|
Differtimento
DAT File
About
|
While working on the SSCT, I imagined what I thought would be a very simple vehicle based on the properties of the differential gear. Three months later, Differtimento gets its name from the "diff"-erential gear and the musical term "divertimento", which defines a piece of free form. Like the SSCT, Differtimento changes shape to gain efficiency as an explorer vehicle - although the advantages gained are completely different in each case. While the SSCT changes shape to adapt itself to the environment around it in a variety of ways for different tasks, Differtimento uses this ability to simply gain traction while turning. Skid-steering vehicles equipped with tracks can turn in place or thereabouts in part due to the low resistance that most tracks offer in the direction perpendicular to regular travel. However, wheeled skid-steering vehicles, specially those equipped with high-grip wheels might offer much more resistance to spin in place. Counteracting that resistance might start a domino effect that can impact the overall design of the vehicle. Let's see a practical example I have entered Differtimento in the Planetary Society's "Build a better mars rover contest", as a Marie Curie Class Rover. Apart from a set of specifications, including the necessary use of two (and only two) Lego motors driven in a skid steer configuration, the organization reminded contestants to keep an eye out for ground clearance and wheel grip. This seemingly simple guidelines actually hide quite a bit of punch. Since the contest also requires that vehicles have a slow motion of travel, if we choose to gear down the combined output of two motors, we might end up with more power that we bargained for. If the lateral resistance on the wheels while turning is too high, the wheels might find it easier to simply pop out of their axles and skip the job altogether. Of course, there are ways to deal with this. For instance, we can simply choose to reinforce the chassis, adding bulk to the vehicle to direct the force of the motors appropriately. But we can also come up with a way of avoiding the resistance instead of countering it.
This functionality has actually been added to one of the latest real-life Mars vehicles designs, NASA's MER - incidentally, while I came with all the mechanisms in Differtimento independently, I am not in the least surprised that there was plenty of precendent. Wheeled skid-steered vehicles equiped with this ability can turn in place effortlessly, instead of wasting power in countering often high lateral resistance. As we will soon see, we can use some of that power to further refine other related mechanisms in the vehicles. As we just saw, the ability of Differtimento to adapt its shape automatically makes it much more useful as a explorer vehicle. Exploring is an activity hard to define narrowly, but that nevertheless requires very specific characteristics. For instance, when we hear "skid-steered", we inmediately imagine a tank or other heavy vehicle carving the ground as it spins. Yet explorer vehicles are generally more efficient if they have less impact on the ground that they tread. Differtimento's is a skid-steered vehicle; but rather than pure bulk relying on bruter force, it transforms its wheel base while turning, transforming in the process the raw motor power into focused and sophisticated agility to lightly tread the ground bellow its wheels. Indeed, an important characteristic of the "steering" mechanism is that the transformation of the wheelbase is accomplished via a very simple method: calculating mechanically the difference in movement between the two motors. Using the differential The key component of Differtimento's shape-shifting system is a differential gear connected to the outputs of both motors. Differential gears can mechanically add or substract the movement of two inputs. In practical terms, the differential gear will turn a number of revolutions proportional to the movement (in revolutions) of both inputs. To be more precise, the movement of the differential will be half the sum of the revolutions of both inputs. For instance, if both inputs turn in the same direction 1 revolution, the differential gear will also turn one revolution (1 revolution + 1 revolution ) / 2 = 1 revolution. Calculating the exact number of revolutions is not critical in the case of Differtimento, but understanding the capabilities of the differential gear as described above is key. The figure below shows a partial view of Differtimento's differential gear setup. Both motors are connected to the differential inputs (the two axles that are also attached to the differential's inner 12 tooh bevel gears). But the intermediate gears reverse the direction of one of the inputs so that when both motors move in the same direction, the sum of the differential inputs is = 0. Why is that, if the differential gear adds revolutions? Take into account that the intermediate gears reverse the movement of one of the motors before sending it to the differential. Thus, the differential detects one input moving forward and the other one moving backwards, which equals 0. 1 positive revolution + 1 negative revolution = 0 revolutions . When both motors turn in the same direction, the differential inputs turn in opposite directions - at the exact same speed, more on that later; and thus the differential casing will not move and the steering/shape-shifting mechanism will not be triggered.
 But if we reverse the direction of one of the motors, the intermediate gears will make both differential inputs turn in the same direction. As we saw earlier, when the differential receives two positive or negative inputs, the sum of which is not zero, and its casing - also called spider - turns proportionally. Since Differtimento's shape-shipting mechanism is connected to it, when the spider moves, the vehicle changes its wheelbase to accommodate turning -all done mechanically. This is the trick that Differtimento uses to stay within the strict description of a skid-steer drive as defined by the rules of the contest - specifically, the motor setup, while using the motor power to avoid and not counter the resitance of regular wheeled skid-steer vehicles to turn.
 I've added to the figure above a crown gear which is not used in Differtimento - see below how the steering works. However this crown gear might help visualize the basics of the system. It is also important to realize that moving the differential, indeed the whole steering mechanism, will only require a fraction of the combined power of two motors. The wheels will keep on turning while the wheelbase automatically changes its shape to better accommodate to their movement.
Clutches and more clutches
That constitutes the basic theory behind Differtimento. I will now go over some key details, some of which will be of interest even for regular skid-steering vehicles. With the setup shown above, the next logical steps are to devise the power-transferring and steering systems, connect them to the the appropriate motors and differential outputs and build a housing to hold it all together.
To accomplish the first two steps, Differtimento also makes use of another "special" Lego gear, the clutch gear. This gear remains solidly connected to its axle until a certain pressure is met, after which if becomes loose from it. This characteristic if of great use in many situations. In Differtimento's case, 3 of those gears are used for 2 different purposes.
First of all, the mechanical adder-subtractors mechanism described earlier requires the motors to turn at the same speed. The differential will detect any difference between the two inputs and will move (or not) accordingly. Incidentally, achieving the exact same speed on both sides of the vehicle when it is moving forward or backward is key to all skid-steer vehicles, since a difference of speed between the two tracks will make the vehicle eventually turn even if the two motors are running in the same direction.
We can mechanically couple the output of two (electric) motors so that the turn at the same speed when moving in the same direction by attatching both motors to one gear, or to two gears shaing the same axle. However, when the motors turn in opposite directions, chaos is ensured: binding gears, stalled motors. Something might break.
Let's now take a look at how the automatic wheel alignment mechanism actually works. A schematic system is pictured at right (the differential gear occupies a slightly different position in the model, changed in the figure for sake of clarity). If the motors (and thus the wheels on each side of the vehicle) move in opposite directions, the differential spider also moves, and this movement is transferred to a clutch gear which is engaged to a mechanism that sends the movement to 4 crown gears. Those gears are connected to cams that move the "steering" levers attached to each wheel.
Why do we need a clutch gear here? To understand this, we need to look at the entire vehicle as one mechanism with subsystems that interact with each other. When the vehicle turns, it will adapt the wheelbase automatically. However, the vehicle can keep on turning indefinitely, while the wheelbase change implies a finite movement. In other words, while both motors can turn for a long time in opposite directions, the clutch gear breaks the linkage between the wheel-turning sub-system and the differential's spider once the wheelbase has changed shape.
Let's see how the complete system works. When the vehicle is advancing in a straight line, backwards or forwards, the wheels should be aligned to the parallel to the sides of the vehicle. Once the motors start turning in different directions, the shape-shifting system kicks in and automatically aligns the wheels like in the second vehicle of the figure at left. Once the wheels are aligned this way, the shape of the vehicle keeps them from turning further. Any further movement in the same direction of the differential spider will be "absorbed" by the clutch gear.
If the motors turn in the opposite direction, the steering system will reverse its direction. The wheels will pass first through the middle position, aligning themselves with the side of the vehicle, and then turn again, reaching once again the stage where they cannot turn further due to their shape. And once again the clutch gear kicks in and breaks any further movement generated in the differential's spider.
This means that the vehicle will adapt the shape of the wheelbase automatically when turning. But once we have completed the turn, we must realign manually the wheels before the vehicle can move in a straight line again. This is achieved by briefly reversing the movement of both motors. That is, after a turn, we activate the control as if to turn in the opposite direction. Before the vehicle moves, the wheels will realign themselves with the vehicle.
In the picture at left, the positions marked with blue dots represent stages where the wheels cannot turn further. Since due to the size of the picture it is difficul to appreciate the positions of the steering levers, I have also included below an animated GIF of the whole sequence. Pay special attention to the positions of the cams and steering levers. You can also see video footage here.
|
|
| Do you like it? | LEGO?, LEGOLAND?, LEGO SYSTEM?, LEGO TECHNIC?, DUPLO?, LEGO PRIMO? and LEGO MINDSTORMS? are trademarks of the LEGO Company which does not sponsor, authorize or endorse this site. |
Enter the clutch gear. The figure at left shows 2 clutch gears connecter to the motor outputs. This device, which is also used in the SSCT, will couple the motor outputs if they rotate in the same direction, making them (and, accordingly, the differential inputs) turn at at the exact same speed. When the motors turn in opposite directions, the pressure overcomes the clutch gear and it becomes loose from its axle. Accordingly, the red axle of the picture will move if both motors turn in the same direction, but will remain still if they turn in different directions. This potentially allows for a similar mechanical adding/substracting mechanism as described earlier, substituting the differential gear for two clutch gears. However, in Differtimento's case, all it does is ensure that both motor outputs maintain the same speed.
