Beginners' guide: One motor, many functions

Experienced builders are well acquainted with many ways to reduce several functions to a single control axle, but the concept is important enough to present it to Technic beginners. Namely, typical approach, which works just fine for majority of MOCs, uses one control axle (or a motor) for each function. However, mobile and remotely controlled models often set strict limits on the number of electric components one can use, and in such situations it is necessary to go for alternate solutions - among which this differential-ratchet-gearbox system is just one of possibilities.

A careful look at the photos should reveal at least a little bit of how this system works ― but let's describe it component by component anyway. The input axle, in this case attached directly to a motor (A) is directly linked to a differential (B) master gear. Each of its two outputs leads to ratchets (C) that limit the rotation in different directions. This twin-ratchet setup ensures that the different directions the input axle is rotated are split among two exiting axles; this can itself be a useful component.

One output axle drives the input of a simple gearbox (D) which, in this case, has three output gears, though designs with four or more shouldn't present a problem. However, there is only one sliding intermediate gear, ensuring that the input rotation can be linked to only one output axle (E) simultaneously. Of course, the other axle from the differential controls the movement of this gear. In this case, it is done with a simple worm gear, spur gear and an off-center set control beam (F).

In other words, driving the motor in one direction changes the active output axle of the gearbox in the 1-2-3-2-1 order. Running it in the other direction transfers its motion to the selected active axle. Of course, the entire system can be built more compact in practice ― this display version was designed to avoid the components being obfuscated.

There are drawbacks one needs to be aware of, of course ― most importantly, the output axles rotate in one direction only (though, if they control a linear motion, the aforementioned off-center set beam connections can help). This system also allows the unlinked axles to rotate freely in one direction, and introduces some friction (thus reducing available torque). It's too large to cram comfortably in smaller models, and finally, it's somewhat noisy. But in certain situations, the benefit of controlling everything with just a single input axle, i.e. one motor, easily overshadows these remarks.

Venturing a little further into theory, it is conceivable to attach each of the output axles to off-center linkages that control Power Function switches connected to the motors attached to further such systems, squaring the number of available output functions (output axles). However, such system would be incredibly cumbersome to use.

Finally, a few auxiliary photos without labels, and a video:

 

 

 

 

Self-steering car chassis, Mark II

Some may remember a very simple self-steering chassis I've tested last February. It was about time to advance it, so here's a new self-steering chassis concept, Mark II.

Let's just quickly remind ourselves what the self-steering chassis is about, and when would one prefer it over typical designs. Namely, if a planned vehicle is limited to using only two motors altogether (due to weight, power requirements, size, simplicity or any other reason), a conventional approach would use one to provide drive, and the other to steer. However, for some particular models it might be troublesome to use only half the available power for providing drive.

An alternative is to have both motors provide drive, one for each wheel. In such configuration, steering is done by having the two motors (and of course, their respective wheels) turn at different speeds, thus producing sufficient tangential force for the chassis to turn. This type of drive, often known as a differential drive, is used by tanks, for example. However, if the chassis is wheeled, it will turn too ― at least if the front wheels have sufficient caster to readily follow it, what was the point of the aforementioned Mark I chassis. But it still suffers from poor reliability unless perfectly balanced and having minimal friction, both of which are sometimes unavoidable. Therefore, an obvious direction for improvement is to introduce a steering control that follows the drive wheels' behaviour.

Mark II concept attempts to solve this problem mechanically. To explain its functioning, let's split it down to a few main components.

Each of the rear wheels' axles (A) is linked to a differential several studs ahead (B). As you have perhaps noticed, there are three gears (C) on one, and four at the other side, resulting in one of the axles spinning in reverse. The differential thus works opposite from its typical application on the half-axle: it actually turns only when the rear wheels aren't turning at the same speeds (i.e. they tend to steer), and its rotation speed is proportional to the difference between the rear wheels' speeds. But as long as the rear wheels turn simultaneously, it remains stationary.

In other words, its turning speed is proportional to the amount of steering that needs to be done by the front wheels ― of course, also obeying the direction. But since it's the speed and not the absolute displacement we're interested in, this differential cannot be directly linked to the steering pinion. Instead, we need a simple sort of a mechanical tachometer, and that is approximately what the front differential (D) does. It has got a very light brake (E) at its main axis, and its second axle leads to a usual steering rack&pinion mechanism (F).

The point is that the brake and the rack&pinion work in balance: the more torque and speed arrive from the rear differential, the more will they be met by the increasing resistance of the brake, thus directing more towards the steering. The steering, of course, imparts its own resistance depending on how sharp turn it's attempting to make, and yet more if it's equipped with a recentering system (G). Exact steering would need a very precise balance between these variables, but in practice it needs no more than a nudge in the correct direction, and then the friction to the ground keeps it in correct geometry. In other words ― it works.

When there is no need to steer anymore (the rear wheels begin turning at the same speed), the rear differential will stop turning, and it will be easier for the front wheels to recenter themselves thanks to a bit of caster. Unless the weight on the front axle is huge and the tyres grippy, this force will be very light ― and that is the main reason why the pinion couldn't be linked to the rear differential through a standard clutch gear: its resistance would almost always be too high, and prevent the front wheels from recentering. Very heavy models may still get away with it, though ― but they rarely have limits on the number of motors anyway. The better but more complex solution is to introduce a recentering mechanism and adjust its force carefully according to the rest of the system, in which case the caster is not necessary.

There are still some particular situations in which Mark II works poorly, such as turning in place (with rear wheels turning in opposite directions), turning slowly in sharp curves, and driving backwards if the steering is centered only by caster (without recentering mechanisms). However, in typical circumstances it works all right.

Flashback: 8865 Test Car

When talking about classic Technic supercars, it's the 8880 which (rightly) usually gets the largest spotlight. However, its predecessor, the 8865 Test Car, was just as advanced and innovative for its time. The one I've built doesn't adhere entirely to the original's colours (some original parts got damaged in those 20+ years), but mechanically it's accurate.

This set was released in 1988 ― of course, deeply in the era of studded beams and brittle toothed connectors. It relies on so many standard parts that it should be fairly simple to build if you've got a good "general" collection of studded Technic, and some special parts such as the front suspension hubs and the old differential can be replaced by their modern equivalents fairly easily. Altogether it's still visibly simpler than the 8880, but curiously, it's a tiny bit longer, wider and higher.

Besides the part assortment, one noticeable difference to the modern Technic sets are the instructions. At 892 parts and featuring many complex systems, modern instructions for the 8865 would be spread into two or even three booklets, while the original's consist of just one booklet which isn't too thick (and shares space with the B model). The fact, provable also through other Technic sets from the time, is that the building steps were much more condensed back then, introducing many sub-steps and handling several components simultaneously. Often requiring very careful inspection of drawings to notice the changes.

The features and functionalities the Test Car offers clearly overshadow its own predecessor, the 8860. It has a V4 engine (the first Technic engine with a V layout), built from old large square pistons and using the offset axle holes in the 24T gears to form a crankshaft. It is connected to a linear axle-sliding gearbox with 3 speeds and neutral, which leads to the differential at the back.

Front independent suspension utilizes special parts relying on ball joints to allow steering, and follows the Ackermann geometry. It is based on usual rack & pinion mechanism, linked to the steering wheel in the cabin. The rear double wishbone suspension doesn't use special parts as an "easy way out", but is instead built from standard parts ― mostly beams, pins and plates, with hinge points precisely adjusted with the U-joints' fulcrums. This structure is quite wide, and together with the need to fit the differential on the same axle, it defines the car total width.

Headlights can be also extended and retracted using a lever in the cabin. The lever rotates a simple crank that raises or lowers the (purely decorative) lights. Finally, almost everything can be adjusted regarding the seats: the back and headrest angle with standard hinges, and the front-back position via rack & pinion.
The bodywork composed mainly of 16L and 12L beams is quite basic, especially in comparison to the modern-age Technic models, but it gives a fairly good idea of the overall outline of the car. It's rather sturdy and, together with a strong chassis, makes the 8865 one seriously shockproof model.

Along the lines of the deep-studded era Technic, there's no real static decoration here. As mentioned, even the headlights are moveable, and the engine fan rotates together with the crankshaft.

However, the model isn't entirely flawless. Changing gears is difficult, and usually impossible if the car is static (it was the later 8880 which introduced a smooth synchronized gearbox with dog-rings). The friction of the parts causes quite a strain on the parts in the first gear if extreme caution was not given to allowing slack in the transmission and suspension, or if the engine crankshaft it offset by the tiniest amount.

But altogether, 8865 is undoubtedly among the all-time Technic flagships. It is also a good demo of how much is possible using mostly regular Technic parts and a few well-known mechanisms.

GALLERY



GENESIS (Timelapse)

Classic Space: 6819 Orbital Shuttle

Don't bother looking for this one in the official catalogues; it's just a mockup, a graphical exercise of recreating the original Classic Space box arts (which I adore) as accurately as possible. The set number is fake, too ― among many other 68xx sets from the theme, the '19' is missing for some reason.

I've tried to keep the Shuttle more or less within the boundaries set by the Classic Space: blue hull, grey machinery, black and transparent yellow details, very simple and clean form-following-function design, and a prominently displayed Space logo both on the minifig and the craft. It's made of 55 parts, not counting the minifig.

The rest is straightforward. Spaceships are usually photographed from the starboard side at an angle and from slightly above, set against a starry background and above a beige planet surface ― tasks which any half-decent bitmap editing program can do with ease, and the background resources are found in abundance on the Internet.

Though the Classic Space is my favourite of all the Legoland/System themes, I like the Space Police Gen I too, and hope to come up with something interesting from that theme as well ― at least when the 6886 Galactic Peace Keeper arrives via Bricklink, which should provide some useful theme-specific parts.

Technic TGB Racing Car

Finally, after several weeks of development frequently interrupted by all sorts of distractions, the TGB is finished. This is the largest Technic car I have built so far. Though not intended as a replica of a real-world car, its component layout and some proportions are very loosely based around the legendary Ferrari 288 GTO ― and hence its name: "TGB" is "GTO" scrambled through ROT-13.
Length: 66 studs
Width: 29 studs (34 including mirrors)
Height: 16.5 studs (bodywork only: 15 studs)
Weight: 2.38 kg
Suspension: Full independent, with front toe-in
Power: PF, 2x Battery pack
Drive: 1x PF XL motor (remote), RWD
Steering: 1x PF M motor (remote) with working steering wheel, Ackermann
Features:

• V12 mid-mounted engine
  
• 6-speed synchronized gearbox
  
• pneumatically liftable nose (with motorized pump)
  
• handbrake
  
• remotely operable headlights
  
• remotely moveable rear wing
  
• openable spring-loaded bonnet
  
• toolbox

Obviously, instead of forming the entire bodywork with panels, I've rather opted for the old-school rough contoured approach using beams and a couple of soft-axles. The model is built completely with studless parts, with sole exception being the rear lights that rely on studded transparent cylinders.
The engine and gearbox consist of standard Technic parts for the purpose: cylinder blocks, pistons and cranks in the engine, and three dog-rings (for six gears) moveable with a stick in the familiar H-pattern for the gearbox. At the final stage there is a standard Technic differential. The car is driven by a single PF XL motor connected to the engine crankshaft, but due to large car weight and many transmission components that introduce significant torque, it isn't blindingly fast.
Independent suspension and its rear half-axles are based on parts from the 8070 Super Car, though with "artificially" extended arms to comply to the desired total chassis width and allow the larger moving extents. Height of the front suspension is determined by two parallel pneumatic cylinders that control the total height of the chassis at the front, but due to the compressibility of air, front wheels can still move with a degree of independency ― and the pneumatic cylinders even provide some damping and elasticity. To raise the car nose, the air is pumped into the cylinders with an onboard motorized pump controlled by the levers in the cockpit. Only the compression is required; when the switch is flicked in the opposite direction, the weight of the car lowers the chassis (compresses the cylinders) itself. The rear suspension is rather standard, with only two soft (light grey) springs per wheel. That is intentional: it lets the car sink approximately 3/4 of total spring extents, for a more realistic and low-laid sporty look.
Steering is based on a typical rack & pinion system, connected through a clutch gear both to a remotely controlled PF medium motor and a steering wheel. One more control available to the driver is the handbrake, positioned to the left of his seat (just like some old Lamborghinis). There wasn't enough space to cram the brakes into the wheels, so it actually pushes a beam against a small rubber wheel attached to the main driveshaft.
Headlights employ standard PF lights (one pair apiece), and the primary concern around them was ensuring that kilometers of their cables do not interfere with the nearby mechanics (suspension and steering). The rear lights are just decorational, but the wing above them isn't: it can raise about 4 studs from its resting cradle which is flush with the bodywork. It is attached to a pair of beams lifted by a PF medium motor placed under the engine. To prevent the motor from stalling when the wing reaches its end positions, a variant of Sariel's 6x1x1 clutch is used.
Along with drive and steering motors, headlights and the rear wing occupy four available remote control receiver ports, while the suspension pneumatic pump can be operated only locally, through the cockpit levers. The power is provided by two PF battery packs ahead of the rear wheels, to spread the thirsty motors' consumption a bit.
As one would expect, the bonnet can be opened to expose the engine and ease the way to the PF batteries, and is spring-loaded in both directions to keep it either firmly shut, or open. Finally, just behind the rear left wheel is a tiny gimmick - an openable compartment with two System-type wrenches.
Despite it incurring some mechanical compromises, I've managed to keep the chassis bottom completely flat (without any protruding studs or gears), the weight distribution close to 50/50 (in fact, it's 48F/52R, but I can live with that), and reasonably small turning circle.  Also perhaps worth noting is a little bit of toe-in on the front wheels.
In general, I am mildly satisfied with how the car turned out, but still I find some of its segments less successful. Here's what should, in my opinion, have been better:

• Bodywork design. I've never had a surplus of designer or visual art talent. Having changed and re-changed it several times, I've half-heartedly settled on this final bodywork visible on the photos, but I'm still convinced it could be significanly better.
  
• Pneumatic pump. It is awkwardly placed and based on an XL motor far too cumbersome for the task. As it was one of the final components, by then I've had no alternative regarding these two parameters, but really it should have been considered in more detail during the planning stage.
  
• Chassis strength. Despite relying on well-pinned four beams throughout the most of its length, the chassis still bends a few millimeters under the car weight. The roof should have been given a structurally more important role.
  
• Gearbox dimensions. Being 7 studs wide, the gearbox pushed the seats far too wide apart. Future designs should perhaps place it somewhere in the engine bay, and have it controlled from the cabin with control axles or even flex system.

These remarks will serve as guidelines for the next Technic car. Other features I'm considering, which this car lacks, are power steering, automatic or sequential gearbox, a full set of disc brakes that also engage the brake lights at the rear, and suspension with (perhaps adjustable?) camber.
Basic views
 
Details

Genesis

Video

Unsolved (or at least not elegantly solved) problem: Power steering

Excuses for a lengthy delay ― a bit of holidays (and then catching up with lots of work meanwhile accumulated) interrupted the posting routine. While I'm toying with a Lego project for which I hope to put a text together in near future, I got intrigued by a problem of simple yet effective Technic power steering.

Sounds simple on the surface, doesn't it? Attach some kind of force meter (probably a differential with a spring) between the rack's pinion and the steering wheel, power the axle with a motor if a given threshold of force is exceeded, and you've got your power steering. Looking something like this (here implemented with NXT rather than Power Functions, but the working principle is the same):

So, as you can see, an arm sufficiently tilted by a differential when the force between the pinion (at the top) and the steering wheel (bottom) becomes large enough, will push on one of the mechanical sensors, depending on the rotation direction. The motor will start spinning accordingly and thus assisting the steering wheel, as long as there is force between the pinion and the steering wheel. When it's gone, the arm will return to the neutral position between the sensors and the motor will interject no longer.

In its basic principle, this does work as a power steering ― it will assist in rotating the pinion against significant resistance, and not activate unless the pinion is resisting the motion set by the steering wheel, or vice versa. Yippee, problem solved, and we can return to playing Portal 2.

No, not really. The trouble is that this sort of system doesn't even remotely provide the continuous and soft feel which it should. It works far too jerky: once the motor threshold is reached, its abrupt intrusion easily turns (and over-turns) the steering wheel and the pinion in the desired direction, releases the arm, and stops. Nothing like the smooth operation found in real cars.

The basic problem here is in the binary operation of the motors and sensors. It can either be on and off, while a smoothly working power steering should have a range of intermediate levels, keeping the motor force proportional to the wheel-pinion tension, and yet sensitive enough to start at low thresholds.

One method could rely on a gradient colour disk being spun according to the tension instead of a pusher arm, which is in turn scanned by an NXT colour sensor. It could determine the force with greater precision and thus run the NXT motor with different power, as required. But this approach would be unacceptably complicated and cumbersome, especially if we confine the project only to standard Lego parts ― in which case building the gradient colour disk would be even more complicated. After all, this system should easily fit in the front of a medium-sized Technic car (say, 8070), and yet keep enough room for the steering, suspension, etc.

Ideas? Post them in the comments if you've got any. If I come up with something, I'll add an update :)