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BSCP "Bississippi"
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"Bississippi" is a simplification of the Shape-Shifting Camera Tank (SSCT for short). It is thus also based on the real vehicle that inspired me to create both models: Inuktun's search and rescue robot Variable Geometry Tracked Vehicle"(VGTV). The main characteristic common to al three vehicles is the ability to change the shape of the tracks and in fact the whole vehicle.
In my chapter of the Mindstorms Masterpieces book, I wrote quite extensively about the concepts behind the specification of these vehicles - please consult those pages for that extensive background. In this paper, I'll stick to explaining the specific mechanical implementations found in BSCP - as well of some of its shortcomings. However, I'd like to start by explaining why I created the "Bississippi" implementation. Basically, the idea is to promote the building of these cool devices by breaking them down to their simplest components. This simplification has also some beneficial side effects for the model. For instance, it's small-ish size makes it much quick when it comes to both movements and response than larger models. On the other hand, I've left some ends open-ended: the chassis is nothing to write home about, but my guess (and wish) is that builders who pick it up from here might want to add further functionality to this bare-bones design and come up with their own their variations adapted to their particular needs. The design presented here incorporates two critical functions of its larger siblings: the ability to move as a regular tracked vehicle would do and the ability to change its shape (by changing the geometry of its tracks). For the first specification, I used two motors coupled via a clutching mechanism. In the SCCT, I used a similar limited-slip clutching set-up as that found in Differtimento, using two of Lego clutch gears to connect the output of both motors. However, since building those vehicles, I've come across other people who had not only used this technique before, but have also found a better variation; which is the one that BSCP incorporates.
 As you can see above, each motor mounts a 8T gear connected to a 40T one. The large gears are mounted on independent axles. The axles are then connected together via a Technic rugged hose, which couples their output when they move in the same direction (making the vehicle move in a straight line), but also allows them to move in opposite directions when turning. When designing treaded vehicles that use an independent motor for each track, we must take into account that their speeds must be equal or it will never travel in a straight line for long! The "folding" mechanism is a slightly more complex one. To better understand how it works, let's break it down into smaller components. The first considerations refer to the actual geometry involved. One obvious pre-requisite is that the wheels that define the form of the tread maintain a certain relationship so that the tread tension remains more or less equal at all time. The full mathematical explanation can be found in the "Mindstorms Masterpieces" book which I once again ask you to consult if you would like to know more. You can also find other combinations empirically by using a few bricks, gears and chain links. Once the positions of the wheels and folding fulcrum (axis of rotation) have been set, the next consideration involves also geometry, but also physics. In order to actually "fold"the vehicle, we need to come up with a way to lift (and lower) its front end. This involves several subsystems in itself.
As it is also obvious from the picture, the change in shape also changes the position at which we apply the force. The higher the arm is, the more force we need to raise it further. Finally, it is also interesting to notice in the picture that while we could rely on gravity when lowering the arm, I have in fact included links (in red) that brace the chassis to the control arm. Once the shape-changing linkages are set, it is time to add power to the control arm. Like in the SSCT, I have included in this design a mechanical "sensor" so that when the frontal part of the vehicle reaches its lower and higher limits, the power is automatically decoupled from the arm.
The picture at left shows the complete mechanism. The problem we are faced with here is pretty paradoxial. On one hand, shape-changing requires quite a bit of force for it to work. On the other hand, we want the mechanism to be able to decouple itself once a certain opposing force is reached. A handy clutch mechanism mechanism once again comes into play. You will notice that the movement of the motor is connected to the lifting arm via a worm gear, housed in a gearbox which connects it to a 24T gear. This effectively divides the movement of the motor by 24, and multiplies its force by the same amount. After this transformation, there is not an easy way to place a clutch mechanism, as the great force coming out of the gear casing will overwhelm it easily. This explains the connection between the motor and the gearbox, which might at first sight might seem non-sensical. Instead of augmenting the force driving the worm gear, we actually reduce it while multiplying the movement of the motor. It's a weak link! But that's exactly what we want. After testing several combinations, I've found the one featured in the picture the bext one for this model. It will "fold"the vehicle confidently, but once the limits are reached, the motor continues spinning but so does the rubber band in the groove of the wheel. The grip of the rubber on the plastic is not capable of countering the force opposing it and the axle of the worm gear will no longer turn. That is basically it: the model works in a very satisfactory manner. And thus it shows very well the strengths of the variable-geometry design: sneaking into tight space with a low profile, and manouvering easily by reducing the amount of track in contact with the ground. Moving forward, I would point 3 outstanding issues not fully sorted out in the BSCP. The easiest and less critical one to tackle is the not rock-solid chassis design. If you decide to add an RCX (or other on-board electronic device) on board, it should be relatively easy to come up with asuperior chassis design. The next issue is the Bississippi's low ground clearance. This is only relatively important in a vehicle of its characteristics. At the end of the day, being able to adopt a low profile is clearly a more critical issue. Thus, it would make little sense to improve ground clearance at the cost of total height. However, if a slightly improved ground clearance is necessary, the bext way to do it it to increase the height of the thacks themselves. But the 40T gears used by Bississippi are the larges ones! Or ...are they? The SSCT uses Technic Turntables to drive the track. Another option is to use the older, pre-Technic (but quite compatible) gears (easy to find) and chain (not so easy to find). As is clearly appreciated in the picture below, the older chains lift the vehicle higher.
 Lastly, the model would be much cooler if the bricks at the front would remain level with the ground at all times. This can be implemented via a paralelogram mechanism. Anyway, enough reading - go build it! |
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The picture at right shows the layout of the "control"arm in relation to the rotating part of the chassis. Using a control arm that rotates around a fulcrum different from that of the folding part of the chassis allows us to apply the necessary force in a more efficient way. The further away from the fulcrum of the chassis that we apply that force, the less force we will need to apply to activate the change in shape.
