Tuesday, April 9, 2019

Some thoughts on Trial Truck construction & design

Having constructed a small parking lot (2-digit number) of Trial Trucks so far, and compared their live performance with their features, there are definitely some lessons to be learned―which I shall briefly cover here, hoping to help other builders possibly struggling with similar problems. And especially to help beginners avoid some tempting ideas which turn out to be cul-de-sacs.

If I had to condense all my Truck Trial experience into one single, most important lesson, it would be to avoid complexity. Features are great for bragging on the forums and in front of the less tech-savvy friends, but in practice they rarely actually help the truck's performance. Among the heaviest mistakes I have done in my early days was to cram tons of advanced mechanisms into a truck, believing they would help just they way they do in the real-life off road cars. But quite simply, they almost never did.

For example, one of my first entries (and worst failures) was an off road SUV which had no less than three independently lockable differentials, independently adjustable ride height at the front and the back axle, five-speed gearbox, four-wheel steering and, well, working headlights―and all these controlled remotely. While it was mechanically sound, in the competition it got easily beaten by the trucks half its power and quarter its features.

Not only was it too heavy, but also all these features unnecessarily raised the truck's center of mass, making it less stable. None of the advanced features actually turned out as advantages―all these differentials and ride heights only improved the performance very slightly, and altogether it was still miles behind competition

So―I can't emphasize this enough―just avoid being seduced by the fancy mechanical contraptions and go simple. A simple but responsive steering, fixed four wheel drive with plenty of power, long-traveling suspension and good ground clearance is all you will ever need. In fact, sometime later I have built a Trial Truck I got almost ashamed of due to its straightforward simplicity―yet it turned out to trample the earlier one in all the criteria important in competition.

Simplicity leads to structural strength which is another point to consider. Simple, multiply reinforced beams with several trusses in critical places will keep your truck strong and resistant to hits and vibrations, will make it easy to repair even if something goes awry, and keeps all the gears and moving parts firmly in place. This is something TLG has done finely with their official Crawler which, despite all its mechanical intricacies, is essentially simply and strongly built.

Don't be afraid to reinforce―indeed, you cannot get away without it―but simplicity makes the truck lighter and stronger, therefore requiring less reinforcements in the first place.

But what about those cases where the rules mandate the minimum truck mass? Sometimes as much as 2 kg? Does it warrant including some more advanced features if there is anyway ample capacity to build them?

Well, it seems that the general answer is no. Again, these features are hardly ever necessary, whereas they introduce issues with strength and reliability that always seem to surface exactly during trials. In such cases with minimum mass requirements, at least for me, it turned much better to devote the extra weight to very strong and reliable suspension and steering, robust transmission, and designing the chassis so that all the heaviest components are as low as possible, while retaining sufficient ground clearance. Of course, in a typical Trial Truck, it's the battery packs and the motors that have a large share in the overall mass.

Ideally, even according to the standard car construction books, the weight distribution should be approximately equal on all wheels for maximum performance. In Trial Trucks, as well as in the real world cars, it is not always possible, or at least it would require too many concessions in other areas. In such cases I've found it helpful to move the center of mass a bit forward rather than back, while it should obviously never move aside. The reason is that the forward center of mass allows for better distribution of weight over all wheels while climbing, which is the most critical operation. It makes descent slightly more unstable, but usually that drawback is largely overshadowed by better climbing performance. Of course, for this to work as intended, the truck should have all-wheel drive, but this is something one should consider as granted for a Trial Truck anyway.

And if there is some mass to be added even after all the necessary components have been completed, it helps to distribute them evenly around the chassis (or again, slightly forward).

Onward, then, to some construction details. Try always to have as few gears in the entire transmission chain as possible. If you opt for a gearbox, make it as straightforward and robust as possible, with two gears usually being enough. And try to rely on the newer, bevel-style gears as they seem to be stronger than the older, spur types. If you absolutely have to go with spurs, the new 24T and 12T seem to be all right, but avoid the small 8T, even in their newer versions. And make sure all torque-bearing gears are properly braced from both sides and mesh cleanly.

Mentioning torque: it is generally wiser to trade a bit torque for speed. What I mean to say is that, instead of letting most of the axles turn slowly but carry large forces, it makes sense to use a couple of gears to make the axles turn faster yet transmit less force (i.e. torque). Of course, one should not go into extremes as friction comes into play at very high rotation speeds. But a transmission which mostly relies on higher speed/lower torque configuration, only to reduce the speed for more torque at the final stage at the wheels (preferably portal axles!) is often a good idea. Less torque also means less reinforcement, and more reliability.

Tuesday, February 19, 2019

LEGO Technic Seismometer Prototype

Well, in its barest concept, a seismometer (device for measuring Earth's surface movement, namely quakes) is a rather simple thing. A large weight, suspended to move freely, attached to something that can note its movements - and that's it. And while assembling a rough one from parts scavenged from an old rusty car at the dump may seem just as simple, building one from LEGO Technic brings its own set of challenges along.

Although a group of dedicated weight bricks (such as 73090a) would have been a "purer" solution and possibly serve its purpose better, I went for the more pragmatic approach and connected a set of several battery packs together - thus indirectly using batteries as weights. Altogether, something the size of a large coffee mug ended up weighing well over one kilogram - more than enough to have the concept proven.

This weight is suspended, hanging on two rubber bands. I agree that, in theory, going for a more complex setup featuring six or eight bands (one for each corner) would have helped with force distribution and rotation, but would have been a nightmare to tune and set up. Two rubber bands allow for sufficient freedom of motion, however, and have a better chance of retaining the seismographer's sanity.

Furthermore, the weight is attached to two independent pieces of thread, one in vertical and another in horizonal plane, both forming a closed loop, which is led through a system of pulleys to translate the weight's movements into proportional movements of the threads. Finally, attaching pencils to the threads and letting a writeable surface slide perpendicularly underneath the pencils, completes the essential seismometer.

Hence, each pencil notes the weight movement in its own direction over time - one vertical, and other horizontal. This is a common approach with real life seismometers as well, because horizontal and vertical movements of the Earth surface lead to different effects and are measured separately. True, this weight could have included a third thread loop measuring the weight's movements in the third direction (sort of like X-Y movement) but that would add lots of complexity while bringing only marginal functionality, and likely go beyond the 48x48 baseplate I decided to use as a caliber for this project.

The writing surface, which is in this case a cascade of white Technic panels, is driven across the direction of the pencils via a rack and pinion system with variable speeds, controlled with a gearbox, allowing for 10 possible settings, balancing between precision in timing and total duration of a single "plate". With only minor adjustments, one could convert it so that a standard roll of paper (e.g. like those used in shop counter printers) can be used.

Finally, apart from dozens of accurately perpendicularly tuned pulleys (perpendicular pulleys retain linearity better), both threads pass through a simple, manually controlled tension mechanism employing a large 40T gear and a worm gear, allowing the seismographer to adjust the correct tension level finely once the seismometer has been set up. The aim is to keep it tense enough so that all weight movements are precisely translated to the movements of pencils running across the "writable" white panels, yet loose enough to avoid friction that would overly hamper the movement of the weights. With some experimentation, I've settled for the tension of about 1.5 Newton, roughly the tension you would get if you suspended your mobile phone off a thread.

The only active component in the entire mechanism is the motor driving the writable panels, mounted on springs to further isolate it along with its vibrations from the rest of the mechanism (though I admit it's not as bad with fixed mounting either). But this could even be done manually if one wanted - the motor at least assures that the graph movement speed is at least somewhat constant. The range of possible speeds, of course, increases if one uses regulated voltage at the input, like I did with the classic 9V train controller.

So, an obvious next question is - how does it behave? The good news is that it proves the concept indeed: having been set upon the table, it is sensitive enough to measure, at least with some consistency, average table bumps, and would surely have no problem measuring a mild earthquake. But it is, on the other hand, far too insensitive to actually measure micro-movements of the Earth surface which are typically imperceivable for people, and which real life seismometers are built to measure. Let alone even finer things like steps of people in the building, opening and closing faucets, bass kicks from someone listening to music somewhere, etc.

I don't say that the latter group could not theoretically be measured with LEGO, but that would require a different design, utilizing more specialized parts, and would likely be far too sensitive for actual quakes. It would probably revolve around a multi-reflection design, featuring a mirror on the weight, with a laser pointer pointed at it, and furthermore with the ray being reflected multiple times to artificially create distance and thus increase resolution. Finally, the laser spot's movements would be monitored by a camera, or by a cascade of Mindstorms' light detectors, or something similar. Such a setup's sensitivity can in theory be increased virtually infinitely, but at the cost of maximum measurable extents' window reducing. Id est, it may well measure the vibrations of the roof being shaken by the wind five stories above, and someone sorting out cutlery in the kitchen in the other wing of the house, yet go completely off scale if breathed into.

Finally, a bit of historical trivia: although commonly thought to be of modern origin, the first seismometers actually got built as early as 2nd century AD, in China. It was a rather simpler approach, with a precariously ballanced ball at the top of an inverted bowl. The fact that the ball dropped from the bowl indicated that there was an earthquake, while the direction of the ball's fall gave a rough idea about where did the earthquake originate from - letting the owners know in which direction should the help and rescue teams be sent.

Built for BrickStory 2019.