that closes the
`skis, and vice versa ifit’s below
`the range. This will speed up or slow down the
`skibot as needed.
`lf you test your skibot in the snow, try to find or create Well—pacl<:ed powder,
`like those normally found on ski runs. Its not able to ski on black diamond runs
`or in loose powder!
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`Creating Gther Vehicles
`
`A
`Here we present you with a list of suggestions for possible projects, their
`common denominator being that all of them are, at least in part, Vehicles.They’rg.
`meant just as starting points.
`
`Elevator
`
`
`
`We briefly discussed an elevator project in Chapter 4, in which we explained that
`a single touch sensor, placed in the elevator car, can control the positioning at an
`unlimited number of floors.We said also that a second touch sensor could serve
`
`the purpose of addressing the car to the proper level, using a simple system where '
`the RCX counts the number of clicks on the sensor.
`
`A Variation on this theme is the car park elevator, where you emulate one of
`those automatic storing systems. It would be nice if your robotic parking could
`decode a sort of ticket, maybe using colors or shapes, so it can return the corre-
`
`sponding vehicle.
`
`Train
`
`The RCX is almost a natural extension to the LEGO 9v electric train system.
`Tlieysime the same Voltage and the same connectors,’ and in fact many train fans i
`currently use one or more RCXS to introduce automation in their layouts.This
`topic is so vast it would require a dedicated book, so we will provide only some
`basic tips.
`
`There are two basic approaches to control a train with the RCX: a) the RCX
`is on the train; b) it supplies power to the tracks.
`
`In case a), you put the RCX in the locomotive or in one car and connect an
`
`out port to the train motor. The train motor is also wired to the wheels, which
`normally draw current from the tracks, so it will happen that your RCX will
`supply power to the tracks, too. Nothing bad happens, but DON ’T connect the
`train speed regulator to the tracks as well; you could damage your RCX.
`If you are the kind of person who likes customizing things, you can open the
`train motor and interrupt the connection to the wheels, so your train will be
`totally independent from external sources. This way you can run many RCX—
`controlled trains on the same track.You can create some external references to
`
`read with sensors, so your train knows when to slow down or stop, or place a
`proximity sensor on the locomotive to avoid collisions.
`
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`In the second approach, b), you substitute the train speed regulator with the
`RCX and ower the tracks from one out ort.You can control three inde en-
`P
`P
`P
`dent tracks or segments with a single RCX, and use the input ports, for example,
`to detect the arrival of the train at the station.
`There are many other devices you can automate in your layout: switching
`points, level crossings, decouplers, semaphores, swing or draw bridges, and so on.
`Cable Railway or Gondola
`
`In a real cable railway, there are two pairs of cables: two supporting ones and two
`pulling ones.The supporting cables are more or less rigidly attached to the lower
`and upper stations, and work as railways for the. cabs that have their pulleys run-
`ning over thern.The pulling cables transfer motion to the cabs: one cable goes
`fiom the first cab to the second across the upper station, while the second con-
`nects the two across the lower station.
`You can place the motor either in the upper or the lower station. lfyou use
`the upper station, you can avoid the second pulling cable. Use touch sensors to
`control when the cab enters the station so you can stop the motor, possibly after
`a short slow down.
`
`Boat
`
`LEGO inventory includes different kinds of propellers, so one might wonder if
`it’s possible to make a robotic boat. It is indeed possible, but it’s not easy to pro-
`vide the necessary flotation lift using only LEGO parts.The two solutions that
`come to our mind require uncommon parts, either single mould boats coming
`from the System product line, or a bunch ofTECl-INIC air tanks.The idea is to
`build a sort of catamaran, with both hulls made of two System boats or a row of
`air tanks.
`7
`7
`T
`i
`i
`N
`..
`Alwslllllprlle,
`and handy non—l_.EGO alternative for the hulls is to Lise
`common soft drink plastic bottles and attach them to long beams with rubber
`bands or duct tape.
`The RCX will stay on the deck, together with the motor that drives the pro-
`peller and the other that controls direction.You can place bumpers on the front
`to make your robotic boat change direction when hitting an obstacle.
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`The RCX, the motors and most other electronic components don't like
`water at all. While distilled water is a good insulator, common tap water
`p or water from the sea, lakes, or pools, conducts electricity extremely well
`and will damage your devices. Take every precaution not to soak or sub- M
`merge them.
`To minimize the risk of damages in case of an accidental bath, put
`the RCX into a small transparent plastic bag, with just the wires coming
`out from the opening, and seal the bag with a rubber band. Run your
`robot in a controlled environment with calm waters, like a pool with no
`people in it.
`
`
`Sailing Tricycle
`
`We are both fans of sailing, and in the Wake of the great success of Luna Rossa,
`the Italian sailboat that won the Luis Vitton Cup 2000, We decided to build a
`robotic sailing tricycle or land yacht.VX/e named it Duna Rossa (Red Dune) to
`_ini_1nic _t]i_e,,origii1al,Luna Rossa,.(Red Moon).
`Though building that tricycle has been a lot fun, we must admit that the per—
`forniances were less than exciting. Witli a strong wind and a favorable slope... it
`moved!
`
`See if you’re able to do better: keep the structure as lightweight as possible,
`use a Very large sail, and reinforce the mast with shrouds, forestays, and baclcstays
`(ropes).
`
`The RCX controls two motors, one to steer the rudder and the other to
`
`operate the Winch for the niainsail.You can detect the wind direction through a
`vane on the masthead connected to a rotation sensor. Monitor the position of the
`boom with a second rotation sensor to adjust it for the proper angle with the
`electric winch. Finally, you’ll need a third sensor (a touch is enough) to control
`the position of the rudder.
`
`You can program your robotic sailing tricycle for two basic behaviors: adjust
`the niainsail to keep with the desired course, or adjust the course to maintain a
`specific sailing point.
`
`
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`33
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`You’re not necessarily required to invent new solutions any time, more ofizen
`you canjust look around you, or on the net, and find something that helps you
`
`robotic vehicles designed for planetary exploration are a good source of inspira-
`tion, and there’s a large quantity of documentation available in the public
`
`domain.
`
`We must confess that our skier was born more for purposes of fiin than to
`demonstrate some general principle. Nevertheless, this small and simple robot has its
`merits, helping us picture the wide range of applications robotics can be used for.